Prohibition of ortho-para transitions in a water molecule

Abstract

A theoretical consideration is given to three types of prohibition of transitions between the rotational states of spin isomers of an H216O molecule, which are based on the molecular symmetry. This is the symmetry of the electron shell of the molecule, of the proton spin functions, of the spatial position of molecular nuclei, and of the rotational Hamiltonian. The prohibition of dipole transitions between isomers and the prohibition of transitions on the basis of the Pauli principle were known earlier. Another prohibition exists which is a consequence of the symmetric position of protons relative to the oxygen nucleus. Conditions are indicated under which the prohibition of ortho-para transitions in a water molecule disappear. In the general form these conditions are realized when the molecule loses the above-listed symmetries. Transitions are allowed in the dipole approximation if the proton spin moments are free and do not form superposition states and the O-H bond lengths are different because of nonlinearity of the molecular vibrations. The transitions are induced by the dipole-moment component due to the deformation of the electron shell of the molecule.