An ab initio SCF MO study of the electronic structure of 7-germanorbornadiene and 7-stannanorbornadiene: two molecules predicted to have an inverted sequence of π levels

Abstract

The geometries of 7-germanorbornadiene (3) and 7-stannanorbornadiene (4) have been optimized, under C_2v symmetry constraint, using the HF/STO-3G and the HF/STO-3G* levels of theory. It is found that both (3) and (4) possess an inverted sequence of π levels, that is, the π-(b₁) orbital lies energetically below the π₊(a₁) orbital. This finding is consistent with the recently calculated inverted sequence of π levels found for 7-silanorbornadiene (2) but is in contrast to that found for the parent hydrocarbon diene, norbornadiene (1) in which the normal sequence of π orbitals (that is, π₊ lies below π_–) obtains. The degree of level inversion, as measured by the energy difference between the π₊ and the π_– canonical MO (CMO) levels, increases along the series: (2)(–0.17 eV) < (3)(–0.26 eV) < (4)(–0.87 eV)(STO-3G). It is proposed that the major causative factor of this trend is the increase in the strength of through-bond (TB) interactions between the π- orbital and the C1–X7–C4 sigma MOs, as X changes from Si to Ge to Sn. In contrast with the π manifold, the sequence of π* levels in (1)–(4) is normal; indeed, the STO-3G energy difference between the π_–* and π₊* CMO levels increases along the series: (1)(1.76 eV)≈(2)(1.70 eV) < (3)(2.02 eV) < (4)(2.67 eV), the value for (4) being remarkably large. This trend is explained in terms of TS and TB interactions in (1)–(4) reinforcing each other in the π* manifold.