Journal of Molecular Spectroscopy最新文献

We produced aniline cations ($c$–C₆H₅ NH₂⁺) and their dehydro- (H-loss) cations inside helium droplets by electron ionization and observed their mass-selected vibrational spectra in the NH stretching region. We observed vibrational bands at m/q = 93 which were identified as aniline cations. These bands showed only a few cm⁻¹ shifts from the symmetric and anti-symmetric NH stretching bands for the amino (NH₂) group in the gas phase. For the H-loss cation at mass m/q = 92, the agreement of the observed NH band frequency with the result of DFT calculations suggests several candidate species, including a seven-membered ring structure, 4-monodehydro azepinylium ($c$–C₆NH₆⁺). A new reaction pathway to this cation was discussed by considering large excess energy in the ionization process.

Perfluoro-n-propane, perfluorocyclobutane, perfluoro-n-butane, and perfluoro-n-pentane are non-ozone-depleting industrial alternatives to chlorofluorocarbons and hydrochlorofluorocarbons. However, these perfluoroalkanes have significant band strength in the atmospheric window from 800 to 1200 cm⁻¹. Coupled with their millennial-scale atmospheric lifetimes, they can lead to significant long-term global warming. Infrared spectra are required to quantify the climate impacts. This work provides a set of high-temperature infrared absorption cross-sections in the range 298–350 K at 0.1 cm⁻¹ resolution from 515 to 1500 cm⁻¹ for each compound. Our cross-sections generally agree with literature measurements except for perfluoro-n-pentane. We use density functional theory to calculate the absorption cross-sections from 0 to 515 cm⁻¹ using the B3LYP functional and several basis sets. The 6-31G(d,p) basis set provides the best results for linear perfluoroalkanes, while the def2-TZVP basis set provides the best results for cyclic perfluoroalkanes. Using experimental cross-sections, we calculate the radiative efficiency and global warming potential for each compound, utilizing the Pinnock curve from Shine and Myhre (2020) and atmospheric lifetimes from Hodnebrog et al. (2020). These quantities are found to be independent of temperature. The average 100-year global warming potential derived from all cross-sections is 9,610±1,260, 10,800±1,420, 10,100±1,330, and 9,380±1,230 for perfluoro-n-propane, perfluorocyclobutane, perfluoro-n-butane, and perfluoro-n-pentane, respectively. Combining the data in this work with our previous measurements reveals that the global warming potential for perfluoroalkanes with an increasing number of CF bonds depends on the ratio of radiative efficiency to molecular weight.

The rotational spectra of two new silicon-bearing carbon chains, H₂C₃Si and HSiCCH have been detected by means of Fourier-transform microwave spectroscopy in a supersonic jet source equipped with an electrical discharge. On the basis of measurements between 10 and 42 GHz, precise spectroscopic constants have been determined for both molecules. Along with H²⁹SiCCH and H³⁰SiCCH, which were detected in natural abundance, several other rare isotopic species of HSiCCH were observed using samples enriched in carbon-13 and D. From this isotopic data, a precise semi-experimental equilibrium structure was derived and compared with a re-evaluated semi-experimental equilibrium structure of the parent silylene SiH₂; the deuterium isotopic measurements also provide important clues as to the formation pathway of HSiCCH in our discharge. Because other small Si-bearing chains have been detected in the circumstellar envelope of the evolved carbon star IRC+10216, both chains may be of astronomical interest.

The vibrational dynamics of diborane have been extensively studied both theoretically and experimentally ever since the bridge structure of diborane was established in the 1950s. Numerous infrared and several Raman spectroscopic studies have followed in the ensuing years at ever increasing levels of spectral resolution. In parallel, ab initio computations of the underlying potential energy surface have progressed as well as the methods to calculate the anharmonic vibration dynamics beyond the double harmonic approximation. Nevertheless, even 70 years after the bridge structure of diborane was established, there are still significant discrepancies between experiment and theory for the fundamental vibrational frequencies of diborane. In this work we use parahydrogen (pH₂) matrix isolation infrared spectroscopy to characterize six fundamental vibrations of B₂H₆ and B₂D₆ and compare them with results from configuration-selective vibrational configuration interaction theory. The calculated frequencies and intensities are in very good agreement with the pH₂ matrix isolation spectra, even several combination bands are well reproduced. We believe that the reason discrepancies have existed for so long is related to the large amount of anharmonicity that is associated with the bridge BH stretching modes. However, the calculated frequencies and intensities reported here for the vibrational modes of all three boron isotopologues of B₂H₆ and B₂D₆ are within ± 2.00 cm⁻¹ and ± 1.44 cm⁻¹, respectively, of the experimental frequencies and therefore a refined vibrational assignment of diborane has been achieved.

New analysis and spectra are reported for the gas-phase ammonia-formic acid complex. Calculations to determine the theoretical barrier to internal rotation were conducted and led to the new internal rotation analysis of the dimer. Using the new analysis and calculations, 12 new lines were measured and assigned and included in the present analysis. This is the first internal rotation analysis for this complex. The measurements were made in the 7–22 GHz range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The complex was analyzed as a hindered rotor and 20 A and 16 E state transitions were fit with the XIAM5 program. The rotational constants were determined to have the following values: A = 11970.19(9) MHz, B = 4331.479(4) MHz, and C = 3227.144(4) MHz. Rotational constants, quadrupole coupling constants, and internal rotor parameters were fit to the spectrum. Double resonance was used to verify line assignments and access higher frequencies. The barrier to internal rotation was found to be 195.18(7) cm⁻¹. High level calculations are in good agreement with experimental values. The calculated V₃ barrier values range from 168.3 to 212.8 cm⁻¹.

