Rotational state populations of ammonia are inferred from chirped pulse spectra of its tunneling doublets in a room temperature K-band waveguide experiment where many tunneling doublets can be addressed by a single chirped pulse excitation. The thermal distribution of states is altered by a pump pulse where the population of the tunneling doublet of a single rotational state is inverted by a π-pulse within roughly 100 ns. The resulting deviation from equilibrium is then propagating to other states due to collisions and interrogated by a probe pulse from which the state populations of many rotational states are inferred at once. From the free induction decays (FID) of the individual states the relaxation time of the radiation-induced superposition state of the two level tunneling system (T2) is inferred. Also the collisional relaxation time (T1) for the difference in the population of the two-level system is determined. These values exhibit a linear pressure dependence, the slope of which agrees very well with previous measurements P. E. Wagner et al 1981 J. Phys. B: At. Mol. Phys., vol. 14, 4763.
Analysis of probe FID signals from these pump-probe experiments reveals the well known hierachy of collisional relaxation in ammonia which was first found by Oka fifty years ago through steady state intensity measurements T. Oka, 1968, J.Chem.Phys., vol 48, 4919 Collision-induced transitions within the tunneling doublet (∆J = 0) determined from T1 measurements are faster than ∆J = ±1 transitions. Of those the ∆K = 0 transitions are much faster than those with ∆K ≠ 0. Due to this hierachy of inelastic processes and thanks to the fast optical pumping experiments state-to-state rates can be measured. As a result, from the pressure dependence of the measured rates state-to-state rate coefficients are determined. Those rate coefficients agree very well with results of simulations of all coupled states which fit with the temporal behavior of the complete pump probe experiments where many individual (J,K) rotational states can be addressed step by step by separate probe-pump-probe pulse sequences.

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P. E. Wagner et al 1981 J. Phys. B: At. Mol. Phys., vol. 14, 4763.. T. Oka, 1968, J.Chem.Phys., vol 48, 4919.

The microwave spectrum of ethyl cyanoformate was recorded at 261 K from 8.7 - 26.5 GHz with waveguide chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW). Ethyl cyanoformate has been studied previously by Suenram, True, and Bohn in a Stark modulation cell.Suenram, R.D., True, N.S., and Bohn, R.K., Journal of Molecular Spectroscopy, 69, 435-444 (1978).n that work, the data showed contributions from two stable ethyl cyanoformate conformers as well as an additional series of broad features ascribed to an exchange-averaged state, in analogy to similar features routinely seen in NMR chemical exchange measurements. In recent years, True has provided estimates for the lifetimes of these exchange species.True, N.S., Journal of Physical Chemistry A, 113, 6936-6946 (2009).ur high resolution spectra of a thermalized sample of ethyl cyanoformate at 261 K show contributions from syn-anti and syn-gauche conformers in agreement with the previous work. Using our standard data collection methods, however, we do not see evidence for the presence of exchange averaged bands. In this talk, we will highlight the similarities and differences of our work to previous work on this molecule and discuss the possibilities of observing these types of species with our spectrometer.

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Suenram, R.D., True, N.S., and Bohn, R.K., Journal of Molecular Spectroscopy, 69, 435-444 (1978).I True, N.S., Journal of Physical Chemistry A, 113, 6936-6946 (2009).O

