intermolecular effects on spin–spin coupling and magnetic shielding constants in gaseous...

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Intermolecular effects on spin– spin coupling and magnetic shielding constants in gaseous difluoromethane Marek Kubiszewski, Wlodzimierz Makulski, Karol Jackowski * Laboratory of NMR Spectroscopy, Department of Chemistry, Warsaw University, ul. Pasteura 1, Warszawa 02-093, Poland Received 12 September 2003; revised 11 November 2003; accepted 11 November 2003 Available online 18 May 2004 Abstract Difluoromethane (CH 2 F 2 ) has been studied using 1 H, 13 C and 19 F nuclear magnetic resonance (NMR) spectra in the gas phase at 300 K. For the first time all indirect spin–spin couplings in CH 2 F 2 were measured as functions of density. 1 H, 13 C and 19 F magnetic shielding constants were also monitored at various densities. After extrapolation to the zero-density limit the relevant values of an isolated CH 2 F 2 molecule (J 0 and s 0 ) were obtained. It was shown that the influence of intermolecular interactions on the NMR spectral parameters of CH 2 F 2 is rather significant. Strong linear density-dependence was found for the 1 J CF and 1 J CH spin – spin coupling constants while the 2 J FH coupling constant was almost independent of density in the gas phase. All the shielding constants of CH 2 F 2 (s H , s C and s F ) had linear density- dependence and they were diminished with the increase of gas density. Isotope effects in 19 F shielding of CH 2 F 2 have been determined when the 13 CH 2 F 2 and CHDF 2 isotopomers were observed at the natural abundance of the heavier nuclei. q 2004 Elsevier B.V. All rights reserved. Keywords: Gas phase; Intermolecular interactions; Spin–spin coupling; Magnetic shielding 1. Introduction It is well-known in nuclear magnetic resonance (NMR) spectroscopy that indirect spin – spin coupling constants are less efficiently dependent on intermolecular interactions than magnetic shielding constants of the same nuclei [1]. Nevertheless, the influence of intermolecular interactions on spin–spin coupling can easily be observed even in the gas phase [2–4], especially when modern techniques of NMR spectroscopy are applied [5–7]. This subject has recently been reviewed and many new examples of density- dependent spin – spin couplings have been reported [8]. It seems that there is only a problem of precision during NMR measurements of gaseous samples because every coupling constant is more or less dependent on intermolecular interactions. Spin – spin coupling constants involving 19 F nuclei are usually density-dependent in the gas phase, the strongest dependence on density has been observed for the 1 J (FC) coupling constant in CD 3 F [9]. Small molecules containing fluorine atoms, especially fluoromethanes CH 42n F n , are good models for various theoretical and experimental studies of NMR spectral parameters. Such chemical compounds can be investigated in the gas phase using three different NMR spectra ( 1 H, 13 C and 19 F) and the measurements of spin – spin coupling constants can easily be verified observing both the two nuclei which are coupled. Early 1 H and 19 F NMR studies of gaseous fluoromethanes have been reviewed by Govil [10]. Later the 19 F shielding in CH 42n F n was extensively studied in the gas phase by Jameson et al. [11–13]. The 13 C NMR spectra of gaseous CH 42n F n have been available since 1977 [14,15]. Rovibrational averaging of NMR parameters of CH 42n F n was discussed by Jameson et al. [16] but the first ab initio calculations of rovibrational effects on the 13 C, 19 F and 1 H shielding in CH 3 F were provided by lately Gee and Raynes [17]. Theoretical investigations of molecular magnetic properties of CH 42n F n are described in the review written by Helgaker et al. [18]. Recently, the spin–spin coupling tensors in fluoromethanes have been calculated by ab initio multiconfiguration selfconsistent-field (MCSCF) [19] and density-functional theory (DFT) [20] methods and the theoretical results have been compared with their own 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.11.059 Journal of Molecular Structure 704 (2004) 211–214 www.elsevier.com/locate/molstruc * Corresponding author. Tel.: þ 48-22-822-02-11x315; fax: þ 48-22-822- 59-96. E-mail address: [email protected] (K. Jackowski).

