Noncovalent Interactions in the Molecular Geometries of 4-Methylthiazole···HO and 5-Methylthiazole···HO Revealed by Microwave Spectroscopy.

The pure rotational spectra of 4-methylthiazole···HO and 5-methylthiazole···HO were recorded by chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. Each complex was generated within the rotationally cold environment of a gas sample undergoing supersonic expansion in the presence of an argon buffer gas. The spectra of five isotopologues of each complex have been measured and analyzed to determine the rotational constants, , , and ; centrifugal distortion constants, , , and ; nuclear quadrupole coupling constants, (N3) and [(N3) - (N3)]; and parameters describing the internal rotation of the CH group, and (,). The experimentally deduced parameters were obtained using the XIAM and the BELGI-C-hyperfine code. For each complex, parameters in the molecular geometry are fitted to experimentally determined moments of inertia. DFT calculations have been performed at the ωB97X-D/aug-cc-pVQZ level in support of the experiments. Each complex contains two hydrogen bonds; a comparatively strong, primary interaction between the N of thiazole and an O-H of HO, and a weaker, secondary interaction between O and either the hydrogen atom attached to C2 (in 5-methylthiazole···HO) or the CH group attached to C4 (in 4-methylthiazole···HO). The barrier to internal rotation of the CH group, , is slightly lower for 4-methylthiazole···HO (XIAM result is 340.05(56) cm) than that for the 4-methylthiazole monomer (357.6 cm). This is likely to be a result of internal charge redistribution within the 4-methylthiazole subunit following its coordination by HO. At the precision of the experiments, of 5-methylthiazole···HO (XIAM result is 325.16(38) cm) is not significantly different from of the 5-methylthiazole monomer (332.0 cm).