We have studied the dynamics of intramolecular vibrational energy redistribution (IVR) from the initially excited mode v(1) (acetylenetype H-C bond) in H-C equivalent to C-CH2Cl molecules in the gaseous phase by means of anti-Stokes spontaneous Raman scattering. The energy relaxation from v, due to IVR was estimated to occur on the time-scale tau approximate to 750 ps, which is one of the slowest IVR time-scales reported so far. The deactivation due to molecular collisions occurred with a rate constant of approximate to 8.5 mu s(-1) Torr(-1), and was slower than the IVR process. A theoretical model which is based on the idea of statistical nature of couplings and incompleteness of IVR (expressed in the form of the so-called dilution factor) is proposed. The model provides rationalization for the observed kinetics and suggests that the IVR process is mediated by not only anharmonic but also vibrational-rotational interactions. (c) 2005 Elsevier B.V. All rights reserved.