There is a myriad of quantum-chemical methods which can be used to predict the OH stretching spectrum of cold, vacuum-isolated hydrate clusters, with a perspective to deepen our understanding of aqueous solution dynamics. They range from uniformly scaled harmonic DFT predictions to fully anharmonic high level wave function theory treatments. Fortuitous error compensation is a major issue in such a situation, in particular if the experimental result is known beforehand. The HyDRA (Hydrate Donor Redshift Anticipation) blind challenge is an effort to circumvent previous knowledge, by inviting theory groups to make predictions for 10 not yet vibrationally characterized organic monohydrates and performing the corresponding experiments in parallel. This contribution will present the procedureTaija L. Fischer, Margarethe Bödecker, Anne Zehnacker-Rentien, Ricardo A. Mata, Martin A. Suhm, ChemRxiv, 2021, DOI: 10.26434 chemrxiv-2021-w8v42.nd discuss results of the recently finished blind challenge HyDRA by comparing the theoretical predictions to our experimental results.

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Taija L. Fischer, Margarethe Bödecker, Anne Zehnacker-Rentien, Ricardo A. Mata, Martin A. Suhm, ChemRxiv, 2021, DOI: 10.26434 chemrxiv-2021-w8v42.a

r0pt Figure Using well-established FTIR and Raman jet spectroscopic set-ups, the gas phase vibrational database of the cyclic formic acid dimer, (FF), has been reviewed and updated for the slow fingerprint vibrations [below 1500 cm⁻¹] of the main and its three symmetrically deuterated isotopologues.A. Nejad, K. A. E. Meyer, F. Kollipost, Z. Xue, M. A. Suhm , J. Chem. Phys. 2021, 155, 224301 and A. Nejad, PhD thesis, submitted (2022).xperimental benchmarks validate the popular second-order vibrational perturbation theory approach in combination with high-level [hybrid] force fields which is shown to provide accurate predictions for moderate excitations of the intermolecular van der WaalsA. Nejad, M. A. Suhm , J. Indian Inst. Sci. 2020, 100, 5.nd intramolecular fingerprint vibrations^a of (FF). The new and extended benchmark-quality database of (FF) is particularly useful to guide recent effortsC. Qu, J. M. Bowman, Phys. Chem. Chem. Phys. 2019, 21, 3397 and A. M. Santa Daría, G. Avila, E. Mátyus, Phys. Chem. Chem. Phys. 2021, 23, 6526.o accurately model the 24-dimensional vibrational dynamics of this prototypical model system in a `bottom-up' approach. As a byproduct, the number of assigned vibrational fundamentals [and selected overtone bands] of the vacuum-isolated formic acid trimer, F(FF), has been drastically increased.^a Since the polar dimer, FF, is a fragment of the trimer, the experimentally validated theoretical description of F(FF) promises to provide reliable spectral predictions for future gas phase spectroscopic searches of FF.

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A. Nejad, K. A. E. Meyer, F. Kollipost, Z. Xue, M. A. Suhm , J. Chem. Phys. 2021, 155, 224301 and A. Nejad, PhD thesis, submitted (2022).E A. Nejad, M. A. Suhm , J. Indian Inst. Sci. 2020, 100, 5.a C. Qu, J. M. Bowman, Phys. Chem. Chem. Phys. 2019, 21, 3397 and A. M. Santa Daría, G. Avila, E. Mátyus, Phys. Chem. Chem. Phys. 2021, 23, 6526.t

