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In this thesis, we propose general, effective strategies for the simulation of vibrational spectra and calculation of accurate molecular geometries. Vibrational and rotational spectroscopies are very powerful tools for investigating the physical-chemical properties of molecular systems, since they allow one to obtain a remarkable set of information related to the structure and dynamics. In particular, concurrent use of several spectroscopies is often needed to obtain a full characterization of the complex systems of current fundamental and technological interest. However, experimental spectra are tuned by several intertwined effects which can make the interpretation of experimental data very challenging or even unaffordable without the support of reliable in silico simulations. As a matter of fact, ongoing developments of hardware and software (not to speak of the underlying physical-mathematical models) are allowing the reproduction of experimental outcomes and their interpretation in terms of stereo-electronic, dynamic, and environmental effects. Despite the unquestionable success of static structure-property relationships and of the basic rigid-rotor / harmonic-oscillator model, accurate results, directly comparable with experiment, can be obtained only employing more refined models, and, in particular, including anharmonic effects. The accuracy of theoretical vibrational spectra can be improved either at the electronic level, through highly correlated methods, able to deliver accurate equilibrium structures, energy and properties, and at the nuclear level, including anharmonic effects in the description of the nuclear motions. In this respect, the second-order vibrational perturbation theory (VPT2) has shown to offer a very effective balance between accuracy and computational cost, thus permitting the study of medium-to-large systems. At the VPT2 level, the semi-diagonal cubic force field is required to compute vibration-rotation interaction constants describing the vibrational dependence of rotational constants. These are needed in order to determine accurate molecular structures by the semi-experimental (SE) approach. The extension to the full cubic and semi-diagonal quartic force field allows the calculation of the anharmonic vibrational frequencies and intensities. A current hurdle is that the actual implementation of VPT2 is fragmented between asymmetric tops (no degeneracy) and symmetric/linear rotors (presence of doubly degenerate modes), which means that anywork done on VPT2 needs to be duplicated. Part of this thesis work has been aimed at demonstrating that the standard VPT2 for Abelian groups can be used also for non-Abelian groups without employing specific equations for two- or three-fold degenerate vibrations but rather handling in the proper way all the degeneracy issues and deriving the peculiar spectroscopic signatures of non-Abelian groups (e.g., -doubling) by a posteriori transformations of the eigenfunctions. Comparison with the results of previous conventional implementations shows a perfect agreement for the vibrational energies of linear and symmetric tops, thus paving the route to the transparent extension of the equations already available for asymmetric tops to the energies of spherical tops and the infrared and Raman intensities of molecules belonging to non-Abelian symmetry groups. The whole procedure has been implemented in our general engine for vibro-rotational computations beyond the rigid rotor/harmonic oscillator model and has been validated on a number of test cases. In the next part of the research activity, we focused on the development and implementation of a methodology to treat medium-sized molecular systems presenting some flexibility, and thus are unsatisfactory described at the VPT2 level. In fact, while the representation of the nuclear potential energy as a truncated Taylor expansion up to the fourth order in terms of normal coordinates has proven to be very accurate for semi-rigid molecules, it is expected to yield unreliable results for flexible molecules that present one or more LAMs. It is noteworthy that for such systems a pure variational treatment would be prohibitive, and even reaching an accuracy comparable to VPT2 could only be done at several times the cost of the latter. Therefore, the best course of action is the formulation of a strategy based on an interplay between both computational methods, in order to take full advantage of each one. From a practical point of view, this operation can be carried out by separating the normal modes, or more generally a set of coordinates, which are suited for VPT2 from those that are not, and then treat the resulting groups separately. For this purpose, a description of molecular vibrations based on internal coordinates is more suitable, usually leading to a smaller coupling between different degrees of freedom. In this thesis, a theoretical derivation of the VPT2 framework has been carried out starting from the available literature. In this context, the main difference with respect to the Cartesian-based formulation is that the kinetic energy operator is not diagonal anymore, and has to be expanded as well, leading to additional terms which have taken into the proper account. It is worth mentioning that each expression derived in the internal-based framework, can be written as a generalization of the corresponding Cartesian-based counterpart, implying a remarkable simplification at the implementation level. The determination of accurate equilibrium molecular structures plays a fundamental role for understanding many physical-chemical properties of molecules, ranging from the precise evaluation of the electronic structure to the analysis of dynamical and environmental effects in tuning their overall behavior. For this purpose the so-called semi-experimental (SE) approach, based on a nonlinear least-squares fit of the moments of inertia associated with a set of available isotopologues, allows one to obtain very accurate results, without the unfavorable computational cost characterizing high-level quantum chemical methods. In this thesis, the MSR (Molecular Structure Refinement) software for the determination of equilibrium structures by means of the SE approach is presented, and its implementation is discussed in some detail. The software, which is interfaced with a powerful graphical user interface, includes different optimization algorithms, an extended error analysis, and a number of advanced features, the most remarkable ones concerning the choice of internal coordinates and the method of predicate observations. In particular, a newblack-box scheme for defining automatically a suitable set of non-redundant internal coordinates of A1 symmetry in place of the customary Z-matrix has been designed and tested. Moreover, different strategies aimed at handling the cases in which the number of experimental data is not sufficient to characterize all structural parameters have been analyzed. The computational framework developed in this thesis, and implemented in the MSR program, has been employed for the determination of the SE structure for molecular systems of biological and astrochemical interest.

