This study was triggered by the results of the microwave investigation of triflic acid (TA), CF₃−SO₂−OH, and its deuterated (TA-d) and ³⁴S (TA-S) isotopologues by Huff et al.A.K. Huff, N. Love, C.J. Smith, K.R. Leopold, (2022) submitted for publication.o fit the observed transitions split by gauche-gauche tunneling due to the CS-OH internal rotation, the interaction Hamiltonians for TA and TA-S required the term F_bc(P_bP_c+P_cP_b). In contrast, the Coriolis term g_aP_a had to be used for TA-d. The F_bc term results from two facts: 1) The moment of inertia tensor I and its inverse, the μ-tensor, of both the right- and left-handed forms of the gauche conformers need to be expressed in the same molecule-fixed axis system; 2) The Cartesian coordinates of some or all atoms and the components of I depend on the internal rotation coordinate τ. If we define a Cartesian axis system with two of the axes in the symmetry plane of the transition state between the two conformers, the third axis is perpendicular to it. When τ changes, the moving atoms will eventually cross the symmetry plane and their 3rd Cartesian coordinate will change its sign and with it also the signs of two off-diagonal components of I and the μ-tensors. The rotational Hamiltonian in a 2-by-2 block format contains the contributions of the right and left gauche conformers (H_R and H_L, respectively) on the diagonal and the Coriolis interactions in the off-diagonal blocks. If the internal rotation wave functions ϕ_R and ϕ_L are symmetrized to ϕ_A′ and ϕ_A" to conform with C_s symmetry, the diagonal blocks of the Hamiltonian become H_A′ and H_A". The off-diagonal blocks contain now the Coriolis terms and the contributions from the sign-changing μ-tensor components. Detailed explanations and a demonstration that this particular interaction may be especially important for K_a=1 energy levels will be presented.

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Footnotes:

A.K. Huff, N. Love, C.J. Smith, K.R. Leopold, (2022) submitted for publication.T

We report and interpret recently recorded high resolution infrared spectra for the fundamentals of the CH₂ scissors and CH stretches at -50 ^oC of gas phase cyclopentane. We extend previous theoretical studies of this molecule, which is known to undergo barierless pseudorotation, by constructing local mode Hamiltonians of the stretching and scissor vibrations for which the frequencies, couplings, and linear dipoles are calculated as functions of the pseudorotation angle using B3LYP/6-311++(d,p) and MP2/cc-pvtz levels of theory. Symmetrization (D_5h) of the vibrational basis sets leads to simple vibration/pseudorotation Hamiltonians whose solutions lead to good agreement with experiment at medium resolution, but which miss interesting line fractionation when compared to the high resolution spectra. In contrast to the scissor motion, pseudorotation leads to significant state mixing of the CH stretches which themselves are Fermi coupled to the scissor overtones.

We report a combination of quantum mechanical calculations and different spectroscopic techniques used to investigate in the gas phase N,N-diethylhydroxylamine (DEHA), an important scavenger compound. The molecule has been first studied by rotational spectroscopy using Pulsed Jet Fourier Transform Microwave (PJ-FTMW) spectrometer in the 6.5-18.5 GHz frequency range and Stark modulated Free Jet Absorption Millimeter-Wave (FJ-AMMW) spectrometer in the 59.6-74.4 GHz range. Three conformers have been overall observed. They are all characterized by the hydroxyl hydrogen atom being in trans isomerism with respect to the bisector of the CNC angle. r0pt Figure For the global minimum, also the ¹³C and ¹⁵N isotopologues have been observed in natural abundance, allowing for a partial structure determination. Ultraviolet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron spectroscopy (XPS) measurements have been performed at the Gas Phase Photoemission beamline (GasPhase) of the Elettra Synchroton light laboratory (Trieste, Italy). The core (C(1s), N(1s) and O(1s)) photoemission spectra have been assigned with the support of SAC-CI/cc-pVTZ calculations. Density Functional Theory (DFT) approaches, including Time-Dependent DFT, have been exploited to reproduce the outer valence electron binding energies and peaks profiles, through the calculation of the Huang-Rhys factors. Interestingly, the structure of the first ionized cation is not that of an amine but it is similar to an N-oxoammonium compound.