The linear radical cation of cyanoacetylene, HC₃N⁺ (²Π), is of fundamental spectroscopic interest due to its strong spin-orbit and Renner-Teller interactions, which have been investigated previously in several high-resolution photoelectron spectroscopic (PES) studiesDai, Z. Sun, W. Wang, J. Mo, Y., J. Chem. Phys. 2015, 143, (5), 054301.^,Desrier, A. Romanzin, C. Lamarre, N. Alcaraz, C. Gans, B. Gauyacq, D. Liévin, J. Boyé−Péronne, S., J. Chem. Phys. 2016, 145, (23), 234310.^,Gans, B. Lamarre, N. Broquier, M. Liévin, J. Boyé-Péronne, S., J. Chem. Phys. 2016, 145, (23), 234309. Here, we present the first broadband vibrational action spectroscopic investigation of this ion through the infrared pre-dissociation (IRPD) method using a Ne tag. Experiments have been performed using the FELion cryogenic ion trap instrument in combination with the Free Electron Lasers for Infrared eXperiments (FELIX) Laboratory at the Radboud University (Nijmegen, The Netherlands)Jusko, P. Brünken, S. Asvany, O. Thorwirth, S. Stoffels, A. van der Meer, L. Berden, G. Redlich, B. Oomens, J. Schlemmer, S., Faraday Discuss. 2019, 217, 172-202. The vibronic splitting patterns of the 3 interacting bending modes (ν₅,ν₆,ν₇), ranging from 180-1600 cm⁻¹, could be fully resolved revealing several bands that were previously unobserved. The associated Renner-Teller and cross-coupling constants were determined by fitting an effective Hamiltonian to the experimental data, and the obtained spectroscopic constants were in reasonable agreement with previous studies of the HC₃N⁺ ion. The influence of the attached Ne atom on the infrared spectrum was investigated by ab initio calculations at the CCSD(T) level of theory, showing that the discrepancies between the IRPD and PES data can be explained by the effect of the Ne binding.

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Dai, Z. Sun, W. Wang, J. Mo, Y., J. Chem. Phys. 2015, 143, (5), 054301.\end Desrier, A. Romanzin, C. Lamarre, N. Alcaraz, C. Gans, B. Gauyacq, D. Liévin, J. Boyé−Péronne, S., J. Chem. Phys. 2016, 145, (23), 234310. Gans, B. Lamarre, N. Broquier, M. Liévin, J. Boyé-Péronne, S., J. Chem. Phys. 2016, 145, (23), 234309.. Jusko, P. Brünken, S. Asvany, O. Thorwirth, S. Stoffels, A. van der Meer, L. Berden, G. Redlich, B. Oomens, J. Schlemmer, S., Faraday Discuss. 2019, 217, 172-202..

While harmonic frequency calculations are widespread across chemistry¹, sparse benchmarking is available to guide users on appropriate model chemistry recommendations (i.e., a method and basis set pair). Instead, studies exploring the dependence of harmonic frequencies on model chemistry have focused on producing multiplicative scaling factors to match the calculated harmonic frequencies to experimental fundamental frequencies². Along with the scaling factor, it is often common to calculate the root-mean-squared error (RMSE) between the scaled harmonic and experimental fundamental frequencies, and use this value as metric of model chemistry performance. We recently compiled a set of over 1,400 scaling factors³ spanning hundreds of methods and basis sets, thus allowing approximate comparisons between different model chemistries². However, initial recommendations from this analysis can only be preliminary, as the differences in the benchmark databases used means that the RMSE metrics cannot be fairly compared across different publications. Here, we introduce a new benchmark database for vibrational frequency calculations (VIBFREQ1292) containing 1,292 experimental fundamental frequencies and CCSD(T)-F12c/cc-pVDZ-F12 harmonic frequencies for 141 molecules. Assuming that our ab initio calculations reduce model chemistry error to a minimum, and noting the importance of using frequency-range-specific scaling factors, our analysis shows that the intrinsic error between the scaled harmonic and experimental frequencies usually lies below 15 cm⁻¹. Thus, using VIBFREQ1292 as our reference set, we have rigorously assessed the performance of over 300 general-purpose model chemistry choices for harmonic frequency calculations. Model chemistry recommendations, as well as expected computational errors will be presented in this talk. 1. Scott, A.; Radon, L., J. Phys. Chem. 1996, 41, 16502-16513. 2. Zapata Trujillo, J. C.; McKemmish, L. K., Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2021, e1584. 3. Zapata Trujillo, J. C.; McKemmish, L. K., Harvard Dataverse, V1, 2021, https://doi.org/10.7910/DVN/SQK6YU.

