Origin invariance in vibrational resonance Raman optical activity

Quantum chemistry study on the hyper-Rayleigh scattering optical activity of R-carvone and (1R,5R)-α-pinene

In this work, the HRS-OA circular differential scattering ratios of medium-size chiral molecules, R-carvone and (1R,5R)-α-pinene, has been computed by means of TD-DFT calculations. The results have been discussed as a function of the basis set size. For R-carvone a conformational analysis has been taken into account. In particular, the three most stable conformations of R-carvone present chiral terms that contribute to the HRS-OA circular differential scattering ratio, with opposite signs. Finally, the modified unit sphere representations (mUSR) have been produced for the molecules under study.

Effective fully polarizable QM/MM approaches to compute Raman and Raman Optical Activity spectra in aqueous solution

Raman and Raman Optical Activity (ROA) signals are amply affected by solvent effects, especially in the presence of strongly solute-solvent interactions such as Hydrogen Bonding (HB). In this work, we extend the fully atomistic polarizable Quantum Mechanics/Molecular Mechanics approach, based on the Fluctuating Charges and Fluctuating Dipoles force field to the calculation of Raman and ROA spectra. Such an approach is able to accurately describe specific HB interactions, by also accounting for anisotropic contributions due to the inclusion of fluctuating dipoles. To highlight the potentiality of the novel approach, Raman and ROA spectra of L-Serine and L-Cysteine dissolved in aqueous solution are computed and compared both with alternative theoretical approaches and experimental measurements.

Monitoring Conformation and Protonation States of Glutathione by Raman Optical Activity and Molecular Dynamics

Glutathione (GSH) is a common antioxidant and its biological activity depends on the conformation and protonation state. We used molecular dynamics, Raman and Raman optical activity (ROA) spectroscopies to investigate GSH structural changes in a broad pH range. Factor analysis of the spectra provided protonation constants (2.05, 3.45, 8.62, 9.41) in good agreement with previously published values. Following the analysis, spectra of differently protonated forms were obtained by extrapolation. The complete deprotonation of the thiol group above pH 11 was clearly visible in the spectra; however, many spectral features did not change much with pH. Experimental spectra at various pH values were decomposed into the simulated ones, which allowed us to study the conformer populations and quality of molecular dynamics (MD). According to this combined ROA/MD analysis conformation of the GSH backbone is affected by the pH changes only in a limited way. The combination of ROA with the computations thus has the potential to improve the MD force field and obtain more accurate populations of the conformer species. The methodology can be used for any molecule, but for a more detailed insight better computational techniques are needed in the future.

Measurement and Theory of Resonance Raman Optical Activity for Gases, Liquids, and Aggregates. What It Tells about Molecules

Resonance Raman Optical Activity (RROA) appeared as a natural extension of the nonresonance branch. It combines the structural sensitivity of chiroptical spectroscopy with the signal enhancement coming from the resonance of molecular electronic transitions with the excitation laser light. However, the idea has been hampered by many technical and theoretical problems that are being clarified only in recent years. We provide the theoretical basis and several examples documenting the problems, achievements, and potential of RROA, in particular in biomolecular studies.

Recognition of the True and False Resonance Raman Optical Activity

Resonance Raman optical activity (RROA) possesses all aspects of a sensitive tool for molecular detection, but its measurement remains challenging. We demonstrate that reliable recording of RROA of chiral colorful compounds is possible, but only after considering the effect of the electronic circular dichroism (ECD) on the ROA spectra induced by the dissolved chiral compound. We show RROA for a number of model vitamin B12 derivatives that are chemically similar but exhibit distinctively different spectroscopic behavior. The ECD/ROA effect is proportional to the concentration and dependent on the optical pathlength of the light propagating through the sample. It can severely alter relative band intensities and signs in the natural RROA spectra. The spectra analyses are supported by computational modeling based on density functional theory. Neglecting the ECD effect during ROA measurement can lead to misinterpretation of the recorded spectra and erroneous conclusions about the molecular structure.

Theory and algorithms for chiroptical properties and spectroscopies of aqueous systems

Chiroptical properties and spectroscopies are valuable tools to study chiral molecules and assign absolute configurations. The spectra that result from chiroptical measurements may be very rich and complex, and hide much of their information content. For this reason, the interplay between experiments and calculations is especially useful, provided that all relevant physico-chemical interactions that are present in the experimental sample are accurately modelled. The inherent difficulty associated to the calculation of chiral signals of systems in aqueous solutions requires the development of specific tools, able to account for the peculiarities of water-solute interactions, and especially its ability to form hydrogen bonds. In this perspective we discuss a multiscale approach, which we have developed and challenged to model the most used chiroptical techniques.

