Hybrid Quantum/Classical simulations of the vibrational relaxation of the Amide I mode of N-methylacetamide in D2O solution

Hybrid quantum/classical molecular dynamics (MD) is applied to simulate the vibrational relaxation (VR) of the amide I mode of deuterated N-methylacetamide (NMAD) in aqueous (D2O) solution. A novel version of the vibrational molecular dynamics with quantum transitions (MDQT) treatment is developed in which the amide I mode is treated quantum mechanically while the remaining degrees of freedom are treated classically. The instantaneous normal modes of the initially excited NMAD molecule (INM0) are used as internal coordinates since they provide a proper initial partition of the system in quantum and classical subsystems. The evolution in time of the energy stored in each individual normal mode is subsequently quantified using the hybrid quantum-classical instantaneous normal modes (INMt). The identities of both the INM0s and the INMts are tracked using the equilibrium normal modes (ENMs) as templates. The results extracted from the hybrid MDQT simulations show that the quantum treatment of the amide I mode accelerates the whole VR process versus pure classical simulations and gives better agreement with experiments. The relaxation of the amide I mode is found to be essentially an intramolecular vibrational redistribution (IVR) process with little contribution from the solvent, in agreement with previous theoretical and experimental studies. Two well-defined relaxation mechanisms are identified. The faster one accounts for ≈40% of the total vibrational energy that flows through the NMAD molecule and involves the participation of the lowest frequency vibrations as short-life intermediate modes. The second and slower mechanism accounts for the remaining ≈60% of the energy released and is associated to the energy flow through specific mid-range and high-frequency modes.Fil: Bastida, Adolfo. Universidad de Murcia; EspañaFil: Soler, Miguel A.. Universidad de Murcia; EspañaFil: Zúñiga, José. Universidad de Murcia; EspañaFil: Requena, Alberto. Universidad de Murcia; EspañaFil: Kalstein, Adrian. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; ArgentinaFil: Fernández Alberti, Sebastián. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Vibrational energy redistribution during donor-acceptor electronic energy transfer: criteria to identify subsets of active normal modes

Photoinduced electronic energy transfer in conjugated donor-acceptor systems is naturally accompanied by intramolecular vibrational energy redistributions accepting an excess of electronic energy. Herein, we simulate these processes in a covalently linked donor-acceptor molecular dyad system by using nonadiabatic excited state molecular dynamics simulations. We analyze different complementary criteria to systematically identify the subset of vibrational normal modes that actively participate on the donoracceptor (S2S1) electronic relaxation. We analyze energy transfer coordinates in terms ofstate-specific normal modes defined according to the different potential energy surfaces (PESs) involved. On one hand, we identify those vibrations that contribute the most to the direction of the main driving force on the nuclei during electronic transitions, represented by the non-adiabatic derivative coupling vector between donor and acceptor electronic states. On the other hand, we monitor normal mode transient accumulations of excess energy and their intramolecular energy redistribution fluxes. We observe that the subset of active modes varies according to the PES on which they belong and these modes experience the most significant rearrangements and mixing. Whereas the nuclear motions that promote donoracceptor energy funneling can be localized mainly on one or two normal modes of the S2 state, they become spread out across multiple normal modes of the S1 state following the energy transfer eventThis work was partially supported by CONICET, UNQ, ANPCyT (PICT-2018-2360), the Universidad Carlos III de Madrid, the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement No. 600371, el Ministerio de Economía, Industria y Competitividad (COFUND2014-51509), el Ministerio de Educación, cultura y Deporte (CEI-15-17), Banco Santander and el Ministerio de Ciencia, Innovación y Universidades (RTI2018-101020-B-I00). We also acknowledge support from the Bavarian University Centre for Latin America (BAYLAT). The work at Los Alamos National Laboratory (LANL) was supported by the Laboratory Directed Research and Development Funds (LDRD) program. This work was done in part at the Center for Nonlinear Studies (CNLS) and the Center for Integrated Nanotechnologies (CINT), a U.S. Department of Energy and Office of Basic Energy Sciences user facility, at LANL. This research used resources provided by the LANL Institutional Computing Program. Los Alamos National Laboratory is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy. This work has received finantial support provided by the Spanish Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER, UE) under Project CTQ2016-79345-P and by the Funda-ción Séneca under Project 20789/PI/18

Molecular dynamics simulations and instantaneous normal-mode analysis of the vibrational relaxation of the C-H stretching modes of N-methylacetamide-d in liquid deuterated water