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Page 1: Intermolecular effects on spin–spin coupling and magnetic shielding constants in gaseous difluoromethane

Intermolecular effects on spin–spin coupling and magnetic shielding

constants in gaseous difluoromethane

Marek Kubiszewski, Włodzimierz Makulski, Karol Jackowski*

Laboratory of NMR Spectroscopy, Department of Chemistry, Warsaw University, ul. Pasteura 1, Warszawa 02-093, Poland

Received 12 September 2003; revised 11 November 2003; accepted 11 November 2003

Available online 18 May 2004

Abstract

Difluoromethane (CH2F2) has been studied using 1H, 13C and 19F nuclear magnetic resonance (NMR) spectra in the gas phase at 300 K.

For the first time all indirect spin–spin couplings in CH2F2 were measured as functions of density. 1H, 13C and 19F magnetic shielding

constants were also monitored at various densities. After extrapolation to the zero-density limit the relevant values of an isolated CH2F2

molecule (J0 and s0) were obtained. It was shown that the influence of intermolecular interactions on the NMR spectral parameters of CH2F2

is rather significant. Strong linear density-dependence was found for the 1JCF and 1JCH spin–spin coupling constants while the 2JFH coupling

constant was almost independent of density in the gas phase. All the shielding constants of CH2F2 (sH, sC and sF) had linear density-

dependence and they were diminished with the increase of gas density. Isotope effects in 19F shielding of CH2F2 have been determined when

the 13CH2F2 and CHDF2 isotopomers were observed at the natural abundance of the heavier nuclei.

q 2004 Elsevier B.V. All rights reserved.

Keywords: Gas phase; Intermolecular interactions; Spin–spin coupling; Magnetic shielding

1. Introduction

It is well-known in nuclear magnetic resonance (NMR)

spectroscopy that indirect spin–spin coupling constants are

less efficiently dependent on intermolecular interactions

than magnetic shielding constants of the same nuclei [1].

Nevertheless, the influence of intermolecular interactions on

spin–spin coupling can easily be observed even in the gas

phase [2–4], especially when modern techniques of NMR

spectroscopy are applied [5–7]. This subject has recently

been reviewed and many new examples of density-

dependent spin–spin couplings have been reported [8]. It

seems that there is only a problem of precision during NMR

measurements of gaseous samples because every coupling

constant is more or less dependent on intermolecular

interactions. Spin–spin coupling constants involving 19F

nuclei are usually density-dependent in the gas phase, the

strongest dependence on density has been observed for the1J(FC) coupling constant in CD3F [9].

Small molecules containing fluorine atoms, especially

fluoromethanes CH42nFn, are good models for various

theoretical and experimental studies of NMR spectral

parameters. Such chemical compounds can be investigated

in the gas phase using three different NMR spectra (1H, 13C

and 19F) and the measurements of spin–spin coupling

constants can easily be verified observing both the two

nuclei which are coupled. Early 1H and 19F NMR studies of

gaseous fluoromethanes have been reviewed by Govil [10].

Later the 19F shielding in CH42nFn was extensively studied

in the gas phase by Jameson et al. [11–13]. The 13C NMR

spectra of gaseous CH42nFn have been available since 1977

[14,15]. Rovibrational averaging of NMR parameters of

CH42nFn was discussed by Jameson et al. [16] but the first

ab initio calculations of rovibrational effects on the 13C, 19F

and 1H shielding in CH3F were provided by lately Gee and

Raynes [17]. Theoretical investigations of molecular

magnetic properties of CH42nFn are described in the review

written by Helgaker et al. [18]. Recently, the spin–spin

coupling tensors in fluoromethanes have been calculated by

ab initio multiconfiguration selfconsistent-field (MCSCF)

[19] and density-functional theory (DFT) [20] methods and

the theoretical results have been compared with their own

0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2003.11.059

Journal of Molecular Structure 704 (2004) 211–214

www.elsevier.com/locate/molstruc

* Corresponding author. Tel.: þ48-22-822-02-11x315; fax: þ48-22-822-

59-96.

E-mail address: [email protected] (K. Jackowski).

Page 2: Intermolecular effects on spin–spin coupling and magnetic shielding constants in gaseous difluoromethane

NMR measurements in liquid crystal solutions (LC NMR)

and gaseous samples at 5 atm.

