Hexasubstituted chloromethylbenzenes. Vibrational spectra and normal coordinate analysis

Bussian, Bernd M.; Eysel, H. H.

doi:10.1016/0584-8539(85)80125-2

Cited by 5 publications

(5 citation statements)

References 29 publications

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“…motions are labeled as follows:  n = out-of-plane bending of a C n -X n bond, it is also named wagging [2],  i = out -of-plane benzene ring torsion of a C i -C i+1 bond and  p = sum of out-of-plane torsion of the methyl C-H bonds around the C p -C mp bond. For deformations inside the methyl groups, r" mp = C mp -H pi for the o-p methyl group stretching (i = a, b or c) when the atom C mp and H pi move out of the ring plane P ring , " p = sum of angles for the C p -H pi -H pi+1 variations, " p = sum of angles for the variations C p -C mp -H i .…”

Section: Labelling Of the Internal Coordinates Used To Describe The Vmentioning

confidence: 99%

“…Among those relative to penta-and hexa-substituted compounds, two extensive papers were published in 1985: one paper compared eleven polymethyl-benzenes (pMeB) [1] and the second described six chloromethyl-benzenes C 6 (CH 3 ,Cl) 6 (ClMeB) [2]. In these studies, the experimental spectra were assigned using normal coordinate calculations to determine a common multi-parameter force field.…”

Section: -Introductionmentioning

confidence: 99%

“…There are large differences between the six intracyclicangles, these angles vary from 115.8° for C 1 -C 2 -C 3 to 123.6° for C 2 -C 3 -C 4 . Thus, using a mean value of 120° as was used in [1,2] should be a very crude hypothesis. Fig.…”

mentioning

confidence: 99%

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

Zeroual

Meinnel

Łapiński

et al. 2013

Vibrational Spectroscopy

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Abstract-The Raman, IR and INS spectra of 1,3-dibromo-2,4,6-trimethylbenzene (DBMH) were recorded in the 80-3200 cm -1 range. The molecular conformation and vibrational spectra of DBMH were computed at the MPW1PW91/LANL2DZ level. Except for the methyl 2 environment, the agreement between the DFT calculations and the neutron diffraction structure is almost perfect (deviations < 0.01 Å for bond lengths, < 0.2° for angles). The frequencies of the internal modes of vibration were calculated with the harmonic and anharmonic approximations; the later method yields results that are in remarkable agreement with the spectroscopic data, resulting in a confident assignment of the vibrational bands. Thus, no scaling is necessary. The coupling, in phase or antiphase, of the motions of symmetrical C-Br and C-Me bonds is highlighted. Our DFT calculations suggest that the torsion of methyl groups 4 and 6 is hindered in deep wells, whereas methyl group 2 is a quasi-free rotor. The failure of the calculations to determine the frequencies of the methyl torsional modes is explained as follows: DFT does not consider the methyl spins and assumes localization of the protons, whereas the methyl groups must be treated as quantum rotors.Keywords: Raman spectroscopy; IR spectroscopy; Inelastic neutron scattering; DFT calculations; 1,3-dibromo-2,4,6-trimethylbenzene 1-IntroductionA tremendous amount of work has been devoted to the spectroscopy of polysubstituted benzenes.Among those relative to penta-and hexa-substituted compounds, two extensive papers were published in 1985: one paper compared eleven polymethyl-benzenes (pMeB) [1] and the second described six chloromethyl-benzenes C 6 (CH 3 ,Cl) 6 (ClMeB) [2]. In these studies, the experimental spectra were assigned using normal coordinate calculations to determine a common multi-parameter force field. Because the molecular conformations were not accurately known the authors assumed that the aromatic ring in all molecules was a regular hexagon, that the bond angles were 120°, and the bond lengths were 1.396 Å for the carbon-carbon aromatic bond, 1.509 Å for the methyl carbonaromatic carbon bond, 1.083 Å for the aromatic carbon-hydrogen bond and 1.096 Å for the methylcarbon hydrogen bond. For the pMeB species, the Raman and infra-red measurements were performed with samples in the gaseous phase at approximately 550 K. A 46-parameter valence force field had been proposed, and the average error between the 736 observed and calculated wavenumbers was 6.4 cm -1 . This agreement suggests that the use of an excessive number of off-diagonal constants (interactions terms) allows us to compensate for the large errors done in the geometrical parameters. According to the authors in these previous studies: "there is a great stability of the internal methyl modes, in a limit of ± 5 cm -1 they are located at: 2930 cm -1 for the C-H stretching, 1445 cm -1 for the asymmetric C-H bending, 1380 cm -1 for the C-H symmetric bending, 1000 and 1060 cm -1 for the C-H rocking". For the "torsion mode" of the meth...

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Section: Labelling Of the Internal Coordinates Used To Describe The Vmentioning

confidence: 99%

Section: -Introductionmentioning

confidence: 99%

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

Zeroual

Meinnel

Łapiński

et al. 2013

Vibrational Spectroscopy

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“…The remaining modes (1A″ and 1E″) correspond to the torsions of the methyl groups that are always located below 200 cm −1 . As a conclusion of the assignment of the HCIMBz spectra, the authors had noticed that “the internal stretching, bending and rocking modes of the methyl groups show an extraordinary stability through the series,” and it was added that “methyl C−H bonds are stretching around 2900 cm −1 , asymmetric bending appears near 1445 cm −1 and symmetric ones near 1379 cm −1 , rocking motions have a frequency near 1064 cm −1 for in plane modes and near 1000 cm −1 for out of plane modes.” This will also be verified for C 6 I 3 (CH 3 ) 3 .…”

Section: Assignment Of the Vibrational Spectra Of Tim At 293 K By Ass...mentioning

confidence: 99%

“…A tremendous amount of works has been devoted to the spectroscopy of polysubstituted benzenes; among them, we will mention two extensive papers published in 1985: one comparing spectroscopic results on 12 polymethyl-benzenes 33 and one studying six chloromethylbenzenes HClMB. 34 In these studies, the experimental spectra were assigned by using normal coordinate calculations in order to determine a 46-parameter modified valence force field. A similar treatment was carried out in our laboratory for tribromo and triiodomesitylene, 19,24 but it has shown a poorer accuracy than that obtained with quantum chemistry programs.…”

Section: Position Of This Work Relative To the Present Knowledge On P...mentioning

confidence: 99%

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

et al. 2008

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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.

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Properties of hexasubstituted benzenes with one or two nitro groups. Dielectric, DSC, X-ray and Raman studies

Kołodziej

Orzechowski

Szostak

et al. 1996

Journal of Molecular Structure

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Hexasubstituted chloromethylbenzenes. Vibrational spectra and normal coordinate analysis

Cited by 5 publications

References 29 publications

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

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

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

Properties of hexasubstituted benzenes with one or two nitro groups. Dielectric, DSC, X-ray and Raman studies

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