Infrared-driven dynamics and scattering mechanisms of NO radicals with propane and butane: impacts of pseudo Jahn-Teller effects.

The topology of multidimensional potential energy surfaces defines the bimolecular collision outcomes of open-shell radicals with molecular partners. Understanding these surfaces is crucial for predicting the inelastic scattering and chemical transformations of increasingly complex radical-molecule collisions. To characterize the inelastic scattering mechanisms of nitric oxide (NO) radicals with large alkanes, we generated the collision complexes comprised of NO with propane or n-butane. The infrared action spectroscopy and infrared-driven dynamics of NO-propane and NO-(n-butane) collision complexes in the CH stretch region were recorded, while also comparing the results to the analogous experiments carried out for NO-CH4 and NO-ethane. The infrared spectroscopy is analyzed using rovibrational simulations to characterize the transition bands and to determine the vibrational predissociation lifetimes of NO-propane and NO-(n-butane). Due to pseudo Jahn-Teller dynamics, the NO-propane and NO-(n-butane) decay mechanisms from IR activation appear similar to those for NO-ethane previously reported from this laboratory (J. P. Davis et al. Faraday Discuss., 2024, 251, 262-278). Furthermore, the NO (X2Π, v'' = 0, J'', Fn, Λ) product state distributions from NO-alkane fragmentation reveal a strong electron-spin polarization and a propensity for NO products to rotate in the plane of the π* molecular orbital, yielding mechanistic insights into the inelastic scattering outcomes. We hypothesize that a geometric phase may be present, impacting the relative population distributions, in addition to the accessible pathway timescales.

Bimolecular collision outcomes on multidimensional potential energy surfaces: infrared spectroscopy and activation of NO-alkane collision complexes.

In bimolecular collisions between open-shell radicals and increasingly-larger alkanes, the relative impact configurations open the possibility of reactive and nonreactive outcomes that are isomer specific. To model the interaction potential between molecular scattering partners, observables are needed from experiments that can quantify both the initial molecular orientations and internal energies on multidimensional potential energy surfaces. Recent work by our group demonstrated that upon infrared (IR) excitation, the dynamics of the nitric oxide-methane collision complex (NO-CH4) are dependent on the initial monomer geometries, as small changes in configuration substantially affect the energies, electronic couplings, and predissociation pathways due to the Jahn-Teller effect. This study focuses on the isomer-specific scattering mechanisms between NO and ethane (C2H6), encoded in the spectroscopic and dynamical signatures of the NO-C2H6 collision complex. IR action spectroscopy with 1 + 1 resonance-enhanced multiphoton ionization of NO products was employed to characterize the fundamental CH stretch transitions of NO-C2H6, as well as to initiate the nonreactive decay mechanisms of the complex. Furthermore, velocity map imaging (VMI) was utilized to explore the dynamics prior to and following IR excitation of NO-C2H6, imprinted on the NO photoproducts. This work compares the dynamics from NO-C2H6 and NO-CH4 vibrational predissociation, in which substantial differences are observed in the energy exchange mechanisms during the evolution of the collision complexes to products.

Photophysical Outcomes of Water-Solvated Heterocycles: Single-Conformation Ultraviolet and Infrared Spectroscopy of Microsolvated 2-Phenylpyrrole.

The molecular chromophores within brown carbon (BrC) aerosols absorb solar radiation at visible and near-ultraviolet wavelengths. This contributes to the overall warming of the troposphere and the photochemical aging of aerosols. In this investigation, we combine a suite of experimental and theoretical methods to reveal the conformation-specific ultraviolet and infrared spectroscopy of 2-phenylpyrrole (2PhPy)─an extended π-conjugated pyrrole derivative and a model BrC chromophore─along with its water microsolvated molecular complexes (2PhPy:nH2O, n = 1-3). Using resonant two-photon ionization and double-resonance holeburning techniques alongside MP3 (ground state) and ADC(3) (excited state) torsional potential energy surfaces and discrete variable representation simulations, we characterized the ultraviolet spectra of 2PhPy and 2PhPy:1H2O. This analysis revealed evidence for Herzberg-Teller vibronic coupling along the CH wagging and NH stretching coordinates of the aromatic rings. Conformation-specific infrared spectroscopy revealed extended hydrogen-bonding networks of the 2PhPy:nH2O complexes. Upon stepwise addition of H2O solvation, the nearest H2O acceptor forms a strong, noncovalent interaction with the pyrrole NH donor, while the second and third H2O partners interface with the phenyl and pyrrole aromatic rings through growing van der Waals π/H atom stabilization. A local-mode Hamiltonian approach was employed for comparison with the experimental spectra, thus identifying the vibrational spectral signatures to specific 2PhPy:nH2O oscillators.

