J. Phys. Chem. 1083, 87,3054-3058
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edge, does not show any tendency to reorient up to its saturation c~ncentration,"~ likewise, due to steric hindrance by the methyl substituents. The data in Table I11 show that C, for 1 and 2 is sensitive to temperature; in particular, the flat-to-edgewise orientational transitions of these compounds begin at lower concentrations when the temperature is increased. The same trend was found for 3 and 4, Figure 2. The decrease in packing density for T2-orientedmonolayers which accompanies an increase in temperature may be understood in terms of models similar to those for insoluble fatty acids on ~ a t e r . The ~ ~ simplest , ~ ~ model assumes a mixture of rigidly flat and rigidly edgewise species, but does not explain why the intermediate plateaus are sharply defined only in a narrow range of temperatures (cf. 45 "C isotherm for hydroquinone). A more appealing model is that of a stack of floppy disks: At low temperatures, the reorientation proceeds directly from rigidly flat to rigidly edgewise structures. But, as the temperature is increased, an intermediate phase appears in which the q2-orientedmolecules undergo librational motion about a plane normal to the surface; librations of f5' with reference to this plane would be sufficient to increase the effective molecular areas corresponding to the measured values. The appearance of well-defined intermediate plateaus only at certain temperatures indicates that the adsorbate structure at these temperatures, T p ,is uniquely stable. This is probably because the molecular motions generated at T pare concerted and may be regarded as a collective librational mode, the energy of which can be estimated from kB(T - To),where kB is the Boltzmann constant, and Tomay &etaken as the temperature at which the orientational transitions are most sharply defined, 273 K. In this approximation, the librational mode energy for 1-4, for which T is about 318 K, is of the order of 30 cm-'; this value may [e compared with those of the rotational and translational modes of solid benzene,@,47 found to be in the range from 40 to 140 cm-'. It is interesting to note that the isotherms at 65 "C do not pass through Ci,(Figure 2), indicating that the structure of the adsorbed layer at this temperature is vastly different from those at lower (46)Harada, I.; Shimanouchi, T. J. Chem. Phys. 1967,46,2708. (47)Bonadeo, H.; Marzocchi, M. P.; Castellucci, E.; Califano, S. J . Chem. Phys. 1972,57,4299.
temperatures. Structural changes in the adsorbed layer may result if the librational amplitudes at 65 "C become very large such that long-range ordering is negated, analogous to the disorder brought about by wild librational motions of the molecules in the liquid crystal p-azoxyanisole near its melting point.48 In contrast to T2-orientedspecies, the $-attached compounds 6 and 7 are quite rigid, as evidenced by the near-constancy of their packing densities. (It is important to mention that the pyridine-type compounds 7 and 8 undergo keto-enol tautomerism in solution; 49 however, metal coordinationm and high t e m p e r a t ~ r have e ~ ~ been shown to favor the enol form.) Apparently, librations such as rotation about the v1 N-Pt and S-Pt are insignificant up to 65 "C. Also, increasing the temperature does not appear to induce the reorientation of the anthraquinone derivative (5). On the other hand, the packing density of the pyridazine derivative (8) postulated to be oriented N-T$ at ambient temperat~res'*~t~ is lower at 65 "C than at 25 "C. In view of the presence of two adjacent equivalent nitrogens in the compound, and since rotational vibrations do not seem to be significant for strongly bound ql-oriented species, the decrease in packing density for 8 at 65 "C might be due to one or both of the following: (i) simple reorientation from an N-ql state to a 1,2-q2 structure; (ii) fluxional motion in which the 9' attachment alternates between the two equivalent nitrogens, the average structure resembling a 1,2-q2orientation. Molecular area calc u l a t i o n ~reveal ~ * ~ that the observed packing density at 65 "C (expressed as area occupied per molecule, u = 28.2 A2) correlates well with the 1,2-q2 orientation (ucdCd= 28.6 A2). Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work, and to the Air Force Office of Scientific Research for additional support. Registry No. 1, 123-31-9; 2,615-90-7; 3,4371-32-8; 4,571-60-8; 5, 117-14-6;6,637-89-8; 7, 16867-04-2;8, 123-33-1;Pt, 7440-06-4. (48)Pynn, R.;Riste, T. In 'Anharmonic Lattices, Structural Transitions and Melting"; Riste, T., Ed.; Noordhoff Publishing: Leiden, 1974; p 363. (49)Elguero, J.; Marzin, C.; Katritzky, A. R.; Linda, P. 'The Tautomerism of Heterocycles"; Academic Press: New York, 1976. (50)Bag, S.P.; Fernando, Q.; Freiser, H. Inog. Chem. 1962,1,887.
