The electronic structure of CaO is complex due to the large number of low-lying energy states and a multitude of rovibronic interactions among them. Several vibrionic bands of the A’¹Π−X¹Σ⁺ CaO have been identified in the visible region using laser induced fluorescence. Analysis of these rotationally resolved data provides more accurate band origins and rotational constants for levels that were previously determined indirectly using perturbation data. Fluorescence decay lifetime measurements were used to determine the radiative decay rate for the A’ state. A previously noted homogeneous perturbation of the C¹Σ⁺ state was examined to determine the identity of the perturbing state. Dispersed fluorescence spectra and fluorescence decay rate measurements were used to show that the perturbation results from the interaction with a state of ³Π(0+) symmetry.

The alkaline earth monohalides are molecules of interest in fields such as astrophysics, high-temperature chemistry, chemiluminescent reactions, spectroscopy, computational chemistry, and cold molecule trapping. The electronic properties of the CaX molecules are well described by Ligand Field Theory. These properties include the relative energies of the X, A, B, and C states, their spin-orbit, spin-rotation, and lambda-doubling constants, and their spin-orbit and L-uncoupling perturbation matrix elements. In this talk we will present some Ligand Field Theory-based ideas: Schmidt orthogonalization, ligand-induced 4p-3d induced polarization, ligand-to-metal charge transfer, and molecular orbital-size based on effective principal quantum number 1/n*⁽3) orbital radius scaling. This paper is dedicated to Joelle Rostas (deceased June 2019) who spent many years studying the alkaline earth monohalide molecules and discussing them with the authors of this talk.

The CaX family of diatomic molecules illustrates concepts developed by inorganic chemists to rationalize the properties of metal-centered complexes. The basic idea is that an atom or an atomic-ion is surrounded by ligands, and that the electronic properties of the complexes are dealt with in a model in which the central metal atom and the ligands are treated as retaining their separated atom or molecule properties perturbed by identifiable and quantifiable metal-ligand interactions. Ligand Field Theory is semi-empirical in the sense that it is a framework for building a systematic understanding of the properties of families of complexes from spectroscopic measurements of the properties of the separated species and the interactions between them. The electronic structures of the CaX molecules are described by atomic-ions-in-molecule ligand field models. The Ca atom is treated as Ca⁺ with a single electron in the 4sσ , 4pσ or π, or 3dσ,π, or δ orbital For X=F, Cl, Br, and I, the ligand is a closed-shell halide ion. For X=O, the ligand is an open-shell O⁻ ion with a single hole in the pπ (π⁻¹) or pσ (σ⁻¹) orbital. The building blocks of the electronic structure model are known by different names in the inorganic chemistry, small-molecule spectroscopy, and quantum chemistry communities. Fine structure (spin-orbit, spin-spin, spin-rotation, and lambda-doubling) and spectroscopic perturbation matrix elements (spin-orbit and L-uncoupling) report on the CaX electronic structure.

2-Cyanopyrimidine is an attractive molecule for interstellar detection via radioastronomy because it is the pyrimidine analog of the detected interstellar molecule, benzonitrile, and has a substantial dipole moment (μ _a = 6.47 D). In the present work, we have analyzed and assigned the rotational spectrum of 2-cyanopyrimidine (C₅H₃N₃) for the first time. Using data in the 130 – 360 GHz frequency region, the vibrational ground state of 2-cyanopyrimidine has been least-squares fit to a partial octic, distorted-rotor Hamiltonian (N_lines  ∼  3400, σ = 40 kHz). The resulting spectroscopic constants are sufficient for spectral prediction outside of the studied frequency range and enable, for the first time, radioastronomical search for this molecule. Work currently continues on the Coriolis-coupled dyad of the two lowest-energy vibrationally excited states, ν₁₈ and ν₂₇. In the present least-squares fit, approximately 2,500 transitions have been assigned for each state. The complete analysis of these states is expected to yield a highly precise energy separation between ν₁₈ and ν₂₇, along with interesting comparison to the analogous dyads of benzonitrile and the cyanopyridines.

