Method MINDO

There are three modihed intermediate neglect of differential overlap (MINDO) methods MINDO/1, MINDO/2, and MINDO/3. The MINDO/3 method is by far the most reliable of these. This method has yielded qualitative results for organic molecules. However its use today has been superseded by that of more accurate methods such as Austin model 1 (AMI) and parameterization method 3 (PM3). MINDO/3 is still sometimes used to obtain an initial guess for ah initio calculations. 

MINDO/3 methods clearly have severe problems with some of the conjugated systems. The MNDO/AM1/PM3 family perform somewhat better, although none of them can predict the correct ordering. The SAMI method is not an improvement for this case. The mean absolute deviation (MAD) for the predicted stabilities is 10kcal/mol, which is a typical accuracy for semi-empirical methods. 

The processes (11) and (12) have been studied by means of the MINDO/3 method optimizing the geometry with respect to the cartesian coordinates of the model compound atoms. The reaction enthalpies AH , calculated for the reactions (11) and (12) are to be seen in Table 11. 

The latter three of the above points are dealt with in the following parts (see parts 4.2-4.5). Experimental investigations of the inner structure of the cations can be supplemented by quantum chemical calculations 104 106). For example, the MINDO/3 method allows the heats of formation of carbocations to be calculated 107). A comparison of some calculated and experimental values (Fig. 6) shows that the reproduction quality of MINDO/3 varies. 

The longer the chain of unbranched carbenium ions is, the more the calculated values deviate from those found experimentally in the direction of higher stability. However, the expected order of ion stability (primary < secondary < tertiary) remains intact. For cations, which are able to delocalize the positive charge due to conjugation in phenyl rings, the calculated stability is too small. The example of the acetyl cation shows that the reliability of the MINDO/3 method decreases, if charged species, especially those containing hetero atoms with free electron pairs, are calculated. 

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

The CNDO/2 and the MINDO/3 methods with complete optimization of geometry were applied during the quantum chemical calculations. 

In Ref.125) the calculation of an activation barrier for reaction (21) in the gas phase is considered to be an error of the MINDO/3 method and the process is assumed to be activationless. But in respect to the medium effect a barrier of 54 k J mol-1 is obtain-ed which agrees again with the results from Huron-Claverie calculations. Bertran et al. calculated the influence of the solvation on the electrophilic attack of a proton 133) or a methyl cation 134,135) on ethene using a MINDO/3 supermolecule model. Smaller reaction enthalpies also result in solution than in the gas phase in addition to the appearance (H+ + ethene) or the increase (CH 4 + ethene) of an activation barrier1361. 

Decreasing reaction enthalpies and an increase in the activation barriers are calculated in the gas phase as the chain length increases. In solution the activation barriers are higher and the reaction enthalpies increase along with chain length. The calculation of activation barriers don t seem to be an error of the MINDO/3 method. 

The heat of formation (13.7 kcal/mol) and the first ionization potential (8.6 eV) of silabenzene have also been calculated using the MINDO/3 method (177). 

However, another study concluded that the changes of the hydrogen-bond stability may be important in biological processes. For these, the influence of local electric fields created by Li+, Na+, and Mg2+ ions on the properties and reactivity of hydrogen bonds in HF and HC1 dimer has been carried out by means of ab initio self-consistent field (SCF) method [33]. A few years later, the effect of intensity and vector direction of the external electric field on activation barriers of unimole-cular reactions were studied using the semiempirical MINDO/3 method [34]. However, both semiempirical and ab initio calculations were performed to study the multiplicity change for carbene-like systems in external electric fields of different configurations (carbene and silylene) and the factor that determines the multiplicity and hence the reactivity of carbene-like structures is the nonuniformity of the field [35]. 

The results of selective hydrolysis together with the percentages of isomers formed suggest that the facility for hydrolytic cleavage is in the order C-10 > C-8 > C-6. This order agrees with the order of bond energies between bromine and carbon atoms as calculated by the MINDO/3 method for the five isomeric monobromobenzo[6]tropoxazines. 

Frontier orbital energies and coefficients for sydnone, calculated by the MINDO/3 method, and the predicted dominant frontier orbital interaction with an electron-deficient alkene. 

A generally better agreement with experimental results appears to be obtained with the more sophisticated PNDO, NDDO, and MINDO/3 methods (76CPL(37)608 76TCA(42)311 81JHC1055 84ZC303 84ZOB674), yet they have only been used on a limited number of acyl heterocycles. 

The dipole moments of oxepin and benzene oxide have been calculated to be in the range 0.76-1.36 D and >1.5 D respectively using the ab initio SCF and MINDO/3 methods (80JA1255). The lower calculated dipole moment would be in accord with experimental observations where the equilibrium was found to favor oxepin (7) in less polar solvents. Coordination between the oxirane oxygen atom and polar solvent molecules would also strengthen the C—C bond of the epoxide and thus lead to a preference for the benzene oxide isomer <72AG(E)825). Thus the proportion of oxepin (7) was found by UV spectral analysis to be higher in isooctane solvent (70%) than in water-methanol (10%). 

The observed order clearly contradicts the results of recent calculations of hydride ion affinity as carried out by the MINDO/3 method, indicating thiophene to be more reactive than furan.190... 

An alternative two-step mechanism involving a spin-paired diradical intermediate has also been considered for 1,3-cycloadditions.18,68,69 However, ab initio calculations70-72 on a wide variety of 1,3-dipoles and dipolarophiles are found to coincide essentially with a synchronous 1,3-cycloaddition mechanism.15,17 On the other hand, a two-step mechanism passing through two transition states separated by an intermediate has been derived using the MINDO/3 method, and found to be compatible with substituent and solvent effects as well as stereospecificity observed in 1,3-cycloadditions.73 However, several factors beyond FMO interactions, such as closed shell repulsions, geometrical distortions, polarization, and secondary orbital interactions, all influence mechanisms, rates, and regioselectivities in cycloaddition reactions.74... 

Several versions of modified INDO (MINDO) that employ such a parameterization have been proposed. These include MINDO/1, MINDO/2, MINDO/2, and MINDO/3, only the last of which resulted in a quantum chemical program that was widely used. MINDO/3151 is parameterized for H, B, C, N, O, F, Si, P, S, and Cl, although certain combinations of these atoms are not parameterized. The MINDO/3 method is no longer heavily used because the parameterized NDDO methods are generally more accurate. 

The Fie1 photoelectron spectra of thioxanthone have been measured and the four ionization energies of 8.04, 9.12, 9.34, and 10.87eV compare favorably with the orbital energies calculated by the MINDO/3 method <1996JE071>. 

---