Isobutane proton affinity

Calculations at various levels allowed to determine slight variations among proton affinities and bond strengths of C-C bonds in straight-chain alkanes " and isobutane,which are relevant to cracking mechanisms. 

A long-chain fatty alcohol of molecular mass 242 Da is ionized via positive-ion chemical ionization with methane, isobutane, hydrogen, water, or methanol as the reagent gas. The two major ions in the spectrum are miz 243 and 225 (the loss of water from the protonated molecule). Predict how the ratio of ion abundances of m/z 243 versus m/z 225 will change with those different reagent gases. Rationalize your answers with a reasonable explanation. The proton affinity of this alcohol is 200 kcalmoD ... 

Another common reagent gas used in Cl is isobutane (1-C4H10). It possesses an even higher proton affinity (196 kcal moP ) than methane [16,17]. In this case, the ionization process is described by the following reactions ... 

When using isobutane in Cl, fragmentation is usually low since the proton affinity of the... 

Three reagent gases are mainly used in positive chemical ionization methane, isobutane, and ammonia. Their proton affinities (in kilojoules per mol) are methane. 

Daughter ions are observable in the spectrum. They are abundant in comparison to other mass spectra recorded in chemical ionization but much less numerous than in the electron ionization spectrum. This indicates that Cl is a soft ionization method compared to El, even if the reagent has low proton affinity. Note that the fragment ions of the Cl spectrum have the same m/z ratios as those recorded in El this is frequent but absolutely not systematic (see Chapter 9). The spectrum recorded in isobutane Cl displays almost exclusively the MH ion. One can furthermore observe a low abundance ion at m/z 302 (M+ ) resulting from a charge exchange reaction. There are almost no fragment ions. The ammonia chemical ionization spectrum shows no... 

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