Infrared Activated Signatures and Jahn-Teller Dynamics of NO-CH4 Collision Complexes.

Bimolecular collision outcomes sensitively depend on the chemical functionality and relative orientations of the colliding partners that define the accessible reactive and nonreactive pathways. Accurate predictions from multidimensional potential energy surfaces demand a full characterization of the available mechanisms. Therefore, there is a need for experimental benchmarks to control and characterize the collision conditions with spectroscopic accuracy to accelerate the predictive modeling of chemical reactivity. To this end, the bimolecular collision outcomes can be investigated systematically by preparing reactants in the entrance channel prior to reaction. Herein, we investigate the vibrational spectroscopy and infrared-driven dynamics of the bimolecular collision complex between nitric oxide and methane (NO-CH4). We recorded the vibrational spectroscopy of NO-CH4 in the CH4 asymmetric stretching region using resonant ion-depletion infrared spectroscopy and infrared action spectroscopy, thus revealing a significantly broad spectrum centered at 3030 cm-1 that extends over 50 cm-1. The asymmetric CH stretch feature of NO-CH4 is explained by CH4 internal rotation and attributed to transitions involving three different nuclear spin isomers of CH4. The vibrational spectra also show extensive homogeneous broadening due to the ultrafast vibrational predissociation of NO-CH4. Additionally, we combine infrared activation of NO-CH4 with velocity map imaging of NO (X2Π, ν″ = 0, J″, Fn, Λ) products to develop a molecular-level understanding of the nonreactive collisions of NO with CH4. The anisotropy of the ion image features is largely determined by the probed rotational quantum number of NO (J″) products. For a subset of NO fragments, the ion images and total kinetic energy release (TKER) distributions show an anisotropic component at low relative translation (∼225 cm-1) indicating a prompt dissociation mechanism. However, for other detected NO products, the ion images and TKER distributions are bimodal, in which the anisotropic component is accompanied by an isotropic feature at high relative translation (∼1400 cm-1) signifying a slow dissociation pathway. In addition to the predissociation dynamics following vibrational excitation, the Jahn-Teller dynamics prior to infrared activation need to be considered to fully describe the product spin-orbit distributions. Therefore, we correlate the Jahn-Teller mechanisms of NO-CH4 to the symmetry-restricted NO (X2Π, ν″ = 0, J″, Fn, Λ) + CH4 (ν″) product outcomes.

Reactive quenching of NO (A2Σ+) with H2O leads to HONO: a theoretical analysis of the reactive and nonreactive electronic quenching mechanisms.

The electronic quenching of NO (A2Σ+) with molecular partners exemplifies the rich non-adiabatic dynamics that occurs on multiple, coupled potential energy surfaces (PESs). The mechanistic details of the electronic quenching depend sensitively on the nature and strength of the intermolecular interactions between NO (A2Σ+) and the molecular partner. In this paper, we reveal the electronic quenching mechanisms of NO (A2Σ+) with H2O, a non-adiabatic process with an extremely large cross section of 121 Å2 near room temperature. In doing so, we demonstrate that the NO (A2Σ+) + H2O PES funnels a wide range of initial intermolecular orientations to the same minimum-energy geometry. Furthermore, we reveal low-energy pathways to conical intersections between NO (A2Σ+) + H2O and NO (X2Π) + H2O that primarily involve decreasing the intermolecular distance and elongating a single O-H bond of H2O. Based on these geometric distortions, we predict that nonreactive electronic quenching will be associated with significant vibrational excitation in a local O-H stretch mode in H2O. Reactive quenching will produce a H-atom and HONO, an important intermediate in atmospheric and combustion chemistry and a precursor to the hydroxyl radical. Overall, our work provides the first detailed theoretical study of the mechanism of the electronic quenching of NO (A2Σ+) with a polyatomic molecular partner, as well as makes concrete predictions to inform future velocity map imaging experiments.

