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

Abstract

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 Ϸ3,330 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 normalmode 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 threestate 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 Ϸ25 ps and is 50 times slower than the microcanonical isomerization rate predicted by the statistical Rice-Ramsperger-Kassel-Marcus theory.IVR ͉ microwave ͉ infrared ͉ double-resonance R otational spectroscopy is one of the most powerful tools in physical chemistry for the determination of gas-phase molecular structure (1). The development of molecular beam spectrometers for rotational spectroscopy-most notably the Fourier-transform microwave (FTMW) spectroscopy technique introduced by Balle and Flygare (2)-has greatly expanded the range of chemical systems that can be studied. Rotational spectroscopy can be routinely used to study challenging structural problems such as the conformations of large molecules (3), weakly bound molecular complexes (4), and radicals (5). In addition, rotational spectroscopy of low-energy vibrational and torsional levels can be used to determine accurate potential energy surfaces for large amplitude motion in ''floppy'' systems (1). Here we demonstrate a fundamentally unique application of rotational spectroscopy. We show that isomerization kinetics can be determined from the rotational spectrum of a highly vibrationally excited molecule.The physical basis of pure rotational spectroscopy is that when the total angular momentum is conserved, the rotational frequency is directly related to the principal moments of inertia of the molecule and, therefore, to its struct...

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