Double proton transfer in the and states of the tropolone • HF complex

Abstract

The ground () and first excited singlet () states of the binary tropolone · HF complex have been examined computationally by exploiting minimal Hartree–Fock (HF/CIS), density functional (DFT/TDDFT), and coupled-cluster (CC/EOM–CC) schemes in conjunction with the aug-cc-pVDZ basis set. This adduct, formed by introducing a hydrogen fluoride ligand into the reaction cleft of the tropolone substrate, affords a model system for probing the nature of double proton-transfer events. Ab initio studies built upon the coupled-cluster ansatz predict a synchronous ground-state reaction barrier of cm−1 height, which represents a 30% drop from the analogous quantity in bare tropolone. Redistribution of charge density upon − () electronic excitation transforms the potential energy landscape markedly, yielding a pronounced tightening of the critical O1−H ··· F−H ··· O2 interaction region (e.g., key heavy-atom distances decrease from Å and Å to Å and Å) and a commensurate reduction in the impediment for hydron migration to cm−1. Intriguingly, the double proton transfer pathway in tropolone · HF shows evidence of non-planarity, notably the presence of twisted transition-state (C2) and global-minimum (C1) configurations, that can be addressed within the framework of the encompassing G4 molecular-symmetry group.