SPECIFIC MOLECULAR-ORBITAL CONTRIBUTIONS TO NUCLEOPHILICITY - THE THIOCARBONYL GROUP AS PRIVILEGED MONITOR TO PINPOINT ACTIVE AND LESS ACTIVE MOLECULAR-ORBITALS IN REACTIONS WITH METHYLATING AGENTS

The rate constants for 41 compounds bearing a C=S function reacting with MeX (X = I, Tos) span 7 orders of magnitude. The PES spectra of these compounds display two very low energy peaks, which stand clearly apart from the other peaks. These two peaks correspond to the pi orbitals of the C-S group; one is its CS pi bonding orbital oriented out of the molecular plane (pi(CS)) and the other its p-type in-plane lone pair orbital (pi(S)). For some of the compounds, the HOMO is the pi(CS) orbital and for others the HOMO is the ns lone pair orbital. The best correlation (R = 0.96) between rate constants h and PES data is obtained when In(k) is plotted against the inverse of PES energy of the pi(S) lone pair orbital. Whether this lone pair orbital is the HOMO or the next lower HOMO has no importance. A modest correlation (R = 0.78) is obtained when In(k) is plotted against the inverse of PES energy of the pi(CS) bonding orbital, An attempt to correlate the calculated energy of the third highest occupied orbital (from AM1 calculations) with In(h) provides a complete scattering of data (R < 0.1), but the calculated energy of the second lone pair orbital sigma(S) (approximate to 90 kcal mol-l deeper than the HOMO) correlates reasonably with In(h) (R = 0.88). The energies of the S 2s and 2p core orbitals (calculated for 13 cyclic compounds with the HF/3-21G technique to be 4000 to 5500 kcal mol(-1) deeper than HOMO) correlate with In(k) (R = 0.86) as well as does that of the second lone pair orbital os. These results are the first where both frontier orbitals and core orbitals display correlation with overall reactivity. They are discussed in terms of direct (perturbational) versus indirect (nonperturbational) concepts.