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.