Adsorption and Thermal Processing of Glycolaldehyde, MethylFormate, and Acetic Acid on Graphite at 20 KDaren J. Burke,*,† Fabrizio Puletti,‡ Paul M. Woods,§ Serena Viti,∥ Ben Slater,‡ and Wendy A. Brown†

†Division of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.‡Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.§Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN,U.K.∥Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K.

*S Supporting Information

ABSTRACT: We present the first detailed comparative study of the adsorptionand thermal processing of the three astrophysically important C2O2H4 isomersglycolaldehyde, methyl formate, and acetic acid adsorbed on a graphitic grainanalogue at 20 K. The ability of the individual molecule to form intermolecularhydrogen bonds is extremely important, dictating the growth modes of the ice onthe surface and the measured desorption energies. Methyl formate forms onlyweak intermolecular bonds and hence wets the graphite surface, formingmonolayer, bilayer, and multilayer ices, with the multilayer having a desorptionenergy of 35 kJ mol−1. In contrast, glycolaldehyde and acetic acid dewet thesurface, forming clusters even at the very lowest coverages. The strength of theintermolecular hydrogen bonding for glycolaldehyde and acetic acid is reflected intheir desorption energies (46.8 and 55 kJ mol−1, respectively), which arecomparable to those measured for other hydrogen-bonded species such as water.Infrared spectra show that all three isomers undergo structural changes as a resultof thermal processing. In the case of acetic acid and glycolaldehyde, this can be assigned to the formation of well-ordered,crystalline, structures where the molecules form chains of hydrogen-bonded moieties. The data reported here are of relevance toastrochemical studies of hot cores and star-forming regions and can be used to model desorption from interstellar ices during thewarm up phase with particular importance for complex organic molecules.

1. INTRODUCTION

The structural isomers of C2O2H4, glycolaldehyde, methylformate, and acetic acid are simple organic molecules that haveattracted attention in a variety of different scientific fields,including astrochemistry, atmospheric chemistry, and heteroge-neous catalysis. Although our particular interest focuses on theirrelevance to astrochemistry, glycolaldehyde and acetic acid areboth important in atmospheric processes.1−3

With respect to astrochemistry, all three molecules have beendetected in the gas phase in the interstellar medium toward star-forming regions of space, which are chemically rich and diverse incomplex organic molecules (COMs).4−6 COMs in general, andthese three isomers in particular, are currently being detected inan increasingly wide range of astrophysical environments,7 andthere is therefore a great deal of interest in the chemistry of theformation and destruction of these species in various regions ofspace.8 It is thought that surface processes contribute to theformation of these molecules within interstellar ices, which arefrozen out on the surface of dust grains at the cold temperatures(20 K) present in the interstellar medium.8−13 Glycolaldehydeand acetic acid are particularly important, because they areclassified as prebiotic molecules which are important to the

origins of life on Earth.14Methyl formate is the simplest ester thathas been detected in astrophysical environments and it has beenproposed that its presence is closely linked to the formation ofdimethyl ether and other important astrophysical molecules.15

Despite their increasing importance and detection in a widerrange of interstellar environments, there remains littleinformation about the exact role of surface chemistry on grainswith respect to the formation and processing of these molecules.Therefore, to investigate the surface chemistry of the threeisomers in model interstellar ices, we have undertaken a detailedstudy of the adsorption, growth modes and thermal processing ofglycolaldehyde, methyl formate, and acetic acid adsorbed on thesurface of HOPG (highly oriented pyrolytic graphite) at 20 K.This work is part of a wider study that explores the thermalprocessing of the three isomers, both when they are pure ices andwhen they are adsorbed in the presence of water ices.16 HOPG isa suitable dust grain analogue, because interstellar dust grains areknown to be carbonaceous and silicaceous in nature,17,18 and has

Received: April 27, 2015Revised: June 7, 2015

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been used for a range of differing astrophysically relevantlaboratory studies (see, for example, refs 19−21). Although thepure ice forms of the isomers are not directly astrophysicallyrelevant, due to the multicomponent nature of interstellar ices,22

understanding the physical processes and interactions withinpure ices is crucial for an understanding of the complexities ofmixed ices. In addition, detailed studies of the desorption of thepure species allows the determination of kinetic parameters thatcan be used to model desorption under interstellar conditions.16

There have been no previous studies of the interaction of theseisomers with graphitic surfaces and only one previous study oftheir adsorption on any surface at the very low temperaturesrelevant to interstellar chemistry.23 In particular, for glycolalde-hyde, the only previous study of relevance to this work wasreported by Hudson et al., who investigated the ion irradiation ofpure glycolaldehyde ice and glycolaldehyde embedded in a rangeof matrixes, including H2O.

24 To date, there have been noreported temperature-programmed desorption (TPD) studies ofglycolaldehyde and we present the first results here and in arelated paper.16 However, several infrared studies have beenconducted to identify the structural conformers of glycolalde-hyde in the gas,25,26 liquid,26 and solid phases,25,27,28 in additionto those of glycolaldehyde embedded in matrixes.29,30 In somecases, these studies were coupled with theoretical calculations toaid the assignment of infrared bands.25,28,29

In contrast, there have been several studies of methyl formateon metallic surfaces. Of relevance to the data described here,studies on Ag{111}31 and clean Cu{110}32 have shown thatmethyl formate adsorbs molecularly as both monolayer andmultilayer. In contrast, on Ni{111},33 dissociative adsorptionoccurs at low coverages, with molecular adsorption beingobserved for multilayers. Work investigating methyl formateadsorption on nonmetallic surfaces is limited. Modica andPalumbo recorded infrared spectra for methyl formate adsorbedon a KBr substrate.23 Methyl formate adsorption onpredeposited H2O ices at liquid nitrogen temperatures has alsobeen studied by Bertin et al. employing TPD and reflection

absorption infrared spectroscopy (RAIRS)34 and by Lattelais etal. using a combination of experiment and theory.35