Our group has been studying the reactions of hydrogen atoms (H atoms) with various molecules (NO, N₂O, CH₃OH) in solid hydrogen for the last several years.F.M. Mutunga, S.E. Follett, D.T. Anderson, J. Chem. Phys. 139, 151104 (2013).M. Ruzi, D.T. Anderson, J. Phys. Chem. A 119, 12270 (2015).M.E. Balabanoff, M. Ruzi, D.T. Anderson, Phys. Chem. Chem. Phys. 20, 422 (2018).ne interesting puzzle that we have been unable to solve is how to detect the concentration of H atoms using FTIR spectroscopy. One possibility to estimate the H atom concentration is to measure the conversion of ortho-H₂ to para-H₂ within the solid that is catalyzed by the presence of H atoms. The H atom is a good ortho-para catalyst because it is paramagnetic and mobile within the solid even at extremely low temperatures. We have recently conducted a number of studies where we purposely synthesize solid para-H₂ samples with approximately 3% ortho-H₂ concentrations (slightly elevated). In the absence of H atoms, the ortho-H₂ concentration in the solid is stable on the order of days due to slow self-conversion. We can quantitatively detect the ortho-H₂ fraction using the overlapping Q₁(0)+S₀(1) and Q₁(1)+S₀(1) double transitions of solid molecular hydrogen. By rapidly generating H atoms via in situ photolysis of various H atom precursor molecules (NO and N₂O), we can initiate ortho-para conversion and follow the ortho-H₂ fraction in real time. This H atom catalyzed ortho-para conversion data therefore has the time dependent H atom concentration encoded in the signal; the challenge is to extract it. We observe qualitative differences in the shape of the ortho-H₂ fraction decay curve depending on the specific precursor used, the specific photolysis conditions, and the temperature of the sample. We will present the latest results and analysis at the meeting.

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F.M. Mutunga, S.E. Follett, D.T. Anderson, J. Chem. Phys. 139, 151104 (2013).

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M.E. Balabanoff, M. Ruzi, D.T. Anderson, Phys. Chem. Chem. Phys. 20, 422 (2018).O

The Criegee intermediates, carbonyl oxides proposed by Criegee in 1949 as key intermediates in the ozonolysis of alkenes, play important roles in organic chemistry and atmospheric chemistry. The simplest Criegee intermediate is CH₂OO. In the reaction of O₃ with C₂H₄, some CH₂OO thus produced are internally excited so that they decompose to form OH, CO, CO₂, and other compounds. Recently a new scheme for production of CH₂OO in laboratories, ultraviolet (UV) irradiation of diiodomethane (CH₂I₂) in O₂, has enabled direct detection of CH₂OO with various methods and stimulated active research on Criegee intermediates. Even though  ∼ 25 % of CH₂OO was reported to decompose at pressure smaller than 60 Torr,W.-L. Ting, C.-H. Chang, Y.-F. Lee, H. Matsui, Y.-P. Lee, and J. J.-M. Lin, J. Chem. Phys. 2014, 141, 104308.nd infrared absorption of internally excited CO and CO₂ was reported,Y. T. Su, H.-Y. Lin, R. Putikam, H. Matsui, M. C. Lin, and Y. P. Lee, Nat. Chem. 2014, 6, 477.o investigation on the dynamics of the decomposition products in the reaction CH₂I + O₂ has been reported. We employed a step-scan Fourier-transform infrared (FTIR) spectrometer to record temporally resolved emission upon irradiation of mixtures of CH₂I₂, O₂, and Ar at 248 and 308 nm. IR emission of CO, CO₂, OH, CH₂I, and H₂CO in the region 1860−4900 cm⁻¹was recorded. At total pressure 8 Torr and irradiation wavelength 248 nm, rotationally resolved lines of CO (v ≤ 11, J ≤ 19) in region 1860−2300 cm⁻¹were observed; the rotational distribution is Boltzmann with temperature near 300 K, but the vibrational distribution is bimodal, with two components having averaged vibrational energies of 99 and 18 kJ mol⁻¹. Emission of OH (v ≤ 3, J ≤ 5.5) in region 2980−3600 cm⁻¹was observed with ambient rotational distribution and average vibrational energy of 41 kJ mol⁻¹. The branching ratio of CO : OH is 60:40. Emission of highly internally excited CO₂ was also observed; its average internal energy was estimated. The effects of pressure and irradiation wavelength on the emission of these species will be discussed.

