A Spectroscopic Study of Some Chalcogenide Ring and Cage Molecules

Abstract

This thesis consists of a spectroscopic study of some selenium and sulfur ring and cage molecules, to investigate intermolecular and intramolecular forces. In particular, the effects on the intramolecular forces of pressure, temperature, coordination and intermolecular forces are studied with infrared (IR), Raman and nuclear magnetic resonance (NMR) spectroscopy; differential scanning calorimetry and X-ray diffraction. The local environments of the C3v cage molecule P4Se3 in the different phases of the compound were analysed with Raman spectroscopy and 31P magic angle spinning (MAS) NMR. The 31P MAS-NMR spectra of the orientationally ordered α and α'-phases have different chemical shifts for the apical P atom (α: 68.0, 86 and 88.0 ppm; α': 75.8 ppm), but similar chemical shifts for the basal P atoms (α: -58.8 ppm, α': -60.0 ppm). When either α or α'-P4Se3 is heated above 358 K, the resulting β-P4Se3 has a well-resolved, liquid-like spectrum, indicating extensive molecular re-orientation. When β-P4Se3 is slowly quenched through the β→α phase transition, it leaves a remnant β-phase mixed with the α-phase as well as P4Se4. A rapidly quenched sample of β-P4Se3 also shows a small remnant β-phase in the α-phase, but also a new phase with sharp resonances at 12.5, 3.6, 0.1 and -12 ppm. These are probably due to a P4Se4 phase that may be orientationally disordered. The Raman spectrum of P4Se3 heated above the α-β phase transition temperature shows a disappearance of the lattice modes and the 373 cm-1 mode as previously reported. It also shows some decomposition to P4Se4. The β-phase reverts into the α-phase on quenching, with only weak remnant bands attributable to P4Se4. The bands of P4Se4 become more prominent as the temperature of the β-phase is raised, but above the β-γ phase transition they are less prominent. The Raman spectrum of P4Se4 has been measured. The strongest band is at 350 cm-1, with the next strongest band at 185 cm-1. The spectra indicate that the dominant isomer is the selenium analogue of α-P4S4 (D2d), confirming previous 31P MAS-NMR studies. Rapidly quenching molten P4Se4 produces an amorphous solid. The existence of a glassy phase was confirmed by x-ray diffraction (XRD). Crystalline and glassy phases of P4Se4 were analysed with Raman spectroscopy, 31P MAS NMR, and differential scanning calorimetry (DSC), and powder XRD. The P4Se4 structural units are retained in the amorphous solid. Temperature dependent Raman spectra show an anomalous small positive pressure coefficient, like many other phosphorus chalcogenides. Raman and Infrared Spectra of CuBrSe3 and CuISe3 have been measured. The fundamentals were assigned by analogy to other adducts of Cu(I) halides and to the Se6 ring molecule. CuBrSe3 has two strong Raman bands at 247 and 272 cm-1; CuISe3 has two strong bands at 243 and 264 cm-1. The bands at ca. 245 cm-1 are assigned to the totally symmetric stretch of the Se6 ring, while the bands above 264 cm-1 are assigned to Cu-Se stretching. The strongest IR bands of CuBrSe3 and CuISe3 are at 78 and 74 cm-1 respectively, and these have been assigned to Cu-X bends. The Cu-X (X = Br or I) stretching frequencies agree well with the empirical correlation found between v(Cu-X) and the Cu-X bond lengths in adducts of phosphine and amine bases. These frequencies are shown to be almost independent of the nature of the coordinating ligands. The pressure dependences from 0 to 20 kbar and temperature dependences from 77-425 K of the Raman-active phonons were measured. In contrast to allotropes of Se, there was no anomalous behaviour of the A1-type stretching modes of the Se6 ring. This shows that the interference of intramolecular Se bonds by intermolecular Se bonds is much reduced by the rings' separation by the (CuBr)x chains or Cu2I2 rhombs. The coefficients (∂v/∂p)T of the external modes are smaller relative to the internal modes than those of rhombohedral Se. In fact, there is not such a clear distinction between internal and external modes. This shows the contrast between molecular solids (e.g. Se allotropes) and covalent network solids (e.g. CuXSe3). Small amounts of sulfur in the reaction mixture can cause sulfur substitution in the Se6 rings of CuXSe3, as shown by the Raman bands at c. 330 cm-1. Reaction of P4S3 with CuBr resulted in a dark brown compound with strong Raman bands at 364 and 472 cm-1; reaction of P4S3 with CuI resulted in a yellow-orange compound with strong bands at 363 and 465 cm-1. The bands at 465 and 472 cm-1 could be the result of the hardening by 20-30 cm-1 of the strongest P4S3 bands upon coordination, while the bands at ca. 363 cm-1 are probably attributable to Cu-P stretches.