S-Nitrosothiols (RSNOs) are derived from the combination of sulfur and nitric oxide (NO) radicals in the Earth's atmosphere and fragment to products following photolysis. Extensive theoretical studies have focused on the thermodynamic and, to a lesser extent, photochemical properties of RSNOs. However, experimental studies of these compounds have been limited due to the inherent instability of RSNOs at room temperature. Using velocity map imaging (VMI), we explore the photodissociation dynamics of jet-cooled S-nitrosothiophenol (PhSNO) from 355 nm photolysis. We report the translational and internal energy distributions of the NO and thiophenoxy (PhS) co-fragments, which are determined by spatial detection of the ionized NO photofragments using 1+1 resonance-enhanced multiphoton ionization (REMPI). The velocity distributions indicate competing PhSNO nonadiabatic dissociation pathways, in which PhS is formed in the ground and first excited electronic states when probing high- and low-energy NO (X²Π_1/2,v",J") rovibrational states, respectively. The results of multireference electronic structure calculations suggest that direct dissociation on the bright S₂ state results in PhS formed in its excited electronic state, whereas intersystem crossing into the triplet manifold leads to population of PhS in its electronic ground state. The dynamical signatures from the dissociation processes are imprinted on the fragments’ quantum states and relative translation, which we explore in rigorous detail using state-resolved imaging and high-level theoretical calculations.

Nitric oxide (NO) is an important pollutant produced in combustion. Laser-induced fluorescence (LIF) on the A²Σ⁺→ X²Π band is a common approach for quantifying the amount of NO and determining the physical conditions inside operating combustion engines. However, NO (A²Σ⁺) is known to undergo reactive and nonreactive electronic quenching with molecular species, providing alternative photochemical pathways that compete with fluorescence. For example, the nonreactive electronic quenching cross sections of NO (A²Σ⁺) with CO and H₂O at 300 K are 6 Ų and 120 Ų, respectively. The underlying photochemical mechanisms responsible for this electronic quenching are not well-understood. In this talk we describe our efforts to develop high-quality potential energy surfaces (PESs) that provide insights into the long-range interactions and conical intersections that facilitate nonreactive electronic quenching of NO (A²Σ⁺) by CO and H₂O. In order to ensure a balanced treatment of the valence and Rydberg electronic states as well as an accurate description of the open-shell character of NO we use the electron-attachment equation of motion coupled cluster with singles and doubles method (EOM-EA-CCSD). Our results demonstrate significant differences between the NO (A²Σ⁺)+CO and NO (A²Σ⁺)+H₂O systems in terms of the strength of the long-range attractive interactions, the impact of neighboring Rydberg states, and the shape of the PESs in the vicinity of the conical intersections responsible for the electronic quenching. In particular, we rationalize the large electronic quenching cross section of NO (A²Σ⁺) with H₂O by the presence of relatively strong long-range attractions that steer the NO (A²Σ⁺)+H₂O system into an excited-state collision complex that is bound by over 4500 cm⁻¹. Overall, this work sheds new light on the mechanisms for nonreactive electronic quenching of NO (A ²Σ⁺) with molecular partners.

Photodissociation is an important loss mechanism for atmospheric hydroperoxides (R-O-OH) leading to the production of OH radicals via the rupture of the weak O-O bond. Photodissociation can either occur through electronic excitation to a repulsive excited electronic state or, alternatively, through unimolecular dissociation on the ground electronic surface. Here we present results from the unimolecular dissociation of peroxyformic acid (PFA) initiated by exciting the molecule in the vicinity of its fifth OH stretching overtone state (6νOH) at both 615 and 626nm. Based on the estimated heat of formation of PFA and its fragments, the O-O bond dissociation energy (D₀) in PFA is estimated to be  45.1 kcal/mol. Thus, exciting room temperature PFA molecules at 615 nm and 626 nm is expected to leave the OH + HCO₂ fragments with roughly 3.3 kcal/mol and 2.5 kcal/mol of available energy, respectively. Using laser induced fluorescence (LIF), we have probed the OH fragments from the near threshold unimolecular dissociation and have determined the partitioning of the available energy in to its internal and translational degrees of freedom. These results, along with the insight they provide regarding the unimolecular dissociation dynamics of HC(O)OOH, will be presented.

