MNDO studies

Quantum chemical predictions of tautomeric equilibria Ab initio multireference Cl studies of tautomers UPS spectra Combined approach to the tautomerism in azaaromatic heterocycles by N, C, and H NMR Fluoro-azoles Experimental data and MNDO studies Tautomerism A review... 

MO calculations were performed rarely for thiopyrans except for an MNDO study of 2 [84ZN(A)267], Charge distribution and orbital interaction concepts were explored in an interpretation of model reactions of thiopyrylium ions with azides giving 68 and the corresponding 3,5-unsub-stituted thiopyrans (84T3549) as well as for the equilibria between 1 and 2 or 167 and 168, respectively (92JOC4431). 

The significantly lower dediazoniation rate of the l/f-3,5-dimethylpyrazole-4-di-azonium ion (8.22) compared with that of the benzenediazonium ion was the central subject of an MNDO study by Brint et al. (1985). The diazonium ion 8.22 has been recovered unchanged after heating for 3 h at 100 °C in aqueous hydrochloric acid. It is not completely decomposed after a similar treatment for 48 h (Reilly and Madden, 1925). Brint et al. calculated the heats of formation of this diazonium ion and of the corresponding heteroaryl cation 8.23 (Scheme 8-16). They found that the values of A//f for the diazonium ion 8.22 and for the benzenediazonium ion are almost identical, whereas that for the cation 8.23 is much greater. The energy required to dissociate the pyrazolediazonium ion is therefore nearly twice that required for the benzenediazonium ion (A//f = 329 and 194 kJ mol-1, respectively). 

A detailed ab initio and modified neglect of diatomic overlap (MNDO) study of structural parameters and pyramidal phosphorus atom inversion and also enthalpy differences between the participants in the conformational equilibrium (axial and equatorial) of 1-R-phosphorinanes (R = H, CH3, C2H5) has been investigated and is in good agreement with the observed NMR and X-ray values <1999IJB660>. 

However, another aspect of these methyl groups pertains to whether or not they serve to stabilize or destabilize a pentadienyl anion. Of course, it is clearly recognized that a stabilizing influence would ensue for pentadienyl cations and radicals. Results of the MNDO study indicate that methylation at an active (i.e., 1,5, or especially 3) position of a pentadienyl anion is accompanied by a stabilizing effect of ca. 1 kcal/mol, while methylation at a 2 or 4 site is destabilizing, by ca. 2. kcal/mol188. For the pentadienyl cation, methylation of the 1, 2, or 3 positions always leads to stabilization, by ca. 6, 1, and 4.5 kcal/mol, respectively. 

Taking the solvation into account in such models is both very difficult and necessary for the best possible understanding of these exceedingly complex phenomena. The disolvation of the amides dimer (one solvent per lithium in THF, THF + HMPA or THF + DMPU)50 seems to be indicated, while trisolvated dimers appear relatively unstable. However, a very extensive semiempirical theoretical (MNDO) study on the various cyclic and open mixed aggregates formed by LDA and LiTMP with LiCl or three different enolates, solvated by discrete molecules of THF or HMPA, showed that general conclusions are almost impossible to draw48. A complex interplay of steric effects, induced by the partners of the aggregate and the solvent, seems to be the dominant influence on the relative stabilities of the species characterized. 

Uchida, N., T. Maekawa, and T. Yokokawa (1986). MNDO studies of basicity of borate glasses. J. Noncryst. Solids 85, 290-308. 

AlOAlO according to ab initio calculations (7) but 27 kcal mol" less stable according to MNDO calculations (8-9). Also possible is rhombic Dgj AlOgAl with an 0-0 bond similar to that predicted by ab initio methods for LlOgLl (14). This geometry was not considered in the ab initio study (7) and showed no minimum in the MNDO study (8). Better calculations are needed to resolve the question. Thermochemical and spectral evidence is inconclusive (15). Uncertainties in the structure and vibrational frequencies cause an uncertainty of 3 cal K mol" in S and -(G -H )/T at 2000 K. Estimated values for the rhombic peroxide (6) are -1 cal K" mol" lower at 2000 K but this difference could be plausibly reversed by use of lower frequencies. 

Singh, R. P., Singh, V., Srivastava, J. N., Bhattacharjee, A. K. MNDO study of Baeyer-Villiger oxidation of indan-1-ones. Indian J. Chem., Sect. B 1996, 35B, 1101-1103. 

