engleski

Near-Infrared Fourier Transform Raman Microspectroscopy at High External Pressure. Crystalline trans-Stilbene and trans-N-Salicylideneaniline

The high-pressure behaviour of molecular solids might be affected by intramolecular properties such as a possibility of internal rotation around single C-C bond. Since the energy needed for a conformational change is comparable to the energy of intermolecular interaction, the equilibrium molecular structure in the solid phase can be influenced by external pressure.Biphenyl and other p-polyphenyls are known examples in this respect. The fact that the shortest intramolecular H-H distance at room temperature and ambient pressure in trans-stilbene (tS) is equal to that of biphenyl (0.206 nm) and that the phenyl rotation is also governed by the two-well potential has prompted us to undertake high-pressure vibrational study of tS. A change in molecular conformation is also expected for various derivatives of N-benzilideneaniline which are, unlike tS and biphenyl, twisted in the crystal phase with the dihedral angle of the aniline ring equal to 40 degrees. Semiempirical AM1 frequency calculations performed for tS predict a 1.5% increase in the C_e=C_e stretching frequency due only to the change of the twist angle from 0 to 90 degrees. Another geometrical factor influencing significantly the double bond stretching which might be altered by external pressure is the Ph-C=C/N bond angle. A gasketed diamond anvil cell was used to generate high pressure. The FT Raman spectra (resolution 4 cm(-1) ) were recorded on a Bruker IFS 66 spectrometer equipped with a FRA 106 FT Raman module. The Raman microscope was coupled to the module by two fiber-optic cables. The main obstacle in using NIR FT Raman microspectroscopy at high external pressures is the lack of an internal pressure standard. One is thus forced to use a secondary standard or to perform IR microspectroscopy prior to Raman measurements. Orthorhombic sulphur is an efficient NIR Raman scatterer. The pressure dependence of its strongest Raman bands at 473 and 219 cm(-1) from 0 to 5 GPa was used to measure the pressure. It is evident that a satisfactory accuracy can be achieved only up to 3 GPa . Therefore the arithmentic mean of the two pressure values was used in the entire region from 0 to 5 GPa. The linear behaviour of the four anthracene bands thus reproduced supports the conclusion that the tS and N-salicylideneaniline (N-S) bands do behave linearly. The most intense tS bands occur at 1639 cm(-1) (C_e=C_e stretching), 1594 cm(-1) (ring 8a), 1193 cm(-1) (Ph-C_e stretching) and 998 cm(-1) (ring 12) and those of N-S at 1617 cm(-1) (C=N stretching), 1591 cm(-1) (ring 8a), 1572 cm(-1) (dominated by the conjugated HOCCCN region), 1457 cm(-1) (Ph-N stretching), 1189 cm(-1) (C-O stretching and O-H bending) and 1001 cm(-1) (ring 12). All the bands were monitored in the entire pressure range and their frequencies change approximately 1-2% from the ambient pressure values. No extra pressure sensitivity could be observed. Vibrational scaling law (correlation between 1/ni_i * dni_i / dP and ni_i) applies to the systems considered as well. The Raman solution band of tS at 960 cm(-1) not seen in the crystal spectrum at ambient pressure and considered as an evidence for the molecular nonplanarity in solution is not induced by pressure changes. One may conclude that tS molecules remain planar up to 5 GPa. Contrary to expectations N-S molecules do not become planar with increasing pressure. Otherwise the observed C=N stretching frequency would show reduced or no pressure sensitivity, as is the case for the benzophenone C=O stretching (the frequency of this mode decreases by 2.3% when the phenyl dihedral angle is changed from 50 to 30 degrees).

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