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

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

Fluorescent small-molecules capable of altering their profiles in response to environmental changes are exceptionally valuable tool compounds throughout the scientific community. The following manuscriipt describes a new class of fluorescent small molecules based on lactam-fused tropolones that are responsive to a dynamic range of environmental changes. These molecules can be easily obtained through a rapid annulation procedure between appropriately functionalized tropolones and primary amines, which is often complete within minutes at room temperature. Molecules generated through this approach have been identified with fluoresence emission across the visible light spectra, and can be tuned based on either the tropolone or amine component. They are also highly responsive to changes in solvent, pH, and certain divalent metal ions. Tropolone-fused lactams thus represent a new class of tunable fluorescent small molecules that could find value throughout the scientific community.

Modifying conformational distribution of chiral tetrahydro-2-furoic acid through its interaction with water: a rotational spectroscopic and theoretical investigation

Rotational spectrum of a binary complex formed between tetrahydro-2-furoic acid (THFA) and water was measured using a chirped pulse Fourier transform microwave spectrometer. A comprehensive theoretical conformational search procedure was carried out using CREST, a conformational searching tool, and DFT calculations to aid the spectral assignment and interpretation. The final conformer ensemble is classified into two structural groups: Type 1 conformers showing a classic carboxylic acid monohydrate structure with two strong hydrogen-bonds formed between the COOH group of cis-THFA and water, and the much less stable Type 2 conformers with trans-THFA and weaker intermolecular interactions with water. The 'cis-' and 'trans-' labels refer to the configurations where the carboxylic C[double bond, length as m-dash]O and OH functional groups are on the same or opposite side, respectively. Only the two most stable Type 2 conformers containing trans-THFA I and II were observed experimentally in a neon jet expansion with an abundance ratio of 1 : 1. This relative abundance observation differs greatly from that of the THFA monomer, i.e. with trans-THFA I : trans-THFA II : cis-THFA III of 10 : 1 : 1 in a neon jet expansion, reported previously. The observation indicates a kinetically controlled formation process of different types of the monohydrates in a jet expansion, whereas a thermodynamically controlled process dominates within each type of structures. The relative stability of the THFA ring conformations is altered by interaction with water, showing a noticeable water induced conformational preference.

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

Despite their importance in diverse chemical and biochemical processes, low-barrier hydrogen bonds remain elusive targets to classify and interpret spectroscopically. Here the correlated nature of hydrogen bonding and proton transfer in the low-barrier regime has been probed for the ground and excited electronic states of 6-hydroxy-2-formylfulvene by acquiring jet-cooled fluorescence spectra of the parent and monodeuterated isotopologs. While excited-state profiles reveal regular vibronic patterns devoid of obvious dynamical signatures, their ground-state counterparts display a radically altered energy landscape characterized by spectral bifurcations comparable in magnitude to typical vibrational spacings (>100 cm^-1). Quantitative analyses yield unusual deuterium kinetic isotope effects that straddle limiting values attributed to above-barrier vibration and below-barrier tunneling of the proton adjoining donor/acceptor sites. Our findings provide compelling experimental evidence for ultrafast hydron-migration events commensurate with the onset of low-barrier hydrogen bonding and afford a trenchant glimpse of molecular phenomena taking place at the "tipping point" between disparate dynamical regimes.