A VMI photoelectron spectroscopy inside an electrostatic ion beam trap (EIBT) is used to probe the time dependent dynamics of rotational states population. The photodetachement of OH⁻ ion results neutral OH in its Π_3/2 and Π_1/2 states and corresponding VMI photoelectron spectra of unresolved P-branch transitions are shown in figure for different times in the EIBT. As storage time increases, the peak radius of P-transitions decreases indicate that population of high rotational levels shift to the low rotational levels. Intersting findinds on internal dynamics of OH⁻ as a function of storage time, change in individual rotational states population as a function of storage time, rate coefficients of such cooling process will be discussed in details during presentation. l18cm

Transition metal oxide and sulfide cations activate C-H bonds in the gas phase; several oxide cations activate methane and convert it to methanol at room temperature. However, a lack of experimental data on the energetics and dynamics of these species makes it difficult to model their reactions with hydrocarbons. We perform photofragment ion imaging experiments and ab initio calculations to determine the bond strength and photodissociation dynamics of the nickel oxide (NiO⁺) and nickel sulfide (NiS⁺) cations. NiO⁺ photodissociates broadly from 25000 to 32000 cm⁻¹via an excited ⁴Σ⁻ state, exclusively forming Ni⁺ (⁴F) + O (³P) products at photolysis energies above  29000 cm⁻¹. Photofragment images of NiO⁺ show resolved Ni⁺ (⁴F_9/2) + O (³P) and Ni⁺ (⁴F_7/2) + O (³P) product spin-orbit channels; the lower energy (J = 9/2) channel dominates at photolysis energies of  29000 to  32000 cm⁻¹. The photofragment spectrum of NiS⁺ from 19800 to 23200 cm⁻¹is highly structured, with  12 distinct vibronic peaks each containing underlying spin-orbit structure. Photofragment images of NiS⁺ collected over this region show slight parallel anisotropy, suggesting the highly structured photodissociation spectrum of NiS⁺ in this region predominantly arises from a ∆Λ = 0 transition. Above 21600 cm⁻¹, the Ni⁺ (²D_5/2) + S (³P) and Ni⁺ (²D_3/2) + S (³P) product spin-orbit channels compete, with a branching ratio of  0.5. Temperature-dependent spectra suggest peaks below 20150 cm⁻¹result from hot bands. Images taken slightly above the dissociation threshold (20600 cm⁻¹) show resolved S (³P₂) and S (³P₁) spin-orbit channels. Our D₀ measurements for NiO⁺ (D₀ = 244.6 +/- 2.4 kJ/mol) and NiS⁺ (D₀ = 240.3 +/- 1.4 kJ/mol) are more precise and closer to each other than previously reported values.

Metal boride compounds have unique properties that make them as chemically interesting as they are relevant in a multitude of disciplines. As more applications are discovered for metal borides, improved chemical models are needed to accurately predict the behavior of these species. To assist in this effort, we have developed a method for the precise and accurate measurement of bond dissociation energies (BDEs) and have applied it to the diatomic transition metal borides (MB), triatomic transition metal diborides (MB2), and triatomic lanthanide metal borides (LnB2). The method relies on the fact that in the open d-subshell and open f-subshell metal boride molecules, there is a large density of electronic states present at energies about the molecule’s dissociation limit. Spin-orbit and adiabatic couplings among the large number of potential energy curves in this region enable the molecules to hop from curve to curve, finding their way to dissociation as soon as sufficient energy is available for this process. In our resonant two-photon ionization experiments, the high density of states in this region leads to a quasi-continuous spectrum below the dissociation energy, followed by a sharp drop to baseline when the molecules are excited above the dissociation limit. The sharp drop in signal occurs at a wavelength that corresponds to the BDE of the molecule. Using this method, the BDEs of 18 diatomic borides and 10 triatomic boride molecules have been measured to high accuracy with the majority of these species having no previous experimental data or observations recorded. These data provide important benchmarks for the development and testing of improved computational methods for these species which will be helpful for modeling larger metal boride clusters and nanoscopic materials. Of equal importance, a comprehensive set of bond dissociation energies for these species allows for a quantitative and qualitative landscape of the transition metal-boron bond to be elucidated.

