Vibrational mode mixing in terminal acetylenes: High-resolution infrared laser study of isolated J states

Abstract

Mode–mode vibrational coupling in the acetylinic CH stretch at 3330 cm−1 of 1-butyne and 1-pentyne is studied via high-resolution, direct absorption infrared spectroscopy. As in our previous study of propyne, mixing of the CH stretch vibration carrying oscillator strength (the bright state) with the bath of multiquantum combination states (the dark, or background, states) manifests itself in the spectrum via fragmentation of the isolated bright state transitions into clusters of closely spaced spectral lines in a ∼0.01 cm−1 window about the zeroth order acetylinic CH stretch position. In the 1-butyne spectrum, we find an experimental density of mixed states of 114±30 states/cm−1 compared to a direct state count prediction of 90 total states/cm−1, and thus quantitatively determine that all possible states appear in the spectrum. The 1-butyne line spacing distribution suggests the Wigner distribution expected for a quantum mechanically ergodic system. Analysis of the mode mixing as a function of J′ shows that anharmonic terms in the potential, rather than Coriolis effects, contribute most strongly to the coupling. The acetylinic CH stretch spectrum of 1-pentyne (2400 states/cm−1) reveals only broad rovibrational transitions with ∼0.01 cm−1 Lorentzian width, even at our 10−4 cm−1 resolution. J′ independent, anharmonic coupling with a minimum of 1/3 of all states must be invoked to reproduce the observed broadening. In contrast, the 1-pentype methyl CH stretch spectrum shows broadening greater than five times larger than that observed at the acetylinic end. Via Fourier transform methods, the spectra for both 1-butyne and 1-pentyne indicate vibrational energy localization in the CH stretch for ∼500 ps. However, for the methyl CH stretch, energy redistribution takes place in <40 ps, clearly indicating the presence of mode specific, nonRRKM vibrational relaxation pathways.

The role of molecular flexibility in accelerating intramolecular vibrational relaxation

“…Therefore matrix elements of eq. [9], when expressed in the product basis, $(a)$@), will yield both low-and high-order couplings between the two internal rotors. Although we do not Can.…”

“…This coherent superposition would then decay in time, resulting in the transfer of vibrational amplitude from the bright state to other vibrational degrees of freedom. The survival probability of the bright state is given by (9,10) where the I; are the intensities of the eigenstates in the clump and the E; are their energies. Each summation in eq.…”

“…The eigenstate-resolved spectra of these been associated with greatly accelerated IVR. As noted in the molecules are the superposition of two pure sequences, which introduction, the methyl C-H vibrations in 1-butyne, propane, must be at least approximately separated for the purpose of isobutane, and trans-2-butene all have longer relaxation times determining lifetimes (1,9,10,12). The effect of such approxthan in the systems where gauche-trans isomerization is posimate separations on the accuracy of the derived IVR lifetimes sible.…”

Vibrationally induced rotational axis switching: A novel mechanism for vibrational mode coupling

“…[4][5][6][7][8] Perturbations in the spectra of many molecules lead to the appearance of clusters of peaks where one would expect to see a single ro-vibrational transition. These clusters or clumps of peaks are direct evidence of state coupling.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] Frequently, IVR is viewed as an explicitly resolved data and discuss in detail the application of these mechanisms to mode-coupling in the mid-sized molecule pyrizine. 10 From these as well as other experiments 4 -13 , it is clear that rotational angular momentum can play an important role in state mixing, and our understanding of the physical mechanism of rotation dependent vibrational mode-coupling is far from complete.…”