Ab initio molecular orbital studies on a singlet-triplet splitting of C 3H 6 and C 4H 8 molecules

Abstract

The vertical singlet-triplet (ST) splitting of C 3H 6 and C 4H 8 molecules containing two sp 2 carbon atoms separated by one or two méthylene groups was investigated by means of ab initio calculations. The molecular geometry was either taken as that corresponding to the UHF/6-31G ∗ energy minimum of the triplet electron configuration, or extracted from the structure of the ferrocarbon crystal, which has been determined previously by the molecular mechanics method. Different basis sets (6-31G ∗ and 6-31 IG ∗∗) and various techniques of taking into account electron correlation (multiconfiguration self-consistent field (MC-SCF) and fourth-order Møller-Plesset (MP4)) were applied. The MP4 method provides more reliable ST splitting values than does the MC-SCF method which involves only pair excitations, but MP4 requires the use of an extremely extended basis set if quantitative estimates are to be obtained. The ST splitting appears to be determined not only by the distance between sp 2 carbon centres but also to a great extent by the mutual orientation of these centres. The presence of three-dimensional ferromagnetic ordering in the previously suggested intermediate graphite-diamond structure is proved.