Polycyclic aromatic hydrocarbons (PAHs) are responsible for the aromatic infrared bands (AIBs) observed in various astronomical objects. Studying their sharp Q-branches associated with the out-of-plane bending vibrational modes under low rotational excitation might be a key for their identification in the interstellar medium (ISM). IR spectrum of naphthalene was recorded around 12.7 μm using the jet-AILES setup, coupled to the Fourier transform spectrometer (Bruker IFS 125 HR) equipping the AILES beamline of the synchrotron SOLEIL. In the jet, an efficient rotational relaxation of naphthalene occurs resulting in a rotational temperature of about 25 K, while the vibrational cooling is limited due to an insufficient number of two-body collisions in the supersonic expansion. This leads to an interesting non-LTE situation, favorable for the detection of hot bands: the low rotational temperature drastically simplifies the rotational structure and magnifies the Q-branches, while the higher vibrational excitation allows the presence of many transitions from moderately excited vibrational states. To assign the observed hot bands we have used the AnharmoniCaOs code, which explicitly considers Fermi and Darling Dennison resonances for a better accuracy of the band positions, and the second order dipole derivatives to simulate intensities of overtone, combination, and difference bands. Our program is unique in the sense that it can produce spectrum at a non-zero kelvin temperature unlike standard commercially available quantum chemistry packages. It also enables us to assign arbitrary (non-thermal) populations to individual vibrational states from which transitions originate, allowing us to simulate non-LTE spectra.

The potential formation of halogen bonded complexes between a donor, heptafluoro-2-iodopropane (HFP), and the three acceptor heterocyclic azines (azabenzenes: pyridine, pyrimidine, and pyridazine) is investigated herein through normal mode analysis via Raman spectroscopy, density functional theory, and natural electron configuration analysis. Theoretical Raman spectra of the halogen bonded complexes are in good agreement with experimental data providing insight into the structure of these complexes. The exhibited shifts in vibrational frequency of as high as 8 cm-1 for each complex demonstrate, in conjunction with NEC analysis, significant evidence of charge transfer from the halogen bond acceptor to donor. Here, an interesting charge flow mechanism is proposed involving a conduit-like flow of electron density from each azabenzenes’ interacting nitrogen atom through the halogen bond and iodine atom to the highly electron-withdrawing fluorine atoms. This mechanism provides further insight into the formation and fundamental nature of halogen bonding and its effects on neighboring atoms. The present findings provide novel and deeper characterization of halogen bonding with applications in supramolecular and organometallic chemistry.

Time resolved Raman studies of halide-thiocyanate dimer radical anions, (X-SCN)^.− (for X=Cl, Br, I), were performed in resonance with their peak of light absorption wavelength at 415 nm. In two of the experiments (for X⁻=Br⁻ or I⁻) the apparent Raman spectrum contains signatures of three hemibonded intermediates present simultaneously in mutual equilibria with their precursor and successor hemibonded radical counterparts: X₂^.− + SCN⁻ = (XSCN)^.− + X⁻ = (SCN)₂^.− + X⁻. In order to extract (X-SCN)^.− (for X=Br, I) from the composite spectrum additional experiments were performed to generate pre-resonance spectra of X₂^.− and (SCN)₂^.− at 415 nm in order to collect and then subtract their contributions from the composite spectrum. Ten Stokes Raman bands of the halide-thiocyanate radical anions (X-SCN)^.− (for X=Br, I) were observed in the 60-2400cm⁻¹region. They were assigned in terms of the strongly enhanced 198 and 174cm⁻¹, weakly enhanced 719.5 and 729cm⁻¹, and moderately enhanced 2069 and 2078cm⁻¹fundamentals, their overtones, and combinations in BrSCN^.− and ISCN^.−, respectively. On attempt to record chloride intermediate only characteristic bands coming from the mixed contributions of Cl₂^.− and (SCN)₂^.− have been apparent. Quantum chemical calculations using a range-separated hybrid density functional (ωB97x) with flexible augmented correlation-consistent basis sets support the spectroscopic assignments of the strongest fundamental vibrations to a predominantly S-X (X = Br, I) stretching mode and the features around 720cm⁻¹and 2070cm⁻¹to CS and CN symmetric stretching modes, respectively. Interestingly, CS and CN bond stretching vibrational frequencies in asymmetrical (X-SCN)^.− anion radicals are shifted a few wavenumbers down or up in comparison to the symmetrical (SCN)₂^.− molecule in BrSCN^.− or ISCN^.−, respectively. Considering that ClSCN^.− seems to have vibrational frequencies almost identical to (SCN)₂^.− does not grant any systematic correlation between hemi-bond polarization in this array of molecules and vibrational frequencies of CS and CN bonds. A possible explanation of such an observation can relate to a counteracting induction and migration effects in σ and π bonds, respectively, upon charge migration across the molecule.

