Anharmonic Computations Meet Experiments (IR, Raman, Neutron Diffraction) for Explaining the Behavior of 1,3,5-Tribromo-2,4,6-trimethylbenzene

Abstract

In the present paper we first show the experimental Raman, infrared, and neutron INS spectra of tribromomesitylene (TBM) measured in the range 50-3200 cm(-1) using crystalline powders at 6 or 4 K. Then, the bond lengths and angles determined by neutron diffraction using a TBM single crystal at 14 K are compared to the computed ones at different levels of theory. Anharmonic computations were then performed on the relaxed structure using the VPT2 approach, and for the lowest normal modes, the HRAO model has led to a remarkable agreement for the assignment of the experimental signatures. A particularity appears for frequencies below 150 cm(-1), and in particular for those concerning the energy levels of "hindered rotation" of the three methyl groups, they must be calculated for one-dimensional symmetrical tops independent of the frame vibrations. This fact is consistent with the structure established by neutron diffraction: the protons of the methyl groups undergoing huge "libration" motions are widely spread in space. The values of the transitions between the librational levels determined by inelastic neutron scattering indicate that the hindering potentials are mainly due to intermolecular interactions different for each methyl group in the triclinic cell.

Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer

“…Computational techniques based on second-order vibrational perturbation theory (VPT2) suitable for medium-size molecular systems have been implemented only recently and were found to considerably improve the match between theory and experiment and to enable assignments of elusive spectral features. 10,11,12,13,14 We report time-resolved FSRS of ground-and lowest electronic excited states of fac-ReCl(CO) 3 (bpy) (bpy = 2,2'-bipyridine) measured across the full chemically relevant range (100-4000 cm -1 ), together with their anharmonic DFT assignments. ReCl(CO) 3 (bpy) is a prototypical member of the rhenium-carbonyl-polypyridine family of versatile redox photosensitizers/photooxidants and luminescent imaging agents.…”

“…It predicts equal energy spacing between vibrational levels and neglects many vibrational couplings and is thus unsuitable for interpretation of diagonal and off-diagonal anharmonicities and cross-peaks in 2-dimensional IR spectra. , Anharmonic vibrational calculations overcome these problems. Computational techniques based on second-order vibrational perturbation theory (VPT2) suitable for medium-size molecular systems have been implemented only recently and were found to considerably improve the match between theory and experiment and to enable assignments of elusive spectral features. − …”

Sum-frequency vibrational spectroscopy of methanol at interfaces due to Fermi resonance

“…69 Based on optimization, we calculate the quadratic force constants to obtain the harmonic frequencies, and compute the cubic and quartic force constants and apply the VPT2 (second-order vibrational perturbation theory) and GVPT2 (generalized VPT2 with adequate dealing with resonances) method developed by Barone et al to achieve the anharmonic vibrational frequencies. 70–83 When calculating the vibrational frequencies, we obtain the first- and second-order derivatives of the electric dipole moments and polarizabilities. We perform the quantum chemistry computations using Gaussian 16 software.…”

“…Barone et al to achieve the anharmonic vibrational frequencies. [70][71][72][73][74][75][76][77][78][79][80][81][82][83] When calculating the vibrational frequencies, we obtain the first-and second-order derivatives of the electric dipole moments and polarizabilities. We perform the quantum chemistry computations using Gaussian 16 software.…”

In Situ Calculation of the Rotation Barriers of the Methyl Groups of Tribromomesitylene Crystals: Theory Meets Experiment

“…It is expected that the hindering potential of the Me group is largely enhanced when molecules go from the gas phase or are isolated in a cage to a crystal, due to packing effects. In order to study the hindering potential experienced by the methyl rotors, methyl proton tunneling of halogeno-methyl benzenes in the solid state has been extensively studied experimentally and theoretically for a long time, particularly by Professor Jean J. Meinnel and his coworkers [2][3][4][5][6][7][8]. In a recent paper, they investigated the methyl rotation barriers in 1,3-dibromo-2,4,6-trimethyl-benzene [9].…”