Meanwhile, the measurements of spin–spin coupling

constants at the zero-density limit have been performed only

for one molecule of fluoromethanes, CD3F [9]. In the

present work we have investigated the spin–spin coupling

constants (1J(CH), 1J(FC) and 2J(FH)) and the three

shielding constants (s(CH2F2), s(C H2F2) and s(CH2F2))

of pure difluoromethane as functions of density in the gas

phase. Our 19F, 13C and 1H NMR measurements have

delivered entirely new results for spin–spin coupling

constants and they have allowed the verification of some

previous shielding parameters of CH2F2.

2. Experimental

NMR spectra were acquired on a Varian UNITYplus-500

FT spectrometer equipped with a Performa I z-PFG unit and

a standard 5 mm ID_PFG probehead. The spectra were

obtained at the 125.88, 470.91 and 500.62 MHz transmitter

frequency for the 13C, 19F and 1H nuclei, respectively.

Chemical shifts were measured using one-dimensional

spectra with standard parameters for each NMR methods.

Liquid TMS and CFCl3 were used as the secondary external

standards. The measurements of spin–spin coupling con-

stants were carried out using modified PFG-HSQC two-

dimensional 19F–13C and 1H–13C spectra as described

earlier [9] and verified using one-dimensional spectra.

Difluoromethane (CH2F2, 99.7%, Aldrich) from a lecture

bottle was used in the present studies without further

purification. Gas samples were prepared by condensation of

CH2F2 gas from the calibrated part of vacuum line into the

4.0 mm o.d. glass tubes (approx. 5.5 cm long) which were

then sealed. The volumes of sample tubes and the vacuum

line were measured using mercury. The gas samples were

fitted into the standard 5 mm o.d. thin-walled NMR tubes

(Wilmad 528-PP) with liquid toluene-d8 in the annular

space. The deuterium CD3 signal from toluene-d8 was used

for the lock system. The 1H, 13C and 19F NMR chemical

shifts were measured relative to liquid TMS and CFCl3,

respectively, as the external reference standards. This study

has been extended on the 19F magnetic shielding in CHDF2

and 13CH2F2 molecules, the commercial CH2F2 product has

contained the latter molecules at the natural abundance and

their concentrations have been sufficient for the 19F NMR

measurements.

3. Results and discussion

In the gas phase at constant temperature the nuclear

shielding of a nucleus in a molecule can be expressed as a

power series in the form of

sðTÞ ¼ s0ðTÞ þ s1ðTÞrþ s2ðTÞr2 þ … ð1Þ

where s0ðTÞ is the shielding for an isolated molecule and the

higher terms (s1ðTÞ;s2ðTÞ…Þ are dependent on the density

r and describe the intermolecular interactions in gases.

Nuclear spin–spin coupling is also modified by interactions

of molecules and the appropriate equation for the spin–spin

coupling is similar to Eq. (1)

JðTÞ ¼ J0ðTÞ þ J1ðTÞrþ J2ðTÞr2 þ … ð2Þ

where J0ðTÞ is the spin–spin coupling for an isolated

molecule and J1ðTÞ; J2ðTÞ… are due to intermolecular

effects in the collisions of molecules. As shown all the

shielding and spin–spin coupling parameters in Eqs. (1) and

(2) are temperature dependent and for this reason NMR

measurements must be performed at constant temperature,

e.g. at 300 K.

Fig. 1 presents the 1J(CH), 2J(FH) and 1J(CH) spin–spin

couplings constants in CH2F2 as functions of density at

300 K. All the three couplings are increased due to

intermolecular interactions in the gas phase though the

change of 2J(FH) is fairly modest. As shown the dependence

on density is linear in every case and it means that J2ðTÞ and

higher coefficients in Eq. (2) can be neglected. The J0ðTÞ

values are obtained by linear extrapolation to the zero-

density points and reveals the spin–spin coupling constants

of an isolated CH2F2 molecule at 300 K. These parameters

have been measured for the first time and they are displayed

Fig. 1. The 1J(CH), 2J(FH) and 1J(CH) spin–spin couplings constants of

CH2F2 as functions of density at 300 K.

