Conformational order of n-alkyl modified silica gels as evaluated by Fourier transform infrared spectroscopy

Abstract

The conformational behaviour of non-deuterated and selectively deuterated alkyl modified silica gels in the dry state is examined by variable temperature FT IR spectroscopy. In the present study, three systems are considered, which are distinguished by the length of the tethered alkyl chains (C 9H 19 , C 18H 37 , C 22H 45 ). The desired information is obtained by the analysis of various conformational-sensitive IR bands, including CH 2 wagging, CD 2 stretching and CD 2 rocking bands. The analysis of the CH 2 wagging bands provides the relative amounts (i.e., integral numbers over the whole chain) of the kink/ gauche– trans– gauche, double- gauche and end- gauche conformers in the tethered alkyl chains. From the analysis of the CD 2 stretching and CD 2 rocking bands information about the conformational order at a specific deuterated methylene segment is available. Here, the CD 2 rocking band data are used to determine the amount of gauche conformers at the deuterated carbon positions C-4 and C-6, and C-12. It is found that the conformational order critically depends on the actual alkyl chain length, chain position and sample temperature. Particular emphasis is given to the impact of the external pressure during sample preparation on the alkyl chain conformations, about which so far no information is available. It is observed that the samples prepared as KBr pellets, which experienced a pressure of about 10 kbar, are characterised by a lower amount of gauche conformers. This substantial increase of conformational order is attributed to better alkyl chain packing along with a gain of intermolecular chain interactions.