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.

Keywords:

• double proton transfer
• excited states
• coupled-cluster theory
• complexes
• hydrogen bonding

Acknowledgements

This paper is dedicated to Prof. Richard N. Zare of Stanford University on the occasion of his 70th birthday, with deep appreciation for his many contributions to Chemistry as well as his tireless efforts on behalf of the greater communities of scientific research and education. The work described herein has been performed under the auspices of a grant provided by the Experimental Physical Chemistry Program in the Directorate for Mathematical and Physical Sciences of the United States National Science Foundation (CHE-0809856). One of the authors (LAB) acknowledges the generous support of the NSF through a Graduate Research Fellowship. The authors wish to thank Prof. Kenneth B. Wiberg of Yale University for valuable discussions. Requisite computations were supported, in part, by the National Center for Supercomputing Applications (under TG-CHE080032N) and utilized the SGI-Altix Cobalt resource.