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

Abstract

The IR spectra of gaseous nitrobenzene (NB) and its 15 N isotopomer have been obtained in the frequency range of 3500-250 cm -1 , and the far-IR spectra of their solutions and the NB neat liquid sample have been recorded in the range of 600-30 cm -1 . A detailed description of the spectra of NB and 1,3,5-trinitrobenzene (sym-TNB) and their isotopomers has been accomplished using the force fields calculated at the MP2(full)/aug-cc-pVTZ and MP2(full)/cc-pVTZ levels. Transferability of the refined scale factors for the calculated force constants obtained by the Pulay technique has been used to provide evidence for validity of both interpreting the NB and sym-TNB spectra and refining the calculated force fields. The direct and inverse spectral problems have been solved by variational technique to determine torsional energy levels and refine the potential function by optimizing the V k coefficients in its Fourier series expansion. The height of the MP2(full) barrier to internal rotation has been reduced from 5.5 to 4.5 kcal/mol due to extension of the used basis set from 6-31(d,p) to aug-cc-pVTZ. The method of joint dynamic structural analysis of the GED, MW, and vibrational spectroscopy, and ab initio data in terms of the PES parameters have been applied to investigation of equilibrium geometry and internal rotation in the nonrigid NB and sym-TNB molecules having one and three coupled internal rotors, respectively. The experimental r e -parameter values of both molecules (C 2v and D 3h point group symmetries) are in agreement with those obtained by ab initio calculations. This paper is dedicated to Professor Magdolna Hargittai on the occasion of her 70th birthday.

Ultrafast Dynamics of Excited Electronic States in Nitrobenzene Measured by Ultrafast Transient Polarization Spectroscopy

“…The process of impulsive stimulated Raman scattering (ISRS) can compete with two-photon absorption of the pump pulse. Most intramolecular Raman modes in nitrobenzene lie beyond the bandwidth of our pump pulse (∼80 meV); however, a few within the bandwidth are of either weak or moderate strength . The relevant moderate strength bands have frequencies at ∼76 meV (611 cm –1 ), 49 meV (394 cm –1 ), and 22 meV (176 cm –1 ).…”

Tetranitromethane: A Nightmare of Molecular Flexibility in the Gaseous and Solid States

“…However,t he four lowest vibrational frequencies (MP2/cc-pVTZ) are located at 57 (E, doubly degenerate), 64 (A), and 84 cm À1 (B), and all correspond to internal rotations of the nitro groups.T he next higher energetic vibration is found at asignificantly higher frequency of 174 cm À1 .T hese data were also supported by vibrational spectroscopy [18] and show that the torsional motions can be adiabatically separated in afourdimensional (4D) dynamic model. [26] Such dynamic models are typically constructed using energyweighted pseudo-conformers. [26] Such dynamic models are typically constructed using energyweighted pseudo-conformers.…”

“…However, this is all but trivial, and to date,t here are less than ten published 2D models [22][23][24][25] and only one 3D model in GED studies. [26] Such dynamic models are typically constructed using energyweighted pseudo-conformers. Thep roblem in the case of TNM is that scans of the potential energy surface (PES) along the torsional coordinates about the C À Nbonds are computationally not feasible.T his is due to the complicated correlation of these motions,which all depend on one another.…”

Tetranitromethan – ein Albtraum molekularer Flexibilität in Gasphase und Festkörper

“…Man beachte,d ass die vier niedrigsten Schwingungsfrequenzen (MP2/cc-pVTZ) bei 57 (E, doppelt entartet), 64 (A) und 84 cm À1 (B) liegen. [26] Solche dynamischen Modelle werden üblicherweise aus energiegewichteten Pseudokonformeren aufgebaut. Die nächsthçhere Frequenz ist erst bei 174 cm À1 zu finden.…”

“…Die Torsionsbewegungen sind also in einem vierdimensionalen (4D) dynamischen Modell adiabatisch abtrennbar.D ies ist jedoch alles andere als trivial, und bis heute fanden weniger als zehn 2D-Modelle [22][23][24][25] und nur ein einziges 3D-Modell in GEB-Studien Anwendung. [26] Solche dynamischen Modelle werden üblicherweise aus energiegewichteten Pseudokonformeren aufgebaut. Im Fall des TNM ist aber das Abtasten der Potentialfläche (PES) entlang der Torsionskoordinaten um die C-N-Bindungen rechnerisch nicht mehr mçglich, was an der komplizierten Wechselbeziehung dieser Bewegungen liegt, die allesamt voneinander abhängen.…”