conformational behavior of ch 3 oc(o)sx (x = cn and scn) pseudohalide congeners. a combined...

Conformational Behavior of CH3OC(O)SX (X ) CN and SCN) Pseudohalide Congeners. ACombined Experimental and Theoretical Study

Sonia Torrico-Vallejos,† Mauricio F. Erben,*,† Mao-Fa Ge,‡ Helge Willner,§ andCarlos O. Della Vedova*,†,|

CEQUINOR (CONICET-UNLP), Departamento de Quımica, Facultad de Ciencias Exactas, UniVersidadNacional de La Plata, C.C. 962, 47 esq. 115, La Plata (B1900AJL), Buenos Aires, Republica Argentina, StateKey Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, ChineseAcademy of Sciences, Beijing 100080, China, Fachbereich C-Anorganische Chemie, Bergische UniVersitat,Wuppertal Gausstrasse 20, 47097 Wuppertal, Germany, and Laboratorio de SerVicios a la Industria y alSistema Cientıfico (LaSeISiC) (UNLP-CIC-CONICET), Camino Centenario e/505 y 508,(1903) Gonnet, Republica Argentina

ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: February 1, 2010

Pure methoxycarbonylsulfenyl cyanide, CH3OC(O)SCN (I), and methoxycarbonylsulfenyl thiocyanate,CH3OC(O)SSCN (II), were prepared by reacting liquid CH3OC(O)SCl with either AgCN or AgSCN,respectively. Compounds I and II were characterized by 1H NMR, CG-MS, and vibrational (FTIR and FT-Raman) techniques. The conformational properties have been studied by using vibrational spectroscopy [infrared(gaseous, liquid, and Ar matrix isolated), Raman (liquid) spectroscopy] together with quantum chemicalcalculations at the B3LYP and MP2 methods with the extended 6-311++G** and aug-cc-pVTZ basis sets.Compound I exhibits a conformational equilibrium at room temperature having the most stable form Cs

symmetry with a synperiplanar (syn) orientation of the carbonyl double bond (CdO) with respect to both theCH3O- and -SCN groups (syn-syn). Several bands assigned to a second conformer have been observed inthe IR matrix spectra. This rotamer presents an antiperiplanar orientation of the thiocyanate group (syn-anti).Evaluating the equilibrium compositions at different temperatures by quenching the gas phase mixtures as Armatrices allowed us to determine the conformational enthalpy difference ∆H0 ) H0

(syn-anti) - H0(syn-syn) )

0.80(18) kcal mol-1. A similar conformational behavior has been determined for compound II. Thermodynamicproperties were also computed at the high-level G2MP2 and G3 model chemistry methods. The importanceof mesomeric (resonance) and anomeric (hyperconjugation) electronic interaction in the conformational behavioris evaluated by using the NBO approach for both species.

Introduction

Recently, we reported the synthesis of the novel com-pounds CH3OC(O)SNCO1 and CH3OC(O)SSCF3

2 derived fromCH3OC(O)SCl. A complete structural study was conductedmainly by focusing in the conformational behavior around thesulfenylcarbonyl moiety [-C(O)S-]. The presence of twoconformers could be clearly determined at room temperature,the syn form (the CdO double bond in synperiplanar orientationwith respect to the S-NCO and S-S bonds, respectively) beingthe preferred conformation, with an important contribution ofthe anti form, of 19(5)% in the case of CH3OC(O)SSCF3.Moreover, the isocyanate species was found to be a veryversatile reagent for the synthesis of carbamates3 and N,N′-disubstituted ureas.4 Thus, following this work, we becameinterested in the two thiocyanate derivatives CH3OC(O)SCNand CH3OC(O)SSCN, which offer the possibility of comparingthe effect that pseudohalide substituents exert on theCH3OC(O)S- group.

Since thiocyanates (R-SCN) are important reagents for thepreparation of sulfur-containing organic compounds, they arehighlighted in several reviews.5–8 To introduce the thiocyanategroup into an organic molecule, a convenient synthetic routeinvolves the reaction of metal thiocyanates with organic halides.The thiocyanate group is thermally unstable and simple chro-matography or a prolonged heating over 50 °C can cause anintramolecular rearrangement to the thermodynamically favoredisothiocyanate isomers.9 This interconversion between theselinkage isomers has been known since 1873 when Gerlich10 andBilleter11 independently observed that allyl thiocyanate isomer-izes during distillation. Correspondingly, the reaction betweenCH3OC(O)Cl with KSCN yields mainly the isothiocyanatederivative [CH3OC(O)NCS], with only a minor contribution ofCH3OC(O)SCN, as reported by Liotta and Engel.12

Thiocyanate compounds with an SCN group attached directlyto the sulfur atom, XSSCN, are well-known. The first thiocy-anate containing a formal single S-S bond, NCSSCN, wassynthesized by Soderback in 1918.13 Halogenated derivatives,such as FC(O)SSCN and ClC(O)SSCN, were also reported byHaas14,15 and Jochims.16 However, these compounds receivedlittle attention until recently, when the electronic properties ofXC(O)SSCN (X ) F17 and CH3O-18) were studied by photo-electron spectroscopy and quantum chemical calculations. Bothcompounds were obtained in situ by the heterogeneous reaction

* To whom correspondence should be addressed. Tel/Fax: +54-221-425-9485. E-mail: [email protected] (M.F.E.) and [email protected] (C.O.D.V.).