There are no previous studies of acetic acid adsorption onmodel grain surfaces at astrophysically relevant temperatures.However, acetic acid adsorption on thin H2O films at liquidnitrogen temperatures has been studied by Bertin et al.,34 by Bahrand co-workers,36−38 by Hellebust et al.3 and by Gao andLeung.39 Using a range of experimental techniques in addition todensity functional theory (DFT), these authors explored thestrong interaction between acetic acid and H2O molecules, withparticular reference to astrophysical and atmospheric ice systems.In this paper, we present the first results of a detailed

comparative TPD and RAIRS study of all three isomers adsorbedon HOPG at ∼20 K. For the coverage dependent TPD studies,numerical analysis has allowed the determination of thedesorption parameters of the isomers, including the desorptionenergies. For glycolaldehyde and acetic acid, these are the firstexperimentally determined values reported in the literature(theoretical values have been previously reported for acetic acidadsorbed on water ices35,37). To compare with our experimentaldata, we have also calculated preliminary adsorption energies forthe three isomers adsorbed on a coronene surface. Tocomplement the TPD study, we have performed RAIRSannealing experiments to explore the structural ordering of theices as a function of temperature. The data reported here are ofrelevance to astrochemical studies of hot cores and star-formingregions. In particular, the TPD desorption parameters and thetrapping behavior of the isomers in water ice, can be used tomodel desorption from interstellar ices during the warm up phaseof a star, as reported elsewhere.16 To fully understand thetrapping and desorption of these isomers within and adsorbed onwater ice (the main constituent of interstellar ices), it is firstnecessary to understand the detailed physical chemistry of theadsorption and thermal processing of the individual pure ices.

Figure 1. TPD spectra following increasing exposures of pure glycolaldehyde adsorbed on HOPG at 23 K. (A) shows the desorption of amorphousglycolaldehyde following exposures of 0.5 to 5 Lm. (B) shows the desorption of glycolaldehyde ranging from 5 to 20 Lm, showing the appearance ofcrystalline ice as a high temperature shoulder. (C) shows the TPD traces following exposures of 20 Lm to 70 Lm, showing the evolution of the crystallinephase with increasing coverage. The inset in panel C shows the divergent nature of the leading edges in more detail.

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2. EXPERIMENTAL METHODOLOGY

The TPD and RAIRS experiments were performed in separatesteel ultrahigh vacuum (UHV) chambers with working basepressures ≤2 × 10−10 mbar. Both chambers are equipped with aclosed-cycled helium refrigerator, allowing cooling of the sampleto base temperatures of 23 and 20 K for the TPD and RAIRSexperiments, respectively. Temperature control and measure-ment was achieved by a Eurotherm 2408 controller coupled to anE-type (TPD) or N-type (RAIRS) thermocouple. HOPGsamples were purchased from Goodfellows and cleaved priorto installation in the chamber by the Scotch tape method.40 Thesamples were cleaned by repeated annealing to 500 K in UHV.Sample cleanliness was confirmed by the absence of anydesorption products during TPD experiments performed withno dosage. In both chambers the ices were grown in situ bydosing via a high precision leak valve. All exposures weremeasured in Langmuir (Lm), where 1 Lm = 10−6 mbar s. For themajority of the exposures, dosing was performed at 1 × 10−7

mbar with adjustments made to the deposition time to vary theice thickness. Only at the lowest (<1 Lm) and highest (>30 Lm)exposures was the background pressure adjusted to ensureaccurate dosing over a reasonable time scale. Methyl formate(Sigma-Aldrich, 99% anhydrous) and acetic acid (Sigma-Aldrich,≥ 99%) were purified by repeated freeze−pump−thaw cyclesprior to deposition onto the HOPG surface. Solid glycolaldehydedimer (Sigma-Aldrich) was placed in a stainless steel vessel andpumped under vacuum (p < 10−3 mbar) for several hours prior toheating to 95 °C until a constant pressure in the gas line wasachieved. To remove residual amounts of air and water from theheated glycolaldehyde sample, the dosing assembly was pumpedagain prior to dosing. To prevent the condensation ofglycolaldehyde onto the walls of the dosing line, the entireassembly was maintained at a temperature of 95 °C. This methodpermitted the dosing of pure monomer glycolaldehyde onto theHOPG sample. The purity of glycolaldehyde was determined bymass spectrometry.RAIR spectra were recorded using a Thermo-Nicolet 6700

Fourier transform infrared spectrometer coupled to a liquidnitrogen cooled mercury cadmium telluride detector. All spectrawere recorded at a resolution of 4 cm−1 and are the result of thecoaddition of 256 scans. For the RAIRS annealing experiments,the sample was raised to the target temperature, held for three 3min and then allowed to cool prior to recording a spectrum. TPDspectra were recorded using a Hiden Analytical HAL 301/PICquadrupole mass spectrometer. A range of masses were recordedfor each species and each exhibited identical behaviors. Hencethe most intense mass fragment is shown in each case: mass 31for methyl formate and glycolaldehyde and mass 43 for aceticacid. The heating rate for all TPD experiments was 0.50 K s−1.

3. RESULTS AND DISCUSSION

3.1. Glycolaldehyde. 3.1.1. Glycolaldehyde TPD. Figure 1shows TPD spectra following increasing exposures of glyco-laldehyde on HOPG at 23 K. Glycolaldehyde desorption fromHOPG exhibits simple behavior at lower exposures (<15 Lm,Figure 1A,B), which becomes more complex when exposuresexceed 20 Lm (Figure 1C). At the lowest exposures, 0.5−5 Lm,desorption gives rise to a single peak (Figure 1A), whichincreases in temperature with increasing exposure, with the TPDtraces sharing a common leading edge. This behavior continueswith increasing exposures up to 15 Lm.