Vibrational spectroscopy represents an active frontier for the identification and characterization of molecular species in the context of astrochemistry and astrobiology. As new missions will provide more data over broader ranges and at higher resolution, especially in the infrared region, which could be complemented with new spectrometers in the future, support from laboratory experiments and theory is crucial. In particular, computational spectroscopy is playing an increasing role in deepening our understanding of the origin and nature of the observed bands in extreme conditions characterizing the interstellar medium or some planetary atmospheres, not easily reproducible on Earth. In this connection, the best compromise between reliability, feasibility and ease of interpretation is still a matter of concern due to the interplay of several factors in determining the final spectral outcome, with larger molecular systems and non-covalent complexes further exacerbating the dichotomy between accuracy and computational cost. In this context, second-order vibrational perturbation theory (VPT2) together with density functional theory (DFT) has become particularly appealing. The well-known problem of the reliability of exchange-correlation functionals, coupled with the treatment of resonances in VPT2, represents a challenge for the determination of standardized or "black-box" protocols, despite successful examples in the literature. With the aim of getting a clear picture of the achievable accuracy and reliability of DFT-based VPT2 calculations, a multi-step study will be carried out here. Beyond the definition of the functional, the impact of the basis set and the influence of the resonance treatment in VPT2 will be analyzed. For a better understanding of the computational aspects and the results, a short summary of vibrational perturbation theory and the overall treatment of resonances for both energies and intensities will be given. The first part of the benchmark will focus on small molecules, for which very accurate experimental and theoretical data are available, to investigate electronic structure calculation methods. Beyond the reliability of energies, widely used for such systems, the issue of intensities will also be investigated in detail. The best performing electronic structure methods will then be used to treat larger molecular systems, with more complex topologies and resonance patterns. Vibrational spectroscopy represents an active frontier for the identification and characterization of molecular species in the context of astrochemistry and astrobiology. As new missions will provide more data over broader ranges and at higher resolution, especially in the infrared region, which could be complemented with new spectrometers in the future, support from laboratory experiments and theory is crucial. In particular, computational spectroscopy is playing an increasing role in deepening our understanding of the origin and nature of the observed bands in extreme conditions characterizing the interstellar medium or some planetary atmospheres, not easily reproducible on Earth. In this connection, the best compromise between reliability, feasibility and ease of interpretation is still a matter of concern due to the interplay of several factors in determining the final spectral outcome, with larger molecular systems and non-covalent complexes further exacerbating the dichotomy between accuracy and computational cost. In this context, second-order vibrational perturbation theory (VPT2) together with density functional theory (DFT) has become particularly appealing. The well-known problem of the reliability of exchange-correlation functionals, coupled with the treatment of resonances in VPT2, represents a challenge for the determination of standardized or “black-box” protocols, despite successful examples in the literature. With the aim of getting a clear picture of the achievable accuracy and reliability of DFT-based VPT2 calculations, a multi-step study will be carried out here. Beyond the definition of the functional, the impact of the basis set and the influence of the resonance treatment in VPT2 will be analyzed. For a better understanding of the computational aspects and the results, a short summary of vibrational perturbation theory and the overall treatment of resonances for both energies and intensities will be given. The first part of the benchmark will focus on small molecules, for which very accurate experimental and theoretical data are available, to investigate electronic structure calculation methods. Beyond the reliability of energies, widely used for such systems, the issue of intensities will also be investigated in detail. The best performing electronic structure methods will then be used to treat larger molecular systems, with more complex topologies and resonance patterns.