† Universidad Nacional de La Plata.‡ Chinese Academy of Sciences.§ Bergische Universitat.| Laboratorio de Servicios a la Industria y al Sistema Cientıfico.

J. Phys. Chem. A 2010, 114, 3703–3712 3703

10.1021/jp912044r 2010 American Chemical SocietyPublished on Web 02/18/2010

between solid AgCN and vapors of the corresponding sulfenylchloride [XC(O)SCl, X ) F and CH3O].

In this work, the synthesis, isolation, and characterization oftwo pseudohalide derivatives, i.e., CH3OC(O)SCN (I) andCH3OC(O)SSCN (II), are presented. Gas and liquid phaseinfrared measurements as well as extensive quantum chemicalcalculations have been performed. For CH3OC(O)SCN, Ar-matrix IR spectra have been also recorded at different temper-atures of the deposition nozzle to evaluate possible conformermixtures in the gas phase. The vibrational analysis is completedwith the Raman spectra of both compounds in the liquid phase.Thus, the structural and vibro-conformational properties weredetermined for both molecules. Finally, the NBO populationanalyses were applied to rationalize the effect of the electronicinteractions on these properties.

Results and Discussion

Quantum Chemical Calculations. To the best of ourknowledge theoretical studies on the CH3OC(O)SCN moleculehave not been reported. In principle, the compound may bepresent in at least four conformers with a planar skeleton,depending on the orientation around the OsC and CsSsingle bonds (see Scheme 1). Taking into consideration stru-ctural studies previously reported for the methoxycarbonylCH3OC(O)- moiety,19–25 a syn orientation of the δ(COsCdO)dihedral angle is preferred, the anti form being higher in energy.The conformational properties of sulfenylcarbonyl compounds,with the general formula XC(O)SY, have been studied and thepreference of a synperiplanar conformation around the C-Ssingle bond was well-established.19,26–28 Furthermore, in a fewexamples, it has been experimentally established that theantiperiplanar conformation appears as a second stable form atambient temperature.29,30 Therefore, the potential energy functionfor internal rotation around the δ(OdCsSC) dihedral angle wascalculated at the B3LYP/6-31G* level by allowing geometryoptimizations with the dihedral angle varying from 0 to 180°in steps of 30°. The potential energy curve is shown inFigure 1.

Two structures correspond to minima in the potential energycurve, corresponding to the syn-syn and syn-anti forms shownin Scheme 1. The most stable one displays a syn orientation ofthe CdO double bond with respect to the S-C bond[δ(OdCsSC) ) 0°] while the anti form (syn-anti), withδ(OdCsSC) ) 180°, is higher in energy by only ca. 0.4 kcalmol-1. Subsequently, full geometry optimizations and frequencycalculations were performed for each of the most stablestructures with the B3LYP and MP2 methods and the 6-31G*,6-311++G**, and aug-cc-pVTZ basis sets. Predicted relativeenergies, ∆E° (corrected by zero-point energy) are listed inTable 1.

The two computational methods predict the structure withsyn orientation around both O-C and C-S single bonds to bethe most stable conformer of CH3OC(O)SCN. The second stableform, higher in energy by 0.31 (B3LYP/6-311++G**) or 0.56

kcal mol-1 (MP2/6-311++G**) corresponds to a conformerwith anti orientation of the δ(OdCsSC) dihedral angle. A thirdform, the anti-syn conformer in Scheme 1, was found to be astable conformer with calculated energies higher than 6 kcalmol-1 with respect to the minimum (see Table 1) and is notexpected to be detected under the conditions used in ourexperiments. On the other hand, the anti-anti rotamer does notcorrespond to a minimum, probably because of steric repulsionsbetween the methyl and thiocyanate groups. Figure 2 showsthe molecular models for the two main forms ofCH3OC(O)SCN.

For compound II, a theoretical study at the B3PW91/6-31+G*level of approximation was recently reported by Du et al.18 Themain objective was the assignment of the photoelectron spectrumthrough the determination of molecular orbital characters.Interestingly, two conformations were found to be very closein energy. The most stable conformer has synperiplanar orienta-tion of both O-C and S-S single bonds with respect to theCdO double bond (syn-syn), while the corresponding formhaving the CdO and S-S bonds in mutual antiperiplanarorientation (syn-anti) lies only 0.81 kcal mol-1 higher in energy(B3LYP/6-311G*) than the most stable form. The PES wasassigned on the basis of the exclusive presence of the most stableconformer.

From the conformational point of view, such a small energydifference between both conformers of compound II is of primeinterest.31–34 Thus, full geometry optimizations and frequencycalculations were performed for each of the most stablestructures of CH3OC(O)SSCN with the B3LYP and the second-order perturbation theory methods and the 6-31G* and6-311++G** basis sets. Our results are in perfect agreementwith those obtained recently by Ge et al.18 One of the importantstructural features of this compound is the dihedral angle aroundthe S-S bond, with a computed value of 87.4° at the B3LYP/6-311++G** level of approximation. Predicted relative ener-gies, ∆E° (corrected by zero-point energy) are listed in TableS1, in the Supporting Information.

