More Like this

1. Unraveling the Vibrational Spectral Signatures of a Dislocated H Atom in Model Proton-Coupled Electron Transfer Dyad Systems

   https://doi.org/10.1021/acs.jpca.3c00524  

   Chen, Liangyi; Sibert, Edwin L.; Fournier, Joseph A. (April 2023, The Journal of Physical Chemistry A)

   Phenol–benzimidazole and phenol–pyridine proton-coupled electron transfer (PCET) dyad systems are computationally investigated to resolve the origins of the asymmetrically broadened H-bonded OH stretch transitions that have been previously reported using cryogenic ion vibrational spectroscopy in the ground electronic state. Two-dimensional (2D) potentials describing the strongly shared H atom are predicted to be very shallow along the H atom transfer coordinate, enabling dislocation of the H atom between the donor and acceptor groups upon excitation of the OH vibrational modes. These soft H atom potentials result in strong coupling between the OH modes, which exhibit significant bend-stretch mixing, and a large number of normal mode coordinates. Vibrational spectra are calculated using a Hamiltonian that linearly and quadratically couples the H atom potentials to over two dozen of the most strongly coupled normal modes treated at the harmonic level. The calculated vibrational spectra qualitatively reproduce the asymmetric shape and breadth of the experimentally observed bands in the 2300–3000 cm–1 range. Interestingly, these transitions fall well above the predicted OH stretch fundamentals, which are computed to be surprisingly red-shifted (<2000 cm–1). Time-dependent calculations predict rapid (<100 fs) relaxation of the excited OH modes and instant response from the lower-frequency normal modes, corroborating the strong coupling predicted by the model Hamiltonian. The results highlight a unique broadening mechanism and complicated anharmonic effects present within these biologically relevant PCET model systems. 

   more » « less
2. Exploring the Spectral Signatures of Large Amplitude Motions in the CH Stretching Region of the Vibrational Spectrum of CH ₅ ⁺

   https://doi.org/10.1021/acs.jpca.6c00147  

   Moonkaen, Pattarapon; McCoy, Anne B (April 2026, The Journal of Physical Chemistry A)

   The spectra of CH+5 and its deuterated analogues are challenging to interpret due to the very flat potential surface and the highly delocalized ground state wave function. In this work, an approach is developed to model the spectra of CH+5 and CH4D+. In this approach, several thousand geometries are sampled from the ground-state probability density, which is evaluated using diffusion Monte Carlo. For each structure, the potential energy is minimized with respect to the five CH bond lengths, and the frequencies and intensities for the five 1–0 transitions involving the CH or CD stretching vibrations are then evaluated using a five-dimensional harmonically coupled anharmonic oscillator model and a sum of one-dimensional cuts through the dipole moment surface. This information is used to obtain the spectrum by two approaches. To model the lower-resolution laser-induced reaction spectrum, the calculated spectrum is obtained by convoluting the spectra obtained at all of the sampled structures. To model the helium droplet spectrum, in which the individual CH vibrational transitions are resolved, the spectrum is obtained by averaging the Hamiltonian matrices and transition moment vectors over the sampled structures. We find that these approaches reproduce the reported spectra. Analysis of the helium droplet spectrum leads us to conclude that, in this environment, the isomerization of CH+5 in its ground state is restricted compared to the gas phase. Overall, this approach provides a simple and physically transparent way to connect fluxional structures to CH-stretch spectral patterns, and offers frameworks for interpreting future spectroscopic studies. 

   more » « less
3. Diffusion Monte Carlo Study of the Structure and Spectroscopy of H ₃ O ^–

   https://doi.org/10.1021/acs.jpca.5c06590  

   Jacobson, Greta M; Webb, Andrew W; Cheng, Lixue; McCoy, Anne B (November 2025, The Journal of Physical Chemistry A)

   A potential energy surface for H3O– has been developed based on the NN+(MOB-ML) approach we developed for studies of complexes of OH– with two or three water molecules. Unlike those systems, H3O– has two low-energy isomers, H–·H2O and OH–·H2, which differ in energy by less than 2.5 kcal mol–1, and which are separated by a barrier of roughly 4.5 kcal mol–1. We find that by training the NN+(MOB-ML) model using structures based on diffusion Monte Carlo (DMC) simulations initiated in the two potential minima, we are able to obtain a potential surface that describes both isomers. Using these potentials, the structure and spectra of H3O– and its deuterated analogues are investigated using DMC. These calculations show that the ground state wave function for H3O– is mainly localized in the H–·H2O minimum in the potential, with a small amount of the probability amplitude (<5%) in the region of the OH–·H2 minimum. The delocalization of the wave function into the secondary minimum is lowered by deuteration of the water molecule, while replacing H– with D– increases the isomerization due to the shortening of the average distance between the hydride ion and the hydrogen atom in water to which it is bound. Introducing one quantum of excitation in the H–···H2O stretching vibration increases the amount of isomerization, while the isomerization decreases with additional excitation of this mode. Excitation of the free OH stretch in the water molecule also increases the amount of isomerization, while excitation of the out-of-plane bending vibration suppresses the isomerization. The effects of partial deuteration on the frequencies for these vibrations are also explored. 

   more » « less
4. Cross Sections and Rate Coefficients for Vibrational Excitation of H2O by Electron Impact

   https://doi.org/10.3390/atoms9030062  

   Ayouz, Mehdi; Faure, Alexandre; Tennyson, Jonathan; Tudorovskaya, Maria; Kokoouline, Viatcheslav (September 2021, Atoms)

   Cross-sections and thermally averaged rate coefficients for vibration (de-)excitation of a water molecule by electron impact are computed; one and two quanta excitations are considered for all three normal modes. The calculations use a theoretical approach that combines the normal mode approximation for vibrational states of water, a vibrational frame transformation employed to evaluate the scattering matrix for vibrational transitions and the UK molecular R-matrix code. The interval of applicability of the rate coefficients is from 10 to 10,000 K. A comprehensive set of calculations is performed to assess uncertainty of the obtained data. The results should help in modelling non-LTE spectra of water in various astrophysical environments. 

   more » « less
5. Nonstatistical Unimolecular Decay of the CH ₂ OO Criegee Intermediate in the Tunneling Regime

   https://doi.org/10.1021/acs.jpclett.4c01401  

   Qian, Yujie; Nguyen, Thanh Lam; Franke, Peter R; Stanton, John F; Lester, Marsha I (June 2024, The Journal of Physical Chemistry Letters)

   Unimolecular decay of the formaldehyde oxide (CH2OO) Criegee intermediate proceeds via a 1,3 ring-closure pathway to dioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected. Vibrational activation of jet-cooled CH2OO with two quanta of CH stretch (17-18 kcal mol-1) leads to unimolecular decay at an energy significantly below the transition state barrier of 19.46  0.25 kcal mol-1, refined utilizing a high-level electronic structure method HEAT-345(Q)Λ. The observed unimolecular decay rate of 1.6 +/- 0.4 x 106 s-1 is two orders of magnitude slower than that predicted by statistical unimolecular reaction theory using several different models for quantum mechanical tunneling. The nonstatistical behavior originates from excitation of a CH stretch vibration that is orthogonal to the heavy atom motions along the reaction coordinate and slow intramolecular vibrational energy redistribution due to the sparse density of states. 

   more » « less