Ivo Cacelli

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.

JOYCE and ULYSSES: integrated and user-friendly tools for the parameterization of intramolecular force fields from quantum mechanical data

The Joyce program is augmented with several new features, including the user friendly Ulysses GUI, the possibility of complete excited state parameterization and a more flexible treatment of the force field electrostatic terms. A first validation is achieved by successfully comparing results obtained with Joyce2.0 to literature ones, obtained for the same set of benchmark molecules. The parameterization protocol is also applied to two other larger molecules, namely nicotine and a coumarin based dye. In the former case, the parameterized force field is employed in molecular dynamics simulations of solvated nicotine, and the solute conformational distribution at room temperature is discussed. Force fields parameterized with Joyce2.0, for both the dyes ground and first excited electronic states, are validated through the calculation of absorption and emission vertical energies with molecular mechanics optimized structures. Finally, the newly implemented procedure to handle polarizable force fields is discussed and applied to the pyrimidine molecule as a test case.

Calculation of the intermolecular energy of large molecules by a fragmentation scheme: Application to the 4-n-pentyl-4′- cyanobiphenyl (5CB) dimer

We present a method for computing intermolecular energies of large molecules based on a suitable fragmentation scheme, which allows one to express the complete interaction energy as a sum of interaction energies between pairs of fragments. The main advantage consists in the possibility of using standard ab initio quantum methods to evaluate the fragment energies. For the 4-n-pentyl-4′-cyanobiphenyl (5CB) dimer, the present results indicate that the most favorite arrangement corresponds to an antiparallel side-by-side geometry with a stabilization energy of about 16 kcal/mol. It is shown that, by the present method, the interaction energy of the 5CB dimer can be evaluated for all geometrical conformations and, in principle, it can be used for bulk simulations.

Molecular photoionization cross sections and asymmetry parameters by L2 basis functions calculations: H2O

A K‐matrix technique using a basis set of square‐integrable functions is applied to the calculation of differential photoionization cross sections in molecules. Continuum orbitals are variationally determined in the static‐exchange approximation of the ion field. Integrated photoionization cross sections and asymmetry parameters β of the three main valence ionization processes in H2O are calculated, in the independent channel approximation, for the photon energy in the range of 14–50 eV and compared with the available experimental data.

Photoionization cross section calculations of HCl by the Stieltjes technique: Effect of the channel coupling

Partial and total valence- and inner-shell photoionization cross sections of HCl are calculated in the random phase approximation employing an extended basis set and the Stieltjes imaging procedure. Results of independent-channel as well as multi-channel calculations in the static-exchange approximation are reported for comparison. The generally good agreement with recent experimental measurements indicates that the random phase approximation provides an efficient description of photoionization channel coupling. The calculated vertical spectra in the ‘discrete’ region of valence- and inner-shell are also reported.

H2S photoabsorption and photoionization cross sections by Stieltjes imaging

Abstract Photoabsorption and photoionization cross sections of H2S have been calculated by using extended basis sets of integrable functions and the Stieltjes imaging procedure. The calculations were performed in static-exchange approximation (SEA) and random-phase approximation (RPA) and the excitation spectrum was obtained either in the single-channel or in the multichannel approach. The gauge dependence of the SEA results tends to inhibit a reliable assessment of the channel interaction effects. The situation is noticeably improved in the RPA where the gauge dependence of the results, even in the single-channel approximation, is considerably lessened.

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.

Gaussian type orbital basis sets for the calculation of continuum properties in molecules: The photoionization cross section of H2

The now well‐established L2 techniques applied to the description of the electronic continuum of small molecules and generally implemented with suitable oscillating basis sets are here adapted and used in connection with Gaussian type orbital (GTO) functions. As a test, a K‐matrix approach associated with extended sets of GTOs is employed to compute with satisfactory accuracy partial wave phase shifts and both the integral and differential photoionization cross sections of the hydrogen molecule. It is apparently possible to greatly extend the range of applicability of the usual L2 techniques beyond the limited set of molecular systems amenable to the one center expansion approximation.

Structure and Dynamics of Ferrocyanide and Ferricyanide Anions in Water and Heavy Water: An Insight by MD Simulations and 2D IR Spectroscopy

Combined computational and experimental techniques were employed to investigate at the microscopic level the structural and dynamic properties of ferro- and ferricyanide ions in aqueous solution. The characterization of the structural patterns and multiscale dynamics taking place within the first solvation spheres in water and heavy water solvents was first achieved through extensive molecular dynamics simulations, performed with refined force fields, specifically parametrized for the cyanide ions under investigation. The information gained about the solute-solvent interactions is then validated through the successful comparison of computed and measured waiting-time-dependent 2D IR spectra. The vibrational patterns resulting from 2D IR measurements were rationalized in terms of the interaction between the ion and the neighboring water molecules described by simulation. It was found that, within the first solvation sphere, the stronger interactions of the solvent with the ferro species are responsible for a delay in the relaxation dynamics, which becomes more and more evident on longer time scales.

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.