Formation measurements

Colgan E G 1995 Activation energy for Pt2Si and PtSi formation measured over a wide range of ramp rates J. Mater. Res. 10 1953... 

As an example of the problem that arises when the mechanism changes, consider the data presented in Figure 10-3. Part (a) depicts the apparent rate constants for semi-carbazone formation measured at an intermediate pH of 3.9. The rate-controlling step is not clearly defined under such conditions, and a curved plot is obtained. 

Radioactive tracer techniques. In electrochemistry, the procedure is essentially the same as in studies of chemical reactions the electroactive substance or medium (solvent, electrolyte) is labelled, the product of the electrode reaction is isolated and its activity is determined, indicating which part of the electroactive substance was incorporated into a given product or which other component of the electrolysed system participated in product formation. Measurement of the exchange current at an amalgam electrode by means of a labelled metal in the amalgam (see page 262) is based on a similar principle. 

For example, consider the near-planar nitrogen in l-azabicyclo[3.3.3]undecane (manxine), N. J. Leonard, J. C. Coll, A. H. -J. Wang, R. J. Missavage and I. C. Paul, J. Amer. Chem. Soc., 93, 4639 (1971). Regrettably, we know of no enthalpy of formation measurements on either this heterocycle or the corresponding hydrocarbon, manxane. 

There is a 30 kJ mol1 difference in the recommended enthalpies of formation of solid 2-aminobiphenyl in the two volumes by Pedley, cf the new measurements reported in W. F. Steele, R. D. Chirico, S. E. Knipmeyer and A. Nguyen, J. Chem. Thermodyn., 23, 957 (1991). This difference attests to the difficulty of making enthalpy-of-formation measurements on amines, a conclusion already intimated in the discussion of the naphthylamines. 

Testa, R., Caglieris, S., Risso, D., Arzani, L., Campo, N., Alvarez, S., Giannini, E., Lantieri, P.B., and Celle, G., Monoethylglycinexylidide formation measurement as a hepatic function test to assess severity of chronic liver disease. Am.. Gastroenterol, 92, 2268-2273, 1997. 

Figure 7. Relationship of oxidation and degree of polyunsaturation. Polyunsaturation is measured as the methylene bridge index (MBI), which is a more precise measure of extent of unsaturation and oxidizability than the double bond index. It is the mean number of 6is-allylic methylene bridge positions per fatty acid (or fatty acyl chain) in a lipid ensemble. The rate of lipid radical formation measures formation of an oxidative product, while O2 consumption (% O2 lost per sec) is a measure of utilization of a reactant. (Drawn using our data abstracted from Wagner, B.A., Buettner, G.R., and Bums, C.P. 1994, Biochemistry 33 4449-4453).

If the enthalpy of formation of 4-lithiobutyl methyl ether is interpolated between the values for the lithiopropyl and the lithiopentyl ethers to be —285 kJ moP, then the enthalpy of isomerization to the less stable 3-lithiobutyl methyl ether is - -10 klmoP, which is about half that of isomerization of n-butyl lithium to 5ec-butyl lithium (-1-21.3 kJmol ). However, a linear interpolation assumes the same strain energy for the 6-membered 4-lithiobutyl ether as for the above 5- and 7-membered cu-lithioalkyl methyl ethers. If it is less strained, then the isomerization enthalpy would be larger. How much of the isomerization enthalpy difference is due to other differences, such as intramolecular complexation and/or aggregation among the various species, is not known. Unfortunately, there is no enthalpy of formation measurement for the delithiated 7-methoxynorbornane. 

We close this section with a statement of surprise and disappointment. For no apparent reason there are no published enthalpy of formation measurements for 1-hydroxy-2,2,6,6-tetramethylpiperidine, the parent species for the above 1-hydroxylated piperidines and the hydroxylamine counterpart to probably the most famous nitroxide radical, TEMPO . 

An admittedly enigmatic result involves the thermolysis of solution phase di-f-butyl diperoxyoxalate and a cyclic counterpart, 7,7,10,10-tetramethyl-l,2,5,6-tetraoxacyclodecane-3,4-dione. While the former reaction was shown to be exothermic by 238.5 8.4 kJmol and the latter by 414.2 8.4 kJmol , the latter is slower by a factor of some 3000. The latter decomposition results in acetone, ethylene, CO2 and 3,3,6,6-tetramethyl-l,2-dioxacyclohexane. The enthalpy of formation measurements of the cyclic peroxyoxalate and the 1,2-dioxane are coupled if we knew the enthalpy of formation of 1,2-dioxane it would allow us to derive the enthalpy of formation of 7,7,10,10-tetramethyl-l,2,5,6-tetraoxacyclodecane-3,4-dione and the other way around. 