At higher exposures, the glycolaldehyde TPD becomes morecomplex, exhibiting many different features during desorption,and ultimately giving rise to a simple peak at the highest exposure(70 Lm, Figure 1C). A 20 Lm exposure sees the appearance of ahigh temperature shoulder on the original TPD trace (Figure1B). This shoulder gradually evolves into the main peak,dominating the spectrum by 70 Lm. In contrast, the original lowtemperature desorption peak (observed at the lower exposures)becomes a less distinct, but clearly discernible, feature on theleading edge of both the 30 Lm and 50 Lm traces. This peak iscompletely absent from the spectrum at 70 Lm.It is clear from Figure 1 that at least two different forms of

glycolaldehyde exist on the surface, the ratio of which is bothcoverage and temperature dependent. The increasingly divergentnature of the leading edges on the TPD spectra (illustrated inFigure 1C) is often characteristic of a phase change occurring inthe ice during heating. Hence, the peak originating as a hightemperature shoulder in Figure 1B can be assigned to desorptionof a crystalline form of glycolaldehyde, and the low coverage peakis assigned to the desorption of an amorphous form. It is clearthat the rate of desorption for the crystalline ice is much lowerthan that of the amorphous form. This observation has also beenreported for the phase change of water ice.41 As theglycolaldehyde coverage increases, the conversion from theamorphous to the crystalline phase increases. This is confirmedby a series of annealing RAIRS experiments (discussed below),which clearly identify a phase change in the glycolaldehyde at 140K.At the lowest exposures, a distinct monolayer peak would

often be expected in the TPD spectrum. However, this is clearlynot the case (Figure 1A), because all TPD share leading edges.This is characteristic of zeroth-order desorption, typically seenfor multilayer ices. This suggests that glycolaldehyde does notwet the HOPG surface at 23 K, preferentially formingintermolecular bonds as opposed to bonding with the substrate.The ability of glycolaldehyde to form intermolecular hydrogenbonds dictates its growth mode on the HOPG substrate, with itpreferentially forming clusters, as opposed to wetting the surface.This has also been reported for the adsorption of otherhydrogen-bonding molecules on a graphene surface, wherezeroth-order desorption was observed for all exposures.42

To quantitatively characterize the desorption behavior ofglycolaldehyde, leading edge analysis was performed on the TPDdata to determine desorption orders, energies, and pre-exponential factors. The methods employed here are fullydescribed elsewhere19 and a comparison of this analysis methodwith others has previously been undertaken for methanol,showing the validity of this technique.43 The data derived fromthis analysis has been employed to model desorption onastrophysical time scales, assisting in the understanding ofthermal chemistry in the interstellar medium.16

A summary of the kinetic parameters determined forglycolaldehyde (and the other ices) is shown in Table 1. Thesevalues have been used to model the desorption of these threeisomers under interstellar conditions.16 The values of theseparameters are derived from the mean values determined fromsets of repeated experimental measurements. The correspondingerrors given in the table are then derived from the standarddeviation of these sets of repeated measurements. Onlymultilayer values are reported for glycolaldehyde due to theobservation of zeroth-order desorption across all exposures. Toour knowledge there are no experimentally determinedparameters for glycolaldehyde desorption from any surface in

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the literature. For glycolaldehyde, our desorption order is zero,irrespective of coverage. The desorption energy of 46.8 ± 8.2 kJmol−1 for glycolaldehyde compares favorably with energies forother hydrogen-bonded molecules showing a similar desorptiontemperature from HOPG, such as methanol44 and water.45 Thedesorption energy derived for glycolaldehyde is an energycorresponding to the desorption of amorphous ice. This energy isderived from TPD spectra for exposures <30 Lm, prior to thecrystalline form dominating the spectrum. Attempts were madeto fit the higher exposures, to obtain a value of the desorptionenergy of crystalline glycolaldehyde. However, the quality of thefits for the two phases was not sufficiently good to allow values tobe determined.3.1.2. Glycolaldehyde RAIRS. Figure 2 shows RAIR spectra

for 50 Lm of glycolaldehyde deposited on HOPG at 20 K.

Experiments were performed for exposures ranging from 20 to100 Lm of glycolaldehyde. All exhibit identical behaviors uponadsorption and subsequent annealing. Hence, the 50 Lm RAIRSannealing sequence was chosen as a representative example. Thespectra in Figure 2 show a range of infrared bands, theassignments for which are outlined in Table 2.Although our data are for glycolaldehyde condensed on a

surface, several bands can still be confidently assigned withreference to existing data for the vapor phase, for crystalline

forms, and in matrixes.25,26,28−30 The dominant CO stretchingmode at 20 K is located at 1753 cm−1. The band in closeproximity to this feature (1713 cm−1) is assigned to the overtoneof the CC stretch. Overtones are typically characterized byweak bands; however, previous work has shown that the intensityof this feature can be assigned to a Fermi resonance between theCO and the CC overtone, leading to intensity stealing bythe weaker band.25,30 Other features that can be confidentlyassigned are the CC stretch (870 cm−1), the CO mode(1117 cm−1), and the CH2 scissor mode (∼1425 cm−1). Higherwavenumber bands include the asymmetric CH2 mode (2856cm−1) and the OH stretching mode (3490 cm−1). Several othermodes are also clearly visible in the spectrum. These have beenassigned differently across the literature, and we therefore onlytentatively assign these features, as detailed in Table 2.To investigate the thermal behavior of the glycolaldehyde ice,

annealing experiments were performed. Annealing spectra wererecorded in 10 K intervals from the base temperature (20 K) upto 160 K, where glycolaldehyde desorbed from the surface. Nochanges were observed in the infrared spectrum for annealingtemperatures ≤90 K. Annealing to 100 K gives rise to a numberof minor changes in the spectrum (not shown). These changesare most likely due to structural rearrangement of the ice duringheating. The spectrum then remains unchanged until 140 K, atwhich point several bands split and shift in wavenumber (Figure2). The CO band exhibits a decrease in intensity and splits togive bands at 1763 and 1755 cm−1. This is coupled with asignificant sharpening and increase in intensity of the CCovertone band, which also shifts to higher wavenumber (1720cm−1). The higher wavenumber features, the OH stretch and theCH2 bands, also split and sharpen. Several features in thefingerprint region of the spectrum also exhibit changes, as

Table 1. Kinetic Parameters for Multilayer Ices of MethylFormate, Amorphous Glycolaldehyde, and Crystalline AceticAcid Adsorbed on HOPG at 23 Ka

methylformate glycolaldehyde acetic acid

desorption order (n) 0 0 0.07 ± 0.03mean desorption energy(Edes)/kJ mol

−135.0 ± 3.6 46.8 ± 8.2 55 ± 2

pre-exponential factor (v)/molecules m−2 s−1 b

8 × 1033±1 1 × 1033±1 8 × 1032±3

aErrors in n, Edes, and v are determined from a spread in the repeatedmeasurements from two/three sets of experimental data. bNote thatfor simplicity, the units for the pre-exponential factor are given asthose for a perfect zeroth-order process.

Figure 2. RAIR spectra showing the effects of sequential annealing of a50 Lm exposure of pure glycolaldehyde ice adsorbed on HOPG at 20 K.The annealing temperatures are shown in the figure.

