Matrix Isolation Spectroscopy and Nuclear Spin Conversion of Propyne Suspended in Solid Parahydrogen
Aaron StromAlejandro Gutiérrez‐QuintanillaMichèle ChevalierJustinas ČeponkusClaudine CrépinD. T. Anderson
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Abstract:
Parahydrogen (pH2) quantum solids are excellent matrix isolation hosts for studying the rovibrational dynamics and nuclear spin conversion (NSC) kinetics of molecules containing indistinguishable nuclei with nonzero spin. The relatively slow NSC kinetics of propyne (CH3CCH) isolated in solid pH2 is employed as a tool to assign the rovibrational spectrum of propyne in the 600-7000 cm-1 region. Detailed analyses of a variety of parallel (ΔK = 0) and perpendicular (ΔK=±1) bands of propyne indicate that the end-over-end rotation of propyne is quenched, but K rotation of the methyl group around the C3 symmetry axis still persists. However, this single-axis K rotation is significantly hindered for propyne trapped in solid pH2 such that the energies of the K rotational states do not obey simple energy-level expressions. The NSC kinetics of propyne follows first-order reversible kinetics with a 287(7) min effective time constant at 1.7 K. Intensity-intensity correlation plots are used to determine the relative line strengths of individual ortho- and para-propyne rovibrational transitions, enabling an independent estimation of the ground vibrational state effective A″ constant of propyne.Keywords:
Propyne
Matrix Isolation
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A detailed understanding of the complex rotation–vibration spectrum of the water molecule is vital for many areas of scientific and human activity, and thus, it is well studied in a number of spectral regions. To enhance our perception of the spectrum of the parent water isotopologue, H216O, a dataset of 270 745 non-redundant measured transitions is assembled, analyzed, and validated, yielding 19 204 rovibrational energy levels with statistically reliable uncertainties. The present study extends considerably an analysis of the rovibrational spectrum of H216O, published in 2013, by employing an improved methodology, considering about one-third more new observations (often with greatly decreased uncertainties), and using a highly accurate first-principles energy list for validation purposes. The database of experimental rovibrational transitions and empirical energy levels of H216O created during this study is called W2020. Some of the new transitions in W2020 allow the improved treatment of many parts of the dataset, especially considering the uncertainties of the experimental line positions and the empirical energy values. The W2020 dataset is examined to assess where measurements are still lacking even for this most thoroughly studied isotopologue of water, and to provide definitive energies for the lower and upper states of many yet-to-be-measured transitions. The W2020 dataset allows the evaluation of several previous compilations of spectroscopic data of water and the accuracy of previous effective Hamiltonian fits.
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ABSTRACT Starting from ab initio electronic structure data, we develop parametrized analytic potential energy surfaces for the HCN and HNC isomers by variationally calculating rovibrational energy levels and adjusting the potential parameters so as to get agreement with experimentally derived transition frequencies to within about 1 cm−1. We also determine an analytic expression in terms of molecular parameters to effortlessly calculate the rovibrational energy levels. We use the obtained empirical potentials to calculate rovibrational levels for eight isotopologues of HCN and eight of HNC up to about 4000 cm−1 above the ground state. The energy levels are estimated to be accurate to within about 3 cm−1 based on comparison to experimental rovibrational transition frequencies for H12C14N, H12C14N, H13C14N, and H12C15N. For all 16 isotopologues, we calculate the zero-point energy and in nine cases we can compare with experimentally derived values. In these comparisons, the variationally obtained ZPE is within 5 cm−1 of the experimentally derived value, while the closed expression gives values within 6 cm−1 of the experimental values. For all 16 isotopologues, we also give molecular parameters from which the energy levels can easily be calculated using the closed expression. Endo- and exoergicities are given for 12 isotopic exchange reactions involving HCN/HNC and some isotopologues together with pre-exponential factors that should be useful in future modelling studies of rare isotopologues.
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An exhaustive review of the measured rovibrational transitions of the 16O12C32S isotopologue of carbonyl sulphide is undertaken. A comprehensive analysis of data from 100 papers is carried out using a consistent set of harmonic oscillator quantum numbers which are recommended for future studies. A corrected compilation of 14,071 OCS transitions is then subjected to a Measured Active Rotational-Vibrational Energy Levels (MARVEL) analysis, resulting in 5729 empirical energy levels. The uncertainties corresponding to these levels are analysed using different procedures; the newly implemented bootstrap method is used to provide final uncertainties.
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This is the third of a series of articles reporting critically evaluated rotational– vibrational line positions, transition intensities, and energy levels, with associated critically reviewed labels and uncertainties, for all the main isotopologues of water. This paper presents experimental line positions, experimental-quality energy levels, and validated labels for rotational–vibrational transitions of the most abundant isotopologue of water, H2 16 O. The latest version of the MARVEL (Measured Active Rotational– Vibrational Energy Levels) line-inversion procedure is used to determine the rovibrational energy levels of the electronic ground state of H2 16
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Rovibrational quantum dynamical computations for deuterated isotopologues of the methane–water dimer
Rovibrational states of methane–water isotopologues are computed in a variational procedure and the wave functions are analyzed in terms of the rigid-rotor and coupled-rotors models.
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