Chiral discrimination in hydrogen-bonded complexes

We report an accurate ab initio study of the effects of chirality on the in termolecular interactions between two small chiral molecules bound by a sin gle 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). For mation 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 co mplexes. There are two distinct contributions to the chirodiastaltic energi es, the diastereofacial contribution which controls the face or side of the acceptor to which the H bond is formed, and the diastereomeric contributio n, which is the energy difference between two complexes formed by (M)- and (P)-HOOH to the same face. The largest chirodiastaltic energy is DeltaE(chi r)= 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 chirodiast altic energy is identical to the diastereomeric energy, and is calculated t o be DeltaE(chir) = 0.36 kcal/mol or 4.5% of the binding energy. (C) 2000 A merican Institute of Physics. [0021-SC(00)31245-4].