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This study reports simulations of the lowest band in the electronic absorption spectrum of pyrazine carried out using a multi-state-multimode vibronic Hamiltonian parameterized using equation-of-motion coupled-cluster methods. The simulations explain the main spectral features and show how peaks of vibronic nature appear. The most complete vibronic model includes four electronic states and six vibrational modes. The simulations reveal that non-adiabatic coupling with bright states located as high as 3 eV above the studied state can lead to discernible features in the absorption spectrum. This study demonstrates the power of fully ab initio treatments of electronic and vibrational structure and their utility in understanding the mechanisms leading to complex molecular spectra. 

Microwave and millimeter-wave rotational spectra of 46 thiophene isotopologues are used to determine a new semi-experimental equilibrium (reSE) structure for thiophene. The large dataset of isotopologues—roughly twice as large as any dataset used for a semi-experimental equilibrium structure determination for a molecule of this size—and the broad frequency coverage enable a detailed investigation concerning previously reported discrepancies between the structural parameters of the reSE and re structures of thiophene. The observation of rotational spectra for 23 new thiophene-dx isotopologues, many with additional 34S or 13C substitution at natural abundance, is made possible by the high sensitivity of the PARIS FTMW spectrometer in the microwave frequency region. Frequency coverage for thiophene and single heavy-atom isotopologues in the millimeter-wave region has been expanded to include the range of 82–750 GHz. Bond distances and bond angles of the new highly precise and accurate reSE structure are determined with precisions of ≤0.0002 Å and ≤0.02°, respectively. This study provides strong evidence that the discrepancies can be addressed only by improvements to the theoretical re structure and that the precision and accuracy of the semi-experimental equilibrium (reSE) structural parameters are approaching the limit of structure determination. 

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. 

Computational studies of small beryllium clusters (Be)n predict dramatic, non-monotonic 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. Frank-Condon diagonal photodetachment was observed with a photoelectron angular distribution consistent with the expected Be4 X2A1→Be4 (1)1A1transition. The electron affinities of Be3 and Be4 were determined to be 11363(30) and 13052(30) cm-1, respectively.