ИСТИНА

Polydimethylsilane (1) was actively studied in 1990th, the all-anti (formerly all-trans) conformation of its silicon chain was proved by X-ray and electron diffraction as well as vibrational spectroscopy methods. However, later this conclusion was exposed to doubt based on this polymer UV band max=342 nm value, which seemed “too low for a fully-extended polymer chain” /1/. That is why we have reinvestigated the structure of 1, using modern-level Raman, IR, and powder X-ray diffraction (XRD) techniques. Comparison of the Raman and IR spectra of 1 in the region of Si-Si and Si-C vibrations reveals operation of the “mutual exclusion” rule. The structure of 1 was solved ab initio and Rietveld refined, successfully describing the whole powder XRD pattern using an all-anti model. The space group of 1 was unambiguously determined as P21/n. Thus, these results re-establish the centrosymmetric planar all-anti conformation of the chain in crystalline 1. DSC data and the results of variable-temperature Raman, UV and XRD studies confirm phase transitions (PT) of 1 at about 150ºC and -40 ºC with small thermal effects. These PT are not thermochromic, preserving the all-anti conformation of the chain, but they lead to small distortions of the unit cell parameters and are probably related to molecular mobility changes. Reference /1/. R. West , in “The chemistry of organic silicon compounds”, Vol. 3, chapter 9, 2001, 547