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

Abstract

The near-ultraviolet π←π 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 [X̃A] and excited [ÃAππ] electronic states. For binary TrOH ⋅ 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 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 Ã-X̃ origin of TrOH ⋅ FA at ν̃=27 484.45cm is displaced by δν̃=+466.76cm 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 X̃A and ÃAππ 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.

Spectral Signatures of Proton-Transfer Dynamics at the Cusp of Low-Barrier Hydrogen Bonding

“…Their DF counterparts show spectral shifts and bifurcations that depend on the nature of the underlying vibrational motion, with the Δ υ X̃ metrics measured for several ground-state features highlighted [ν 0 ( a 1 ) denotes the zero-point level]. These results build on laser-spectroscopic techniques and HFF-synthesis procedures , reported elsewhere (cf. Supporting Information for details).…”

Hydrogen-Bonding Motifs and Proton-Transfer Dynamics in Electronically Excited 6-Hydroxy-2-formylfulvene

“…Acquisition of rotationally resolved LIF data for the HFF Ã – X̃ origin band exploited a similar configuration based on a higher resolution dye-laser source (Lambda Physik FL3002E; Exciton 428 dye dissolved in p -dioxane; tunable intracavity etalon yielding a time-averaged bandwidth of ≤0.03 cm –1 fwhm) that was pumped by the third harmonic of a separate injection-seeded Nd:YAG system (Spectra-Physics PRO-250-20; 20 pps repetition rate, ∼10 ns pulse duration). Both tunable dye-laser platforms were combined to conduct a limited series of fluorescence hole-burning (FHB) studies designed to assess the common parentage of observed spectral signals …”

“…Both tunable dye-laser platforms were combined to conduct a limited series of fluorescence hole-burning (FHB) studies designed to assess the common parentage of observed spectral signals. 66 Experiments were performed in a stainless-steel vacuum vessel evacuated to a base pressure of <10 −7 Torr by means of a liquidnitrogen baffled oil diffusion pump (Varian VHS-6 with 362−6 cold trap) backed by a mechanical rotary pump (Leybold D65B). Target molecules were cooled rovibrationally through entrainment of their ambient (25 °C) vapor in a supersonic freejet expansion of ultrapure helium carrier gas as formed by a pulsed valve of the current-loop actuated design (R. M. Jordan C-211SS; modified for 20pps operation).…”

“…The basic apparatus employed for the laser-induced fluorescence (LIF) measurements performed on HFF has been described in detail elsewhere 66 and only the most salient aspects for the present study are presented here. Requisite light spanning the 400−432 nm region was generated by using the third harmonic of an injection-seeded Nd:YAG system (Spectra-Physics PRO-270-20; 20pps repetition rate, ∼10 ns pulse duration) to pump a tunable dye laser (Sirah PSCAN-D-30 with dual littrow/grazing gratings of 2400/3000 lines mm −1 yielding a time-averaged bandwidth of ∼0.06 cm −1 fwhm) containing a solvated-dye gain medium (Exciton Exalite 411, 417, or 428 dye dissolved in p-dioxane).…”

Lactam-fused tropolones: a new tunable, environmentally sensitive fluorophore class

“…8 For nearly as long, tropolone ( 5 ) has been known to be weakly fluorescent, and its absorbance and fluorescence intensity varies based on pH and solvent. 9 In more recent years, however, tropolone fluorescence has been found to be responsive to various environmental changes, including gases, 10 metals, 11 and other exogenous additives 12 that could make it a valuable tool for fluorescence small molecule development. However, tropolone's fluorescence is weak, with a reported quantum yield in cyclohexane of only 0.5% and even weaker in aqueous solutions.…”