Torsional vibrations and internal rotation are the one kind of large amplitude motion in polyatomic molecules and clusters. It is well known that the standard approach within the frame of harmonic approximation to calculating the frequencies of torsional vibrations is unworkable. In this case, one must calculate full potential energy surface (PES) while varying torsional coordinates throughout all intervals of their determination. Recently, when calculating PES, some authors took into account zero-point vibrational energy (ZPVE). Sometimes it leads to an improvement in the agreement between calculated and experimental data, and sometimes it rather worsens the agreement between them [1,2]. To obtain more complete information on the efficiency of taking into account ZPVE, we have made a calculation of energy of stationary torsional states for hydrogen peroxide and methyl alcohol molecules with and without taking into account ZPVE. It is well known that there is much more experimental data about the energy of excited torsional states for these molecules than for any others. The calculations were performed on several levels of theory. [1] S. Dalbouha, M.L. Senent, N. Komiha J.Chem.Phys., 142(7) 2015 074304. [2] G.A. Pitsevich, A.E. Malevich, U.V. Lazicki, U.U. Sapeshka Journal of the Belarusian State University. Physics. 2 2021 15.

Propylene oxide, CH₃C₂H₃O, is a stable chiral molecule that gained new attention through its recent radio astronomical discovery in the interstellar medium toward the galactic centerB.A. McGuire, P.B. Carroll, R.A. Loomis, I.A. Finneran, P.R. Jewell, A.J. Remijan, G., A. Blake, Science 352, 1449–1452 (2016) Subsequently, extensive laboratory data on rotational transitions in the ground state and in the lowest vibrationally excited ν₂₄ torsion state were publishedA.J. Mesko, L. Zou, P.B. Carroll, S.L. Widicus Weaver, J. Mol. Spectrosc. 335, 49–53 (2017)^,P. Stahl, B. E. Arenas, O. Zingsheim, M. Schnell, L. Margulès, R. A. Motiyenko, G. W. Fuchs, T. F. Giesen, J. Mol. Spectrosc. 378, 111445 (2021) Previously, only the 3 m spectral range of the four C−H stretching vibration modes was measured with high spectral resolution at mid−infrared wavelengthF.X. Sunahori, Z. Su, C. Kang, Y. Xu. Chem. Phys. Lett. 494, 14−20 (2010) In the present study we used two quantum cascade laser spectrometers at 8 and 10 m to record ro−vibrational spectra of the ν₁₇ fundamental mode (CH₂ rock) at 1023 cm⁻¹and the ν₁₂ ring breathing mode at 1266 cm⁻¹. The spectra were measured in a static cell at room temperature and in a supersonic jet expansion at low temperatures. The room temperature measurement allowed a quick assignment via graphical techniques (Loomis−Wood diagram) and determination of the molecular parameters using the SPFIT/SPCAT program packageH.M. Pickett, J. Mol. Spectrosc. 148, 371–377 (1991) In the supersonic jet spectrum line splittings could be observed for certain transitions. The combination of measurements at low temperature (30K) and at room temperature conditions led to an assignment of hundreds of transitions of the very dense infrared spectrum and covers quantum numbers from lowest J and K up to J = 55 and K_a P. Stahl, B. E. Arenas, O. Zingsheim, M. Schnell, L. Margulès, R. A. Motiyenko, G. W. Fuchs, T. F. Giesen, J. Mol. Spectrosc. 378, 111445 (2021).F.X. Sunahori, Z. Su, C. Kang, Y. Xu. Chem. Phys. Lett. 494, 14-20 (2010).H.M. Pickett, J. Mol. Spectrosc. 148, 371–377 (1991).

The characterization of the byproducts of biomass pyrolysis is an integral part in the development of viable biofuels and renewable energy sources. 4-Pyrone, (IUPAC name: 4-pyran-1-one) is one of the byproducts observed in the pyrolysis of many forms of biomass, such as wood chips, straw, and cotton husks. Using the technique of argon matrix-isolation FT-IR spectroscopy, the pyrolysis products of 4-pyrone were characterized by passing a diluted sample of 4-pyrone through a heated silicon carbide tube onto a cold window that captures the products and allows for their analysis via FT-IR spectroscopy. Computational analysis using Gaussian 09 was also utilized to model the unimolecular decomposition pathways, and these results were compared to the experimental spectra for product identification. Current data collected at pyrolysis temperatures ranging between 900 K and 1400 K indicate the formation of acetylene, vinylacetylene, propyne, and carbon monoxide. The formation of formylketene is also likely, as some peaks have been observed that match computational predictions.

Matrix-isolation spectroscopy is used to characterize the weakly-bound complex(es) of hydrogen cyanide with methyl chloride, two astrophysically relevant molecules. HCN and its polymers captivate interstellar discussions of prebiotic monomers and other life-bearing polymers, while CH₃Cl leads as the first organohalogen detected in space. This highlights the importance of studying their reactivity. In this talk, we will describe our new matrix-isolation instrument, constructed at the University of Maryland, and how we identify the structure of the weakly-bound complexes [(HCN)_nCH₃Cl] that form upon co-condensation of HCN and CH₃Cl in an argon matrix. Infrared spectroscopy is used in tandem with quantum chemistry calculations to characterize the vibrational spectrum of the resulting complexes. Our work reveals preferential formation of matrix-isolated HCN trimer species in the presence of CH₃Cl, qualitatively characterized by non-covalent interactions though natural bond orbital calculations. Finally, we will discuss the astrochemical implications of the resulting complexes and HCN trimer formation.

