Nuclear spin symmetry conservation provides strong selection rules in radiative transitions, inelastic collisions, and chemical reactionsV. Horka-Zelenkova, G. Seyfang, P. Dietiker and M. Quack, J. Phys. Chem. A, 2019, 123, 6160-6174.nd yet when a molecule is trapped in a cryocrystal at low temperatures, nuclear spin symmetry conversion is observed. In this study we report observations of the nuclear spin conversion (NSC) of monodeuterated methane (CH₃D) trapped in solid parahydrogen (pH₂) samples at approximately 1.7 K via high-resolution Fourier transform infrared spectroscopy. CH₃D doped pH₂ crystals are prepared using a rapid vapor deposition technique to co-deposit room temperature CH₃D and precooled pH₂ gases onto a cold substrate. This project focuses on the ν₂ (CD stretch) and ν₆ (CH₃ rock) modes corresponding to parallel and perpendicular rovibrational bands, respectively. The temporal changes in peak intensities extracted from the IR spectra are characteristic of NSC and can be used to assign peaks originating from the ortho (J,K = 0,0, I = 3/2) and para (J,K = 1,1, I = 1/2) nuclear spin states. Measurements performed on as-deposited and annealed samples show comparable first-order NSC dynamics with an average time constant τ = 311(33) min. The measured NSC time constants for a variety of methyl containing molecules, CH₃F, CH₃OH, and CH₃CCH isolated in pH₂ display a range of time constants.Y.-P. Lee, Y.-J. Wu and J. T. Hougen, J. Chem. Phys., 2008, 129.Y.-P. Lee, Y.-J. Wu, R. M. Lees, L.-H. Xu and J. T. Hougen, Science, 2006, 311, 365-368.A. I. Strom, A. Gutiérrez-Quintanilla, M. Chevalier, J. Ceponkus, C. Crépin and D. T. Anderson, J. Phys. Chem. A, 2020, 124, 4471-4483.y systematically studying the NSC of methyl containing molecules trapped in solid pH₂ we hope to better understand how solvation/confinement of the molecule results in a breakdown of the nuclear spin symmetry conservation rules. V. Horka-Zelenkova, G. Seyfang, P. Dietiker and M. Quack, J. Phys. Chem. A, 2019, 123, 6160-6174.a Y.-P. Lee, Y.-J. Wu and J. T. Hougen, J. Chem. Phys., 2008, 129.

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A. I. Strom, A. Gutiérrez-Quintanilla, M. Chevalier, J. Ceponkus, C. Crépin and D. T. Anderson, J. Phys. Chem. A, 2020, 124, 4471-4483.B

Our group recently showed that the rate constant for the diffusion-limited H + N₂O reaction in solid parahydrogen (pH₂) is inversely related to the N₂O concentration.F. M. Mutunga et al., J. Chem. Phys 154 (2021) 014302.his finding was rationalized by the idea that chemical impurities disrupt the long-range order of the crystal and slow the H-atom quantum diffusion rate. We investigated this finding further by studying the H + CO → HCO reaction in solid pH₂. To explore how the reaction rate constant depends on the CO concentration, we prepared solid pH₂ matrix samples co-doped with CO and Cl₂ using the Rapid Vapor Deposition technique,M. E. Fajardo and Simon Tam, J. Chem. Phys. 108.10 (1998) 4237.-atoms are produced in situ using a UV-IR method described previously.S. C. Kettwich, P. L. Raston, and D. T. Anderson, J. Phys. Chem. A 113 (2009) 7621.e performed kinetic studies at 1.56(3) and 4.00(2) K with a range of CO concentrations (10−250 ppm). We observe a similar inverse relationship between the rate constant and the CO concentration. Furthermore, it looks like the rate constant decreases from 1.56(3) to 4.00(2) K. This work is ongoing, and the latest results and analysis will be presented at the meeting. F. M. Mutunga et al., J. Chem. Phys 154 (2021) 014302.T M. E. Fajardo and Simon Tam, J. Chem. Phys. 108.10 (1998) 4237.H S. C. Kettwich, P. L. Raston, and D. T. Anderson, J. Phys. Chem. A 113 (2009) 7621.W

Molecular clouds are known to host a rich chemistry despite seemingly unfavorable conditions. In particular, a wide variety of cyano-polyynes and cyano-aromatic species have been detected in the past years. These nitrile compounds are also present in the atmospheres of various moons such as Titan. This suggests that the CN radical may be a vector of a rich chemistry in astrophysical media. To further understand the abundances of these molecules, astrochemical models are needed. These require accurate kinetic parameters, including rate coefficients and branching ratios measured at low temperature. Here, we present a new implementation of the Chirped Pulse in Uniform Flow (CPUF) technique, designed for branching ratio measurements. It combines the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) technique to reach low temperatures, with molecular beam sampling into an E-band chirped pulse Fourier transform millimeter wave spectrometer to detect reaction products. For the reaction between CN and propene we have measured the branching fraction of the vinyl cyanide channel at 35 and 50 K. Upper limits for all the different cyanopropene isomer channels were also determined. Theoretical calculations of the reaction potential energy surface and product branching fractions will be also be presented along with our latest results on other CN + hydrocarbon reactive systems.

