The B3LYP/6-31+G(d,p) method was used to calculate the proton affinities of n-alkylamines, n-alkyl thiols, and n-alcohols and the ammonium affinities of the n-alcohols up to C-18. These affinities and the gas-phase acidities of the n-alcohols were all found to correlate linearly with the quotient n/(n + 1), where n is the number of carbon atoms in the alkyl chain. This correlation leads to a limiting value of Delta H for very long alkyl chains: for the amines, thiols, and alcohols, the calculated maximum proton affinities PA(298,max)(RX) were 938.7, 828.2, and 816.9 kJ mol(-1), respectively. The maximum ammonium affinity, -Delta H-298,H-max. Of the n-alcohols is 115.1 kJ mol(-1).