The determination of accurate equilibrium molecular structures plays a fundamental role for understanding many physical-chemical properties of molecules, ranging from the precise evaluation of the electronic structure to the analysis of dynamical and environmental effects in tuning their overall behavior. For this purpose the so-called semiexperimental approach, based on a nonlinear least-squares fit of the moments of inertia associated with a set of available isotopologues, allows one to obtain very accurate results, without the unfavorable computational cost characterizing high-level quantum chemical methods. In the present work the MSR (Molecular Structure Refinement) software for the determination of equilibrium structures by means of the semiexperimental approach is presented, and its implementation is discussed in some detail. The software, which is interfaced with a powerful graphical user interface, includes different optimization algorithms, an extended error analysis, and a number of advanced features, the most remarkable ones concerning the choice of internal coordinates and the method of predicate observations. In particular, a new black-box scheme for defining automatically a suitable set of nonredundant internal coordinates of A1 symmetry in place of the customary Z-matrix has been designed and tested. Finally, the implementation of the method of the predicate observations is discussed and validated for a set of test molecules. As an original application, the method is employed for the determination of the semiexperimental structure for the most stable conformer of glycine.

In this work semi-experimental and theoretical equilibrium geometries of 10 sulfur-containing organic molecules, as well as 4 oxygenated ones, are determined by means of a computational protocol based on density functional theory. The results collected in the present paper further enhance our online database of accurate semi-experimental equilibrium molecular geometries, adding 13 new molecules containing up to 8 atoms, for 12 of which the first semi-experimental equilibrium structure is reported, to the best of our knowledge. We focus in particular on sulfur-containing compounds, aiming both to provide new accurate data on some rather important chemical moieties, only marginally represented in the literature of the field, and to examine the structural features of carbon-sulfur bonds in the light of the previously presented linear regression approach. The structural changes issuing from substitution of oxygen by sulfur are discussed to get deeper insights on how modifications in electronic structure and nuclear potential can affect equilibrium geometries. With respect to our previous works, we perform non-linear constrained optimizations of equilibrium SE structures with a new general and user-friendly software under development in our group with updated definition of useful statistical indicators.

