Infrared spectra and structures of aniline⁺-furan and aniline⁺-phenol. Preference between π-type and σ-type hydrogen-bonded structures

Infrared photodissociation spectra of aniline⁺-M (M=thiophene, furan and phenol) are measured in the 2700-3700 cm^-1 region and analyzed by density functional theory calculations. Only a structure involving a π-type hydrogen bond is found for aniline⁺- thiophene. Two structural isomers are identified for aniline⁺-furan and aniline⁺-phenol, which have either a π-type or a σ-type hydrogen bond, where an amino proton of aniline⁺ interacts with the π-electrons or the oxygen atom of the neutral molecules, respectively. The isomer with a σ-type hydrogen bond is more stable for aniline⁺-phenol, while less stable for aniline⁺-furan.