Nonequilibrium molecular dynamics (MD) simulations and instantaneous normal mode (INMs) analyses are used to study the vibrational relaxation of the C-H stretching modes (s(CH3)) of deuterated N-methylacetamide (NMAD) in aqueous (D2O) solution. The INMs are identified unequivocally in terms of the equilibrium normal modes (ENMs), or groups of them, using a restricted version of the recently proposed Min-Cost assignment method. After excitation of the parent s(CH3) modes with one vibrational quantum, the vibrational energy is shown to dissipate through both intramolecular vibrational redistribution (IVR) and intermolecular vibrational energy transfer (VET). The decay of the vibrational energy of the s(CH3) modes is well fitted to a triple exponential function, with each characterizing a well-defined stage of the entire relaxation process. The first, and major, relaxation stage corresponds to a coherent ultrashort (τrel = 0.07 ps) energy transfer from the parent s(CH3) modes to the methyl bending modes δ(CH 3), so that the initially excited state rapidly evolves into a mixed stretch-bend state. In the second stage, characterized by a time of 0.92 ps, the vibrational energy flows through IVR to a number of mid-range-energy vibrations of the solute. In the third stage, the vibrational energy accumulated in the excited modes dissipates into the bath through an indirect VET process mediated by lower-energy modes, on a time scale of 10.6 ps. All the specific relaxation channels participating in the whole relaxation process are properly identified. The results from the simulations are finally compared with the recent experimental measurements of the s(CH3) vibrational energy relaxation in NMAD/D2O(l) reported by Dlott et al. (J. Phys. Chem. A 2009, 113, 75.) using ultrafast infrared-Raman spectroscopy. © 2010 American Chemical Society

Instantaneous normal modes, resonances, and decay channels in the vibrational relaxation of the amide i mode of N -methylacetamide-D in liquid deuterated water

A nonequilibrium molecular dynamics (MD) study of the vibrational relaxation of the amide I mode of deuterated N -methylacetamide (NMAD) in aqueous (D2 O) solution is carried out using instantaneous normal modes (INMs). The identification of the INMs as they evolve over time, which is necessary to analyze the energy fluxes, is made by using a novel algorithm which allows us to assign unequivocally each INM to an individual equilibrium normal mode (ENM) or to a group of ENMs during the MD simulations. The time evolution of the energy stored in each INM is monitored and the occurrence of resonances during the relaxation process is then investigated. The decay of the amide I mode, initially excited with one vibrational quantum, is confirmed to fit well to a biexponential function, implying that the relaxation process involves at least two mechanisms with different rate constants. By freezing the internal motions of the solvent, it is shown that the intermolecular vibration-vibration channel to the bending modes of the solvent is closed. The INM analysis reveals then the existence of a major and faster decay channel, which corresponds to an intramolecular vibrational redistribution process and a minor, and slower, decay channel which involves the participation of the librational motions of the solvent. The faster relaxation pathway can be rationalized in turn using a sequential kinetic mechanism of the type P→M+L→L, where P (parent) is the initially excited amide I mode, and M (medium) and L (low) are specific midrange and lower-frequency NMAD vibrational modes, respectively. © 2010 American Institute of Physics

Hybrid quantum/classical simulations of the vibrational relaxation of the amide i mode of N -methylacetamide in D2O solution

Hybrid quantum/classical molecular dynamics (MD) is applied to simulate the vibrational relaxation (VR) of the amide I mode of deuterated N-methylacetamide (NMAD) in aqueous (D2O) solution. A novel version of the vibrational molecular dynamics with quantum transitions (MDQT) treatment is developed in which the amide I mode is treated quantum mechanically while the remaining degrees of freedom are treated classically. The instantaneous normal modes of the initially excited NMAD molecule (INM0) are used as internal coordinates since they provide a proper initial partition of the system in quantum and classical subsystems. The evolution in time of the energy stored in each individual normal mode is subsequently quantified using the hybrid quantum-classical instantaneous normal modes (INMt). The identities of both the INM0s and the INMts are tracked using the equilibrium normal modes (ENMs) as templates. The results extracted from the hybrid MDQT simulations show that the quantum treatment of the amide I mode accelerates the whole VR process versus pure classical simulations and gives better agreement with experiments. The relaxation of the amide I mode is found to be essentially an intramolecular vibrational redistribution (IVR) process with little contribution from the solvent, in agreement with previous theoretical and experimental studies. Two well-defined relaxation mechanisms are identified. The faster one accounts for ≈40% of the total vibrational energy that flows through the NMAD molecule and involves the participation of the lowest frequency vibrations as short-life intermediate modes. The second and slower mechanism accounts for the remaining ≈60% of the energy released and is associated to the energy flow through specific mid-range and high-frequency modes. © 2012 American Chemical Society

Vibrational dynamics of polyatomic molecules in solution: Assignment, time evolution and mixing of instantaneous normal modes

Intramolecular vibrational dynamics of polyatomic molecules in solution can be addressed through normal mode analysis based on either equilibrium normal modes (ENMs) or instantaneous normal modes (INMs). While the former offers a straightforward way of examining experimental spectra, the latter provides a decoupled short-time description of the vibrational motions of the molecule. In order to reconcile both representations, a realistic assignment of the INMs in terms of the ENMs is needed. In this paper, we describe a novel method to assign the INMs using the ENMs as templates, which provides a unique relationship between the two sets of normal modes. The method is based specifically on the use of the so-called Min-Cost or Min-Sum algorithm, duly adapted to our problem, to maximize the overlaps between the two sets of modes. The identification of the INMs as the system evolves with time then allows us to quantify the vibrational energy stored in each INM and so monitor the flows of intramolecular vibrational energy within the solute molecule. We also discuss the degree of mixing of the INMs and characterize the way they change with time by means of the corresponding autocorrelation functions. The usefulness of the method is illustrated by carrying out equilibrium molecular dynamics (MD) simulations of the deuterated N-methylacetamide (NMAD) molecule in D2O solution. © 2010 Springer-Verlag