Theoretical determination of molecular structure and conformation

Abstract

Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradiene, lH-azepine, benzenimine (azanorcardiene), oxepine and benzene oxide (oxanorcaradiene) employing the STO-3G, 4-31G and 6-31G* basis sets. Theoretical geometries, conformations and barriers to ring inversion have been obtained and compared with the available experimental structural data. It has been found that the cyclotrienes possess a boat conformation with a constant admixture of 22% chair character leading to a flattening of the triene part and n-electron delocalization typical for a planar polyene. The structural data obtained in this work suggest that cyclic delocalization of either 6~ (homoaromaticity) or 8~ electrons (antiaromaticity) is not present in the three cyclotrienes. The former possibility, however, cannot be excluded in the case of the three norcaradienes. 5 .6 :, X7 p" (c D 2 X=CHz X 4 X=NH 6 X=0 2 ' CHT may obtain some aromatic character by 1,6 overlap of 2pn-orbitals and a homoconjugative closure of the triene part. In NCD, however, interactions between the n-molecular orbit& (n-MOs) of the diene part and the

Valence isomerization of cyclohepta-1,3,5-triene and its heteroelement analogues

“…Although the boat form prevails for the monocyclic heteropines 1 , 3 and 5 , Cremer et al showed that this represents an incomplete picture [43–44]. In fact, they are “perturbed” boats with at least 22% chair character, leading to an almost similar boat puckering for all.…”

Theoretical studies of conjugated systems containing CC and CN fragments and their alkyl and amino derivatives

“…However, rarely are dipole moments measured in the gas phase. Uncertainty in experimental data can be illustrated by considering the experimental dipole moment of aniline: there are 45 dimethylpyridine molecule also shows an exceptionally large deviation i n dipole moment, whereas t h e calculated dipole moments for pyridine, the picolines, and the lutidines are in good agreement with experimental values. Since we cannot postulate a reasonable explanation for these deviations a t this time, we suggest that redetermination of experimental data may be necessary.…”

Vibrational Overtone Spectroscopy of Cycloheptatriene

“…No experimental evidence for the existence of the "chair" conformer has been found. However, calculations 14 suggest that cycloheptatriene possesses a "boat" conformation with a constant admixture of 23% "chair" character to the "boat" form. In cycloheptatriene the C 3 C 4 and the methylenic group are forced above the plane formed by C 1 C 2 C 5 C 6 , producing a "boat" conformer (Figure 1).…”

“…Electron diffraction 12 and X Abstract published in AdVance ACS Abstracts, November 1, 1997. microwave data 13 suggest that cycloheptatriene in the gas phase possesses the "boat" conformation, although the microwave spectrum 13 indicates the possibility that puckering of the ring may be important. The most stable calculated conformation 14 of cycloheptatriene is a "perturbed boat" conformation, which is a linear combination of 77% "boat" and 23% "chair" (Figure 1). The barrier for the ring inversion was calculated to be about 6 kcal/mol, 14 but no energies were determined for the puckering of cycloheptatriene from "boat" to "chair".…”