Theoretical Calculations of the Proton Affinities of n-Alkyl Systems

J. Phys. Chem. A, 104 (38), 8739-8743, 2000.

Theoretical Calculations of the Proton Affinities of n-Alkylamines, n-Alkyl Thiols, and n-Alcohols and the Ammonium Affinities of the n-Alcohols

Axel Patrick Ligon

Department of Analytical Chemistry, University of Wuppertal, Gauss-Strasse 20, 42097 Wuppertal, Germany

Received: January 14, 2000

In Final Form: May 17, 2000

Abstract:

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 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, -H_298,max, of the n-alcohols is 115.1 kJ mol^-1.