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Torsional Barrier in N,N-Dimethylthioformamide
Dynamic Nuclear Magnetic Resonance in the Gas Phase. The Torsional Barrier in N,N-Dimethylthioformamide Torbjarn Drakenberg Division of Physical Chemistry 2, The Lund Instttute of Technology, Chemical Center, S-22007 Lund 7, Sweden (Received November 4, 1975)
The torsional barrier in gaseous N,N-dimethylthioformamide has been obtained from a total bandshape analysis of the proton NMR spectrum. The height of the same barrier for nonpolar solutions has also been estimated. It was found that the barrier was increased considerably by solute-solvent or solute-solute interaction, especially in the neat liquid. The gas phase barrier was found to be ca. 22.5 kcal/mol (AG*), whereas the barrier in the neat liquid is greater than 25 kcal/mol.
Introduction The dynamic NMR methods, especially the total bandshape analysis technique, have been very useful tools for the study of interconversion barriers with activation energies (AG*) ranging from ca. 5 to ca. 25 kcal/mol.1,2 In almost every study neat liquids or solutions have been used, and only in very few cases has the gas phase been studied.3-6 The lack of data for the gas phase is easily understood as being due to the very low sensitivity of the NMR method, and furthermore the relaxation time in the gas phase is often short as a result of efficient spin-rotation relaxation. At least the problem with the low sensitivity can nowadays be partly overcome with the use of the pulsed Fourier transform technique, which was recently used in the study of the inversion barrier in gaseous aziridine.6 The lack of reliable gas phase data for most kinds of interconversion processes feasible for dynamic NMR studies is rather unfortunate, since most theoretical calculations, used for comparison with experimental data, refer to isolated molecules, which is best realized in the gas phase. This work is a continuation of the attempts to obtain reasonably reliable data on various kinds of interconversion barriers, in the gas phase, and deals with the torsional barrier in N,N-dimethylthioformamide (DMTF), which has previously been studied as the neat liquid or in concentrated solution^.^ Attempts have also been made to study N,N-dimethylformamide, but up to now only a single line has been observed for all the methyl protons, probably due to chemical shift equivalence. To test if data from nonpolar solvents could be used to estimate the true gas phase barrier, DMTF has also been studied in m-dichlorobenzene and decalin solutions. Results a n d Discussion In Figure 1 a few spectra of DMTF in the gas phase are reproduced. As can be seen, it is possible to obtain a reasonable signal-to-noise ratio even at temperatures far below the boiling point of the compound studied. It may be clearly seen from these spectra that the coalescence is not due to a temperature dependence in the chemical shift difference, but is caused by an exchange between the two methyl groups. From the spectra it was not possible to directly obtain the spin coupling between the methyl protons and the formyl proton. It was, however, possible from the high temperature spectrum, to show that the mean cou-
pling constant is less than or equal to 0.6 Hz, in good agreement with the coupling constants observed for DMTF in m-dichlorobenzene (mDCB) or decalin solutions. The coupling to the high-field methyl signal was found to be 0.5 Hz, and that to the low-field methyl signal 0.7 Hz. For both solvents used, the DMTF methyl proton chemical shift difference is concentration dependent, as can be seen from Table I. The negative sign for the neat liquid indicates that the order of the chemical shifts has been inverted, as judged from the relative magnitudes of the coupling constants. The pronounced concentration dependence of the chemical shift difference for the mDCB solution is expected, due to the so-called ASIS effect.8 The concentration dependence of the chemical shift difference for the decalin solutions is most easily explained if it is assumed that the thioamide homoassociation causes an upfield shift of the signal due to the methyl protons cis to sulfur. This problem is, however, not the main interest of this paper and will not be discussed further. From the observed chemical shift differences (see Table I), it is obvious that the assignment of the gas phase signals must be the same as for the dilute decalin solution, and in the total bandshape analysis performed to determine the activation energy of the hindered rotation, coupling constants of 0.5 and 0.7 Hz were used for the high- and lowfield signals, respectively. The resulting free energy of activation is given in Table I, together with data for the liquid state. From this table it is obvious that the torsional barrier in DMTF is considerably higher in the liquid state than in the gas phase. This is in agreement with the data for N,Ndimethylnitr~samine.