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W.-L. Ting, C.-H. Chang, Y.-F. Lee, H. Matsui, Y.-P. Lee, and J. J.-M. Lin, J. Chem. Phys. 2014, 141, 104308.a Y. T. Su, H.-Y. Lin, R. Putikam, H. Matsui, M. C. Lin, and Y. P. Lee, Nat. Chem. 2014, 6, 477.n

Carbocyclic geminal dibromides are important intermediates in synthesis of complex heterocyclic molecules and natural products.Ganesh, N. V.; Jayaraman, N. J. Org. Chem. 2007, 72, 5500-5504.ltrafast time-resolved techniques can help to obtain information about the electronic structure of the intermediates which appears in the chemistry and photochemistry of these molecules. Also, relaxation dynamics and reactivity of these intermediates in different solvents can be characterized. This information can be used for the design of different compounds with desired properties. In the current work, 1,1-dibromocycloalkanes with 3-, 4-, and 5-member rings were synthesized using previously described procedures.Napolitano, E.; Fiachi, R.; Mastrorilli, E. Synt. Comm. 1986, 122-125.ingh, R. K.; Danishefsky, S. J.Org.Chem. 1975, 40, 2969-2970. Blankenship, C.; Paquette, L. A. Synt. Comm. 1984, 14, 983-987.or all three samples, ultrafast time-resolved absorption experiments with UV-excitation were performed in acetonitrile and methylcyclohexane solvents. It was shown that excitation of dibromocycloalkane solutions with 250 nm short (40 fs) pulses forms excited state absorption (ESA) of parent molecules in the spectral range from 360 to 760 nm. Within 800 fs, ESA decays with beginning of formation product bands. Next within 50 ps, these broad bands reach their maximum intensity and form well-defined broad peaks centered at 550, 600 and 400 nm for 3-, 4-, and 5-member rings respectively. These product species are long-lived and begin to decay at 1 ns. Obtained results suggest the formation of isomer products (CH₂)n−C−Br−Br (n=1-3), similar to the isomeric species (HBrCBr-Br) observed in isomerization of bromoform.Pal, S. K.; Mereshchenko, A. S.; Butaeva, E. V.; El-Khoury, P. Z.; Tarnovsky, A. N. J. Chem. Phys. 2013, 138, 124501.Mereshchenko, A. S.; Butaeva, E. V.; Borin, V. A.; Eyzips, A.; Tarnovsky, A. N. Nat. Chem. 2015, 7, 562-568.

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Ganesh, N. V.; Jayaraman, N. J. Org. Chem. 2007, 72, 5500-5504.U Napolitano, E.; Fiachi, R.; Mastrorilli, E. Synt. Comm. 1986, 122-125.

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Blankenship, C.; Paquette, L. A. Synt. Comm. 1984, 14, 983-987.F Pal, S. K.; Mereshchenko, A. S.; Butaeva, E. V.; El-Khoury, P. Z.; Tarnovsky, A. N. J. Chem. Phys. 2013, 138, 124501.

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Semi-classical transition state theory (SCTST) is a powerful tool for calculating quantitative chemical reaction rates based on local properties of the transition state (TS) itself without detailed knowledge of the entire reaction path. The widely successful variant of SCTST based on second-order vibrational perturbation theory (VPT2) routinely provides accurate kinetic rate predictions. However, the fundamental formulation and approximations of VPT2 (and higher order extensions) often cause it to fail in the presence of significant anharmonicity near the TS. Curvilinear vibrational self consistent field theory (VSCF) together with its second order perturbative extension (VMP2) are an effective alternative to standard VPT2 for the bound state vibrational problem of highly anharmonic PES minima, suggesting similar improvements are possible at the TS. This talk will present some ideas and exploratory results towards applying complex scaled VSCF to the TS problem.

Water, an essential ingredient of life, is prevalent in space and various media. H₂O in the gas phase is the major polyatomic species in the interstellar medium (ISM) and a primary target of current studies of collisional dynamics. In recent years a number of theoretical and experimental studies have been devoted to H₂O-X (with X=He, H₂, D₂, Ar, …) elastic and inelastic collisions in an effort to understand rotational distributions of H₂O in molecular clouds. In this work we are following those studies and will present benchmark calculations of rovibrational states and resonances of the H₂O-H₂ cluster in the rigid rotor approximation using the MultiConfiguration Time Dependent Hartree (MCTDH) approach. We will also present the first state-to-state inelastic scattering results of the H₂O+H₂ process in the rigid rotor approximation using the MCTDH approach. These calculations will serve as a foundation for similar triatomic – linear molecule interactions which are usually computationally expensive using standard calculations methods.