Vibrational optical activity for structural characterization of natural products

This review presents the recent progress towards elucidating the structures of chiral natural products and applications using vibrational optical activity (VOA) spectroscopy.

A combined experimental and theoretical study of optical rotatory dispersion for (R)-glycidyl methyl ether in aqueous solution

The dispersive optical activity for aqueous solutions of non-rigid (R)-glycidyl methyl ether (R-GME) has been explored synergistically from experimental and theoretical perspectives. Density functional theory analyses performed with the polarizable continuum model for implicit solvation identified nine low-lying stable conformers that are interconverted by rotation about two large-amplitude torsional coordinates. The antagonistic chiroptical signatures predicted for these structural isomers were averaged under a Boltzmann-weighting ansatz to estimate the behavior expected for a thermally equilibrated ensemble. This led to optical rotatory dispersion profiles that reproduced the overall shape of observations but failed to achieve uniform agreement with measured specific-rotation values even when anharmonic vibrational corrections were applied. A mixed QM/FQ paradigm, whereby quantum-mechanical (QM) calculations of optical activity were combined with classical molecular dynamics simulations of explicit solvation that included mutual-polarization effects by means of fluctuating charges (FQ), was enlisted to elucidate the microsolvation environment and gauge its impact upon conformer distributions and response properties. Although quantitative accord with experiments remained elusive, this approach revealed strong variations in the magnitude and sign of rotatory powers for R-GME as the configuration of surrounding water molecules evolved, thereby highlighting the inherently dynamical nature of the solvated chiroptical response, calling into question the validity of "static" descriptions based on the presumption of distinct energy minima, and giving insight into the inherent complexity posed by the modeling of such properties for solvated systems.

Vibrational circular dichroism under the quantum magnifying glass: from the electronic flow to the spectroscopic observable

A chemically intuitive method to analyse and interpret vibrational circular dichroism spectra based on the vibrational transition current density.

Time-Dependent Formulation of Resonance Raman Optical Activity Spectroscopy

In this work, we extend the theoretical framework recently developed for the simulation of resonance Raman (RR) spectra of medium-to-large sized systems to its chiral counterpart, namely, resonance Raman optical activity (RROA). The theory is based on a time-dependent (TD) formulation, with the transition tensors obtained as half-Fourier transforms of the appropriate cross-correlation functions. The implementation has been kept as general as possible, supporting adiabatic and vertical models for the PES representation, both in Cartesian and internal coordinates, with the possible inclusion of Herzberg-Teller (HT) effects. Thanks to the integration of this TD-RROA procedure within a general-purpose quantum-chemistry program, both solvation and leading anharmonicity effects can be included in an effective way. The implementation is validated on one of the smallest chiral molecule (methyloxirane). Practical applications are illustrated with three medium-size organic molecules (naproxen-OCD₃, quinidine and 2-Br-hexahelicene), whose simulated spectra are compared to the corresponding experimental data.

Computational simulation of vibrationally resolved spectra for spin-forbidden transitions

In this computational study, we illustrate a method for computing phosphorescence and circularly polarized phosphorescence spectra of molecular systems, which takes into account vibronic effects including both Franck-Condon and Herzberg-Teller contributions. The singlet and triplet states involved in the phosphorescent emission are described within the harmonic approximation, and the method fully takes mode-mixing effects into account when evaluating Franck-Condon integrals. Spin-orbit couplings, which are responsible for these otherwise forbidden phenomena, are accounted for by means of a relativistic two-component time-dependent density functional theory method. The model is applied to two types of chiral systems: camphorquinone, a rigid organic system that allows for an extensive benchmark, and some members of a class of iridium complexes. The merits and shortcomings of the methods are discussed, and some perspectives for future developments are offered.

Can the Resonance Raman Optical Activity Spectrum Display Sign Alternation?

The monosignate character of resonance Raman optical activity (RROA) spectra has been often taken as granted in experimental and computational approaches, on the basis of basic theoretical approximations only considering resonance with a single electronic state of the molecule and the scattering process to be governed by the Franck-Condon mechanism. We show in this letter for the first time that, by resorting to a fully quantum mechanical (QM) methodology able to take into account all terms entering the general definition of RROA, and which considers excited state interference and Herzberg-Teller effects, sign alternation and at the same time intensity enhancement in RROA spectra is obtained. Such features constitute an important milestone toward the exploration of RROA of a wide range of chiral biological molecules.

