1.14 eV) than was found (with more limited basis sets) for 7 ionization of benzene9 (0.4 eV) or pyridine10 (0.6 eV). We therefore expect that tt excitation to low-lying states in these (and other) larger -electron systems will cause rearrangement of relatively less importance than the already small amount found in ethylene. This will then mean that one can indeed use the fixed core, -electron approximation as a quantitatively accurate quantum chemical method for such problems (this is implicit in previous work1112). It would be most interesting to see accurate ab initio calculations for larger tt systems (e.g., naphthalene)using the procedures we used for ethylene: it may be that for such large systems even tt excitation to Rydberg states or tt ionization would be moderately well represented with the fixed -core restriction.Acknowledgment. . E. S. acknowledges partial support of this work through a grant from the Petroleum Research Fund, administered by the American Chemical Society. We are also grateful to University Computing Company for a grant from the Chemical Applications Department, which enabled us to perform these calculations. Thanks are due Dr. R. Gilman for a preprint of his paper.12
The question must now be answered as to why the observed kinetic behavior is markedly different in CCU-DMSO compared to CHCI3-DMSO. Obviously the difference must be due to the relative hydrogen bonding capabilities of chloroform and carbon tetrachloride. Chloroform is known to form weak hydrogen bonds;3 e.g., the association constant between DMSO and chloroform in carbon tetrachloride has been estimated to be 31 M-1.17 This accounts for the lower stability of the dimer in chloroform relative to carbon tetrachloride. The differences in kinetic behavior in CCl4-DMSO and CHCI3-DMSO, imply that the breakdown and formation of a DMSO-2-pyridone hydrogen bond in the dimer is more rapid in chloroform than in carbon tetrachloride. This is probably due to the fact that this step is facilitated (17) P.
increased stability of the parent ion on increased methyl substitution could be attributed to the ability of the methyl group to release election density into the borazine ring thus stabilizing the parent ion.The general substituent effects on the relative stability of the parent ion observed for the B-substituted vV-trimethylborazines also obtains for the B-substituted borazines. Mass spectral data available in the literature were analyzed by the method described here and the results summarized in Table X. The stability of the parent ion of B-monosubstituted borazines increases in the order F < Cl < OCH3 < CH3 < Br; for the known B-disubstitüted derivatives the same order is observed. The results in both series of compounds are in general agreement with the data obtained for the corresponding N-trimethylborazine derivatives. A comparison of the results in Tables IV and VIII indicates that the loss of a hydrogen atom from an //-methyl group is more facile than from a B-methyl group, which is in agreement with the more ready formation of an immonium ion (II).
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