We present a combined millimeter-wave and high-resolution infrared (FTIR) analysis of the the spectra of DN₃. We have observed the infrared spectrum of DN₃ at a resolution of 0.0009 cm⁻¹using the synchrotron at the Canadian Light Source between 30 and 5000 cm⁻¹at several pressures between 1 and 100 mTorr. We have also measured the millimeter-wave spectrum of DN₃ at Wisconsin and at Prague, covering a frequency range from 130 to 730 GHz. Using classical linear least-squares fitting and combination differences, we have assigned the spectra for the ground state and 15 lowest-energy vibrationally excited states (including 8 not previously studied). The latter include the four combination states of either ν₃ or ν₄ with either ν₅ or ν₆ and the tetrad of states involving three quanta of ν₅ and ν₆. In fact, all sixteen of these vibrational states are a complex polyad coupled by Coriolis, Darling-Dennison, and Fermi interactions that strongly perturb the observed transition frequencies. We are working toward combining all of this spectral data to achieve a global sixteen-state fit using SPFIT.

Rotational spectra of vibrationally excited species are important tracers for the temperature structure of astronomical objects such as high-mass star-forming regions and are also assumed to be the reason for many yet unassigned spectroscopic features observed in line-rich sources. Thus, they bear high astronomical interest but are so far largely unexplored. Here, the ground state and 17 vibrationally excited states of the trans conformer of glycialdehyde (C₃H₄O₂) are examined. The low-lying torsional motion of the aldehyde group trans to the oxirane ring at about 140 ±10 cm⁻¹ and the three non-zero dipole moment components lead to a dense and complicated mm-wave spectrum. High-resolution broadband spectra were recorded in the frequency ranges of 75−170 GHz and 500−750 GHz with an in-house synthesized sample. The broadband spectra alone were not sufficient for identifying the weak rotational transitions of vibrationally excited states high in energy and therefore were supported by double-resonance double-modulation (DM-DR) measurements for which an updated experimental implementation was used.O. Zingsheim et al., J. Mol. Spectrosc. 381 (2021) 111519M-DR measurements are a convenient method to derive the relationships of transitions by identifying transitions that share a common energy level. This greatly simplified the assignment of series of perturbed or weak vibrational satellites. Candidates were identified either by visual inspection in Loomis-Wood plots in our LLWP softwareL. Bonah et al., J. Mol. Spectrosc. 388 (2022) 111674; https://llwp.astro.uni-koeln.de/r automated with peaklists. The preliminary results of the analysis including the interaction analysis of selected states will be presented.

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Footnotes:

O. Zingsheim et al., J. Mol. Spectrosc. 381 (2021) 111519D L. Bonah et al., J. Mol. Spectrosc. 388 (2022) 111674; https://llwp.astro.uni-koeln.de/o

Propylene oxide (also known as methyloxirane) is one of the simplest chiral molecules and also shows effects of internal rotation of its methyl group. It is a much studied molecule and has received much scientific attention in the past.
Propylene oxide has 24 fundamental vibrational modes, with each vibrational mode consisting out of thousands of ro-vibrational transitions.
So far, ro-vibrationally resolved experimental data for the vibrational spectrum however, have been quite sparse. To address this problem, there has recently been a strong effort in our research group to obtain jet-cooled ro-vibrationally resolved spectra of propylene oxide. In this way the CH₃ torsion, CH₂, CH₃ rocking and ring breathing fundamental vibrational modes have been investigated with different experimental techniques.
In this work we will present our efforts in obtaining a continuous jet-cooled spectrum of propylene oxide in the 3μm region with a cw-OPO cavity ringdown spectrometer and report about our latest progress in analysing the C-H stretching group in the 3μm region. In the measured spectrum, various combination bands have also been observed, whose impact on neighbouring vibration bands will also be discussed.