open 12 si MIUR “PRIN 2015” funds (Grant Number 2015F59J3R) By combining rotational spectroscopy in supersonic expansion with the capability of state-of-the-art quantum-chemical computations in accurately determining structural and energetic properties, the genuine nature of a sulfur–sulfur chalcogen bond between dimethyl sulfide and sulfur dioxide has been unveiled in a gas-jet environment free from collision, solvent and matrix perturbations. A SAPT analysis pointed out that electrostatic S⋅⋅⋅S interactions play the dominant role in determining the stability of the complex, largely overcoming dispersion and C−H⋅⋅⋅O hydrogen-bond contributions. Indeed, in agreement with the analysis of the quadrupole-coupling constants and of the methyl internal rotation barrier, the NBO and NOCV/CD approaches show a marked charge transfer between the sulfur atoms. Based on the assignment of the rotational spectra for 7 isotopologues, an accurate semi-experimental equilibrium structure for the heavy-atom backbone of the molecular complex has been determined, which is characterized by a S⋅⋅⋅S distance (2.947(3) Å) well below the sum of van der Waals radii. embargoed_20191010 Obenchain, Daniel A.; Spada, Lorenzo; Alessandrini, Silvia; Rampino, Sergio; Herbers, Sven; Tasinato, Nicola; Mendolicchio, Marco; Kraus, Peter; Gauss, Jürgen; Puzzarini, Cristina; Grabow, Jens-Uwe*; Barone, Vincenzo Obenchain, Daniel A.; Spada, Lorenzo; Alessandrini, Silvia; Rampino, Sergio; Herbers, Sven; Tasinato, Nicola; Mendolicchio, Marco; Kraus, Peter; Gauss, Jürgen; Puzzarini, Cristina; Grabow, Jens-Uwe*; Barone, Vincenzo

Using quantum chemical methods, we investigate the possible outcomes of H- reactions with acetylene and diacetylene molecules. We find both reactions to be exothermic reactions without barriers, yielding stable anions of the corresponding polyynes: C2H- and C4H-. We show in this work that the computed chemical rates in the case of the formation of the C4H- anion would be larger than those existing for the direct radiative electron attachment (REA) process, the main mechanism generally suggested for their formation. In the case of the C2H- anion, however, the present chemical rates of formation at low T are even lower than those known for its REA process, both mechanisms being inefficient for its formation under astrochemical conditions. The present results are discussed in view of their consequences on the issue of the possible presence of such anions in the ISM environments. They clearly indicate the present chemical route to C2H- formation to be inefficient at the expected temperatures of a dark molecular cloud, whereas this is found not to be the case for the C4H-, in line with the available experimental findings.

Ethanimine, a possible precursor of amino acids, is considered an important prebiotic molecule and thus may play important roles in the formation of biological building blocks in the interstellar medium. In addition, its identification in Titan's atmosphere would be important for understanding the abiotic synthesis of organic species. An accurate computational characterization of the molecular structure, energetics, and spectroscopic properties of the E and Z isomers of ethanimine, CH3CHNH, has been carried out by means of a composite scheme based on coupled-cluster techniques, which also account for extrapolation to the complete basis-set limit and core-valence correlation correction, combined with density functional theory for the treatment of vibrational anharmonic effects. By combining the computational results with new millimeter-wave measurements up to 300 GHz, the rotational spectrum of both isomers can be accurately predicted up to 500 GHz. Furthermore, our computations allowed us to revise the infrared spectrum of both E- and Z-CH3CHNH, thus predicting all fundamental bands with high accuracy.

In the present work, the near edge X-ray absorption spectroscopy (NEXAFS) spectra at both C and N K-edges of pyridine, 2-fluoropyridine, and 2,6-difluoropyridine have been studied both experimentally and theoretically. From an electronic point of view, both transition potential density functional theory and time-dependent density functional theory approaches lead to reliable results provided that suitable basis sets and density functionals are employed. In this connection, the global hybrid B3LYP functional in conjunction with the EPR-III basis set appears particularly suitable after constant scaling of the band positions. For the N K-edge, vertical energies obtained at these levels and broadened by symmetric Gaussian distributions provide spectra in reasonable agreement with the experiment. Vibronic contributions further modulate the band-shapes leading to a better agreement with the experimental results, but are not strictly necessary for semi-quantitative investigations. On the other hand, vibronic contributions are responsible for strong intensity redistribution in the NEXAFS C K-edge spectra, and their inclusion is thus mandatory for a proper description of experiments. In this connection, the simple vertical gradient model is particularly appealing in view of its sufficient reliability and low computational cost. For more quantitative results, the more refined vertical Hessian approach can be employed, and its effectiveness has been improved thanks to a new least-squares fitting approach.