Bifunctional Hydrogen Bonding of Imidazole with Water Explored by Rotational Spectroscopy and DFT Calculations

Laser vaporization of imidazole in the presence of an argon buffer gas has allowed the generation and isolation of two isomers of an imidazole monohydrate complex, denoted herein as imid···H 2 O and H 2 O···imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid···H 2 O, but these roles are reversed in the H 2 O···imid complex. Both isomers have been characterized by chirped-pulse Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The ground-state rotational spectra of four isotopologues of imid···H 2 O and three isotopologues of H 2 O···imid have been measured. All spectra have been assigned and fitted to determine rotational ( A 0, B 0, C 0 ), centrifugal distortion ( D J, D JK ), and nuclear quadrupole coupling constants ( χ aa (N1), [χ bb (N1) – χ cc (N1)], χ aa (N3), and [χ bb (N3) – χ cc (N3)]). Structural parameters ( r 0 and r s ) have been accurately determined from measured rotational constants for each isomer. The imid···H 2 O complex contains a nonlinear hydrogen bond (∠(O–H b ···N3) = 172.1(26)° in the experimentally determined, r 0 geometry) between the pyridinic nitrogen of imidazole and a hydrogen atom of H 2 O. The DFT calculations find that the H 2 O···imid complex also contains a nonlinear hydrogen bond between the oxygen atom of water and the hydrogen attached to the pyrrolic nitrogen of imidazole (∠(O···H1–N1) = 174.7°). Two states observed in the spectrum of H 2 O···imid, assigned as 0 – and 0 + states, confirm that large amplitude motions occur on the time scale of the molecular rotation. Density functional theory has been performed to characterize these large amplitude motions.