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1. Vibronic spectrum of pyrazine: New insights from multi-state-multi-mode simulations parameterized with equation-of-motion coupled-cluster methods

   https://doi.org/10.1063/5.0280659  

   Wójcik, Paweł; Reisler, Hanna; Stanton, John F; Krylov, Anna I (August 2025, The Journal of Chemical Physics)

   This study reports simulations of the lowest band in the electronic absorption spectrum of pyrazine carried out using a multi-state-multimode vibronic Hamiltonian parameterized using equation-of-motion coupled-cluster methods. The simulations explain the main spectral features and show how peaks of vibronic nature appear. The most complete vibronic model includes four electronic states and six vibrational modes. The simulations reveal that non-adiabatic coupling with bright states located as high as 3 eV above the studied state can lead to discernible features in the absorption spectrum. This study demonstrates the power of fully ab initio treatments of electronic and vibrational structure and their utility in understanding the mechanisms leading to complex molecular spectra. 

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2. A Theoretical Study of Polarization Selective Two-Dimensional Vibronic Spectroscopies of Multimode Systems

   https://doi.org/10.1364/UP.2020.M4B.20  

   Weakly, Robert B.; Gaynor, James D.; Khalil, Munira (January 2020, Optica Publishing Group)

   A model vibronic Hamiltonian composed of coupled anharmonic vibrational modes and two electronic states provides a quantitative framework for understanding how vibronic couplings and dipole orientations are encoded in two-dimensional vibronic spectroscopy. 

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3. Two-dimensional electronic–vibrational sum frequency spectroscopy for interactions of electronic and nuclear motions at interfaces

   https://doi.org/10.1073/pnas.2100608118  

   Deng, Gang-Hua; Qian, Yuqin; Zhang, Tong; Han, Jian; Chen, Hanning; Rao, Yi (August 2021, Proceedings of the National Academy of Sciences)

   Interactions of electronic and vibrational degrees of freedom are essential for understanding excited-states relaxation pathways of molecular systems at interfaces and surfaces. Here, we present the development of interface-specific two-dimensional electronic–vibrational sum frequency generation (2D-EVSFG) spectroscopy for electronic–vibrational couplings for excited states at interfaces and surfaces. We demonstrate this 2D-EVSFG technique by investigating photoexcited interface-active ( E )-4-((4-(dihexylamino) phenyl)diazinyl)-1-methylpyridin-1- lum (AP3) molecules at the air–water interface as an example. Our 2D-EVSFG experiments show strong vibronic couplings of interfacial AP3 molecules upon photoexcitation and subsequent relaxation of a locally excited (LE) state. Time-dependent 2D-EVSFG experiments indicate that the relaxation of the LE state, S 2 , is strongly coupled with two high-frequency modes of 1,529.1 and 1,568.1 cm −1 . Quantum chemistry calculations further verify that the strong vibronic couplings of the two vibrations promote the transition from the S 2 state to the lower excited state S 1 . We believe that this development of 2D-EVSFG opens up an avenue of understanding excited-state dynamics related to interfaces and surfaces. 

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4. Communication: Electronic transition of the l–C ₆ ⁺ cation at 417 nm

   https://doi.org/10.1063/5.0106183  

   Colley, Jason E.; Orr, Dylan S.; Duncan, Michael A. (September 2022, The Journal of Chemical Physics)

   A new electronic transition is reported for the linear C 6 + cation with an origin at 416.8 nm. This spectrum can be compared to the matrix isolation spectra at lower energies reported previously by Fulara et al. [J. Chem. Phys. 123, 044305 (2005)], which assigned linear and cyclic isomers, and to the gas phase spectrum reported previously by Campbell and Dunk [Rev. Sci. Instrum. 90, 103101 (2019)], which detected the same cyclic-isomer spectrum reported by Fulara. Comparisons to electronically excited states and vibrations predicted by various forms of theory allow assignment of the spectrum to a new electronic state of linear C 6 + . The spectrum consists of a strong origin band, two vibronic progression members at higher energy and four hot bands at lower energies. The hot bands provide the first gas phase information on ground state vibrational frequencies. The vibrational and electronic structure of C 6 + provide a severe challenge to computational chemistry. 

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5. Theory for proton-coupled energy transfer

   https://doi.org/10.1063/5.0217546  

   Cui, Kai; Hammes-Schiffer, Sharon (July 2024, The Journal of Chemical Physics)

   In the recently discovered proton-coupled energy transfer (PCEnT) mechanism, the transfer of electronic excitation energy between donor and acceptor chromophores is coupled to a proton transfer reaction. Herein, we develop a general theory for PCEnT and derive an analytical expression for the nonadiabatic PCEnT rate constant. This theory treats the transferring hydrogen nucleus quantum mechanically and describes the PCEnT process in terms of nonadiabatic transitions between reactant and product electron–proton vibronic states. The rate constant is expressed as a summation over these vibronic states, and the contribution of each pair of vibronic states depends on the square of the vibronic coupling as well as the spectral convolution integral, which can be viewed as a generalization of the Förster-type spectral overlap integral for vibronic rather than electronic states. The convolution integral also accounts for the common vibrational modes shared by the donor and acceptor chromophores for intramolecular PCEnT. We apply this theory to model systems to investigate the key features of PCEnT processes. The excited vibronic states can contribute significantly to the total PCEnT rate constant, and the common modes can either slow down or speed up the process. Because the pairs of vibronic states that contribute the most to the PCEnT rate constant may correspond to spectroscopically dark states, PCEnT could occur even when there is no apparent overlap between the donor emission and acceptor absorption spectra. This theory will assist in the interpretation of experimental data and will guide the design of additional PCEnT systems. 

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