Vibrational Analysis of CH3OC(O)SCN. The vibrationaland conformational properties of CH3OC(O)NCS were studiedby Campbell et al.35 On the other hand, due to the inherentinstability and the difficulties to isolate the thiocyanate, little isknown about the linkage isomer CH3OC(O)SCN.

The IR (liquid) and Raman (liquid) spectra of CH3OC(O)SCNare shown in Figure 3. A tentative assignment of the observedbands was carried out by comparison with calculated wave-

SCHEME 1: Representation of the Conformers ofCH3OC(O)SCN

Figure 1. Calculated potential function (B3LYP/6-31G*) for internalrotation around the C-S single bond in CH3OC(O)SCN.

3704 J. Phys. Chem. A, Vol. 114, No. 10, 2010 Torrico-Vallejos et al.

numbers and on comparison with spectra of related molecules,specially XC(O)SNCO (X ) Cl, F and CH3O)14,36,27 and XSCN(X ) H, Cl, Br).37 Furthermore, the potential energy distribution(PED) associated with each normal vibrational mode wascalculated under the harmonic assumption. Experimental andcalculated [MP2/6-311++G**] frequencies, intensities, andtheir tentative assignments are given in Table 2. This table alsolists the data obtained from the isolated Ar-matrix infraredspectrum.

The characteristic mode of vibration of thiocyanate com-pounds corresponds to a band with weak intensity located at2176 cm-1 assigned to the ν(SCtN) stretching mode in theliquid-phase IR spectrum, with its counterpart at 2178 cm-1 inthe liquid Raman spectrum as the most intense signal.38,39 Thisfundamental is absent in the gas-phase spectrum and is observedas a very low intensity absorption in the Ar-matrix spectrum.Characteristic absorptions corresponding to the stretching modeof vibrations of OCH3 and CdO groups appear at 2970 and1794/1765 cm-1, respectively.

Good agreement is obtained when the experimental vibra-tional frequencies and the calculated values are compared. Fromthe PED analysis, the characteristic fundamentals are clearlyestablished, but many other vibrations are mixed. It becomesapparent that absorptions observed in the gas IR spectrum andin the Ar-matrix spectrum can be assigned, in principle, to thepresence of a second conformer of CH3OC(O)SCN at roomtemperature. This topic will be investigated in more detail inthe following section.

Conformational Equilibrium

The occurrence of two bands in the region of the CdO groupin the IR (gaseous an Ar matrix isolated) spectrum (Figure 4)suggests that an equilibrium of two conformers in similarconcentrations is present at room temperature. It is well-knownthat the ν(CdO) normal mode of carbonyl compounds is verysensitive to conformational properties.35,40 The carbonyl stretch-ing region (1900-1700 cm-1) of the infrared spectra ofCH3OC(O)SCN in the vapor phase and isolated in an Ar-matrixare shown in detail in Figure 4. Two intense absorptions areevident; one band in the gaseous spectrum occurs at 1794 cm-1

and the other at 1765 cm-1. The well-defined vibrational-rotational contour of the blue-shifted band allows assigning thisabsorption to the syn-syn form in a straightforward manner.The almost parallel orientation of the carbonyl oscillator (dipolederivative vector) with respect to the principal axis of inertiaB, a B-type band is expected for the ν(CdO) normal mode ofthe syn-syn form. In the same way, from the band contour ofthe absorption at 1765 cm-1, the syn-anti form is proposed.Because of the orientation of the carbonyl oscillator with respectto the principal axis of inertia A and B, an AB hybrid bandtype is expected for the ν(CdO) normal mode of the syn-anticonformer. The inset of Figure 4 shows the principal axis ofinertia for both conformers.

Following the classical procedure proposed by Seth-Paul,41

a semiquantitative analysis of the band contours expected forboth prolate asymmetric tops syn-syn and syn-anti rotamers,has been further performed.42 The observed P-R separationsfor the B-type band at 1975 cm-1 (10 cm-1) and for the AB-type band at 1765 cm-1 (11 cm-1) are good reproduced by thevalues calculated for the syn and anti forms of CH3OC(O)SCNwhose values are 9 and 11 cm-1, respectively. The detailedprocedure for the calculation of P-R separation is given asSupporting Information.

Additionally, the calculated wavenumber difference [B3LYP/6-311++G**] between the CdO stretching modes of the twoconformers is 27 cm-1 (Table 1), in good agreement with theexperimentally observed value of 29 cm-1 in both the Ar-matrixand gas-phase spectra. This correspondence is observed in Figure4, where the simulated spectrum from the MP2/6-311++G**calculated frequencies is shown, assuming a mixture of thesyn-syn and syn-anti forms in a 75:25 ratio.

