Subject experimental considerations

It will be noted that because of the low self-diffusion coefficients the numerical values for representations of self-diffusion in silicon and germanium by Anhenius expressions are subject to considerable uncertainty. It does appear, however, that if this representation is used to average most of the experimental data the equations are for silicon... 

Fixed-bed bubble-flow operation has been the subject of considerably less experimental work than has trickle-flow operation. This reflects the fact that bubble-flow operation has been of much more limited industrial importance. 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

Because these pyrograms are derived statistically from data on coals with a range of thermal properties and whose petrographic specifications are subject to considerable experimental uncertainty (29), they are quantitatively Imprecise and can be interpreted only in a broad qualitative manner. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

Janke, W., Debus, G. Experimental studies on antianxiety agents with normal subjects methodological considerations and review of the main effects. In Efron, D.H., Cole, J.O., Levine, J.R., Wittenborn, J.R. (eds) Psychupharmacologv a Review of Progress 1957-1967. US Public Health Service Publication No. 1836. US Government Printing Office, Washington, DC, 1968, pp. 205-230. 

XPS is more useful than UPS for the purpose of characterisation, although in this role it rarely provides information that cannot be obtained from cheaper and more readily-accessible techniques. It is subject to considerable experimental difficulties and sources of error (e.g. surface effects) which limit its popularity. However, the growing body of XPS data is a most valuable contribution to inorganic chemistry. Compared with UPS, XPS gives poor resolution and vibrational structure is rarely observed. On the other hand, as previously noted, XPS is amenable to the study of solids while UPS is largely restricted to gases. 

All the cuprates described till now are hole superconductors. The nature of holes has been subject of considerable discussion (Chakraverty et al. 1988 Rao et al. 19896 Sarma Rao 1989). There has been no experimental evidence for the presence of Cum type species in the doped cuprates. Instead, there is considerable evidence from electron and X-ray spectroscopies for the presence of hybridized oxygen holes which can be represented as O-. The detailed description of the holes in terms of the d and p characters has been investigated (Bianconi 1990). Essentially, the mobile holes in the cuprates are present in the in-plane n band which has 0-2p character. The concentration of holes (in all but the T1 cuprates) are easily determined by iodometry or Fen-Fem titrations (Rao el al. 1991a Shafer Penney 1990). Since the Hall coefficients are temperature dependent, the chemical titration method becomes invaluable. 

With an E° value of —0.75 V, entry no. 19 of Table 17, reaction between alkyl halides and alkyllithium compounds, represents a strongly exergonic electron-transfer reaction which is expected to proceed at a diffusion-controlled tate. Experimental rate constants are not available, but such reactions are qualitatively known to be very fast. As we proceed to entry no. 21, two model cases of the nucleophilic displacement mechanism, it can first be noted that the nosylate/[nosylate]- couple is electrochemically reversible the radical anion can be generated cathodically and is easily detected by esr spectroscopy (Maki and Geske, 1961). Hence its E° = —0.61 V is a reasonably accurate value. E° (PhS /PhS-) is known with considerably less accuracy since it refers to an electrochemically irreversible process (Dessy et al., 1966). The calculated rate constant is therefore subject to considerable uncertainty and it cannot at present be decided whether the Marcus theory is compatible with this type of electron-transfer step. In the absence of quantitative experimental data, the same applies to entry no. 22 of Table 17. For the PhS-/BuBr reaction we again suffer from the inaccuracy of E° (PhS /PhS-) what can be concluded is that for an electron-transfer step to be feasible the higher E° value (—0.74 V) should be the preferred one. The reality of an electron-transfer mechanism has certainly been strongly disputed, however (Kornblum, 1975). 

There is considerable interest in the nature of solute-solvent interactions in SCFs, and a variety of experimental and theoretical techniques have been brought to bear on the subject. Experimental work near the critical point often shows interesting anomalies. One of the groundbreaking studies found that partial molal volumes of infinitely dilute solutes become large and negative in the compressible region around the critical point... 

Several studies on the quaternary systems of CaO-SiOj-HiO with NajO or K,0 have been reported (K19,S52,M52). Alkali greatly lowers the concentrations of CaO in the solution and raises those of Si02. The solid phase compositions are difficult to study. Determinations based on changes in concentration on adding CH to alkali silicate solutions are subject to considerable experimental errors, while direct analyses of the solid are difficult to interpret because the alkali cations are easily removed by washing. Suzuki ei al. (S52) considered that they were adsorbed. Macpheeef /. (M52) reported TEM analyses of the C-S-H in washed preparations obtained by reaction ofCjS (lOg) in water or NaOH solutions (250 ml). The C-S-H obtained with water had a mean Ca/Si ratio of 1.77 that obtained with 0.8 M NaOH had a mean Ca/Si ratio of 1.5 and a mean NujO/SiOj ratio of 0.5. These results do not appear to be directly relevant to cement pastes. The pore solutions of the latter may be 0.8 M or even higher in alkali... 

The Mills-Nixon hypothesis had, as its foundation, certain differences in the chemical behaviour of indan (3) and tetralin (4) from which a localization of the aromatic 7r-bonds was predicted to occur in the direction depicted by la rather lb. The original experimental evidence upon which the effect was based was shown to be erroneous, but calculations at various levels of theory indicated that aromatic bond localization should exist and become more pronounced as the size of the annelated ring decreases In essence one can recognize that the structure of benzene has a symmetry such that both Kekule structures must contribute equally. With Q tetralin (4) (and the lower homologues) no such symmetry requirement exists and ring annelation could induce bond length alternation within the arene nucleus. As the strain imposed by the fused ring increases, the Mills-Nixon effect should increase. The hypothesis has been the subject of considerable discussion and the controversy is far from settled. 

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