Molecular systems of increasing size and complexity from small prebiotic molecules of astrochemical interest [1], medium size chromophores important for technology applications [2] to larger bio-molecules such as proteins [3,4] are nowadays studied by broad range of experimental techniques, involving different parts of electromagnetic spectrum (see figure). However, it is seldom straightforward to link the rich experimental data to the desired information on the specific structure and properties of complex molecular systems. In this context there is increasing synergy between experiment and theory, exploiting methodological and technological advances on both sides. I will discuss status and perspective of the project aimed at development, validation and application of QM based computational protocols supporting to decode and analyze experimental data based on light-matter interaction. c0pt Figure [1] Y. Zhao, M. Hochlaf, M. Biczysko "Identification of DNA bases and their cations in Astrochemical environments: Computational spectroscopy of Thymine as a test case" Front. Astron. Space Sci. 8, 757007 (2021) [2] X. Li, X. Yin, Y-L. Bai, M. Biczysko "Interpretation and prediction of optical properties: novel fluorescent dyes as a test case” Front. Phys. 11, 1236987 (2023) [3] Y. Wang, H. Kruse, N.W. Moriarty, M.P. Waller, P.V. Afonine, M. Biczysko "Optimal clustering for quantum refinement of biomolecular structures: Q - R #4" Theo. Chem. Acc. 142, 100 (2023) [4] Y. Liu, M. Biczysko, N.W. Moriarty "A radical approach to radicals" Acta Cryst. D78, 43-51 (2022)

Second order vibrational perturbation theory has proved to be a powerful tool for calculating spectra for transitions that involve one or two quanta of excitation in small to moderate sized molecules and ions. As part of these calculations, the third and fourth derivatives of the potential with respect to the normal modes are evaluated numerically. In this talk, I will discuss several ways in which we can use this information to obtain additional insight into the spectrum. I will also discuss reasons why sometimes intensities obtained from VPT2 calculations may show signs of near degeneracies, while frequencies look reasonable, and strategies for correcting for these problematic intensities. The discussion will focus on recent studies in our group including work in our group, including studies of the pyrene anion, protonated water clusters, and complexes of Octamethyl Calix[4]Pyrrole with halide ions.

Diffusion Monte Carlo (DMC) is a stochastic method that can be used to obtain the zero-point energy and the ground state wavefunction for a molecule of interest. The advantage of DMC is that it is general, and can produce accurate results even with molecules with highly anharmonic vibrations. The downside is that it is a ground state method, so we cannot obtain spectroscopic results. Also, even if we have the excited state information, calculating intensity of the transition is not straightforward since the wave function is represented by an ensemble of localized functions, the locations of which will be different for the ground and excited state. Introducing a guiding function can help address both issues. We can use excited state guiding function to run guided DMC calculation and obtain the excited state information which is needed to evaluate the vibrational frequencies and intensities. Since we simulate the ground state and the excited state separately, the ground state and the excited state wave function will be sampled at different coordinates in space, so we cannot obtain the transition dipole moment from simple multiplication of these wave functions. In this talk, we will present two ways to calculate intensities of vibrational transitions. In the first, we use the fact that the guiding function should provide a good approximation to the wave function for the state of interest, and we can approximate the transition dipole matrix element which involves both ground state and excited state wave function by replacing one of the wave functions with the guiding function. In the second, we use descendant weighing to obtain the ratio of desired product of the ground and excited state wave function. We use these methods to calculate the vibrational frequencies and intensities of several small molecules including H₂O, H₃O₂⁻ and H₅O₂⁺, and explore the advantages and disadvantages of the two approaches.