Variable Temperature Conformational EquilibriaQuenched by Matrix Isolation

Taking into account the experimental evidence coming fromthe gas-phase infrared spectrum about the presence of more than

TABLE 1: Calculated Relative Energies Corrected by Zero-Point Energy (kcal mol-1) and Vibrational Frequency of the CdOStretching Mode (cm-1) with IR Intensities (km mol-1, in Parentheses) for Three Conformers of CH3OC(O)SCN

syn-syn syn-anti anti-syn

method of calculation ∆E° ν(CdO) ∆E° ν(CdO) ∆E° ν(CdO)

B3LYP/6-31G* 0.00a 1870(221) 0.42 1849(326) 6.64 1899(315)B3LYP/6-311++G** 0.00b 1840(272) 0.31 1813(421) 6.68 1872(380)B3LYP/aug-cc-pVTZ 0.00c 1829(249) 0.24 1801(387)MP2/6-31G* 0.00d 1848(180) 0.70 1831(267) 7.48 1865(275)MP2/6-311++G** 0.00e 1824(225) 0.56 1801(351)MP2/aug-cc-pVTZ 0.00f 0.44G2MP2 0.00g 0.37G3 0.00h 0.39CCSD(T)/6-311++G** 0.00i 0.54

a E° ) -719.421522 hartree. b E° ) -719.547432 hartree. c E° ) -719.588356 hartree. d E° ) -717.969509 hartree. e E° ) -718.171574hartree. f E ) -718.4966263 hartree, zero-point correction at the MP2/6-311++G** level. g G2MP2 energy ) -718.564018 hartree. h G3energy ) -719.159740 hartree. i E ) -718.3057792 hartree, optimized geometry and zero-point correction at the MP2/6-311++G** level.

Figure 2. Molecular models for the two main conformers ofCH3OC(O)SCN.

Conformational Behavior of CH3OC(O)SX J. Phys. Chem. A, Vol. 114, No. 10, 2010 3705

one conformer, and that rich conformational equilibria areenvisaged by the quantum chemical calculations, further ex-perimental evidence is desirable to fully characterize theconformational properties of compound I. If the barrier betweenthe rotamers is <10 kJ mol-1, it was demonstrated that the gasphase equilibrium is not disturbed by quenching of the molecularbeam as a matrix.43,44 Thus, matrix-isolation IR spectroscopyis reported as a technique to investigate the conformationalpreferences of volatile compounds.45–47

Infrared spectra of Ar-matrix isolated CH3OC(O)SCN atdifferent temperatures of the spray-on nozzle have beenmeasured. When the mixture in Ar is passed through the heatedspray-on nozzle and the resulting mixture is deposited as amatrix, some IR matrix bands initially present in the roomtemperature spectrum increase in their relative intensity. Thesebands can be assigned with confidence to the higher energysyn-anti conformer of compound I and are also listed in Table2.

Because the syn-anti conformational change involves sig-nificant variations mainly in the CdO bond strength, theintegrated absorbance ratios of the carbonyl stretching bandsbelonging to both conformers have been plotted on a logarithmicscale as a function of the reciprocal absolute temperature. Theseratio values correspond closely to the concentration ratios ofthe two conformers.48 Other fundamental modes, for instancesthe 1195/1201 cm-1 pair of absorptions, show a very similarbehavior. Such van’t Hoff plots represented in Figure 5 yieldeda mean value of standard enthalpy difference ∆H0 ) 0.80 (18)kcal mol-1 for the syn T anti conformational equilibrium ofCH3OC(O)SCN. Taking into account the calculated (MP2/6-311++G**) entropy difference for both conformers [∆S0 )

0.39 cal/(K mol)], the standard free energy difference value of∆G0 ) 0.68 kcal mol-1 is obtained. From this value, anabundance of 24% for the less stable anti form at 298 K iscalculated.

The two computational methods used through this work, evenusing the triple-� quality basis sets expanding with diffuse andpolarization functions yield energy difference values which aredefinitively too low, with ∆E0 of 0.31 and 0.56 kcal mol-1 atthe B3LYP and MP2 methods with the 6-311++G** basis sets,respectively (Table 1). Thus, high-level quantum chemicalcalculations have been performed, including the G2MP2 andG3 composite methods and the coupled-cluster CCSD(T)/6-311++G** level of approximation. Similar energy values arecomputed at the G2MP2 and G3 methods, which are too low(0.37 and 0.39 kcal mol-1, respectively) when compared withthe experimentally determined ∆H0 value. Better agreement isobtained with the CCSD(T)/6-311++G** calculation, with a∆E0 ) 0.54 kcal mol-1 (zero point energy and thermal energycomputed at the MP2 level).

CH3OC(O)SSCN

The IR and Raman (liquid) spectra of CH3OC(O)SSCN areshown in Figure 6. A tentative assignment of the observed bandswas carried out by comparison with calculated wavenumbers,as well as on comparison with spectra of related molecules.40,49

Experimental and calculated [B3LYP/6-311++G**] frequen-cies, intensities, and their tentative assignments are given asSupporting Information (Table S1). C1 symmetry is expectedfor the feasible rotamers of CH3OC(O)SSCN. Thus, the 27normal modes of vibration are both infrared and Raman activemodes.

Figure 3. Liquid IR (top) and liquid Raman (bottom) spectra of CH3OC(O)SCN.


The general features of the vibrational spectra ofCH3OC(O)SSCN can be interpreted on the basis of the presenceof the syn-syn conformer. However, the carbonyl stretchingregion of the liquid phase infrared spectrum shows the presenceof a very broad absorption, which could indicate more than oneconformer present in liquid CH3OC(O)SSCN. The calculatedwavenumber difference (B3LYP/6-311++G**) for this modebetween the two conformers is 30 cm-1, with the ν(CdO) modeof the most stable syn-syn conformer located at high wave-numbers in comparison with the ν(CdO) of the second stablesyn-anti form. The low vapor pressure of compound II makesthe measurement of either gas-phase or matrix infrared spectrumdifficult. Considering that at least two conformations are veryclose in energy, further calculations at a higher level ofapproximation have been conducted. Thus, the G2MP2 and G3composite methods were used to compute the energy differencebetween the two main conformers of CH3OC(O)SSCN. Ac-cording to these calculations, the syn-syn conformer is morestable than the syn-anti form by 0.81 and 0.91 kcal/mol, forthe G3 and G2MP2 methods, respectively.

