Frances S. Rees

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm 1 of internal energy measured by dynamic rotational spectroscopy

We demonstrate the application of molecular rotational spectroscopy to measure the conformation isomerization rate of vibrationally excited pent-1-en-4-yne (pentenyne). The rotational spectra of single quantum states of pentenyne are acquired by using a combination of IR–Fourier transform microwave double-resonance spectroscopy and high-resolution, single-photon IR spectroscopy. The quantum states probed in these experiments have energy eigenvalues of ≈3,330 cm−1 and lie above the barrier to conformational isomerization. At this energy, the presence of intramolecular vibrational energy redistribution (IVR) is indicated through the extensive local perturbations found in the high-resolution rotation–vibration spectrum of the acetylenic C–H stretch normal-mode fundamental. The fact that the IVR process produces isomerization is deduced through a qualitatively different appearance of the excited-state rotational spectra compared with the pure rotational spectra of pentenyne. The rotational spectra of the vibrationally excited molecular eigenstates display coalescence between the characteristic rotational frequencies of the stable cis and skew conformations of the molecule. This coalescence is observed for quantum states prepared from laser excitation originating in the ground vibrational state of either of the two stable conformers. Experimental isomerization rates are extracted by using a three-state Bloch model of the dynamic rotational spectra that includes the effects of chemical exchange between the stable conformations. The time scale for the conformational isomerization rate of pentenyne at total energy of 3,330 cm−1 is ≈25 ps and is 50 times slower than the microcanonical isomerization rate predicted by the statistical Rice–Ramsperger–Kassel–Marcus theory.

Rotational spectroscopy of vibrationally excited states by infrared-Fourier transform microwave–microwave triple-resonance spectroscopy

A Fourier transform microwave (FTMW) spectroscopy-based technique for measuring the rotational spectrum of vibrational excited states is demonstrated. A pulsed infrared laser is used to prepare the excited state outside the FTMW cavity. Following laser excitation, the molecules drift into the FTMW cavity region. The FTMW spectrometer is used to monitor a single rotational transition in the excited state. The rotational spectrum of one of the states involved in the transition monitored by the FTMW spectrometer is obtained through the Autler–Townes splitting of the quantum state caused by the application of resonant microwave radiation to the cavity region.

Structure and vibrational dynamics of the halogen-substituted butynes HC=CCH2CH2X (X=F,Cl,Br) from molecular-beam high-resolution microwave and infrared spectroscopy

The pure rotational and high-resolution acetylenic C–H stretch rovibrational spectra of a series of substituted butynes, HCCCH2CH2X (X = F,Cl,Br), are reported. For each of the molecules the pure rotational spectrum of two conformational isomers (trans and gauche) has been assigned. For the Cl and Br substituted compounds the pure rotational spectrum of two isotopic species has been assigned (35Cl, 37Cl and 79Br, 81Br) for each conformer. An analysis of the nuclear quadrupole hyperfine structure in the pure rotational spectrum shows good agreement with structural parameters obtained through electronic structure calculations. The rotational transitions are used to obtain full rotational assignment of the acetylenic C–H stretch vibrational band of the more stable trans conformers through infrared-microwave double-resonance spectroscopy. The rotational band contours are predominantly a-type for all three halobutynes. The rotational structure of the band displays separations characteristic of the trans conformer. This result reflects the fact that the bright state for the vibrational spectrum retains the ground state conformation. From the eigenstate-resolved spectra we determine the timescale for intramolecular vibrational energy redistribution (IVR). The IVR rate obtained from the high-resolution spectrum provides an upper limit to the isomerization rate following coherent vibrational excitation. The IVR lifetimes of the acetylenic C–H stretch have been determined to be 1.5 ns for trans-4-fluorobut-1-yne, 3.5 ns for trans-4-chlorobut-1-yne, and 2.0 ns for trans-4-bromobut-1-yne. In all cases, the upper limit isomerization rate inferred from the vibrational spectrum is three orders-of-magnitude slower than the RRKM rate calculated using the ab initio barrier heights. Although direct dynamical information about the isomerization rate cannot be obtained from the spectrum, evidence is found for conformational isomerization occurring through the J-dependent growth of the measured rovibrational state density.