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