Formation pathways and mechanisms of unsaturated nitrogen-containing compounds at very low temperatures are of great interest in the field of astrochemistry. The reaction of butadiene isomers with CN is one such reaction class that can be a key elementary step in the formation of large nitrogen rich molecules of potential biological importance in astrochemical environments. We present a joint experimental and theoretical investigation of reaction kinetics of vibrationally excited CN with 1,3 and 1,2-butadiene isomers at 70 K. The experimental method involves a newly built apparatus that couples a pulsed uniform supersonic flow with a continuous wave cavity ringdown spectrometer (UF-CRDS). The well-matched hydrodynamic time of the uniform flow with the long ringdown time decay allows the measurement of the complete kinetics over observation times up to 200 μs on each ringdown decay, an approach termed SKaR (Simultaneous kinetics and ringdown). The vibrationally excited CN is produced by photolyzing BrCN with a 248 nm excimer laser. The adjustable butadiene density greatly exceeds that of the photolytic CN radicals, ensuring pseudo first order conditions for the loss of CN. The time-dependent density of CN(v=1, j=5.5) is probed by tuning the DFB diode laser at 7070.24 cm⁻¹, the R₁(5.5), line of the (0-1) band of A²Π–X²Σ⁺ electronic transition.The measured bimolecular rates are (3.96 ± 0.28) x 10⁻¹⁰ and (3.06 ± 0.35) x 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ for 1,3 and 1,2-butadiene, respectively. The experimental rates were modeled by VRC-TST (Variable Reaction Coordinate Transition State Theory) calculations on a high-level multireference potential energy surface. For the 1,2-butadiene case the entrance channel branching was combined with earlier predictions of branching following the decays of the initial adducts to predict the overall branching.

Formic acid, one of the most abundant acids in the Earth's atmosphere, plays an important role in atmospheric acidity. Here, we report the direct observation of formic acid generated by the reaction of the simplest Criegee intermediate (CH₂OO) with formaldehyde (HCHO). By employing mid-infrared comb lasers and synchronized two-color time-resolved dual-comb spectroscopy, we investigated the kinetics of the reaction CH₂OO + HCHO as well as the yields of the reaction products HCOOH + HCHO and CO + H₂O + HCHO. The potential implications of the reaction CH₂OO + HCHO in the atmospheric chemistry are also discussed in this work.

Nucleation is a key process in cloud and dust particle formation in different systems including planetary atmospheres and circumstellar shells. It consists of a succession of unions of small gas-phase molecules leading to the formation and growth of droplets and particles. Under the appropriate conditions, low temperatures and/or high degrees of supersaturation, nucleation can become a barrierless process, and dimerization, the complexation of the two smallest molecular species involved, becomes its rate-limiting step. The kinetics of formation of homodimers have been investigated in a few previous studies, using the CRESU (reaction kinetics in uniform supersonic flow) technique mainly coupled with mass spectrometric detection. Here we have employed a completely new detection scheme, chirped-pulse Fourier transform mm-wave spectroscopy, to study for the first time the kinetics of formation of heterodimers. This innovative technique combines the ability to generate continuous cold uniform supersonic flows with the high selectivity and general applicability of rotational spectroscopy, allowing us to follow both reactant and product concentrations simultaneously. Furthermore, the high sensitivity achieved has allowed us to employ pseudo-first-order conditions to obtain absolute rate constants. We will report the first measurements of rate constants for the formation of a set of heterodimers, including formic acid, CO2 and water among others, at temperatures between 35 and 150 K.

Noncovalent interactions including π-π stacking, CH/π interactions and halogen bonding play a significant role in many chemical and biological processes. Understanding their nature and behavior gives insight into how they influence processes such as protein folding, molecular self-assembly, and drug-substrate interactions. Using a combination of velocity mapped ion imaging, two-color photoionization, and theoretical calculations we probe the chlorobenzene-ammonia cluster system. From our previous studies, we have insight into the reactive pathways as well as the possible geometries of the σ-type reaction intermediate (Reid, et al. 2013 S.A Reid, S. Nyambo, A. Kalume, B. Uhler, C. Karshenas, L. Muzangwa, J.Phys. Chem. A 2013, 117, 12429-12437. This study is focusing on probing the reaction pathways around the activation energy. The first photon is for excitation and a time delayed second photon is for ionization to initiate the radical reaction. The dimer cation radical reacts via Cl- atom or HCl loss pathways forming protonated aniline and aniline cation respectively or H-atom loss forming ortho-chloro protonated aniline. The energy of the photons can be controlled enabling the experiment to determine threshold energy of the reaction as well as confirm the primary reaction channel. S.A Reid, S. Nyambo, A. Kalume, B. Uhler, C. Karshenas, L. Muzangwa, J.Phys. Chem. A 2013, 117, 12429-12437)