The conformational landscapes of allyl ethyl ether (AEE) and allyl ethyl sulfide (AES) were investigated using Fourier transform microwave spectroscopy in the frequency range of 5-23 GHz aided by the density functional theory (DFT) B3LYP-D3(BJ)-aug-cc-pVTZ calculations. The quantum chemical calculations identified a rich conformational equilibrium with 14 stable conformers for AEE and 13 for AES within 14 kJ/mol. Experimentally, rotational transitions corresponding to three low energy conformers of AEE and two forms of AES were assigned. Interconversion pathways were modelled to understand the relaxation of the higher energy conformers to the more stable forms in the supersonic jet expansion and to explain the observed tunneling splitting in one conformer of AEE. To better understand the stereoelectronic effects and topology of the interactions governing the conformational preferences of AEE and AES, natural bond orbital (NBO) and non-covalent interaction (NCI) analyses were performed. For the lowest energy conformers of AEE and AES, ground state effective (r₀) and substitution (r_s) structures were derived using the spectral data collected for the ¹³C and ³⁴S isotopologues in natural abundance. The results confirm the theoretical predictions that the lowest energy geometries are surprisingly different depending on the identity of the bridging chalcogen atom.

Recently, the ground state spectroscopic constants of three cyanobutadiene isomers (C₅H₅N) were reported as plausible targets for detection in the interstellar medium.¹ Many hundreds of transitions were collected in the 130 – 375 GHz range for each isomer, but s-trans-Z-1-cyano-1,3-butadiene is the only isomer with an isolated ground state. s-trans-Z-1-Cyano-1,3-butadiene (μ_a = 3.6 D, μ_b = 2.3 D, MP2/6-311+G(2d,p) is a prolate (κ = −0.91), asymmetric top with C_s symmetry, and its ground state was fit to a partial octic, distorted rotor Hamiltonian model with over 5500 distinct transitions. The lowest-energy vibrationally excited state (ν₁₉, B₁) is predicted to be 129 cm⁻¹higher in energy than the ground state and is close in energy to the second-lowest energy vibrationally excited state (ν₂₇, B₂, 135 cm⁻¹, MP2/6-311+G(2d,p)). These states form a Coriolis coupled dyad with intense coupling along the a- and b- principal axes causing the formation of sharp local resonances hundreds of MHz away from their unperturbed predicted frequencies. This presentation will detail the effects of strong Coriolis coupling in a dyad of vibrational states < 6 cm⁻¹apart and the strategies used to assign and least-squares fit these transitions. 1. Zdanovskaia, M. A.; Dorman, P. M.; Orr, V. L.; Owen, A. N.; Kougias, S. M.; Esselman, B. J.; Woods, R. C.; McMahon, R. J. Rotational Spectra of Three Cyanobutadiene Isomers (C₅H₅N) of Relevance to Astrochemistry and Other Harsh Reaction Environments. J. Am Chem. Soc. 2021, 143, 9551-9564.

2-Cyanopyrimidine and 2-cyanopyrazine are heterocyclic analogs of the known interstellar molecule benzonitrile. They are attractive molecules for interstellar detection via radioastronomy because they have substantial dipole moments (μ_a = 6.47 D and μ_a = 4.22 D, respectively) and could serve as tracer molecules for both pyrimidine and pyrazine. In the present work, we analyzed and assigned the rotational spectra of 2-cyanopyrimidine (C₅H₃N₃) and 2-cyanopyrazine (C₅H₃N₃) for the first time. From 130 – 500 GHz, the vibrational ground state transitions of each molecule have been least-squares fit to partial octic, distorted-rotor Hamiltonian with each fit containing several thousand transitions. The resulting spectroscopic constants are sufficient for radioastronomical searches for these molecules. Further similar to benzonitrile, the two lowest-energy vibrationally excited states form Coriolis coupled dyads for both 2-cyanopyrimidine and 2-cyanopyrazine. Several thousand transitions of each vibrationally excited state have been assigned and analyzed with a partial octic, distorted-rotor Hamiltonian with a- and/or b-axis Coriolis coupling. The complete analysis of these states is expected to yield a highly precise energy separation between these vibrationally excited states and allow for an interesting comparison to the analogous dyads of benzonitrile and the cyanopyridines.

