1765
J\lAKVIN
c.
calculated moments, of o-chlorotoluene indicates that the “ortho” effect, which is usually ascribed to mutual induction, is absent or a t least too small to be detected by dipole moment measurements in this compound. The dipole moment of 3,3,3-trifluoropropene, 2.45, is intermediate in value between the vapor moment of benzotrifluoride, 2.86, and the moment,
Vol. 75 2.36, of trifl~oromethylacetylene,’~ determined from microwave spectra. It is interesting to note that the moment of the acetylenic compound containing the trifluoromethyl group is close to the vapor dipole moment, 2.34, of l,l,l-trifluoroethane. (14) J. N. Shoolery, R. G. Shulman, W. F. Sheehan, Jr., V. Schomaker and D. PIX. Yost, J . Chcm. P h y s . , 19, 1364 (1951). PITTSBURGH
19, PENNA.
[COSTRIBVTION FROM ARTHURD. LITTLE,INC.]
An Assignment of Frequencies for the Methyl Halomethanes and Silanes (CHJXY,, (CH3)zxy2 and (CH3)3xy BY XLRVIN C. TOBIN RECEIVED XOVEMBER 32, 1952 Complete assignments uf frequencies based on spectroscopic data found in the literature are given for thirteen methyl halomethanes and silanes of the types (CHs)XY3, (CH3hXY2 and (CH3hXY. I t is pointed out that in these compounds there is relatively little interaction between methyl and skeletal frequencies.
I n spite of the numerous difficulties involved, it TABLE I has occasionally proved possible to make assignSORMAL MODECLASSIFIC !ATIONS ments of frequencies for large molecules, especially (CH 4)XYa when relatively high symmetry is present. This AI Az has been done, for example, by Young, Koehler and CH3 asym. stretching McKinney’ for the group I V tetramethyl com- CHI sym. stretching VI pounds. A modification of their assignment, based CH, asym. bending on new experimental data, has been used by Lip- CHI sym. bending V1 pincott and Tobin? to calculate the thermodynamic CH, rocking functions of lead, tin and germanium tetramethyls. C-X stretchiiig v3 In this paper, assignments of fundamental fre- X-Y stretching v4 quencies are given for those methyl chloro-, bromo- SY3 rocking and iodomethanes and silanes for which spectro- SY3 bending V5 scopic data have been published, considering the CH, torsion V.5 molecules to have C% or Csv symmetry. Follow- Activity R . pol., I. Inactive ing Young, Koehler and McKinney, internal vibra(CH3)zXYz tions of the methyl groups are assumed to lie above AI 1200 a“’. For the methanes, the CHs rocking CHJ asym. stretching v1 frequencies lie roughly in the region 1000-1200 CH, sym. stretching Vi cm.-’, skeletal frequencies below 1000 ctn.-’. For CH3 asytn. bending V3 the silanes, the CH3rocking frequencies lie roughly CH3 sym. bending v4 in the region 800-900 cm.-’, skeletal frequencies CI& rocking V3 below about 800 cm.-’. C-X stretching V6 Within any one of these regions, it is assullied S-Y stretching Y7 that: (1) Frequencies corresponding to similar S C Zbetiding V8 normal modes, such as symmetric C-H stretching, XY2 bending YQ fall in the same region of the spectrum, regardless Skeletal twisting via of symmetry species. (2) For a given type of vi- Skeletal rocking v21 bration within a given symmetry species, symmetric CH3 torsion Y14 modes lie below asymmetric modes. ( 3 ) Angle Activity R. pol., I. R. R., I. bending frequencies lie below bond stretching fre( C H B ~ X Y quencies. (4) Characteristic bond frequencies do AI Ar not change much in going along the series X(CH:
