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The microwave spectrum of methyl silane has been reinvestigated. Seven symmetric top and ten asymmetric top isotopic species have been studied. These molecules, together with the six symmetric tops studied by Lide and Coles, yield a total of thirty-three rotational constants. A least-squares analysis of these constants gave the following structural parameters:     rSiC=1.8668±0.0005 A,    rCH=1.093±0.005 A,    rSiH=1.485±0.005 A,∠HCH=107∘40′±30′,∠HSiH=108∘15′±30′.Analysis of the spectra of the isotopic species CH2D–SiH2D and CH2D–SiHD2 shows conclusively that methyl silane in its equilibrium configuration has the methyl group staggered with respect to the silyl group. Certain transitions in the asymmetric top isotopic species, e.g., CH3SiH2D, are split into doublets because of coupling of over-all and internal rotation. These splittings were used to determine the barrier to internal rotation. The form of the potential hindering internal rotation was taken to be V = ½V3(1 — cos3α)+½V6(1 — cos6α), giving the result V3 = 1700±100 cal, V6∼0–150 cal. This is in reasonable agreement with the result of Kivelson's analysis of the J = 0→1 torsional satellites of CH3–SiH3.

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The microwave spectra of five isotopic species of dimethyl sulfide are reported. Changes in rotational constants with isotopic substitution yield the following structural parameters: CS 1.802 A; CSC=98°52′; CH 1.901 A; HCH=109°34′; 2θ=104°22′, where 2θ is the angle between the symmetry axes of the methyl groups. The equilibrium conformation of both methyl groups is the staggered one, i.e., staggered with respect to the adjacent CS bond axis. From Stark effect measurements the dipole moment of dimethyl sulfide is found to be 1.50±0.01 debye. Fine structure in the ground-state rotational spectrum of (CH3)2S and an excited torsional state of CH3SCD3 has been resolved and analyzed. This fine structure results from coupling of internal and over-all rotation and is affected by top-top coupling terms in the kinetic and potential energy portions of the Hamiltonian. Neglecting only the potential energy coupling terms, the (CH3)2S and CH3SCD3 splittings yield as the barrier to internal rotation 2132±6 and 2118±3 cal/mole, respectively. Estimates of the potential energy coupling parameters are made. They are found to be an order of magnitude smaller than the main term of ∼2100 cal/mole in the Fourier expansion of the potential energy.

5406fying specific structural features suggested by a given approximate molecular wave function. For example, in selecting a wave function for naphthalene which gives the most favorable agreement between the calculated and experimental carbon-13 chemical shift values, one has a description of this molecule which predicts that the highest charge density (both cr and T ) is at the 0 carbon. As the a position is more chemically reactive than the 0, it is concluded that the free valency term, which is largest in the a! position, dominates charge-density considerations in the chemistry of naphthalene. The cr and T charges obtained from the extended HMO treatment of phenanthrene failed to give the correct ordering of shifts at C-1 and C-2, and one can only hope that use of more refined treatments of this molecule will lead t o the better agreement noted in the naphthalene case. SummaryThe relatively narrow range of values (-13.0 to +5.5) noted for the several aromatics studied in this work is in itself an unusual observation when one considers that the shifts in simple aliphatic hydrocarbons extend over a range of more than 50 ppm.28 This emphasizes that the electronic environment of a carbon atom in various aromatic systems is remarkably similar and that variations are minor when compared t o changes noted in other classes of compounds. Furthermore, the reasonable theoretical correlation of these small differences lends support t o the accuracy of even simple wave mechanical treatments for these molecules. Recent inclusion of cr electrons into such theoretical treatments appears to result in even better agreement. The use of carbon-13 chemical shift data as a means of selecting between alternative approximate wave functions indicates that these data are superior t o the corresponding proton values in ellucidating the details of the electronic structure of molecules.Acknowledgment.Abstract: The microwave spectrum of axial cyclohexyl fluoride has been investigated in the frequency region 10 to 37 Gc/sec. Twenty-four absorption frequencies are reported which can be attributed to rotational transitions of aCBHllF in its ground vibrational state; rotational constants derived from the measured frequencies are A = 3562.962, B = 2628.608, C = 1980.869 Mc/sec. Assuming that the bonded CC and CH distances are the same in a-C6H11F as they are in propane and that the ring structure has the same symmetry as cyclohexane, the following structural parameters are obtained: CF = 1.399, CC = 1.526, CH = 1.096 A, LCCC --111" 42', LHCH = 107" 21', LHCF = 107" 51', LCCF = 110" 0', LCCH = 109" 24', /3 = 54" 5', where /3 = the dihedral angle for