Solvent-Mediated Charge Transfer Dynamics of a Model Brown Carbon Aerosol Chromophore: Photophysics of 1-Phenylpyrrole Induced by Water Solvation.

Nitrogen heterocycles are known to be important light-absorbing chromophores in a newly discovered class of aerosols, commonly referred to as "brown carbon" (BrC) aerosols. Due to their significant absorption and spectral overlap with the solar actinic flux, these BrC chromophores steer the physical and optical properties of aerosols. To model the local aqueous solvation environment surrounding BrC chromophores, we generated cold molecular complexes with water and a prototypical BrC chromophore, 1-phenylpyrrole (1PhPy), using supersonic jet-cooling and explored their intermolecular interactions using single-conformation spectroscopy. Herein, we utilized resonant two-photon ionization (R2PI) and UV holeburning (UV HB) double-resonance spectroscopies to obtain a molecular-level understanding of the role of water microsolvation in charge transfer upon photoexcitation of 1PhPy. Quantum chemical calculations and one-dimensional discrete variable representation simulations revealed insights into the charge transfer efficacy of 1PhPy with and without addition of a single water molecule. Taken together, our results indicate that the intermolecular interactions with water guide the geometry of 1PhPy to adopt a more twisted intramolecular charge transfer (TICT) configuration, thus facilitating charge transfer from the pyrrole donor to the phenyl ring acceptor. Furthermore, the water network surrounding 1PhPy reports on the charge transfer such that the H2O solvent primarily interacts with the pyrrole ring donor in the ground state, whereas it preferentially interacts with the phenyl ring acceptor in the excited state. Large Franck-Condon activity is evident in the 1PhPy + 1H2O excitation spectrum for the water-migration vibronic bands, supporting H2O solvent reorganization upon excitation of the 1PhPy chromophore. Fluorescence measurements with increasing H2O % volume corroborated our gas-phase studies by indicating that a polar water solvation environment stabilizes the TICT configuration of 1PhPy in the excited electronic state, from which emission is observed at a lower energy compared to the locally excited configuration.

Stereodynamic Control of Collision-Induced Nonadiabatic Dynamics of NO (A2Σ+) with H2, N2, and CO: Intermolecular Interactions Drive Collision Outcomes.

Intermolecular interactions, stereodynamics, and coupled potential energy surfaces (PESs) all play a significant role in determining the outcomes of molecular collisions. A detailed knowledge of such processes is often essential for a proper interpretation of spectroscopic observations. For example, nitric oxide (NO), an important radical in combustion and atmospheric chemistry, is commonly quantified using laser-induced fluorescence on the A2Σ+ ← X2Π transition band. However, the electronic quenching of NO (A2Σ+) with other molecular species provides alternative nonradiative pathways that compete with fluorescence. While the cross sections and rate constants of NO (A2Σ+) electronic quenching have been experimentally measured for a number of important molecular collision partners, the underlying photochemical mechanisms responsible for the electronic quenching are not well understood. In this paper, we describe the development of high-quality PESs that provide new physical insights into the intermolecular interactions and conical intersections that facilitate the branching between the electronic quenching and scattering of NO (A2Σ+) with H2, N2, and CO. The PESs are calculated at the EOM-EA-CCSD/d-aug-cc-pVTZ//EOM-EA-CCSD/aug-cc-pVDZ level of theory, an approach that ensures a balanced treatment of the valence and Rydberg electronic states and an accurate description of the open-shell character of NO. Our PESs show that H2 is incapable of electronically quenching NO (A2Σ+) at low collision energies; instead, the two molecules will likely undergo scattering. The PESs of NO (A2Σ+) with N2 and CO are highly anisotropic and demonstrate evidence of electron transfer from NO (A2Σ+) into the lowest unoccupied molecular orbital of the collision partner, that is, the harpoon mechanism. In the case of ON + CO, the PES becomes strongly attractive at longer intermolecular distances and funnels population to a conical intersection between NO (A2Σ+) + CO and NO (X2Π) + CO. In contrast, for ON + N2, the conical intersection is preceded by an ∼0.40 eV barrier. Overall, our work shines new light into the impact of coupled PESs on the nonadiabatic dynamics of open-shell systems.

