Hyperconjugation as a driving force for the different conformations of symmetric triethyl derivatives of boroxin, borazine, benzene, and triazine.
The mol. structure of the sym. tri-Et derivs. of boroxin, borazine (2), s-triazine, and benzene (4) have been studied by a combination of X-ray structure detns. and ab initio methods. A single crystal structure detn. of 2 and 4 has been carried out. The mol. symmetry of 1,3,5-triethylbenzene (Cs) has been exptl. detd. to be different in comparison to triethylboroxin and B,B',B''-triethylborazine (C3h). Ab initio optimizations imposing both symmetries have been carried out at the HF/6-31G(d) level of theory. Natural bond orbital analyses were utilized to establish hyperconjugation to be responsible for the arrangement of the Et groups with respect to the plane of the six-membered rings. Various conformers for triethylboroxin have been taken into account. MP2/6-31G(d) calcns. were performed to est. the relative stability of the different conformers.
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