Franco Egidi

Implementation of a graphical user interface for the virtual multifrequency spectrometer: The VMS‐Draw tool

This article presents the setup and implementation of a graphical user interface (VMS‐Draw) for a virtual multifrequency spectrometer. Special attention is paid to ease of use, generality and robustness for a panel of spectroscopic techniques and quantum mechanical approaches. Depending on the kind of data to be analyzed, VMS‐Draw produces different types of graphical representations, including two‐dimensional or three‐dimesional (3D) plots, bar charts, or heat maps. Among other integrated features, one may quote the convolution of stick spectra to obtain realistic line‐shapes. It is also possible to analyze and visualize, together with the structure, the molecular orbitals and/or the vibrational motions of molecular systems thanks to 3D interactive tools. On these grounds, VMS‐Draw could represent a useful additional tool for spectroscopic studies integrating measurements and computer simulations.

The Optical Rotation of Methyloxirane in Aqueous Solution: A Never Ending Story?

The long-standing problem of the calculation of the optical rotation (OR) of (R)-methyloxirane in aqueous solution at different wavelengths is solved by means of a novel gauge-invariant computational protocol able to take into account at the same time for intramolecular averaging specific and bulk solvent effects, leading for the first time to a quantitative agreement (both sign and absolute value) between computed and experimental OR values at several frequencies.

Toward an Accurate Modeling of Optical Rotation for Solvated Systems: Anharmonic Vibrational Contributions Coupled to the Polarizable Continuum Model

We present a newly implemented methodology to evaluate vibrational contributions (harmonic and anharmonic) to the optical rotation of solvated systems described by means of the polarizable continuum model (PCM). Proper account of an incomplete solvation regime in the treatment of both the electronic property and the molecular vibrations is considered, as well as the inclusion of cavity field effects. In order to assess the quality of our approach, test calculations on (R)-methyloxirane in various solvents and (S)-N-acetylproline amide in cyclohexane and aqueous solution are presented. The comparison with experimental findings is also shown.

General strategy for computing nonlinear optical properties of large neutral and cationic organic chromophores in solution.

Tuning of nonlinear optical (NLO) properties of organic chromophores (OCs) by stereo-electronic and environmental effects has been widely documented by different experimental techniques and theoretical studies. Disentanglement and analysis of the different contributions requires, however, the availability of effective yet accurate quantum mechanical approaches for medium to large size systems in their natural environment. As a first step, we have shortly reviewed the phenomenological models still used by experimentalists to interpret their results and shown that a quantum mechanical approach based on the density functional theory (DFT) and the polarizable continuum model (PCM) should be able to overcome most theoretical limitations allowing, at the same time, the study of large systems with reasonable computational resources and the analysis of the results in terms of well-defined physical-chemical effects. After validation of the most suitable density functional/basis set in conjunction with the PCM description of bulk solvent effects, we have performed a systematic study of representative OCs, especially cationic ones, with special reference to their first order hyperpolarizability. The internal consistency of the results and their good agreement with experiment paves the route toward integrated experimental/computational studies of NLO properties taking together physical soundness, feasibility, reliability, and ease of interpretation.

Development of a Virtual Spectrometer for Chiroptical Spectroscopies: the Case of Nicotine

The impressive advances of computational spectroscopy in most recent years are providing robust and user-friendly multifrequency virtual spectrometers, which can also be used by nonspecialists to complement experimental studies. At the heart of these developments there are latest-generation models based on Density Functional Theory for the proper treatment of stereo-electronic effects, coupled to the polarizable continuum model to deal with bulk solvent effects, and low-order perturbative treatments of anharmonic effects. Continuing our efforts to increase the range of application of virtual spectrometers, we report here about chiroptical spectroscopies with special reference to optical rotation and vibrational circular dichroism. The capabilities and possible limitations of our latest tool will be analyzed for the specific case of (S)-nicotine in vacuo and in different solvents.

