HYDROGEN-BONDING IN THE SYMMETRY-EQUIVALENT C2H DIMER OF 2-PYRIDONE IN ITS S0 AND S2 ELECTRONIC STATES - EFFECT OF DEUTERIUM SUBSTITUTION

The properties of the two intermolecular N-H...O bonds that are responsible for the formation of the cis-peptide-like dimer of 2-pyridone (2PY) have been examined using deuterium substitution of the bridging hydrogen atoms as a probe. Studies of the fully resolved S2 <-- S0 electronic spectrum of (2PY)2 in a molecular beam show that the protonated dimer has a symmetry-equivalent planar C2h structure in its S0 (1A(g)) and S2 (1B(u)) states. Analogous studies of (2PY)2-d1 and (2PY)2-d2 show that (2PY)2 and (2PY)2-d2 are energy delocalized dimers in their S2 states, with an exciton splitting of less than 20 cm-1. Effective structures of the symmetric dimers in both states are derived from the measured rotational constants. A comparison of these structures shows that there is a distortion of the hydrogen-bonding geometry when hydrogen is replaced by deuterium, along both in-plane and out-of-plane coordinates. DELTA-R(N-H...O) = 0.008 angstrom, DELTA-theta[C = O...(H) - N] = 0-degrees, and DELTA-phi (the dihedral angle) = 0.96-degrees in S0 (2PY)2-d2 and DELTA-R = 0.003 angstrom, DELTA-theta = 0-degrees, and DELTA-phi = 0.86-degrees in S2 (2PY)2-d2. The implications of these findings for currently accepted models of the hydrogen bond are discussed.