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 center dot center dot center dot H2O and H2O center dot center dot center dot imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid center dot center dot center dot H2O, but these roles are reversed in the H2O center dot center dot center dot 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 center dot center dot center dot H2O and three isotopologues of H2O center dot center dot center dot imid have been measured. All spectra have been assigned and fitted to determine rotational (A(0), B-0, C0()), centrifugal distortion (D-J, D-JK), and nuclear quadrupole coupling constants (chi(aa)(N1), [chi(bb)(N1) - chi(cc)(N1)], chi(aa)(N3), and [chi(bb)(N3) - chi(cc)(N3)]). Structural parameters (r(0) and r(s)) have been accurately determined from measured rotational constants for each isomer. The imid center dot center dot center dot H2O complex contains a nonlinear hydrogen bond (<(O-H-b center dot center dot center dot N-3) = 172.1(26)degrees 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 center dot center dot center dot 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 center dot center dot center dot H1-N1) = 174.7 degrees). Two states observed in the spectrum of H2O center dot center dot center dot 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.