CHAPTER 1
Theoretical Aspects of Organosulphur, Organoselenium, and Organotellurium Compounds
1 Introduction
This report covers the period April 1969 — March 1970. However, for the sake of completeness some earlier and later work is also discussed.
The title is to some extent misleading, since there has been little of theoretical significance published concerning organo-selenium or -tellurium compounds, and indeed it seems unlikely that the situation will improve dramatically in the near future, at least for the latter. For the most part, therefore, we shall be concerned with organosulphur compounds. Fortunately there have been a few important developments in this area.
Although good SCF wave-functions have been available for various electronic states of the sulphur, selenium, and tellurium atoms for some considerable time, even as late as 1966 the only non-empirical calculations on molecules were restricted to H2S and C0S. Even semi-empirical PPP π-electron calculations on sulphur-containing heterocycles were few and far between at that stage, and discussions of bonding in organosulphur compounds were mostly restricted to Huckel treatments (for a review see ref. 2).
In the past two years in particular, however, the situation has improved dramatically as far as our understanding of bonding in organosulphur compounds is concerned, although the situation with regard to selenium and tellurium compounds has remained roughly static. Before discussing in some detail the results, it is worth digressing on the reason for this relatively sudden improvement.
The past decade has seen a tremendous change in the level of sophistication of theoretical treatments of molecules of interest to organic chemists. Thus, even as late as 1960, ab initio treatments of organic molecules were restricted to those containing just one carbon atom. With the lack of any semi-empirical treatments to include σ-electrons as well, organic chemists had to be satisfied with the vast body of π-electron calculations within the Huckel or Pariser–Parr–Pople (PPP) SCF formalism. However, in the past four years in particular, the situation has changed dramatically.
The extension of Huckel theory to include all valence electrons and the development of semi-empirical SCF all-valence-electron treatments has transformed the situation, and semi-empirical all-valence-electron calculations on a wide variety of medium-sized organic molecules have appeared in the literature. The spectacular advance in computer capability, coupled with the use of Gaussian Type (GTF) instead of Slater Type (STF) basis functions in the expansion method, has seen the development of non-empirical all-electron treatments on large numbers of polyatomic molecules.
Not unnaturally, the increased problems of dealing with second-row atoms has meant that there has been much less attention paid to organosulphur compounds than to, say, organo-nitrogen or -oxygen compounds. With regard to non-empirical calculations, whilst optimized orbital exponents for Slater type orbitals for sulphur have been available for some time, calculations on polyatomic sulphur-containing molecules have been limited by the lack of efficient programs for the evaluation of three- and four-centre two-electron integrals. The situation is already improving, however, and good calculations have appeared on hydrogen sulphide and thiiran. In contrast, whilst efficient programs for evaluation of multicentre integrals over Gaussian functions have been available for some time, it is only within the past year or so that optimised exponents have become available, and good calculations on a wide variety of sulphur compounds have appeared. Whilst the situation looks very hopeful for improving our knowledge of bonding in organosulphur compounds over the next few years, it seems unlikely that good non-empirical wave-functions for organo-selenium and -tellurium compounds will become available. The main hope in these cases must be in the development of adequate approximate SCF MO treatments. In this connection, the experimental results obtained by photoelectron spectroscopy, both X-ray and u.v., are likely to be of considerable importance.
2 d-Orbital Participation in the Ground States of Organosulphur Compounds
Reviews relevant to this section are given in refs. 24 — 27.
The role of d-orbital participation in the ground state of organosulphur compounds has interested chemists for many years. Despite numerous theoretica and experimental investigations, it is only now that a clear picture of the situation is emerging. Most theoretical investigations until 1968 had concentrated on π-bonding between divalent sulphur and carbon ; the interesting feature being that there was little measure of agreement between various workers as to the importance of d-orbital participation. This is not too surprising, since both the Hückel and PPP π SCF MO procedures are unsuitable for attempting to establish the extent to which the d-orbitals are involved in bonding, because of the large number of parameters whose values must be estimated but which are decisive in deriving orbital occupation numbers.
The question of d-orbital participation can really only adequately be dealt with by non-empirical quantum mechanical treatments, and perhaps the most significant theoretical advance has been in this area. We start off with a detailed discussion of recent work on H2S, which for the purist, no doubt, is classified as an inorganic molecule. Nonetheless, a number of very important points are illustrated in this work.
Calculations with §later-type Basis Functions. — Non-empirical LCAO MO SCF calculations using a minimal Slater basis set have been reported by two groups of workers, and the results are shown in Table 1. (The discrepancy between the atomic...