Additionally to the CdO normal mode of vibration, otherabsorptions also are important to describe. The structure of thetitle disulfide compound can be understood as formed bydisulfide and thiocyanate groups. So, one of the characteristicmode of vibration of this type of compounds corresponds to a

band with medium intensity located at 2158 cm-1, assigned tothe ν(SCtN) stretching mode in the IR spectrum, with itscounterpart in the Raman spectrum as the most intense signal,also centered at 2158 cm-1, in agreement with related species.50,51

The most intense liquid-phase IR band at 1148 cm-1 is assignedto the antisymmetric stretching mode of the CH3sOsCgroup,19,27,52 and the stretching mode ν(SsCN) is assigned to amedium intensity band in the IR at 666 cm-1 (668 cm-1 inRaman).53 It is well-known that the ν(SsS) stretching modeis usually observed in the Raman spectrum and can beused to characterize disulfide containing compounds. ForCH3OC(O)SSCN the medium intensity signal appearing at 526cm-1 is assigned to ν(SsS), in agreement with reported valuesfor other disulfide compounds.34,54

NBO Analysis

For a better understanding of the conformational preferencesof the compounds here examined, a natural bond orbital (NBO)analysis at DFT level of theory has been performed. NBOmethod makes possible to examine hyperconjugative interactionsdue to electron transfers from either bonding or lone pair filledorbitals (donors) to empty antibonding (acceptor) orbitals.Significant donor-acceptor NBO interactions and their second-order perturbation stabilization energies (in kcal mol-1), cal-

TABLE 2: Observed and Calculated Vibrational Data (cm-1) for CH3OC(O)SCN

experimental calculatedc

IR (gas)a IR (Ar matrix) MP2/6-311++G**

mode syn-syn syn-anti syn-syn syn-anti IR (liquid) Ramanb syn-syn syn-antitentative assignment

(PED)/symmetry

ν1 3022 vw 3019 3056 vw 3241 (1) 3242 (1) νas(CH3) (100)/A′ν2 2970 w 2969 2964 w 2966 m 3207 (2) 3209 (3) νas(CH3) (100)/A′′ν3 2857 vw 2847 2844 vw 3108 (4) 3109 (6) νs(CH3) (100)/A′ν4 2183 2176 2176 w 2178 vs 2124 (<1) 2118 (<1) ν(CtN) (90) + ν(SsCN) (10)/A′ν5 1799 R 1770 R 1787 1758 1875 s 1772 vw 1824 (41) 1801 (100) ν(CdO) (100)/A′

1789 P 1765 Q1760 P

ν6 1444 vw 1465 1451 1453 vw 1520 (2) 1518 (3) δas(CH3) (85) + Fs(CH3) (10)/ A′ν7 1438 vvw 1447 1436 1433 w 1505 (2) 1504 (3) δas(CH3) (95) + Fas(CH3) (5)/ A′ν8 1330 vvw 1351 1328 vw 1494 (2) 1493 (4) δs(CH3) (95) + δas(CH3) (5)/A′ν9 1198 m 1202 m 1195 1201 1201 s 1235 (41) 1240 (87) Fs(CH3) (60) + νas(CsOsC) (25) +

δas(CH3) (5)/A′ν10 1154 vs 1151 1171 1147 vs 1153 vw 1195 (100) 1205 (85) Fas(CH3) (90) + δas(CH3) (5)/A′′ν11 1156 1193 (<1) 1192 (<1) νas(CsOsC) (60) + Fs(CH3) (20) +

F(CdO) (20)/A′ν12 962 vvw 934 vw 935 w 998 (1) 1006 (1) νs(CsOsC) (85) + νas(CsOsC) (10)/A′ν13 812 m 813 817 814 w 815 m 831 (9) 836 (17) F(CdO) (40) + δ(CsOsC) (20) +

ν(SsCN) (20)/A′ν14 697 688 697 vw 719 (1) 716 (1) ν(SsCN) (90) + ν(CtN) (5)/A′ν15 666 vw 663 661 662 w 698 w 661 (2) 662 (3) oop(CdO) (90) + τ(OsC) (5)/A′′ν16 523 vw 518 vw 519 vw 531 (1) 496 (1) δ(CSC) (45) + ν(SsCN) (25) +

F(CdO) (15)/A′ν17 481 vvw 494 vvw 488 vvw 489 vvw 398 (1) 430 (<1) ν(CsS) (40) + δ(SCN) (20) +

δ(CSC) (20)/A′ν18 435 416 vw 364 (3) 355 (3) δ(CdO) (35) + δ(CsOsC) (30) +

δ (CdO) (15)/A′ν19 393 m 316 (<1) τ(SsC)/A′′

364 vw 313 (<1)ν20 259 vw 254 (1) 263 (3) F(CdO) (45) + δ(CsOsC) (35) +

δ(SCN) (15)/A′ν21 157 (<1) 156 (<1) τ(CH3sO)/A′′ν22 134 w 127 (1) 136 (1) τ(OsC)/A′′ν23 116 (<1) 118 (1) δ(CsSC) (60) + δ(SCN) (35)/A′ν24 85 w 58 (<1) 47 (1) τ(CsS)/A′′

a Band intensity: vs ) very strong, s ) strong, m ) medium, sh ) shoulder, w ) weak, vw ) very weak, vvw ) very very weak. b Liquidat room temperature. c In parentheses relative band strengths for the two most stable forms, IR intensities [100% ≡ 554 km/mol for thesyn-syn form] and [100% ≡ 351 km/mol for the syn-anti form].


culated in vacuum at B3LYP/6-311++G** level of theory forthe main conformers of compounds I and II are showed inTable 3.

