Nonadiabatic Vibrational Resonance Raman Spectra from Quantum Dynamics Propagations with LVC Models. Application to Thymine

Abstract

We present a viable protocol to compute vibrational resonance Raman (vRR) spectra for systems with several closelying and potentially coupled electronic states. It is based on the parametrization of linear vibronic coupling (LVC) models from time-dependent density functional theory (TD-DFT) calculations and quantum dynamics propagations of vibronic wavepackets with the multilayer version of the multiconfiguration time-dependent Hartree (ML-MCTDH) method. Our approach is applied to thymine considering seven coupled electronic states, comprising the three lowest bright states, and all vibrational coordinates. Computed vRR at different excitation wavelengths are in good agreement with the available experimental data. Up to 250 nm the signal is dominated by the lowest HOMO → LUMO transition, whereas at 233 nm, in the valley between the two lowest energy absorption bands, the contributions of all the three bright states, and their interferences and couplings, are important. Inclusion of solvent (water) effects improves the agreement with experiment, reproducing the coalescence of vibrational bands due to CC and C�O stretchings. With our approach we disentangle and assess the effect of interferences between the contribution of different quasi-resonant states to the transition polarizability and the effect of interstate couplings. Our findings strongly suggest that in cases of close-lying and potentially coupled states a simple inclusion of interference effects is not sufficient, and a fully nonadiabatic computation should instead be performed. We also document that for systems with strong couplings and quasi-degenerate states, the use of HT perturbative approach, not designed for these cases, may lead to large artifacts.

Computational Model for Electrochemical Surface-Enhanced Raman Scattering: Key Role of the Surface Charges and Synergy between Electromagnetic and Charge-Transfer Enhancement Mechanisms

“…70 Progress in this direction are in development for RR of molecules in solution. 71 As an estimate, we used the perturbative Herzberg−Teller approach which suggested that further enhancement can be expected (but not limited) to a 1 modes of Py, in line with SERS selection rules 6 and previous studies. 72 For BP4VA, the EC-SERS spectra were dominated by the RR contribution due to a bright state of the molecule (RR mechanism), as already discussed in the literature.…”

“…In addition, to properly introduce the electronic and vibronic coupling in the theoretical model, a nonadiabatic approach would be necessary, but further methodological advances are required because they were only reported for small systems, or using low-dimensionality models . Progress in this direction are in development for RR of molecules in solution . As an estimate, we used the perturbative Herzberg–Teller approach which suggested that further enhancement can be expected (but not limited) to a 1 modes of Py, in line with SERS selection rules and previous studies …”

UV-Resonance Raman Spectra of Systems in Complex Environments: A Multiscale Modeling Applied to Doxorubicin Intercalated into DNA

“…RR offers a unique selectivity as well as a high sensitivity to experimentally detect even traces of compounds, and thus, it finds analytical applications in agriculture, life sciences, explosive detection, art, archeology, and forensics, with additional current research in carbon nanotubes . The key ingredient in the simulation of RR spectra of isolated systems is the transition polarizability tensor, − which can be obtained by exploiting a variety of approaches. − …”

“… 3 The key ingredient in the simulation of RR spectra of isolated systems is the transition polarizability tensor, 4 − 7 which can be obtained by exploiting a variety of approaches. 8 − 22 …”

Mind the GAP: quantifying the breakdown of the linear vibronic coupling Hamiltonian

“…Importantly, the simplicity of this approach makes it possible parameterise an effective Hamiltonian with very few quantum chemistry calculations 28 , providing a mechanism to achieve a detailed understanding of excited state dynamics through simplified models, which ultimately facilitates extraction of the critical parameters affecting function. These properties have recently been exploited to perform excited state dynamics using LVC models across a range of difference materials 1,2, [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] . While the advantage of the LVC is that it can be relatively straightfor-ward to obtain, the disadvantage is that it becomes inaccurate for larger displacements away from the starting geometry when anharmonic effects become more important.…”