Cm-1 Raman Band Research Articles

Tetrahedral Displacement: The Molecular Mechanism behind the Debye Relaxation in Water

The arguments for and against a single-molecule rotation mechanism for dielectric relaxation of water are surveyed. It is concluded that two distinct molecular mechanisms are operative in water. Single-molecule rotation is faster than the Debye relaxation time, τD, and possesses a smaller activation energy. It governs the abnormally fast proton mobility in water. The temperature dependence of τD agrees with that of water self-diffusion assuming a water hopping distance of 3.3 Å, the separation between an occupied and unoccupied corners of a cube binding the pentawater tetrahedron. This slower translational mechanism controls the ordinary transport phenomena in water. “Tetrahedral displacement” correlates with two tetrahedral normal modes: the antisymmetric stretch in extended tetrahedral structures at low temperatures and a torsion mode in loosely bound tetrahedra at high temperatures. The temperature dependence of the 180 cm-1 Raman band is in quantitative agreement with the activation energy for water reorientation and, in the framework of a two-dimensional model, also explains the activation energy for τD.

Phase transitions in Pb3(P1-xAsxO4)2: influence of the central peak and flip mode on the Raman scattering of hard modes

The integrated intensities and full width at half maximum (FWHM) of the 80 cm-1 Raman bands (hard modes) of ferroelastic Pb3(PO4)2 and Pb3(P0.77As0.23O4)2 were investigated in both the ferro and 'intermediate' phases for different polarisations as functions of temperature. The observed behaviour of these quantities has been interpreted in terms of interactions of hard modes with the central peak, flip mode and soft mode. The consequences of the three-component order parameter interpretation of the phase transition are also discussed. The integrated intensity curves in the 'intermediate' phase revealed a 'precursor order parameter' type of dependence. The numerical value of this 'precursor order parameter' at Tc is also estimated for Pb3(PO4)2. From the FWHM versus temperature curves in the 'intermediate' phase, the flip mode frequency is estimated to be 3*1010 Hz.

The infra-red spectra of the Borate esters

The infra-red spectra of a series of alkyl and aryl esters of boric acid have been obtained in the region from 1800 to 670 cm-1. These all show a strong characteristic absorption band at 1340 � 10 cm-1 which is assigned to the asymmetrical stretching frequency of the BO3 group. Comparison of the infra-red and Raman spectra of trimethyl borate suggests the assignment of the 728 cm-1 Raman band to the symmetrical stretching mode. The non-coincidence of infra-red and Raman bands indicates a fairly high degree of symmetry with restriction of the rotation of the methyl groups round the B-O link. This contrasts with other data on the behaviour in the vapour where such rotation may occur.