The hydrate formed by photolysis of this substance is one of the few such products (the others are uracil hydrate, 5-fluorouracil hydrate, and uridine hydrate) that have actually been isolated and compared with authentic material of known structure.7 It is nearly the only product formed in the photolysis, is definitely stable at room temperature and neutral pH, and the thermal reversal to dimethyluracil is nearly quantitative. These properties, as Moore observed, make the reaction ideal for mechanistic investigation. Burr and Park have investigated the reaction mechanism by measuring the photolysis rate of dimethyluracil in mixtures of water with several nonaqueous, nonreactive solvents as a function of water concentration.64 The photolysis rate for 10" iM DMU was found to be the same in water-saturated cyclohexane (about 5 x 10-3M in water) as in dry cyclohexane. The photolysis rate in dry, highly purified dioxane was quite insensitive to water, and it was observed that hydrate formation (measured by thin-layer chromatography and by thermal absorbance reversal) became appreciable only at water concentrations above 40%. 

Moriguchi et al. (43) noted a correlation between the change in the basal plane spacing with H2S exposure of CdSt, and the ionic radius of sulfide, and used this as evidence for CdS monolayer formation. Measurements of high lateral conductivity in metal ion fatty acid films exposed to H2S (20,23) and of photoelectric properties (21) have also been used to invoke the concept of continuous sheets of metal sulfide forming. 

N. L. Allinger, op. cit., cited in Reference 24. Admittedly, there is an earlier enthalpy of formation measurement, that of T. L. Flitcroftand H. A. Skinner, Trans. Faraday Soc., 54,644(1958) from which we would likewise have derived a (5 (Ph, Cypr —C=CH) value of 33.7 4.8 kJmol-1. 

J Templar, K Arrigan, WJ Simpson. Formation, measurement and significance of lightstruck flavor in beer. A review. Brewers Digest 70(5) 18-25, 1995. 

This reaction enthalpy probes the superstrain associated with the two, trigonal, bridgehead carbons beyond that found in the individual rings. As may simply be concluded by looking at the molecular structure, the m = n = 1 species is clearly highly strained (quantum chemical calculations show the additional structural feature of non-planarity77). Experimental enthalpy-of-formation measurements of bicyclobutene show this compound to be accompanied by an exothermicity of reaction 18 of 42 42 kJ mol 1. 

The origins of the 9 kJ mol-1 error and file much larger discrepancy between the two literature results is not obvious. However, we do note that for both naphthalene and 1,2,3,4-tetrahydronaphfiialene, much better known hydrocarbons with file same carbon skeleton as the m = n = 4 species, our primary archive cites alternative enfiialpy-of-formation measurements that deviate by ca 10 and 40 kJ mol-1 from those recommended by this source. 

Figure 9.6 Comparison of the weight percentage of carbon formation measured by TEOM and TPO-IR (Carbon Analyzer) after reaction over 25 mg ICI R15513 catalysts at 0.1 MPa and 923 K at different feed gas compositions simulating (1) initial C02...

Freyssinet, J. M., Torbet.J., Hudry-Clergeon, G., and Maret, G. (1983). Fibrinogen and fibrin structure and fibrin formation measured by using magnetic orientation. Proc. Natl. Acad. Sci. USA 80, 1616-1620. 

Fig. 3. (a) Schematic of the thin film target for the d/xt formation measurements consisting of the emission, the moderation and the reaction layers, which are prepared by rapidly freezing hydrogen isotopes on the gold foils (not shown) held in vacuum at 3.5 K. [13] The layer thickness (3.43 mg-cm-2, 96 pg-cm-2, and 21 pg-cm-2, respectively) were measured off-line via a particle energy loss (see Fig. 5) (b) Measured Si energy spectra with prompt (I t > 0.02 ps) and delayed (II t > 1.5 ps) time cuts. Fusion in DS reaction layer is separated from that in US D2 due to the pf TOF across the vacuum... 

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