ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 26, Number 4
JKCSEZ 26(4)
August 20, 1982 

Semi-empirical MO calculations, EHT, CNDO/2, MINDO/3, and MNDO met hods, were performed on various geometries of n-butane, n-alkyl radical and tetramethylene diracal (triplet) in order to compare eigenvalue and eigenvector properties with those obtained by STO-3G method. All methods predicted the same relative order of stabilities of various geometries for n-butane; geometrical preferences were found to be dominated by one-electron factor, π-orbital energy changes being more impotant in the semi-empirical methods. The hyperconjugative energy changes accompanying structural changes from (n-σ*)trans to (n-σ*)cis were underestimated in the EHT, CNDO/2 and MINDO/3, whereas those were overestimated in the MNDO. The net destabilizing effect of (n-σ*)trans structure was mainly due to the large internuclear energy involved in the structure. Through-space interaction between n1 and n2 orbitals of diradical caused energy gap narrowing of ΔEsp and Δε=ε0-εav; through-space interaction had opposing effect to that of through-bond interaction. Due to the less severe neglect of differential overlaps in the MNDO, this energy gap narrowing effect appeared amplified in the MNDO. In general orbital properties were found to be reproduced satisfactorily, but eigenvalue properties were not, in all the semi-empirical methods especially when σ-σ* and n-σ*interactions were involved.