Electron Diffraction Analysis for the Molecules with Multiple Large-Amplitude Motions. 3-Nitrostyrene—A Molecule with Two Internal Rotors

Abstract

Dynamic structural analysis of the molecules possessing large-amplitude degrees of freedom has been attempted by many researchers; however, so far, electron diffraction investigations involved only one large-amplitude coordinate (internal rotation or bending). The current state of computational facilities allows extending of the general dynamic approach to the systems possessing two or more large-amplitude motions. This paper presents the first practical implementation of the theoretical method developed previously by the authors for solving the dynamic-structural problem with two or more large-amplitude coordinates; the procedure is applied to a molecule of 3-nitrostyrene. The molecule is represented as a set of pseudoconformers built on a two-dimensional grid corresponding to both internal rotation coordinates present in the molecule (with 10-30° steps by each angle); altogether, up to 342 pseudoconformers were used. Structural analysis was based on the experimental electron diffraction data supported by quantum chemical calculations (at the MP2 and B3LYP levels of theory) and molecular spectroscopy data. Quantum chemistry predicts the planar structure of both syn- and anti- stable conformations with close energies and weak interaction between internal rotations of nitro and vinyl groups. The gas-phase electron diffraction experimental data are compatible with the quantum chemical predictions. The principal equilibrium geometry parameters of the molecule (syn- conformation) have been determined as follows: r(e)(C-C)(ring, avg.) = 1.391(1) Å, r(e)(C-C) = 1.477(5) Å, r(e)(C═C) = 1.333(7) Å, r(e)(C-N) = 1.463(5) Å, r(e)(N═O) = 1.227(3) Å, ∠e(O═N═O) = 124.3 (4)°. Experimental data for this molecule are insufficient to make estimates of the barrier heights of internal rotation; the population ratio of syn- and anti- conformations is evaluated as 50 ± 20%. Results of our investigation confirm the presence of significant internal rotations in the 3-nitrostyrene molecule.

IR spectra of nitrobenzene and nitrobenzene-15N in the gas phase, ab initio analysis of vibrational spectra and reliable force fields of nitrobenzene and 1,3,5-trinitrobenzene. Investigation of equilibrium geometry and internal rotation in these simplest aromatic nitro compounds with one and three rotors by means of electron diffraction, spectroscopic, and quantum chemistry data

“…General molecular model is applicable to both spectroscopic and structural studies making possible their joint treatment. This approach presupposing analysis of the molecular Hamiltonian and adiabatic separation of different molecular degrees of freedom was initially developed for semirigid molecular systems with SAM and later extended to nonrigid polyatomic molecules with one or more coordinates of LAM [51][52][53][54][55][56]. The basic idea of subdividing molecular translations, rotations, and vibrations is supplemented with rigorous adiabatic separation of the LAM coordinates.…”

“…[73,74]. Structural analysis of the GED data jointly with the spectroscopic experimental data and quantum chemical calculation results has been performed with the SYMM/DISP/ELDIFF/LARGE software package [51][52][53][54][55][56], whose molecular model is based entirely on the parameters of the PES and advantage of which in the investigation of structure of flexible molecules is briefly characterized above. The necessary ab initio calculations, including the calculation of quadratic and cubic force constants, have been performed by the MP2(full) method using the aug-cc-pVTZ and ccpVTZ basis sets.…”

“…The barrier height value in kcal/mol is given in braces, DZPE is the difference of zero-point vibrational energies for equilibrium and transition states f The proposed estimate Formulation of general problem on GED analysis for nonrigid molecules using potential procedure Within the framework of the potential procedure [49,50], general formulation of the problem of GED data analysis for molecules with one or more LAM coordinates as well as the resulting equations and methods of their solution were presented in Refs. [51][52][53][54][55][56]. Here, we give a brief description of the approach and its main distinctions for the case of symmetric internal coordinates of LAM.…”

“…The potential procedure uses adiabatic separation of motions of these types described strictly and in detail in Refs. [53,56]. The Hamiltonian of the system is divided into the ''vibrational'' part determined by SAM and the part connected with LAM.…”

Internal rotation and equilibrium structure of 2-chloro-3-nitrothiophene from gas electron diffraction and quantum chemistry

“…The equilibrium C e N bond length equal to 1.438(3) Å is small compared to nitrobenzene (1.468(5)Å, [18]) and 3-nitrostyrene (1.463(5)Å, [9]) molecules. The NO 2 group is a specifically rigid structure.…”