Rotation Barriers of Amides in the Gas Phase Martin Felgelt Department of Chemistry and Blology, University of Bremen, P2800 Bremen 33, West Germany (Received: December 2 I, 1982)
The line shape analysis of the gas-phase 'H NMR spectra of six N,N-dialkylamides (R'CONR2: R = CH3, R =F, C1, CH3,CF,; R = LPr, R' = CH3; R = CzH5,R' = H) and of N,N-dimethylthioacetamidegive standard free energies of activation which are at least 1-2.3 kcal/mol smaller than the values in various solvents reported previously. The differences are not correlated with the structure and the barrier heights of the amides. In liquids, dipolar interactions and the internal pressure of the solvent increase the rigidity of the amide bond. Introduction The hindered internal rotation about the partial double bond in simple amides has been measured by NMR line Present address: Institut fur Organische Chemie, Universitat Erlangen-Nurnberg, Henkestr. Heukstr. 42,D-8520Erlangen, West Germany.
0022-3654/03/2087-3054$0 1.50/0
shape analysis for a quarter century.' However, all the studies were restricted to solutions due to the low sensi(1) (a) Phillips, W. D. J. Chem. Phys. 1955,23,1363-4.Gutowski, H. S.;Holm, C. H. Ibid. 1956,25,1228-34. (b) For a review see: Jackman,
L. M. "Dynamic NMR Spectroscopy";Jackman, L. M., Cotton, F. A. Ed.; Academic Press, New York, 1975;pp 203-52. 1983 American Chemical Society
Rotation Barriers of Amides In the Gas Phase
tivity of the NMR experiment. Self-association of the amide molecules, specific association with the solvent, solvent polarity, and internal pressure change the rotation rate and may mask the influence of substituents. We have now measured the kinetics of some N,N-disubstituted amides in the gas phase, hoping to get inherent molecule parameters suitable as starting points to interpret the effect of the medium on the rigidity of the amide bond. Only the amides of low molecular weight have vapor pressures which are sufficient for obtaining NMR signals at the sensitivity of the presently available spectrometers. We have selected NJV-dimethylformamide (DMA), N,Ndiethylformamide (DEF), N,N-dimethylacetamide (DMA),, and N,N-diisopropylacetamide(DIA) to check on increasing steric repulsion between the carbonyl and the nitrogen substituents; NJV-dimethylcarbamoyl fluoride (DMCF), NJV-dimethylcarbamoyl chloride (DMCCl), and Nfl-dimethyltrifluoroacetamide(DMTFA) may show the influence of electronic factors on the rate, and finally we have selected N,N-dimethylthioacetamide(DMTA), the thio analogue of DMA.
Experimental Section DMF, DMA, DMCCl, and DEF were commercial available (Merck); DIA was prepared from acetyl chloride and diisopropylamine,DMTFA from ethyl trifluoroacetate and dimethylamine. DMCF was obtained from DMCCl with HBr followed by treatment with SbFP3 DMTA was prepared from DMA and P4Sl0and sublimated in vacuo. The liquid amides were distilled prior to use. The glass NMR tubes were filled on a vacuum line with the vapor of the amide at room temperature. Known amounts of dimeth~1-d~ ether or methane-d4 were condensed in the probe to effect line narrowing and to provide a deuterium spectrometer lock. After the probe was sealed and warmed, the total pressure was estimated by assuming ideal gas conditions (0.5-24 bar, see Table I, error limits ca. 20%). The partial pressure of the amides corresponds their vapor pressure at about 300 K4 but it is reduced by condensation at measuring temperatures lower than 300 K. Two types of samples were used: Optimum sensitivity was obtained with a 10-mm 0.d. glass NMR tube which was constricted ca. 4 cm above the bottom, connected at the bottom to the vacuum line, and sealed at this point. Lower sensitivity but better resolution was observed with 4-cm-long sealed samples of approximately 9 mm 0.d. tube. Both sample geometries which fit into a IO-" prevent a diffusion of the gas out of the thermostated region of the probe. The FT 'H NMR spectra were recorded at 360 MHz. The short Tl of the gaseous amides at pressures lower than 24 bar5 allows the use of 60-70° pulse angles combined with repetition intervals longer than 5 s. Usually 2 000 to 10000 scans were accumulated at temperatures where the spectra are broadened by exchange and at low temperatures where the signals are reduced by condensation of amide vapor at the walls of the measuring cell. We were (2) Preliminary results on DMA have been published: Feigel, M. Chem. Commun 1980, 456-7. (3) Reeves, L. W.; Shaw, K. N. Can. J . Chem. 1971, 49, 3671-82. Reeves, L. W.; Shaddick, R. C.; Shaw, K. N. Can. J. Chem. 1971, 49, 3683-91. (4) Data for DMA, DMF, and DEF Carli, A.; Di Cave, S.; Sebastiani, E. Chem. Eng. Sci. 1972,27, 993-1001. Boublik, T.; Fried, V.; Hala, E. 'The Vapor Pressure of Pure Substances"; Elsevier, Amsterdam, 1973; pp 127, 172, 221. (5) Inversion recovery experiments on DMA and DMTFA give an increase of T1with the total pressure; DMA: pCDaWD8 = 2.5 bar, T , = 1.2 6.9 bar, Ti = 1.7 S; PCD = 24.5 bar, TI 6.2 S. DMTFA: pcD4 S; P C D = 7.7 bar, T1= 2.5 s (T = 300 8).
The Journal of Physical Chemistry, Vol. 87, No. 16, 1983 3055
able to obtain spectra in a reasonable time (
