The quasi-free rotation of the small molecule in inert matrices at low temperatures attracted researchers’ attention for a long time. The nature of the changing “effective” rotational constants of these molecules in different inert matrices is still rather unclear. Recently, we analyzed intensity redistribution of the vibration- rotational IR bands of the water molecule in the Ar matrix under the influence of the temperature variation in the ν₂ [1] and ν₁, ν₃ [2] spectral regions. In doing so, we have used literature experimental data. This time we performed the experimental study of the water molecule rotation in the Xe matrix at different temperatures and concentrations. The FTIR spectra were obtained in the ν₂ spectral region immediately after matrix formation and after a few hours of estimation processes of the spin relaxation. Based on the experimental data, the “effective” rotational constants in the ground and exited ν₂ vibrational states of water molecule in Xe matrix were found. The relative intensities of the components of the vibration-rotational ν₂ band were calculated at different temperatures and different ratios of the ortho- and para- water molecules. These data were compared with experimental ones. [1] G. Pitsevich, I. Doroshenko, A. Malevich, E. Shalamberidze, V. Sapeshko, V. Pogorelov, L.G.M. Pettesson, Spectrochim. Acta Part A, 172 (2017) 83-90 [2] G. Pitsevich, E. Shalamberidze, A. Malevich, V. Sablinskas, V. Balevicius, L.G.M. Pettesson, Mol.Phys., 115 (2017) 2605-2613.

The transduction of protons between distinct donor and acceptor sites, as often directed by attendant hydrogen bonds, is a ubiquitous chemical transformation essential for all of acid/base chemistry, yet the role of selective nuclear displacements in this deceptively simple process remains a topic of active investigation. Polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy was employed to investigate the vibrational specificity of proton-transfer dynamics in the lowest-lying singlet excited state, ùB₂ (π^*π), of 6-hydroxy-2-formylfulvene (HFF), with judicious selection of incident and detected polarization geometries allowing for the quantitative extraction of refined rotation-tunneling parameters. While the zero-point level of the ground electronic state [~X¹A₁] straddles the barrier crest for hydron migration and thus affords a prototypical example of low-barrier hydrogen bonding,Z. N. Vealey, L. Foguel and P. H. Vaccaro, J. Phys. Chem. Lett. 9, 4949 (2018).his model system also has been found to undergo a dramatic change in dynamics upon π^*← π electron promotion – as reflected by a > 1000-fold decrease in tunneling rates.Z. N. Vealey, L. Foguel and P. H. Vaccaro, J. Phys. Chem. A. 123, 6506 (2019).his talk will present complementary experimental and computational analyses of vibronic levels in the pertinent ùB₂ (π^*π) manifold designed to unravel the dependence of unimolecular reactivity on heavy atom motion and to elucidate the disparate behavior observed for analogous ~X¹A₁ features.

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Footnotes:

Z. N. Vealey, L. Foguel and P. H. Vaccaro, J. Phys. Chem. Lett. 9, 4949 (2018).t Z. N. Vealey, L. Foguel and P. H. Vaccaro, J. Phys. Chem. A. 123, 6506 (2019).T

Proton transport is ubiquitous, insights into its underlying mechanism and manifestation in vibrational spectroscopy are important for a variety of scientific fields. One way to probe these motions is through the studies of protonated water clusters, H⁺(H2O)_n . Gaining insights of the interplay between the OH stretch of one of the hydrogen atoms on the hydronium core and the OO vibration involving the oxygen atom in its coordinated water molecule is critical in understanding proton transport through these hydrogen bonded networks. It has been shown theoretically and experimentally that the ability of a proton to undergo transfer through the hydrogen-bonded network is highly influenced by the distance between the donor and acceptor oxygen atoms. The simplest model that incorporates this coupling is based on an adiabatic treatment of the Hamiltonian in which the OH and OO stretches are described by two harmonic oscillators which are coupled through a cubic term in the potential. This model has been shown to replicate experimental spectral features of these protonated water clusters. This raises questions of why such a simple model works so well and how the adiabatic treatment, or inclusion of higher order terms in the potential and dipole surface affect the results. To investigate these questions, we have introduced higher order terms in the potential and dipole surfaces within an adiabatic treatment, where we first add only higher order terms to the potential and then add the higher order terms of both the potential and the dipole surface. In addition, we have considered the effects of the adiabatic treatment itself by implementing a full 2D calculation for describing the OO and OH stretching motions. In comparing the results of the adiabatic treatment to experiment, the cubic harmonic model, and the full 2D calculation, the leading correction comes from the higher order terms in the potential. The calculation of intensity shows small, yet notable, corrections from higher order terms in the dipole moments.

