Mindo/3 and mndo calculations of closed‐ and open‐shell cations containing C, H, N, and O

Abstract

Heats of formation of 119 closed- and open-shell carbocations calculated by the semiempirical quantum chemical methods MINDO/3 and MNDO are reported and compared with experimental data. With proper consideration of failures in specific areas, both methods can be used for the thermodynamics of carbocations containing C, H, N, and O. MINDO/3 predicts unrealistic values for nitrogen containing cations with nitrogen multiple bonds and is not suited for closed-shell cations containing oxygen. Saturated acyclic hydrocarbon radical cations often are computed with abnormally long CC bonds by MNDO. Otherwise, the standard deviation of the two methods is not very different, being in the range of ±13 kcal/mol. MINDO/3 tends to overestimate the cation stabilities, whereas MNDO calculates cations usually too high in energy. Some of the errors which were found in the calculations of the ions are related to the computed values for the parent neutral structures, but others are not.

Formation Enthalpies of Ions: Routine Prediction Using Atom Equivalents

“…All in all, present NDDO results for ions are not significantly better than those obtained using obsolete schemes such as MINDO/3 or MNDO. 98 Indeed, accordingly to the RMSD data in Table 11, typical errors are about 12 kcal/mol, or somewhat lower for neutrals using PM3. They are at best about 4-6 times larger than G3MP2B3 errors.…”

Correlation of the acidity of substituted phenols, anilines, and benzoic acids calculated by MNDO, AM1, and PM3 with Hammett‐type substituent constants

“…The results listed in Table VI (entries [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] show that the electronic charges on the acidic hydrogens and on the anionic heteroatoms are a good index for the electronic effect of substitu-ents on aqueous and gas phase acidities of phenol and aniline systems. Moreover, the comparison of the energy differences (6 AH;) for processes l a and l b (Tables I and 111) with those obtained for processes 2a and 2b indicates well the decomposition of the substituent effect into the effect on the neutral molecule and the appropriate anionic species.…”

Semiempirical calculations of gas-phase basicities for compounds containing carbon, hydrogen, nitrogen, and oxygen applied to mass spectrometry

“…The heats of formation for protonated organics (MH+) are of particular interest, but less theoretical work has been done in this area than for species resulting from charge transfer (M+) or other processes. MINDO/3 and MNDO calculations have been performed on a variety of cations (M+), giving errors of-7 and +9 kcal/mol, respectively, when compared to experimental values (8). The present work expands the number and variety of compounds examined with the AMI parameterization by calculating heats of formation for 127 species in neutral and protonated forms.…”