Proton affinities of molecules containing nitrogen and oxygen: comparing ab initio and semiempirical results to experiments

We report a comparison of theoretical and experimental proton affinities at nitrogen and oxygen sites within a series of small molecules. The calculated proton affinities are determined using the semiempirical methods AM 1, MNDO , and PM 3; the ab initio Hartree–Fock method at the following basis levels: 3‐21 G //3‐21 G , 3‐21+ G //3‐21 G , 6‐31 G *//6‐31 G *, and 6‐31+ G ( d , p )//6‐31 G *; and Møller–Plesset perturbation calculations: MP 2/6‐31 G *//6‐31 G *, MP 3/6‐31 G *//6‐31 G *, MP 2/6‐31 G +( d , p )//6‐31 G *, MP 3/6‐31 G +( d , p )//6‐31 G *, and MP 4( SDTQ )/6‐31 G + G ( d , p )//6‐31 G *. The semiempirical methods have more nonsystematic scatter from the experimental values, compared to even the minimal 3‐21 G level ab initio calculations. The thermodynamically corrected 6‐31 G *//6‐31 G * proton affinities provide acceptable results compared to experiment, and we see no significant improvement over 6‐31 G *//6‐31 G * in the proton affinities with any of the higher‐level calculations. © 1992 John Wiley & Sons, Inc.