Chloromethane-water adduct: rotational spectrum, weak hydrogen bonds, and internal dynamics

The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, O-H⋅⋅⋅Cl (R(H⋅⋅⋅Cl) =2.638(2) Å) and C-H⋅⋅⋅O (R(H⋅⋅⋅O) =2.501(2) Å), forming a five-membered ring. All transitions display the hyperfine structure due to the (35)Cl (or (37)Cl) nuclear quadrupole effects. Dynamical features in the spectrum are caused by two large-amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V(2) =320(10) cm(-1). Finally, an unexpected small value of the inertial defect (-0.96 uÅ(2) rather than -3.22 uÅ(2)) allowed the estimation of the barrier to the internal rotation of the CH(3) group, V(3) ≈8 cm(-1).