In this experimental investigation, collision broadening effects on the Q(J)-branch vibration-inversion-rotation spectral lines of ammonia (NH₃) within the fundamental ${\nu }_4$ asymmetric bending vibrational band in the 6.2 µm mid-IR region are reported. A continuous-wave external-cavity quantum cascade laser coupled with a high-sensitive cavity ring-down spectrometer was employed to selectively probe 10 transitions in high-resolution, including few inversion doublets of gaseous NH₃. Pressure-broadening coefficients, ${\gamma }_{N{H}_3-X_i}$ ($X_i$ = He, Ar, N_2, O_2, zero-air) in cm⁻¹ atm⁻¹, characterizing the collision interaction between NH₃ and various external perturbing gases, including helium, argon, nitrogen, oxygen, and zero-air, were determined at room temperature (296 K). Mean collision times and optical collision diameters of each collision partner were explored to gain deeper insight into the perturber-induced collision dynamics. This investigation elucidates the intricate intermolecular interactions and collision phenomena induced by various foreign perturbers with NH₃, with potential implications for future spectroscopic and atmospheric research of this polyatomic molecule.

Laser Induced Fluorescence (LIF) has been used to study the spectroscopy of ruthenium monofluoride (RuF) in the UNB laser-ablation molecular-jet apparatus. High-resolution spectra of three band systems have been obtained, with a linewidth (FWHM) of ∼0.004 cm⁻¹, in the near infrared (NIR), green and blue regions at ∼ 760 nm, 548 nm and 450 nm respectively. Electronic states have been assigned based on observation of first lines in the R and P branches and the excited states are labelled A, B and C in increasing energy order. Three transitions have been assigned to the green system as spin–orbit components, B⁴Γ_5.5 – X⁴Φ_4.5 and B⁴Γ_4.5 – X⁴Φ_3.5 of the (0,0) band, and B⁴Γ_5.5 – X⁴Φ_4.5 of the (1,1) band of the B – X system. The single NIR and blue bands were assigned as A⁴Δ_3.5 - X⁴Φ_4.5 (0,0) and C⁴Δ_3.5 - X⁴Φ_4.5 (0,0) respectively. Rotational structure of four individual isotopologues ⁹⁶RuF, ¹⁰⁰RuF, ¹⁰²RuF and ¹⁰⁴RuF was well resolved and used to examine the rotational and vibrational isotope effects. The rotational lines were doubled by hyperfine structure due to the nuclear spin I = ½ of fluorine. The hyperfine structure due to the nuclear spin I = 5/2 of ⁹⁹Ru and ¹⁰¹Ru was not resolved resulting in weak broadened ⁹⁹RuF and ¹⁰¹RuF lines that were overlapped by the stronger lines of the other isotopologues and could only be assigned in the C⁴Δ_3.5 - X⁴Φ_4.5 (0,0) band. The fluorine hyperfine structure was used to estimate the fraction of F 2sσ and 2pπ in the Ru-centered σ and π molecular orbitals.

We observed ten pseudo a-type transitions in the E state of N-ethylacetamide (NEAA) using a Fourier transform microwave spectrometer. We found triplets caused by the coupling between the two methyl groups for the observed pseudo a-type transitions. Also, we succeeded in observing similar triplets for the 5 a-type Q-branch transitions, 20 b-type and 7 c-type transitions of the E state of NEAA. These observed triplets allowed us to determinate the potential barrier V₃ to internal rotation of the ethyl-methyl group to be 1078 (12) cm⁻¹. In addition, though the observed intensities of the c-type transitions in the A state were weak, the c-type transition in the E state were observed to be as intense as the b-type transitions of the E state. We explained in this paper the abnormal intensities of the c-type transitions and pseudo a-type transitions in the E state of NEAA.

In this study, the microwave spectra of 2-, 3-, and 4-picolyamine were recorded in a pulsed supersonic jet. With the aid of Natural Bond Orbital analysis, NCIplot, and Multiwfn, the structural effects of substitution were explored and then quantified by comparing semi-experimental geometries. The conformational landscape of each molecule is dominated by Van der Waals interactions between the amino group and the pyridine ring. Spectra of ²H, ¹³C, and ¹⁵N were also collected and analyzed.

Cantilever-enhanced Photoacoustic Spectroscopy in combination with Wavelength Modulation Spectroscopy was used for the multicomponent analysis of gases. Distributed feedback Quantum Cascade Laser tunable in the region of ∼ 1045 – 1048 cm⁻¹ was used as the excitation source. Principal component analysis (PCA) and Partial Least Squares (PLS) (the pls package in the statistical software R) methods were applied to a mixture of selected substances (methanol and ethanol) in the gas phase to both simulated (www.spectraplot.com) and experimentally acquired spectra, PCA predicting the number of substances and PLS its concentrations.