Magnesium Sulfide (MgS) is an astrophysically interesting molecule. Its solid form is the main component of the mineral niningerite (found in enstatite chondrite meteorites) and the MgS component of solid dust grains has generally been agreed upon as the carrier of the 30 μm feature seen in the emission spectra of some carbon-rich stars^a; however, investigations of the visible spectrum of gas phase MgS remain relatively sparse in the literature. The first analysis of a rotationally-resolved spectrum of MgS in the gas phase was undertaken by Marcano and Barrow in 1970^b, who investigated the B¹Σ⁺−X¹Σ⁺ system in absorption. Our group at UNB previously reported the first experimental observation and rotational analysis of the low-lying A²Π state of MgS at 4531.94 cm⁻¹ from a series of laser-induced dispersed fluorescence spectra taken using a grating spectrometer. Since then we have extended this work by recording a series of dispersed fluorescence spectra of the B¹Σ⁺−A¹Π and B¹Σ⁺−X¹Σ⁺ systems at higher resolution using a BOMEM DA3 Fourier transform interferometer. The weak B¹Σ⁺−a³Π₁ transition was also observed. We present here our extended analyses of the B¹Σ⁺−A¹Π and B¹Σ⁺−X¹Σ⁺ systems, as well as the first analysis of the B¹Σ⁺−a³Π₁ system in MgS.
^aVolk, Kevin; Sloan, G. C.; Kraemer, Kathleen E. (2020). The 21 μm and 30 μm emission features in carbon-rich objects. Astrophysics and Space Science, Volume 365 (Issue 5), article id.88. DOI: 10.1007/s10509-020-03798-2. ^bMarcano, M; Barrow, R. F. (1970). Rotational Analysis of Bands of the B¹Σ⁺−X¹Σ⁺ system of gaseous MgS. Transactions of the Faraday Society, Volume 66, pages 2936-2938. DOI: 10.1039/TF9706602936.

The electronic spectrum of CaO is so complicated that it had been dismissed as uninterpretable random fragments from a polyatomic molecule. An atomic-ions-in-molecule model, which employs foundational concepts from Inorganic Chemistry, provides the “why” as well as the “what.” There are two oxidation states, Ca²⁺O²⁻ and Ca⁺O⁻, the latter manifest in O⁻ 2pπ-hole (π⁻¹) and 2pσ-hole (σ⁻¹) “hard/soft” forms. These three families of electronic structure states are co-present in the low-energy region, and their large differences in molecular structure (R_e and ω_e) result in a dense web of perturbations. But all is now understood. Going from CaO to ScO, the addition of a single valence electron awakens the sleeping giant of complexity, bellowing “you ain’t seen nothin’ yet.” The number of low-lying electronic states in each of the three families increases significantly. Can an atomic-ions-in-molecule model guide the interpretation of the ScO spectrum? New Laser Induced Fluorescence (LIF), Dispersed LIF, and lifetime-gated LIF spectra offer insights into the electronic structure of ScO. These spectra sample the ScO A²Π, C²Π and D²Σ⁺ states over a wide range of vibrational levels. Of special importance is the A(v=6) C(v=16) perturbation and two previously unobserved, closely-spaced, long-lived, Ω=1/2 states that lie near, and are probably made visible by their interaction with the C²Π_1/2(v=6) and A²Π_3/2(v=16) states.

Despite being subject to numerous single reference computations, Europium Oxide (EuO) to date has not had its electronic structure studied with multireference methods. High-level ab initio approaches were performed detailing its numerous excited states, and spin multiplicities. Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) was utilized to compute the ground and excited state properties of EuO. The potential energy curves for the ground, excited states, and different dissociation channels are explored. Spin-orbit corrections were performed with the Breit-Pauli hamiltonian. When available comparisons to experiment are made.

High-level ab initio approaches were performed on LrF and LrO detailing their numerous excited states, and spin multiplicities. Herein, multi reference methodologies such as the complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) were utilized to calculate ground and excited state properties of LrF and LrO. The potential energy curves for the ground, several excited states, and different dissociation channels are explored at CASSCF and MRCI+Q. Spin-orbit corrections were performed by diagonalizing the MRCI wavefunction on the basis of the Breit-Pauli Hamiltonian. For the second part of this work the bond dissocation energies (BDEs) of LrO and LrF were performed at different levels of theory using a range of basis sets. Core-valence, relativistic effects and spin-orbit contributions to the ground state are discussed. In addition, density functional theory (DFT) is also compared against wavefunction methods. Detailed spectra for intricate diatomic complexes such as actinide oxides and fluorides are essential for future experimental studies on heavy metal containing species.

