Knowledge Bank

The microwave spectrum of the water-CO complex has been observed using both molecular beam electric resonance and Fourier transform microwave spectroscopy. The water is hydrogen bonded to the carbon of the carbon monoxide. A tunneling motion between the two equivalent hydrogen bonded structures gives rise to two states. These states are unambiguously assigned based on hyperfine structure arising from either the proton spin-spin interaction $(H2O)$ or deutcrium quadrupole hyperfine $(D2O)$. A barrier to the tunneling motion of about $200cm-1$ is obtained from the difference in the dipole moments of the two states using a simple one dimensional tunneling model. Although the water is hydrogen bonded to the CO, the O-H bond of water is tilted away from a linear hydrogen bonded configuration by about $12\circ$. (The direction is such that the lone pairs on the oxygen, rather than the other hydrogen, are brought into the carbon.) Electrostatic models predict a linear hydrogen bonded structure, with a barrier to tunneling of between 400 and $900cm-1$ depending on the choice of distributed multipole model.