For compound I, the NBO population analysis shows thepresence of two lone pairs formally located in the divalent sulfuratom, i.e., lpπ(S) and lpσ(S), with occupancies of 1.78e and1.96e, respectively, indicating that the electron donor capacityis mainly due to the lpπ(S). They interact preferably with theCdO and CtN antibonding orbitals, the most intense interac-tions are due to the donation of the HOMO orbital lpπ(S) tothe LUMO orbital π*(CdO) [lpπ(S) f π*(CdO)] and to theπ*(CtN) [lpπ(S) f π*(CtN)]. Also electron donation fromthe lpπ(O) orbital in the methoxycarbonyl group into the

π*(CdO) is observed. These interactions are responsible forthe high occupancy of the two acceptor orbitals π*(CdO)(0.28e) and π*(CtN) (0.11e), which contributes to the stabi-lization by resonance (mesomeric effect), favoring the localplanar structure around the -OC(O)SCN moiety. These interac-tions are mostly not affected by the syn or anti orientation aroundthe CsS bond. However, the second lone pair formally locatedat the sulfur atom [lpσ(S)] interacts in a different way dependingon the conformation of the molecule. Thus, lpσ(S) mainlyinteracts with the σ*(CdO) orbital in the syn form [lpσ(S) fσ*(CdO)] or with the σ*(OsC) orbital of the anti form [lpσ(S)f σ*(OsC)] (anomeric effect). The syn form is favored by2.3 kcal mol-1 (see Table 3). Obviously, differences between

Figure 4. IR spectrum in the carbonyl stretching region for gaseous and for Ar-matrix isolated CH3OC(O)SCN, together with the simulatedspectrum from the calculated frequencies at the MP2/6-311++G**. Principal moments of inertia are shown for molecular models of both syn-synand syn-anti conformers (the C-axis is perpendicular to the plane formed by the A- and B-axes).


these conformations are not limited to hyperconjugation effectssince the bonds around the -C(O)S- group are quite polar.NBO calculations predict net atomic charges about -0.54e and+0.27e on the O and S atoms, respectively, whereas the C atomcarry a +0.68e charge.

For compound II, the resonance interaction involving theπ*(CdO) and π*(CtN) acceptor orbitals is disrupted by thepresence of the disulfide bond and the concomitant loss of localplanarity. Thus, the occurrence of an extended π systemincluding both the -OC(O)S- and -SCN groups is precluded.Nevertheless, the anomeric interactions that account for thesyn-anti conformational equilibrium are very similar to thoseobserved for compound I, as listed in Table 3. For comparison,equivalent second-order donor-acceptor interaction forCH3OC(O)SCl are also included in Table 3. Very similar valuesare observed for the three molecules. This fact denotes that both-CN and -SCN groups and the chlorine atom have a similareffect, acting thus, as pseudohalogenide groups.

Conclusions

From the analysis of the vibrational spectra of CH3OC(O)SCN,the existence of two conformers in the gas phase is well-established.These forms differ in the relative orientation of the CdO doublebond respect to the SsC single bond being the syn-syn conformerthe most stable form, having a planar skeleton structure with theCdO double bond in mutual synperiplanar orientation with respectto both the methoxy and thiocyanate groups. According to NBOanalysis, resonant (mesomeric) interactions seem to be responsiblefor the planar structure, while hyperconjugative interactions tendto stabilize the syn-syn conformer with respect to the syn-antione. From studies of the Ar-matrix spectrum at variable temper-atures the conformational enthalpy difference ∆H0 ) H0

(anti-syn) -H0

(syn-syn) ) has been determined to be 0.80(18) kcal mol-1.Quantum chemical calculations, even at high levels of approxi-mationm yield energy differences that are fairly lower than theexperimental values; the better agreement is obtained with the MP2and CCSD(T) methods with the extended 6-311++G** basis sets,with an estimated ∆E0 value of 0.56 and 0.54 kcal mol-1,respectively.

For compound II, the coexistence of also two conformers thatdiffer in the relative orientation of the carbonyl group with

respect to the S-S single bond is suggested, the syn-synconformer being the most important form.

