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

Abstract

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 ...

Computational simulation of vibrationally resolved spectra for spin‐forbidden transitions

“…Simulation of vibronic spectra is carried out by using models based on the FC principle, a good framework to describe transitions of semirigid systems, which do not undergo significant structural changes upon the electronic transition. Herein, the band‐shape can be generated as a sum of individual transitions between vibrational states of the initial and final electronic states in the so‐called time‐independent (TI) approach (details can be found in previous studies). For one‐photon phosphorescence and CPP, the spectrum intensity is computed as where and are the transition integrals between the initial …”

“…This is often insufficient, in particular for sensitive, chiroptical spectroscopies, since experimental band‐shapes actually have a vibrational structure, which may be well visible or partially hidden depending on the quality of the measurements. In recent years, methods of various levels of sophistication have been developed and implemented in standalone or general‐purpose computational chemistry packages to include this information and simulate more reliable, vibrationally resolved electronic spectra, also called vibronic spectra . In this context, some of us have developed a general computational tool for the simulation of different kinds of one‐photon spectroscopies; this tool is based on the harmonic approximation supporting both internal and Cartesian coordinates with the possible inclusion of mode‐mixings, as well as Franck‐Condon (FC) and Herzberg‐Teller (HT) effects .…”

“Divide and conquer” semiclassical molecular dynamics: A practical method for spectroscopic calculations of high dimensional molecular systems

“…[1][2][3][4][5] Alternatively, perturbative quantum methods have also been successfully applied to many systems, but they are intrinsically limited to a single reference geometry. [6][7][8][9][10][11] High dimensional systems, such as peptides, are instead usually simulated through ad-hoc scaled harmonic approaches or by means of classical mechanics, either using force fields [12][13][14] or employing ab initio molecular dynamics (AIMD) [15][16][17][18][19][20][21] approaches in which the nuclear forces are calculated using electronic structure codes. In classical simulations the curse of dimensionality is significantly tamed with respect to quantum mechanical counterparts.…”

Toward a general mixed quantum/classical method for the calculation of the vibronic ECD of a flexible dye molecule with different stable conformers: Revisiting the case of 2,2,2‐trifluoro‐anthrylethanol

“…If the molecule of interest is rigid, meaning that its potential energy surfaces (PESs) can be adequately described within the harmonic approximation, and if its electronic states are not affected by strong nonadiabatic couplings, the simulation of the ECD spectrum is now rather standard . Conversely, when the internal dynamics of the molecules is also characterized by slow, large amplitude motions, e.g., torsions around σ bonds, the accurate calculation of ECD spectra still presents significant challenges.…”