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 wi th MeX (X = I, Tos) span 7 orders of magnitude. The PES spectra of the se compounds display two very low energy peaks, which stand clearly ap art from the other peaks. These two peaks correspond to the pi orbital s of the C-S group; one is its CS pi bonding orbital oriented out of t he molecular plane (pi(CS)) and the other its p-type in-plane lone pai r orbital (pi(S)). For some of the compounds, the HOMO is the pi(CS) o rbital and for others the HOMO is the ns lone pair orbital. The best c orrelation (R = 0.96) between rate constants h and PES data is obtaine d 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 th e pi(CS) bonding orbital, An attempt to correlate the calculated energ y of the third highest occupied orbital (from AM1 calculations) with I n(h) provides a complete scattering of data (R < 0.1), but the calcula ted 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 kc al mol(-1) deeper than HOMO) correlate with In(k) (R = 0.86) as well a s does that of the second lone pair orbital os. These results are the first where both frontier orbitals and core orbitals display correlati on with overall reactivity. They are discussed in terms of direct (per turbational) versus indirect (nonperturbational) concepts.