Criegee intermediates play an important role in the lower levels of the atmosphere, as they provide a non-photolytic source of OH radicals. In this work we re-examined the vibrational spectrum of methyl-Criegee (CH3C=OO⁺) in the CH-stretching overtone region (2νCH) after the development of a new experimental method known as IR VUV ion-dip spectroscopy. By utilizing a general implementation of vibrational perturbation theory (VPT) [1,2], we gain insights into the motions responsible for the peaks in the experimental spectrum. We find that after shifting the harmonic frequencies based on differences between experiment and the calculated spectrum in the fundamental CH-stretching region, we can achieve excellent agreement between experiment and the VPT2 spectrum. This empirical approach was implemented using three different levels of electronic structure theory/basis sets: B2PLYP-D3/cc-pVTZ, B3LYP/aug-cc-pVTZ, and MP2/aug-cc-pVTZ. For all levels of electronic structure theory, the resulting spectra are virtually indistinguishable from each other, illustrating the robustness of this approach. References: Boyer, M. A.; McCoy, A. B. A flexible approach to vibrational perturbation theory using sparse matrix methods. The Journal of Chemical Physics 2022, 156, 054107. Boyer, M. A.; McCoy, A. B. A wave function correction-based approach to the identification of resonances for vibrational perturbation theory. The Journal of Chemical Physics 2022, 157, 164113.

The gas-phase rotational spectrum from 8 to 750 GHz and the high-resolution infrared spectrum of pyridazine (o-C₄H₄N₂) have been analyzed for the ground and four lowest-energy vibrationally excited states. A combined global fit of the rotational and infrared data has been obtained using a sextic, centrifugally distorted rotor Hamiltonian with Coriolis coupling between appropriate states. For the first time, the Coriolis coupling has been addressed in the two lowest-energy coupled dyads (ν₁₆, ν₁₃ and ν₂₄, ν₉). Utilizing the Coriolis coupling between the vibrational states of each dyad and the analysis of the infrared spectrum for ν₁₆ and ν₉, we have determined precise band origins for each of these fundamental states: ν₁₆ (B1) = 361.2132927 (17) cm⁻¹, ν₁₃ (A2) = 361.2840824 (17) cm⁻¹, ν₂₄ (B2) = 618.969096 (26) cm⁻¹, and ν₉ (A1) = 664.7233784 (27) cm⁻¹. Notably, the energy separation in the ν₁₆-ν₂₄ Coriolis-coupled dyad is one of the smallest spectroscopically measured energy separations between vibrational states: only 2122.222 (72) MHz or 0.0707897 (24) cm⁻¹. Additionally, the spectra of pyridazine-dx isotopologues generated for a previous semi-experimental equilibrium (reSE) structure determination allowed us to analyze the two lowest-energy vibrational states of pyridazine for all pyridazine-dx isotopologues. Six of these formed coupled dyads, while three were amenable to fitting with single-state Hamiltonians. Spectroscopic constants and energy differences determined in this work are compared to their CCSD(T)/cc-pCVTZ computed values.

Group 10 element cation(Ni⁺, Pd⁺, Pt⁺) interactions with acetylene are studied with high-level ab initio quantum mechanical methods. Geometries were optimized with the CCSD(T) method with the cc-pVTZ-PP and aug-cc-pVTZ-PP basis sets with small core pseudopotentials. Harmonic vibrational frequencies were computed with the same level of theory to compare to the experimental IR photodissociation spectrum measured via argon tagging. The binding energies were determined at the CCSD(T)/CBS level of theory via the Focal Point Analysis method with harmonic zero-point vibrational energy corrections.

Due to a large change in geometry upon ionization of NF₃, the corresponding photoelectron spectrum observed by Berkowitz and Greene exhibits a long vibrational progression spanning roughly 1.5 eV. The ionization threshold is assigned at 13.0 eV, consistent with previous experimental determinations but at considerable odds with mid-level calculations which predict an ionization energy nearly 0.4 eV lower. We perform high-accuracy thermochemical calculations (HEAT) to determine the ionization threshold and find it is consistent with previously calculated estimates, a fact which is not entirely surprising given the difficulty in determining the onset experimentally due to small Franck-Condon factors in the vicinity of the origin in ionization processes associated with a significant geometry change. More interestingly, the width of the overall progression is enough to surmount the barrier to planarity in NF₃⁺, assigned experimentally to an observed change in vibrational spacing on either side of the peak maximum. Again, high-accuracy calculations determine a value much lower in energy (0.24 eV) than that predicted by experiment, throwing into question the explanation for the change (or loss) of observed spacing on the higher energy side of the peak maximum. Attempts to alleviate discrepancies between calculated and experimental assignments, relying on the aforementioned thermochemical calculations and Franck-Condon simulations are discussed.

