l0pt Figure The aromatic, six-membered, heterocyclic molecule with two nitrogen atoms in the para positions, C₄N₂H₄, is called pyrazine. Pyrazine is commonly found in various food products contributing to their taste and flavor.¹ Pyrazine is also a molecule that is interesting for computational and experimental spectroscopy; as a 10-atom, 42-electron molecule, pyrazine is in the middle ground of being amenable to advanced simulation methods while featuring a rich vibronic spectrum. In my talk I will present simulations of the first absorption band of pyrazine. The simulations follow the Köppel-Domcke-Cederbaum approach for constructing the multi-mode multi-state vibronic Hamiltonians.² We use ab initio model parameters that are derived by following the definition of diabatic states of Ichino, Gauss and Stanton.³ References: 1. J.A. Maga, Pyrazine Update. Food Rev. Int. 8, 479–558 (1992). 2. H. Köppel, W. Domcke, and L.S. Cederbaum, Multimode Molecular Dynamics Beyond the Born-Oppenheimer Approximation. In Advances in Chemical Physics (eds I. Prigogine and S.A. Rice). (1984). 3. T. Ichino, J. Gauss, and J.F. Stanton, Quasidiabatic States Described by Coupled-Cluster Theory. J. Chem. Phys. 130, 174105 (2009).

r0pt Figure UV pump-probe experiments were performed to measure the gain profiles and numerical gain coefficients of stimulated Brillouin scattering in low density molecular gases. Pairs of counterpropagating pulses, each around 5 eV, were passed through a gas chamber and analyzed to determine gain profiles. Frequency adjustments made to the probe laser seeder allowed for a tuning range of ±10 GHz. Access to both seeders allows for an even larger range of tunability while maintaining narrowband behavior. Computer simulations of intersecting angled laser pulses were then used to calculate an effective interaction length for various beam configurations, allowing for numerical coupling coefficients to be calculated. Both gain profile shapes and calculated numerical values are in agreement with the kinetic theory of Brillouin gain. Examples of the recovered gain profiles and normalized theoretical curves for molecular nitrogen are shown in the figure to the right.

In the usual case, computational chemistry calcs are done with Gaussian-type orbitals (GTO) in either spherical or Cartesian form, or, like as not, done in spherical form and then reported in Cartesian form, or, perhaps again, reported in one form in one part of the output and in other parts, the other. The forms are equivalent, but Cartesian forms treat all 3 coordinate directions identically, while the axial spherical harmonic forms have a unique axis with two rotation angles. Basis functions used in computations are grouped by angular momentum L (s,p,d,f,g,h,...). Angular coordinates are chosen spherical or Cartesian with radial falloff determined by L. Although they are equivalent, the Cartesian and spherical expansions are derived from diffeent starting points: the first from the 3D Taylor series and the second from a sum rule on renormalized spherical harmonics giving the Coulomb potential. Integrating Cartesian polynomials over the charge density give terms in the Taylor series, also known as the primitive or traced expansion. The primitive expansion is reduced to the traceless form by using permutation invariance of derivatives and applying the Laplace equation. This leads to a traceless Cartesian expansions of energies, multipole moments and polarizabiities that is spectroscopically useful. We describe the use of polynomial integrals over the charge density as degeneracy-weighted terms in the traced Cartesian expansion of the molecular electric potential, the transformation to the differently-weighted traceless Cartesian multipoles and the relation to the spherical multipoles. Despite predictions that traced terms apply in some cases, recent experiments show that the traceless treatment is support by experiment, so primitive moments are not of spectroscopic utility. We show some diagnostic applications of multipoles in spectroscopy and the use of them in determining the bond-length quenching of atomic polarizbility in diatomics.

l0pt Figure Hydrogen-bound complexes play important roles in the atmosphere, but are inherently difficult to study at ambient temperatures. We present a conceptually simple model for understanding the significant changes that occur with temperature in the infrared spectra of such complexes. We have measured room temperature spectra of methanol dimer and two deuterated isotopologues in the OH(D)-stretching region. We correctly predict spectral changes observed in the gas phase for the bound OH-stretch in methanol dimer from jet to room temperature, and corroborate this with experimental and theoretical results for the deuterated isotopologues. The origin of the observed spectral features is explained based on a reduced-dimensional vibrational model, which includes the two high-frequency OH-stretches, the two methyl torsions and the six intermolecular low-frequency vibrations. Key to the success of the model is a new coordinate definition to describe the intrinsic large-amplitude curvilinear motion of the low-frequency vibrations. Despite the deceivingly simple appearance of the room temperature bound OH-stretching fundamental band, it consists of  ∼ 10⁷ vibrational transitions. The model is applicable to the atmospherically relevant hydrated complexes.