M. Kubiszewski et al. / Journal of Molecular Structure 704 (2004) 211–214212

Page 3: Intermolecular effects on spin–spin coupling and magnetic shielding constants in gaseous difluoromethane

in Table 1 together with other measurements performed

earlier. In our opinion the present J0ðTÞ constants are

superior as the experimental standards because they have

been determined from correlation PFG-HSQC spectra and

verified by appropriate one-dimensional spectra. It is

interesting to compare our J0ðTÞ coupling constants with

recent calculations performed for the equilibrium geometry

of a CH2F2 molecule [19,20]. The MCSCF results are as

follows: 1Je(FC) ¼ 2220.7 Hz, 1Je(CH) ¼ 175.7 Hz and2Je(FH) ¼ 51.9 Hz [19]. Here the agreement with

the experimental J0 values in Table 1 is really good because

the rovibrational corrections can be responsible for the

discrepancy between the theoretical and experimental

results. It was shown for an acetylene molecule by

Wigglesworth et al. [21,22] that rovibrational corrections

to spin–spin coupling constants can be from few to several

hertz if the increase of temperature to 300 K is required.

Unfortunately, such rovibrational corrections for difloro-

methane are still unknown and the exact comparison of the

J0 theoretical and experimental results cannot be accom-

plished. On the other hand the DFT calculations give the1Je(FC) value from 2324.8 to 2309.9 Hz [20], it is so far

from the experimental coupling constant

(1J0(FC) ¼ 2220.7 Hz at 300 K, cf. Table 1) that we can

safely assume low precision of the latter calculations. Table

1 also presents the J1 coefficients due to the influence of

intermolecular interactions on spin–spin coupling constants

in the gas phase. As shown the one-bond coupling constants

of CH2F2 are distinctly dependent on density, cf. the 1J1(FC)

and 1J1(CH) values in Table 1. In contrast, the spin–spin

coupling along two chemical bonds (2J(FH)) is almost

independent of density.

In the present study the 1H, 13C and 19F nuclear

magnetic shielding of CH2F2 were also monitored. The 19F

NMR signals of gaseous samples were so strong that we

could observe the 13C and 2H isotopomers of difluor-

omethane at the natural abundance of these isotopes. Fig.

2 shows the 19F density-dependent magnetic shielding in

all the three isotopomers: 12CH2F2, 13CH2F2 and12CHDF2. These functions are linear and the determination

of s0 and s1 shielding parameters is simple, their values

are given in Table 1. The absolute 19F shielding constant

Table 1

Spin–spin coupling and nuclear magnetic shielding parameters of CH2F2

measured in the gas phase at 300 K

Parameter Reference

Spin–spin coupling of CH2F21J0(FC) (Hz) 2234.55(5) This work1J(FC) (Hz) 2233.91(11)a Lantto et al. [19]

2232.7b Jackowski and Raynes [14]1J0(CH) (Hz) 180.42(5) This work1J(CH) (Hz) 180.38(4)a Lantto et al. [19]2J0(FH) (Hz) 50.24(5) This work2J(FH) (Hz) 49.06(13)a Lantto et al. [19]

50.09c Smith and Raynes [21]

50.5d Jameson et al. [13]1J1(FC) (Hz ml mol21) 363(75) This work1J1(CH) (Hz ml mol21) 705(52) This work2J1(FH) (Hz ml mol21) 86(65) This work

1H, 13C and 19F magnetic shielding

s0 (CH2F2) (ppm) 25.291(3)e This work

s0 (C H2F2) (ppm) 77.726(4)f This work

s0 (12CH2F2) (ppm) 338.935(2)g This work

339.1d Jameson et al. [23]

s0 (13CH2F2) (ppm) 339.054(4)g This work

s0 (12CHDF2) (ppm) 339.642(5)g This work

(s1)b (ppm ml mol21) 102.6 h Jameson et al. [13]

ðs1Þint (CH2F2)

(ppm ml mol21)

240(3) This work

235.3(18)c Smith and Raynes [26]

241.7 Meinzer, cited in Ref. [26]

ðs1Þint (C H2F2)

(ppm ml mol21)

2359(3) This work

ðs1Þint (12CH2F2)

(ppm ml mol21)g

2118(3) This work

2118d Jameson et al. [13]

ðs1Þint (13CH2F2)

