Microwave spectra, molecular geometries, and internal rotation of CH 3 in N -methylimidazole⋯H 2 O and 2-methylimidazole⋯H 2 O Complexes.

Broadband microwave spectra have been recorded between 7.0 and 18.5 GHz for N -methylimidazole⋯H 2 O and 2-methylimidazole⋯H 2 O complexes. Each complex was generated by co-expansion of low concentrations of methylimidazole and H 2 O in argon buffer gas. The rotational spectra of five isotopologues of each complex have been assigned and analysed to determine rotational constants ( A 0 , B 0 , C 0 ), centrifugal distortion constants ( D J , D JK ) and parameters that describe the internal rotation of the CH 3 group. The results allow the determination of parameters in the ( r 0 ) molecular geometry of each complex. H 2 O is the hydrogen bond donor and the pyridinic nitrogen of imidazole is the hydrogen bond acceptor in each case. The ∠(O-H b ⋯N3) angles are 177(5)° and 166.3(28)° for N -methylimidazole⋯H 2 O and 2-methylimidazole⋯H 2 O respectively. These results are consistent with the presence of a weak electrostatic interaction between the oxygen atom of H 2 O and the hydrogen atom (or CH 3 group) attached to the C2 carbon atom of imidazole, and with the results of density functional theory calculations. The ( V 3 ) barrier to internal rotation of the CH 3 group within N -methylimidazole⋯H 2 O is essentially unchanged from the value of this parameter for the N -methylimidazole monomer. The same parameter is significantly higher for the 2-methylimidazole⋯H 2 O complex than for the 2-methylimidazole monomer as a consequence of the weak electrostatic interaction between the O atom and the CH 3 group of 2-methylimidazole.