I recently published a paper¹ that analyzed near-resonant two-photon absorption (TPA) spectra of polyatomic molecules excited in the vibrational fundamentals. For each molecule, a very small number of TPA lines are greatly enhanced due to near resonance. It is well known that TPA in a standing wave is dominated by a Doppler Free absorption. In this talk, the theory will be summarised and extensions to the published paper will be presented, including: (1) Theory of high finesse optical cavities with TPA loss, including the peak transmission and spectral shape of cavity modes (which are not Lorenztian with TPA). (2) Extension of the theory of saturation that includes the consequences of slow vibrational relaxation, which invalidates the traditional optical Bloch equations. (3) Theory of the TPA lineshape in the limit of low pressure such that the transition width is limited by the finite time of flight of the absorbers through the TEM₀₀ mode of the cavity. 1. K.K. Lehmann, J. Chem. Phys. 151, 144201 (2019)

As recently discussed by K.K. Lehmann in a theoretical analysis of two-photon absorption (TPA) [1], the combination of high circulating power and counterpropagating fields associated with the cavity ring-down spectroscopy (CRDS) technique can yield relatively prominent, two-photon, Doppler-free absorption features for near-resonant, three-level rovibrational systems. Here, we present CRDS measurements of TPA by N₂O near λ =4.5 μm. The experiment comprises a quantum cascade laser which is frequency-coupled to a high-finesse optical resonator through controlled optical feedback. Consistent with Lehmann's model, we observe CRDS decay signals that depend upon two characteristic decay rates: a constant single-photon value and an intracavity-power-dependent two-photon absorption rate. The resulting spectra exhibit strong Doppler-free features, saturation effects at low pressure, and TPA cross-section and collisional broadening coefficient consistent with theoretical predictions. 1. K.K. Lehmann, "Resonance enhanced two-photon cavity ring-down spectroscopy of vibrational overtone bands: A proposal," J Chem Phys 151, 144201 (2019).

The rotational spectrum of the benzofuran-formaldehyde complex has been investigated by high resolution Fourier transform microwave spectroscopy. In addition to the parent species, the rotational spectra of all mono-substituted ¹³C isotopologues and the complex with H₂C¹⁸O have been also measured. This extensive set of rotational constants for isotopic species allowed an accurate structural determination exploiting the so-called “semi-experimental” approach.
In the observed isomer, the two subunits orient themselves almost parallel to each other. The non-covalent bonding distance between the carbon atom of formaldehyde and the nearest carbon atom of benzofuran has been found to be well within the sum of van der Waals radius of carbon. The joint experimental and computational study pointed out that the two moieties are linked through a π-π interaction. The interaction energy is calculated to be about 20 kJ mol⁻¹ and, according to a SAPT analysis, is dominated by dispersion.

Spectral complexity can prove to be a challenge when investigating the rotational spectra of complicated systems and developments in automated fitting tools can reduce the time spent fitting these spectra. In this talk, we discuss our progress towards incorporating the algorithm from RAARR (Robust Automated Assignment of Rigid Rotors)Yeh, L.; Satterthwaite, L.; Patterson, D., "Automated, context-free assignment of asymmetric rotor microwave spectra," J. Chem. Phys. 150(20), 204122 (2019).nto Autofit and the performance of the combined approaches on typical rotational spectra collected from our lab of small molecules near room temperature where contributions from multiple vibrational states are present. We will also highlight a few recent improvements to Autofit, such as an improved graphical user interface.