(ppm ml mol21)g

2126(4) This work

ðs1Þint (12CHDF2)

(ppm ml mol21)g

2133(6) This work

The 19F shielding parameters of CHDF2 and 13CH2F2 are also included.a NMR measurements in the gas phase at 5 atm.b A single 13C NMR measurement in the gas phase at ,40 atm.c 1H NMR measurements in the gas phase at 293 K.d 19F NMR measurements of gaseous samples.e Absolute shielding assuming: s0(CH4) ¼ 30.611(24) ppm [24].f Absolute 13C shielding assuming s0(CO) ¼ 0.6(9) ppm [25].g Absolute 19F shielding assuming s(liq. CFCl3, B0k) ¼ 192.7 ppm,

cf. s(liq. CFCl3, B0’) ¼ 188.7 ppm [27].h 2ð4p=3ÞxM; where xM ¼ 224:5 ppm ml mol21 is the molar suscepti-

bility of CH2F2 taken from Ref. [13].

Fig. 2. The 19F shielding constants of 12CH2F2, 13CH2F2 and 12CHDF2 as

functions of density in the gas phase at 300 K. 13CH2F2 and 12CHDF2 were

observed at the natural abundance of 13C and 2H nuclei.

M. Kubiszewski et al. / Journal of Molecular Structure 704 (2004) 211–214 213

Page 4: Intermolecular effects on spin–spin coupling and magnetic shielding constants in gaseous difluoromethane

of an isolated 12CH2F2 molecule is determined and

remains in agreement with the previous value measured

by Jameson et al. [23], the other s0’s allow one to establish

the real magnitudes of isotopic shifts: 2D19F(13/12C) ¼

20.119(6) ppm and 2D19F(2/1H) ¼ 20.707 ppm. It is

worth noting that the s1 coefficient of Eq. (1) consists of

the sum of the bulk susceptibility correction ðsbÞ and the

intermolecular term ðsintÞ; in Table 1 their values are given

separately. Intermolecular interactions are responsible for

the decrease of the 19F shielding in all the isotopomers of

difluoromethane but isotopic effects are also well seen for

the s1 parameters, the most negative value is observed for12CHDF2 (2133(6) ppm ml mol21), the next for 13CH2F2

(2126(4) ppm ml mol21) and for 12CH2F2 (2 -

118(2) ppm ml mol21). Table 1 also presents the 1H and13C shielding parameters of CH2F2. They have been

determined from the linear dependence on density. The

appropriate s0 shielding constants were measured also in

the absolute 1H and 13C shielding scales assuming as the

standards: s0(CH4) ¼ 30.611(24) ppm [24] and

s0(CO) ¼ 0.6(9) ppm [25]. The present shielding constants

of isolated molecules can be directly used for any

comparison with the results of quantum chemical calcu-

lations. The values are valid for molecules at 300 K,

otherwise the rovibrational corrections to shielding must be

calculated, as it has been done for a CH3F molecule by Gee

and Raynes [17].

4. Conclusions

The present work shows that the spin–spin coupling

constants of difluoromethane are dependent on density in

the gas phase. This dependence on density is so significant

for the one-bond coupling constants (1J(FC) and 1J(CH))

that it can not be ignored. The accurate measurements of

these coupling constants have been performed after the

extrapolation to the zero-density point. Similar results have

been obtained for all the three shielding constant of CH2F2.

To our knowledge the 13C shielding of this compound have

been measured as a function of density for the first time. It

has allowed us to determine the absolute 13C, 1H and 19F

shielding constants of an isolated CH2F2 molecule when the

present results have been compared with the best

reference standards of shielding scale for the 13C, 1H and19F nuclei, respectively. Our 19F NMR measurements were

extended on the 13CH2F2 and 12CHDF2 isotopomers and

the secondary isotope effects (2D19F(13/12C) and2D19F(2/1H)) have been determined on the level of isolated

molecules. All the present new data can be used as

the experimental NMR standards for the verification of

theoretical results.

Acknowledgements

This work was partially supported by the Polish State

Committee for Scientific Research (to M.K. and K.J.) as the

research grant number 4 T09A 120 25 available in years

2003–2005. The authors thank Dr W. Kozminski for his

modified PFG-HSQC sequence of pulses used in the present

work.

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