Journal of Molecular Spectroscopy最新文献

The potential-energy and quadrupole-moment functions of the H${}_2$ ground electronic state are well known in literature (Komasa et al., 2019; Wolniewicz et al., 1998), and the line list of the vibrational–rotational transitions was calculated (Roueff et al., 2019). In this paper, we analyze the calculated intensities in order to learn how the intensities will change when analytic quadrupole-moment functions fitted to the ab initio and experimental data are used instead of spline-interpolated functions. We found that the use of splines does not deteriorate the intensities and does not lead to nonphysical saturation, as in heavier molecules, owing to the high precision of the ab initio data and the high density of the grid. The accuracy of the calculated intensities is estimated up to high overtones. Extraction of new spectroscopic information from the observational data that supplements the laboratory measurements is performed. The laboratory and observational data do not help increase the quality of the analytic functions. Numerous anomalies resulting from the destructive interference are identified in the calculated line lists, some of them being situated within the recently observed spectral regions, 1.5–$2.5\mu m$. The intensities of these anomalies can be sensitive to the form of the molecular functions as well as to the proton-to-electron mass ratio. In this connection, the similar Le Roy anomalies (Brown and LeRoy, 1973; Le Roy and Vrscay, 1975) also arising due to the destructive interference in the Lyman and Werner systems are discussed.

The microwave spectra for a hydrogen-bonded trans-2 glyoxylic acid–water complex were measured in the 6–16 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants for the dimer were determined to have the following values: A = 9384.2354(31), B = 1707.63973(73), and C = 1447.44879(56) MHz. The hydrogen bonded structures and rotational constants were calculated for the lowest energy dimglyoxylic acid - water using DFT, MP2 and CCSD calculations with various basis sets. The B3LYP/aug-cc-PVQZ-DG3 calculations yielded rotational constants of A = 9393.59, B = 1713.76, and C = 1453.23 MHz, in very good agreement with experimental values. The calculations show two feasible tunneling motions involving hydrogen atoms in this complex.

Fourier analysis of the kinetic and potential interactions of non-coaxial internal tops in the molecules C₆H₄(OH)₂, HO(CH₂)OH, HOOOH, HOSOH, HSSSH, and HSOSH was carried out. It was found that for the all six molecules, in which both tops are characterized by a period 2 $\pi$, the harmonics with a period of $\pi$ are dominate, with its contribution to the Fourier representation of potential surfaces increasing in the sequence mentioned above. Contributions of internal tops interactions to the kinetic and potential energies were determined for analyzed molecules. The influence of potential and kinetic interactions of internal tops on 1) the features of tunneling splitting of the ground and excited torsional states of molecules, 2) the landscape of 2D potential energy surfaces, 3) frequencies of fundamental torsional vibrations of hydroxyl and thiol groups, 4) the ratio of trans - and cis - conformer energies is also analyzed.

A combined rotational spectroscopy, thermochemistry, and kinetic modeling study explores the mechanism of acetaldehyde (ethanal), vinyl alcohol (ethenol), and ethylene oxide (oxirane) formation in the oxidation of ethylene (ethene). Multiplexed quantitative detection of oxidation intermediates and products exiting a SiO₂/SiC microreactor at 1700 K is demonstrated with the BrightSpec W-band chirped-pulse Fourier transform millimeter-wave spectrometer. The broadband rotational spectrum contains transitions of formaldehyde (CH₂O), methoxy (CH₃O), methanol (CH₃OH), ketene (CH₂CO), acetaldehyde (CH₃CHO), syn-vinyl alcohol (syn-CH₂CHOH), anti-vinyl alcohol (anti-CH₂CHOH), oxirane (c-CH₂OCH₂), propyne (CH₃CCH), and syn-propanal (syn-CH₃CH₂CHO). We focus on the three C₂H₄O species and deduce their concentration ratio [CH₃CHO]:[CH₂CHOH]:[c-CH₂OCH₂] = 1:0.7(2):0.06(2) by comparing the observed line intensities to those simulated with the PGOPHER software. Detailed thermochemistry of the C₂H₄O isomers and conformers is provided by Active Thermochemical Tables. The observed excess concentrations of vinyl alcohol and oxirane relative to the more stable acetaldehyde compared to the equilibrium ratio at 1700 K (1:0.087:0.000024), point to direct chemical pathways to these higher energy isomers. The mechanism for the formation of the three C₂H₄O isomers is analyzed using a 0-D homogenous reactor kinetics simulation for ethylene oxidation. The ratios of the C₂H₄O isomers concentrations predicted by the kinetic model are compared to the experimental values.

