The role of molecular flexibility in accelerating intramolecular vibrational relaxation

Abstract

Can. J. Chem. 72, 652 (1994). 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 I-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, I-butene, n-butane, methyl formate, and propargyl amine) and for some "rigid" molecules (I-butyne, isobutane, propyne, trans-2-butene, and tert-butylacetylene). The lifetimes for the halogenated molecules, 2-fluoroethanol, 1,2-difluoroethane, trans-lchloro-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. On prksente des donnkes pour dkmontrer que la relaxation vibrationnelle intramolkculaire (RVI) est plus rapide dans les molCcules flexibles lorsque la vibration prCparCe initialement est proche de la liaison autour de laquelle le mouvement de grande amplitude se produit. Dans le pent-I-yne, l'kthanol et l'alcool propargylique, les temps de vie des RVI des deux modes diffkrents d'Clongation de l'hydrure sont connus et, pour chaque molkcule, la rotation interne permet de joindre les conformkres gauche et trans. Dans chaque cas, la vibration la plus proche du centre de flexibilitk prksente la relaxation la plus rapide. Cette tendance est supportke par les temps de vie RVI disponibles pour d'autres molCcules flexibles (peroxyde d'hydrogkne, but-I-kne, n-butane, formate de methyle et amine propargylique) et pour quelques molCcules << rigides D (but-I-yne, isobutane, propyne, trans-but-2-kne et tert-butylacktylkne). Les temps de vies des molCcules halogknkes, 2-fluoroCthanol, 1,2-difluorokthane, trans-I-chloro-2-fluoroethane et trifluoropropyne sont tous de l'ordre de grandeur attendu pour des molkcules rigides. On prksente un algorithme qui permet de calculer d'une facon cohkrente les temps de vie RVI ?I partir de spectres discrets rksolus en frkquence qui couvrent les cas des couplages faibles ?I intermkdiaires. Dans tous les cas possibles, les temps de vie rapportis ont Ct C calculks (ou recalculks) B partir des positions et des intensitks originales des bandes. On peut comparer les temps de vie directement B ceux dkduits ?I partir de caractkristiques spectrales Clargies d'une facon homogkne avec un contour lorentzie...

Probing the origins of vibrational mode specificity in intramolecular dynamics through picosecond time-resolved photoelectron imaging studies

“…However, although this rule holds in some cases 8 exceptions have been welldocumented for many years, 6,[9][10][11][12][13][14] and factors such as mode-dependence, 15 existence of critical "doorway" states, 4,6,16 molecular flexibility, 17, 18 molecular symmetry 5 and the nature of substituents 8 have all been claimed to play a critical role in determining the course of the dynamics.…”

Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis→trans tunneling rate

“…21 In the gas phase, the presence of low barrier internal rotations have been postulated to be responsible for the decrease of the lifetime of excited states associated with vibrations close to the center of flexibility, due to the enhancement of the coupling of vibrational and internal rotational states. [23][24][25][26][27] In the solid phase, the methyl rotor may also enhance couplings between the intramolecular vibrational states and the phonon bath. Thus, the comparison between the isomerization quantum yields for formic and acetic acids in similar experimental conditions aids to evaluate the role of the methyl rotor in the dynamics of vibrational energy relaxation.…”

Vibrational overtone initiated unimolecular dissociation of HOCH2OOH and HOCD2OOH: Evidence for mode selective behavior

“…The ability of different functional groups to promote IVR to different extents has been reported in the literature with several studies noting that IVR is faster when the initially excited vibration is in close proximity to a methyl rotor or to a bond about which torsional motion occurs. [39][40][41] So our observation of the differences in the dissociation rate for the alcohol versus the peroxide OH stretch in HMHP and HMHP-d 2 is likely a manifestation of these effects. Further experiments on jet cooled HMHP samples, where inhomogeneous broadening can be minimized, are planned in the future to more directly probe the IVR rates of the 4 OH stretching states.…”