The vast majority of gas-phase techniques typically employed for studying reaction kinetics follow only the time dependence of one of the reactants or the products and do not allow to distinguish between different isomers, conformers or vibrationally excited states presented in the reaction. However, this has changed in the recent years with the development of the chirped-pulse microwave spectroscopy in uniform supersonic flow (CPUF) technique to study gas phase reaction kinetics. This technique as implemented in Rennes employs the CRESU (a French acronym standing for reaction kinetics in uniform supersonic flow) method coupled with chirped-pulse Fourier transform microwave spectroscopy, combining the power to generate continuous cold uniform supersonic flows with the high selectivity and general applicability of rotational spectroscopy. As a result, it is possible to simultaneously study the time dependence of various species involved in the reaction and to distinguish between different conformers, isomers and vibrationally excited states. The uniform CRESU conditions permit frequent enough collisions to preserve local thermodynamic equilibrium in the flow with regard to translational and rotational degrees of freedom, but vibrational relaxation of some molecules may not be complete. This is a double-edged sword: a challenge for the analysis of product branching ratios, especially for strongly exothermic reactions; but also an opportunity to study vibrational relaxation of polyatomic molecules at low temperatures, a subject of significant theoretical and astrophysical interest. Here we present the time dependent collisional relaxation of some vibrationally excited states of both small molecules produced either by photolysis or chemical reaction in the cold uniform flow, as well as low frequency modes of larger molecules.

Non-covalently bound clusters have long been a target of study in microwave spectroscopy, however, the typical method of forming these small clusters precludes observation of that formation. Here, I present observation of the conformer-selected formation of ethanol-methanol dimers in a cryogenic buffer gas cell via microwave spectroscopy. Use of a buffer gas cell allows for observation of a complete time-domain picture of the reaction of two monomers to form a dimer, as the dimers are formed in the interaction region of the experiment as opposed to just after a pulsed valve. Relaxation cross sections and collisional cross sections are also presented for ethanol.

Low temperature reactions play a critical role in the chemistry of the interstellar medium (ISM). Measuring the kinetics of these reactions is key to constraining models of ISM chemistry, and to understanding ISM chemistry at large. However, measuring chemical kinetics at temperatures relevant to the ISM presents numerous experimental challenges, including creating homogeneous and cooled reactants. Buffer gas cooling offers a near universal method of achieving uniform electronic, vibrational, rotational, and translational cooling, while consuming minimal sample. When combined with microwave spectroscopy, buffer gas cooling cells offer a unique method for probing reactions occurring at very low temperatures. We will report our progress in building and characterizing a buffer gas cell configuration capable of measuring bimolecular reactions of thermalized species occurring within the cell, and discuss its applications for studying ISM chemistry.

Some of the most important molecular architectures in nature, such as light harvesting antennae, feature the presence of several nearly identical electronic chromophores in close proximity, in which directed electronic energy transfer plays a key part in the initial events following absorption of a visible photon. This is an area in which spectroscopy and dynamics are inextricably linked, and for which gas phase spectroscopy can play a role in testing model systems in a way not possible in their natural environments. We have studied the UV photofragment spectroscopy of a series of cryo-cooled ions in the gas phase that are close analogs of protonated Leu-enkephalin, the pentapeptide Tyr-Gly-Gly-Phe-Leu-OMe (in short-hand notation, YGGFL-OMe). This protonated ion has been studied previously, and folds into a single peptide backbone conformation that incorporates a beta-turn. We replace the Tyr and Phe UV chromophores with other chromophores chosen to bring their electronic absorptions into near degeneracy. UV photofragmentation reports on the location of the electronic energy via a unique fragmentation pathway involving loss of the resonance-stabilized aromatic, CH2-Phe-X. We identify the chromophore responsible for the UV absorption and map out the efficiency of electronic energy transfer as a function of vibronic state via the fragmentation behavior, a fragmentation based version of fluorescence resonance energy transfer (FRET).

The weakly bound complex between two hydrogen sulfide molecules and one water molecule, ( H₂S)₂ ( H₂O), was identified from its rotational spectrum observed at conditions of supersonic expansion. The spectra of parent species were obtained using a chirped-pulse Fourier transform microwave spectrometer (Newcastle, UK). The isotopologues were identified with Balle-Flygare Fourier transform microwave spectrometer (Bangalore, India). Distinct physical properties of H₂O and H₂S under ambient settings have long been recognized as a result of their significantly different hydrogen-bonding capabilities. It has conclusively shown ( H₂S)₂ is hydrogen-bonded similar to ( H₂O)₂ at very low temperatureA. Das, P. K. Mandal, F. J. Lovas, C. Medcraft, N. R. Walker, and E. Arunan. Angewandte Chemie International Edition, 2018, 57, 15199-15203. The break with axial molecular symmetry and the simplified internal dynamics allowed us to investigate ( H₂S)₂ ( H₂O) at a level of structural detail that has not yet been possible for ( H₂O)₃ and ( H₂S)₃ with rotational spectroscopy due to their zero-dipole moment. The rotational spectrum of ( H₂S)₂ ( H₂O) shows a doubling of the lines, close to 1:3 relative intensity for the parent species, caused by the internal rotation of the H₂O moiety about its C₂ axis. Analysis of experimental results reveals that the three monomers are bound in a triangular arrangement through the S-H···S, O-H···S and S-H···O hydrogen bonds. The r_s and r₀ structural parameters have been evaluated, and the three heavy atom distances r_s(S-H···S)=4.067(2)Å, r_s(O-H···S)=3.412(11)Å and r_s(S-H···O)=3.454(11)Å are appreciably shorter than the respective distances in ( H₂S)₂, HOH···S H₂ and HSH···O H₂P. K. Mandal, Ph.D. Dissertation, Indian Institute of Science, 2005. The geometry contains numerous characteristics that indicate the cooperative nature of the intermolecular interaction. The experimental results for all observables determinable from the rotational spectrum are found to be in excellent agreement with ab initio predictions.

