Interpretation Of Structural Properties Research Articles

Diving for Accurate Structures in the Ocean of Molecular Systems with the Help of Spectroscopy and Quantum Chemistry.

The prediction and interpretation of structural properties are the starting points for a deep understanding of thermochemistry, kinetics, and spectroscopic signatures of molecular systems. To give an example, detailed knowledge of the conformational behavior of the main building blocks of biomolecules in the gas phase (i.e., without the perturbing effect of the environment) is a mandatory prerequisite toward the understanding of the role played by different interactions in determining the biological activity in terms of structure-activity relationships. The first step to take is an unambiguous definition of molecular structure. We address the so-called Born-Oppenheimer equilibrium structure, which is defined in a rigorous manner and isotopically independent, and the target accuracy. For the latter, we aim at so-called "spectroscopic" accuracy, which implies uncertainties of a few milliangstroms for bond lengths and smaller than a tenth of degree for angles. If on one side the continuous enhancements of the experimental techniques give access to new and unprecedented spectroscopic determinations, on the other side they require increasing efforts for an unbiased interpretation and analysis. Among the pieces of information, accurate molecular structures play a particularly important role. Indeed, there is a strong relationship between the experimental outcome and the electronic structure of the system. Spectroscopic techniques, in particular those exploited in the gas phase, are therefore accurate and reliable sources for structural information. However, it is seldom straightforward to derive molecular structures directly from the experimental information. Indeed, even in the favorable case of investigations in the gas phase, vibrational effects are always present, and disentangling their contributions requires collection of information for all vibrational modes, a nearly impossible task. To overcome these limitations, joint theory-spectroscopy strategies can be identified, which are referred to as "top-down" and "bottom-up". The first approach, denoted as the semiexperimental approach, relies on extracting from experimental outcomes the equilibrium structure by using quantum-chemical computations to recover vibrational effects. The bottom-up approach consists in verifying the computed equilibrium geometry by means of a comparison between calculated and experimental spectroscopic parameters that probe structural characteristics. In this contribution, we try to review the most important challenges in accurate molecular structure determinations, with particular emphasis on the "solution" provided by a joint theoretical-experimental approach and on the current state of the art. Starting from the illustration of different strategies, we proceed by addressing the increasing complexity in the derivation of equilibrium geometries: we start from the construction of a database of accurate structures, we then face the problem of extending the dimension of the systems amenable to accurate structural determinations, and finally we move to the challenge of understanding the nature of intermolecular interactions.

Quantum Mechanical Computations and Spectroscopy: From Small Rigid Molecules in the Gas Phase to Large Flexible Molecules in Solution

Interpretation of structural properties and dynamic behavior of molecules in solution is of fundamental importance to understand their stability, chemical reactivity, and catalytic action. While information can be gained, in principle, by a variety of spectroscopic techniques, the interpretation of the rich indirect information that can be inferred from the analysis of experimental spectra is seldom straightforward because of the subtle interplay of several different effects, whose specific role is not easy to separate and evaluate. In such a complex scenario, theoretical studies can be very helpful at two different levels: (i) supporting and complementing experimental results to determine the structure of the target molecule starting from its spectral properties; (ii) dissecting and evaluating the role of different effects in determining the observed spectroscopic properties. This is the reason why computational spectroscopy is rapidly evolving from a highly specialized research field into a versatile and widespread tool for the assignment of experimental spectra and their interpretation in terms of chemical physical effects. In such a situation, it becomes important that both computationally and experimentally oriented chemists are aware that new methodological advances and integrated computational strategies are available, providing reliable estimates of fundamental spectral parameters not only for relatively small molecules in the gas phase but also for large and flexible molecules in condensed phases. In this Account, we review the most significant methodological contributions from our research group in this field, and by exploiting some recent results of their application to the computation of IR, UV-vis, NMR, and EPR spectral parameters, we discuss the microscopic mechanisms underlying solvent and vibrational effects on the spectral parameters. After reporting some recent achievements for the study of excited states by first principle quantum mechanical approaches, we focus on the treatment of environmental effects by means of mixed discrete-continuum solvent models and on effective methods for computing vibronic contributions to the spectra. We then discuss some new developments, mainly based on time-dependent approaches, allowing us to go beyond the determination of spectroscopic parameters toward the simulation of line widths and shapes. Although further developments are surely needed to improve the accuracy and effectiveness of several items in the proposed approach, we try to show that the first important steps toward a direct comparison between the results obtained in vitro and those obtained in silico have been made, making easier fruitful crossovers among experiments, computations and theoretical models, which would be decisive for a deeper understanding of the spectral behavior associated with complex systems and processes.

Toward an integrated computational approach to CW-ESR spectra of free radicals

Interpretation of structural properties and dynamic behaviour of molecules in solution is of fundamental importance to understand their stability, chemical reactivity and catalytic action. Information can be gained, in principle, by a variety of spectroscopic techniques, magnetic as well as optical. In particular, continuous wave electron spin resonance (CW-ESR) measurements are highly informative. However, the wealth of structural and dynamic information which can be extracted from ESR spectroscopy is, at present, limited by the necessity of employing computationally efficient models, which are increasingly complex as they need to take into account diverse relaxation processes affecting the spectrum. In this paper, we address the basic theoretical tools needed to predict, essentially ab initio, CW-ESR spectra observables according to the stochastic Liouville equation (SLE) approach, combined with quantum mechanical and hybrid methods for the accurate and efficient computation of structural, spectroscopic and magnetic properties of molecular systems. We shall discuss, on one hand, the quantum mechanical calculation of magnetic observables, via density functional theory (DFT), time-dependent DFT (TD-DFT) and application of the polarizable continuum model (PCM) for the description of environmental effects, including anisotropic environments and systems where different regions are characterized by different dielectric constants. One the other hand, the explicit evaluation of dynamical effects will be discussed based on the numerically exact treatment of the SLE in the presence of several relaxation processes, which has been proven to be a challenging task.