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 AM1, MNDO, and PM3; the ab initio Hartree–Fock method at the following basis levels: 3‐21G//3‐21G, 3‐21+G//3‐21G, 6‐31G*//6‐31G*, and 6‐31+G(d, p)//6‐31G*; and Møller–Plesset perturbation calculations: MP2/6‐31G*//6‐31G*, MP3/6‐31G*//6‐31G*, MP2/6‐31G+(d, p)//6‐31G*, MP3/6‐31G+(d, p)//6‐31G*, and MP4(SDTQ)/6‐31G+G(d, p)//6‐31G*. The semiempirical methods have more nonsystematic scatter from the experimental values, compared to even the minimal 3‐21G level ab initio calculations. The thermodynamically corrected 6‐31G*//6‐31G* proton affinities provide acceptable results compared to experiment, and we see no significant improvement over 6‐31G*//6‐31G* in the proton affinities with any of the higher‐level calculations. © 1992 John Wiley & Sons, Inc.