Saa, J. M., Ballester, P., Deya, P. M., Capo, M., Garcias, X. Metal-Induced Reductive Cleavage Reactions An Experimental and Theoretical (MNDO) Study on the Stereochemical Puzzle of Birch and Vinylogous Birch Processes. J. Org. Chem. 1996, 61,1035-1046. 

Dewar, M. J. S., McKee, M. L. Ground states of molecules. 50. MNDO study of hydroboration and borohydride reduction. Implications concerning cyclic conjugation and pericyclic reactions. J. Am. Chem. Soc. 1978,100, 7499-7505. 

Perhaps the first example of a computationally simulated Bronsted correlation was the semiempirical MNDO study of Anhede, Bergman and Kresge (ABK) [34] for a series of PTreactions involving fluoroethanols, Eq. (19.11), with n= 0-3. 

An obvious candidate for a stable noncyclic carbenium ion is the tert-butyl cation observed in superacidic media. Even if the proton affinity of isobutene (Table 22.1) does not make it very likely that tert-butyl cations will exist in zeolites, several quantum chemical studies have localized stationary points for tert-butyl cations in zeolite and found that they are less stable than the adsorption complex, but are similar in stability to surface butoxides. Because of technical limitations vibrational analysis, which could prove that this cation is a local minimum on the potential energy surface, that is a metastable species, have only recently been made. Within a periodic DFT study of isobutene/H-FER a complete vibrational analysis for all atoms in the unit cell was made [48], and as part of a hybrid QM/MNDO study on an embedded cluster model of isobutene/H-MOR a vibrational analysis was made with a limited number of atoms [49]. Both reached the... 

These compounds were the least studied theoretically. In 1984, Olivella et al. published an MNDO study of 14N,N-biazoles including quaternary salts (84CJC687). The most important conclusions are summarized in Figure 9. Averaged values (depending on the p atoms) show the nature of the interactions. 

Fenestranes larger than the [4.4.4.4] system have not been the subject of published ab initio calculations, but there are MNDO studies of several of these hydrocarbons ... 

Theoraical studies have been carried out on (a) Li2C=CLi2 (at the MP2/6-316 levd)," (b) (CH2=CHCH2Li)2 (MNDO calculation),(c) pentadienyl-lithium (MNDO study) and (d) MeCH= H H2,M+ (13 M=Li, Na, K or Rb) ab initio calculation uang diffuse function augmented split valence basis sets) c -geometries ae favoured for (13). 

Glidewell C, Lloyd D (1984) MNDO study of bond oidtrts in some conjugated bi- and tri-cyclic hydrocarbons. Tetrahedron 40 4455-4472... 

Hydroboration attracted several theoretical studies in 1978. Publications which have appeared include an ab initio study of the reaction of BHs with ethylene, a CNDO/2 study of the nature of the ethylene-borane complex, the transition state in the hydroboration reaction, a MNDO study of hydroboration of alkenes and alkynes, and a MNDO study of hydroboration and borohydride reduction with implications concerning cyclic conjugation and pericyclic reactions. Finally, HaBCHO was one of the molecules which has been studied with regard to banana bonds of the carbonyl group. ... 

M. J. S. Dewar and M. L. McKee, Inorg. Chem., 1978,17, 1569. Ground states of molecules 47. MNDO studies of boron hydrides and boron hydride anions. 

Synthetic strategies towards Ceo- Molecular mechanics and MNDO study on sumanene and related structures. Journal of the Chemical Society, Perkin Transactions 2,1867-1871. 

The Science Citation Index lists 4605 citations of the original MNDO paper in the period 1977-1996, with an average of 341 per year over the last ten years, which indicates the widespread use of MNDO in computational chemistry. It is thus obviously impractical to attempt a comprehensive survey of published MNDO studies. Early MNDO applications up to 1982 have been listed " based on the entries in Chemical Abstracts, and selected MNDO studies on chemical reactions up to 1987 have been summarized in a later review. ... 

With the introduction of AMI in 1985 and PM3 in 1989, there are now three established semiempirical methods available that are based on the MNDO model. It is legitimate and common practice to use the approach that is most suitable for a given application. Since AMI and PM3 are, in a statistical sense, somewhat more accurate than MNDO (due to the more refined parametrization), they are employed more often than MNDO in current work, even though there are still many MNDO papers being published. For example, there have been numerous MNDO studies on the structures, stabilities, spectra, and reactions of fullerenes since 1990, which have been reviewed recently... 

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