Experimental Section

Synthesis. Following the early method reported for relatedspecies by Haas and Reinke15 and the recent work of Ge etal.18 CH3OC(O)SSCN and CH3OC(O)SCN have been synthe-sized by the methathesis reaction of CH3OC(O)SCl and thecorresponding silver salts according to eq 1:

Physical Properties and Spectroscopic Characterization.Compounds I and II are colorless liquids, with the characteristicoverpowering sulfenylcarbonyl odor. In the liquid state,CH3OC(O)SSCN is stable for several hours at air and roomtemperature. The thiocyanate compound isomerizes after severalhours at room temperature to the respective isothiocyanateisomer [CH3OC(O)NCS].55

Isolation of both compounds from the reaction mixture canbe performed by vacuum distillation. The 1H NMR spectra ofcompounds I and II only shows singlet signals located at δ )4.0 ppm that corresponds to the CH3O protons. Previous studiesfor CH3OC(O)SCN reported a value of 4.2 ppm for the protonsignal.12 The 13C NMR spectrum of the CH3OC(O)SSCN showstwo quartet and a singlet signals. Thus, chemical shifts at δ )164.7 ppm (q,3JCH ) 3.7 Hz), δ ) 110.3 ppm (s) and δ ) 57.4ppm (q, 1JCH ) 150.3 Hz) are assigned to the CdO, CtN, andCH3 carbons, respectively. These chemical shift values andcoupling constants are in good agreement with reported datafor related compounds.16,19,27,56,57

The GC chromatogram of compound II shows a peak at aretention time of 6.1 min (measured in CHCl3). In the electronimpact GC-MS spectra the compound shows the molecular ion(M•+) peak as a very weak intensity signal at m/z 149. Severalionic fragments arising from logical ruptures appear at m/z[relative intensity, fragment] at 118 [10, C(O)SSCN+], 90 [15,SSCN+], 64 [70, SS+], 60 [30, SCO+], 59 [100, CH3OC(O)+],32 [15, S+], and 15 [70, CH3

+]. This fragmentation pattern isin agreement with the reported photoionization (He I) massspectrum acquired together with the photoelectron spectrum ofII, where peaks at m/z 149, 118, and 90 were observed.18

Additional evidence for identifying both compounds comesfrom the analysis of their IR and Raman (liquid) spectra, ascommented above. In summary, the complement of bothtechniques allows for an easy characterization of these thiocy-anate species. The characteristic mode of vibration of thethiocyanate group ν(SCtN) present in both molecules isobserved as the most intense signal in the Raman spectra at2178 and 2158 cm-1 for I and II, respectively, with theircounterparts at 2176 (weak) and 2158 cm-1 (medium) in theIR spectrum of the liquid substances. The most intense bandobserved in the IR(liquid) spectra of both compounds, respec-tively, is centered at 1147 and 1154 cm-1 assigned to theantisymmetric stretching mode (νas CsOsC) in the methoxy-carbonyl group. For compound II, the characteristic disulfidestretching mode is observed in the Raman (liquid) spectrum asa strong signal at 526 cm-1.

General Procedure and Reagents. In a Carius tube adaptedwith a Young’s valve kept in a cold bath at -25 °C, 20 mmolof CH3OC(O)SCl was condensed on 32 mmol of finelypowdered AgSCN or AgCN, which previously were dried in

Figure 5. van’t Hoff plots using the absorbance ratios of the IR Ar-matrix band pairs at 1787/1758 (b) and 1201/1195 (0) cm-1 obtainedafter quenching the rotamer equilibria of CH3OC(O)SCN at differentdeposition temperatures.

CH3OC(O)SC1(1) + AgX(s) f CH3OC(O)SX(1) +AgC1(s) X ) CN, SCN (1)


vacuum at 80 °C for 2 h. The reaction mixture was kept withstirring for 1 h at low temperature and one more hour at roomtemperature. The reaction is followed by observing the vanishingof the pale yellow color (due to CH3OC(O)SCl) of the reactionmixture. The completion of the reaction was followed byinfrared spectroscopy and was confirmed mainly checking theband at 550 cm-1 due to the νS-Cl stretching mode ofCH3OC(O)SCl.40 After this, the volatile components werefractionated under dynamic vacuum trap-to-trap distillationthrough traps held at -20, -60, and -196 °C. A 2.0 g (17mmol) and 2.80 g (19 mmol) amount of pure CH3OC(O)SCNand CH3OC(O)SSCN were collected as a colorless liquids inthe -20 °C trap, representing a yield of ca. 85 and 95%,respectively. Only minor quantities of OCS and CO2 wereobserved as decomposition products in the U-trap at -196 °C.The final purity was checked by IR and 1H NMR spectroscopy.

Volatile materials were manipulated in a glass vacuum-lineequipped with PTFE valves (Young, London, U.K.) andcapacitance pressure gauge (680 A, Setra System) for the controlof the pressure. The vacuum line has a connection to an IR cell(optical path length 10 cm, Si windows 0.5 mm thick). Thisarrangement allowed us to observe the course of the reactionby FTIR spectroscopy (EQUINOX 55, Bruker). Pure compounds

were stored in flame-sealed glass ampules under liquid nitrogenin a long-term Dewar vessel. CH3OC(O)SCl (97% Aldrich) waspurified by fractional trap-to-trap condensation. AgCN andAgSCN were purchased from commercial source (Aldrich,estimated purity better than 99%).

Instrumentation. (A) Vibrational Spectroscopy. Gas andliquid phase infrared spectra were recorded with a resolutionof 1 and 2 cm-1, respectively, in the range 4000-400 cm-1 onthe Bruker EQUINOX 55 FTIR spectrometer. FT-Ramanspectrum of the liquid substances were run with a Bruker 66with FRA 106 Raman accessory at a resolution of 2 cm-1;samples in a 4 mm glass capillary was excited with 5 mW ofa 1064 nm Nd:YAG laser.

