5314027762

POSTER SESSION. H16

SOME FINER THEORETICAL DETAILS OF CALCULATIONS IN METHYL ROTOR MOLECULES

J. T. HOUGEN AND M. A. MEKHTIEV, Optical Technology Di-vision. National Institute oj Standards and Technology, Gaithersburg,

MD 20899, USA; R. M. LEES, Department oj Physics, Uniuersity oj New Brunswick, Fredeńcton, NB, Canada E3B 5A3\ LI-HONG XU, Department of Physical Sciences, Unioersity oj New Brunswick, Saint John. NB, Canada E2L ĄL5.

This poster will prescnt a progress report on our thinking on smali (but philosophically and nuinerically important) questions which have arisen while carrying out various rho-axis-method theoretical calculations for the torsion-rotation problem in methanol. At the time of abstract preparatiori sonie of this thinking is still in flux, so only the questions will be given below. (1) How is the quantum chemistry "intrinsic reaction coordinate" rclated to the spectroscopie internal rotation angle 7? What are the consequences of ignoring this (juestion? The intrinsic reaction coordinate is defined using only a knowledge of the potent ial energy surface gradients. The spectroscopie internal rotation angle appearing in the fitting Hamiltonians is often defined in terms of a rigid C3„-top model, or in terms of the int uitively appealing average HOCH, dihedral angle in the top, i.e., (71 + 72 + 73)/3. or in terms of a large-amplitude coordinate satisfying certain constraint-like cquations. Are these s(»veral definitions cxactly equivalent, merely compatable, or actually inconsistent? (2) How should one determine a top-axis system from a quantum chemistry structurc in which the niethyl rotor does not have exactly Ca,, symmetry? Is it really necessary to find the top-axis system? Many spectroscopie fitting paraineters are traditionally defined from a knowledge of: the principal moments of inertia of the molecule, the moment of inert ia of the methyl top about its C3 axis, and the direction cosines of this C3 axis in the principal axis system. This clearly leads to questions w hen trying to compare spectroscopie constants from data fits with those predicted by quantum chemistry calculations of the equilibrium structurc at the bottom of the well, sińce the latter do not normally give an exactly €3,, top. (3) How should the signs of n_Q. Pb-Pc, D_ab, p, etc. in the torsion-rotation problem be related in order to avoid inconsistencies when intensity calculations involving constructive and destructivc interference efTects among the dipole moment components are performed? What is the physical meaning of the different possibilities for relativc sign choices? The qucstion of positive directions for the a,b and c axes, as well as the epiestion of positive sense for changes in the internal rotation angle and the K-rotation angle can normally be ignored. However, for intensity calculations in which interference efFects occur. the relative signs of p_a,//j>, and p_c are important. The sign of D_aj> and p*, but not that of p_u, is changed by (a,b,c) -> (a. -6, -c); the sign of P-, and p_c (whose first nonvanishing 1'ouricr cxpansion term is stn37) is changed by 7 —► —7.