Classical and nonclassical isomers of tropylium, silatropylium, and germatropylium cations. Descending the periodic table increases the preference for nonclassical structures

Abstract

Ab initio calculations at the G2(MP2,SVP) level predict that there is a striking transition from a preference for classical isomers on the C7H7+ surface to a preference for nonclassical isomers on the C6H7Ge+ surface. The situation for C6H7Si+ is intermediate between these extremes, with classical and nonclassical isomers tending to have comparable energies despite their significant geometric differences.