We previously presented the least-squares fit of the benzonitrile (C₆H₅CN, C_2v, μ_a = 4.5 D) ground vibrational state, which was fit to a partial octic Hamiltonian, as well as the first analysis of its two Coriolis-coupled, lowest-energy fundamental states, ν₂₂ and ν₃₃.¹ Herein, we present the continuation of this work: the analysis and fitting of the Coriolis- and Fermi-coupled triad of the overtone and combination states (ν₂₂ = 2, ν₃₃ = 2, and ν₂₂ + ν₃₃) near 315 cm⁻¹. Approximately 3000 transitions are assigned for each state, including multiple resonances, and their least-squares fitting results in precise energy separations that are in agreement with those expected based on the energy separation determined for ν₂₂ and ν₃₃. 1. Zdanovskaia, M. A.; Esselman, B. J.; Lau, H. S.; Bates, D. M.; Woods, R. C.; McMahon, R. J.; Kisiel, Z. The 103 - 360 GHz Rotational Spectrum of Benzonitrile, the First Interstellar Benzene Derivative Detected by Radioastronomy. J. Mol. Spectrosc. 2018, 351, 39-48.

Understanding the nuances of electron correlation is a central challenge to the development and practical implementation of strongly correlated electronic materials with applications spanning from spintronics to single molecule magnets and beyond. Electron correlation is also important in governing photodetachment and photoionization processes; previously reported photoelectron spectra of small samarium suboxide cluster anions revealed an increase in excited state band intensities with decreasing photon energies, opposite of what is expected to arise from threshold effects. This phenomenon has been attributed to strong photoelectron-valence electron interactions, facilitated by the high density of accessible electronic states in narrow energy windows for these clusters. Similar behavior is observed for the Gd₂O triatomic anion, despite the anticipation of a simpler electronic structure from the half-filled 4f subshell on each Gd center. Herein, the anion photoelectron spectra of Gd₂O were collected with photon energies in the range of 2.033 to 3.495 eV, and various explanations for this interesting behavior are explored.

We have synthesized and collected the 135 – 375 GHz rotational spectrum of Z-1-cyano-1,3-butadiene (C₅H₅N), an open-chain isomer of pyridine. Its substantial dipole moments (μ_a = 3.3 D, μ_b = 2.3 D, B3LYP/6-311+G(2d,p)) make it an attractive species for detection by radioastronomy. We have fit the ground vibrational state to a low-error (σ_fit = 0.039 MHz), partial octic, distorted-rotor Hamiltonian with 5,500+ distinct transitions in our frequency region, along with 34 hyperfine-resolved microwave transitions collected by the McCarthy Group in both a benzene/N₂ discharge and in a butadiene/CH₃CN discharge. In collaboration, we were able to determine the nuclear hyperfine coupling constants and confirm the presence of Z-1-cyano-1,3-butadiene in the discharge experiments. This analysis provides the fundamental spectroscopic constants needed to support an astronomical search of Z-1-cyano-1,3-butadiene in the interstellar medium. Presently, we are analyzing the two lowest-energy fundamentals, ν₁₉ and ν₂₇ (∆E = 7.5 cm⁻¹, B3LYP/6-311+G(2d,p)). These fundamental states comprise a Coriolis-coupled dyad exhibiting a− and b-type coupling, and this presentation will describe the progress of the dyad least-squares fit in the millimeter-wave analysis of Z-1-cyano-1,3-butadiene.