Gas-phase CH-stretching overtone bands of the two volatile organic compounds methyl acetate and ethyl acetate were detected for up to v = 6–0 by incoherent broad-band cavity-enhanced absorption spectroscopy. To obtain high sensitivity, short-pass and long-pass filters were used to cut background light out of a high reflection range of the dielectric multi-layer mirrors consisting of the cavity. Hence, the signal-to-noise ratios of v = 4–0 were achieved to be 40–50 by the 100s integration time, suggesting an application possibility of this spectroscopic technique for environmental monitoring. Profiles of the observed overtone bands were analyzed in detail with the aid of theoretical calculations and their prominent peaks were assigned to the progressions starting from the bundles of the symmetric CH-stretching bands starting at 2964 cm⁻¹. Based on the local-mode analysis, the harmonic frequencies and the anharmonicities were determined and the dissociation energies were derived.

Many molecular compounds of spectroscopic interest are difficult to produce or can only be produced in situ with low production rates, e.g., transient species. One way to produce such species is to use a discharge nozzle in combination with a supersonic jet expansion. The hereby rotationally cooled spectra increase the line intensity at low rotational quantum numbers and ease the detection of absorption features. Nevertheless, the detection of rare species remains difficult and requires an extremely sensitive detection scheme. The Cavity Ringdown (CRD) technique with its high sensitivity is ideally suited to address this kind of problem. In addition, CRD spectroscopy can also be used to detect very weak molecular rovibrational transitions of otherwise well-known stable molecules. While CRD spectroscopy of supersonic jets is no new idea, the application to wavelengths in the mid-IR (2.5μm - 4.5μm), where -O-H, -N-H and ≡ C-H stretching vibrations can be investigated, only got possible in recent times with the availability of suitable laser sources and highly reflective dielectric mirrors for this wavelength region.
Here, we report about our progress in building up a CRD spectrometer operating in the mid infrared range utilizing a tunable cw-OPO laser system with high-quality cavity mirrors (R > 99.99%) between 3μm - 3.4μm. First spectroscopic results will be presented.

Proton-coupled electron transfer (PCET) has been of great interest in chemical and biochemical catalysis. The electron transfer process in numerous biomimetic systems has been investigated in solution, but direct interrogation of the proton transfer coordinate remains largely unexplored. We have measured cryogenic ion vibrational spectra of a series of phenol-benzimidazole PCET model compounds to explore the nature of the strong OH-N H-bond in the ground electronic state. Highly redshifted and broadened H-bonded OH stretch transitions were observed throughout the model series. Isotopic substitution and anharmonic vibrational calculations suggest that the breadth arises from an interplay between strong OH stretch-bend Fermi resonance interactions and coupling of the OH stretch to low frequency H-bond soft-mode motions accessible at the zero-point level. The effects of steric hindrance, resonance stabilization and charge distribution are also investigated through systematic structural variation of the model compounds.

4-benzoylbenzoate (4BBA⁻, C₁₄H₉O₃) serves as a model system for the marine organic material in sea-spray aerosols (SSAs), which are highly heterogeneous and complex. SSAs are primarily composed of salt water, which affects the behavior of the marine organic material contained within it. Here, we investigate how addition of metal ions and solvent (H₂O, CH₃CN) modify the structure of 4BBA⁻ by use of cryogenic ion vibrational predissociation spectroscopy. Upon addition of Ca²⁺ to 4BBA⁻, we observe the collapse of the asymmetric and symmetric CO₂ stretching modes due to the bidentate complexation of Ca²⁺ to the carboxylate head group. Upon addition of high dielectric solvent (H₂O or CH₃CN) the vibrational modes are seen to slowly relax towards the vibrational modes of 4BBA⁻. This behavior is explained by electronic structure calculations, showing that the skeletal structure of 4BBA⁻ relaxes towards its original structure with increasing solvation.

The diverse tunability of gold nanoclusters via size, geometry, and ligand chemistry allows them to be optimized for greater catalytic activity, selectivity, and optoelectronic properties. The binding of a hydride to Au₉(PPh₃)₈³⁺ to form Au₉(PPh₃)₈H²⁺ has raised the question of whether the hydride behaves as a metal dopant which donates its two electrons to the Au core or whether it behaves as an electron-withdrawing ligand such as Cl⁻ and Br⁻. We previously showed significant similarities between its electronic absorption spectrum to that of Au₉(PPh₃)₈Cl²⁺ and Au₉(PPh₃)₈Br²⁺, but follow-up theoretical work suggested that this was a coincidence. Here we analyze the infrared absorption spectra of Au₉(PPh₃)₈H²⁺ with a single N₂ or H₂O molecule physiosorbed onto the cluster to further elucidate the role of the hydride in Au₉(PPh₃)₈H²⁺.

The electron impact ionization of helium droplets doped with ethylene molecules and clusters yields diverse C_XH_Y⁺ cations embedded in the droplets. The ionization primarily produces C₂H₂⁺, C₂H₃⁺, C₂H₄⁺, and CH₂⁺, whereas larger carbocations are produced upon the reactions of the primary ions with ethylene molecules. The vibrational excitation of the cations leads to the release of bare cations and cations with a few helium atoms attached. The laser excitation spectra of the embedded cations show well resolved vibrational bands with a few wavenumber widths—an order of magnitude less than those previously obtained in solid matrices or molecular beams by tagging techniques. Comparison with the previous studies of free and tagged CH₂⁺, CH₃⁺, C₂H₂⁺, C₂H₃⁺, and C₂H₄⁺ cations shows that the helium matrix typically introduces a shift in the vibrational frequencies of less than about 20 cm-1, enabling direct comparisons with the results of quantum chemical calculations for structure determination. This work demonstrates a facile technique for the production and spectroscopic study of diverse carbocations, which act as important intermediates in gas and condensed phases.