Bond dissociation energies pose a major challenge for modern computational methods. This is largely due to the need to treat electron correlation to the same level of accuracy in the molecule as in the separated atoms. This becomes especially difficult in transition metal and lanthanide containing species, as the electronic state densities are particularly high. As important as this issue is, the problem has been largely unassailable due to a long-standing dearth of highly accurate experimental data that is suitable for use as computational benchmarks. In this study, we use the sharp onset of predissociation in a highly congested, quasicontinuous optical spectrum to precisely measure the bond dissociation energies of 10 diatomic lanthanide sulfides, LnS, along with the corresponding lanthanide selenides, LnSe. These studies provide lanthanide thermochemistry of unparalleled accuracy, while also providing an opportunity to analyze the bonding in terms of the separated atom states to which the ground states diabatically correlate. From this analysis, it is proposed that most of the LnS and LnSe molecules diabatically correlate to the Ln⁺ (4f^N 5d¹) + S⁻(3s² 3p⁵)/ Se⁻(4s² 4p⁵) separated ion limit. Our previous observation that the bond dissociation energies of the diatomic transition metal sulfides are universally about 15% stronger than their selenide analogs is extended to these 10 LnS/LnSe molecules, and the trend is observed to hold.

r0pt Figure The destruction of molecules by photodissociation influences the composition and dynamics of exoplanets, particularly in the presence of a UV-rich stellar environments, where molecules are hot. Current photodissociation calculations generally assume T = 0 K, appropriate for the cold interstellar medium but inadequate for hot (exo)planets and stars. We compute temperature-dependent photodissociation cross sections for molecules found in these atmospheres. The cross sections are calculated by solving the nuclear-motion Schrödinger equation as part of the ExoMol project using codes Duo and ExocrossYurchenko et al Comput Phys Commun 2016 202 262–275;Yurchenko et al A&A 2018 614 A131 and averaging results obtained as discrete spectra. We benchmark our results for three systems. The general validity of the methodology is tested considering the A¹Π← X¹Σ⁺ transition of HCl at low T. Our cross sections and rates agree with published resultsDishoeck et al J. Chem. Phys. 1982 77 36932.31*10⁻¹⁰ s⁻¹ versus 2.10*10⁻¹⁰ s⁻¹ respectively ). We also reproduce calculated photodissociation cross section of BeH⁺ at T=1800 K.Yang et al J Quant Spectrosc Radiat Transf 2020 254 107203e performed the simulations considering the three excited states, A¹Σ⁺, B¹Π and C¹Σ⁺, see figure. The photodissociation spectrum of NaCl varies consistently with T: at T=100 K, there two peaks at 240 nm and 260 nm; while at T=1500 K the peaks merge. Yurchenko et al Comput Phys Commun 2016 202 262–275;Yurchenko et al A&A 2018 614 A131 , Dishoeck et al J. Chem. Phys. 1982 77 3693( Yang et al J Quant Spectrosc Radiat Transf 2020 254 107203W

