Conformational analysis by UV spectroscopy: the decisive contribution of environment-induced electronic Stark effects

Abstract

UV chromophores are frequently used as probes of the molecular structure. In particular, they are sensitive to the electric field generated by the molecular environment, resulting in the observation of Stark effects on UV spectra. While these environment-induced electronic Stark effects (EI-ESE) are already used for conformational analysis in the condensed phase, this work explores the potential of such an approach when performed at much higher conformational resolution in the gas phase. By investigating model alkali benzylacetate and 4-phenylbutyrate ion pairs, where the electric field applied to the phenyl ring is chemically tuned by changing the nature of the alkali cation, this work demonstrates that precise conformational assignments can be proposed based on the correlation between the conformation-dependent calculated electric fields and the frequency of the electronic transitions observed in the experimental UV spectra. Remarkably, the sole analysis of Stark effects and fragmentation patterns in mass-selected UV spectra provided an accurate and complete conformational analysis, where spectral differences as small as a few cm⁻¹ between electronic transitions were rationalized. This case study illustrates that the identification of EI-ESE together with their interpretation at the modest cost of a ground state electric field calculation qualify UV spectroscopy as a powerful tool for conformational analysis.