Dynamical signatures from competing, nonadiabatic fragmentation pathways of S-nitrosothiophenol.

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 (X2Π1/2, 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.

Imaging the nonreactive collisional quenching dynamics of NO (A2Σ+) radicals with O2 (X3Σg -).

Nitric oxide (NO) radicals are ubiquitous chemical intermediates present in the atmosphere and in combustion processes, where laser-induced fluorescence is extensively used on the NO (A2Σ+ ← X2Π) band to report on fuel-burning properties. However, accurate fluorescence quantum yields and NO concentration measurements are impeded by electronic quenching of NO (A2Σ+) to NO (X2Π) with colliding atomic and molecular species. To improve predictive combustion models and develop a molecular-level understanding of NO (A2Σ+) quenching, we report the velocity map ion images and product state distributions of NO (X2Π, v″ = 0, J″, Fn, Λ) following nonreactive collisional quenching of NO (A2Σ+) with molecular oxygen, O2 (X3Σg -). A novel dual-flow pulse valve nozzle is constructed and implemented to carry out the NO (A2Σ+) electronic quenching studies and to limit NO2 formation. The isotropic ion images reveal that the NO-O2 system evolves through a long-lived NO3 collision complex prior to formation of products. Furthermore, the corresponding total kinetic energy release distributions support that O2 collision coproducts are formed primarily in the c1Σu - electronic state with NO (X2Π, v″ = 0, J″, Fn, Λ). The product state distributions also indicate that NO (X2Π) is generated with a propensity to occupy the Π(A″) Λ-doublet state, which is consistent with the NO π* orbital aligned perpendicular to nuclear rotation. The deviations between experimental results and statistical phase space theory simulations illustrate the key role that the conical intersection plays in the quenching dynamics to funnel population to product rovibronic levels.

The effects of site asymmetry on near-degenerate state-to-state vibronic mixing in flexible bichromophores.

Laser-induced fluorescence excitation and dispersed fluorescence spectra of a model flexible bichromophore, 1,1-diphenylethane (DPE), have been recorded under jet-cooled conditions in the gas phase in the region near the first pair of near-degenerate excited states (S1 and S2). The S1 and S2 origin transitions have been identified at 37 397 and 37 510 cm-1, a splitting of 113 cm-1. This splitting is four times smaller than the excitonic splitting calculated by ab initio methods at the EOM-CCSD/cc-pVDZ level of theory (410 cm-1), which necessarily relies on the Born-Oppenheimer approximation. Dispersed fluorescence spectra provide a state-to-state picture of the vibronic coupling. These results are compared with the results of a multimode vibronic coupling model capable of treating chromophores in asymmetric environments. This model was used to predict the splitting between S1 and S2 origins close to the experiment, reduced from its pure excitonic value by Franck-Condon quenching. Quantitative accuracy is achieved by the model, lending insight into the state-to-state mixing that occurs between individual S1 and S2 vibronic levels. The S2 origin is determined to be mixed with S1(v) levels by two mechanisms common to internal conversion in almost any setting; namely, (i) mixing involving near-degenerate levels with large vibrational quantum number changes that are not governed by Δv = 1 Herzberg-Teller (HT) selection rules, and (ii) mixing with levels with larger energy gaps that do follow these selection rules. In DPE, the asymmetric ring flapping vibrational mode R¯ dominates the HT coupling.

Nonstatistical Dissociation Dynamics of Nitroaromatic Chromophores.

