Journal of Molecular Spectroscopy最新文献

The microwave spectrum of 1-chloromethyl-1-fluorosilacyclopentane has been recorded for the first time using the chirped pulse, Fourier transform microwave technique. Quantum chemical calculations show the two lowest energy conformers as being a twist-trans and a gauche form with the gauche form previously being shown as having two separate conformations, a gauche+ (lower in energy) and gauche- (higher in energy) form. Analysis of the spectrum provided the observation of the twist-trans conformer only, with 253 and 85 transitions being assigned to the ³⁵Cl and ³⁷Cl isotopologues, respectively. $R$-branch, $a$- and $b$-type transitions were observed. The spectrum was fit to a Watson S-reduced Hamiltonian and consisted of rotational constants, quartic centrifugal distortion constants, and nuclear quadrupole coupling constants, including the determination of the off-diagonal nuclear quadrupole coupling constant, ${\chi }_{ab}$. Interpretation of the structure was provided using second moments and is found to have a similar ring structure to other known silacyclopentanes. Analysis of the ${\chi }_{zz}$ has been carried out and compared to other similar molecules. An investigation of the known quantum chemical energies of the gauche conformer reveals that the reported B3LYP energies do not align with the observed microwave results.

Laser-based experiments facilitate numerous strategies for interrogating the complex non-adiabatic dynamics operating in the excited states of molecules. Measurements may be broadly separated into frequency- and time-resolved variants, with a combination of different approaches (with different associated observables) typically being required to reveal a complete mechanistic picture. In the former category, quantum state-resolved information may often be obtained using narrow linewidth lasers. This provides detailed information relating to the starting point on the photochemical reaction coordinate (via the absorption spectrum) and the asymptotic endpoints (i.e. the photoproducts). Direct observation of the intermediate pathways connecting these two limits is often not possible, however, due to the inherently long temporal duration of the laser pulses relative to the typical timescales of non-adiabatic energy redistribution processes. It is therefore desirable to obtain complementary information that monitors real-time evolution along the reaction coordinate as excited state population traverses the potential energy landscape. This may be achieved in time-resolved pump–probe experiments conducted using ultrafast (i.e. sub-picosecond) laser pulses. The use of valence state photoionization for the probe step is a commonly employed methodology and has proved highly instructive in revealing subtle mechanistic details of key energy redistribution pathways operating in many different molecular systems. One frequent limitation here, however, is the restricted “view” along the reaction coordinate(s) connecting the initially prepared excited states to various photoproducts. Guided by examples drawn from recent work using time-resolved photoelectron imaging, this review will discuss such issues in detail and highlight some strategies that potentially help overcome them – with particular emphasis placed on the advantages of projecting as deeply as possible into the ionization continuum. The role of complementary measurements using other spectroscopic techniques and the importance of high-level supporting theory to guide data interpretation will also be reinforced.

Jet-cooled rovibrational absorption spectra of SO₂–H₂O and SO₂–D₂O complexes have been measured in the v₁ symmetric stretching region of SO₂ around 1151 cm⁻¹ using a segmented rapid-scan quantum cascade laser spectrometer. The rovibrational spectrum of SO₂–D₂O has also been measured in the v₂ bending region of D₂O around 1178 cm⁻¹. The tunneling splittings caused by the internal rotation of the water unit could not be resolved under our experimental conditions. The observed rovibrational transitions for these two complexes are analyzed using a standard A-reduced Watson asymmetric top Hamiltonian, yielding precise molecular constants for the excited vibrational states. The experimental band-origins are 1156.713866(70) cm⁻¹ for SO₂–H₂O and 1156.819797(66) cm⁻¹ for SO₂–D₂O in the v₁ region of SO₂, and 1180.177901(58) cm⁻¹ for SO₂–D₂O in the v₂ region of D₂O.

It has been shown previously (P. R. Bunker and Per Jensen, J. Quant. Spectrosc. Radiat. Transf., 243 (2020) 106835) that if we choose angles to have dimension, we have to distinguish between the Planck constant $h$, having the dimension of $\mathrm{action}{\mathrm{angle}}^{-1}$, and the Planck constant of action $h_A$, having the dimension of $\mathrm{action}$. In the present paper, we show that a further implication that results from choosing angles to have dimension is that the Kibble balance equation relating the mass weighed to the Planck constant has to involve both of the distinct fundamental constants $h$ and $h_A$. We derive that new equation here and show how it compares to the equation that is obtained if one chooses angles to be dimensionless as required in SI.