Our previous work [Rivera-Rivera et al. J. Chem. Phys. 142, 014303 (2015)] used classical molecular dynamics simulations to study the pressure effects on the relaxation of a nitromethane (CH₃NO₂) molecule in an argon bath at 300 K and pressure ranging from 10 to 400 atm. The molecule was instantaneously excited by statistically distributing 50 kcal/mol among all its internal degrees of freedom. The saved CH₃NO₂ positions and momenta are then used to separate the vibrational and rotational energy of the molecule following the methodology developed by Rhee and Kim [J. Chem. Phys. 107, 1394 (1997)]. The vibrational, rotational, and Coriolis energies exhibited multi-exponential decay. It is also found, that at later times the three energies decay approximately exponentially with similar decay rates. The mode-specific decomposition of these three energies produces, for each of the eight studied pressures, approximately 30 separate decay curves whose signal rises above statistical noise. Which vibrational and rotational modes these decay curves represent, and how their pressure dependence varies, gives insight into how excess energy equilibrates in CH₃NO₂.

Protein conformational dynamics are at the root of many biological processes but are difficult to characterize experimentally because they occur at timescales that can range from picoseconds to milliseconds and longer. Sophisticated techniques must be used to measure dynamics occurring on fast timescales (fs or ps), such as dynamics of protein side chains or solvent in protein microenvironments. Two-dimensional infrared (2D IR) spectroscopy has emerged as a powerful tool for the measurement of protein dynamics and conformational heterogeneity at the picosecond timescale due to its high temporal and spatial resolution. However, the IR spectrum of a protein is typically severely congested due to the large number of similarly bonded atoms. For this reason, protein 2D IR is paired with site-specific incorporation of spectrally resolved IR probes that are active in the transparent frequency region ( 1800-2500 cm⁻¹) and thus act as vibrational reporters. Putidaredoxin is known to play an effector role on cytochrome P450cam, however the conformation of the cytochrome P450cam-putidaredoxin (P450-Pdx) complex is currently debated. The conformational dynamics of the P450-Pdx complex were measured using heme-bound CO as a vibrational probe of local environment. To further examine the proposed conformational state of the P450-Pdx complex, the dynamics of a P450cam mutant (L358P) thought to behave similarly to the putidaredoxin complex were also measured. The information gathered from 2D IR experiments has provided new insight into the conformational states exhibited by the P450-Pdx complex.

Extreme ultraviolet (XUV) spectroscopy is an inner shell technique that probes the M_2,3 -edge excitation of atoms. Absorption of the XUV photon causes a 3p → 3d transition, the energy and multiplet of which is directly related to the element and ligand environment. This in-lab technique is thus element-, oxidation state-, spin state-, and ligand field specific and is a useful tool for the study of electron and energy transfer processes in materials and chemical biology. With the use of this technique and semi-empirical simulations, I have collected ultrafast transient M_2,3 -edge absorption data of four different metalloporphyrinates (M = Fe, Co, Ni, Mn) in order to resolve the early time relaxation mechanism of these catalytically-relevant coordination complexes with femtosecond time resolution. This is the first instance of using tabletop transient XUV/VUV spectroscopy on coordination complexes and furthermore highlights the importance of directly probing of the metal center in these systems. I will additionally present ongoing work on applying this technique to the study of heterobimetallic systems with directly-interacting dual metal centers within a non-innocent ligand scaffold. The relation of function to metal-specific photodynamics will help lay essential groundwork for the development of multimetallic catalysts with efficiencies comparable to those found in nature.

Extreme ultraviolet (XUV) transient absorption spectroscopy is emerging as a powerful, element-specific tool for measuring femtosecond to attosecond dynamics in molecular and solid-state systems. Tabletop XUV transient absorption spectra retain the element specificity of hard x-ray absorption while providing a straightforward mapping of the unoccupied valence and conduction band density of states. The presence of distinct signals for holes and electrons in the XUV region is especially powerful, as the dynamics of these carriers are often convolved in transient UV/visible measurements. In this work we measure the rate of charge transfer across TiO2/CH3NH3PbI3 and CH3NH3PbI3/NiO interfaces, and highlight the competition between carrier cooling in the perovskite absorber and charge injection into the electron/hole collection layers