Table 2. RAIRS Assignments for Glycolaldehyde IceDeposited on HOPG at 20 K and Following the Formation ofCrystalline Glycolaldehyde after Annealing the Ice to 140 Ka

wavenumber/cm−1

assignmentamorphous glycolaldehyde on

HOPG at 20 Kcrystalline glycolaldehyde

on HOPG >140 K

νsCC 870 870*βOH 1072νsCO 1117 1115/1124*δCOH 1196 1192*τCH2

orβCOH

1240 1245/1269

*δCH orδCH2

1340 1342

*δCH orδCH2

1377 1377/1387

δCH 1414δCH2

1425

*δCH21443 1442

2νCC 1713 1720νsCO 1753 1755/1763νasCH2

2856 2852/2872

*νsCH 2914 2912νOH 3490 3249/3411aThe band assignments refer to the infrared spectra shown in Figure 2.The asterisk indicates tentative assignments. Symbols: ν, stretching, δ,bending, β, in-plane bending mode, τ, torsional mode, s, symmetric, as,asymmetric.

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detailed in Table 2. On heating to 150 K (not shown), all bandsdecrease in intensity, before desorption at 160 K.The observed sharpening and splitting of bands upon heating

is assigned to the formation of crystalline glycolaldehyde.Crystallization has been observed for many large organicsadsorbed on HOPG following annealing.44,46 The resultingstructure of crystalline glycolaldehyde is not fully understood;however, various authors have suggested that it can exist in theform of long hydrogen-bonded chains of monomers25 or asdimers with long-range order.28 Evidence of a distinctive phasechange in the glycolaldehyde ice was also observed in TPDspectra.3.2. Methyl Formate. 3.2.1. Methyl Formate TPD. Methyl

formate gives the most complex TPD spectra of the threeisomers, with the TPD showing peaks that can be assigned to thedesorption of monolayer, bilayer, and multilayer species withincreasing coverage (Figure 3). At the lowest exposures (0.1−0.7Lm) a single peak is observed which is assigned to monolayerdesorption (Figure 3A).Figure 3B shows the development of a bilayer peak which

occurs at exposures >1 Lm and is seen concurrently with thecontinuous growth of the multilayer peak. The multilayer peakgoes on to dominate the TPD spectrum at exposures >7 Lm(Figure 3C). The bilayer is clearly visible between 1 and 7 Lm (T= 105−108 K) and is distinguishable from themonolayer peak bya clearly identifiable common leading edge. Even at lowexposures, multilayer formation is detectable via a hightemperature shoulder on the bilayer peak. Multilayer growthoccurs concurrent with bilayer formation but begins to dominate>5 Lm. Above exposures of 7 Lm the TPD spectrum ischaracterized by a single multilayer peak. This has a temperaturethat increases from 109 to 116 K and exhibits zeroth-orderdesorption kinetics. At the highest exposures (>20 Lm), there isevidence of an increasingly divergent desorption rate as indicatedby the separation of the leading edge during desorption (Figure3C). The simultaneous evolution of both monolayer and bilayerTPD peaks suggests that methyl formate wets the HOPG surfaceand forms 2D monolayer and 3D bilayers, prior to multilayer

growth. In contrast to glycolaldehyde, methyl formate cannotform strong intermolecular bonds and therefore it wets HOPG,initially forming a monolayer.The assignment of methyl formate bilayers is based upon

similar, although not identical, behaviors observed for ethanoldesorption fromHOPG at 80 K, which clearly shows a distinctivebilayer component in the TPD at intermediate exposures.46

Bertin et al. also observed the presence of two distinct features intheir TPD spectra for methyl formate on amorphous andcrystalline water ice.34 In contrast to our assignment, theyascribed the two peaks to the desorption of an amorphousmethylformate phase (at the lower temperature) followed by acrystalline phase at higher temperature, analogous to the well-known amorphous to crystalline phase transition observedduring H2O desorption.47 However, if this assignment is correct,then with higher exposure the phase change should still beevident as a shoulder on the leading edge of the TPD trace, asobserved for H2O ices.41 It is clear from Figure 3B,C that ourassigned bilayer peak is only visible at intermediate exposures,disappearing from the TPD at higher exposures (as in the case ofethanol ices). We therefore assign the low temperature shoulderto the formation of a methyl formate bilayer and the hightemperature peak to the desorption of a methyl formatemultilayer.Similarly to glycolaldehyde, analysis of TPD spectra was

performed to determine the kinetic parameters shown in Table 1.The methyl formate multilayer desorption energy of 35 ± 3.6 kJmol−1 compares favorably to previously reported values.Schwaner et al.31 determined a multilayer value of 34 kJ mol−1

using first-order kinetics for methyl formate on Ag{111} at 90 K.The same authors derived a monolayer desorption energy of 37kJ mol−1. This energy correlated well with monolayer energiesderived for methyl formate desorption from amorphous andcrystalline water surfaces of 37 ± 4 and 38 ± 4 kJ mol−1, wherethe authors assumed zeroth-order kinetics and a typical first-order approximation for the pre-exponential factor.34 Themultilayer energy for methyl formate is lower than that ofglycolaldehyde (Table 1), therefore underlining the importance

Figure 3. TPD spectra following increasing exposures of methyl formate adsorbed on HOPG at 23 K. (A) shows monolayer exposures (0.1−0.7 Lm),(B) focuses on the bilayer and multilayer regime at low exposures (0.7−5 Lm), and (C) shows multilayer ices at high exposures (7−50 Lm).