General formulation of vibronic spectroscopy in internal coordinates

Our general platform integrating time-independent and time-dependent evaluations of vibronic effects at the harmonic level for different kinds of absorption and emission one-photon, conventional and chiral spectroscopies has been extended to support various sets of internal coordinates. Thanks to the implementation of analytical first and second derivatives of different internal coordinates with respect to cartesian ones, both vertical and adiabatic models are available, with the inclusion of mode mixing and, possibly, Herzberg-Teller contributions. Furthermore, all supported non-redundant sets of coordinates are built from a fully automatized algorithm using only a primitive redundant set derived from a bond order-based molecular topology. Together with conventional stretching, bending, and torsion coordinates, the availability of additional coordinates (including linear and out-of-plane bendings) allows a proper treatment of specific systems, including, for instance, inter-molecular hydrogen bridges. A number of case studies are analysed, showing that cartesian and internal coordinates are nearly equivalent for semi-rigid systems not experiencing significant geometry distortions between initial and final electronic states. At variance, delocalized (possibly weighted) internal coordinates become much more effective than their cartesian counterparts for flexible systems and/or in the presence of significant geometry distortions accompanying electronic transitions.

Simulation of Vacuum UV Absorption and Electronic Circular Dichroism Spectra of Methyl Oxirane: The Role of Vibrational Effects

Vibrationally resolved one-photon absorption and electronic circular dichroism spectra of (R)-methyl oxirane were calculated with different electronic and vibronic models selecting, through an analysis of the convergence of the results, the best compromise between reliability and computational cost. Linear-response TD-DFT/CAM-B3LYP/SNST electronic computations in conjunction with the simple vertical gradient vibronic model were chosen and employed for systematic comparison with the available experimental data. Remarkable agreement between simulated and experimental spectra was achieved for both one-photon absorption and circular dichroism concerning peak positions, relative intensities, and general spectral shapes considering the computational efficiency of the chosen theoretical approach. The significant improvement of the results with respect to smearing of vertical electronic transitions by phenomenological Gaussian functions and the possible inclusion of solvent effects by polarizable continuum models at a negligible additional cost paves the route toward the simulation and analysis of spectral shapes of complex molecular systems in their natural environment.

The Virtual Multifrequency Spectrometer: a new paradigm for spectroscopy

On going developments of hardware and software are changing computational spectroscopy from a strongly specialized research area to a general tool in the inventory of most researchers. Increased interactions between experimentally-oriented users and theoretically-oriented developers of new methods and models would result in more robust, flexible and reliable tools and studies for the systems of increasing complexity, which are of current scientific and technological interest. This is the philosophy behind this review, which presents the development of a so-called virtual multi-frequency spectrometer (VMS) including state-of-the-art approaches in a user-friendly frame. The current status of the VMS tool will be illustrated by a number of case studies with special reference to infrared and UV-vis regions of the electro-magnetic spectrum including also chiral spectroscopies. Only the basic theoretical background will be provided avoiding explicit equations as far as possible, and pointing out the most recent advancements beyond the standard rigid-rotor harmonic-oscillator model coupled to vertical electronic excitation energies.

Anharmonic Effects on Vibrational Spectra Intensities: Infrared, Raman, Vibrational Circular Dichroism, and Raman Optical Activity

The aim of this paper is 2-fold. First, we want to report the extension of our virtual multifrequency spectrometer (VMS) to anharmonic intensities for Raman optical activity (ROA) with the full inclusion of first- and second-order resonances for both frequencies and intensities in the framework of the generalized second-order vibrational perturbation theory (GVPT2) for all kinds of vibrational spectroscopies. Then, from a more general point of view, we want to present and validate the performance of VMS for the parallel analysis of different vibrational spectra for medium-sized molecules (IR, Raman, VCD, ROA) including both mechanical and electric/magnetic anharmonicity. For the well-known methyloxirane benchmark, careful selection of density functional, basis set, and resonance thresholds permitted us to reach qualitative and quantitative agreement between experimental and computed band positions and shapes. Next, the whole series of halogenated azetidinones is analyzed, showing that it is now possible to interpret different spectra in terms of mass, electronegativity, polarizability, and hindrance variation between closely related substituents, chiral spectroscopies being particular effective in this connection.