Determination of Gas Phase Basicities and Proton Affinities

The methods for the determination of GBs and PAs (Chap. 2.12) make use of their relation to (Eq. 9.23) and the shift of upon change of [AH] or B, respectively [187,188]. Basically, the value of GB or PA is bracketed by measuring eq with a series of several reference bases ranging from lower to higher GB than the unknown. There are two methods we should address in brief, a detailed treatment of the topic being beyond the scope of the present book, however. The kinetic method makes use of the dissociation of proton-bound heterodimers, and the ther-mokinetic method determines the equilibrium constant of the acid-base reaction of gaseous ions. In general, proton transfer plays a crucial role in the formation of protonated molecules, e.g., in positive-ion chemical ionization mass spectromehy (Chap. 7). 

The kinetic method [189-192] compares the relative rates of the competitive dissociations of a proton-bound adduct [A-H-B] formed by admitting a mixture of A and B to a Cl ion source [189,190]. There, the proton-bound adduct [A-H-B] is generated amongst other products such as [AH] and [BH]. Using standard tandem MS techniques, e.g., MIKES, the cluster ion [A-H-Bref] is selected and allowed to undergo metastable decomposition  

The relative intensities of the products [AH] and [B fH] are then used as a measure of relative rate constants of the competing reactions. In case the PA of the unknown was equal to that of the reference, both peaks would be of equal intensity. As this will almost never be the case, a series of reference bases is employed instead, and PA is determined by interpolation. The value of EA(a) is obtained from a plot of ln[AH] V[BrefH] versus pA(b) at ln[AH]V[BrefH] = 0. 

The thermokinetic method [193,194] uses the measurement of the forward rate constant of the equilibrium 

The thermokinetic method takes advantage of the correlation observed between fcejqj and AG2 through the relationship 

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