MO SCF Methods

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

The predictions of relative stability obtained by the various approaches diverge more widely when nonbenzenoid systems are considered. The simple Hiickel method using total n delocalization energies relative to an isolated double-bond reference energy (a + fi) fails. This approach predicts stabilization of the same order of magnitude for such unstable systems as pentalene and fulvalene as it does for much more stable aromatics. The HMO, RE, and SCF-MO methods, which use polyene reference energies, do much better. All show drastically reduced stabilization for such systems and, in fact, indicate destabilization of systems such as butalene and pentalene (Scheme 9.2). 

Ground States of Molecules XXV MINDO/3. An Improved Version of the MINDO Semiempirical SCF-MO Method Richard C. Bingham, Michael J. S. Dewar and Donald H. Lo Journal of the American Chemical Society 97 (1975) 1285-1293... 

If the full molecular symmetry is assumed, the ground states of the cation radical of fulvalene and the anion radical of heptafulvalene are both predicted to be of symmetry by using the semiempirical open-shell SCF MO method The lowest excited states of both radicals are of symmetry and are predicted to be very close to the ground states in the framework of the Hiickel approximation these states are degenerate in both cases (Fig. 4). Therefore, it is expected that in both these radicals the ground state interacts strongly with the lowest excited state through the nuclear deformation of symmetry ( — with the result that the initially assumed molecular... 

Bingham RC, Dewar MJS, Lo DH. Ground states of molecules. XXV. MINDO/3. Improved version of the MINDO semiempirical SCF-MO method. / Am Chem Soc 1975 97 1285-93. 

An ab initio calculation uses the correct molecular electronic Hamiltonian (1.275) and does not introduce experimental data (other than the values of the fundamental physical constants) into the calculation. A semiempirical calculation uses a Hamiltonian simpler than the correct one, and takes some of the integrals as parameters whose values are determined using experimental data. The Hartree-Fock SCF MO method seeks the orbital wave function 0 that minimizes the variational integral <(4> //el initio method. Semiempirical methods were developed because of the difficulties involved in ab initio calculation of medium-sized and large molecules. We can... 

Computer programs for the HMO, EH, PPP, CNDO, INDO, MINDO, and ab initio SCF MO methods can be obtained from the Quantum Chemistry Program Exchange, Chemistry Department, Indiana University, Bloomington, Indiana, 47401. 

The 77-SCF MO method and the Del Re procedure for u-electrons have been applied by Danilov148 to evaluate the enthalpy changes on tautomeric conversion 2 -> 6 in the ground state and the first excited singlet state for cytosine and its anionic and cationic forms. The results show that although in the ground state of cytosine the tautomeric... 

Energies of Highest Occupied (HOMO) and Lowest Empty (LEMO) Molecular Orbitals of Adenine, Thymine, and Their Pair Calculated by w-SCF MO Method (Closed-Shell)"... 

The values of the ionization potentials of a few minor pyrimidine bases were estimated from the charge transfer spectra255 (Table XXXIII). These values compare well with the ionization potentials calculated by means of the 77-SCF MO method.156 However, the relative order of the ionization potentials given by the calculations (77-SCF MO and CNDO/2 methods, Tables XXXIII and XXXIV)—thiocytosine... 

Electronic energy levels can be satisfactorily calculated using the PPP-SCF-MO method and the results used to assign bands in the observed UV spectra. Comparison of results for... 

With sodium borohydride and catalytic amounts of titanyl acetoacetonate, a,fi-unsaturated carbonyl compounds give allyl alcohols regioselectively, whereas a-diketones and acyloins are reduced to vicinal diols.325 Enantioselectivities in the reduction of acetophenone, catalysed by 1,3,2-oxazaborolidones, have been examined using the AM1-SCF MO method. The optimized geometries, thermal enthalpies, and entropies of R and S transition states in the stereo-controlling steps of the reduction have been obtained.326... 

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