Phosphorus and sulfur are integral to life on Earth, and their role in the chemistry of the interstellar medium is highly debated and unknown. Only a handful of phosphorus-bearing species have been detected thus far, with the most recent confirmed detection taking place in 2014. The simultaneous detection of molecules such as PO, SH, and OH indicate the possibility of reactive intermediate species existing in the interstellar medium and circumstellar envelopes of evolved stars. To explore this possibility, we have characterized the [H, P, S, O] tetratomic isomer family using high level ab initio methods. We provide rotational, vibrational, and electronic spectroscopic data to help drive experimental and observational detection of new phosphorus and sulfur-bearing molecules and explore chemical and photochemical pathways to explain possible reservoirs and sources for the as of yet undetected PH and PS diatomic molecules.

Nitrogen is an essential ingredient in molecules that support life. Its presence also typically leads to the delocalization of electrons, causing large dipole and quadrupole moments. Such molecules are sometimes able to form negative ions through the electrostatic binding of an excess electron via a process known as Rydberg Charge Exchange. These so-called multipole-bound (dipole-bound, quadrupole-bound, etc.) anions have been shown to be important in radiation damage in biology as well as electron transport processes. Here, we present our recent computational and experimental results studying the creation of new multipole-bound anions.

The chemi-ionization reactions of atomic lanthanides M + O → MO + e⁻ are currently being investigated as a method to artificially increase the ion density in the ionosphere for uniform radio wave propagation. Recent experiments involving the release of atomic neodymium (Nd) into the upper atmosphere have resulted in the production of a cloud with green emission[1]. Based on the cloud emission, it is believed that NdO was the primary product, but spectroscopic characterization of NdO is needed to properly identify the emitting species. While NdO is well characterized above 590 nm, little spectroscopic data exits at emission wavelengths below 590 nm[2,3]. In this work, jet-cooled NdO was produced and low- and high-resolution laser-induced fluorescence (LIF) and dispersed laser-induced fluorescence (DLIF) techniques were used to characterize the electronic structure of NdO from 15,500-21,000 cm⁻¹. Congested DLIF spectra allowed vibrational characterization of the ground X4 state as well as five low-lying states for the first time. By employing high-resolution LIF, the hyperfine structure of the ground X4 state was obtained. Data and analysis of the ground and low-lying states of NdO will be presented.
[1] Ard, S.G. et al. J. Chem. Phys.2015, 143, 204303. [2] Kaledin, L.A. et al. Acta Physica Hungarica 1983, 54, 189-212. [3] Linton, C. et al. J. Mol. Spec. 2004, 225, 132-144. [4] VanGundy, R.A. et al. J. Chem. Phys. 2019, 114302.

Replication of the long lifetimes of 4d transition metal complexes in their 3d counterparts is desirable for both cost reduction and environmental concerns. Fe(rac-HMTI)(CN)₂ is an Fe complex with a remarkable nanosecond lifetime metal-to-ligand charge transfer (MLCT) state in low polarity solvents. Architecturally, the Fe center is ligated to axially-oriented strong field cyano ligands, and equatorially to a rigid [14]-tetracene-N4 macrocycle. This rigidity enforces poor vibrational overlap of excited states, significantly raising the barrier of vibrational relaxation and extending their lifetimes. Fe(HMTI)(CN)₂ is studied by optical transient absorption, and spectroelectrochemical studies reproduce the features of the OTA at potentials consistent with metal oxidation and ligand reduction, confirming the attribution of MLCT character to the transition. DFT, TDDFT and CASSCF computational methods are also used to create a theoretical potential energy manifold, to better describe the deactivation mechanism of the excited state. Further understanding of this molecule's photophysics will allow for more targeted development of longer-lived Fe complexes.