Recent combined multi-experimental and theoretical vibrational spectroscopic efforts have tremendously contributed to and extended our understanding of the internal dynamics of formic acid (HCOOH), the smallest carboxylic acid.A. K. Kelemen and S. Luber, Phys. Chem. Chem. Phys. 2022, 24, 28109.ne important result is that the well-known ν₅-2ν₉ OH bend-torsion Fermi resonance plays a far more important role than previously believed.A. Nejad and E. L. Sibert III, J Chem. Phys. 2021, 154, 064301.he OH stretch has long been known to be strongly perturbed by several resonances, leading to a significant intensity redistribution. However, the analysis of its rotationally-resolved spectrum was achieved only as recently as 2023,Y.-C. Chan and D. J. Nesbitt, J. Mol. Spectrosc. 2023, 392, 111743.lmost two decades after its first measurement.Hurtmans et al., J. Chem. Phys. 2000, 113, 1535.sing high-level full-dimensional perturbative (Canonical Van Vleck) and variational (GENIUSH-Smolyak) vibrational models in combination with full-dimensional potential energy and property surfaces, it is found that the perturbers belong to a large network of coupled vibrational states that build on the well-known ν₅-2ν₉ Fermi resonance. Many of the observed OH stretch perturbers are highly-excited multi-quantum vibrational states (n ≥ 3) so that normal mode labels become inadequate to properly label the bands. The question arises of how - if at all - the interaction can be understood in chemical bonding terms which is addressed in light of the underlying OH bend-torsion ν₅-2ν₉ Fermi resonance.

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Footnotes:

A. K. Kelemen and S. Luber, Phys. Chem. Chem. Phys. 2022, 24, 28109.O A. Nejad and E. L. Sibert III, J Chem. Phys. 2021, 154, 064301.T Y.-C. Chan and D. J. Nesbitt, J. Mol. Spectrosc. 2023, 392, 111743.a Hurtmans et al., J. Chem. Phys. 2000, 113, 1535.U

The nitrate radical (NO₃) may have been one of the first radicals spectroscopically detected [1]. Nevertheless, the spectroscopy of its X ²A₂^′ electronic ground state is extremely complex, largely due to extensive vibronic coupling with and within the B ²E^′ excited state. Vibronic coupling models predict the rather unintuitive result that the fundamental of the degenerate N-O stretching mode (ν₃) has negligible IR intensity and is located near 1050 cm⁻¹ [2]. A strongly IR-active feature at 1492 cm⁻¹, which was originally assigned to the ν₃ fundamental [3], is then attributed to the ν₃+ν₄ combination band. This reassignment has resulted in a long-lasting and still ongoing controversy about the correct ν₃ frequency [4]. Here, we use the IR-cryo-SEVI technique, which was only recently extended to polyatomic molecules [5], to record high-resolution photoelectron spectra of vibrationally excited NO₃⁻. By selectively exciting the ν₃ and 2ν₃ levels with an infrared laser prior to photodetachment, additional transitions involving the ν₃ mode can be observed that do not appear in the photoelectron spectra of vibrationally cold NO₃⁻ [6]. The resulting IR-cryo-SEVI spectra are in excellent agreement with simulations predicted by the vibronic coupling model. Most importantly, we observe a feature that-even without the help of theory-clearly indicates a ν₃ fundamental frequency close to 1050 cm⁻¹. [1] J. Chappuis, Ann. Sci. Ec. Norm. Super. 11, 137-186 (1882). [2] J. F. Stanton, Mol. Phys. 107, 1059-1075 (2009). [3] T. Ishiwata et al., J. Chem. Phys. 82, 2196-2205 (1985). [4] M. Fukushima, J. Mol. Spectrosc. 387, 111646 (2022). [5] J. A. Lau, M. DeWitt, et al., J. Phys. Chem. A 127, 3133-3147 (2023). [6] M. C. Babin et al., J. Phys. Chem. Lett. 11, 395-400 (2020).