A careful, principally experimental Raman study which puts the focus on the ${\nu }_5$ bending vibrational band by gaseous room-temperature sulfur hexafluoride is reported. In essence, our results boil down in the following two findings. Firstly, the anisotropic Raman-allowed spectrum by that band, which for the first time was isolated from the recordings and calibrated on an absolute scale. Secondly, an absolute-calibrated binary collision-induced component, which was also for the first time extracted from the crude measurements in the wavenumber range 475–570 cm⁻¹. Both spectra are depolarized and lead to parameters in agreement with the existing literature. The reported findings bring us one step closer to understanding how vibration mechanisms work in S$F_6$ and show among other things that, at the ${\nu }_5$ frequency, collisions between molecules enhance light scattering by 0.8% at 1 atm and by 13% at 15 atm of sulfur hexafluoride gas pressure.

The millimeter-wave rotational spectrum of 2-cyanopyridine was collected from 130 to 750 GHz, and the ground and two lowest-energy excited vibrational states were analyzed. In total, over 20,000 rotational transitions were least-squares fit for the three vibrational states to partial-octic, distorted-rotor Hamiltonians with low error (σ_fit < 50 kHz). For the ground state, the many thousands of newly measured rotational transitions enabled substantial refinement of the rotational constants and determination of the centrifugal distortion constants. The rotational spectrum was collected at room temperature, permitting the observation of the two lowest-energy fundamental modes, ν₃₀ (A″, 154 cm⁻¹) and ν₂₁ (A′, 175 cm⁻¹), and determination of their spectroscopic constants. The two excited vibrational states are Coriolis coupled and require a two-state Hamiltonian. Eight Coriolis-coupling parameters ($G_a,G_a^J,G_a^K,G_a^{\mathrm{JJ}},F_{\mathrm{bc}},F_{\mathrm{bc}}^J,G_b,$and $G_b^J$) have been determined, as well as a precise energy difference of 26.524 312 6 (40) cm⁻¹ between the vibrational states. A comparison of the ground-state spectroscopic constants, as well as the Coriolis coupling-related parameters of analogous dyads is presented for multiple cyanoarenes.

Semi-empirical methods offer a cost-effective means of computing explicit, anharmonic vibrational frequencies for large molecules, such as polycyclic aromatic hydrocarbons (PAHs), but their default parameters produce insufficiently accurate results for comparison to experiment, especially in the hydride stretching region where the NIRSpec instrument on JWST is most effective. This work delivers several reparameterized variants of the PM6 semi-empirical method trained to reproduce the experimental vibrational frequencies of 5 small hydrocarbon molecules. When benchmarked on the experimental frequencies of benzene and naphthalene, these reparameterized methods match the empirical values to within 38.7 cm${}^{-1}$ on average. Combining these values with the default PM6 frequencies below 3000 cm${}^{-1}$ brings the average deviation below 22 cm${}^{-1}$ for naphthalene, comparing favorably with the existing state of the art in B3LYP/4-31G, for a two order of magnitude decrease in the computational cost. As such, the present work offers a promising means of extending the computation of explicit, anharmonic vibrational frequencies to PAHs larger than those that can be examined anharmonically via B3LYP. Such large and accurate data sets are necessary to disentangle the unidentified spectral features observed around myriad astronomical bodies and the influx of observational data from JWST.

The carbon-rich molecular ions C₃H^＋ and HC₃O^＋ have been studied for the first time at high spectral resolution in the $5\mu m$ wavelength regime employing leak-out spectroscopy of their ${\nu }_2$ (C₃H^＋) and ${\nu }_3$ (HC₃O^＋) vibrational fundamentals. All measurements were performed in a newly built cryogenic 22-pole ion trap apparatus, the COLtrap II instrument. Besides the action spectroscopic characterization the new measurements have also been used towards optimization and an improved understanding of the leak-out process via a semi-quantitative kinetics model for which some rates are determined or estimated and rationalized.