Acetic acid CH₃C(O)OH plays an important role in the acidity in the troposphere. The reaction of Criegee intermediate with CH₃C(O)OH was proposed to be a potential source of secondary organic aerosol in the atmosphere. We investigated the detailed mechanism and kinetics of the reaction of Criegee intermediate CH₂OO with CH₃C(O)OH. The time-resolved infrared absorption spectra of transient species produced upon irradiation at 308 nm of a flowing mixture of CH₂I₂/O₂/CH₃C(O)OH at 298 K were recorded with a step-scan Fourier-transform infrared spectrometer. Bands of CH₂OO were observed initially upon irradiation; their decrease in intensity was accompanied with the appearance of bands near 886, 971, 1021, 1078, 1160, 1225, 1377, 1402, 1434, and 1777 cm⁻¹, assigned to the absorption of hydroperoxymethyl acetate [CH₃C(O)OCH₂OOH, HPMA], the hydrogen-transferred adduct of CH₂OO and CH₃C(O)OH. Two conformers of HPMA, an open form and an intramolecularly hydrogen-bonded form, were identified. At a later reaction period, bands of the open-form HPMA became diminished and new bands appeared at 930, 1045, 1200, 1378, 1792, and 1810 cm⁻¹, assigned to the formic acetic anhydride [CH₃C(O)OC(O)H, FAA], a dehydrolysis product of HPMA. The intramolecularly hydrogen-bonded HPMA is stable. From the temporal profiles of HPMA and FAA, we derived a rate coefficient k = (1.3 ± 0.3) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ for the reaction CH₂OO + CH₃C(O)OH to form HPMA and a rate coefficient k = 980 ± 40 s⁻¹ for the dehydration of the open-form HPMA to form FAA. Theoretical calculations were performed to elucidate the CH₂OO + CH₃C(O)OH reaction pathway and to understand the different reactivity of the two forms of HPMA.

Recent microwave studies in our laboratory have explored a series of carboxylic sulfuric anhydrides (RCOOSO₂OH, CSAs) that are formed from a cyclic reaction between carboxylic acids and SO₃. These studies have shown that the reaction occurs readily with a wide range of carboxylic acids. Moreover, the zero-point corrected activation energies are typically small and, in some cases even negative, but there remains uncertainty as to the factors which control the size and sign of the barrier. In this talk we present chirped-pulse and cavity microwave spectra of pivalic sulfuric anhydride, (CH₃)₃CCOOSO₂OH (PivSA), and explore the reaction pathway for its formation using computational chemistry. The reaction is found to be best described as a pericyclic heteroene reaction coupled with a 60 degree rotation of the t-butyl group. The process can occur through either a sequential (two-step) or a concerted (one-step) pathway. Based on zero-point corrected single-point CCSD(T) calculations, the sequential pathway has the lowest energy transition state, with a value of -0.52 kcal/mol relative to that of a pivalic acid - SO₃ precursor complex. This value represents the lowest barrier for SO₃ + carboxylic acid reactions studied to date. When compared with CF₃COOSO₂OH, which has the highest barrier among the systems previously studied, the results provide insight into the relative influence of the electronic and mass effects on the reaction energetics. Additional computational studies further explore the effects of the R group of the RCOOH reactants. Finally, as a precursor to the experimental work on PivSA, the microwave spectrum of the pivalic acid monomer was also recorded and is reported here as well.

Nitrogen-containing organic molecules play significant roles within atmospheric and combustion chemistry due to their presence as volatiles emitted during wildfires, significant components of crude biofuels, and their use in carbon capture technology. The historical focus of gas-phase nitrogen chemistry was on small molecules formed during high-temperature combustion, where the molecular structure of fuel-N has a limited effect on the chemistry due to significant fragmentation. At lower temperatures relevant to Earth's atmosphere and modern combustion technology, oxidation out-competes thermal decomposition and fuel-N structure can have a substantial effect on reactivity. However, the degree to which chemical pathways differ between N-containing compounds is unclear due to the lack of detailed kinetic studies and potential energy surfaces, resulting in poor representation of these pathways within chemical kinetics models. Here, we present the first set of results in a research program designed to build a comprehensive understanding of the structure and reactivity relationships for model nitrogen-containing compounds, with a focus on differences between radical isomers. Five-membered rings pyrrole (c−C₄H₅N) and pyrrolidine (c−C₄H₉N) serve as model compounds due to their broad chemical importance and ability to form three different radicals. Using KinBot, a computational tool that automatically locates kinetically important stationary points, we have developed potential energy surfaces for reactions of the pyrrolyl and pyrrolidinyl radicals with O₂ at the B3LYP/6-31G level of theory for reaction searches, conformational analyses, and IRC calculations, followed by reoptimization of the most relevant stationary points at ωB97X-D/6-311++G(d,p) with energies refined at CCSD(T)-F12/cc-pVDZ-F12. These results indicate stark differences in reactivity between radical isomers and support our ongoing experimental kinetics studies using a high-pressure laser photolysis reactor coupled to a photoionization mass spectrometer. Characterization of pyrrole and pyrrolidine oxidation will not only provide a detailed knowledge base for future studies of N-containing compounds, but also will allow for comparison to well-studied oxidation pathways of isoelectronic species, such as furan and tetrahydrofuran. Such comparisons will give us a better understanding of the key differences in reactivity between nitrogenated, oxygenated, and pure hydrocarbon volatiles.