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Structures of Azomethane, 1,1,1-Trifluoroazomethane,and Hexafluoroazomethane, Determined by Electron Diffraction C. H. Chang, R. F. Porter, and S. H. Bauer Contribution f r o m the Department of Chemistry, Cornell University, Ithaca, New York 14850. Received February 13, 1970
Abstract: Analysis of electron diffraction patterns has shown that the samples of azomethane and l,l,l-trifluoroazomethane which were investigated were in the trans configurations about the N-N bond, but that the sample of hexafluoroazomethane was in the cis configuration. The geometTic parameters are for CH3N=NCH3, N=N = 1.254 zk 0.003, C-N = 1.474 f 0.003, C-H = 1.107 f 0.006 A, LNNC = 111.9 i 0.5", LNCH = 109.4 f 1.5"; forCH3N=NCF3, N=N = 1.219 f !.008,C--N(H) = 1.440 f 0.012,C-N(F) = 1.476 f 0.012, C-H = 1.107 (assumed), C-F = 1.322 f 0.003 A, LNNC(H) = 110.5 =t4", LNNC(F) = 126.2 2', LNCH = 109.4' (assumed), LNCF = 109.3 f 0.6"; and for CF3N-NCF3, N=N = 1.236 f 0.015, C-N = 1.490 f 0.006, C-F = 1.326 f 0.002 A, LNNC = 133.0 =k l o , and LNCF = 109.3 f 0.3'. The estimatederrorsare three times the standard deviations obtained from least-squares fitting of th? intensity data. A summary of N=N bond lengths is presented; the reported values cover the ranges 1.21-1.29 A.
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hether hexafluoroazomethane, CF3N=NCF3, as initially prepared,l was in the cis or trans configuration has not been satisfactorily resolved. An early electron diffraction study of azomethane, CH3N=NCH3, indicated a trans configuration about the N=N bond. The proposed trans configuration for the perfluoro compound was based on apparent similarities between (CF&N2 and (CHJ2N2 of their chemical properties, of their uv absorption spectra, and of product analysis following thermal decomp o ~ i t i o n . ~Although Ramaswamy5 did find a dipole moment of 0.46 D for (CF3)?N2,this was given no weight in view of the possibility that his sample contained impurities. Low-temperature infrared spectra of the perfluoro compound reported by Freitag6 showed no absorption in the 1450-1650-cm-' region, assigned in most azo compounds to the N=N stretching frequency. However, Pritchard, et and Dubov, et al.,8 did observe bands at 1581, 1562, and 1532 cm-'. Preparation of the partially fluorinated compound, 1, 1,l-trifluoroazomethane, CH3N=NCF3, has been reported r e ~ e n t l y . ~Azomethane and its partially and totally fluorinated d e r i v a t e s p r o v i d e an interesting series of compounds for observation of structural changes as affected by substitution of CF, for CH, groups. This report covers the results of an electron diffraction study of these three compounds. Experimental Section Azomethane. Azomethane was prepared from 1,Zdimethylhydrazine hydrochloride and cupric chloride by the procedure of Jahn.10 The 60-MHz nmr spectrum of the purified sample in TMS solution indicated a single proton resonance with a chemical shift (6) of 3.73 ppm downfield from the reference. The sample was stored in a 500-ml bulb at a pressure of 20 cm. To prevent photo0. Ruff and M. Ciese, Chem. Ber., 69, 598 (1936). H. Boersch, Montash., 65, 311 (1935). 0.Ruff and W. Willenberg, Chem. Ber., 73, 724 (1940). D . Clark and H . 0. Pritchard, J . Chem. Soc., 2136 (1956). ( 5 ) K. L. Ramaswamy, Proc. Indian Acud. Sci., Secr. A , 2,364 (1935). (6) W. 0.Freitag, Ph.D. Thesis, University of Pennsylvania, 1955. (7) G. 0.Pritchard, H . 0.Pritchard, H . I. Schiff, and A. F. TrotmanDickenson, Trans. Faraday SOC.,52, 849 (1956). (8) S. S. Dubov, et al., Zh. Vses. Khim. Obsh., 6, 596 (1961). (9) A. H . Dinwoodie and R. N. Haszeldine, J. Chem. Soc., 2266 (1965). (10) F. P. Jahn, J. Amer. Chem. Soc., 59, 1761 (1937). (1) (2) (3) (4)
Chang, Porter, Bauer
chemical deterioration of the sample, the bulb was covered with black tape and left in the dark until used. This amount is adequate for obtaining a complete set of electron diffraction photographs. Hexafluoroazomethane. Hexafluoroazomethane was obtained from Merck Sharp and Dohme of Canada. The l9F nmr spectrum of the compound in CC14 solution indicated a single narrow resonance. The chemical shift at a frequency of 94.6 MHz was 74.0 ppm upfield from the CFCla reference. This shift is somewhat less than the value of 79.7 ppm given by Mitsch and Ogden,11f12but is (74.7 close to the values for CF3 groups in CF3-N=N-CFZCI ppm) and CFa-N=N-CF2Br (74.5 ppm). The infrared spectrum taken with this sample showed the same triplet as was previously reported.7~9 l,l,l-Trifluoroazomethane. Trifluoroazomethane was prepared from trifluoronitrosomethane (CFaNO) and methylamine by a procedure similar to that used by Dinwoodie and Haszeldine.9 The CFINO gas was first transferred in vacuo into a 500-ml bulb at a pressure of 10 cm. The reagent was then frozen with liquid nitrogen and an equivalent amount of CHlNHl was condensed in the bulb. The mixture was then allowed to warm slowly. After repeated condensation and rewarming the reaction mixture was transferred into a - 80" trap which was pumped for a few minutes to remove unreacted CFINO and volatile impurities. The material The hyrecovered from the -80" trap was CFs-N=N-CH,. drogen nmr spectrum of the compound in CC14solution indicated a narrowly spaced 1 :3 :3 :1 quartet, centered slightly downfield from azomethane, and a chemical shift (6) relative to TMS of 4.07 ppm. The quartet structure, which is presumed to be due to the long-range coupling of the H and F atoms, gave a coupling constant of 1.8 cps. The 19F nmr spectrum of the compound indicates a line without resolved structure slightly upfield from CFaN=NCF3. The chemical shift relative to CFCI, is 74.7 ppm. Electron Diffraction Photographs. Sectored electron diffraction photographs of these three compounds were taken with the samples at room temperature, using the apparatus previously described. Optical densities as dependent on radial position were obtained with a digital microphotometer. The data reduction procedure followed in this laboratory has been published.l4
