Accurate molecular structures of small‐ and medium‐sized molecules

Abstract

Molecular structure is one of the most relevant concepts in chemistry. It plays a central role in determining molecular and spectroscopic properties: a mandatory prerequisite for a thorough understanding of the chemical and physical properties of molecules is in fact represented by the knowledge of their geometrical structures. While in some fields a qualitative description of the molecular structure might be sufficient, in many others, like for example spectroscopy, a quantitative, and accurate determination is mandatory. Nowadays, the most advanced computational methodologies allow reliable structural predictions able to fulfil the proper accuracy requirements.This contribution provides an overview on this topic, focusing on the computational strategies that permit accurate equilibrium structure determinations for systems ranging from small molecules to medium-sized building-blocks of biomolecules. K E Y W O R D Scomposite scheme, equilibrium structure, semiexperimental equilibrium geometry, quantum chemistry | I N T R O D U C T I O NThe determination of molecular structure is one of the principal aims in many areas of chemistry. To give an example, in physical chemistry the prediction and interpretation of structural properties and dynamic behavior of molecules are unavoidable for a deeper understanding of their stability and chemical reactivity. To give a more specific example, in the field of biomolecules (amino acids, nucleic bases, carbohydrates, etc.), a detailed knowledge of the conformational behavior of their main building blocks 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. Moving to molecular spectroscopy, there is a strong relationship between the experimental outcome and the electronic structure of the system (see e.g., references 1 and 2). Spectroscopic techniques, in particular those exploited in the gas phase, are therefore accurate and reliable source for structural information. However, it is seldom straightforward to derive molecular structure from the experimental information and quantum chemistry is often required to either support or complement such determinations (see e.g., references 1 and 2). Nowadays, computational methodologies greatly facilitate the prediction of molecular structures and of the corresponding spectroscopic signatures, which in turn can be used to verify the reliability and accuracy of the theoretical predictions. [3] As far as quantum-chemical calculations are concerned, the accuracy that can be reached depends on the proper account of electron correlation and basis-set convergence. [4] For small-to medium-sized molecules, in the absence of large multireference character, it is nowadays well established that highly accurate structural parameters can be obtained by employing coupled-cluster (CC) techniques. Composite schemes have been formulated where CC methods are used in conjunction with hierarchical series of basis sets, ...

Highly accurate equilibrium structure of the C2h symmetric N1‐to‐O2 hydrogen‐bonded uracil‐dimer

“…Over the last years serious efforts have been put into researching approximate, yet reliable, schemes to obtain high‐level ab initio structures that promise a similar sense of “gold‐standard” accuracy as the CCSD(T) method assures for single‐reference ground state energies . It was shown that additive composite schemes for geometries converge to the results from a direct high‐level calculation to satisfactory accuracy.…”

“…It was shown that additive composite schemes for geometries converge to the results from a direct high‐level calculation to satisfactory accuracy. For the composite approaches one distinguishes between “gradient” and “geometry” schemes . The theoretically more rigorous “gradient” scheme sums all separable components (e.g., HF gradient, MP2 gradient, CCSD(T) gradient, etc.)…”

Correct Modeling of Cisplatin: a Paradigmatic Case

“…[6,7,9,10] These studies are often contradictory and only semi-quantitative predictions have been achieved, likely because experimental outcomes obtained using solid compounds have been simulated through calculations performed on molecules in the gas phase.S uch an approach neglects intermolecular interactions,e specially Thea im of the present study is to demonstrate that an improved description of the physicochemical properties of cisplatin can be achieved by adding some physical flavors to the model. Thep roposed approach relies on recovering the most important intermolecular interactions occurring in the solid phase by using the cis-[Pt(NH 3 ) 2 Cl 2 ]d imer for the simulation, and on introducing af ull quantum-mechanical treatment of anharmonic effects.Inaddition, it is shown that, when the model does not grasp the essential physical elements,i mproving the electronic structure treatment is useless.In the last decade,t he CCSD(T) method [11] has become the "gold standard" for the accurate prediction of structural, thermochemical, and spectroscopic properties.E ven more accurate results,r ivaling the most refined experimental techniques,c an be obtained by employing composite schemes.[12] We now have determined the equilibrium structure and harmonic vibrational frequencies of cisplatin with a"cheap" computational protocol, [13] which is detailed in the Supporting Information (SI). As to vibrational frequencies, best-estimated harmonic values have been combined with anharmonic effects at the DFT level (B3PW91-D3 [14] and B2PLYP-D3…”

“…[12] We now have determined the equilibrium structure and harmonic vibrational frequencies of cisplatin with a"cheap" computational protocol, [13] which is detailed in the Supporting Information (SI). As to vibrational frequencies, best-estimated harmonic values have been combined with anharmonic effects at the DFT level (B3PW91-D3 [14] and B2PLYP-D3…”

Theoretical and computational chemistry in Italy

“…Accurate geometric structures are mandatory for the computation of reliable physical‐chemical properties and also provide the most important contributions (via inertia moments) to microwave spectra. In her contribution, Cristina Puzzarini analyzes the status and perspectives of accurate structure computations for molecules of increasing dimensions . Computational spectroscopy is becoming more and more important due to the difficulty of interpreting experimental results without the help of quantum mechanical simulations and to the continuous increase of accuracy and feasibility of this kind of computations also for non‐specialists.…”