Isobutane fragmentation

The ratio of the amount of n-butane-2-13C to the amount of isobutane produced was, provided measurements were made under conditions where secondary reactions were unimportant (i.e., initial reaction products), constant and independent of temperature, and this ratio was 1/4. At the same time, no scrambling of the 13C occurred i.e, all of the isotopically substituted molecules remained singly labeled. Anderson and Baker (68) speculated that the butane isomerization might have occurred by a recombination of adsorbed surface residues produced by fragmentation of the... 

The techniques based on the direct introduction of the sample in the mass spectrometer can also be used with CL This allows just a small amount of fragmentation. Due to the fact that less energy is used than in the El mode, Cl produces relatively stable molecular ions and ion fragments, and does not give as extensive fragmentation as readily as El does. Figure 3.8 compares the mass spectra of 7-oxo-dehydroabietic acid obtained by DE-MS using El at 70 eV and Cl with isobutane. In both these... 

Isobutane is an especially versatile reagent gas, because i) it provides low-fragmentation PICI spectra of all but the most unpolar analytes, ii) gives almost exclusively one well-defined adduct ([Mh-C4H9], [M+57]) if any (Fig. 7.7), and iii) can also be employed for electron capture (Chap. 7.4). 

Fig. 7.7. Comparison of (a) 70 eV El and (b) isobutane-d spectrum of glycerol. Instead of a molecular ion, an [M+H] quasimolecular ion is observed in El mode, too. In addition to [M+H], the Cl spectrum shows few fragment ions and a weak [2M+H] cluster ion signal.

Example In an overdose case where evidence was available for the ingestion of Percodan (a mixture of several common drugs) the isobutane-CI mass spectrum of the gastric extract was obtained (Fig. 7.8). [29] All drugs give rise to an [Mh-H] ion. Due to the low exothermicity of protonation by the tert-C Hi) ion, most [Mh-H]" ions do not show fragmentation. Solely that of aspirin shows intense... 

Example The extraordinary stable trityl ion, PhsC, m/z 243, tends to dominate mass spectra (Chap. 6.6.2). Thus, neither the El spectrum of chlorotriphenyl-methane nor that of its impurity triphenylmethanol show molecular ions (Fig. 8.11). An isobutane PICI spectrum also shows the trityl ion almost exclusively, although some hint is obtained from the Ph2COH ion, m/z 183, that cannot be explained as a fragment of a chlorotriphenylmethane ion. Only FD reveals the presence of the alcohol by its molecular ion at m/z 260 while that of the chloride is detected at m/z 278. Both molecular ions undergo some OH or Cl loss, respectively, to yield the Ph3C fragment ion of minor intensity. 

The concept of site isolation is important in catalysis. On metal particles one usually assumes that ensembles of metal atoms are necessary to activate bonds and to accommodate the fragments of molecules that tend to dissociate or to recombine. We present here three examples of such effects the dehydrogenation of decane into 1-decene, the dehydrogenation of isobutane into isobutene and the hydrogenolysis of acids or esters into aldehydes and alcohols. In most cases the effect of tin, present as a surface alloy, wiU be to dilute the active sites, reducing thereby the yield of competitive reactions. 

The electron impact (Figure 9) and chemical ionization (figures 10 and 11) spectra of benzoic acid were obtained using a Finnigan Mat 8200 mass spectrometer. Methane and isobutane were used as the reactants for the Cl methods. The studies were performed at 220°C(EI) and 130-140°C (Cl), 0.05 mA, and 70 eV. The assignment of the main fragments is presented in Table 6. 

Hence RS of isobutane undergoes fragmentation more readily than does RS of n-butane, and fewer RS fragments of isobutane survive. Consequently, isobutane, typical of branched-chain alkanes, has a low-intensity RS peak compared with/i-butane. 

Thus by choice of reagent gas, we can control the tendency of the Cl-produced M + H+ ion to fragment. For example, when methane is the carrier gas, dioctyl phthalate shows its M + H+ peak (m/z 391) as the base peak more importantly, the fragment peaks (e.g., m/z 113 and 149) are 30-60% of the intensity of the base peak. When isobutane is used, the M + H+ peak is large and the fragment peaks are only roughly 5% as intense as the M + H+ peak. 

In many laboratories, the El spectrum and the Cl spectrum (with methane or isobutane) are obtained routinely since they are complementary. The Cl spectrum will frequently provide the [M + H]+ peak when the El spectrum shows only a weak or undetectable M + peak. The [M - H]+ peak may also appear in the Cl spectrum by hydride abstraction. The Cl fragmentation pattern is usually difficult to predict or rationalize. Note that the nitrogen rule (see Section 2) does not apply to the [M + H]+ or the [M - H]+ peaks neither does it apply to the Cl fragmentation ions. 

When isobutane is added to such an experiment, acetone and methanol are still the major products, and the concentrations of all products increase with the concentrations of isobutane and terf-Bu202. About one molecule of isobutane is consumed per initiating tert-BuO radical. The higher consumption of oxygen is due mostly to oxidation of the initiating fragments (Experiment 51). 

Chemical ionization produces less fragmentation than electron ionization. For chemical ionization, the ionization source is filled with a reagent gas such as methane, isobutane, or ammonia, at a pressure of 1 mbar. Energetic electrons (100-200 eV) convert CH4 into a variety of reactive products ... 

It is significant that the mixture yielded propane as the major product (Table III). As noted in our earlier paper on catalytic cracking (6), the predominance of C3 fragments in the cracked products and the absence of isobutane appeared to be a unique property of erionite. Our present data indicate that this is also true for hydrocracking over a dual function erionite. The only exception was that when n-pentane alone was hydro-cracked, equimolal quantities of ethane and propane were found. This shift in product distribution in the presence of n-hexane, a second crackable component, indicated that the reaction path within the intracrystalline space was complicated. 

Under photo-stimulation, isoindolyloxyl radical (5) abstracts primary, secondary, or tertiary hydrogens from unactivated hydrocarbons including cyclohexane, isobutane, or n-butane (Scheme l).23 The nitroxide (5) traps the resultant carbon-centred radical (R ) and so afford the A -aI koxyisoindo les (6). Blank photolysis experiments with no added hydrocarbon have shown some unprecedented / -fragmentation of (5) to afford the nitrone (7). A number of C60 nitroxide derivatives have been synthesized and characterized by ESR spectroscopy which show features common to nitroxide radicals.24 Reaction of nitroxide and thionitroxide radicals with thiyl radicals have been observed, from which sulfinyl, sulfonyl, and sulfonyloxy radicals were generated.25 The diisopropyl nitroxide radical was generated in the reaction of lithium diisopropylamide with a-fluoroacetate esters.26... 

---