The vapor-phase and Raman spectra of 3-cyclopenten-1-ol (3CPOL) and 3-cyclopenten-1-amine (3CPAM) have been recorded and analyzed. The spectra demonstrate the existence of six conformers for each molecule, two pairs of which are mirror images of each other and with equivalent vibrational modes. The conformational minima are achieved for selected values of the ring-puckering coordinate and the internal rotation coordinate of either the -OH or -NH2 group. Theoretical CCSD/cc-pVTZ computations for both molecules were carried out and their two-dimensional potential energy surfaces (PEFs) were calculated. The PEFs confirm the existence of all conformers and provide the relative energies for these. The structures with the π-type intramolecular hydrogen bonding are at the lowest calculated energies and 301 to 411 cm⁻¹lower than the other conformations for 3CPOL, and 165 to 197 cm⁻¹lower for 3CPAM.

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 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). dramatic change in dynamics has been suggested to accompany π^*← π electron promotion, as reflected by a > 1000-fold decrease in tunneling rate.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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Z. N. Vealey, L. Foguel and P. H. Vaccaro, J. Phys. Chem. Lett. 9, 4949 (2018).a Z. N. Vealey, L. Foguel and P. H. Vaccaro, J. Phys. Chem. A. 123, 6506 (2019).T

The ground and excited state IR vibronic spectra of jet-cooled methyl anthranilate (MA) in the hydride stretch (2400-3800 cm⁻¹ and mid-IR (1400-1800 cm⁻¹) regions have been recorded and many of the peaks have been assigned. The key exception, and the subject of this talk, is the H-bonded NH stretch on the S₁ excited surface. In contrast to the S₀ surface, where the NH stretches of -NH₂ can be modeled assuming symmetric and asymmetric stretch vibrations, on the S₁ surface only the free NH stretch is observed. Time-dependent density functional electronic structure calculations combined with both normal mode and VPT2 results predict an extremely bright transition between 2900 cm⁻¹ and 3100 cm⁻¹ depending on the level of theory for the hydrogen bonded NH stretch. No corresponding transition is observed experimentally. To explain the discrepancy between the experimental and calculated intensities of the dislocated NH stretch transition in the S₁ excited state a model is proposed based on the adiabatic separation of the NH stretch and other internal coordinates. In this model, the excitation of the NH stretch leads to dramatic structural reorganization, this leading to many Franck-Condon factors that, in turn, lead to substantial shared intensity of the initial bright state over hundreds a wavenumbers, thereby diluting the band sufficiently that it is no longer apparent in the spectra.

In the work we present our recent works focusing on the chemical enhancement effect of Raman cross section of the wagging vibrations in aromatic amines and benzyl radical adsorbed on metal surfaces. These molecules can display giant chemical enhancement effect in Raman intensity of the specific wagging vibrational modes. To understand the origin of the Raman intensity enhancement, we calculated the potential energy surfaces along the vibrational mode and analysed the change of polarizability due to the binding interaction. Our results also showed that the change of the polarizability derivatives is closely associated with the binding interaction and the orbital hybridization. Furthermore, we also analysed the anharmonicity of the potential energy surface and the vibrational coupling effect of the wagging vibration contributing to the Raman intensity. The Raman features of these special vibrational modes significantly change in vibrational frequency and the relative Raman signal. Finally, we further predict the contribution of the charge transfer state to the relative Raman intensity based on the density functional theoretical calculations.

Tobacco addiction plagues society and stems from the neuroactivity of nicotine, a natural compound that is abundant in tobacco leaves. Recent studies have determined that the protonation sites for nicotine are different in its gas (at the pyridine ring) and aqueous (at the pyrrolidine ring) states. These two different protonation sites are expected to trigger different biological activity in the native, anhydrous environment of the binding pocket of the human brain. Infrared (IR) spectroscopy is a powerful tool, which records the molecular vibrations associated with a specific bond and its environment. With the aid of first principles (ab-initio) electronic structure calculations, the IR spectra can be decoded and associated to underlying molecular structures. In this context, protonation in different sites is associated with spectral bands of different frequencies. We have relied on different hierarchical approximations of electronic structure methods, such as Density Functional Theory (DFT), Second Order Møller-Plesset Perturbation Theory (MP2) and Coupled Cluster Theory [CCSD(T)] to compute the IR spectra of nicotine, nornicotine, and their hydrated clusters to investigate the transition of the protonation site from the pyridine to the pyrrolidine rings with an increasing number of water molecules in the cluster. The experimentally observed stabilization of the pyrrolidine protonation site is facilitated by the formation of strong hydrogen bonds to both nitrogen centers, which requires multiple waters. The result is a strong red-shift in the acidic N-H vibrational bands. Notably, nicotine favors the pyrrolidine protonation site more than nornicotine at each hydration level. This increased stability of the pyrrolidine protonation in nicotine is suggested to support stronger binding of nicotine to tryptophan in the nicotinic acetylcholine receptor. The quantification of the incremental hydration for nicotine and nornicotine manifests the multiple energetically relevant hydrated states, which is currently considered in biological applications, such as binding strength to neuroreceptors.

