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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. 

Computational studies of small beryllium clusters (BeN) predict dramatic, nonmonotonic changes in the bonding mechanisms and per-atom cohesion energies with increasing N. To date, experimental tests of these quantum chemistry models are lacking for all but the Be2 molecule. In the present study, we report spectroscopic data for Be3 and Be4 obtained via anion photodetachment spectroscopy. The trimer is predicted to have D3h symmetric equilibrium structures for both the neutral molecule and the anion. Photodetachment spectra reveal transitions that originate from the X2A2″ ground state and the (1)2A1′ electronically excited state. The state symmetries were assigned on the basis of anisotropic photoelectron angular distributions. The neutral and anionic forms of Be4 are predicted to be tetrahedral. Franck−Condon diagonal photodetachment was observed with a photoelectron angular distribution consistent with the expected Be4−X2A1 → Be4X1A1 transition. The electron affinities of Be3 and Be4 were determined to be 11363 ± 60 and 13052 ± 50 cm−1, respectively 

The millimeter-wave rotational spectrum of ketene (H2C=C=O) has been collected and analyzed from 130 to 750 GHz, providing highly precise spectroscopic constants from a sextic, S-reduced Hamiltonian in the Ir representation. The chemical synthesis of deuteriated samples allowed spectroscopic measurements of five previously unstudied ketene isotopologues. Combined with previous work, these data provide a new, highly precise, and accurate semi-experimental (reSE) structure for ketene from 32 independent moments of inertia. This reSE structure was determined with the experimental rotational constants of each available isotopologue, together with computed vibration–rotation interaction and electron-mass distribution corrections from coupled-cluster calculations with single, double, and perturbative triple excitations [CCSD(T)/cc-pCVTZ]. The 2σ uncertainties of the reSE parameters are ≤0.0007 Å and 0.014° for the bond distances and angle, respectively. Only S-reduced spectroscopic constants were used in the structure determination due to a breakdown in the A-reduction of the Hamiltonian for the highly prolate ketene species. All four reSE structural parameters agree with the “best theoretical estimate” (BTE) values, which are derived from a high-level computed re structure [CCSD(T)/cc-pCV6Z] with corrections for the use of a finite basis set, the incomplete treatment of electron correlation, relativistic effects, and the diagonal Born–Oppenheimer breakdown. In each case, the computed value of the geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The discrepancies between the BTE structure and the reSE structure are 0.0003, 0.0000, and 0.0004 Å for rC–C, rC–H, and rC–O, respectively, and 0.009° for θC–C–H. 

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.