Journal of Molecular Spectroscopy最新文献

We present the database of spectroscopic constants of diatomic molecules (DSCDM), a website dedicated to the spectroscopic constants of ground and excited states of diatomics (https://dscdm.physics.stonybrook.edu). The database can be improved based on community feedback: the community as contributors may upload new data. Additionally, it counts on an Application Programming Interface (API) for molecular spectroscopy data retrieval to make the search process more efficient. On the other hand, the website presents a machine learning predictor of spectroscopic constants based on atomic properties and a new plotting tool to study the statistical distribution of spectroscopic constants.

A collection of 6759 pure-rotational and vibrational–rotational spectroscopic line positions available for seven isotopologues of the ground $X{}^1{\Sigma }^{+}$ electronic state of carbon monosulfide has been employed in a weighted nonlinear least-squares direct fit to the potential energy function of ¹²C³²S and supplementary functions describing breakdown of the Born-Oppenheimer approximation. All radial functions are represented by compact analytical models having proper theoretical asymptotic behavior. The spectroscopic line positions are reproduced on average to within the associated experimental uncertainties by the quantum–mechanical eigenvalues of the derived Hamiltonians for individual isotopologues. The potential energy function is constrained to obey the theoretical radial behavior both at short-range and at long-range. Accurate quantum–mechanical vibrational term values and rotational and centrifugal distortion constants are provided for all stable isotopologues of CS included in the least-squares fits.

A comprehensive assessment of available literature spectroscopic data and molecular constants for the X ¹Σ⁺ ground electronic states of isotopologues ³⁵ClF and ³⁷ClF is undertaken. Three different approaches are employed in the analysis of the available information. The first approach involves merging molecular constants from various studies to yield an optimized set of Dunham coefficients {Y₀₁, Y₀₂, Y₀₃, Y₁₀, Y₁₁, Y₂₁, Y₀₂ and Y₁₂}. Utilizing these updated constants, vibrational energies (G_υ) and rotational constants (B_υ) for υ = 0–9 are calculated, and RKR potentials are determined for both isotopologues. The second approach involves calculating synthetic spectroscopic line positions using literature molecular constants, with normally distributed random errors added on, and subjecting these to a modern direct-potential-fit analysis. This analysis produces a precise analytical potential energy function for ³⁵ClF, and Born-Oppenheimer breakdown functions that characterize adiabatic and non-adiabatic corrections. The third approach involves fitting the synthetic spectroscopic data directly to Dunham coefficients. The results of the three approaches are compared and discussed.

The microwave spectrum of the sesamol (CH₂O₂C₆H₃OH) monomer has been collected using CP-FTMW spectroscopy in the 5.5–18.75 GHz region of the electromagnetic spectrum. Density functional theory calculations of the monomer have been performed. B3LYP|cc-pVTZ predicts the global minimum geometry to be s-trans-sesamol where the hydroxyl group is oriented ${180}^{°}$ away from the dioxole ring. A second, local minimum, geometry for s-cis-sesamol was calculated to have the hydroxyl group oriented $0^{°}$ towards the dioxole ring, and the zero-point energy difference between the two geometries is predicted to be 1.4 kJ/mol. Both unique conformers have been identified in the experimental spectrum. In accordance with the predicted dipole moments, only b-type transitions have been observed for s-trans-sesamol, while a- and b-type transitions have been assigned to s-cis-sesamol. The relative intensities in the observed spectrum indicate that the molecular beam contains a higher population of the s-trans-sesamol monomer. Satellite transitions were observed for both sesamol conformers. Treating the parent and satellite transitions as coupled vibrational states, the energy differences and Coriolis coupling constants were determined for each conformer. The magnitude of the coupling constants, the energy difference between the states, and a comparison to a similar state observed in 1,3-benzodioxole indicate that the excited states correspond with CH₂ puckering of the dioxiole ring. The second moments of inertia and their contributions from the ring puckering have been analyzed for a series of functionalized molecules including piperonal, 1,3-benzodioxole and sesamol.

This work aims at theoretical consideration of weak effects, some of which are well-known in molecular spectroscopy, and some are not yet fully elaborated. A cohesive element of our consideration consists in the use of the original perturbative approach, which significantly facilitates the derivation of the sought expressions and their employment for applications. We consider several examples of perturbative problems concerning permitted or forbidden electric dipole and permitted magnetic-dipole transitions. The suggested approach can be generalized to other effects involving electronic, spin, and higher-order perturbations in molecular spectra.

