Analysis of the Principles Governing Proton-Transfer Reactions. Carboxyl Group

The participation of the -COOH group in proton-transfer processes is investigated by ab initio calculations with a 4-31G* basis set. Of particular interest is the influence of the -OH group upon the attached C=O functionality, extracted by comparison with the simpler H2CO molecule. The primary effect of the -OH is to increase the proton affinity of C=O, raising the barrier for proton transfer to an acceptor molecule for any given H bond length. As in the case of H2C0, displacement of the proton acceptor away from an O lone pair and toward the C=O axis provides an impetus for the proton to be transferred away from HCOOH. The strength of this impetus, however, is found to depend upon the specific orientation of the OH group of HCOOH. This added level of complexity, absent in H2CO, can be simply explained on the basis of two factors: (i) electrostatic interactions between the subunits involved in the H bond and (ii) removal of a proton from HC(OH)2+ to leave behind two possible conformers of HCOOH (cis or trans) which differ in intrinsic stabi!ity. Proton-transfer energetics of HCOOH are found to be less sensitive than H2CO to H-bond deformations out of the molecular plane. With regard to the OH portion of the carboxyl group, its proton affinity is lowered relative to HOH, making it a much less likely proton acceptor, especially if the proton is positioned trans to the C-H bon