Raman frequency shifts for the rotatory lattice mode close to the melting point in ammonia solid I

We correlate here the thermal expansivity alphap to the frequency shifts 1/nu(partial differential nu/partial differential P)T for the rotatory lattice (librational) mode in ammonia solid I close to the melting point. This is carried out for the pressures of 0, 1.93, and 3.07 kbar at various temperatures for this solid structure. By obtaining linear plots of alphap versus 1/nu(partial differential nu/partial differential P)T for the pressures studied, we extract the values of the slope dPm/dT according to our spectroscopic relation. Our calculated values of dPm/dT can be compared with the experimental ones for ammonia solid I close to the melting point.     

Spectroscopic modification of the Pippard relation applied for the translational mode in ammonia solid II near the melting point

This study gives our calculation for the frequency shifts 1v partial differentialv partial differentialT(P) and the specific heat C(P) near the melting point in the ammonia solid II. We establish a linear relationship between C(P) and 1v partial differentialv partial differentialT(P) using the Raman frequencies of the translational mode which we calculated in this system. This leads to the validity of the spectroscopic modification of the first Pippard relation in the ammonia solid II near the melting point. From this linear variation of C(P) with the 1v partial differentialv partial differentialT(P) we deduce the slope values of dP(m)dT near the melting point for the fixed pressures of 3.65, 5.02 and 6.57 kbar in the ammonia solid II. They are compared with the experimental dP(m)dT values for this system.     

A linear variation of the thermal expansivity with frequency shifts for the translational mode in ammonia solid II near the melting point

We examine here our spectroscopic modification of the Pippard relation which is a linear variation of the thermal expansivity alpha(p) with the frequency shifts (1/nu)(partial differentialnu/partial differentialp)(T) close to the melting point in ammonia solid II. For this we use our calculated frequencies for the Raman mode of nu (51 cm(-1)) in ammonia solid II for the pressures of 3.65, 5.02 and 6.57 kbars. We establish this linearity between alpha(p) and (1/nu)(partial differentialnu/ partial differentialp)(T) for the pressures studied in ammonia solid II close to the melting point. The observed behaviour of ammonia solid II is explained in terms of our spectroscopic relation given here.     

Temperature and pressure dependence of the Raman frequency shifts near the melting point in ice I

This study examines the validity of the spectroscopic modification of the Pippard relations for the hexagonal ice (ice I) close to the melting point. A linear variation of the specific heat CP with the frequency shifts EQUATION: SEE TEXT is obtained for ice I. This linearity is also obtained between thermal expansivity alphaP and the frequency shifts EQUATION: SEE TEXT close to the melting point in this crystal.     

Calculation of the Raman frequencies of the translational mode in ammonia solid II

We report here our calculated Raman frequencies of the translational mode as a function of temperature for the fixed pressures of 3.65, 5.02 and 6.57 kbars in the ammonia solid II. They were calculated by means of our Grüneisen relation using the volume data from the literature for all the pressures indicated within the temperature regions close to the melting point in this system. Our calculated frequencies are in very good agreement with those observed experimentally for this translational mode of the ammonia solid II. This shows that the observed behaviour of ammonia solid II can be described adequately by means of the calculation employed here.     

Raman frequency shifts of an internal mode near the tricritical and second order phase transitions in NH4Cl

This study gives our analysis for the frequency shifts of the v2 (1708 cm-1) Raman mode in NH4Cl close to its tricritical (P=1.6 kbar) and second order (P=2.8 kbar) phase transitions. From our analysis, we extract the values of the critical exponent which describes the critical behavior of the Raman frequency shifts for this internal mode for the pressure conditions studied in NH4Cl. Our exponent value of alpha approximately 0.2 for the tricritical phase transition is close to the values of 1/16 (TTc) for the specific heat, predicted from a 3D Ising model. Our exponent values for the second order phase transition (P=2.8 kbar) for TTc are comparable with those reported in earlier studies.     

In situ FT‐Raman spectroscopic study of the conformational changes occurring in isotactic polypropylene during its melting and crystallization processes

The conformational changes occurring in isotactic polypropylene during the melting and crystallization processes have been carefully investigated using FT‐Raman spectroscopy at temperatures below, at, and above the polymer melting point. Results confirmed the retention of some crystallinity up to +210 °C, which is 50 °C above the melting point. It was found that, at temperatures just above the melting point (1–10 °C), there is still some short range order of at least 12 monomer units long in certain regions of the melt. At 10 °C above the melting point, the short range order drops below 12 monomer units resulting in the disappearance of the Raman band at 841 cm^–1. Vice versa, the experimental measurements show that the iPP melt system is stable when the persistence length of helical sequences is less than 12 monomer units. As soon as the helix length exceeds 12 units, the 3₁ helix conformation extends quickly and then crystallization occurs. These results are discussed in terms of Imai's microphase separation theory and it agreed very well with it. Also, from our observations for correlation splitting, Raman bands related to conformational states were identified. This analysis indicates the existence of three different conformational states at 808, 830, and 841 cm^–1. The 808 cm^–1 band was assigned to helical chains within crystals (representing crystalline phase). The 841 cm^–1 band was shown to be composed of a band at 841 cm^–1, assigned to shorter chains in helical conformation with isomeric defects (representing the isomeric defect phase), and a broader band at 830 cm^–1 assigned to chains in nonhelical conformation (representing the melt‐like amorphous phase). This indicates the detection of a three‐phase structure in iPP, where a third phase could be due to the presence of defect regions within the crystalline region, or due to the presence of an amorphous–crystal interphase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2173–2182, 2006     

Stoichiometry and conformation of the azacrown moiety in sodium complexes of azacrown ethers. A Raman/IR spectroscopic study. Part II: Complexes of 4,13-diaza-15-crown-5

4,13-Diaza-15-crown-5 and three of its sodium complexes (bromide, iodide and thiocyanate) were studied using Raman and IR spectroscopy and normal coordinate calculations, following the corresponding study on the sodium complexes of 4,13-diaza-18-crown-6 in the preceding paper. Complex formation was again accompanied by a characteristic shift of the bands, especially of those in the 800–900 cm^–1 region. The complexes of 4-13-diaza-15-crown-5 were distinct from those of 4-13-diaza-18-crown-6, in that both of the bands at 830 and 890 cm^–1 of the parent azacrown were affected on complex formation and in that only the 11 complex was formed. Normal mode calculations were made to predict conformations of the azacrown ring of the parent 4,13-diaza-15-crown-5 and its sodium complexes. Attention was paid to the different extent of mismatch in size of a sodium ion and azacrown cavities.     

Quasiharmonic treatment of infrared and raman vibrational frequency shifts induced by tensile deformation of polymer chains. II. Application to the polyoxymethylene and isotactic polypropylene single chains and the three-dimensional orthorhombic polyethylene crystal

Quasiharmonic equations are derived for stress-induced vibrational frequency shifts in the infrared and Raman spectra of polymer chains subjected to a tensile stress. The expressions are applied to the helical chains of polyoxymethylene and isotactic polypropylene. Observed frequency shifts can be reproduced well by using reasonable anharmonic force constants. A semiquantitative interpretation is given for the close relationship between stress-induced vibrational frequency shifts and the deformation mechanism of the polymer chains. Stress-induced frequency shifts are also calculated for an orthorhombic polyethylene crystal subjected to uniaxial tension along the chain axis or to hydrostatic pressure. The results consistently and reasonably reproduce observed data, not only for the intramolecular vibrational modes but also for the external lattice modes. © 1992 John Wiley & Sons, Inc.