(B) Matrix Spectroscopy. In a stainless steel vacuum line(1.1 L volume), a small amount of CH3OC(O)SCN (ca. 0.05mmol) was mixed with an 1:1000 excess of Ar. For eachexperiment ca. 0.6 mmol of this mixture was passed via astainless steel capillary through a heated quartz nozzle, whichwas placed directly in front of the matrix support. Thetemperature of the matrix support was held at 14 K and thenozzle temperature was adjusted at the several temperatures inthe range 20 < T < 340 °C. Details of the matrix apparatus havebeen given elsewhere.58 Matrix IR spectra were recorded on an

Figure 6. Liquid IR (top) and Raman (bottom) spectra for CH3OC(O)SSCN.

TABLE 3: Stabilization Energies (kcal mol-1) for Orbital Interactions for Syn and Anti Conformers of Compounds I and IIand the Related CH3OC(O)SCl Species, Using the B3LYP/6-311++G** Approximation

CH3OC(O)SCN CH3OC(O)SSCN CH3OC(O)SCla

interaction syn-syn syn-anti syn-syn syn-anti syn-syn syn-anti

lpπS f π*CtN 27.56 27.97 28.00 28.15lpπS f π*CdO 23.18 22.75 22.20 20.10 23.95 24.61lpπO f π*CdO 49.57 52.52 49.12 48.03 47.13 51.56lpσS f σ*CdO 4.64 4.47 4.51lpσS f σ*OsC 2.35 2.30 2.45

a Similar values are reported in ref 19 by using the B3LYP/6-31+G* level of approximation.


IFS66v/S FT spectrometer (Bruker, Karlsruhe, Germany) in thereflectance mode with a transfer optic. A DTGS detector witha KBr/Ge beam splitter in the region ν ) 4000-400 cm-1 wasused. In this region 64 scans were coadded for each spectrumby means of apodized resolution of 1 cm-1.

(C) NMR Spectroscopy. The 1H and 13C NMR measurementswere recorded with a Mercury-200 spectrometer operating at200 and 50 MHz, respectively, for CH3OC(O)SCN and with aBruker Avance DRX-300 spectrometer operating at 300 and75 MHz, respectively, for CH3OC(O)SSCN. Pure sample weredissolved in CDCl3 using Si(CH3)4 as internal reference.

(D) GC-MS Determination. The GC-MS measurementswere recorded in a GCMS-QP2010 SHIMADZU instrumentusing gaseous helium as mobile phase with the pressure in thecolumn head equal to 100 kPa. The column used was a 19091J-433 HP-5 of 30 m × 0.32 mm × 0.25 mm film thickness.Approximately 1 µL volume of the compound dissolved inCHCl3 was chromatographed under the following conditions:the injection temperature was 200 °C, the initial columntemperature (70 °C) was held for 2 min and then increased to200 at 7 °C/min and held for 2 min after elevated to 300 at 5°C/min and held for 2 min more. In the spectrometer the sourcewas kept at 200 °C.

(E) Theoretical Calculations. All quantum chemical calcula-tions were performed with the GAUSSIAN 03 program pack-age.59 Full geometry optimizations were done by applying abinitio (MP2) and DFT (B3LYP) methods with standard basissets up to the Pople-type 6-311++G** and the augmentedDunning’s correlation-consistent basis sets of valence triple-�(aug-cc-pVTZ). For the normal coordinate analysis, transforma-tions of the ab initio Cartesian harmonic force constants to themolecule-fixed internal coordinates system were performed, asdescribed by Hedberg and Mills and implemented in theASYM40 program.60 This procedure evaluates the potentialenergy distribution (PED) associated with each normal vibra-tional mode under the harmonic assumption. The internal andsymmetry coordinates used to perform the normal coordinateanalysis are defined in Figure S2 and Table S4, respectively,given as Supporting Information.

Acknowledgment. This work is part of the Postdoctoral workof S.T.V., who is a Postdoctoral fellow of CONICET. C.O.D.V.and M.F.E. are members of the Carrera del Investigador ofCONICET, Republica Argentina. Financial support by theVolkswagen-Stiftung and the Deutsche Forschungsgemeinschaftis gratefully acknowledged. The Argentinean authors thank theANPCYT-DAAD for the German-Argentinean cooperationAwards (PROALAR) and the DAAD Regional Program ofChemistry for Argentina. They also thank the Consejo Nacionalde Investigaciones Cientıficas y Tecnicas (CONICET), toComision de Investigaciones Cientıficas de la Provincia deBuenos Aires (CIC), Republica Argentina. They are indebtedto the Facultad de Ciencias Exactas, Universidad Nacional deLa Plata, for financial support.

Supporting Information Available: Calculated relativeenergies and vibrational data for compound II are given inTables S1 and S2, respectively. A detailed description for thecalculations of the P-R band separation with the calculatedasymmetry parameters are given in Table S3. The symmetryand internal coordinates for compound I are given in Table S4and Figure S2, respectively. This material is available free ofcharge via the Internet at http://pubs.acs.org.

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conformational behavior of ch 3 oc(o)sx (x = cn and scn) pseudohalide congeners. a combined...

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