MNDO calculations of proton and methyl‐and ethyl‐cation affinities of neutral carbon, nitrogen, and oxygen bases

Abstract

Proton affinities (PA) of 80 neutral bases were calculated using the semiempirical molecular orbital procedure MNDO. These were compared with the corresponding experimental and, where available, ab initio STO-SG and 4-31G data. For the 12 bases studied which led to ions which were not hyperconjugatively stabilized, the mean absolute error between the calculated and experimental values was 7.2 kcal mol-'. However, a plot of these data revealed a clear tendency of MNDO to underestimate the PAS of the more basic molecules. Where hyperconjugative stabilization of the ion was possible, the calculated PAS were underestimated by a further 7-10 kcal mot1 for each attached alkyl group. Calculated and observed methyl-and ethyl-cation affinities were compared for 18 bases with qualitatively similar results.

Prototropic changes in cationic base‐pair adducts. I. Guanine protonation

“…AM^ proton affinities are very similar to experimental estimates of 223.8 and 223 kcal mol-' for the two bases, respectively [36]. As is generally the case, MNDO proton affinities are underestimated, while the ~~/ 4 -3 1~ proton affinities appear to be too large [22].…”

“…Calculated data for the protonation of both bases are collected in Table 11. For comparison with other estimates, these were computed using the experimental heat of formation of the proton (365.7 kcal mol-' [19]) rather than the theoretical value for reasons elaborated before [22].…”

An MNDO molecular orbital study of the reactions of protonated oxirane derivatives (XCHCH2OH+, X = CN, Cl, CH3, Ph) with simple nucleophiles. Implications for regioselectivity in the reactions of electrophiles with nucleic acid bases

“…The methyl and ethyl affinities of a wide variety of oxygen and nitrogen bases are almost perfectly correlated and show no special stability of the 0-ethyl species. 27 On the basis of our earlier MNDO calculations we argued that these subtle site selectivities could only be understood in terms of the energetic influences on the transition states themselvesa2* From a consideration of the calculated gas-phase reaction profiles and transition state geometries involved in the bimolecular alkylation of a group of simple oxygen and nitrogen nucleophiles by the methane and ethanediazonium ions (CH3N2+ and CH3CH2N2') we were able to develop a coherent qualitative explanation of the higher proportions of 0 -alkylation found in the reactions of agents where the related SN1 reactions were more facile. Similar calculations on the deoxynucleosides themselves were subsequently shown to give an approximately quantitative account of the relative activation energies for each of the nucleic acid base sites alkylated by the methyl and ethylnitroso~reas.~~ The only exceptions applied to sites directly adjacent to the sugar-phosphate backbone where other factors neglected in the model calculations on the isolated deoxynucleosides are believed to be imp~rtant.~'…”

“…eProton affinity of neutral parent based on AH&H+) = 365.7 kcal/mol (c.f. ref 27). fCalculated .rr-ionization potential.…”

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

“…(Sanz et al [lo], in addition to 3-216 and 3 -2 1 +~, also give MNDO and AM^ comparisons for a large series of azines.) These studies are able to encompass a larger number of molecules than the ab initio attempts, and the authors, in some cases, have been able to scale their results to experiment [12,19,20]. The recent release of the P M~ option has not been tested for its accuracies in calculating proton affinities with respect to experimental re-In this work, we focus on molecules for which protonation occurs on nitrogen or oxygen sites.…”