After the first highly accurate IR intensity study on CO₂ 20012-00001 band was published in 2015, [Polyansky et al, PRL 114,243001 (2015)], reliable intensity data have been accumulated experimentally for several other bands in the 2001x and 3001x series, with uncertainties less than 0.1-0.5%. Compared to these data, earlier published theoretical CO₂ IR intensities were found with deviations usually around 0.5-1%. To further reduce deviations and uncertainties in the theoretical IR intensities computed using the semi-empirically refined potential energy surfaces (PES) and ab initio dipole moment surfaces (DMS), we have carried out systematic studies to estimate the magnitude of various contributions from the PES, the DMS, and the experimental data. We will present the procedure through which we have achieved 0.2% uncertainty, and discuss why we may need an even more accurate PES, a carefully built (or selected) high quality DMS, and more consistent experimental data, to finally achieve 0.1% or better uncertainty.

We report calculations of vibronic levels and branching ratios for laser-coolable linear polyatomic molecules to an accuracy and completeness to be useful to guide experimental studies. The present computational scheme consists of a multi-state quasidiabatic Hamiltonian with relevant perturbations such as Renner-Teller, linear vibronic, and spin-orbit coupling, coupled-cluster calculations for adiabatic potential energy surfaces and molecular parameters, and discrete variable representation calculations for vibronic levels and wave functions. The computed levels and branching ratios for the A²Π_1/2→ X²Σ_1/2 transitions of CaOH, SrOH, and YbOH show promising agreement with experimental measurements. Based on the computed branching ratios, laser-cooling SrOH requires fewer repumping lasers than CaOH. The calculations also elucidate intensity-borrowing mechanisms for nominally symmetry-forbidden transitions. A close inspection of computational results further reveals it beneficial to avoid Fermi resonances in designing laser-coolable molecules.

Frequency combs and cavity-enhanced optical techniques have revolutionized molecular spectroscopy: their combination allows recording saturated Doppler-free lines with ultrahigh precision. Despite these advancements, precision spectroscopy has not been employed systematically to improve the quality of comprehensive spectroscopic databases as individual observations are often time consuming and lacks impact. Here we present a new spectroscopic method using network theory, based on the generalized Ritz principle, which offers a powerful tool for the intelligent design and validation of precision-spectroscopy experiments and the subsequent derivation of accurate energy differences. As a proof of concept, 351 carefully-selected near-infrared transitions (at 1.4 μm) are detected for both H₂¹⁶O R. Tóbiás et al., Spectroscopic-network-assisted precision spectroscopy and its application to water, Nature Comm. 11, 1708 (2020)nd H₂¹⁸O M.L. Diouf et al., Network-based design of near-infrared Lamb-dip experiments and determination of pure rotational energies of H₂¹⁸O at kHz accuracy, in preparation (2021) two benchmark species of molecular spectroscopy. Transition frequencies of the Lamb-dips are measured up to kHz accuracy using our frequency comb referenced NICE-OHMS spectrometer. These measurements, augmented with extremely-accurate literature lines to ensure overall connectivity, allow the precise determination of the lowest ortho-energy and 348 energy levels of both isotopes with unprecedented accuracy. Based on the limited number of observed transitions, 2765 calibration-quality lines are obtained in a wide wavenumber interval. These can be used to improve spectroscopic information systems and applied to frequency metrology, astrophysics, atmospheric sensing, and combustion chemistry.

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Footnotes:

R. Tóbiás et al., Spectroscopic-network-assisted precision spectroscopy and its application to water, Nature Comm. 11, 1708 (2020)a M.L. Diouf et al., Network-based design of near-infrared Lamb-dip experiments and determination of pure rotational energies of H₂¹⁸O at kHz accuracy, in preparation (2021),

