Dual hydrogen-bonding motifs in complexes formed between tropolone and formic acid

作者：Nemchick Deacon J; Cohen Michael K; Vaccaro Patrick H^*

来源：Journal of Chemical Physics, 2016, 145(20): 204303.

DOI：10.1063/1.4967253

The near-ultraviolet pi*<-pi absorption system of weakly bound complexes formed between tropolone (TrOH) and formic acid (FA) under cryogenic free-jet expansion conditions has been interrogated by exploiting a variety of fluorescence-based laser-spectroscopic probes, with synergistic quantum-chemical calculations built upon diverse model chemistries being enlisted to unravel the structural and dynamical properties of the pertinent ground [(X) over tilde (1)A'] and excited [(A) over tilde (1)A' (pi*pi)] electronic states. For binary TrOH center dot FA adducts, the presence of dual hydrogen-bond linkages gives rise to three low-lying isomers designated (in relative energy order) as INT, EXT1, and EXT2 depending on whether docking of the FA ligand to the TrOH substrate takes place internal or external to the five-membered C-O-H center dot center dot center dot O=C reaction cleft of tropolone. While the symmetric double-minimum topography predicted for the INT potential surface mediates an intermolecular double proton-transfer event, the EXT1 and EXT2 structures are interconverted by an asymmetric single proton-transfer process that is TrOH-centric in nature. The (A) over tilde - (X) over tilde origin of TrOH center dot FA at (nu) over tilde (00) = 27 484.45 cm(-1) is displaced by delta(nu) over tilde (00) = + 466.76 cm(-1) with respect to the analogous feature for bare tropolone and displays a hybrid type - a/b rotational contour that reflects the configuration of binding. A comprehensive analysis of vibrational landscapes supported by the optically connected (X) over tilde (1)A' and (A) over tilde (1)A' (pi*pi) manifolds, including the characteristic isotopic shifts incurred by partial deuteration of the labile TrOH and FA protons, has been performed leading to the uniform assignment of numerous intermolecular (viz., modulating hydrogen-bond linkages) and intramolecular (viz., localized on monomer subunits) degrees of freedom. The holistic interpretation of all experimental and computational findings affords compelling evidence that an external-binding motif (attributed to EXT1), rather than the thermodynamically more stable cleft-bound (INT) form, was the carrier of fluorescence signals observed during the present work. Published by AIP Publishing.

• 出版日期2016-11-28