David A. McWhorter

Molecular rotation in the presence of intramolecular vibrational energy redistribution

At high energy, the vibrational dynamics of a polyatomic molecule are qualitatively different from the separable normal-mode dynamics that characterize the low energy region of the spectrum. Once the total rovibrational state density exceeds 10-100 states cm-1, the effects of intramolecular vibrational energy redistribution (IVR) are readily observed in the frequency-domain spectrum. In an energy region where IVR occurs, the time scale for the flow of vibrational energy is comparable to the time scale for molecular rotation. The jostling of nuclear positions caused by the IVR dynamics leads to a time-dependent moment of inertia for the molecular rotation. The time-dependent modulation of the moment of inertia, in turn, affects the appearance of the rotational spectrum of the molecule. These effects can be described by the motional narrowing formalism first developed for nuclear magnetic resonance spectroscopy. We present a basic description of the rotational problem for the case where the molecule has a single energetically accessible nuclear geometry and the case where the total energy of the molecule is above the barrier to isomerization. In the latter case, the microcanonical isomerization rate can be obtained from the overall line shape of the rotational spectrum. An example of using rotational spectroscopy to measure the isomerization rate of 4-chlorobut-1-yne at 3330 cm-1 is presented.

The rotational spectra of single molecular eigenstates of 2-fluoroethanol: Measurement of the conformational isomerization rate at 2980 cm−1

The rotational spectroscopy of single molecular eigenstates has been used to measure the conformational isomerization rate in 2-fluoroethanol. Eigenstates in the asymmetric –CH2(F) stretch spectrum of the Gg′ conformer near 2980 cm−1 are prepared with an infrared laser. These eigenstates are approximately 2000 cm−1 above the barrier to Gg′−Tt conformational isomerization. The rotational spectrum is measured using an infrared-microwave double-resonance technique based on the Autler–Townes splitting of states in a strong microwave field. This technique does not require saturation of the infrared preparation step. Two types of rotational transitions are observed. These are assigned to rotational transitions from vibrational states with Tt conformation (near 15.8 GHz) and to “isomerization states” (near 17.1 GHz) where the torsional wave functions are above the isomerization barrier. The isomerization kinetics are obtained from the linewidth of the ensemble eigenstate rotational spectrum. The lifetime for the...

Doorway state enhanced intramolecular vibrational energy redistribution in the asymmetric =CH2 hydride stretch of methyl vinyl ether

The microwave-infrared double-resonance capabilities of an electric-resonance optothermal spectrometer have been used to assign the high resolution (5 MHz) infrared spectrum of the asymmetric =CH2 stretch of the cis conformer of methyl vinyl ether near 3130 cm−1. This vibrational state is anharmonically coupled to a near-resonant bath state by a 0.69 cm−1 matrix element resulting in two vibrational bands separated by about 1.44 cm−1. The two mixed states resulting from this interaction are further coupled to other near-resonant bath states with an average matrix element of about 0.01 cm−1. The coupled state density increases weakly with the total angular momentum, J, however, the intramolecular vibrational energy distribution (IVR) rate is approximately independent of the total angular momentum quantum number. Therefore, the rotationally mediated coupling mechanisms are weaker than the anharmonic terms in the redistribution process. A two-state analysis of the strong coupling, which includes a phenomenolo...

Intramolecular vibrational energy redistribution in the acetylenic C–H and hydroxyl stretches of propynol

The high-resolution infrared spectra of the acetylenic C–H and O–H stretches of propynol have been measured using an electric-resonance optothermal molecular beam spectrometer (EROS). Both spectra display extensive fragmentation of the hydride-stretch oscillator strength characteristic of the intramolecular vibrational energy redistribution (IVR) process. The IVR lifetime is strongly mode-specific. The IVR lifetime of the acetylenic C–H stretch is approximately 400 ps, with a slight increase in the lifetime with increasing values of the Ka quantum number. The lifetime of the O–H stretch is 60 ps and is independent of the rotational quantum numbers. The experimental upper limit for the anharmonic state densities are 30 and 40 states/cm−1 for the acetylenic C–H and O–H stretches, respectively. These values are in good agreement with the values obtained by a direct state count (19 and 32 states/cm−1, respectively). The measured density of states increases with an approximate (2J+1)-dependence. These results ...

The rotational spectrum of highly vibrationally mixed quantum states of propynol near 3330 cm−1

The rotational spectra of molecular eigenstates of propynol in the region of the acetylenic C–H stretch (3330 cm−1) have been measured using infrared-microwave saturation spectroscopy. These spectra illustrate the basic properties of the rotational spectra of highly vibrationally mixed quantum states. From the measurements we are able to measure the average value of the rotational constant and the width of the rotational constant distribution. We determine that the average value of the quantity 12 (B+C) is 17 MHz smaller than the ground state value (a decrease of 0.4%). The width of the distribution (FHWM) is 90 MHz (1% of the ground state value). The distribution is approximately Gaussian. Narrowing of the rotational spectrum of single eigenstates by intramolecular vibrational energy redistribution (IVR) exchange processes is observed for the Ka=2 eigenstates. From the spectral narrowing we determine that the average IVR lifetime for vibrational states with Ka=2 near 3330 cm−1 is approximately 75 ps, abo...

Structurally mixed molecular eigenstates of 2-fluoroethanol resulting from conformational isomerization

Using a newly developed method of molecular-beam, infrared-microwave double-resonance spectroscopy, we are able to measure the rotational spectrum of a single molecular eigenstate of the molecular Hamiltonian near 3000 cm−1 of energy above the ground rovibrational state. This energy lies above the barrier to conformation isomerization in many molecules. In the –CH2F asymmetric C–H stretch of the Gg conformer of 2-fluoroethanol, near 2983 cm−1, we demonstrate the contribution of vibrational states localized around the Tt conformer structural minimum to the individual molecular eigenstates. The measurement demonstrates the ability of isolated molecules to use vibrational excitation to achieve geometrical rearrangement.