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of intermolecular interactions in dictating the desorption ofphysisorbed species.Figure 4 shows the desorption energies for monolayer methyl

formate as a function of exposure, derived from the data shown in

Figure 3A. Figure 4 clearly shows that the monolayer desorptionenergy is dependent on the initial exposure. The lowest exposure,0.1 Lm, has an energy of 18.1 ± 1.2 kJ mol−1, which rises withincreasing dose until it reaches a saturation value equivalent tothe multilayer desorption energy of 35 ± 3.6 kJ mol−1 for anexposure of 1 Lm.The increasing energy suggests that there are attractive

interactions between the methyl formate molecules that lead tothe increasing desorption energy. Unlike glycolaldehyde andacetic acid, methyl formate cannot hydrogen bond. However, theincreasing value of the desorption energy suggests that there arefavorable interactions between the molecules. This agrees withthe observation of bilayer formation, as indicated in the TPDspectra. This also correlates with the RAIR spectra (shownbelow), which show that methyl formate, though incapable ofhydrogen bonding, orders upon annealing. The values of methylformate monolayer energies derived here are lower than thosepreviously determined for desorption from Ag{111}31 andwater35 surfaces at low coverages. However, in both of thesecases, methyl formate can form weak bonds with the substrate,either hydrogen bonds in the case of a water surface or an oxygenlone pair interaction with the Ag{111} substrate.31 However,when adsorbed on HOPG, such bonding is not possible.3.2.2. Methyl Formate RAIRS. Figure 5 shows RAIR spectra

for 50 Lm of methyl formate adsorbed on HOPG at 20 K. Notethat the methyl formate spectrum is split into four individualfigures for clarity, because different regions of the spectrum havedifferent band intensities. As for glycolaldehyde, a range ofexposures were studied, all corresponding to multilayer ices.Similar trends were observed for all exposures, and hence a 50 Lmmethyl formate ice has been selected as an illustrative example.Methyl formate adsorption on HOPG at 20 K is characterized

by 10 infrared bands, 9 of which can be confidently assigned bycomparison with the literature.23,31−34,48 The assignments are

detailed in Table 3. The most intense band in the spectrum is theCO feature (1737 cm−1, Figure 5C). Other strong bands at1228, 1172 (Figure 5B), and 914 cm−1 (Figure 5A) are assignedto the CCO symmetric stretch, the CH3 rocking mode and theCO symmetric stretch, respectively. Broad low intensity featurescorresponding to CH3 vibrations are located at 1439 and 1450cm−1 (Figure 5B) and at 2963 and 3016 cm−1 (Figure 5D). A lowintensity feature at 1390 cm−1 is assigned to the CH deformationmode.34 A further vibrational band located at 1460 cm−1 remainsunassigned.Annealing the methyl formate ice from 20 to 90 K reveals

subtle changes in the CO and CCO bands. Both featuressharpen and increase in intensity slightly from 70 K, with theCCO band exhibiting a minor shift to higher wavenumberfrom 1229 to 1233 cm−1. Heating the ice to 95 K sees significantchanges across the entire spectrum, giving rise to changes in bandprofile, intensity, and position as well as to splitting of the originalbands. This can be clearly seen in Figure 5 and also in Figure 6,which shows the normalized integrated band intensities for the1737, 1228, and 1172 cm−1 features in the methyl formatespectra shown in Figure 5. Figures 5 and 6 show that the CO,CH3, CCO, and CO bands all exhibit a significant decreasein intensity at 95 K, coupled with splitting of the original bandinto two or three components (listed in Table 3). The remainingbands exhibit an increase in intensity and sharpening when theice is heated to 95 K. The CH band at 1390 cm−1 sharpens alongwith the 1451 cm−1 feature, with a new band appearing at 1473cm−1. The CH3 symmetric stretch at 3016 cm−1 becomes moreprominent at 95 K, with the CH3 band at 2962 cm

−1 splitting intoa doublet at 105 K. Heating the ice beyond 105 K sees a decreasein the intensity of all bands, with desorption observed at 115 K.The splitting of infrared bands following the annealing of

amorphous ices is characteristically attributed to a structuralchange, often as a result of the formation of a highly ordered(crystalline) structure. This band splitting is usually accompaniedby band sharpening and increases in peak intensity, as observedfor the annealing of acetic acid ices (discussed later). However,methyl formate does not adhere to this trend. Clearly there issignificant peak splitting in the spectra shown in Figure 5, but thisis generally accompanied by a reduction in peak intensity,particularly in the case of the CO stretch. This is not a result ofdesorption, because the changes observed in the spectrum occurat 95 K, which is 10−15 K prior to desorption being observed inthe TPD. Bertin et al.,34 in their study of methyl formateadsorbed on crystalline and amorphous water ice, ascribed anobserved blue shift in the CO stretch upon annealing to theformation of a crystalline structure. This assignment was basedon the correlation of this spectral shift to that of infraredmeasurements of highly oriented polycrystalline samples ofmethyl formate prepared by liquid nitrogen cooling of the liquidphase.49 However, we cannot definitively assign our ice to adistinct crystal form, because our infrared spectra do notcorrelate with the sharpened bands reported by Katon.49 Instead,we attribute the changes observed in the RAIR spectra in Figure 5to a structural change of the ice via a modification to the existingamorphous structure deposited at 23 K. This structural changecould arise due to a glass transition of the methyl formate ice,which has been observed for acetic acid.38,50

3.3. Acetic Acid. 3.3.1. Acetic Acid TPD. Compared tomethyl formate and glycolaldehyde, acetic acid yields thesimplest TPD of all three isomers, contrasting the complexityobserved in the RAIRS annealing spectra (see below). Followingexposures of 1−50 Lm (Figure 7), a single peak is observed in the

Figure 4. Plot showing the variation of desorption energy with exposurefor submonolayer doses of methyl formate adsorbed on HOPG at 23 K.The absolute values and error bars for the data are shown on the figure.

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TPD spectrum. This shows a steady increase in temperature withincreasing exposure, with peak temperatures ranging from 152 to166 K from 1 to 50 Lm. All TPD traces share a common leadingedge, indicating zeroth-order desorption, which is confirmed byleading edge analysis (Table 1). Similarly to the glycolaldehydeTPD, there is no indication of monolayer formation, even at thelowest exposures. This suggests that acetic acid also does not wetthe HOPG surface at 23 K, preferentially bonding to other aceticacid molecules as opposed to bonding with the substrate. TheTPD spectra in Figure 7 are assigned to the desorption ofcrystalline acetic acid as detailed in the following RAIRS section.Kinetic parameters derived from analysis of the acetic acid

multilayer TPD spectra are shown in Table 1. Similar toglycolaldehyde, acetic acid does not form monolayers on HOPGat 23 K. As expected, the desorption energy of acetic acid is thehighest of the three isomers at 55 ± 2 kJ mol−1, with desorptionfollowing fractional order kinetics (n = 0.07). These values reflect

the ability of acetic acid to form ordered and crystallinehydrogen-bonded chains prior to desorption, which is observedin the annealing RAIR spectra (see below). As a result, the kineticparameters correspond to the desorption of crystalline acetic acidfrom HOPG.Previous DFT studies have shown that acetic acid bonds to

water with adsorption energies ranging from 68 kJ mol−1, in thecase of a single acetic acid molecule, to 38 kJ mol−1, for the aceticacid dimer.35,37 Allouche and Bahr37 also calculated the cohesiveenergy of a series of acetic acid/water configurations, includingmonomers, dimers, trimers, and hydrates, all of which haveenergies in the range 55−56 kJ mol−1. Although these values donot strictly correspond to the adsorption and desorption of pureacetic acid, they are in good agreement with the desorptionenergy reported here. Because both acetic acid/acetic acid andwater/acetic acid configurations share double hydrogen bonds,the agreement is not unsurprising. Furthermore, the desorption

