Vibrational Spectra of Triiodomesitylene: Combination of DFT Calculations and Experimental Studies. Effects of the Environment

Abstract

A study of the internal vibrations of triiodomesitylene (TIM) is presented. It is known from X-rays diffraction at 293 K that the molecule has nearly D(3h) symmetry because of the large delocalization of the methyl protons. By using Raman and infrared spectra recorded at room temperature, a first assignment is done by comparing TIM vibrations with those of 1,3,5-triiodo- and 1,3,5-trimethyl-benzene. This assignment is supported by DFT calculations by using the MPW1PW91 functional with the LanL2DZ(d,p) basis set and assuming C(3h) symmetry. The agreement between the calculated and experimental frequencies is very good: always better than 97% for the observed skeletal vibrations. The calculations overestimate the methyl frequencies by 7%, and experiment shows only broad features for these excitations. Because a neutron diffraction study had established that the TIM conformation at 14 K is not exactly trigonal, new theoretical calculations were done with C(s) symmetry. This shows that all previous E' and E'' modes of vibration are split by 2-12 cm(-1). This is confirmed by infrared, Raman, and inelastic neutron scattering spectra recorded below 10 K. Apart from two frequencies, all the TIM skeleton vibrations have been detected and assigned by using C(s) symmetry. For the methyl vibrations, experiment has confirmed the splitting of the previously degenerate modes; only some small discrepancies remain in the assignment. This is partly due to the difference of the model conformation used in the calculations and the crystallographic one. All these results confirm that each of the three methyl groups has not only its own tunnel splitting but also a different specific spectroscopic behavior for all the molecular modes.

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

“…In several papers devoted to IR and Raman spectroscopies and reported by some of us, it has been shown that using the so-called anharmonic oscillator (AO) level to compute vibrations is much more accurate than the use of an ad-hoc scale factors and so for a relative weak computational cost. 7,[9][10][11][12][13][14][15][16]24 Furthermore, only full account of both mechanical and electrical anharmonicity allows us to assign the whole IR and Raman spectra (including overtones and combination bands). 11,13 So cubic and semidiagonal quartic force constants have been computed by finite differences of analytical Hessians at the B3LYP-D3 to obtain anharmonic frequencies with the GVPT2 model taking into the proper account possible Fermi and Darling−Dennison resonances, and the symmetry of the molecule.…”

“…Computations have been performed enforcing the C 3 h symmetry. In several papers devoted to IR and Raman spectroscopies and reported by some of us, it has been shown that using the so-called anharmonic oscillator (AO) level to compute vibrations is much more accurate than the use of an ad-hoc scale factors and so for a relative weak computational cost. ,− , Furthermore, only full account of both mechanical and electrical anharmonicity allows us to assign the whole IR and Raman spectra (including overtones and combination bands). , So cubic and semidiagonal quartic force constants have been computed by finite differences of analytical Hessians at the B3LYP-D3 to obtain anharmonic frequencies with the GVPT2 model taking into the proper account possible Fermi and Darling–Dennison resonances, and the symmetry of the molecule. ,,, For the lowest frequencies, anharmonic contributions have been obtained using the so-called hindered rotor (HR) treatment, , which gives access to the HRAO model. , Some extra computations have also been performed using another hybrid functional, namely the MPW1PW91 − in conjunction with the LANL2DZ+polarization basis set on all atoms. − Hereafter, we denote this level of computation as II .…”

“…Below 300 K it crystallizes in the triclinic system; our goal was to establish if the asymmetrical environment is able to perturb the molecular 3-fold symmetry and in particular what are the consequences on the spectroscopic properties. TBM is a derivative member of the restricted group of molecules M 6 for which the barrier hindering the methyl rotation has a 6-fold symmetry among which we may quote several substituted benzenes: toluene (Tol), p -fluorotoluene (pFT), p -xylene (pX), 2,6-difluorotoluene (26dFT), 1,3,5-trimethylbenzene (mesitylene or Mes), and others that have been investigated in the past. − The presence of three symmetrical tops (the three methyl groups) raises the question of their independence with the framework and also of their eventual coupling. Our previous studies of the fully hydrogenated TBM by inelastic neutron scattering (INS) spectroscopy in the range 0.01–100 cm –1 had established that the three methyl groups of TBM are independent and are tunneling at different energies, so the question is to investigate if this disrespect of the 3-fold symmetry is also encountered for other physical properties.…”

Vibrational spectroscopy and DFT calculations of 1,3-dibromo-2,4,6-trimethylbenzene: Anharmonicity, coupling and methyl group tunneling

“…Neutron diffraction studies of the molecular conformations have shown additional consequences: not only the proton probability density but also the orientation differs for each methyl group [12,13,14]. These deviations from the ideal symmetry have visible consequences in the Raman and IR spectra at low temperatures (< 50 K): the inactive modes for C 3h symmetry become visible whereas the previously degenerate modes E" and E" are split [15]. The influence of intra and intermolecular symmetry on the tunneling properties of the methyl groups is still more spectacular in the case of the fully hydrogenated dibromomesitylene (C 9 Br 2 H 10 or DBMH) [16].…”

Symmetry breakings in a metal organic framework with a confined guest

“…In our case, we applied a scaling factor of 0.98 for the frequency range 1000 -1300 cm −1 shown in the present manuscript [28]. This scaling factor corrects partly for the neglect of anharmonic corrections [29][30][31]. Most importantly, the used program allows us to fix the spin state of the system, so that in our case the calculations were conducted in both high spin and low spin states.…”