Abstract
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 the role played by dynamical and environmental effects in tuning their overall behavior. For small semi-rigid systems in the gas phase, state-of-the-art quantum chemical computations rival the most sophisticated experimental (from, for example, high-resolution spectroscopy) results. For larger molecules, more effective computational approaches must be devised. To this end, we have further enlarged the compilation of available semi-experimental (SE) equilibrium structures, now covering the most important fragments containing H, B, C, N, O, F, P, S, and Cl atoms collected in the new SE100 database. Next, comparison with geometries optimized by methods rooted in the density functional theory showed that the already remarkable results delivered by PW6B95 and, especially, rev-DSDPBEP86 functionals can be further improved by a linear regression (LR) approach. Use of template fragments (taken from the SE100 library) together with LR estimates for the missing interfragment parameters paves the route toward accurate structures of large molecules, as witnessed by the very small deviations between computed and experimental rotational constants. The whole approach has been implemented in a user-friendly tool, termed nano-LEGO, and applied to a number of demanding case studies.
Highlights
The knowledge of detailed molecular geometries in the gas phase is the mandatory prerequisite for the study of their physical−chemical properties and for the disentanglement of stereo-electronic, vibrational, and environmental effects that define the overall experimental observables.[1−3] accurate geometries of isolated molecules provide the best benchmarks for the validation of different quantum mechanical (QM) approaches[4−9] and for the development of accurate force fields to be used in molecular mechanics (MM)[10] or multi-level QM/MM models for the study of systems too large to be amenable to the most accurate QM studies
Triple-ζ basis sets in conjunction with the B2PLYP double-hybrid functional[28] have been demonstrated to provide accurate predictions of geometries, rotational spectroscopic parameters, and vibrational properties.[7,20,29−32] if not explicitly indicated, both PW6B95 and rev-DSDPBEP86 were always augmented for dispersion contributions by means of the Grimme’s density functional theory (DFT)-D3 scheme[33] with Becke-Johnson damping,[34] even if the bare PW6B95 functional can already provide a satisfactory description of dispersion forces.[35]
LRA was originally developed for the B3LYP/SNSD42−44 model chemistry based on a set of 47 SE equilibrium structures,[19] which was later extended by new SE equilibrium geometries and employed to parameterize LRA for the B2PLYP/cc-pVTZ level of theory.[20]
Summary
The knowledge of detailed molecular geometries in the gas phase is the mandatory prerequisite for the study of their physical−chemical properties and for the disentanglement of stereo-electronic, vibrational, and environmental effects that define the overall experimental observables.[1−3] accurate geometries of isolated molecules provide the best benchmarks for the validation of different quantum mechanical (QM) approaches[4−9] and for the development of accurate force fields to be used in molecular mechanics (MM)[10] or multi-level QM/MM models for the study of systems too large to be amenable to the most accurate QM studies. Accurate equilibrium structures can be computed by means of high-level post-Hartree−Fock approaches, with the coupled-cluster model including single and double excitations together with a perturbative inclusion of triples, CCSD(T), being the so-called “gold standard” for molecules not involving too strong static correlation effects.[11] this approach shows a very unfavorable scaling with the dimension of the investigated system, especially when complete basis set extrapolation and core−valence contributions are taken into account. The direct experimental outcomes are the rotational constants, which are proportional to the inverse of the inertia moments in the Eckart frame. Since they depend on both the coordinates and the masses of the atoms in the molecule, measurements performed for a sufficient number of isotopologues provide the information needed for determining all the averaged geometrical parameters of the corresponding vibrational states. In order to move to the equilibrium configuration, vibrational contributions need to be considered and the rotational constants of the equilibrium geometry have to be Received: August 5, 2021 Published: October 20, 2021
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