Methylamine is a molecule performing two strongly coupled large amplitude motions: CH₃ internal rotation and NH₂ inversion. The rovibrational spectrum of the methylamine molecule has been extensively studied both experimentally and theoretically. The analyses of infrared bands such as NH₂ inversion or CN stretching show significant perturbations from highly excited torsional states. In order to untangle the interactions in the 700-1200 cm⁻¹region of the methylamine spectrum, it is crucial to assign the perturbing excited torsional states (3ν₁₅ and 4ν₁₅). Both states are located well above the top of the torsional barrier. Thus, the splittings between the lower and upper sublevels are very large (80 to 180 cm⁻¹) and only low lying sublevels of 3ν₁₅ or 4ν₁₅ will be experimentally identified. The spectra were recorded with a resolution of 0.00125 cm⁻¹using Bruker IFS-120HR spectrometer at the University of Oulu. The accurate energy levels of the first excited torsional state, ν₁₅, I. Gulaczyk, M. Kreglewski, V-M. Horneman, J. Mol. Spectrosc. 342 (2017) 25-30I. Gulaczyk, M. Kreglewski, JQSRT 252 (2020) 107097 were used as reference values for lower state combination differences in the assignments of the third and fourth torsional hot bands, 3ν₁₅-ν₁₅ and 4ν₁₅-ν₁₅. After the complete analysis in the second torsional overtone region (360-720 cm⁻¹) was performed I. Gulaczyk, M. Kreglewski, V-M. Horneman, JQSRT 217 (2018) 321–328 many of the remaining unassigned lines in this region could be assigned to v=3-1 and v=4-1 bands. Earlier, about 200 transitions of B, E₁₊₁ and E₁₋₁ symmetry for the 3ν₁₅ and 28 transitions of B symmetry for 4ν₁₅ were found N. Ohashi, H. Shimada, W. B. Olson, K. Kawaguchi, J. Mol. Spectrosc. 152 (1992) 298 On the basis of the calculated energy levels for the third and fourth excited torsional states, many transitions of the hot band v=3-1, not assigned previously, have been identified (over 1500 transitions for all symmetry species). As for v=4-1, so far, the previously assigned series were only extended to higher J values (over 100 transitions assigned of B symmetry), but the analysis is in progress. All the assignments were confirmed by the LSCD. Each set of the experimental data was fit to a single state model based on the group theoretical formalism N. Ohashi and J.T. Hougen, J. Mol. Spectrosc. 121 (1987) 474

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I. Gulaczyk, M. Kreglewski, V-M. Horneman, J. Mol. Spectrosc. 342 (2017) 25-30

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I. Gulaczyk, M. Kreglewski, V-M. Horneman, JQSRT 217 (2018) 321–328, N. Ohashi, H. Shimada, W. B. Olson, K. Kawaguchi, J. Mol. Spectrosc. 152 (1992) 298. N. Ohashi and J.T. Hougen, J. Mol. Spectrosc. 121 (1987) 474.

