Alessandro Ferretti

Quantum mechanical and semiclassical dynamics at a conical intersection

We present simulations of wave‐packet dynamics for a model of a conical intersection in two dimensions. The potential energy surfaces and couplings are functions of a total symmetrical coordinate and of a symmetry breaking one. The wave packet crosses the coupling region once, moving essentially in the direction of the symmetrical coordinate. The dynamics are determined by two methods, one quantum mechanical and the other semiclassical, based on trajectories and surface hopping. The semiclassical approximation is quite adequate for low coupling strengths in the diabatic representation, less so for larger couplings. Approximate analytic solutions for the two‐dimensional problem and for one‐dimensional analogs are provided, in order to generalize the numerical results and to analyze the reasons of the discrepancies between semiclassical and quantum mechanical results.

Integrated computational approaches for spectroscopic studies of molecular systems in the gas phase and in solution: pyrimidine as a test case

An integrated computational approach built on quantum mechanical (QM) methods, purposely tailored inter- and intra-molecular force fields and continuum solvent models combined with time-independent and time-dependent schemes to account for nuclear motion effects is applied to the spectroscopic investigation of pyrimidine in the gas phase as well as in aqueous and CCl4 solutions. Accurate post-Hartree–Fock methodologies are employed to compute molecular structure, harmonic vibrational frequencies, energies and oscillator strengths for electronic transitions in order to validate the accuracy of approaches rooted into density functional theory with emphasis also on hybrid QM/QM′ models. Within the time-independent approaches, IR spectra are computed including anharmonicities through perturbative corrections while UV–vis line-shapes are simulated accounting for the vibrational structure; in both cases, the environmental effects are described by continuum models. The effects of conformational flexibility, including solvent dynamics, are described through time-dependent models based on purposely DFT-tailored force fields applied to molecular dynamics simulations and on QM computations of spectroscopic properties. Such procedures are exploited to simulate IR and UV–vis spectra of pyrimidine in the gas phase and in solutions, leading in all cases to good agreement with experimental observations and allowing to dissect different effects underlying spectral phenomena.

An Integrated Protocol for the Accurate Calculation of Magnetic Interactions in Organic Magnets

A new, fast, and efficient computational protocol for the accurate calculation of singlet-triplet magnetic splittings in organic diradicals is tested and validated. This procedure essentially consists of three steps: the adoption of modified virtual orbitals (MVO) and a mixed variational-perturbational approach (CSPA) are now combined with a third method that exploits the reduction of the configurational space dimensions achieved by fragmentation/localization criteria. This innovative approach is successfully tested on four different substituted m-phenylene bis(tert-butyl) nitroxides, which show paramagnetic behavior, by computing singlet-triplet energy gaps and comparing them with their experimental counterparts.

Magnetic coupling in bis-nitronylnitroxide radicals: The role of aromatic bridges

Configuration interaction calculations have been applied to the study of the magnetic coupling in bis-nitronyl nitroxide radicals with benzene bridges. Molecular orbitals obtained with different localization schemes have been considered in the generation of the CI space, with the aim of investigating the role played by the various fragments in the magnetic interaction. The aromatic bridge is found significant, while fragments outside the magnetic-bridge-magnetic moiety can be neglected. Using simplified model molecular species, an accurate analysis of the ferromagnetic/antiferromagnetic coupling in the meta and para diradicals is reported.

Structure-Properties Relationships in Triplet Ground State Organic Diradicals: A Computational Study.

A fast and efficient computational protocol, devised for the accurate calculation of singlet-triplet magnetic splittings in organic diradicals, is here applied to several promising organic magnets, recently considered in the literature. The very good agreement with the measured values, obtained for all investigated compounds, suggests that the present approach could successfully flank the experiment in the design of novel magnetic materials. Indeed, some structure-magnetic properties relationships were rationalized thanks to the theoretical soundness of the adopted multireference approach. In particular the different effects of N· and NO· magnetic moieties, as well as the role of lateral aliphatic chains and phenyl pendant substituents, are discussed in detail.

Toward an effective yet reliable many-body computation of magnetic couplings in bisnitronyl nitroxide biradicals

Configuration interaction calculations have been applied to the study of the magnetic coupling in a series of bisnitronyl nitroxide diradicals. Molecular orbitals obtained with different localization schemes have been considered in the generation of the configuration interaction space, with the aim of investigating the role played by the various fragments in the magnetic interaction. Polyene spacers are found significant, while fragments outside the magnetic-bridge-magnetic moiety can be neglected.

Magnetic Interactions in Phenyl-Bridged Nitroxide Diradicals: Conformational Effects by Multireference and Broken Symmetry DFT Approaches

In the present paper we report the key results of a comprehensive computational study aimed at investigating the dependence of the singlet-triplet energy gap in phenyl-bridged bis-nitroxide diradicals, on the basis set and on soft structural parameters like torsion and pyramidalization. We have compared the BS-DFT technique with the post-Hartree-Fock DDCI2 multireference approach. With this latter method we have also studied the different role that sigma and pi core and virtual orbitals have in the resulting singlet-triplet energy gap.The results obtained represent one step forward in the definition of a protocol for an efficient and reliable computation of spin-spin coupling in diradical systems.

Singlet–triplet energy gap of a diarylnitroxide diradical by an accurate many-body perturbative approach

We report on the calculation of the spin-spin coupling term J in a diarylnitroxide diradical system which is a slightly simplified version of a recently synthesized stable species with a triplet ground state. We have applied our complementary space perturbative approach using virtual orbitals appropriately modified with the aim of investigating, through molecular fragmentation, the effects of the non-bridge aryls. We show how by using multireference CI techniques it is now possible to obtain accurate and reliable values of J even for large organic radicals, the basic units of longer chain polyradical systems.

Accurate yet feasible post-Hartree–Fock computation of magnetic interactions in large biradicals through a combined variational/perturbative approach: Setup and validation

We present a scheme for the calculation of the spin-spin coupling term J in diradicals which is quantitatively accurate and computationally cheap. The method exploits the use of modified virtual orbitals and perturbation theory, incorporated in a multireference configuration interaction approach. The results obtained for model diradical species which exhibit ferromagnetic and antiferromagnetic coupling are fully satisfactory and very promising for future applications of the method to larger molecular systems of technological interest in magnetic-based devices.

Intermolecular interactions in eumelanins: a computational bottom-up approach. I. small building blocks

The non-covalent interactions between pairs of the smallest eumelanins building blocks, 5,6-dihydroxy-indole (DHI) and its redox derivatives, are subjected to a systematic theoretical investigation, elucidating their nature and commenting on some of their possible effects on the layered structure of eumelanin. An accurate yet feasible protocol, based on second order perturbation theory, was set up and validated herein, and thereafter used to sample the intermolecular potential energy surfaces of several DHI related dimers. From the analysis of the resulting local minima, the crucial role of stacking interactions is assessed, evidencing strong effects on the geometrical arrangement of the dimer. Furthermore, the absorption spectra of the considered dimers in their most stable arrangements are computed and discussed in relation to the well known eumelanin broadband features. The present findings may help in elucidating several eumelanin features, supporting the recently proposed geometrical order/disorder model (Chen et al., Nat. Commun. 2014, 5, 3859).