Pyrolysis Spectra

Pyrolysis spectra become distorted with respect to their diagnostic features for two major sets of reasons. The first is variations in instrument operation (e.g., heat transfer efficiency from wire to sample, ion source temperature, MAB gas identity, analyzer calibration, tuning, and ion transmission discrimination attributable to contaminated optics). Most of these factors can be controlled... 

The main feature of the pyrolysis spectra of polyethylene, polypropylene and isoprene can be interpreted on the basis of simultaneous breakage of carbon-carbon bonds in the main polymer chain. For a more detailed interpretation of the pyrolysis spectrum it is necessary to assume a series of radical-chain reactions in which intramolecular hydrogen abstraction plays an important role. 

Muchall et al. (98CC238) have recently investigated the gas-phase thermolysis of 2,5-dihydro-2,2-dimethoxy-2,5,5-trimethyl-l//-l,2,4-oxadiazole (75) by PE spectroscopy. Decomposition of 75 was induced by means of a continuous wave (CW) C02 laser as directed heat source at 26 W, which corresponds to a temperature of 500 50°C. When the PE spectra of acetone, tetramethoxyethene, and dimethyl oxalate were subtracted from the pyrolysis spectrum, a sim-ple spectrum remained that could be identified as that of dimethoxycarbene. Thermolysis in solution (94JA1161) had shown formation of tetramethoxyethene, and FVP experiments (92JA8751) gave dimethyl oxalate, both of which arise from the common precursor, dimethoxycarbene. Thermolysis of oxadiazolines similar to 75 in solution affords dialkoxycarbenes via an intermediate carbonyl ylide (94JOC5071). 

Diels-Alder reactions, 4, 842 flash vapour phase pyrolysis, 4, 846 reactions with 6-dimethylaminofuKenov, 4, 844 reactions with JV,n-diphenylnitrone, 4, 841 reactions with mesitonitrile oxide, 4, 841 structure, 4, 715, 725 synthesis, 4, 725, 767-769, 930 theoretical methods, 4, 3 tricarbonyl iron complexes, 4, 847 dipole moments, 4, 716 n-directing effect, 4, 44 2,5-disubstituted synthesis, 4, 116-117 from l,3-dithiolylium-4-olates, 6, 826 electrocyclization, 4, 748-750 electron bombardment, 4, 739 electronic deformation, 4, 722-723 electronic structure, 4, 715 electrophilic substitution, 4, 43, 44, 717-719, 751 directing effects, 4, 752-753 fluorescence spectra, 4, 735-736 fluorinated derivatives, 4, 679 H NMR, 4, 731 Friedel-Crafts acylation, 4, 777 with fused six-membered heterocyclic rings, 4, 973-1036 fused small rings structure, 4, 720-721 gas phase UV spectrum, 4, 734 H NMR, 4, 7, 728-731, 939 solvent effects, 4, 730 substituent constants, 4, 731 halo... 

The mechanism of this reaction has not been thoroughly explored. Some work has been done in analysis of potential intermediates for the reaction, although these intermediates were generated using flash vacuum pyrolysis (FVP). Materials in this experiment were trapped and IR spectrum suggested the formation of a ketene prior to cyclization. 

Attempts to prepare 6-hydroxybenzofuroxan by demethylation of 5-methoxybenzofuroxan, by pyrolysis of 4-azido-3-nitrophenol, and by hypochlorite oxidation of 4-amino-3-nitrophenoD failed. This rather unstable compound was finally prepared by hydrolysis of 5-acetoxybenzofuroxan its tautomeric possibilities are numerous, but from the similarity of its ultraviolet spectrum to that of 5-methoxybenzofuroxan it was considered to be largely in the hydroxy form. It is a fairly strong acid, of pK 6.76 (cf. 5-hydroxybenzo-furazan, pK 7.28). 7-Hydroxy-4,6-dinitrobenzofuroxan has been reported as arising from oxidation and nitration of dinitrosoresorcinol monooxime (tetraoxocyclohexene trioxime). ... 

The high temperature pyrolysis of PE, PP and mixtures of these polymers was studied in a novel bench-scale rotating eone reaetor to identify the optimal operating eonditions for this reaetor. It was shown that the effect of the sand or reaetor temperature on the product spectrum obtained was large eompared with the effect of other parameters, e.g. residenee time. 15 refs. 

The results of low-temperature matrix-isolation studies with 6 [41a] are quite consistent with the photochemical formation of cyclo-Cif, via 1,2-diketene intermediates [59] and subsequent loss of six CO molecules. When 6 was irradiated at A > 338 nm in a glass of 1,2-dichloroethane at 15 K, the strong cyclobut-3-ene-1,2-dione C=0 band at 1792 cm in the FT-IR spectrum disappeared quickly and a strong new band at 2115 cm appeared, which was assigned to 1,2-diketene substructures. Irradiation at A > 280 nm led to a gradual decrease in the intensity of the ketene absorption at 2115 cm and to the appearance of a weak new band at 2138 cm which was assigned to the CO molecules extruded photo-chemically from the 1,2-diketene intermediates. Attempts to isolate cyclo-Cig preparatively by flash vacuum pyrolysis of 6 or low-temperature photolysis of 6 in 2-methyltetrahydrofuran in NMR tubes at liquid-nitrogen temperature have not been successful. 

UV irradiation (A>300nm) of an argon matrix containing tetra-fluoromethane led to the formation of difluorocarbene CF2 (Milligan and Jacox, 1968a). It was shown that the IR spectrum of this species contains three bands at 1222 (i i), 1102 (v ), and 668 (i 2)cm . Some time later difluorocarbene was stabilized in a neon matrix at 4.2 K from the gas phase after vacuum flash pyrolysis (1300°C) of perfluoroethene (Snelson, 1970b). In this case the IR bands of CF2 differed from those in an argon matrix by less than 2 cm . ... 

