Knowledge Bank

As shown by the $author1$ the maximum separation between a large-amplitude internal motion and a mall-amplitude vibrations can be reached by the use of so-called quasi-equilibrium semirigid model a uniquely determined by the gradient of the adiabatic potential. In contrast with semirigid bender models suggested by Bunker and $Landsberg2$ quasi-equilibrium model depend on nuclear masses; and as a result asymmetric isotopic substitution breaks their point symmetry. The role of such symmetry breaking is analyzed for deuteron-derivatives of water and ammonia molecules. Vibrations energy levels calculated for different cuts of the 3-dimenaional potential function are compared with the accurate calculations carried out by Carney at $al3$ for various isotopes of water. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$G.A. Natanson, Molec. Phys., 46, 481, 1982. $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$P. R. Bunker, B. M. Landsberg, J. Molec, Spectrosc., 67, 374 1977. $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$G.D. Carney, L.A. Curtiss, and S.R. Langhoff, Appl. Spectrosc. 30, 453, 1976.