Chiral discrimination in hydrogen-bonded complexes

We report an accurate ab initio study of the effects of chirality on the intermolecular interactions between two small chiral molecules bound by a single hydrogen bond. The methods used are second-order Moller-Plesset theory (MP2), as well as density functional theory with the B3LYP functional. The differential interaction energy between two homochiral molecules, e.g., R . . .R' and the analogous heterochiral molecules RS' measures the degree of chiral discrimination, termed the chirodiastaltic energy, DeltaE(chir). Formation of the O-H . . .O hydrogen bond between the chiral H-bond donor HOOH and the chiral H acceptor 2-methyl oxirane leads to four diastereomeric complexes. There are two distinct contributions to the chirodiastaltic energies, the diastereofacial contribution which controls the face or side of the acceptor to which the H bond is formed, and the diastereomeric contribution, which is the energy difference between two complexes formed by (M)- and (P)-HOOH to the same face. The largest chirodiastaltic energy is DeltaE(chir)= 0.46 kcal/mol (6% of the binding energy) between the syn-(M)- and syn-(P)-HOOH 2-methyl oxirane complexes. The chiral 2,3-dimethyloxirane acceptor is C-2 symmetric and hence offers two identical faces. Here the chirodiastaltic energy is identical to the diastereomeric energy, and is calculated to be DeltaE(chir) = 0.36 kcal/mol or 4.5% of the binding energy. (C) 2000 American Institute of Physics. [0021-SC(00)31245-4].