Low frequency internal modes of 1,2,4,5-tetramethylbenzene, tetramethylpyrazine and tetramethyl-1,4-benzoquinone: INS, Raman, infrared and theoretical DFT studies

doi:10.1016/j.saa.2005.06.030

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Volume 63, Issue 3, March 2006, Pages 766-773

https://doi.org/10.1016/j.saa.2005.06.030 Get rights and content

Abstract

The results of inelastic neutron scattering (INS), Raman and infrared (IR) studies on 1,2,4,5-tetramethylbenzene (durene), tetramethylpyrazine (TMP) and tetramethyl-1,4-benzoquinone (TMBQ) in the solid state are reported. The observed frequencies are analyzed on the basis of DFT calculations. The low frequency region, below 400 cm⁻¹, related to the torsional and bending out-of-plane vibrations of the CH₃ groups, is of particular interest. The detailed analysis is possible due to the simulation of the INS spectra by using the auntie-CLIMAX program. It is shown that the observed low frequency INS bands are dramatically shifted, compared to the calculated ones, towards higher frequencies. Although one cannot exclude deficiencies of theoretical methods as applied to low frequency modes, it seems more probable the interpretation based on an existence of non-conventional C single bond H^…π, CH^…N, CH^…O hydrogen bonds formed by the methyl groups in crystalline phases.

Introduction

As follows from the critical reviews, devoted to the applications of the INS technique in the molecular spectroscopy [1], [2], the region of low frequencies, corresponding to the deformation modes with participation of the H-atoms, seems to be of particular interest. This is due to the large incoherent scattering cross-section of hydrogen and the high amplitudes of these vibrations. The methyl derivatives became the subject of our interest, since they are characterized by the high intensities of the CH₃ torsional and C single bond CH₃ wagging modes. The detailed studies on durene [3], [4] seem to be a very good example, which has shown that the INS spectra can be useful in the analysis of the internal and external molecular interactions. The searches for the most useful models, describing the behavior of the modes in condensed matter, appeared most important. However, in our opinion, these models are not sufficiently precise to predict the INS spectra in the low frequency region. On the other hand, as follows from the reviews [5], [6], the torsional potential is related to the rotational barrier which rules the tunnel splitting.

In our strategy, accepted in this paper, we have chosen for comparison three dynamically similar molecules (of the same D_2h symmetry) containing four CH₃ groups in the phenyl ring. The molecules are characterized by different either intra or intermolecular interactions in the crystalline lattice. The packing of the molecules is very well known for 1,2,4,5-tetramethylbenzene (durene) [4], tetramethylpyrazine (TMP) [7], [8] and tetramethyl-1,4-benzoquinone (TMBQ) [9]. In analysis of the solid state of durene, one can distinguish the interaction between methyl groups and π-electrons of the phenyl rings (non-conventional C single bond H–π hydrogen bonds), while in the cases of TMP and TMBQ, there are interactions via CH^…O and CH^…N hydrogen bonds. One should stress here that these interactions are considered as blue-shifting hydrogen bonds. In contrast to conventional hydrogen bonds, the CH^…Y bridges are characterized by an increase of the stretching ν(C single bond H) frequency. Although one cannot exclude other lattice effect upon the dynamics of CH₃ groups, it seems that the direct contacts between the CH bonds and the proton accepting centers are the main factors affecting this dynamics. It is now commonly accepted approach based on theoretical and experimental arguments, to treat such contacts as unconventional hydrogen bonds. There is a rich literature devoted to various aspects of C single bond H^…Y interactions, as follows from reviews [10], [11], [12].

In this paper, we will try to show the effect of the unconventional hydrogen bonds on the frequencies of the CH₃ torsional and C single bond CH₃ wagging modes. The values of the frequencies obtained experimentally for the crystalline state will be compared with those modeled by using the DFT calculations. This comparison yields sufficiently good data with respect to the structure optimization and dynamics [13].

One should emphasize that the INS technique yields the best information enabling the correct assignment of the observed INS peaks to corresponding modes recognized from the DFT calculations. Our approach, presented in this paper, we treat as a first step in recognition of the unconventional hydrogen bonds by analyzing the dynamics of CH₃ groups in a given environment. For more complete recognition, we decided to compare the INS results with those based on Raman and infrared (IR) spectra.

Section snippets

Experimental and calculations

The compounds of 98% purity from Aldrich were used without additional treatments.

Neutron scattering data were collected at the pulsed IBR-2 reactor in Dubna using the inverted time-of-flight spectrometer NERA-PR [14] at 20 K. The spectra were converted from neutron per channel to the S(Q,ω) function density of states. For energies between 5 and 100 meV, the relative INS resolution was estimated to be ca. 3%.

The IR spectra were recorded at room temperature in the KBr discs or in Nujol as well as

Results and discussion

Calculated and experimental INS, IR and Raman frequencies for three investigated compounds are compared in Table 1, Table 2, Table 3. From the collected data, it clearly follows that in the Raman and IR spectra, only few modes are recorded.

Due to either selection rules or extremely low intensities, some of the modes are not active both in Raman and IR spectra. In contrast, practically all modes below 1000 cm⁻¹ are well reflected in the INS spectra. Even more, modes in the region of the lowest

Conclusions

The analysis of modes related to torsional vibrations of the CH₃ groups in tetramethylpyrazine, 1,2,4,5-tetramethylbenzene (durene) and tetramethyl-p-benzoquinone possessing the same symmetry, allows us to distinguish, among seven low frequency modes, four ones, into which neat CH₃ torsional vibrations contribute. These modes are practically not visible in IR and Raman spectra due to either selection rules or extreme low intensities. In contrast to IR and Raman spectra, these modes are

Acknowledgments

A partial financial support by the Polish Ministry of Science and Informatics (Grant 4 T09A 05125) and the Plenipotentiary Representative of Polish Republic in JINR is acknowledged. Thanks are to Dr. B.Czarnik-Matusewicz for fruitful discussion.

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Low frequency internal modes of 1,2,4,5-tetramethylbenzene, tetramethylpyrazine and tetramethyl-1,4-benzoquinone: INS, Raman, infrared and theoretical DFT studies

Abstract

Introduction

Section snippets

Experimental and calculations

Results and discussion

Conclusions

Acknowledgments

Chem. Phys.

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J. Phys. Chem.

Trends Appl. Spectrosc.

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Chem. Rev.

Annu. Rev. Phys. Chem.

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New J. Chem.

J. Chem. Soc.

An Introduction to Hydrogen Bonding