Spectroscopy, Internet Invited Lecture

INVESTIGATION OF THE EFFECT OF HYDROGEN BONDING IN MOLECULAR CRYSTALS ON ABSORPTION AND RAMAN SPECTRA

O.P.Cherkasova(1), I.N.Smirnova(2), M.M.Nazarov(2),
E.V.Fedulova(2), A.V.Kargovsky(2), A.P.Shkurinov(2)

(1) Institute of Laser Physics SB RAS, Novosibirsk, Russia
(2) Departament of Physics and International Laser Center, Moscow State University, Moscow, Russia

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ABSTRACT

The bands observed in vibrational spectra of molecular crystals in THz range correspond both to external and to internal vibrational modes. The vibrational bands are determined both by structure of molecule itself and by the structure of crystal. The estimation of roles of external and internal impacts in observed vibrational bands and their assignment are among the main topics of low-frequency spectroscopy. The corticosteroids are aggregated into the molecular crystals of space group P212121. The molecules could be bound by van der Waals and/or stronger hydrogen forces in molecular crystals. The influence of a crystal lattice motions on the THz spectra can be analyzed and the spectral features associated molecular motions can be highlighted. Simultaneous application of time-domain method for absorption spectra (THz-TDS) and Raman spectra as well as the use of quantum chemical calculations allow performing a complete analysis of vibrational spectra of molecular crystals in THz range. The aim of this work is study absorption and Raman spectra of progesterone, 17α-hydroxyprogesterone and cortisone in the frequency range 3-110 cm-1 in the temperature range from 18 K to 300 K and to estimate the role of hydrogen bonding in molecular crystals on hormones spectra. The density functional theory (DFT) calculations are used for the qualitative interpretation of the experimental data. All observed bands were assigned using DMol3 simulations.

We used the substances in the form of poly-crystalline powders without further purification purchased from Koch-Light Laboratories Ltd, UK. The description of the THz-TDS apparatus was given in [1]. The Raman spectra were recorded as in [2].

We obtained that substances under study have several intense spectral features, which were assigned using DMol3 simulations. The estimation of external (translations and librations) and internal (bending and twisting) impacts in the observed vibrational bands were made. We observed the different temperature dynamics of the THz and Raman band positions when the temperature goes down. The types of bonds that the molecules can form with their neighbors are different in our substances. So the absence of hydrogen bonds in progesterone can explain high mobility of the molecules in this crystal and maximum band shifts. In the crystal of 17α-hydroxyprogesterone there is one hydrogen bond that leads to a rigid position of molecules within the layer and to the absence of band shifts. The range of translations gets narrow and the range of librations shifts to the lower frequencies. Cortisone crystal has two different types of hydrogen bonds which lead to a weakening of tight coupling in the layer and to the appearance of coupling between the layers. Those molecules move one against another in the plane hardly and there is only two possible translation modes, and even in the low frequency region all molecules are bound together so hard, that any libration movement leads to twisting. The maximal temperature shift is observed for twisting motions.

Having assigned intermolecular resonances with numerical simulation and transmission experiments with polycrystalline samples we returned to solution experiments (with ethanol solvent) and showed that even known intermolecular resonances are not resolvable in solutions in THz transmission spectroscopy. Careful comparison of smooth spectra amplitude and shape changes allows to estimate concentration level of “something” in the solvent. Further investigations of biomolecules in natural environment (water solution) should be done.

The absorption spectra solid state substances in THz frequency range are found to be sensitive to small structure changes of the molecules and to the presence of intermolecular hydrogen bonds while spatial symmetry is preserved. The frequencies of observed intra- and intermolecular bands are not constant and independent, but are rather influenced by each other.

The research was supported RFBR grant N 11-02-12248-OFI-M.

References

1. M.M. Nazarov, A.P. Shkurinov, E.A.Kuleshov, V.V.Tuchin. Terahertz time-domain spectroscopy of biological tissue // Quantum. Electron. 2008. V. 38 (7). P. 647-655.
2. O. P. Cherkasova, V.A. Volodin, V.A. Minaeva, B.F. Minaev, G.V. Baryshnikov. Temperature dynamics for low-frequency Raman spectra of progesterone, 17α-hydroxyprogesterone and cortisone // Vestnik Novosibirsk State University. Series: Physics. 2010. V. 5, issue 4. P. 176- 180.

Representing author

photo

Prof. Olga Pavlovna Cherkasova

Institute of Laser Physics SB RAS, Novosibirsk State Technical University, Head of Biophysics Laboratory
Novosibirsk, Russia

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