Abinitio Studies of Structural Features Not Easily Amenable to Experiment .43. Basis set dependence of carbon-oxygen single bonds and comment on the microwave substitution structure of methyl ethyl ether.

by Klimkowski, V. J.; Siam, K.; Ewbank, J. D.; Schaefer, Lothar; Geise, H. J.; VanAlsenoy, C.

The geometries of CH3OH, CH3OCH3, C2H5OCH3, and ethylene oxide were refined by Pulay's ab initio gradient method on the 6-513G** level, in order to comment on a previously noted irregularity in the differences between the 4-21G and rs C-O bond distances of C2H5OCH3 and ethylene oxide. In the case of ethylene oxide, the unusually large difference of 0.049 Å between the 4-21G and the rs C-O bond lengths is mainly due to deficiencies of the 4-21G basis in predicting the structures of strained cyclic org. systems contg. hetero-atoms. In the case of C2H5OCH3, however, the most credible interpretation of the currently available data is that exptl. uncertainties contribute significantly to the relatively large difference of 0.040 Å between the 4-21G and rs C(methylene)-O bonds, and the rs parameters very likely are not a true measure of the relative extension of the C(methyl)-O and C(methylene)-O bond distances in this compd. An rg C-O bond distance of 1.428(9) Å is tentatively predicted for ethylene oxide. Furthermore it is found that, in agreement with previous studies, differences between C-H bond distances within a Me group adjacent to electron lone pairs are practically independent of the basis set. The study confirms that, in general, ab initio gradient geometries can be a useful supplement in crit. evaluations of exptl. structural information.

Journal of Molecular Structure-Theochem
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