After the first unsuccessful attempts to record a matrix IR spectrum of the methyl radical, reliable data were obtained by the use of the vacuum pyrolysis method. IR spectra of the radicals CH3 and CD3 frozen in neon matrices were measured among the products of dissociation of CH3I, (CH3)2Hg and CD3I (Snelson, 1970a). The spectra contained three absorptions at 3162 (1 3), 1396 V2) and 617 cm (I l) belonging to the radical CH3 and three bands 2381, 1026 and 463 cm assigned to the radical CD3. Normal coordinate analysis of these intermediates was performed and a valence force field calculated. In accordance with the calculations, methyl radical is a planar species having symmetry >31,. 

An informative IR spectrum of the t-butyl radical, containing 18 bands, has been recorded after freezing of the products of vacuum pyrolysis of azoisobutane [110] and 2-nitrosoisobutane [111] in an argon matrix at 10 K (Pacansky and Chang, 1981). This spectrum is in agreement with a pyramidal structure of the radical (CH3)3C (symmetry C3v) which has elongated CH bonds in positions trans to the radical centre. On the basis of experimental vibrational frequencies and ab initio calculations of the radical geometry the enthalpy value [// (300)] of its formation has been calculated as 44 kJ moP. ... 

Attempts have also been made to obtain the radicals (CF3)3C and CeFs as products of vacuum pyrolysis of (CF3)3CI and CeFsI (Butler and Snelson, 1980b). However, only perfluoroisobutene was observed in an IR spectrum of pyrolysis products of (CF3)3CI. Thermolysis of CeFsl led to formation of CF4, CF3 and CF2 as a result of decomposition of the aromatic ring. This behaviour was explained as due to catalytic effects which take place on the platinum reactor surface. 

Perfluorobenzylradical has been obtained similarly (Scheme 5) by vacuum pyrolysis of perfluorobenzyl iodide or of perfluorodibenzyl (Baskir et al., 1989). The matrix IR spectrum of C6F5CF2 radical contained the following bands 1597, 1501, 1485, 1312, 1267, 892 cm. Upon warming of the matrix from 12 K to 40 K the disappearance of these bands and the appearance of perfluorobibenzyl bands were observed. 

To use the DCI probe, 1-2 xL of the sample (in solution) are applied to the probe tip, composed of a small platinum coil, and after the solvent has been allowed to evaporate at room temperature, the probe is inserted into the source. DCI probes have the capability of very fast temperature ramping from 20 to 700 °C over several seconds, in order to volatilise the sample before it thermally decomposes. With slower temperature gradients, samples containing a mixture of components can be fractionally desorbed. The temperature ramp can be reproduced accurately. It is important to use as volatile a solvent as possible, so as to minimise the time required to wait for solvent evaporation, which leaves a thin layer of sample covering the coil. The observed spectrum is likely to be the superposition of various phenomena evaporation of the sample with rapid ionisation direct ionisation on the filament surface direct desorption of ions and, at higher temperature, pyrolysis followed by ionisation. 

Various methods of analysis exert different thermal stress on a material (Table 6.39). Direct heating in the inlet of a mass spectrometer in order to obtain a mass spectrum of the total pyrolysate is an example of thermochemical analysis. Mass spectrometry has been used quite extensively as a means of obtaining accurate information regarding breakdown products produced upon pyrolysis of polymers. Low residence times allow detection of high masses. 

The photodissociation of jet-cooled methyl from a flash pyrolysis source has also been studied at 216.3 nm using the hign-n Rydberg atom time-of-flight technique by Wilson and co-workers.113 The H-atom time-of-flight spectrum indicates that predissociation of Cl l s(/)2 ) at 216.3 nm predom-... 

Figure 5.5 Typical average pyrolysis Ar MAB/Tof mass spectrum from 0.5 pi, about 50,000 cells, of a tdh+ strain of V. parahaemolyticus serotype 04 K12.

An ANN is an array of three or more interconnected layers of cells called nodes (much like columns of cells in a spreadsheet). Data are introduced to the ANN through the nodes of the input layer. For instance, each input layer node can contain the relative intensity of one of the m/z values from a bacterial pyrolysis mass spectrum. The output layer nodes can be assigned to iden-... 

Figure 62 The unreacted (100), partially reacted (101), and completely reacted (102) diacetylene group-containing oligomers in the product from the reaction between 95 and Cp2Mo2(CO)6 (top). The X-ray diffraction spectrum (middle, left), TEM micrographs (middle, right), and conductivity plots (bottom) of the product from the reaction between 95 and Cp2Mo2(CO)6 after pyrolysis to 1000°C. (Adapted from ref. 130.)...

As an example of the form of the information that may be derived from a pyrolysis-MS, Figure 26 [69] shows the structure of the polycarbonate (PC) and the EI-MS spectra of pyrolysis compounds obtained by DPMS of poly(bisphenol-A-carbonate) at three different probe temperatures corresponding to the three TIC (total ion current) maxima shown in Figure 27(b) Figure 27 compares the MS-TIC curve with those obtained from thermogravimetry. (The TIC trace is the sum of the relative abundances of all the ions in each mass spectrum plotted against the time (or number of scans) in a data collection sequence [70].)... 

The spectrum of silicon based polymers is enriched by high tech ceramics like silicon nitride and carbide, respectively. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, polycarbosilanes and polysilazanes (preceramics). These synthetic ceramics display a certain analogy to silicates, having SiC, SiN, or Si(C,N) as structural subunits instead ofSiO. 

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