The role of molecular flexibility in accelerating intramolecular vibrational relaxation

Evidence is presented to show that intramolecular vibrational relaxation (IVR) is faster in flexible molecules when the initially prepared vibration is close to the bond about which the large-amplitude motion occurs. In each of 1-pentyne, ethanol, and propargyl alcohol, IVR lifetimes are known for two different hydride stretches and in each molecule internal rotation connects gauche and trans conformers. In each case the vibration that is closer to the center of flexibility shows faster relaxation. This trend is supported by the available IVR lifetimes for other flexible molecules (hydrogen peroxide, 1-butene, n-butane, methyl formate, and propargyl amine) and for some "rigid" molecules (1-butyne, isobutane, propyne, trans-2-butene, and tert-butylacetylene). The lifetimes for the halogenated molecules, 2-fluoroethanol, 1,2-difluoroethane, trans-1-chloro-2-fluoroethane, and trifluoropropyne are all in the range expected for rigid molecules. An algorithm is presented for the consistent calculation of IVR lifetimes from discrete frequency-resolved spectra, which range from the sparse through intermediate coupling cases. Wherever possible, the reported lifetimes have been calculated (or recalculated) from the original line positions and intensities. The lifetimes may be compared directly to those deduced from homogeneously broadened spectral features with a Lorentzian contour.