The Electronic Circular Dichroism of Nicotine in Aqueous Solution: A Test Case for Continuum and Mixed Explicit-Continuum Solvation Approaches

In this paper, we extend an integrated QM/MM/polarizable continuum model (PCM) method, which combines a fluctuating charge (FQ) approach to the MM polarization with the PCM, to describe electronic circular dichroism (ECD) spectra of systems in aqueous solution. The main features of the approach are presented, and then applications to the UV and ECD spectra of neutral (S)-nicotine in aqueous solution are reported. The performance of the QM/FQ/PCM is compared with that of the PCM against newly measured UV ECD spectra, which are in agreement with previous findings. The inclusion of specific solvation effects via the QM/FQ/PCM method leads to an improvement in the calculated spectra compared to the experimental findings, though the pure PCM results are still qualitatively correct and are a useful tool for the characterization of the states.

Real time propagation of the exact two component time-dependent density functional theory

We report the development of a real time propagation method for solving the time-dependent relativistic exact two-component density functional theory equations (RT-X2C-TDDFT). The method is fundamentally non-perturbative and may be employed to study nonlinear responses for heavy elements which require a relativistic Hamiltonian. We apply the method to several group 12 atoms as well as heavy-element hydrides, comparing with the extensive theoretical and experimental studies on this system, which demonstrates the correctness of our approach. Because the exact two-component Hamiltonian contains spin-orbit operators, the method is able to describe the non-zero transition moment of otherwise spin-forbidden processes in non-relativistic theory. Furthermore, the two-component approach is more cost effective than the full four-component approach, with similar accuracy. The RT-X2C-TDDFT will be useful in future studies of systems containing heavy elements interacting with strong external fields.

Optical rotatory dispersion of methyloxirane in aqueous solution: assessing the performance of density functional theory in combination with a fully polarizable QM/MM/PCM approach

We report a study on the performance of a recently developed fully polarizable QM/MM/PCM approach based on Fluctuating Charges (FQ) combined with 11 different Density Functionals for the description of the Optical Rotation at different wavelengths of (R)-Methyloxirane in aqueous solution. The results are compared with those obtained for the isolated system and for the solvated one as described by the Polarizable Continuum Model. In all cases, a comparison with experimental data is also shown. The results show that the effect of the solvent is much more significant than the effect of the density functional.

Origin invariance in vibrational resonance Raman optical activity

A theoretical investigation on the origin dependence of the vibronic polarizabilities, isotropic and anisotropic rotational invariants, and scattering cross sections in Resonance Raman Optical Activity (RROA) spectroscopy is presented. Expressions showing the origin dependence of these polarizabilities were written in the resonance regime using the Franck-Condon (FC) and Herzberg-Teller (HT) approximations for the electronic transition moments. Differently from the far-from-resonance scattering regime, where the origin dependent terms cancel out when the rotational invariants are calculated, RROA spectrum can exhibit some origin dependence even for eigenfunctions of the electronic Hamiltonian. At the FC level, the RROA spectrum is completely origin invariant if the polarizabilities are calculated using a single excited state or for a set of degenerate states. Otherwise, some origin effects can be observed in the spectrum. At the HT level, RROA spectrum is origin dependent even when the polarizabilities are evaluated from a single excited state but the origin effect is expected to be small in this case. Numerical calculations performed for (S)-methyloxirane, (2R,3R)-dimethyloxirane, and (R)-4-F-2-azetidinone at both FC and HT levels using the velocity representation of the electric dipole and quadrupole transition moments confirm the predictions of the theory and show the extent of origin effects and the effectiveness of suggested ways to remove them.

A benchmark study of electronic excitation energies, transition moments, and excited-state energy gradients on the nicotine molecule

In this work, we report a comparative study of computed excitation energies, oscillator strengths, and excited-state energy gradients of (S)-nicotine, chosen as a test case, using multireference methods, coupled cluster singles and doubles, and methods based on time-dependent density functional theory. This system was chosen because its apparent simplicity hides a complex electronic structure, as several different types of valence excitations are possible, including n-π(*), π-π(*), and charge-transfer states, and in order to simulate its spectrum it is necessary to describe all of them consistently well by the chosen method.