Isomer Stability Dependence of Hydrogen-Bonded Benzoxazole Clusters on Solvent Molecules

The structures of hydrogen-bonded benzoxazole clusters with methanol and ammonia, BO-(CH₃OH) (n = 1-3) and BO-(NH₃) (n = 1, 2), in a supersonic jet have been investigated by measuring the S₁-S₀ electronic spectra and isomer-selected vibrational spectra with the aid of quantum chemical calculations. Similar to BO-(H₂O)₁, two isomers of BO-(NH₃)₁ were observed, which form two types of hydrogen bond networks starting from the CH bond at the 2-/7-position to the nitrogen atom of BO (C2HN/C7HN). The relative stability of these isomers strongly depends on solvent molecules. Natural bond orbital analysis reveals that the OH···N hydrogen bond is dominant in BO-(H₂O)₁ and that intermolecular interaction between the CH group and the nitrogen atom of ammonia, especially C2H···N, is significantly enhanced, resulting in a more stable C2HN isomer. Symmetry-adapted perturbation theory calculations indicate that the dispersion interaction between the methyl group of methanol and π electron cloud on the BO ring is responsible for the extreme stability of the C7HN of BO-(CH₃OH)₁. Furthermore, using time-dependent density functional theory calculations, the isomer tendency of the electronic transition shifts from the monomer origin is reproduced and it is proposed that the significant blue shift in C2HN is due to the shortened C2H bond length upon electronic excitation.