Dicarbon (C₂) is one of the most abundant molecules in space and has been detected in different astronomical environments, including the interstellar medium, comets, and stars. In diffuse clouds, the dominant destruction pathway for is photodissociation by UV photons through the F ¹Π_u state and other higher ¹Π_u and ¹Σ_u⁺ states excited from the ground X ¹Σ_g⁺ state. However, the only laboratory study of the F ¹Π_u state was more than half a century ago and did not provide detailed information about its photodissociation, while no MRCI+Q level calculation has been done on the F state to date. Thus, considerable uncertainty exists about the photodissociation rate of in space and its atomic branching ratios, limiting the accuracy of simulations given by astrochemical models. Here we present a high-level ab initio study of photodissociation, focusing on the F ¹Π_u − X ¹Σ_g⁺ transition. Potential energy curves of electronic states were calculated at the SA-CASSCF/MRCI+Q level using the aug-cc-pV5Z basis set with additional diffuse functions. To represent the Rydberg state nature of F state, the active space consisted of the valence orbitals and several additional σ_g orbitals. A total of 57 potential energy curves for singlet, triplet and quintet states were calculated, as well as transition dipole moments, nonadiabatic coupling matrix elements, and spin-orbit couplings. The F state lies near three ³Π_u states that are likely responsible for its predissociation via spin-orbit coupling.

Charge-transfer processes, particularly in hydrated iodide and salt clusters, depend sensitively on the chemical environment and number of water molecules solvated around the iodide ion. Studying such charge-transfer behaviour is ideally suited to gas-phase clusters, whereby the size and chemical composition, along with number of water molecules, can be controlled. For example, when hydrated iodide interacts with ultraviolet light, the electron fully separates from the iodine ion, forming a solvated electron. Charge-transfer transitions are also observed in ionic systems such as metal-halide clusters. To understand these charge-transfer processes at a molecular level, laser spectroscopic measurements in the ultraviolet and visible region are utilised and, initially, focussed on the ionic salt systems. Electrospray ionization is employed producing salt clusters, which can then be stored in the cell of a Fourier transform ion cyclotron resonance mass spectrometer. Laser systems using optical parametric oscillators are implemented, providing intense tuneable laser light in the 225 – 2600 nm region. For each size-selected salt cluster, evaporation of stoichiometric and non-stoichiometric fragments are recorded, elucidating photochemical pathways connected to charge-transfer transitions. These evaporation channels are revealed by mass spectrometry, whereby an electronic absorption spectrum can be generated in each case, in addition to wavelength-specific photochemistry. These experiments are complemented with simulated spectra generated using quantum chemical calculations. Unanswered questions such as where the charge is located, and where it moves to within the cluster, along with whether the charge is localised to one atom, are yet to be fully understood. Isolating size-selected hydrated iodide alongside a systematic series of salt clusters and exploring their photochemistry offers a targeted approach to tackle such questions. Studying these systems not only provides fundamental insight into charge-transfer mechanisms in cluster physics, but also offers a laboratory model system for a molecular level understanding of reactions occurring during marine aerosol ageing or radiation-induced cell damage. Figure

Protonation or deprotonation sites mediate many cases of biological photochemistry. For example, flavin ions have several photobiological functions such as initiating DNA repair (occurring in some bacteria and frogs for example) and are central to quantum mechanical compass used by birds for navigation. These pathways involve various stages of flavin protonation and deprotonation, isomers and redox states. Furthermore, intermediate flavin radicals (which are formed in the biological photoredox pathways) have several possible protonation isomers (protomers) that are challenging to characterize due to their reactive nature and the additional challenge of separating isomeric species. Using photodissociation (PD) action spectroscopy coupled with mass spectrometry, reactive species (such as flavin radical ions) can be isolated in a vacuum and characterized based on their spectroscopic properties without reacting away. By photodissociating flavin adenine dinucleotide dianion (FAD) in an ion trap, multiple flavin radical anion fragments are formed. Photoproduct flavin ions in three redox states can each be separately re-isolated in the ion trap, in turn, and assigned by photodissociation action spectroscopy. Franck Condon simulations confirm that noncanonical protomers of these flavin radical ion are formed by direct photodissociation of larger flavins. These results reveal the structure of radical-anion products and provide assigned gas-phase spectra for reference and with these in hand, can provide new insights into the complex redox chemistry of FAD in biochemical systems.

