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

Abstract

The ground (X 1 A 1 ) and first excited singlet (à 1 B 2 ) states of the binary tropolone Á 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 DEX pt ¼ 1740:5 cm À1 height, which represents a 30% drop from the analogous quantity in bare tropolone. Redistribution of charge density upoñ A 1 B 2 ÀX 1 A 1 ( à ) electronic excitation transforms the potential energy landscape markedly, yielding a pronounced tightening of the critical O 1 ÀH Á Á Á FÀH Á Á Á O 2 interaction region (e.g., key heavy-atom distances decrease from rX O1ÁÁÁF ¼ 2:89 Å and rX O2ÁÁÁF ¼ 2:54 Å to rà O1ÁÁÁF ¼ 2:67 Å and rà O2ÁÁÁF ¼ 2:48 Å ) and a commensurate reduction in the impediment for hydron migration to DEà pt ¼ 1292:8 cm À1 . Intriguingly, the double proton transfer pathway inà 1 B 2 tropolone Á HF shows evidence of non-planarity, notably the presence of twisted transition-state (C 2 ) and global-minimum (C 1 ) configurations, that can be addressed within the framework of the encompassing G 4 molecular-symmetry group.