The rotational spectra of 2-, 3-, and 4-cyanopyridine (C₆H₄N₂) were recorded from 130 – 360 GHz, and an analysis of the ground vibrational state and two lowest-energy excited vibrational states for each isomer was completed. Each ground state was least-squares fit to a partial octic, distorted-rotor Hamiltonian with several thousand distinct transitions assigned (σ_fit < 50 kHz). The B3LYP/6-311+G(2d,p)-predicted quartic and sextic distortion constants are in excellent agreement with their experimentally determined values for all three isomers. Given the large dipole moments of 2-, 3-, and 4-cyanopyridine (μ = 5.8 D, 3.7 D, 2.0 D, respectively) and the astrochemical community’s interest in detecting nitriles, the cyanopyridines are an attractive set of aromatic species for detection by radioastronomy. This work provides the fundamental constants needed to support an astronomical search of the cyanopyridine isomers in the interstellar medium. We also examined the two lowest-energy fundamentals of each isomer, which correspond to the in-plane and out-of-plane CN bending modes for all three species. These states are  ∼  150 cm⁻¹higher in energy than the ground state and are separated by less than 30 cm⁻¹, an energy separation small enough that each corresponding pair exhibits Coriolis coupling. The dyads display a-type Coriolis coupling for 4-cyanopyridine (C_2v) and both a− and b-type Coriolis coupling in 2- and 3-cyanopyridine (C_s), resulting in both global and local perturbations to the rotational transitions. Each isomer required the inclusion of several coupling terms in the two-state least-squares fit in order to fit all observed rotational transitions, including numerous resonances. Fitting each dyad’s spectroscopic parameters allowed for the precise experimental determination of each dyad’s energy separation, the Coriolis coupling coefficients, and the deperturbed vibration-rotation interaction constants. Combined with the recently published work on the analogous dyad of benzonitrile, this coupling in the cyanopyridines provides an opportunity to compare the Coriolis interactions of cyano-substituted aromatic molecules in greater detail.

Nitriles are of interest to the astrochemical community due to their prevalence in the interstellar medium, their possible link to polyaromatic nitrogen heterocycles and prebiotic molecules, and their frequently strong dipole moments and consequent detectability. We have recently prepared four cyanobutadiene isomers of pyridine, E-1-cyano-1,3-butadiene, Z-1-cyano-1,3-butadiene, 1-cyano-2,3-butadiene, and 2-cyano-1,3-butadiene. Herein, we present the first analysis of the rotational spectrum of 2-cyano-1,3-butadiene (C₅H₅N, μ_a = 3.2 D, μ_b = 2.3 D), including over 4000 transitions from the 130 - 360 GHz frequency region least-squares fit using an octic Hamiltonian. The resulting spectroscopic constants and measured transitions will enable the first radioastronomical search for this species in the interstellar medium. We also present the analysis of the Corioils-coupled dyad of its two lowest-energy fundamental vibrational states (ν₂₇ and ν₁₉), which includes over 3200 transitions for each state. The coupling results in remarkably intense a-type and b-type resonances and nominal interstate transitions, and the analysis results in a highly precise determination of the energy separation between the two states.

We have developed a method for automatically generating predictions of Coriolis vibration-rotation coupling coefficients from Gaussian or CFOUR programs using previously known formulas, allowing us to make predictions for coupling coefficients G_α and F_βγ for ∑_i|∆v_i| ≤ 3 and G_α^J, G_α^K, F_βγ^J, F_βγ^K for ∑_i|∆v_i| = 2. Herein, we compare the results of these predictions to experimentally determined coupled-state fits that have been published. For example, the predictions of the lower order constants (G_α and F_βγ) are in qualitative agreement (within 10%) of the recently published fits of benzonitrile, phenyl isocyanide, and phenyl acetylene. In the course of implementing the predictions for the higher order coupling coefficients (G_α^J, G_α^K, F_βγ^J, F_βγ^K), we found it necessary to apply reductions to the formulas to facilitate direct comparison to experiment (akin to that done for A-reduced or S-reduced centrifugal distortion constants). Since both the centrifugal distortions and the coupling coefficients require reductions in the Coriolis coupled distorted rotors fit, the question arises whether the choice in the reduction for the centrifugal distortions impacts the choice in the reduction for the coupling coefficients. Thus, in addition to comparing our predictions to experimentally determined values, we explore the implications of the choice for the reductions of the centrifugal distortion constants and of the coupling coefficients.