Figure 5.RAIR spectra showing the effects of sequential annealing of a 50 Lm exposure of methyl formate adsorbed onHOPG at 20 K. (A) shows the lowfrequency region between 1000 and 820 cm−1. (B) shows the region between 1550 and 1050 cm−1. (C) shows the carbonyl region between 1800 and1680 cm−1. (D) shows the region between 3200 and 2800 cm−1. The annealing temperatures and band strengths in the respective regions are shown inthe figure.

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energy determined for acetic acid correlates well to that of otherhydrogen-bonded systems, for example ethanol (56.3 kJmol−1).46

3.3.2. Acetic Acid RAIRS. RAIR spectra were recorded fordoses of acetic acid between 20 and 100 Lm adsorbed on HOPGat 20 K. The layer was then annealed to various temperatures.The observed RAIRS behaviors were identical over this coveragerange and hence we show the 100 Lm spectrum, as this gives theclearest infrared bands. Figure 8 shows the RAIR spectrarecorded for 100 Lm of acetic acid adsorbed on HOPG as afunction of annealing temperature and is divided into differentspectral regions, based on the differing intensities of the bands.Comparison with previous studies34,38,39 allows the assignmentof many of the vibrational features observed. A list of the

assignments is detailed in Table 4. The table also shows how thebands change upon annealing the acetic acid ice to 130 K.Acetic acid exists in several forms in the condensed phase. In

the liquid phase, it exists mostly as cyclic dimers,51 althoughevidence of open-chain polymers is also reported.52 In the solidphase, previous infrared studies39,53 show that it exists mainly as acombination of cyclic dimers and chainlike polymers. The dimerconfiguration is dominant when the acid is deposited at lowtemperatures,34 whereas the polymeric configuration is favoredwhen deposition occurs at higher temperatures (typically above120−130 K).34,39 When the transition from low to hightemperature has been monitored, evidence of a phase changefrom the cyclic dimer to a crystalline chainlike polymer structure

Table 3. Assignment of the Spectral Features for Multilayer Methyl Formate Adsorbed on HOPG at 20 K and Thermally Annealedto 95 K, As Seen in Figure 5a

wavenumber/cm−1

assignment HOPG, 20 K (this work) HOPG, 95 K (this work) Cu{110}, 90 K32 Ni{111}, 86 K33 Ag{111}, 90 K31 KBr, 16 K23

νCO 914 906 920 916 910900

ρCH31172 1161 1170 1182 1159 1164

11671176 1175

νCcO 1228 1214 1230 1220 1211 1210

12311269 1240

δCH 1390 1394 1383δsCH3

1439 1442 1435

δasCH31450 1451 1450 1460 1451 1450

1460 1473 1446νCO 1737 1729 1720 1730 1733 1720

1706νsCH3

2963 2962 2985 2959

2984νasCH3

3016 3018 3042 3015 3010, 3038

aThe table also includes data from the literature to aid assignments. Symbols: ρ, rocking, ν, stretching, δ, bending; s, symmetric, as, asymmetric.

Figure 6. Plot showing the normalized peak areas of selected vibrationalbands of methyl formate as a function of annealing temperature: opencircles, the CH3 rocking mode (1172 cm

−1); open triangles, the CCOmode (1228 cm−1); open squares, the CO stretch (1737 cm−1).

Figure 7.TPD spectra following increasing exposures of acetic acid ontoHOPG at 23 K. The ice exposures range from 1 to 50 Lm.

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has been found.34,39,53 Given this information, it is likely thatacetic acid dimers are formed on the HOPG surface upondeposition at 20 K, and this is confirmed by the infrared

spectrum. The most prominent feature is the CO stretchingvibration (Figure 8B) found in the range from 1650 to 1800cm−1. This feature is particularly sensitive to the structure of

Figure 8. RAIR spectra showing the effects of sequential annealing of a 100 Lm exposure of acetic acid adsorbed on HOPG at 20 K. (A) is the lowfrequency and fingerprint region between 1550 and 850 cm−1. (B) is the carbonyl region highlighting the formation of monomers and cyclic dimers inthe range 1820−1620 cm−1. (C) is the high frequency region between 3500 and 2400 cm−1. The annealing temperatures and band strengths in therespective regions are shown in the figure.

Table 4. RAIRS Assignments for Acetic Acid Adsorbed on HOPG at 20 K (Figure 8)a

wavenumber/cm−1

assignment HOPG at 20 K (this work) HOPG at 130 K (this work) crystalline water at 80 K34 Cu at 123 K39 solid phase53

952ρsCH3

1020 1028 1019

ρasCH31055 1056 1049

νCO 1288 1287 1281 1284 1269νCO 1308 1306/1317/1329 1300 1314δsCH3

1363 1364 1360 1363 1365

δasCH31417 1418 1410 1418

δCOH 1437 1435/1443 1431 1436 1406νCO (d) 1718 1723/1730 (broad) 1643 1646 1659

1734 1666 1732 17151699

νCO (m) 1761 1753 1755 (bulk) 1749−17901797 1790 (surface)1832

2νCO 2577 2581 2572 2576 2519νCO + δsCH3

2644 2642 2638 2640 2614

νCO + δsOH 2696 2693 2687 2691 26642708

*2δsCH32753 2739

*2δasCH32814 2799

*2δCOH 2870 2855νsCH3

2941 2930 2920 2924

*νasCH32976 2982

2998νOH 3133−3103 3031 3024 3028 3028

∼3070aAdditional bands appearing at 130 K are also listed. Data from the literature are also included for comparison to aid the assignment of the bands.For some of the new bands growing in following annealing to 130 K, the assignment is unclear and hence has not been made. Tentative assignmentsare indicated by asterisks. Symbols: ρ, rocking, ν, stretching, δ, bending; s, symmetric, as, asymmetric, d, dimer, m, monomer.

