A thermochemical study of relations between proton affinities and core electron binding energies

The correlation between the core ionization energy and the proton affinity of the same compound (Martin-Shirley correlation) is investigated from the thermochemical point of view. The thermochemical model is based on the equivalent core approximation, so that a core ionized compound is related to an isoelectronic protonated compound. The Martin-Shirley correlation may be attributed to two observed trends: (1) the core ionization chemical shifts of the isoelectronic compounds (e.g. C1s and N1s shifts of methane and ammonia) correlate; (2) the difference in formation enthalpies between the two compounds in the thermochemical model (e.g. methane and ammonia) is fairly constant. The correlation between core ionization energies of isoelectric compounds is manifested already for atomic ionization potentials, where the slope may be approximated by (Z/Z + 1)², e.g. ( 6 7)² in the case of carbon vs. nitrogen.