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Footnotes:

A. Das, P. K. Mandal, F. J. Lovas, C. Medcraft, N. R. Walker, and E. Arunan. Angewandte Chemie International Edition, 2018, 57, 15199-15203.. P. K. Mandal, Ph.D. Dissertation, Indian Institute of Science, 2005..

Formaldehyde (FA) is a fascinating molecule for astrochemists due to the complex mechanistic pathways leading to its formation. Being a very important prebiotic precursor, understanding its participation in various weak interactions is crucial. FA is well established to form hydrogen bonds. In the present work, the interaction of FA with Nitromethane (NM) was studied at low temperature and supported by ab initio theoretical calculations. The heterodimers of NM and FA, NM-FA, were generated within Ar and N₂ matrices and characterized using infrared spectroscopy. Perturbation in the ν₃ mode of NM and ν₂ mode of FA due to the formation of heterodimers has been investigated, as these infrared spectral signatures were shifted from the monomer absorption of NM and FA. The variation of the intensity of these features, in response to the variation in concentration of FA and NM, additionally supported with computations, affirms the formation NM-FA. The red shifts observed, agree well with the predictions by harmonic frequency calculations on the pnicogen-hydrogen-tetrel bound geometry. Computations indicated three minima on the potential energy surface at MP2/CBS and B2PLYP-GD3/CBS levels of theory. The most stable heterodimer, observed experimentally, was stabilized by cooperative pnicogen (O…N), hydrogen (O…H) and tetrel (O…C) bonds as confirmed by QTAIM and NBO analyses. Dominance of electrostatics over other effects in forming the bonds has been established by energy decomposition analysis (EDA). The ability of FA, as a potential electron donor to pnicogen bonding while being a weak tetrel donor too, in addition to its expected participation in hydrogen bonding, stands established experimentally and computationally.

r0pt Figure In quantum crystals such as solid parahydrogen (pH₂), there is considerable overlap between the wavefunctions of molecules in neighboring lattice sites, such that added chemical impurities can exchange positions with nearest-neighbor pH₂ molecules and thereby quantum diffuse through the solid. Our group and others have taken advantage of the quantum diffusion of hydrogen atoms in solid pH₂ to study various low temperature hydrogenation reactions.F. M. Mutunga et al., J. Chem. Phys. 154 (2021) 014302.n this talk, we report the first experimental evidence of atomic oxygen diffusion in solid pH₂. O₂ doped pH₂ samples are irradiated at 193 nm to produce O(³P) atoms, and repeated FTIR spectra are collected to map out the temporal behavior during and after photolysis. The experimental proof of mobile O-atoms is provided by the formation of ozone (O₃), which forms via the barrierless O + O₂ + M → O₃ + M reaction. After photolysis, while the system is kept in the dark, continued growth in the O₃ concentration with time is detected, indicating that O-atoms are mobile and reacting with O₂ present in the solid. The O₃ growth after photolysis is fit to first-order kinetics equations to extract the rate constant. Kinetics measurements show that the O-atom reaction rate more than doubles in annealed crystals compared to as-deposited crystals. This finding is consistent with the expectation that quantum diffusion is more facile in homogeneous samples with minimum defects. In fact, some proportion of the photo-produced O-atoms get trapped in as-deposited samples and can only be made mobile by annealing the sample. Currently, we are studying the effects of the photolysis conditions, temperature, and doped O₂ concentration on the reaction rate constant. This study shows that O-atoms can be isolated in solid pH₂ and that they are delocalized. Through double doping experiments, we hope to develop this method to study O-atom reactions with other species under controlled low temperature conditions.

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Footnotes:

F. M. Mutunga et al., J. Chem. Phys. 154 (2021) 014302.I

Both in experiment and simulations, the kinetics of the mutual interconversion of states in a complex many-body system are usually much more difficult to determine than the equilibrium distribution of states (EDSs). Therefore, it is tempting to find out whether the knowledge of the EDS of a system allows us to obtain information about its kinetics, and if so, to what extent. For this, it is proposed to use the Metropolis Monte Carlo (MMC) method. The EDS plays a roles of the potential of mean force that determines the acceptance probabilities of new states in the MMC simulations.
The approach is illustrated by the protein folding/unfolding reaction. Specifically, two proteins are considered - a model β-hairpin and helical α₃D protein. For β-hairpin, the free-energy surfaces and free-energy profiles for a set of temperatures are used as the EDSs. It has been found that the rate constants and first-passage time (FPT) distributions obtained in the MMC simulations change with temperature in good agreement with those obtained from molecular dynamics simulations. For α₃D, whose equilibrium folding/unfolding was studied by single-molecule FRET (Chung et al., J. Phys. Chem. A, 115, 2011, 3642), the experimental FRET-efficiency histograms at different denaturant concentrations were used as the EDSs. The rate constants for folding and unfolding obtained in the MMC simulations have been found to change with denaturant concentration in reasonable agreement with the rate constants extracted from the photon trajectories on the basis of theoretical models.
The promising feature of the present approach is that it does not require introducing any additional parameters to perform simulations, which suggests its applicability to other complex systems.