Global analysis of a-, b-, and c-type transitions involving tunneling components of K=0 and 1 states of the methanol dimer

Spectral data on K = 0 and 1 levels of the methanol dimer available from pr evious and present Fourier transform microwave measurements have been inter preted globally, using a group-theoretically derived effective Hamiltonian and corresponding tunneling matrix elements to describe the splittings aris ing from a large number of tunneling motions. In the present work, 302 new measurements (40 K = 1-1 and 262 K = 1-0 transitions) were added to the pre vious data set to give a total of 584 assigned transitions with J less than or equal to 6. As a result of the rather complete K = 0, 1 data set for J less than or equal to 4, the lone-pair exchange tunneling splittings were o btained experimentally. Matrix element expansions in J(J + 1) used in the p revious K = 0 formalism were modified to apply to K > 0, essentially by mak ing a number of real coefficients complex, as required by the generalized i nternal-axis-method tunneling formalism. To reduce the number of adjustable parameters to an acceptable level in both the K = 0 and K = 1 effective Ha miltonians (used in separate K = 0 and K = 1 least-squares fits), a rather large number of assumptions concerning probably negligible parameters had t o be made. The present fitting results should thus be considered as providi ng assurance of the group-theoretical line assignments as well as a nearly quantitative global interpretation of the tunneling splittings, even though they do not yet unambiguously determine the relative contributions from al l 25 group-theoretically inequivalent tunneling motions in this complex, no r do they permit quantitative extrapolation to higher K levels. (C) 1999 Ac ademic Press.