More Like this

1. Sub-Doppler infrared spectroscopy of resonance-stabilized hydrocarbon intermediates: ν ₃ / ν ₄ CH stretch modes and CH ₂ internal rotor dynamics of benzyl radical

   https://doi.org/10.1039/c7cp05776h  

   Kortyna, A. ; Samin, A. J. ; Miller, T. A. ; Nesbitt, D. J. ( January 2017 , Physical Chemistry Chemical Physics)

   Highly reactive benzyl radicals are generated by electron dissociative attachment to benzyl chloride doped into a neon–hydrogen–helium discharge and immediately cooled to T rot = 15 K in a high density, supersonic slit expansion environment. The sub-Doppler spectra are fit to an asymmetric-top rotational Hamiltonian, thereby yielding spectroscopic constants for the ground ( v = 0) and first excited ( v = 1, ν 3 , ν 4 ) vibrational levels of the ground electronic state. The rotational constants obtained for the ground state are in good agreement with previous laser induced fluorescence measurements (LIF), with vibrational band origins ( ν 3 = 3073.2350 ± 0.0006 cm −1 , ν 4 = 3067.0576 ± 0.0006 cm −1 ) in agreement with anharmonically corrected density functional theory calculations. To assist in detection of benzyl radical in the interstellar medium, we have also significantly improved the precision of the ground state rotational constants through combined analysis of the ground state IR and LIF combination differences. Of dynamical interest, there is no evidence in the sub-Doppler spectra for tunneling splittings due to internal rotation of the CH 2 methylene subunit, which implies a significant rotational barrier consistent with partial double bond character inmore »the CC bond. This is further confirmed with high level ab initio calculations at the CCSD(T)-f12b/ccpVdZ-f12 level, which predict a zero-point energy corrected barrier to internal rotation of Δ E tun ≈ 11.45 kcal mol −1 or 4005 cm −1 . In summary, the high-resolution infrared spectra are in excellent agreement with simple physical organic chemistry pictures of a strongly resonance-stabilized benzyl radical with a nearly rigid planar structure due to electron delocalization around the aromatic ring.« less
2. Hidden complexities in the reaction of H ₂ O ₂ and HNO revealed by ab initio quantum chemical investigations

   https://doi.org/10.1039/c7cp05883g  

   Beckett, Daniel ; Edelmann, Marc ; Raff, Jonathan D. ; Raghavachari, Krishnan ( January 2017 , Physical Chemistry Chemical Physics)

   Nitroxyl (HNO) and hydrogen peroxide have both been implicated in a variety of reactions relevant to environmental and physiological processes and may contribute to a unique, unexplored, pathway for the production of nitrous acid (HONO) in soil. To investigate the potential for this reaction, we report an in-depth investigation of the reaction pathway of H 2 O 2 and HNO forming HONO and water. We find the breaking of the peroxide bond and a coupled proton transfer in the first step leads to hydrogen nitryl (HNO 2 ) and an endogenous water, with an extrapolated NEVPT2 (multireference perturbation theory) barrier of 29.3 kcal mol −1 . The first transition state is shown to possess diradical character linking the far peroxide oxygen to the bridging, reacting, peroxide oxygen. The energy of this first step, when calculated using hybrid density functional theory, is shown to depend heavily on the amount of Hartree–Fock exchange in the functional, with higher amounts leading to a higher barrier and more diradical character. Additionally, high amounts of spin contamination cause CCSD(T) to significantly overestimate the TS1 barrier with a value of 36.2 kcal mol −1 when using the stable UHF wavefunction as the reference wavefunction. However, whenmore »using the restricted Hartree–Fock reference wavefunction, the TS1 CCSD(T) energy is lowered to yield a barrier of 31.2 kcal mol −1 , in much better agreement with the NEVPT2 result. The second step in the reaction is the isomerization of HNO 2 to trans -HONO through a Grotthuss-like mechanism accepting a proton from and donating a proton to the endogenous water. This new mechanism for the isomerization of HNO 2 is shown to have an NEVPT2 barrier of 23.3 kcal mol −1 , much lower than previous unimolecular estimates not including an explicit water. Finally, inclusion of an additional explicit water is shown to lower the HNO 2 isomerization barrier even further.« less
3. High-resolution CH stretch spectroscopy of jet-cooled cyclopentyl radical: First insights into equilibrium structure, out-of-plane puckering, and IVR dynamics

   https://doi.org/10.1063/5.0096946  

   Kortyna, Andrew ; Reber, Melanie A. ; Nesbitt, David J. ( July 2022 , The Journal of Chemical Physics)