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acetic acid.39 On HOPG, the spectrum in Figure 8B shows threemain components at 1718, 1734, and 1761 cm−1. The first two ofthese are assigned to acetic acid in the dimeric form and thehighest wavenumber band is assigned to monomeric aceticacid.38,39,53

As clearly shown in Figure 8, annealing the acetic acid ice hasprofound effects on the infrared spectrum. Initial annealing fromthe base temperature to 70 K immediately gives rise to changes inseveral of the main bands. These changes continue as the ice isfurther annealed up to 110 K. Upon further heating to ∼125 K(not shown) very minor changes are observed. Followingannealing of the ice to 130 K, very distinct changes are seenacross the entire spectral range. These changes continue toevolve, with bands growing, disappearing, and splitting withcontinued heating up to 160 K. Desorption is observed followingheating to 170 K.The profound changes that occur in the spectrum on heating

to 130−160 K are assigned to the crystallization of the acetic acidinto long polymeric chains. The onset of these structural changesat 130 K can be assigned to an initial glass transition within theacetic acid ice as previously reported.38,50 The subsequentcrystallization is evidenced by the appearance of bands at 1643and 1660 cm−1 at 160 K, assigned to the CO stretch of bulkpolymers in the solid.34,39 At 130 K, a band appears at around1755 cm−1, which shifts down to 1749 cm−1 and dominates thespectrum at 160 K. In addition, a new band at 1794 cm−1 appearsfollowing annealing to 130 K. This new feature is assigned to theCO stretch of acetic acid monomers on the surface of the ice,as opposed to the 1749 cm−1 feature that is assigned tomonomers trapped in the bulk of the ice.39 At 160 K, the COstretch of bulk trapped monomers (1749 cm−1) becomes themost prominent feature, consistent with crystallization.39

Although the changes in the CO region of the spectrum arethe most prominent, there are clearly distinct changes in allspectral regions on heating from 130−160 K and all of these canbe assigned to the crystallization of the acetic acid. Exactassignments of the new bands observed have been made wherepossible and are detailed in Table 4. The ability of the acetic acidto crystallize and to form long chain polymers occurs as a result ofits ability to form intermolecular hydrogen bonds. It is interestingto note that TPD spectra for pure acetic acid, do not show anyevidence of this crystallization process because the phase changetakes place prior to the onset of desorption.3.4. Comparison of the Adsorption and Desorption

Behavior of the Isomers. It is clear that the three isomers allbehave differently when adsorbed on HOPG. Methyl formate inparticular behaves differently, when compared to the other twoisomers, with respect to both TPD and RAIRS. In the TPD,methyl formate shows evidence of monolayer, bilayer, andmultilayer desorption and has the lowest desorption temper-atures and energies of the three isomers. Glycolaldehyde andacetic acid, on the contrary, show zeroth-order desorption inTPD, irrespective of the surface coverage. This contrastingbehavior can be understood in terms of the intermolecularbonding that can occur for each of the three isomers. To furtherquantify the interactions of the three isomers with themselvesand with carbonaceous surfaces, we have undertaken apreliminary DFT study (Supporting Information) of theinteractions and adsorption energies of acetic acid, methylformate, and glycolaldehyde. Initial calculations show that the gasphase dimer binding energy for methyl formate is −18.2 kJmol−1, substantially less than the same binding energy forglycolaldehyde (−53.5 kJ mol−1) and acetic acid (−82.4 kJ

mol−1). This low dimer binding of methyl formate in the gasphase can be explained by the fact that it cannot hydrogen bondand hence the binding is largely due to dispersive interactionsrather than electrostatic hydrogen bonds. Acetic acid andglycolaldehyde, on the contrary, can hydrogen bond, andtherefore an acetic acid molecule, for example, preferentiallybonds to another acetic acid molecule compared to bonding tothe HOPG surface. This assertion is again supported by DFTcalculations of monomer adsorption (Supporting Information),using coronene as a proxy for HOPG. Adsorption of a monomerof acetic acid on coronene gives an adsorption energy of−25.9 kJmol−1, but the dimer binding energy is −82.4 kJ mol−1,confirming a very strong tendency for acetic acid to clusterrather than wet the HOPG, as indicated by the TPD data.Adsorption of methyl formate on coronene yields an adsorptionenergy of −24.4 kJ mol−1, i.e., larger than the gas phase dimerbinding energy of methyl formate, indicating that impingingmethyl formate will preferentially stick to the bare carbonaceoussurface, rather than to another methyl formate molecule, again asshown by TPD data. Similarly, a glycolaldehyde molecule alsohydrogen bonds preferentially to another glycolaldehydemolecule (gas phase dimer binding energy of −53.5 kJ mol−1),rather than binding to the HOPG surface (monomer adsorptionenergy −25.5 kJ mol−1). Hence for acetic acid andglycolaldehyde, our preliminary DFT calculations confirm thatislanding and/or multilayer formation occurs immediately uponadsorption, in agreement with experimental TPD data, asopposed to surface wetting and monolayer formation beingobserved. Methyl formate, in contrast, cannot hydrogen bond toitself and therefore instead binds to the HOPG surface as alsoseen in the TPD spectra. Because of this lack of hydrogenbonding, methyl formate wets the graphite surface, forming amonolayer, a bilayer, and then subsequently a multilayer.Interestingly, the calculated monomer adsorption energy of−24.4 kJ mol−1 for methyl formate on coronene is in goodagreement with the low coverage desorption energy reported inFigure 4 of 18.1 ± 1.2 kJ mol−1. The calculated dimer adsorptionenergy of−39.9 kJ mol−1 for methyl formate is also in reasonableagreement with the higher exposures corresponding to multi-layer ices (>1 Lm), which have a desorption energy of 35 ± 3.6 kJmol−1.The adsorption behavior observed for acetic acid and

