Accurate molecular structures and infrared spectra of trans-2,3-dideuterooxirane, methyloxirane, and trans-2,3-dimethyloxirane

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In this tutorial review, we present some effective methodologies available for the simulation of vibrational and vibrationally resolved electronic spectra of medium-to-large molecules. They have been integrated into a unified platform and extended to support a wide range of spectroscopies.The resulting tool is particularly useful in assisting the extensive characterization of molecules, often achieved by combining multiple types of measurements. A correct assessment of the reliability of theoretical calculations is a necessary prelude to the interpretation of their results. For this reason, the key concepts of the underlying theories will be first presented and then illustrated through the study of thiophene and its smallest oligomer, bithiophene. While doing so, a complete computational protocol will be detailed, with emphasis on the strengths and potential shortcomings of the models employed here. Guidelines are also provided for performing similar studies on different molecular systems, with comments on the more common pitfalls and ways to overcome them. Finally, extensions to other cases, like chiral spectroscopies or mixtures, are also discussed. K E Y W O R D Sanharmonic vibrational spectroscopy, computational spectroscopy, large amplitude motion, medium-to-large molecules, vibronic spectroscopy | I N T R O D U C T I O NMolecular properties are commonly probed with a wide panel of spectroscopies, which can be combined to offer a unique picture, reflection of the property, its environment, and the experimental conditions. [1][2][3][4][5][6][7][8] The wealth of information contained in the recorded spectrum or spectra can become complex to analyze in a purely phenomenological way and theoretical models can provide a valuable aid in understanding the origin of the features observed in the band-shapes. [9][10][11] As a matter of fact, the predictive and interpretative power of computational spectroscopy has been clearly demonstrated already for small molecular systems in the gas phase, by comparison with the most sophisticated experimental techniques. [1,[12][13][14] However, depending on the target system and the required accuracy, the theoretical spectroscopic models can vary greatly in complexity. The situation is even more challenging when considering their actual implementation, often done in ad hoc, standalone programs, whose levels of completion, versatility and ease of use may be very different. As a result, getting an extensive picture of the experimental spectra can become a challenge as it may require multiple packages with their own usage and peculiarities.Hence, to facilitate the simulation and interpretation of multiple spectra, several conditions need to be met. A first challenge is to provide a simple interface, lowering the barrier to the broad adoption of state-of-the-art methods able to produce accurate data for the problems at hand. [11,[15][16][17] This can be achieved by automatizing most aspects of the calculations. Due to the inherent complexity of some theoretical models, such ...

“…The most accurate structures, both at equilibrium and with the vibrational corection, are obtained at the B2PLYP(‐D3BJ)/maug‐cc‐pVTZ level, with MAE and

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Section: Resultssupporting

confidence: 56%

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Aiming at an accurate prediction of vibrational and electronic spectra for medium‐to‐large molecules: An overview

Bloino

Baiardi

Biczysko

2016

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182

196

“…A scheme analogous to the geometry‐based correction (here denoted “wavenumber‐based correction”) has been recently applied to calculate the frequencies of the methyl cyclopropyl cation, with Equation adjusted accordingly. The capability to extrapolate frequencies by combining analytical Hessians of arbitrary components, or by finite differences of the component gradients, is missing from most computational packages (with the notable exception of Cfour ), especially if more complex corrections (such as the effects of diffuse functions in basis sets and all‐electron correlation in the so‐called “cheap” scheme, or even relativistic Hamiltionians) are required. The update to the cbs () routine of Psi4 performed as part of the current work brings this functionality to this open‐source program.…”

Section: Resultsmentioning

confidence: 99%

Validating additive correction schemes against gradient‐based extrapolations

Kraus

Frank

2019

The use of additive correction schemes to obtain structures and vibrational frequencies of increasingly larger molecules is becoming more common. Such approaches, based on the cubic extrapolation formula applied directly to the quantity of interest, have been successfully validated only at the highest levels of computational accuracy: for coupled cluster methods with comparably large basis sets. Here, a systematic validation of geometries and vibrational frequencies is carried out, including more affordable and relevant levels of theory, such as the Møller‐Plesset perturbation theory applied with smaller basis sets. Comparisons of such additive schemes against the more rigorous gradient‐based extrapolation are presented. The cbs() routine of the open‐source quantum‐chemistry package Psi4 has been extended for this purpose. The results confirm that geometries and frequencies of covalently bound species obtained with additive correction schemes are in an excellent agreement with the results of gradient‐based extrapolations. However, when applied to systems involving noncovalent interactions, the errors due to such schemes are significantly larger. In general, we propose the application of gradient‐based extrapolations, as they incur no extra cost compared to additive schemes.

“…The approach here summarized is also denoted as “cheap geometry” scheme to stress its reduced computational cost. While there is no theoretical justification for the inclusion of diffuse function effects (Δ r (aug)) once extrapolation to the CBS limit is performed, the latter correction is often introduced to recover on an empirical basis the limitations affecting extrapolation procedures carried out with small‐ to medium‐sized basis sets.…”

Section: Discussionmentioning

confidence: 99%

Accurate molecular structures of small‐ and medium‐sized molecules

Puzzarini

2016

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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, ...