Organic carbon in the atmosphere is emitted from biogenic and anthropogenic sources and plays a key role in atmospheric chemistry, air quality, and climate. Recent studies have identified several of the major nitroaromatic chromophores embedded in organic "brown carbon" (BrC) aerosols. Indeed, nitroaromatic chromophores are responsible for the enhanced solar absorption of BrC aerosols, extending into the near UV (300-400 nm) and visible regions. Furthermore, BrC chromophores serve as temporary reservoirs of important oxidizing intermediates including hydroxyl (OH) and nitric oxide (NO) radicals that are released upon electronic excitation. The present work represents the first study of the 355 nm photolysis of known BrC chromophores ortho-nitrophenol and 2-nitroresorcinol, as well as the prototypical nitroaromatic, nitrobenzene. Experiments are carried out in a pulsed supersonic jet expansion with velocity map imaging of NO X2Π (ν″ = 0, J″) fragments to report on the photodissociation dynamics. The total kinetic energy release (TKER) distributions and the NO X2Π (ν″ = 0, J″) product state distributions deviate significantly from Prior simulations, indicating that energy is partitioned nonstatistically following dissociation. Experiments are conducted in tandem with complementary calculations using multireference Møller-Plesset second-order perturbation theory (MRMPT2) for stationary points obtained by using multiconfiguration self-consistent field (MCSCF) with an aug-cc-pVDZ basis on the ground and lowest energy triplet electronic states. Furthermore, insights into the partitioning of energy upon photodissociation are achieved by using relaxed scans at the MCSCF/aug-cc-pVDZ level of theory. As a whole, the results suggest that upon excitation to S1, all three nitroaromatics share a common overall mechanism for NO production involving isomerization of the nitro group, nonradiative relaxation to S0, and dissociation to form rotationally hot NO.

Imaging the Dynamics of CH2BrI Photodissociation in the Near Ultraviolet Region.

The photodissociation dynamics of jet-cooled CH2BrI were investigated in the near-ultraviolet (UV) region from 280-310 nm using velocity map imaging. We report the translational and internal energy distributions of the CH2Br radical and ground state I (2 P3/2) or spin-orbit excited I (2 P1/2) fragments determined by velocity map imaging of the ionized iodine fragments following 2 + 1 resonance-enhanced multiphoton ionization of the nascent neutral iodine products. The velocity distributions indicate that most of the available energy is partitioned into the internal energy of the CH2Br radical with only modest translational excitation imparted to the cofragments, which is consistent with a simple impulsive model. Furthermore, from extrapolation of the velocity distribution results, the first determination of the C-I bond dissociation energy of CH2BrI is presented in this work to be D0 = 16 790 ± 590 cm-1. The ion images appear anisotropic, indicative of a prompt dissociation, and the derived anisotropy parameters are consistently positive. Additionally, the angular distributions report on the electronic excited state dynamics, which validate recent works characterizing the electronic states responsible for the first absorption band of CH2BrI. In the current work, photolysis of CH2BrI on the red edge of the absorption spectrum reveals an additional channel producing I (2 P3/2) fragments.

Velocity map imaging of OH radical products from IR activated (CH3)2COO Criegee intermediates.

The unimolecular dissociation dynamics of the dimethyl-substituted Criegee intermediate (CH3)2COO is examined experimentally using velocity map imaging to ascertain the translational and internal energy distributions of the OH and H2CC(CH3)O radical products. The energy profile of key features along the reaction coordinate is also evaluated theoretically. Unimolecular decay of (CH3)2COO is initiated by vibrational activation in the CH stretch overtone region and the resultant OH X(2)Π3/2 (v = 0) products are state-selectively ionized and imaged. Analysis reveals an isotropic spatial distribution, indicative of a 3 ps lower limit for the timescale of dissociation, and a broad and unstructured total kinetic energy release distribution. The energy released to products is partitioned principally as internal excitation of the H2CC(CH3)O fragments with modest translational excitation of the fragments and a small degree of OH rotational excitation. The total kinetic energy release distribution observed for (CH3)2COO is compared with that predicted for statistical partitioning over product quantum states, and contrasted with recent experimental and quasi-classical trajectory results for syn-CH3CHOO [N. M. Kidwell et al., Nat. Chem. 8, 509 (2016)].