~ Harris and Spragg reported for this compound a difference in AG* of ca. 2 kcal/mol between the gas phase and the neat liquid, with the gas phase lower. Similar results have been reported for the aldehyde rotation in benzaldehydes obtained from ir dataeg It was of interest to determine whether a dilute solution in an inert, nonpolar solvent could be used to estimate “gas phase” data in the case of DMTF as shown to be the case for some a ~ i r i d i n e sThus, .~ the barrier to internal rotation in DMTF dissolved in decalin and mDCB was obtained from bandshape analysis, resulting in the data given in Table I. mDCB is not a nonpolar solvent, and in these solutions, even at the lowest concentration used, the barrier is considerably higher than in the gas phase. For decalin, on the other hand, it was difficult to obtain usable spectra at very low concentrations of DMTF due to the closeness of The Journal of Physical Chemistry, Vol. 80pNo. 9, 1976
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Torbjorn Drakenberg TABLE I: Chemical Shift Difference between the Methyl Proton NMR Signals and the Barrier to Internal Rotation in N,N-Dimethylthioformamide
T
T
381 K
111 K
Sample composition
Au, ppm
DMTF gas t TMS gas DMTF liquid 1 M in mDCB 0.1 M in mDCB 0.01 M in mDCB 0.1 M in decalin 0.01 M in decalin
0.09 -0.12 0.07 0.14 0.15 0.05 0.08
AG*, (T,K) kcal/mol
(T,K)
(381) (303) (303) (303) (303) (303) (303)
(420) (473) (443) (443) (443) (431)
22.5 25.5 24.8 24.6 24.2 23.5
ficiently low concentration has been used and that the solvent really is nonpolar. The work on dynamic NMR studies in the gas phase will continue with studies on other types of interconversion barriers. Experimental Section The solvents, m-dichlorobenzene and decalin, were commercial products used without purification, and the N,Ndimethylthioformamide was a generous gift from J. Sandstrom, University of Lund. The gas phase samples were prepared in glass ampoules that fitted perfectly in 12-mm NMR tubes. The ampoules were filled on a vacuum line with DMTF and TMS and were sealed off under vacuum at dry-ice temperature. At temperatures above 150 OC the sample were completely gaseous. The NMR spectra were recorded on a Varian XL-100 spectrometer operating in the Fourier transform mode and using external proton lock. The temperature was measured with a thermocouple as described p r e v i o ~ s l yTypical .~ settings for the parameters used for the FT spectra were: spectral width 2000 Hz,acquisition time 2 s, number of transients 2000, and pulse width 80 HS (45" flip angle).
i \
T
4 7 3 ~
i \ 3.Q
ppm from T M B
3.0
Acknowledgments. Professor J. Sandstrom is heartily thanked for the gift of the N,N-dimethylthioformamide compound, and Dr. R. E. Carter for valuable linguistic criticism. The cost of the XL-100 spectrometer was partly defrayed by a grant from the Knut and Alice Wallenberg Foundation. This work was made possible by financial support from the Swedish Natural Science Research Council.
Figure 1. Proton NMR spectra of the methyl proton signal from N,K dimethylthioformamide at a few temperatures In the gas phase.
References a n d Notes
the solvent NMR signals. However, for the lowest concentration, much higher than for the mDCB solution, the barrier is only ca. 1 kcal/mol higher than for the gas phase. From the variation of the chemical shift difference with concentration, it is plausible that there is an interaction between the DMTF molecules, responsible for the difference in both the chemical shift difference and the barrier between the solution and the gas phase. It thus seems possible to estimate gas phase barriers for thioamides (and probably amides) from data on dilute solutions in nonpolar solvents. Care must however be taken to ensure that a suf-
( 1 ) G. Binsch In "Topics In Stereochemistry", Vol. 3, E. L. Ellel and N. L. Allinger, Ed., interscience, New York, N.Y., 1966. (2) I. 0. Sutherland in "Annual Reports on NMR Spectroscopy", Vol. 4, E. F. Moone , Ed., Academic Press, London, 1971. (3) R. K. arris and R. A. Spragg, Chem. Commun., 362 (1967). T. Drakenberg and J. M. Lehn, J. Chem. Soc., Pekin Trans. 2, 532 (1972). R. E. Carter and T. Drakenberg, J. Chem. SOC., Chem. Commun., 582 (1972). R. E. Carter and T. Drakenberg, J. Am. Chsm. Soc., 97,6990 (1975). A. Loewensteln. A. Melera, P. Rlgny, and W. Walter, J. Phys. Chem., 68, 1597 (1964). P. Laszlo in "Progress in Nuclear Magnetic Resonance Spectroscopy", Vol. 3, J. W. Emsley, J. Feeney, and L. H. Sutcliffe, Ed., Pergamon Press, London, 1967. W. G. Fatela, R. K. Harris, F. A. Milter, and R. E. Witkowski, Spectrochim. Acta, 21, 231 (1965).
The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
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