Catechol molecule C₆H₄(OH)₂ is the representative of a separate class of aromatic hydrocarbons. It is an important component of biochemical, industrial and commercial products. In a recently published article [1] rotationally resolved Fourier Transform far-infrared spectrum of the free and bonded O−H groups forming the intramolecular hydrogen bond was recorded. The authors were also able to measure pure rotational spectrum in the 70 – 220 GHz frequency range using a millimeter–wave spectrometer. Splitting due to tunneling was resolved for free O−H torsional state. Having this solid experimental background, we have performed extensive calculations of the kinetic parameters and 2D potential energy surfaces formed by variation of the O−H torsional coordinates of the catechol molecule at few levels of theory. It was found that almost all used levels of theory very well predict the frequency value of the fundamental torsional vibration of the free hydroxyl group: 224 – 227 cm⁻¹, while experimental value is 221.9 cm⁻¹. At the same time calculated frequencies of the H−bonded hydroxyl group torsional fundamental vibration is higher (422 – 425 cm⁻¹) than experimental one (415 cm⁻¹). We associate this difference with overestimating intramolecular H−bond energy in the frame of used levels of theory. It is also worth noting that there was a good agreement between calculated and experimental tunneling splitting of the first excited torsional state of free O−H group (3.6 ×10⁻⁶ and 1 ×10⁻⁵ respectively) . Besides, complete information about the second conformer of the catechol molecule with C_2V symmetry has been obtained. [1] J. Bruckhuisen and etc, Molecules 26 (2021) 3645

Water and hydrogen bonding have been actively studied for many years. The unique features of hydrogen bonding lead to processes such as proton transport which is essential to a variety of chemical, biochemical, and electrochemical processes. Despite the long-standing interest, many questions still remain surrounding the intricacies of hydrogen bonding. One goal of this study is to understand how the strength and other properties of hydrogen bonding can be extracted from the spectral signatures near 5000 cm⁻¹, which corresponds to combination transitions involving the HOH bend and the OH stretch. This region is of interest because, like the bend region, the intensities are relatively insensitive to the hydrogen bonding environment. At the same time, unlike the bend fundamental region, the frequencies of the transitions reflect the hydrogen bond strength as they follow the trends for the ν=1 levels of the OH stretches. This makes interpretation easier and introduces an interesting question, what are the origins of the intensities of the bend and the combination transitions? In order to investigate this question, we use various structures of water clusters, and dissect the intensity of the combination transition into mechanical and electrical contributions. Through this process, we are able to gain insights into the origin of the intensities. Further for the electrical contributions, we explore how the bending vibration is tuning the transition moment for the OH stretch.

The neurotransmitters Serotonin and Dopamine are thought to play a distinct role in brain chemistry and human behavior. In the Present work we will discuss the Conformer specific spectroscopic signatures of protonated Serotonin and Dopamine in the gas phase and aqueous media. A comparison of the computed frequencies of protonated Dopamine with the observed Raman spectrum indicates that gauche and trans conformers coexist in the liquid dopamine and exhibit population redistribution upon a change in pH during stress. Since the trans-conformers have more affinity for the receptor site than the gauche conformers, the higher population distribution of trans conformers is hypothesized to be associated with efficient neurotransmission and normal human behavior. The strong cation-pi interaction in the isolated gauche conformers of serotonin in the gas phase is predicted to be a possible cause of human aggression. 1. Vipin Bahadur Singh, ACS Chem Neuroscience, 12,613-625 (2021)

2-(Phenylsulfonyl)vinylbenzene (2PVB) and 2-(Tosyl)vinylbenzene(2TVB) molecules were characterized, by recording their FT-IR(4000-400 cm⁻¹) and FT Raman(4000-50 cm⁻¹) spectra. Torsional potentials, barrier to internal rotation around phenyl-sulfonyl C-S bond, sulfonyl-vinyl S-C bond, vinyl-benzene C-C bond, and phenyl-methyl C-C bond (this bond is relevant for 2TVB only). Optimized structure parameters, general valence force field, harmonic vibrational fundamentals, potential energy distribution, infrared and Raman intensities, frontier molecular orbital parameters, NLO behaviour and NBO characteristics were determined using density functional theory, employing B3LYP exchange-correlation in conjunction with functional 6-311++G(d,p) basis set. Time dependent DFT was made use of to compute absorption maxima (λmax) and oscillator strengths, for both molecules, in their electronic transitions, in DMSO-d6 solution. Good agreement was found, between measured and computed parameters involving structure parameters, IR and Raman spectra and UV-Vis transitions. The rms error between experimental and theoretical vibrational frequencies was 6.4 and 4.35 cm⁻¹), for 2PVB and 2TVB, respectively. With the help of PED and eigenvectors, all vibrational fundamentals of both the molecules were assigned for the first time. The computations demonstrated that both the molecules were good for NLO applications, that was substantiated by NBO analysis for both 2PVB and 2TVB.