Cobalt cation-acetylene complexes are size-selected and studied with infrared laser photodissociation spectroscopy. Co⁺(C₂H₂)_n complexes are produced via laser vaporization in a pulsed supersonic expansion of argon seeded with acetylene. These complexes are mass-selected in a reflectron time-of-flight mass spectrometer, and their infrared spectra are measured in the C–H stretching region (2800 - 3400 cm⁻¹) with photodissociation spectroscopy. A coordination number of three is found for cobalt cation. Density functional theory calculations are performed with the B3LYP functional with the Def2TZVP basis set to support experimental spectra. Bands are observed for both the asymmetric and symmetric stretch of acetylene and are red-shifted from those of free acetylene. The presence of reacted and cation-π structures is investigated by comparing experiment to theory.

The vibrational frequencies of water molecules that are incorporated into water clusters encode an abundance of information about the strength of inter- and intramolecular interactions, and how these interactions are affected by the hydrogen-bonding environment. The addition of intensity information provides insights on how the electronic structure changes with molecular vibrations. In this talk, we will focus on the spectra of water and water clusters, and will explore how the strength and types of hydrogen bonds are encoded in these spectra. We start by considering the 1-0 transitions of the OH oscillators in the 3000 – 3500 cm⁻¹ region. A challenge with this spectral region comes from the fact that with increased hydrogen bond strength, the OH stretch frequency becomes more red-shifted, the intensity increases, and the transitions become more spread out. This makes it difficult to use the shape of spectral envelope in this region to enumerate the number of water molecules that sample a specific hydrogen bonding environment. In contrast, the sensitivity of the intensity to the hydrogen bonding environment is much weaker for transitions to states with one quantum of excitation in both the OH stretching and the HOH bending vibrations in the same water molecule. We will explore the origins of the differences in the sensitivity of the intensities of these two types of transitions to the hydrogen bonding environment, and how we can use the shape of thee spectral envelope in the 5200 cm⁻¹ region to enumerate of the hydrogen bonding environments sampled by individual water molecules.

Water clusters provide a set of systems where the influence of nuclear quantum effects on the stability of the hydrogen-bonding network can be studied both experimentally and theoretically. While electronic structures calculations indicate that the lowest energy isomer of (H₂O)₆ is the prism structure, analysis of the spectra of (H₂O)₆ by Johnson and Pate and their co-workers showed that the cage isomer is the lowest energy structure for (H₂O)₆. However, when deuterated, the prism isomer becomes the lowest energy structure again. This was further explored by our group using diffusion Monte Carlo, where we found the introduction of zero-point energy to (D₂O)₆ leads to the cage and prism isomers being nearly degenerate as opposed to (H₂O)₆ where the cage isomer is significantly lower in energy than the prism isomer. Similar effects are seen in protonated water clusters. While smaller protonated water clusters, H⁺(H₂O)₂₋₅, show no observable change in the populations of the isomers observed in the spectra upon deuteration, the spectra of H⁺(H₂O)₆ and D⁺(D₂O)₆ have significant differences due to changes in the populations of isomers that are present. Through a combination of high-level electronic structure calculations performed by the Jordan and Xantheas groups, vibrational perturbation theory, and diffusion Monte Carlo studies we investigated the effect that deuteration has on the spectrum of H⁺(H₂O)₆. Vibrational perturbation theory provides some insights into how the spectral signatures of each of the isomers of H⁺(H₂O)₆ changes. The challenge of vibrational perturbation theory is that it does not handle large amplitude motions well. Using diffusion Monte Carlo, we obtained an exact anharmonic ground state wave function and relative zero-point energies of several isomers of H⁺(H₂O)₆. The ground state wave functions were found to be localized near the stationary point that each simulation started from. This allows us to use mappings between the structural information, such as OO and OH distances, to the frequencies of hydrogen-bonded OH stretches to explore the contributions of various isomers to the observed spectrum.