We report our combined theoretical and spectroscopic studies on 3,5-difluorobenzoic acid. Using a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer, we recorded and analyzed the rotational spectrum spanning the frequency range of 6 – 12.5 GHz. Quantum chemical calculations were employed to analyze the conformational changes and landscapes of 3,5-difluorobenzoic acid. These calculations focused on studying the potential energy surfaces along the CCCO and OCOH dihedral angles at the B3LYP/6-311G level. Based on the computational results, we identified the global minimum conformer 1 as well as the local minimum conformer 2. We discussed and interpreted the geometric structures of the relevant conformations, with a particular emphasis on the interactions between the carboxylic group and the substituted fluorine atoms. Furthermore, these findings were compared to benzoic acid in internal strains. In our spectral analysis, we successfully identified conformer 1 and its seven ¹³C singly substituted isotopologues. We derived highly accurate rotational constants for 3,5-difluorobenzoic acid, displaying good agreement with computational results. We established the effective structure of its ground vibrational state using Kraitchman’s equations. Similar to benzoic acid, the global minimum conformation of 3,5-difluorobenzoic acid adopts a planar structure, corroborating our computational outcomes.

The study starts with a detailed analysis of all the high-resolution rovibrational transitions measured for the minor hypochlorous acid isotopologue H¹⁶O³⁷Cl, employing the Marvel (Measured Active Rotational-Vibrational Energy Levels) procedure. The survey considers 16 sources, the wavenumber coverage of the transitions extends from the microwave to the visible region, corresponding to 13 vibrational parent states. Experimental transitions data from older (1994, 1996, and 2000) sources, not published explicitly in the original papers, are presented in here for the first time; the newly reported transitions represent about one third of all the transitions assigned for H¹⁶O³⁷Cl. Then a minor update to the transitions and levels of H¹⁶O³⁵Cl published in J. Mol. Spectrosc. 384 (2022) 111561 is provided. Proper labelling and internal consistency of the transitions and the empirical rovibrational energies of H¹⁶O³⁵Cl and H¹⁶O³⁷Cl is enforced utilizing effective Hamiltonian (EH) and first-principles variational nuclear-motion computation results. An iteratively updated line list, based on empirical and EH rovibrational energy levels and first-principles intensities, is used to assign a significant number of new transitions in the $2{\nu }_2$ band (2300–2800 cm⁻¹) for both isotopologues. At the end, for H¹⁶O³⁵Cl/H¹⁶O³⁷Cl, from the total of 20 349/10 266 experimentally measured transitions considered, 20 119/10 124 could be validated, allowing the derivation of 5760/3933 empirical rovibrational energy levels with well-defined uncertainties. Improved parameters for an effective Hamiltonian corresponding to the $2{\nu }_2$ state are given. A comparison with data in the canonical spectroscopic line-by-line database HITRAN is also provided.

The microwave spectrum of n-butyl nitrate was recorded in the 5 to 20 GHz frequency range using broadband chirp and narrowband pulse excitation molecular jet Fourier transform microwave spectrometers. A quantum chemistry structural analysis yielded thirteen stable conformers. Among them, the five most energetically stable conformers were observed in the experimental spectra. The most stable conformer features a butyl chain with an anti-gauche-anti conformation (AGA) where the γ-carbon atom is about 64° out of the nitrate plane. For this conformer, spectra of all ¹³C and ¹⁵N minor isotopologues could be measured. The conformer with a straight butyl chain (AAA), and three other conformers (GAA, GGA, and AGG) were also observed. Accurate rotational constants, centrifugal distortion constants, and ¹⁴N nuclear quadrupole coupling constants could be deduced and compared to the theoretical values.

Perfluoro-n-hexane and perfluoro-2-methylpentane are fully fluorinated alkanes used as non-ozone-depleting alternatives to chlorofluorocarbons and hydrochlorofluorocarbons. These compounds are long-lived and potent greenhouse gases due to their strong CF bonds and infrared absorption in the atmospheric window. Infrared absorption cross-sections are required to quantify the climate impact of these compounds via the radiative efficiency and global warming potential. To our knowledge, there are only two experimental measurements for perfluoro-n-hexane, and there are no experimental measurements for perfluoro-2-methylpentane in the infrared. In this work, we provide a set of absorption cross-sections in the range 515–1500 cm⁻¹, at 0.1 cm⁻¹ resolution from 298 to 350 K for each compound. We calculate the absorption cross-section between 0 and 515 cm⁻¹ using density functional theory with various basis sets. The 6-31,G(d,p) basis set with the B3LYP functional is found to provide the best results. Using both measurements and calculations combined, we calculate the radiative efficiency and global warming potential for each compound. No significant temperature dependence was observed in these quantities. The average radiative efficiency derived from all cross-sections is 0.48 ± 0.06 W m⁻² ppbv⁻¹ for perfluoro-n-hexane and 0.46 ± 0.06 W m⁻² ppbv⁻¹ for perfluoro-2-methylpentane. The average 100-year global warming potential derived from all cross-sections is 9590 ± 1260 for perfluoro-n-hexane and 9220 ± 1210 for perfluoro-2-methylpentane.