There have been a number of theoretical papers on the structures and energetics of CO₂-Ar_n clusters. But in terms of experiment, the only previous spectroscopic results are for n = 1 (extensive work on the CO₂-Ar dimer) and n = 2 (microwave and infrared spectra of CO₂-Ar₂). We have now obtained and analyzed infrared spectra in the CO₂ ν₃ region for a number of clusters in the range n = 3 to 17. Notable among these are CO₂-Ar₁₅ and CO₂-Ar₁₇, which mark completion of the first solvation shell for CO₂ in argon. These clusters have highly symmetric structures with D_3h and D_5h symmetry, respectively, in good agreement with theory. For n = 15, CO₂ is surrounded by five argon rings, each containing three Ar atoms. For n = 17, there are three rings of five atoms each, plus two additional Ar atoms located on the symmetry axis at each end. The observed spectra are symmetric top parallel bands, and both exhibit distinct intensity alternation which helps to confirm their assignment. Observed B-values are 69.93 MHz for CO₂-Ar₁₅ and 54.52 MHz for CO₂-Ar₁₇. As usual for symmetric rotors, the spectra are not sensitive to the A constant, but we do obtain precise values for the band origins, and hence the vibrational shifts (relative to free CO₂) as induced by the argon cages.

Few polyatomic molecules have been the subject of more ab initio studies than B₂H₆, diborane. The earliest studies were focused on elucidating the structure of its prototypical three-center, two-electron “banana” bonds. Information about the force field and motion of atoms in B₂H₆ can be most directly derived from its vibrational frequencies. However, with eight atoms, high symmetry, and significant vibrational anharmonicities, an exclusively spectroscopic determination of its anharmonic force field is nearly an intractable problem. However, with advances in ab initio methods and the development of methods to treat vibrational frequencies and intensities beyond the harmonic approximation, this challenging system is now amenable to deeper understanding. We decided to use parahydrogen (pH₂) matrix isolation infrared spectroscopy to measure the vibrational wavenumbers and intensities of as many infrared absorptions of B₂H₆ as possible in the 800 to 5000 cm⁻¹region to compare with more recent ab initio studies by Ziegler and Rauhut that go beyond the double harmonic approximation.B. Ziegler, G. Rauhut, J. Phys. Chem. A 123, 3367 (2019).ur studies show nearly quantitative agreement between theory and experiment for the allowed infrared vibrational modes in the surveyed region. We devised a scheme to assign peaks in our spectra that then can be compared directly with computational predictions. Indeed, earlier spectroscopic assignments were hampered by not knowing the anharmonic contributions to the measured vibrational frequencies. We are currently investigating the analogous spectra of the B₂D₆ isotopolog and will present out latest findings and comparisons with available theory at the meeting.

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B. Ziegler, G. Rauhut, J. Phys. Chem. A 123, 3367 (2019).O

r0pt Figure Accurate and comprehensive diatomic molecular spectroscopic data is essential to the measuring and monitoring of gaseous environments, the computational benchmarking of theoretical approaches and, increasingly, in ultracold physics. The recent search for unusual transition metal diatomics, such as TiO and VO, in hot Jupiter exoplanets has demanded spectra of sub 0.1 cm⁻¹accuracy. This experimental need has motivated significant developments in line list construction. A line list contains the assigned rovibronic energy levels of a molecule, as well as the transition frequencies and intensities between these energy levels. Here, I will discuss the new trihybrid construction of line lists, specifically for NH and ZrO. This trihybrid methodology is advantageous as precedence is given to experimental energy levels that independently form a self-consistent network. This list of energy levels is subsequently interpolated with perturbative calculations using model Hamiltonians and extrapolated with variational calculations using fitted potential energy and coupling curves. The exemplar cases of NH and ZrO highlight the diversity of electronic structures encountered in line list construction. For NH, only the uncoupled ground and first excited triplet electronic states are considered, as direct transitions to other states are either forbidden or negligible in intensity. Alternatively, for ZrO, eleven highly coupled electronic states are considered as many transitions are allowed and intense, especially in the hot stellar environment of S-type stars that ZrO characterises.