Using the NICE-OHMS (noise-immune-cavity-enhanced optical-heterodyne-molecular spectroscopy) technique, a large number of rovibrational transitions have been observed in the 7000-7350 cm⁻¹ window for H₂¹⁶O [Nat. Commun. 2020, 11, 1708.] and H₂¹⁸O [in preparation], at the kHz-accuracy level. This talk focuses on the theoretical methods that were applied in the design of high-precision NICE-OHMS experiments and the extraction of accurate spectroscopic information from the measured lines. In spectroscopic networks (SN), the vertices and edges are the energy levels and the transitions, respectively. We used the SNAPS (spectroscopic-network-assisted precision spectroscopy) method [Nat. Commun. 2020, 11, 1708.] to select the target lines for measurement and provide initial line positions. The internal consistency of the ultraprecise experimental transitions was verified using cycles of the SN, while paths allow to extract high-accuracy energy levels and derive benchmark-quality predicted lines. The SN of H₂¹⁶O and H₂¹⁸O consists of four subcomponents, corresponding to the ortho and para nuclear-spin isomers combined with even and odd parity. For a given nuclear-spin isomer, the levels with even and odd parity are not connected by the vibrational measurements, their connectivity relies on the inclusion of purely rotational transitions. No transitions connecting the ortho and para states have been observed. Thus, the ortho and para components of the SNs are not connected, and the energy of the ortho states is known only relative to the lowest ortho state and not to the para ground state. The energy of the lowest ortho state was determined in two ways: 1) an effective-Hamiltonian fit to a set of energy differences (taken from the literature and calculated from NICE-OHMS lines) within the ground vibrational band; 2) a network-based approach: paths of NICE-OHMS transitions augmented with diminishing ortho-para energy splittings obtained from first-principles quantum-chemical computations. Notably the quantum-chemical computations resulted in highly accurate splittings without the consideration of any experimental data.

Doppler-free saturation techniques have revolutionized the precision and resolution of molecular spectroscopy. It sets the standard for improving the accuracy of the level structure of molecules by several orders of magnitude. In our studies we employ a NICE-OHMS setup for measuring spectra of water isotopologues in vibrational excitation to the 4^th polyad, at a wavelength of 1.4 μm R. Tobias, T. Furtenbacher, I. Simko, A.G. Csaszar, M.L. Diouf, F.M.J. Cozijn, J.M.A. Staa, E.J. Salumbides, W. Ubachs, Spectroscopic-network-assisted precision spectroscopy and its application to water, Nature Comm. 11, 1708 (2020) The high intra-cavity circulating laser power allows for saturating the vibrational lines producing Lamb dips at typical widths of 400 kHz. Here we present our work on two different water isotopologues HD¹⁶O and H₂¹⁷O, in which various physical effects significantly alter the ordinary saturated Lamb-dip. In H₂¹⁷O, thanks to the spin of the ¹⁷O, we resolve the hyperfine structure, which can be exploited to extract hyperfine constants from a rovibrational transition. In HD¹⁶O, where the deuteron breaks the para/ortho symmetry, perturbed spectral line shapes are observed, in particular for lines that exhibit small K_c splittings between lines of opposite parity. It is hypothesized that AC Stark effects are the cause for breaking up the line into a multi-component structure.

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Footnotes:

R. Tobias, T. Furtenbacher, I. Simko, A.G. Csaszar, M.L. Diouf, F.M.J. Cozijn, J.M.A. Staa, E.J. Salumbides, W. Ubachs, Spectroscopic-network-assisted precision spectroscopy and its application to water, Nature Comm. 11, 1708 (2020).

Whether in ionic or covalent bonding, in weakly bound clusters, in liquids, or in solids, perturbed electronic structure contributes to spectroscopy, electromagnetic interactions and enhanced reactivity. For Rydberg states, effects of current interest include vibrational autoionization as discussed by Ed Eyler and by Timothy Barnum The range of importance of classical force fields extends through VdW clusters up to biological problems such as enhanced protein folding from fluorine-substitution in proteins. For smaller molecules and Rydberg ion cores such as NO+ or alkaline earth mono-halide cations, accurate coupled cluster methods are quite feasible at the CCSD(T) or CCSDT level, using augmented basis sets up to the quadruple-zeta level. These methods can be used in the CFOUR ab-initio suite to determine polarizability expansions giving the local force field out to quite high order. Several approaches will be described, including arrays of test-charges, the use of implemented analytical derivatives, and static field perturbations. This presentation will focus on results useful for modeling Rydberg vibrational autoionization in NO Rydberg states.

In high angular momentum (l ≥ 3) Rydberg states, the centrifugal barrier hinders close approach of the Rydberg electron to the ion-core. As a result, these core-nonpenetrating Rydberg states can be well described by a simplified model in which the Rydberg electron is only perturbed by the long-range electric properties of the ion-core. We have developed a long-range model to describe the vibrational autoionization dynamics of high-l Rydberg states of nitric oxide. In particular, our model explains the extensive angular momentum exchange between the ion-core and Rydberg electron previously observed in vibrational autoionization of nf Rydberg states. In addition, we have measured total decay lifetimes of ng Rydberg states and find that the decay of ng states is dominated by autoionization rather than predissociation. We examine the predicted ion rotational state distributions generated by vibrational autoionization of ng states and discuss a potential application in the production of quantum state-selected molecular ions.