Buckminsterfullerene (C₆₀) represents the largest and highest symmetry (I_h) molecule that has been observed with complete rovibrational resolution. Obtaining high-resolution gas-phase infrared spectrum of C₆₀ is challenging due to its negligible vapor pressure and enormous vibrational partition function ( > 10³) at room temperature. Overcoming these challenges requires methods to efficiently generate and cool gas phase C₆₀. While oven sublimation of C₆₀ ( ≈ 900 K) coupled with a cryogenic cell enables cooling down to ≈ 150 K, this is still too hot (due to radiative heating of the cryogenic cell) to observe the absolute rovibrational ground state via infrared spectroscopy and severely limits the resolution of electronic spectroscopy. In this work, we demonstrate a laser desorption C₆₀ source coupled to an improved cryogenic cell design. A soft laser pulse desorbs C₆₀ from a thin film into a cryogenic cell filled with cold Ar buffer gas ( ≈ 82 K). Crucially, the uniformity of C₆₀ thin film and soft laser desorption process allow stable pulse-to-pulse C₆₀ yield, which facilitates high-resolution infrared measurements of state-resolved population distributions and nuclear quantum statistics. We characterize the temperature via Voigt analysis of rovibrational transitions and find the translation is thermalized to the buffer gas temperature, while the rotational temperature is only slightly higher (100 K). The improved rotational cooling conditions allow us to observe the low-J regime, in which some R(J) lines (e.g. J = 13, 14, 17) are forbidden due to permutation symmetry of 60 identical bosonic carbon atoms. Finally, this laser desorption source has negligible heat load and can be easily coupled to a liquid-helium cryostat to achieve potentially much lower C₆₀ temperatures. The soft desorption process is also generalizable to gas phase generation of a wide range of large molecules. For example, quantum-state resolved spectroscopy of even larger fullerenes (e.g. C₇₀) becomes possible.

r0pt Figure Extractants containing soft Lewis base donor atoms have been employed in lanthanide(III)/actinide(III) separations. The advantage of these complexing agents is attributed to favorable covalent interactions between the actinides and the soft donor ligands. There has been intense focus on determining if covalency is the driving force behind this differential behavior, the role that the 5f orbitals play in this preference, and the impact on the spectroscopic properties. Americium, located in the middle of the actinide series, has been of particular interest because of its contracted 5f shell and the need of separating it from its isoradial and isoelectronic neodymium(III) and europium(III) analogs. Density functional theory and multireference analysis on Am(III), Nd(III) and Eu(III) coordinated to chalcogen (S, Se) containing ligands suggests that there is an enhanced metal-ligand participation for the actinide congener, and that going from S to Se, there is further perturbation of the valence shell orbitals of the americium centers. This is reflected on low-energy charge transfer bands not observed before in these types of systems. The authors gratefully acknowledges the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry program (2020LANLE372, DE-AC52-06NA25396) and the postdoctoral support provided by the Glenn T. Seaborg Institute. LA-UR-24-22095html:<hr /><h3>Footnotes: The authors gratefully acknowledges the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry program (2020LANLE372, DE-AC52-06NA25396) and the postdoctoral support provided by the Glenn T. Seaborg Institute. LA-UR-24-22095

Complex autoionization processes following x-ray-matter interactions involve the ejection of electrons via the decay of unstable states created by x-rays. This fundamental process affects the determination of structure and its temporal evolution across the biological and physical sciences. Furthermore, autoionization dominates the decay of light elements (first row p-block) following x-ray absorption. The high detection yields of the ejected electrons provides a powerful observable which is sensitive to the local structural environment of the absorbing atom site. Thus far, the complex nature of x-ray induced autoionization limits quantum molecular simulations to isolated molecules and simple condensed phase systems. This work presents how efficient and accurate quantum chemical methods can generate datasets to train deep-neural networks predicting autoionization spectra from molecular structures. Our novel research direction initiates the development of AI applications for accelerating both the predictive and analytical strength available to x-ray induced autoionization simulation.