The nucleic acid bases that we know today are thought to have originated from simple precursors also referred to as proto-biotic RNA. These molecular ancestors of RNA and DNA may have formed from a vast number of organic compounds found on early Earth and/or delivered by meteorite infall. Understanding the evolution of chemical synthesis ranging from those precursors to today’s canonical nucleobases is essential in answering questions to the chemical origins of life. Among other factors, ultraviolet radiation (UVR) from the sun should have played a key role in shaping and selecting the building blocks of life. Whether the carbonyl and/or amino group substituents played any role in regulating the photostability of the canonical pyrimidine nucleobases is currently unknown. The biological relevance of the pyrimidine chromophore and its carbonyl- and amino-substituted derivatives make these molecules excellent candidates for investigating how their interaction with UVR may have enabled the selection of the RNA and DNA pyrimidine nucleobases on early Earth. Time-resolved spectroscopic results, complemented with quantum-chemical calculations, will be presented, which lend support to the idea that functionalization at the C2 and C4 positions of the pyrimidine chromophore has a preponderant role in controlling the inherent electronic relaxation mechanisms and photostability of the DNA and RNA pyrimidine nucleobases and their derivatives.
The authors acknowledge the National Science Foundation (Grant No. CHE-1800052) for financial support.

The natural RNA and DNA nucleobases absorb harmful ultraviolet radiation but the ability to dissipate this excess electronic energy efficiently to the ground state makes them highly photostable. Understanding how minor structural modifications affect their photochemical properties is important for advancement in biological applications. For example, 7-deazaguanosine (7dza) has been used for decades to probe the charge transfer dynamics in DNA, due to its lowered oxidation potential relative to guanosine. However, the electronic relaxation mechanism of 7dza has not been investigated until recently. Using steady-state and time-resolved electronic spectroscopic techniques, combined with quantum-chemical calculations, we have investigated the excited-state dynamics of guanosine 5’-monophosphate (GMP) and 7dza in in aqueous solution and in a mixture of methanol and water following excitation at 267 nm. The following relaxation mechanism has been proposed for both molecules: L_b ⇒ L_a ⇒ ¹πσ*(ICT) ⇒ S₀, where the ¹πσ*(ICT) stands for an intramolecular charge transfer excited singlet state with significant πσ* character. In 7dza, however, the relaxation dynamics is slightly slowed compared to GMP, which adjudicate to stabilization of the two lowest-energy singlet states and to the alteration of the topology of the excited state potential energy surfaces.
The authors acknowledge the National Science Foundation (Grant No. CHE-1800052) for financial support and the High-Performance Computing Resource in the Core Facility for Advanced Research Computing at Case Western Reserve University.

Localized surface plasmon resonance (LSPR) has offered a unique way to trigger photocatalyzed chemical reactions on nanoscale. 8-Bromoadenine was one of the halogenated nucleobases could be applied in the tumor tissue upon irradiation as a potential DNA radio-sensitizer. To utilize the high spatial resolution properties of noble metal nanostructure and improve the selectivity of plasmon catalytic reaction, we investigated the cleavage of C-Br bond in 8-bromoadenine in electrochemical interface. Surface-enhanced Raman spectroscopy (SERS) provides an excellent opportunity to probe and monitor plasmonic photochemical reactions in situ and in real-time. The EC-SERS spectra contour map can explain the potential effect in the reaction, the plasmon catalyzed reaction rate constant in different potentials. Analyzing the intensity change of the characteristic peak of adenine in electrochemical SERS (EC-SERS) spectra was used to investigate the reaction kinetics of the cleavage of C-Br bond could be estimated by intensity integration of the SERS peak.