Infrared and Electronic Spectroscopy of the Jet-Cooled 5-Methyl-2-furanylmethyl Radical Derived from the Biofuel 2,5-Dimethylfuran.

The electronic and infrared spectra of the 5-methyl-2-furanylmethyl (MFM) radical have been characterized under jet-cooled conditions in the gas phase. This resonance-stabilized radical is formed by H atom loss from one of the methyl groups of 2,5-dimethylfuran (DMF), a promising second-generation biofuel. As a resonance-stabilized radical, it plays an important role in the flame chemistry of DMF. The D0-D1 transition was studied using two-color resonant two-photon ionization (2C-R2PI) spectroscopy. The electronic origin is in the middle of the visible spectrum (21934 cm(-1) = 455.9 nm) and is accompanied by Franck-Condon activity involving the hindered methyl rotor. The frequencies and intensities are fit to a one-dimensional methyl rotor potential, using the calculated form of the ground state potential. The methyl rotor reports sensitively on the local electronic environment and how it changes with electronic excitation, shifting from a preferred ground state orientation with one CH in-plane and anti to the furan oxygen, to an orientation in the excited state in which one CH group is axial to the plane of the furan ring. Ground and excited state alkyl CH stretch infrared spectra are recorded using resonant ion-dip infrared (RIDIR) spectroscopy, offering a complementary view of the methyl group and its response to electronic excitation. Dramatic changes in the CH stretch transitions with electronic state reflect the changing preference for the methyl group orientation.

Unimolecular dissociation dynamics of vibrationally activated CH3CHOO Criegee intermediates to OH radical products.

The hydroxyl radical is an important atmospheric oxidant, and a significant source of its production occurs through alkene ozonolysis. This takes place via a cycloaddition reaction and subsequent fragmentation to form an energized carbonyl oxide (for example, CH3CHOO), known as a Criegee intermediate, which can then either react with another atmospheric species or decay and, in doing so, produce the hydroxyl radical. Here, we examine the dissociation dynamics of a prototypical Criegee intermediate by characterizing the translational and internal energy distributions of the OH radical products, which reflect critical configurations along the reaction pathway. Experimentally, the kinetic energy release to OH products is ascertained through velocity map imaging. Theoretically, quasi-classical trajectories are performed on a new full-dimensional, ab initio potential energy surface. Both experiment and theory show that most of the available energy flows into internal excitation of the vinoxy products. The isotropic angular distribution of OH fragments indicates that dissociation occurs in ≥2 ps, in agreement with theory.

UV Photodissociation Dynamics of the CH3CHOO Criegee Intermediate: Action Spectroscopy and Velocity Map Imaging of O-Atom Products.

UV excitation of jet-cooled CH3CHOO on the B(1)A'-X(1)A' transition results in dissociation to two spin-allowed product channels: CH3CHO X(1)A' + O (1)D and CH3CHO a(3)A″ + O (3)P. The O (1)D and O (3)P products are detected using 2 + 1 REMPI at 205 and 226 nm, respectively, for action spectroscopy and velocity map imaging studies. The O (1)D action spectrum closely follows the previously reported UV absorption spectrum for jet-cooled CH3CHOO [Beames et al. J. Chem. Phys. 2013 , 138 , 244307]. Velocity map images of the O (1)D products following excitation of CH3CHOO at 305, 320, and 350 nm exhibit anisotropic angular distributions indicative of rapid (ps) dissociation, along with broad and unstructured total kinetic energy (TKER) distributions that reflect the internal energy distribution of the CH3CHO X(1)A' coproducts. The O (3)P action spectrum turns on near the peak of the UV absorption spectrum (ca. 324 nm) and extends to higher energy with steadily increasing O (3)P yield. Excitation of CH3CHOO at 305 nm, attributed to absorption of the more stable syn-conformer, also results in an anisotropic angular distribution of O (3)P products arising from rapid (ps) dissociation, but a narrower TKER distribution since less energy is available to the CH3CHO a(3)A″ + O (3)P products. The threshold for the higher energy CH3CHO a(3)A″ + O (3)P product channel is determined to be ca. 88.4 kcal mol(-1) from the termination of the TKER distribution and the onset of the O (3)P action spectrum. This threshold is combined with the singlet-triplet spacings of O atoms and acetaldehyde to establish the dissociation energy for syn-CH3HOO X(1)A' to the lowest spin-allowed product channel, CH3CHO X(1)A' + O (1)D, of ≤55.9 ± 0.4 kcal mol(-1). A harmonic normal-mode analysis is utilized to identify the vibrational modes of CH3CHO likely to be excited upon dissociation into the two product channels.

