Oblique phonons in uniaxial molecular crystals: Raman spectra of s‐trioxane

Abstract

Using Raman scattering the angular dispersion of extraordinary phonons in s-trioxane, a uniaxial crystal of trigonal structure, has been measured. It has been found that the transitions are of the type TO -+ LO or LO -+ TO so that the anisotropic short-range forces predominate over the long range electrostatic ones. The LO-TO splittings are of the order of 10 cm-' or less as expected for molecular crystals. For phonon propagation direction along the optic axis extraordinary and ordinary phonons of E species coincide.

Phase-Dominant Pressure-Induced Planar Molecular Conformation of S-Trioxane

“…High-pressure Raman spectra of S -trioxane are shown in Figure . The ambient pressure Raman spectrum of S -trioxane observed by us accorded with those reported in the literature . The most prominent Raman-active modes are O–C–O and H–C–H scissoring, ring breathing, and symmetric and antisymmetric CH 2 stretching motions.…”

“…The ambient pressure Raman spectrum of S-trioxane observed by us accorded with those reported in the literature. 16 The most prominent Raman-active modes are O−C−O and H−C−H scissoring, ring breathing, and symmetric and antisymmetric CH 2 stretching motions. Their exact peak positions are listed in Table 1 along with the appropriate vibrational assignments.…”

“…Because uniaxial molecular crystal S -trioxane is the cyclic trimer of formaldehyde, (CH 2 O) 3 , with a puckered structure(shown in Figure ), it has been the subject of many studies both as a cyclic ether and as a model compound for polyoxymethylene. Its molecular structure has been examined by means of X-ray and electron diffraction, infrared, Raman, and microwave spectroscopies. − Under the ambient condition, the molecules are packed in rows paralleled to the c axis. In each row they are separated by 1/2 c and alternately rotated from each other by +48.6° so that each carbon is only 11.4° from being exactly superimposed on an oxygen atom and each polar hydrogen atom points nearly toward an oxygen atom.…”