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···H₂O and H₂O···imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid···H₂O, but these roles are reversed in the H₂O···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···H₂O and three isotopologues of H₂O···imid have been measured. All spectra have been assigned and fitted to determine rotational (A₀, B₀, C₀), centrifugal distortion (D, D), and nuclear quadrupole coupling constants (χ(N1), [χ(N1) - χ(N1)], χ(N3), and [χ(N3) - χ(N3)]). Structural parameters (r₀ and r_s) have been accurately determined from measured rotational constants for each isomer. The imid···H₂O complex contains a nonlinear hydrogen bond (∠(O-H_b···N3) = 172.1(26)° in the experimentally determined, r₀ geometry) between the pyridinic nitrogen of imidazole and a hydrogen atom of H₂O. The DFT calculations find that the H₂O···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 H₂O···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.