Fermi resonance effects in the vibrational spectroscopy of methyl and methoxy groups.

A theoretical model Hamiltonian [J. Chem. Phys. 2013, 138, 064308] for describing vibrational spectra associated with the CH stretch of CH2 groups is extended to molecules containing methyl and methoxy groups. Results are compared to the infrared (IR) spectroscopy of four molecules studied under supersonic expansion cooling in gas phase conditions. The molecules include 1,1-diphenylethane (DPE), 1,1-diphenylpropane (DPP), 2-methoxyphenol (guaiacol), and 1,3-dimethoxy-2-hydroxybenzene (syringol). Transforming the bending normal mode vibrations of CH3 groups to local scissor vibrations leads to model Hamiltonians which share many features present in our model Hamiltonian for the stretching vibrations of CH2 Fermi coupled to scissor modes. The central difference arises from the greater scissor-scissor coupling present in the CH3 case. Comparing anharmonic couplings between these modes and the stretch-bend Fermi coupling for a variety of systems, it is observed that the anharmonic couplings are robust; their values are similar for the four molecules studied as well as for ethane and methanol. Similar results are obtained with both density functional theory and coupled-cluster calculations. This robustness suggests a new parametrization of the model Hamiltonian that reduces the number of fitting parameters. In contrast, the harmonic contributions to the Hamiltonian vary substantially between the molecules leading to important changes in the spectra. The resulting Hamiltonian predicts most of the major spectral features considered in this study and provides insights into mode mixing and the consequences of the mixing on dynamical processes that follow ultrafast CH stretch excitation.

Vibronic coupling in asymmetric bichromophores: experimental investigation of diphenylmethane-d5.

Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d5 (DPM-d5) are reported in the isolated-molecule environment of a supersonic expansion. While small, the asymmetry induced by deuteration of one of the aromatic rings is sufficient to cause several important effects that change the principle mechanism of vibronic coupling between the close-lying S1 and S2 states, and spectroscopic signatures such coupling produces. The splitting between S1 and S2 origins is 186 cm(-1), about 50% greater than its value in DPM-d0 (123 cm(-1)), and an amount sufficient to bring the S2 zero-point level into near-resonance with the v = 1 level in the S1 state of a low-frequency phenyl flapping mode, ν(R) = 191 cm(-1). Dispersed fluorescence spectra bear clear evidence that Δv(R) = 1 Herzberg-Teller coupling dominates the near-resonant internal mixing between the S1 and S2 manifolds. The fluorescence into each pair of Franck-Condon active ring modes shows an asymmetry that suggests incorrectly that the S1 and S2 states may be electronically localized. From rotationally resolved studies, the S0 and S1 states have been well-fit to asymmetric rotor Hamiltonians while the S2 state is perturbed and not fit. The transition dipole moment (TDM) orientation of the S1 state is nearly perpendicular to the C2 symmetry axes with 66(2)%:3(1)%:34(2)% a:b:c hybrid-type character while that of the S2 origin contains 50(10)% a:c-type (S1) and 50(10)% b-type (S2) character. A model is put forward that explains qualitatively the TDM compositions and dispersed emission patterns without the need to invoke electronic localization. The experimental data discussed here serve as a foundation for a multi-mode vibronic coupling model capable of being applied to asymmetric bichromophores, as presented in the work of B. Nebgen and L. V. Slipchenko ["Vibronic coupling in asymmetric bichromophores: Theory and application to diphenylmethane-d5," J. Chem. Phys. (submitted)].