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Unimolecular decay of the formaldehyde oxide (CH2OO) Criegee intermediate proceeds via a 1,3 ring-closure pathway to dioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected. Vibrational activation of jet-cooled CH2OO with two quanta of CH stretch (17-18 kcal mol-1) leads to unimolecular decay at an energy significantly below the transition state barrier of 19.46  0.25 kcal mol-1, refined utilizing a high-level electronic structure method HEAT-345(Q)Λ. The observed unimolecular decay rate of 1.6 +/- 0.4 x 106 s-1 is two orders of magnitude slower than that predicted by statistical unimolecular reaction theory using several different models for quantum mechanical tunneling. The nonstatistical behavior originates from excitation of a CH stretch vibration that is orthogonal to the heavy atom motions along the reaction coordinate and slow intramolecular vibrational energy redistribution due to the sparse density of states. 

We report the hyperfine-resolved rotational spectrum of gas-phase phenoxy radical in the 8–25 GHz frequency range using cavity Fourier transform microwave spectroscopy. A complete assignment of its complex but well-resolved fine and hyperfine splittings has yielded a precisely determined set of rotational constants, spin-rotation parameters, and nuclear hyperfine coupling constants. These results are interpreted with support from high-level quantum chemical calculations to gain detailed insight into the distribution of the unpaired π electron in this prototypical resonance-stabilized radical. The accurate laboratory rest frequencies enable studies of the chemistry of phenoxy in both the laboratory and in space. The prospects of extending the present experimental and theoretical techniques to investigate the rotational spectra of isotopic variants and structural isomers of phenoxy and other important gas-phase radical intermediates yet undetected at radio wavelengths are discussed. 

In this work, we present the spectral analysis of 1 H- and 2 H-1,2,3-triazole vibrationally excited states alongside provisional and practical computational predictions of the excited-state quartic centrifugal distortion constants. The low-energy fundamental vibrational states of 1 H-1,2,3-triazole and five of its deuteriated isotopologues ([1- 2 H]-, [4- 2 H]-, [5- 2 H]-, [4,5- 2 H]-, and [1,4,5- 2 H]-1 H-1,2,3-triazole), as well as those of 2 H-1,2,3-triazole and five of its deuteriated isotopologues ([2- 2 H]-, [4- 2 H]-, [2,4- 2 H]-, [4,5- 2 H]-, and [2,4,5- 2 H]-2 H-1,2,3-triazole), are studied using millimeter-wave spectroscopy in the 130–375 GHz frequency region. The normal and [2- 2 H]-isotopologues of 2 H-1,2,3-triazole are also analyzed using high-resolution infrared spectroscopy, determining the precise energies of three of their low-energy fundamental states. The resulting spectroscopic constants for each of the vibrationally excited states are reported for the first time. Coupled-cluster vibration–rotation interaction constants are compared with each of their experimentally determined values, often showing agreement within 500 kHz. Newly available coupled-cluster predictions of the excited-state quartic centrifugal distortion constants based on fourth-order vibrational perturbation theory are benchmarked using a large number of the 1,2,3-triazole tautomer isotopologues and vibrationally excited states studied.