Carbocations are a class of important organic intermediates, which exist in hydrocarbon plasmas and are believed to play a role in the chemistry in the interstellar medium. Protonated ethylene () is one such carbocation, which is formed from the smallest alkene. It is also important in mass spectrometry as it appears in the mass spectra of many organic molecules and it is used as the protonating agent in chemical-ionization mass spectrometry. High-level electronic structure calculations predict that the minimum energy structure is the non-classical one in which the excess proton is equidistant from the two carbon atoms. This was confirmed by the IR spectrum of obtained by the Dopfer and Duncan groups. In this work, we use fixed-node Diffusion Monte Carlo (DMC), based on a potential with CCSD(T)-level accuracy to obtain excited state wave functions and vibration frequencies. The analysis of excited state wave functions confirms that the ion is localized in the non-classical minimum on the potential. It also allows us to explore the couplings among the motions of the hydrogen atoms. The frequencies obtained from the DMC calculations are compared to the experimental frequencies in the IR spectrum. Based on this, one of the weak features between 2500 and 3000 cm⁻¹ in the spectrum is assigned to the combination band of the proton transfer and breathing mode. Lastly, extensions of DMC to calculations of intensity will be discussed

Two closely related compounds, the H₃O· radical (HR) and H₃O⁻ anion (HA), are of significant interest in astrophysics. Both species exist as pyramidal isomers, but are not stable at room temperature. However, they can be stable at low temperatures and have been observed in interstellar space and comets. Similar to the ammonia molecule, HR and HA exist in two equivalent configurations, which makes tunneling between them possible. However, to our knowledge, no attempts have been made to calculate tunneling splittings even for the ground vibrational states of these species. In this work, a non-standard set of vibrational coordinates was utilized to describe the inversion motion. The first coordinate used was the distance (h) between the oxygen atom and the plane formed by three hydrogen atoms. The second coordinate was a fully symmetrical coordinate q₁₂₃, which was composed of three coordinates (q₁, q₂, q₃) describing the distances between valence-free hydrogen atoms. These coordinates were tested on NH₃ and H₃O⁺ species, and tunneling splittings of the ground states were obtained for both at the CCSD(T)/Aug-cc-pVQZ level of theory, yielding values of 0.78 and 53.5 cm⁻¹, respectively. For the HR and HA species, calculations of the two-dimensional potential energy surface (2D PES) were performed at the UHF CCSD(T)/d-aug-cc-pVQZ, UHF CCSD(T)/aug-cc-pVQZ, and CCSD(T)/aug-cc-pVQZ, CCSD(T)/d-aug-cc-pVQZ levels of theory. The calculated tunneling splittings in the ground states of the hydronium anion (4.1 cm⁻¹) and radical (3.4 cm⁻¹) were found to be significantly lower than the tunneling splitting in the ground state of the hydronium cation (53.5 cm⁻¹).

Chirality is an important property of biosystems and can be found also in technologies such as 3D displays or energy storage. The chirality is reflected in the chiroptical signal. For a full understanding, it is important to know the link between the observed optical spectra and the structure. To this aim, computational spectroscopy plays a crucial role. However, common protocols, which rely on pure electronic transitions, have limited accuracy, and are generally insufficient to predict chiroptical properties, such as electronic circular dichroism or circularly polarized luminescence. Our previous studies have shown the importance of considering the vibrational contributions in simulations of UV-visible spectra of chiral molecules as well. However, the inclusion of vibronic contributions significantly increases the computational cost, and the underlying harmonic approximation is particularly problematic for flexible compounds. We present a systematic approach to account for the vibronic effects in large chiral molecules. Starting from a prototypical molecule to highlight the importance of vibronic effects in chiroptical spectra, a general protocol is tested on Ir-complex molecules[1]. The procedure is then applied to a chiral boron dipyrromethene dye (BODIPY), which exhibits a variety of conformations and crossing of excited electronic states[2]. To analyze the results, graphical tools were developed and used. The data generated by the simulations and potential sources of inaccuracy can be overviewed more easily. These visualization techniques can provide new insights into the origin of the chiroptical signal and help design more efficient chiral molecules of technological interest.