We report calculations of double core-hole (DCH) shake-up states as well as double shake-up states in Ar and Kr using a recently implemented two-component multi-configuration self-consistent field (MCSCF) scheme using the exact two-component atomic-mean-field (X2CAMF) Hamiltonian. With this new computational tool, we achieved sub-eV agreement with experiment assignment in Ar KL edge DCH states. Our calculations on Kr suggest a significant contribution of double shake-up states in both KL and KM edge signals, therefore providing new insights into the assignment of spectra.

3-(4-Fluorophenyl)-5-phenyl-1H-pyrazole (FPHP) and 3-(4-Chlorophenyl)-5-phenyl-1H-pyrazole (CPHP) molecules were characterized by their Fourier transform infrared spectra in the range of 4000-400cm⁻¹, Fourier transform Raman spectra in the range 4000-50cm⁻¹and Ultraviolet-Visible spectra in the range 200-400 nm. Torsional potential scans and barrier heights were computed around rotating bonds [ C-C (phenyl-pyrazole) bond in both the molecules; C-C (pyrazole-fluorophenyl (FPHP); (pyrazole-chlorophenyl (CPHP))]. These barrier heights were compared with those of closely related molecules. Geometry optimization, harmonic frequencies, potential energy distribution (PED), general valence force field, Raman and infrared intensities, frontier molecular orbital (FMO) parameters, nonlinear optical (NLO) behaviour, NBO (natural bond orbital) properties were computed with B3LYP exchange correlation functional and 6-311++G(d, p) basis set using DFT. Fukui functions, MESP (molecular electrostatic potential), ELF (electron localized function), localized orbital location (LOL) parameters were calculated using the same level of theory for understanding chemical reactivity of both the molecules. The time -dependent density functional theory (TD-DFT) was made use of to calculate absorption maxima and their corresponding oscillator strengths for their electronic transitions in a solution of DMSO-d6 for both the molecules and compared with their observed values. All the fundamental vibrations were assigned for the first time for both the molecules. The drug characteristics of the molecules were evaluated by using drug-likeness. The target protein and the ligand reactions were investigated using molecular docking.

The infrared spectroscopy of cryptands containing Na⁺, K⁺, Rb⁺, Sr⁺², and Ba⁺² ions are studied theoretically and experimentally, extending our initial study¹ that included the K⁺ and Ba⁺² ions. In these studies, [2, 2, 2] cryptand complexes are modified to contain a phenyl group. This chromophore provides a UV absorption that allows us to record a UV photofragment spectrum, a spectrum which shifts to unique wavelengths depending on size and charge of the embedded ion. Infrared spectra in the alkyl CH stretch region are recorded using IR-UV double resonance. The spectra are theoretically modelled by identifying energetically low-lying conformers and modeling their CH stretch fundamentals with local mode Hamiltonians. These Hamiltonians include Fermi coupling between CH stretches and nearly degenerate states. Energetic and spectroscopic results are calculated at the M062x and B3LYP functionals. Two key factors for understanding the CH stretch region, and which are highlighted in our work, are the effects of the N atoms lone pairs and the resonance interactions with CH₂ wagging degrees of freedom. The theoretical spectra are used to identify those conformers observed experimentally and to explore the role of the central ion’s size in modifying the structure of the crypt and, consequently, the peak patterns in the CH stretch spectral region.
1. Molecular Cage Reports on Its Contents: Spectroscopic Signatures of Cryo-Cooled K⁺- and Ba²⁺-Benzocryptand Complexes, C. D. Foley, C. D. Allen, K. Au, C. Lee, S. B. Rempe, P. Ren, E. L. Sibert III, and T. S. Zwier, J. Phys. Chem. A 2023 127 (30), 6227-6240.