   First, high-resolution sub-Doppler infrared spectroscopic results for cyclopentyl radical (C 5 H 9 ) are reported on the α-CH stretch fundamental with suppression of spectral congestion achieved by adiabatic cooling to T rot ≈ 19(4) K in a slit jet expansion. Surprisingly, cyclopentyl radical exhibits a rotationally assignable infrared spectrum, despite 3N − 6 = 36 vibrational modes and an upper vibrational state density (ρ ≈ 40–90 #/cm −1 ) in the critical regime (ρ ≈ 100 #/cm −1 ) necessary for onset of intramolecular vibrational relaxation (IVR) dynamics. Such high-resolution data for cyclopentyl radical permit detailed fits to a rigid-rotor asymmetric top Hamiltonian, initial structural information for ground and vibrationally excited states, and opportunities for detailed comparison with theoretical predictions. Specifically, high level ab initio calculations at the coupled-cluster singles, doubles, and perturbative triples (CCSD(T))/ANO0, 1 level are used to calculate an out-of-plane bending potential, which reveals a C 2 symmetry double minimum 1D energy surface over a C 2v transition state. The inversion barrier [V barrier ≈ 3.7(1) kcal/mol] is much larger than the effective moment of inertia for out-of-plane bending, resulting in localization of the cyclopentyl wavefunction near its C 2 symmetry equilibrium geometry and tunneling splittingsmore »for the ground state too small (<1 MHz) to be resolved under sub-Doppler slit jet conditions. The persistence of fully resolved high-resolution infrared spectroscopy for such large cyclic polyatomic radicals at high vibrational state densities suggests a “deceleration” of IVR for a cycloalkane ring topology, much as low frequency torsion/methyl rotation degrees of freedom have demonstrated a corresponding “acceleration” of IVR processes in linear hydrocarbons.« less
4. Energy- and conformer-dependent excited-state relaxation of an E / Z photoswitchable thienyl-ethene

   https://doi.org/10.1039/c9cp01226e  

   Young, Jamie D. ; Honick, Chana R. ; Zhou, Jiawang ; Pitts, Cody R. ; Ghorbani, Fereshte ; Peters, Garvin M. ; Lectka, Thomas ; Tovar, John D. ; Bragg, Arthur E. ( July 2019 , Physical Chemistry Chemical Physics)

   Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E / Z photoisomerization following absorption of visible light. cis -to- trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565–420 nm) transient absorption (TA) studies to probe the photophysics of cis -to- trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (P A and P B ) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the P B conformer is found to contribute to triplet population and the switching of cis -4TCE: high-energy excitation (<470 nm) of P B involves direct excitation to S 2 , relaxation from which prepares an ISC-active S 1 geometry (ISC QY 0.4–0.67, k ISC ∼ 1.6–2.6 × 10 −9 s −1 ) thatmore »is the gateway to triplet population and isomerization. We ascribe low cis -to- trans isomerization yield to excitation of the nonreactive P A conformer (75–85% loss) as well as loses along the P B S 2 → S 1 → T 1 cascade (10–20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive “antiparallel” ring conformation.« less
5. Automatic discovery of photoisomerization mechanisms with nanosecond machine learning photodynamics simulations

   https://doi.org/10.1039/D0SC05610C  

   Li, Jingbai ; Reiser, Patrick ; Boswell, Benjamin R. ; Eberhard, André ; Burns, Noah Z. ; Friederich, Pascal ; Lopez, Steven A. ( April 2021 , Chemical Science)

   Photochemical reactions are widely used by academic and industrial researchers to construct complex molecular architectures via mechanisms that often require harsh reaction conditions. Photodynamics simulations provide time-resolved snapshots of molecular excited-state structures required to understand and predict reactivities and chemoselectivities. Molecular excited-states are often nearly degenerate and require computationally intensive multiconfigurational quantum mechanical methods, especially at conical intersections. Non-adiabatic molecular dynamics require thousands of these computations per trajectory, which limits simulations to ∼1 picosecond for most organic photochemical reactions. Westermayr et al. recently introduced a neural-network-based method to accelerate the predictions of electronic properties and pushed the simulation limit to 1 ns for the model system, methylenimmonium cation (CH 2 NH 2 + ). We have adapted this methodology to develop the Python-based, Python Rapid Artificial Intelligence Ab Initio Molecular Dynamics (PyRAI 2 MD) software for the cis – trans isomerization of trans -hexafluoro-2-butene and the 4π-electrocyclic ring-closing of a norbornyl hexacyclodiene. We performed a 10 ns simulation for trans -hexafluoro-2-butene in just 2 days. The same simulation would take approximately 58 years with traditional multiconfigurational photodynamics simulations. We generated training data by combining Wigner sampling, geometrical interpolations, and short-time quantum chemical trajectories to adaptively sample sparse data regions alongmore »reaction coordinates. The final data set of the cis – trans isomerization and the 4π-electrocyclic ring-closing model has 6207 and 6267 data points, respectively. The training errors in energy using feedforward neural networks achieved chemical accuracy (0.023–0.032 eV). The neural network photodynamics simulations of trans -hexafluoro-2-butene agree with the quantum chemical calculations showing the formation of the cis -product and reactive carbene intermediate. The neural network trajectories of the norbornyl cyclohexadiene corroborate the low-yielding syn -product, which was absent in the quantum chemical trajectories, and revealed subsequent thermal reactions in 1 ns.« less