Substituent effects on acetylene stability. A comparison of STO-3G, 6-31G, 6-31G**, and 6-311G** calculations

Abstract

Abinitio calculations using STO-3G, 6-31G, 6-31G**, and 6-311G** basis sets and standard geometries, and 6-31G and 6-311G** basis sets with 6-31G optimized geometries have been performed on a series of acetylene derivatives (H—C≡C—R; –R is —CH3, —Li, —C≡N, [Formula: see text] —NH2,—C≡CH, —CH—CH2, and —F). Substituent effects on stability have been studied using isodesmic methyl exchange reactions (H—C≡C—R + CH4 → H—C≡C—H + CH3R). STO-3G and 6-31G calculations have been carried out for methyl exchange reactions involving several large substituents including phenyl, and for the tert-butyl exchange reactions of 10 substituents (H—C≡C—R + Me3CH → H—C≡C—H + Me3CR).Geometry optimization has an appreciable effect only on the π donor substituents OCH3 and NH2. None of the smaller basis sets reproduces the orderof stabilities or the absolute values of the 6-311G**//6-31G calculations. The latter are in satisfactory agreement with the very few experimentally based enthalpies of methyl exchange.