rb0pt Figure Large-amplitude motion (LAM) may induce a substantial conformational change in molecules, which is deeply relevant to molecular functionality e.g., in biomolecules. However, the creation and manipulation of vibrational wave packets pertinent to LAM in isolated molecular systems have been rarely realized, and most studies on molecules in the electronic ground states deal with one-dimensional LAMC. B. Madsen, et al., J. Chem. Phys. 130, 234310 (2009).M. Nikaido, et al., 2021 International Symposium on Molecular Spectroscopy, RH06 (2021).. Here we study diphenylmethane (DPM), which has two degrees of freedom for LAM, i.e., symmetric and anti-symmetric torsional vibration, T (20 cm⁻¹) and ―T (16 cm⁻¹). Adiabatically cooled DPM was irradiated by a pair of femtosecond laser pulses (pumps) to excite modes T and ―T through impulsive stimulated Raman scattering. Then, resonance two-photon ionization (R2PI) spectra were obtained by nanosecond UV pulses ( ∼ 268 nm, probe). Populations in v = 1 of both vibrational modes were evaluated from the integrated intensities of hot bands appearing in the R2PI spectra. The experimental results show that the populations in v = 1 of mode T and ―T oscillate against the double pump interval as a result of the wave-packet interference (Figure). By utilizing the difference in these oscillation periods, mode selective excitation was realized (dashed lines). We also conducted semiclassical calculations, where molecular vibration is treated quantum mechanically while molecular rotation is simulated by classical trajectory calculation, to show a good match-up with the observed time-evolution of populations.

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C. B. Madsen, et al., J. Chem. Phys. 130, 234310 (2009).

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Proton transfer reactions are essential in many biological and chemical processes, but are notoriously difficult to study directly due to strong anharmonic effects and ultrafast vibrational relaxation dynamics. Here, we present ultrafast transient absorption (TA) and two-dimensional infrared (2DIR) spectroscopies to characterize the intramolecular H-bonding interactions and proton transfer dynamics in a series of acetylacetone derivatives and their deuterated isotopologues. Strong 2DIR cross peaks are observed in the OH/OD bend region, indicating significant coupling between the OH/OD bends and other fingerprint modes. The OH/OD stretch regions display broad, homogeneous ground-state bleach signals that decay within 100 fs to intramolecular modes, resulting in long-lived hot-ground state signatures. Interestingly, polarization-dependent measurements reveal anisotropic signals that persist beyond the vibrational lifetimes with large isotopic differences. In general, the OH stretch anisotropies decay within about 200 fs, equivalent to one period of the intramolecular O-O stretch H-bond soft mode. The OD stretch anisotropies decay within 1-2 ps and correlate with the H-bond strength. The observed anisotropy timescales are interpreted in terms of the delocalization and transfer timescales of the proton/deuteron upon vibrational excitation.

Identifying molecular-level details of the solvent reorganizations that facilitate photo-induced excited-state proton transfer is challenging for current experimental and theoretical approaches. One distinct challenge for computations is accessing the long time scales required to get frequency-resolved data. In this talk, I will discuss our recent work modelling optical pump THz probe experiments using molecular dynamics simulations to elucidate the ultrafast changes in the solvation environment for three derivatives of pyranine: the photoacid HPTS, the methoxy derivative MPTS, and the photobase OPTS. The experiments show damped oscillations in the THz signal at short times for all three derivatives and our simulations enable their assignment to vibrational energy transfer beatings between the photoexcited chromophore and nearby solvent molecules. Additionally, the simulations of HPTS reveal strikingly efficient sub-ps energy transfer into a particular solvent mode, that is active near 4 THz, and which can provide the requisite energy required for solvent reorganization promoting proton transfer.

The phycobilisome is the principal light-harvesting chromoprotein complex in cyanobacteria and red algae. We have employed broadband multidimensional electronic spectroscopy with 6.7-fs laser pulses for the first time to study the excitation energy transfer mechanisms in intact phycobilisomes isolated from Fremyella diplosiphon. The results show that excitation energy transfer pathways include delocalized optical excitations of bilin (linear tetrapyrrole) chromophores, which transfer excitation energy along the rods in < 600 fs. Excitation energy moves more slowly from the rods to the core on the > 10 ps time scale, indicating that excitation energy is localized on individual bilin chromophores in the allophycocyanin-containing segments of the phycobilisome. The intramolecular charge-transfer character of the β84 chromophore in allophycocyanin would strongly favor dynamic exciton localization upon transfer of excitation energy from the rod segments. This phenomenon contributes to a kinetic bottleneck, which allows photoregulatory mechanisms, including that involving binding of the orange carotenoid protein, to operate efficiently in the core. In phycobilisomes isolated from Fremyella diplosiphon grown under red light, in contrast to those grown under white light, the terminal emitting APC680 segments exhibit significantly shorter excited-state lifetimes. These findings further show that trapping bilin sites are accumulated in the core of the phycobilisome during growth as part of a chromatic adaptation response. (Work supported by grant awards DE-SC0010847 (W.F.B. laboratory) and DE-SC0020606 (C.A.K. laboratory) from the Photosynthetic Systems program of the Office of Basic Energy Sciences, U.S. Department of Energy.)