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
(DJ, DJK), and nuclear quadrupole coupling
constants (χaa(N1),
[χbb(N1) – χcc(N1)], χaa(N3), and [χbb(N3) – χcc(N3)]). Structural
parameters (r0 and rs) have been accurately determined from measured rotational
constants for each isomer. The imid···H2O complex contains a nonlinear hydrogen bond (∠(O–Hb···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.