Intramolecular Vibrational Dynamics of the Asymmetric &dbond;CH2 Hydride Stretch of Isobutene

Conformer lifetimes of ethyl cyanoformate from exchange-averaged rotational spectra

Ethyl cyanoformate exists as a mixture of two conformers but displays three R-branch a-type band series in its rotational spectrum. Simulations with population fractions 0.37 at 210 K and 0.70 at 297 K undergoing conformer exchange with average conformer lifetimes, <tau(iso)>, shorter than approximately 40 ps at approximately 210 K and shorter than approximately 37 ps at 297 K reproduce the experimental spectra between 26.5 and 38 GHz, the exchanging species accounting for the third set of bands. The upper-limit <tau(iso)>'s are 1 order of magnitude longer than RRKM theory predictions and the population fractions are consistent with the total population with energy above 700 cm(-1), approximately twice the conformer interconversion barrier height. Model calculations indicate that extensive K-sublevel mixing in individual molecular eigenstates can produce the large population and the narrow distribution of the rotational-constant sum, B + C, consistent with the observed exchange-averaged band series.

Decoding the dynamical information embedded in highly mixed quantum states

The standard description of the vibrational and rotational motion of polyatomic molecules, as expressed by the distortable rotor/harmonic oscillator approximation, provides an adequate description of the molecular quantum states only in regions of low total state density. When the total state density is large, exceeding 100 states/cm(-1), the vibrational dynamics are "dissipative" and the fundamental process of intramolecular vibrational energy redistribution is operative. The presence of intramolecular vibrational energy redistribution leads to molecular quantum states of a qualitatively different nature. With respect to a normal-mode vibrational basis, these quantum states are "highly mixed" in their vibrational character and represent nuclear motion that is a combination of all the normal-mode motions. This review describes frequency domain spectroscopy techniques designed to probe the vibrational, rotational, and structural composition of these eigenstates. Recent work that investigates spectroscopy between highly mixed states is also reviewed.