Metastable atomic hydrogen reactions

The problem of non-specific decomposition is exemplified by the study of 4-methyl-l, 3-dioxolane [657]. Following El the d2 molecule labelled at the 2 position (the ring C between the O s) gave a metastable peak for loss of D- but not for loss of H In the mass spectrum, however, the peak intensity for H loss was greater than that for D loss, suggesting that there were not just one, but at least two, reaction channels for atomic hydrogen loss. 

H3+(D3+) + N. The ions were obtained from hydrogen or deuterium by electron bombardment [6, 8,10,11], by electric discharge [7], and in reactions with rare gas ions or metastable atoms [5, 9]. The tabulated rate constant determined with the flowing afterglow technique... 

Despite this superficial similarity, however, subtle differences between the behaviour of ionized amines and the analogous ionized alcohols and ethers remain. Thus, metastable ionized 2-butylamine loses 80% ethane in contrast, ionized 2-butanol eliminates both ethane (35%) and methane (40%)85. The latter reaction corresponds to loss of the smaller methyl group and an a-hydrogen atom from the larger ethyl substituent at the branch point. Methane loss does not occur from ionized amines with a methyl substituent on the -carbon, with the solitary exception of ionized isopropylamine which does expel methane (10%). However, ionized 3-hexylamine eliminates both ethane (35%) and propane (20%)85. 

Instead of carboxylic acids, other carbonyl compounds can be used acid halides, esters, amides, etc. The commonly accepted general mechanism for these reactions consists of the initial nucleophilic addition of an active hydrogen compound to the electron-poor carbonyl carbon atom of the R COOI I molecule, with the formation of a metastable intermediate that can undergo a subsequent elimination reaction ... 

The propene ion is one system in which determination of isotope effects is hampered by hydrogen randomisation. Nevertheless, deuterium isotope effects upon ion abundances following El have been obtained by making allowances for the extent of hydrogen randomisation [see Sect. 7.5.1(f)]. For hydrogen atom elimination, the isotope effect/H//D has been put at 1.7—2.0 for source reactions [372] and 2.3—3.0 and 3.6 (first and second field-free regions, respectively) for metastable ions [510]. For hydrogen atom elimination from propenoic acid ions, the isotope effect /H//D has been put at 4.3 for metastable ions [587]. 

The earliest attempt to detect the presence of free radicals in this way was through the catalytic conversion of ortho-para hydrogen mixtures. At equilibrium at room temperature, ordinary hydrogen consists of a mixture of 75 per cent ortho-H2 (nuclear spins parallel) and 25 per cent para-H2 (nuclear spins antiparallel). At low temperatures (<90°K) equilibrium mixtures may be prepared which contain up to 100 per cent pure para-H2. The latter mixtures are metastable below 500 C and are slowly converted to the stable composition above that temperature. The thermal reaction has been well studied " and corresponds to a catalytic conversion by H atoms present at these temperatures. 

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