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Footnotes:

Yeh, L.; Satterthwaite, L.; Patterson, D., "Automated, context-free assignment of asymmetric rotor microwave spectra," J. Chem. Phys. 150(20), 204122 (2019).i

Catalytic thermal cracking of O₂ is employed to dope helium droplets with O(³P) atoms. Sequential capture of O(³P) and HCN leads to the production of a hydrogen-bound O-HCN complex in a ³Σ electronic state, as determined via comparisons of experimental and theoretical rovibrational Stark spectroscopy. Ab initio computations of the three lowest lying intermolecular potential energy surfaces reveal two isomers, the hydrogen-bound (³Σ) O-HCN complex and a nitrogen-bound (³Π) HCN-O complex, lying 300 cm⁻¹ higher in energy. The non-relativistic HCN-O to O-HCN interconversion barrier is predicted to be only about 40 cm⁻¹. Moreover, the barrier is reduced upon explicit consideration of spin-orbit coupling. Consistent with the prediction of a relatively small interconversion barrier, there is no experimental evidence for the production of the nitrogen-bound species upon sequential capture of O(³P) and HCN. Rigorous atom-molecule bound-state computations are also performed, considering the couplings of various angular momenta, providing support for the lack of existence of HCN-O.

Study of the photophysics and electronic structure properties of quinoline and its derivatives have been the subject of considerable interest because of their commercial and pharmaceutical applications. Some of the quinoline derivatives have been found as a potential probes for measuring the polarity of microenvironment in chemical and biological systems. Diffuse interstellar bands in the emission spectrum of the interstellar medium, indicate the presence of quinoline and other small polycyclic aromatic nitrogen heterocycles, which readily dissociate under exposure to interstellar radiation. Since the quinoline ring is the basic fluorophor unit in all its derivatives, it is important to understand the change in dynamics and electronic structure of quinoline in presence some external perturbation. Being isoelectronic with naphthalene, these molecules provide useful comparisons for checking the electronic and vibrational state assignments, ionization potentials, and other properties of the parent hydrocarbon. In addition, these molecules possess nonbonding electrons which give rise to n– π^* states. The location and characterization of these states are of both theoretical and practical significance. Further, solvents have an important influence on the fluorescence property of N-heterocyclic compounds. Experimentally, Quinoline shows vibronic absorption spectrum and corresponding large Stoke shifted broad fluorescence emission spectrum having very low quantum yield and dual decay time, however protonated quinoline shows red shifted fluorescence spectrum with increase in quantum yield and fluorescence decay become mono-exponential. To understand the vibronic structure of electronic absorption spectra and photophysics of protonation of quinoline, both vibronic and electronic structure studies of quinoline (Q) and protonated quinoline (QH⁺) were carried out along with vibrational calculations for absorption and fluorescence bands at B3LYP 6-311++G(d,p) level in ground and excited state by density functional methods (DFT) and (time-dependent density functional) TD-DFT methods respectively with the help of Gaussian 09 software. Normal mode mixing is taken into account by the Duschinsky transformation. The vibronic structure of strongly dipole-allowed transitions is calculated within the Franck–Condon approximation. A good correlation between computational spectroscopic calculations and experiment results are found to understand the photo-physics of protonation of quinoline.

The structure of Naphthazarin (NZ) has been considered in spectroscopic for a long time. However, there are some ambiguity in spectroscopic data up to now. In this work, an easy and feasible method has been introduced for studying the proton tunneling in NZ to assess symmetry. For this purpose, a two-dimensional potential energy surface which has been capable to explain the movement of hydrogen atom with high accuracy has been used. This potential energy function, which couples OH stretching with in-plane bending modes has been applied on a fixed skeleton geometry of NZ calculates the tunneling splitting, OH stretching, and in-plane bending frequencies. To study the effect of the saddle point and the bending force constant of OH on transition frequencies of hydrogen bond, a differentiation was applied on two dimensional function towards X and Y, respectively. The tunneling splitting of 15.3 and 0.8cm -1 and the barrier height of 65.3 and 127.3 kJ/mol for stepwise and concerted proton transfer, respectively, indicate that both proton transfers are probable to happen, but NZ of D-2h symmetry will last for the longer time. In addition, as the saddle point position and the bending force constant increase, the tunneling frequency increases. It seems that the interaction between these two modes (the bending force constant) is responsible for higher OH stretching and lower OH bending frequencies in the bent hydrogen bonded systems compared to those in linear hydrogen bonded systems. This coupling, also, predicts a strong fermi resonance between OH stretching and the second overtone of OH bending modes in NZ and explains the cause of broading for OH stretching frequency in the bent H-bonded systems such as those in the enol form of -diketones.