Mechanisms of the gas-phase acyl group transfers, Cl- + R(XY)Cl, involving various acyl functional groups, >XY with X = C, S, or P and Y = O or S, are investigated theoretically at the MP2/6-31+G* and B3LYP/6-31+G* levels (additionally with extended basis sets of B3LYP/6-311+G(3df,2p)), and the effects of solvent (ε = 78.5) are calculated with the SCIPCM model at the isodensity level of 0.0004 au. The tetrahedral adducts formed in the carbonyl (RCO) and thiocarbonyl (RCS) group transfers are either transition states (double-well PES) or intermediates (single- or triple-well PES) depending on R, a stronger electron acceptor R favoring the intermediate. However, all of the sulfonyl (RSO2) and phosphoryl ((RO)2PO) transfers proceed with trigonal bipyramid (TBP)-type transition states, in contrast to the stepwise mechanism through TBP-type intermediates for the sulfinyl (RSO) (and sulfonyl transfers between F-) transfers. The most important factor determining whether an adduct in an acyl-group-transfer reaction is the transition state or intermediate is the energy gap between the and orbitals. The possibility of reacting through an intermediate is greater for lower and higher levels. The backside σ-attack pathway is favored over the π-attack pathway only when a low-lying orbital, preferably below the level, is available. In general, the results are in good agreement with those of experiments. The solvent effect elevates the barrier height almost uniformly so that the relative orders of gas-phase activation barriers between different R groups are maintained in solution.

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Similar expansion involving o, and ApK, leads to a mixed type interaction constant h,,, equation lc 26 Obviously the following definitions hold.These parameters are variously referred to as q, C, or p by several worker^,^ and are known to reflect the intensity of interaction between the two reacting fragments comprising the TS (Scheme 1). The magnitude of the Hammett type constants I pl, I represents the intensity of indirect interaction between two substituents, i(ol) andj(o,) through the respective reaction centres, R, and R,, when the two fragments are involved in forming or breaking of a bond, r,,, between the two reaction centres in the TS. Thus the I plJl should be related inversely to the distance between the two substituents 0, and o,, since it represents the intensity of interaction between them through reaction centres; in fact it has been shown that the distance r,, is related to I pV I by equation 5, where a and p are positive constants, assuming the rigidity of the fragment's skeleton in the react~on.~ According to equation 2a, the magnitude of pi, is subject to fall-off by a factor of 2.4 -2.8 when a nonconjugating group, like CH2 (or CO), intervenes in one of the fragments (i) between the substituent, o,, and the reaction centre, R,, since ' (a) S I Miller,

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