Large-amplitude motion in 1,4-cyclohexadiene and 1,4-dioxin: theoretical background for joint treatment of spectroscopic, electron diffraction and ab initio data

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.…”

“…If the LAM energy is much greater than the energy of rotation of the molecule as a whole, the contribution of molecular rotation in the Hamiltonian for both above motions can be neglected [53]. This condition is satisfied in the case of hindered internal rotation.…”