Water hydrogen bonding: The structure of the water–carbon monoxide complex

Rotational transitions between J≤3 levels within the K=0 manifold have been observed for H2O–CO, HDO–CO, D2O–CO, H2O–13CO, HDO–13CO, and H217O–CO using the molecular beam electric resonance and Fourier transform microwave absorption techniques. ΔMJ=0→1 transitions within the J=1 level were also measured at high electric fields. A tunneling motion which exchanges the equivalent hydrogens gives rise to two states in the H2O and D2O complexes. The spectroscopic parameters for H2O–CO in the spatially symmetric tunneling state are [∼(B0) =2749.130(2)MHz, D0=20.9(2)kHz, and μa=1.055 32(2)D] and in the spatially antisymmetric state are [∼(B0) =2750.508(1)MHz, D0=20.5(1)kHz, and μa=1.033 07(1)D]. Hyperfine structure is resolved for all isotopes. The equilibrium structure of the complex has the heavy atoms approximately collinear. The water is hydrogen bonded to the carbon of CO; however the bond is nonlinear. At equilibrium, the O–H bond of water makes an angle of 11.5° with the a axis of the complex; the C2v axis of water is 64° from the a axis of the complex. The hydrogen bond length is about 2.41 Å. The barrier to exchange of the bound and free hydrogens is determined as 210(20) cm−1 (600 cal/mol) from the dipole moment differences between the symmetric and antisymmetric states. The tunneling proceeds through a saddle point, with C2v structure, with the hydrogen directed towards the CO subunit. The equilibrium tilt away from a linear hydrogen bond is in the direction opposite to the tunneling path.