Microwave spectra, molecular geometries, and internal rotation of CH3 in N-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O and 2-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O Complexes

Broadband microwave spectra have been recorded between 7.0 and 18.5 GHz for N-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O and 2-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O complexes. Each complex was generated by co-expansion of low concentrations of methylimidazole and H2O 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 CH3 group. The results allow the determination of parameters in the (r(0)) molecular geometry of each complex. H2O is the hydrogen bond donor and the pyridinic nitrogen of imidazole is the hydrogen bond acceptor in each case. The angle(O-HbMIDLINE HORIZONTAL ELLIPSISN3) angles are 177(5)degrees and 166.3(28)degrees for N-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O and 2-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O respectively. These results are consistent with the presence of a weak electrostatic interaction between the oxygen atom of H2O and the hydrogen atom (or CH3 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 CH3 group within N-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O is essentially unchanged from the value of this parameter for the N-methylimidazole monomer. The same parameter is significantly higher for the 2-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O complex than for the 2-methylimidazole monomer as a consequence of the weak electrostatic interaction between the O atom and the CH3 group of 2-methylimidazole.

Automated summary

The microwave spectra of N-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O and 2-methylimidazoleMIDLINE HORIZONTAL ELLIPSISH2O complexes were recorded and analyzed to determine their molecular geometries and internal rotation properties, revealing the presence of weak electrostatic interactions.