Combustion measurements

The stmcture of DPXN was determined in 1953 from x-ray diffraction studies (22). There is considerable strain energy in the buckled aromatic rings and distorted bond angles. The strain has been experimentally quantified at 130 kj/mol (31 kcal/mol) by careful determination of the formation enthalpy through heat of combustion measurements (23). The release of this strain energy is doubtiess the principal reason for success in the particularly convenient preparation of monomer in the parylene process. 

N. Chigier, Combustion Measurements, Hemisphere Publishing Corp., New York, 1991. 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

Diagnostics in Combustion Measurements to Unravel Combustion Chemistry... 

Here, selected, more established laser diagnostic techniques will only be briefly named with their principal area of application in combustion measurements, but not further described details on these and other methods can be found in the above-mentioned literature and references cited therein. Workhorse techniques in combustion diagnostics include Raman and Rayleigh... 

Figure 7.1.15 shows a sample of turbulence fields, with and without combustion, measured in this configuration. The turbulence levels are significantly higher for the reactive than for the nonreactive flow. The authors... 

All this is in line with the most recent finding for triquinacene, for which the direct determination of its AHy(g) from its experimentally measured heat of combustion finally corroborated the results of the most advanced computational studies that triquinacene is not homoaromatic [35]. Evidently, heat of combustion measurements should also be carried out for some representative [nlpericyclines to finally settle the quest for their neutral homoaromaticity. 

Numerically, the above sum to ca 40 kJmol-1. The experimentally measured enthalpy of formation for liquid 83, derived from a combustion measurement, is —24 kJmol-1. Using our standard protocol to estimate the necessary enthalpy of vaporization results in an additional 50 kJmol-1. A value of 26 kJmol-1 is thus predicted for the enthalpy of formation of gaseous 83. The source of the 14 kJmol-1 discrepancy evades us58. 

It is interesting to note that there are enthalpy of formation data for solid and gaseous tetra-f-butyltetrahedrane, but not for its more stable valence isomer, tetra-t-butylcyclobutadiene cf G. Maier, Angew. Chem., Int. Ed. Engl., 27, 309 (1988). This review cites unpublished enthalpy of combustion measurements (M. Mansson) and enthalpy of sublimation measurements (C. Riichardt, H.-D. Beckhaus and B. Dogan). We admit our surprise that details of these measurements remain unpublished. 

M. Laffitte. Trends in Combustion Calorimetry. The Use of the Tian-Calvet Microcalorimeter for Combustion Measurements. In Experimental Chemical Thermodynamics, vol. 1 S. Sunner, M. Mansson, Eds. IUPAC-Pergamon Press Oxford, 1979 chapter 17 3. 

There are data only for R = H and R1 = H, resulting in difference quantities of 24.4 and 27.0 kJmol-1. The enthalpy of formation of iV-ethylaniline calculated using the average of 15 and i6 and each of the two literature values for IV-methylaniline would be ca 59 and 70 kJmol-1. The former value is closer to the literature value. Perhaps we should not be surprised because the same problems that may beset combustion measurements of the enthalpy of formation of tV-mcthylanilinc are likely to be applicable to /V-cthylaniline, especially since measurements of both species appear in the same literature source. 

Table 4 gives a list of nitro-PAH arising directly from combustion measured in different cities of the world. This is presented as air concentration and as nitro-PAH levels in soot. 

DIODE LASER SENSORS FOR COMBUSTION MEASUREMENTS AND CONTROL... 

About 30 years ago, an enthalpy of formation was reported for 3,3,4,4-tetramethyl-l,2-dioxetane . Both by direct microcalorimetric combustion measurements of the neat solid and by reaction calorimetry (of the solid itself, and in acetone solution to form acetone), a consensus value was derived. Now, is the enthalpy of formation plausible , notwithstanding the very large error bars Consider reaction 6 for the dioxetane that produces 2,3-dimethyl-2,3-butanediol . The liquid phase enthalpy of reaction is —329 kJmoU. It is remarkable that this value is compatible with that for the dialkyl peroxides, ca —335 kJmoU, despite the ring strain that might be expected. 

Since the procedure involves combustion, measurement of the isotopic distribution at specific sites in the molecule is not possible. When this information is required, NMR is used to determine the D/H ratio at each site. 

We adopt the enthalpies of formation from Stull, Westrum and Sinke, the combustion measurements of G. Becker and W. A. Roth, Chem. Ber., 67, 627 (1934) and the structural assignments of K. Alder and G. Stein, Chem. Ber., 67, 612 (1934). The a- and /3-isomers were also called iendo and Lexo> in this last reference to reflect the stereochemistry of the terminal 5-membered ring we note the fusion of the norbornane units. 

Enthalpies of formation are determined experimentally in a number of ways. Perhaps the most common way is through precise oxygen combustion measurements. Where data do not exist in tabulations, estimation techniques—either empirical or based on quantum chemical calculations—are available. 

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