The Resonance Raman Spectrum of Cytosine in Water: Analysis of the Effect of Specific Solute–Solvent Interactions and Non-Adiabatic Couplings

Abstract

In this contribution, we report a computational study of the vibrational Resonance Raman (vRR) spectra of cytosine in water, on the grounds of potential energy surfaces (PES) computed by time-dependent density functional theory (TD-DFT) and CAM-B3LYP and PBE0 functionals. Cytosine is interesting because it is characterized by several close-lying and coupled electronic states, challenging the approach commonly used to compute the vRR for systems where the excitation frequency is in quasi-resonance with a single state. We adopt two recently developed time-dependent approaches, based either on quantum dynamical numerical propagations of vibronic wavepackets on coupled PES or on analytical correlation functions for cases in which inter-state couplings were neglected. In this way, we compute the vRR spectra, considering the quasi-resonance with the eight lowest-energy excited states, disentangling the role of their inter-state couplings from the mere interference of their different contributions to the transition polarizability. We show that these effects are only moderate in the excitation energy range explored by experiments, where the spectral patterns can be rationalized from the simple analysis of displacements of the equilibrium positions along the different states. Conversely, at higher energies, interference and inter-state couplings play a major role, and the adoption of a fully non-adiabatic approach is strongly recommended. We also investigate the effect of specific solute–solvent interactions on the vRR spectra, by considering a cluster of cytosine, hydrogen-bonded by six water molecules, and embedded in a polarizable continuum. We show that their inclusion remarkably improves the agreement with the experiments, mainly altering the composition of the normal modes, in terms of internal valence coordinates. We also document cases, mostly for low-frequency modes, in which a cluster model is not sufficient, and more elaborate mixed quantum classical approaches, in explicit solvent models, need to be applied.

Linear Vibronic Coupling Approach for Surface-Enhanced Raman Scattering: Quantifying the Charge-Transfer Enhancement Mechanism

“…The intensity of a RR process from the ground vibrational state | g ; 0⟩ to the final vibrational state | g ; f ⟩ depends on the corresponding transition polarizability tensor components . Their sum-overstate expression can be transformed into a time-dependent framework (see the Supporting Information for full derivation and refs and for details) obtaining the following expression α ρ σ f 0 ( ω I ) = 1 ℏ ∑ k m μ ρ g k μ σ g m ∫ 0 ∞ normald t e − i t ( − E g 0 − ℏ ω I ) / normalℏ − γ t ⟨ d k ; v g f | normale − i H t / ℏ | d m ; v g 0 ⟩ where ρ and σ are Cartesian indices, ω I is the incident radiation frequency, k and m are indices for all the relevant excited electronic states (i.e., those approximately resonant with the excitation frequency and all those coupled to them), E g 0 is the energy of the initial state, and γ is the damping factor related to the lifetime of the electronic excited states, which in the following is considered identical for all states for simplicity. Notice that for the diabatic states of our basis set, ideally independent of the coordinates, we adopted the Condon approximation, which assumes that the Cartesian components of transition dipole moments are independent of the coordinates.…”

“…The effectiveness of this nonadiabatic approach in a simpler intramolecular context has been recently proved for the RR spectra of several molecules exhibiting contributions from nearby dark states. 39,40 On the other hand, being based on a variational approach, it can naturally describe both regimes of strong and weak couplings and also introduce an intensity-borrowing mechanism triggered by molecular vibrations. In this way, it is expected to solve possible issues encountered by adiabatic FC and HT vibronic approaches when applied to adiabatic states whose PL/CT mixing is variable and depends on the different tuning sensitivities of the respective energies caused by the applied bias.…”

“…The selective Raman enhancement of vibrational bands of the molecule related to metal-to-molecule CT resonant processes is explained by the very fast and extremely efficient population transfer from PL to CT states. The effectiveness of this nonadiabatic approach in a simpler intramolecular context has been recently proved for the RR spectra of several molecules exhibiting contributions from nearby dark states. , …”

The Role of Hydrogen Bonding in the Raman Spectral Signals of Caffeine in Aqueous Solution

“…Raman and in particular RR spectra [30][31][32][33][34][35]. Some modeling with isolated caffeine in the gas phase or describing the condensed phase using the Polarizable Continuum Model (PCM) [36] or a microsolvation approach has been used in several works to assist the peak assignments of Raman spectra [19,29,[37][38][39][40][41][42].…”

Hyper-Raman spectroscopy of biomolecules

“…While both IR and RS have advantages for specific applications, RS has become the dominant optical technique for probing chemical signatures in biological systems. The sensitivity of RS can be dramatically increased via techniques such as resonant Raman, 9,10 surface enhancement, 11 coherent anti-Stokes Raman scattering, 12–14 etc . These sensitivity enhancements have been found to even allow for the detection of individual molecules.…”