Rotational recurrences in thermal ensembles of nonrigid molecules

An approach has been developed to calculate the periods of rotational recurrences (RRs) in thermal ensembles of nonrigid molecules. Starting from the standard nonrigid rotor Hamiltonian with centrifugal distortion (CD) terms, the explicit analytical formulas were derived for CD-induced corrections to the periods of J, H, and K transients for symmetric top molecules. The corrections are shown to be proportional to the CD constants and the temperature. In addition, they depend considerably on the rotational constants of the molecule. Generally, the CDs are demonstrated to induce a shift in the periods of the RRs, which depends on the order (n = 1, 2,...) of RRs. Only the first few RRs exhibit n-independent shifts, and this is the situation in which our theory is exact. For higher order RRs, our approach starts to exaggerate slightly the values of the CD-induced corrections, giving their upper boundary. (C) 2003 Elsevier Science B.V. All rights reserved.