Hydrogen bonding in the symmetry-equivalent C2h dimer of 2-pyridone in its S and S2 electronic states. Effect of deuterium substitution

Abstract

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(1Ag) and S2(1Bu) 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. ΔR(N–H⋅⋅⋅O)=0.008 Å, Δθ[C=O⋅⋅⋅(H)–N]=0°, and Δφ (the dihedral angle)=0.96° in S0 (2PY)2–d2 and ΔR=0.003 Å, Δθ=0°, and Δφ=0.86° in S2 (2PY)2–d2. The implications of these findings for currently accepted models of the hydrogen bond are discussed.