glycolaldehyde has also been reported for other molecules thatpreferentially hydrogen bond to each other. Water is well-knownto preferentially form islands and does not wet the surface whenadsorbed on HOPG.45 Methanol only shows zeroth-orderkinetics when adsorbed on Au43 or on a graphene layer boundto Pt{111} at very low temperatures.42 Similarly, ethanol alsoshows zeroth-order kinetics, irrespective of coverage, whenadsorbed on a graphene surface at 25 K.42 The comparativelylower desorption energy of methyl formate (Table 1) is also inline with its lack of hydrogen bonding, because the hydrogen-bonded structures of glycolaldehyde and acetic acid give rise tohigher desorption energies. Further evidence illustrating theinability of acetic acid to form a monolayer on HOPG and itspreferential formation of hydrogen bonds is demonstrated whenit is adsorbed onto an amorphous water ice surface. In this case,acetic acid forms a monolayer on the surface in contrast to thecase of the pure ice as reported here, because it can hydrogenbond directly to the water surface.34

Methyl formate also behaves somewhat differently whencompared to acetic acid and glycolaldehyde following a RAIRSannealing experiment. There is very clear evidence of

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crystallization for both acetic acid (Figure 8) and glycolaldehyde(Figure 2), with band sharpening and splitting being observed inthe RAIRS in both cases following annealing of the ice. For bothmolecules, this can be assigned to the formation of hydrogen-bonded chains/polymers of molecules. Methyl formate, on thecontrary, clearly shows evidence of a structural change, asevidenced by the appearance of the RAIR spectra followingannealing to 95 K (Figure 5). However, the appearance of theRAIR spectra for methyl formate is somewhat different fromthose for acetic acid and glycolaldehyde, with most bandsdecreasing in intensity and broadening (rather than sharpening)of bands being observed in most cases. The observed changes inthe appearance of the methyl formate infrared spectra followingannealing have previously been assigned to the formation ofcrystalline methyl formate.23 However, we believe that this is notstrictly a crystallization process in the same manner as thatobserved for the other two isomers but rather occurs as a result ofan ordering of the methyl formate molecules. Methyl formatecannot hydrogen bond to itself and therefore cannot form long-range ordered, and hydrogen-bonded, ice structures, like thoseformed by glyocolaldehyde and acetic acid. Indeed, to the best ofour knowledge, the crystal structure of methyl formate (if oneexists) has not yet been determined, although Katon and Ranierihave recorded infrared spectra for polycrystalline methylformate.49 Further evidence demonstrating the inability ofmethyl formate to form intermolecular hydrogen-bonded icescomes from the observed differences in the RAIR spectracompared to those of other hydrogen-bonded molecules such asmethanol44 and ethanol.46 In these cases, both ices clearly exhibitband intensity increases, sharpening, and splitting uponannealing, which is a known signature of crystallization in ices.3.5. Astrophysical Implications. Though pure ices of

glycolaldehyde, acetic acid, and methyl formate are not observedin interstellar space, the data derived here can still be used to giveinformation about the adsorption and desorption of ices underinterstellar conditions. In particular, the desorption energiesderived from the TPD spectra, in conjunction with experimentsdetermining the trapping and desorption behavior of the isomersin water ice (reported elsewhere), allow detailed modeling of thedesorption of interstellar ices.16 Data concerning the adsorption,desorption and thermal processing of COMS under conditionsrelevant to interstellar chemistry are currently sorely lacking,despite the increasing importance of these species to ourunderstanding of star and planet formation. Hence the datadescribed here help to contribute to our understanding of thechemistry of these species on dust grain analogue surfaces. Thedata reported here also give important insights into the role ofintermolecular bonding in the formation and growth of COMs.

4. SUMMARY AND CONCLUSIONSThe adsorption and thermal desorption of methyl formate,glycolaldehyde, and acetic acid adsorbed on HOPG at temper-atures of ∼20 K have been studied. Methyl formate exhibitsdifferent adsorption and desorption behavior than glycolalde-hyde and acetic acid because of its intrinsic inability to formhydrogen bonds. Methyl formate wets the HOPG surface,forming monolayers prior to the growth of bilayer and thenmultilayer ices. It has the lowest desorption energy of the threeisomers of 18.1 kJ mol−1, for submonolayer exposures, whichsubsequently converges to 35 kJ mol−1 for multilayer ices. Theexperimental values are in agreement with our preliminary DFTcalculations of methyl formate monomer adsorption oncoronene. In contrast, glycolaldehyde and acetic acid preferen-

tially cluster on the HOPG surface upon adsorption andconsequently have higher desorption energies of 46.8 and 55kJ mol−1, respectively. Again, preliminary DFT calculationssupport experimental observations, with these two moleculesshowing a greater preference to form dimers when compared tomethyl formate. RAIRS shows that both glycolaldehyde andacetic acid undergo structural changes during heating, giving riseto crystalline structures with long-range order prior todesorption. As a result, acetic acid desorbs as a crystalline ordimeric form for all exposures, whereas glycolaldehyde formslong hydrogen-bonded chains at sufficient exposures. Methylformate also undergoes a structural change upon heating;however, RAIRS suggests that this is via short-range reordering,rather than a distinct crystallization as observed for glycolalde-hyde and acetic acid. The data described here reveal the details ofthe physical changes that occur upon thermal processing of pureglycolaldehyde, methyl formate, and acetic acid ices. These dataare of importance for understanding the formation anddesorption of COMs in the interstellar medium, potentiallycontributing to our understanding of star and planet formation.Furthermore, these data provide the basis for understanding theinteraction of these species with water ice, which is crucial tounderstanding interstellar chemistry.

■ ASSOCIATED CONTENT*S Supporting InformationSupporting Information describes the methodology used and theresults obtained for preliminary DFT calculations performed toinvestigate the interactions between individual isomer molecules,and their monomer adsorption energy on a coronene surface.The Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpca.5b04010.

■ AUTHOR INFORMATIONCorresponding Author*D. J. Burke. Tel: +44 1273 678402. E-mail: darenburke@hotmail.com.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe Leverhulme Trust are thanked for funding D.J.B. andP.M.W. to undertake this research at UCL and the University ofSussex are thanked for further funding for DJB at Sussex. F.P.acknowledges support from the European Community’s SeventhFramework Programme FP7/2007-2013 under grant agreementNo. 238258.

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