Small per-deuterated

Table II Peak assignments for the GC/EI-MS spectra of the small per-deuterated cyclic dimethylsiloxanes.

Gas Chromatography/Ammonia Chemical Ionisation Mass Spectroscopic Analysis Of The Small Per-Deuterated Dimethylsiioxane Cyclics. The Cl spectra were also recorded of the small perdeuterated dimethylsiioxane cyclics using foe same instrument as previously with ammonia Three competing processes are known to occur with ammonia in foe case of PDMS [70]. Quasimolecular ions are formed either by hydrogen or ammonium ion addition as well as subsequent trideuteriomefoyl loss. Ions can be observed at M +18, M + 2(18 - 19), M + 1, and M -18. Each of these processes are observed for foe cyclics upto foe decamer and spectra were obtained for foe pentamer up to foe decamer inclusive (Figures 33 and 34 show those of the hexamer and foe nonamer respectively). 

The successfiil preparation of foe small per-deuterated cyclic dimethylsiloxanes now meant that larger cyclics and linears could in theory be prepared in a recognised fashion from the small rings via either foe Brown and Sluzarczuk [4] or foe Chojnowski [30] [31] [32] [33] methods. Die investigations in to foe triflic acid reaction with dimethyldiphenylsilane, however, showed the possibility that foe reaction might be catalytic and so this was attempted with deuterated materials and is described below. 

We can exploit mixtures of per-deuterated and per-hydrogenated polymers in small-angle neutron scattering measurements to reveal information on the configuration of individual chains as well as assemblies of chains in the case of phase separating blends, block copolymers, and other inhomo-... 

Fig. 1.17 Small-angle neutron scattering pattern for a sample of per-deuterated lightly branched polyethylene containing 10% of hydrogen containing linear polyethylene. The sample is molten and has just been subjected to deformation in a channel die. The flow axis is horizontal and the effective extension ratio is 3.

Preparation Of Small Cyclic Per-Deuterated Dimethylsiloxanes. Investigations have shown triflic acid to be capable of removing phenyl groups from dimethyldiphenylsilane and polymerisation of the hydrolysis products is possible in situ. The reaction was now carried out using bis(trideuteriomethyl)diphenylsilane prepared from CD3I via a Grignard to produce per-deuterated dimethylsiloxanes. 

The traces obtained from the GLC analysis of the reaction (Figure 25) clearly show the production of what have been identified, through the use of GC/EI-MS, as linear phenyl terminated poly(dimethylsiloxane) oligomers (Ph-(Si(CE)j)20) -Si(CD3)2Ph). The subsequent removal of the terminal phenyl groups upon further reaction with triflic acid was seen to result in a rearrangement to small cyclic oligomers of per-deuterated dimethylsiloxane. 

Chain-per-deuterated THE5 is a good model sample for studying spin cluster with a limited number of protons (see molecular stmcture in Figure 4-6). For example, the core C4 carbon dipolar coupled to two protons, H4 and H5, represents a relatively isolated H-C-H three spin system with one large ( 5 kHz) and one small ( 0.4 kHz) couplings. As shown in Figure 4-6, the resolution produced... 

Since the bond to the isotopic atom is not formed or broken in the transition state of the rate-determining step of the reaction, the difference between the rate constant for the reaction of the undeuterated and deuterated substrates is usually small. As a result, secondary deuterium KIEs are usually close to unity, i.e. the maximum secondary deuterium KIE is 1.25 per deuterium (Shiner, 1970a) and most of these KIEs are less than 1.10 (Westaway, 1987a). Therefore, careful kinetic measurements with an error of approximately 1 % in each rate constant or specially designed competitive methods are required to determine them with an acceptable degree of accuracy. 

While not directly relevant to H-bonds per se, a recent work compared the H/D fractionation factors for a number of small molecules, as computed at various levels of theory . The results indicated that the relative free energy of protiated versus deuterated species is rather insensitive to choice of basis set. Best results are achieved if polarization functions are added to all atoms, and correlation is recommended, but even SCF computations with a basis set as small as 3-21G can provide quite reasonable results. 

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