The rotational spectrum of glycolic acid has been extended to frequencies from 318 to 1000 GHz. Analysis of this spectrum builds upon previous experimental and theoretical studies performed on glycolic acid conformers. Extensive new assignments were made for the ground vibrational state of the $SSC$ conformer of glycolic acid in addition to the two lowest vibrational states. Assignments were also expanded for the $AAT$ conformer of glycolic acid. This work improves the precision of some rotational constants and nearly all of the centrifugal distortion constants for each conformer and vibrational state. The spectral predictions based on these refined parameters will facilitate the search for glycolic acid in the interstellar medium at frequencies covered by submillimeter observatories.

The gas-phase doubly hydrogen-bonded glyoxylic acid - formic acid hydrogen-bonded complex was obtained by mixing a heated sample of glyoxylic acid with room-temperature formic acid in argon. High-level DFT and MP2 calculations with various basis sets were performed and the structures and rotational constants were determined for the lowest energy dimers of glyoxylic acid - formic acid. The microwave spectrum was measured in the 6-12 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants were determined to have the following values: A = 5533.911(3), B = 923.3883 (5), and C = 792.1132(6) MHz using 18 transitions. B3LYP/ cc-pVQZ calculations for the lowest energy dimer yielded calculated rotational constants of A = 5551.2, B = 922.9 and C = 791.3 MHz.

The microwave spectrum of a second doubly hydrogen-bonded glyoxylic acid - formic acid hydrogen-bonded complex was measured in the 6–12 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants were determined to have the following values: A = 4113.081(4), B = 1026.9957(5), and C = 822.9584(6) MHz. The doubly hydrogen bonded structures and rotational constants were calculated for the lowest energy dimers of glyoxylic acid - formic acid using DFT and MP2 calculations with various basis sets. M11/ def2QZVPP calculations for the third lowest energy dimer yielded rotational constants of A = 4248.7, C = 1031.4, and C = 829.9 MHz, in good agreement with experimental values.

The rotational spectra of [³⁵Cl]- and [³⁷Cl]-chlorobenzene (C₆H₅Cl) have been studied from 2 to 18 GHz and 130–360 GHz, resulting in the measurement, assignment, and least-squares fitting of almost 40,000 transitions of twenty-one vibrational states. Previously measured [³⁵Cl]- and [³⁷Cl]-chlorobenzene ground-state transitions were combined with newly measured transitions and fit to A-reduced, partial sextic Hamiltonian models with low-error (σ_{total fit} <0.05 MHz). Analysis of the 2–18 GHz spectrum allowed for refinement of the nuclear hyperfine coupling constants for the ground-state spectra of both isotopologues, while measurement of the 130–360 GHz spectrum provided sufficient information to determine the sextic centrifugal distortion constants of the ground states for the first time. From these millimeter-wave data collected at room temperature, the spectroscopic constants for the lowest-energy fundamentals of [³⁵Cl]-chlorobenzene (ν₂₀, ν₃₀, ν₁₁, ν₁₄, ν₁₉, ν₂₉ and ν₁₈) and [³⁷Cl]-chlorobenzene (ν₂₀, ν₃₀, ν₁₁) were determined. As with previous studies of chloroarenes, the computed (B3LYP/6–311+G(2d,p)) spectroscopic constants show quite close agreement with the experimentally determined values.

The microwave spectra of (Z)-1-chloro-3,3,3-trifluoropropene and its gas-phase heterodimer with the argon atom in the 5.6 to 18.1 GHz frequency range are first observed and assigned using broadband, chirped pulse, Fourier transform microwave (FTMW) spectroscopy. Subsequent analysis of higher-resolution spectra obtained between 5 and 21 GHz with a narrowband, Balle-Flygare FTMW instrument provides spectroscopic constants, including nuclear quadrupole coupling constants, for five isotopologues of the propene molecule and two isotopologues of the complex with argon. Structural parameters obtained from these spectra show the existence of an intramolecular hydrogen bond between one of the fluorine atoms of the trifluoromethyl group and the hydrogen atom on the adjacent carbon atom. No evidence is seen for internal rotation of the trifluoromethyl group. The location of the argon atom in the heterodimer is consistent with the expectation that it will be positioned so to interact with the greatest number of heavy atoms, and in particular, the polarizable chlorine atom.