AB Initio Studies on Intramolecular Proton Transfer in Tropolone and Its Hydrogen Fluoride Complex.

Tropolone affords a model system for investigating the interrelated phenomena of proton transfer and hydrogen bonding, being small enough to retain computational tractability yet still exhibiting the multimode dynamics that govern the behavior of substantially larger molecular complexes. A series of ab initio calculations, built primarily upon techniques that allow for extensive treatment of electron-correlation effects, have been performed, with particular emphasis directed toward elucidation of the structural and vibrational properties that mediate unimolecular dynamics. Barring anomalous predictions of nonplanarity engendered by certain Pople-style basis sets, geometry optimization procedures and harmonic force-field calculations predict both the ground and the lowest singlet excited electronic surfaces to support a global minimum-energy configuration of rigorously planar (Cs) symmetry. In contrast to minimal Hartree-Fock (HF/CIS) and density functional schemes (DFT/TDDFT) which yield inconsistent theoretical results for the X~ 1A1 and A 1B 2 manifolds, coupled-cluster methods (CC/EOM-CC) predict proton-transfer barrier heights that are in accordance with the experimentally observed increase in tunneling efficiency following pi* ← pi electron promotion (triples-corrected barrier crests of DEX&d5;pt = 2557.0 cm-1 and DEA&d5;pt = 1316.1 cm-1). Detailed analyses show that the reduction in elevation stems most notably from an overall contraction of the C--O--H---O = C reaction site (whereby the equilibrium donor-acceptor distance decreases from rX&d5;O˙˙ ˙O = 2.53 A to rA&d5;O˙˙ ˙O = 2.46 A). It is concluded that coupled cluster structures, manifesting hindered proton transfer and dynamic nonplanarity, are required for a balanced description of this intramolecularly hydrogen-bonded species. The tropolone-HF complex, formed by introducing a hydrogen fluoride molecule into the tropolone reaction cleft, serves as a model adduct for probing the sequential versus concerted nature of double proton-transfer processes. Ab initio studies utilizing the coupled cluster ansatz have predicted a synchronous ground-state reaction barrier of DEX&d5;pt = 1740.5 cm-1, representing a 30% drop from the analogous value for tropolone. Redistribution of charge density upon pi* ← pi electronic excitation transforms the potential energy surface, yielding a markedly constricted O1--H---F--H---O 2 interaction region (key heavy-atom separation diminishes from rX&d5;O1 ˙˙˙F = 2.89 A to rA&d5;O1 ˙˙˙F = 2.67 A) and a reduced barrier of DEA&d5;pt = 1292.8 cm-1 height. Intriguingly, the pathway for double proton transfer in A 1B 2 tropolone-HF shows evidence of nonplanarity, notably the presence of twisted transition-state (C2) and global-minimum (C1) structures, that is addressed within the framework of the encompassing G4 molecular symmetry group.