The complex tunnelling dynamics of double proton transfer in carboxylic acid dimers has been the focus of many theoretical and experimental studies.^1,2 Here we combine spectroscopic and computational approaches to model and understand how functional groups in substituted benzoic acid heterodimers can influence these dynamics. Dimers of benzoic acid with its 4-chloro-, 4-nitro-, and 4-amino-analogues were studied using a 2 to 6 GHz chirped-pulse Fourier transform microwave spectrometer, which is based on the design by Pates et al.,³ to obtain experimental tunneling line splittings. Jacobi field instanton theory (JFI)^4,5 was used to compute tunneling splittings in the ground vibrational state. The use of the JFI method, which necessitates a smaller number of potential energy and gradient calculations compared to other methods, enabled us to use ab initio on-the-fly potentials and compute the splitting in full dimensionality, in spite of the large system sizes. Furthermore, final expressions for the tunneling splittings provided a way to examine the influence of substituents on both the potential energy barrier height and shape, and on the vibrational modes, which can either promote or inhibit tunnelling. (1) Evangelisti, L.; Écija, P.; Cocinero, E. J.; Castaño, F.; Lesarri, A.; Caminati, W.; Meyer, R. J. Phys. Chem. Lett. 2012, 3 (24), 3770–3775. (2) Tautermann, C. S.; Voegele, A. F.; Liedl, K. R. J. Chem. Phys. 2004, 120 (2), 631–637. (3) Pérez, C.; Lobsiger, S.; Seifert, N. A.; Zaleski, D. P.; Temelso, B.; Shields, G. C.; Kisiel, Z.; Pate, B. H. Chem. Phys. Lett. 2013, 571, 1–15. (4) Eraković, M.; Vaillant, C. L.; Cvitaš, M. T. J. Chem. Phys. 2020, 152 (8), 084111. (5) Mil’nikov, G. V.; Nakamura, H. J. Chem. Phys. 2001, 115 (15), 6881–6897.

Electron propagator methods (EPMs) are well known in the physical-chemical community as a useful tool for the identification of signals observed in ultraviolet photoelectron spectra (UPS) of isolated organic molecules. However, to completely reproduce an experimental UPS the vibrational signature associated to each electronic signal of interest should be computed. In the first part of this contribution, the implementation of a simple protocol for the simulation of vibrationally resolved UPS is briefly described and its application to the calculation of the spectra of seven semi-rigid organic molecules is proposed.A. Baiardi, L. Paoloni, V. Barone, V. G. Zakrzewski, J. V. Ortiz, J. Chem. Theory Comput., 2017, 13, 3120-3135.,L. Paoloni, M. Fusè, A. Baiardi, V. Barone, J. Chem. Theory Comput., 2020, 16, 5218-5226.hemical community is particularly interested in processes that occur in solutions, and therefore the energy which is needed to remove (or to add) an electron from a molecular system is often measured by means of electrochemical techniques. As a consequence, in this case the experimental observables of interest are oxidation and reduction potentials, which differ from the ionization potentials and the electron affinities computed with EPMs. In the second part of this contribution, experimental redox potentials of 12 organic dyes are compared with ionization potentials and electron affinities computed through EPMs. Differences between computed and observed values are rationalised in terms of polarization and solvation effects, and the estimation of redox potentials through the employment of suitable corrections to the values calculated with EPMs is discussed.

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Footnotes:

A. Baiardi, L. Paoloni, V. Barone, V. G. Zakrzewski, J. V. Ortiz, J. Chem. Theory Comput., 2017, 13, 3120-3135. L. Paoloni, M. Fusè, A. Baiardi, V. Barone, J. Chem. Theory Comput., 2020, 16, 5218-5226.C