Dewar-type theory

The Dewar-type theories are parametrized so as to yield good values of the 25°C gas-phase standard enthalpy of formation AHf ss- These theories calculate as... 

The Dewar-type theories treat the molecule as a collection of valence electrons and atomic cores, where each core consists of an atomic nucleus and the inner-shell (core) electrons. For example, the core of a carbon atom consists of the nucleus and the two Is electrons. The simplest approach would be to take Vcc = 2b>a 2aKc,ab... 

Ihe parameter values in a Dewar-type theory are chosen as follows. One decides on a set of elements (for example, C, H, O, N, P, Si, S, F, Q, Br, I) for which the theory is to be parametrized, and one chooses a set of a few dozen molecules (containing only these elements) for which A///298 and the molecular geometry and dipole moment are... 

MINDO/3 is based on the INDO approximation. The remaining theories of this subsection are based on the far more justifiable NDDO approximation (see the preceding subsection). Tliese NDDO-based Dewar-type theories give significantly better results than MINDO/3, and MINDO/3 is essentiaUy obsolete, being little used nowadays. 

The Dewar-type theories treat the molecule as a collection of valence electrons and atomic cores, where each core consists of an atomic nucleus and the inner-shell (core) electrons. For example, the core of a carbon atom consists of the nncleus and the two Is electrons. The simplest approach would be to take Vcc = 2b>a2aVcc,ab = 2B>A2ACACB/f AB. where Ca and Cb are the core charges of cores A and B. For example, for a carbon atom, Ca = 6 — 2 = 4, the number of valence electtons. Although this form of V c is used in CNDO and INDO, it is more consistent with the approximations used to evaluate the electron-core interaction integrals in Dewar-type theories to take... 

The first useful Dewar-type theory was the MINDO/3 (third version of the modified INDO) method, published in 1975. For a sample of compounds containing no elements other than C, H, O, and N, the average absolute errors in MINDO/3 calculated properties are 11 kcal/mol in heats of formation, 0.022 A in bond lengths, 5.6° in bond angles, 0.49 D in dipole moments, and 0.7 eV in ionization energies. The errors in A.H°f 29S are larger than Dewar was aiming for. 

The positive charge in 28 is stabilized by /j-cr-C-C-hyperconj ligation with the C-C-ring bonds of the two cyclopropyl moieties. In the parlance of VB theory this is described by resonance of 28 with non-bonding resonance limiting structures, the homoallenyl cation type structure 28a, the homopropargyl cation type structure 28b and the Dewar-type limiting resonance structure 28c. 

Dewar s theory is still the most acceptable explanation of the bonding in silver-olefin complexes the relative contributions of the component M -ol (a o-type bond) and M- ol (a jr-bond) co-ordinate bonds is, however, uncertain. Spectroscopic evidence tends to suggest that the o-bonding is more important than the 7r-bonding. However the chemical shifts in H n.m.r. studies may be explained by either polarization effects or oH-M charge transfer (o-bond formation). Transfer of 0.1 to 0.2 electrons could account for the entire chemical shifts found but, since polarization effects cannot be calculated accurately, conclusions on the importance of the o-bonding can be only tentative I28>. As-symetric substitution at the double bond results in a non-symmetric orientation of the Ag+ ion (from H n.m.r.) but the shifts found could result from steric as well as electronic effects 127>-... 

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well established that the accurate description of the potential surface for a pericyclic reaction requires a much more complicated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large number of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qualitative ideas similar to the Woodward-Hoffmann rules virtually impossible. 

None of the theories proposed before 1951 to explain the nature of the bonding in metal-olefin complexes was entirely satisfactory (35). Chatt (S3) suggested that, in addition to the ordinary coordinate bond, some sort of bond involving the filled d-orbitals of the metal atom was essential for coordination of the olefin, but such a bond was difficult to formulate until Dewar (64) described it in terms of molecular orbitals. The structure which he proposed for the silver-olefin complexes, and that subsequently proposed for the platinum-olefin complexes by Chatt and Duncanson (35) are shown schematically in structures (I) and (II). The type bond, which has also been called a ji-bond (64, 4), is formed by the overlap of the filled bonding... 

This is the central idea of the theory proposed by Herndon and Hosoya to simulate the resonance energy obtained from the SCFMO calculation by parametrizing the values of a, b, and k [23], They took the reference values from the PPP type calculation by Dewar and de Llano [26] for the set of ten typical benzenoid hydrocarbons, and... 

Semiempirical molecular orbital methods23-25 incorporate parameters derived from experimental data into molecular orbital theory to reduce the time-consuming calculation of two-electron integrals and correlation effects. Examples of semiempirical molecular orbital methods include Dewar s AMI, MNDO, and MINDO/3. Of the three quantum chemical types, the semiempirical molecular orbital methods are the least sophisticated and thus require the least amount of computational resources. However, these methods can be reasonably accurate for molecules with standard bond types. 

Dewar suggested one further type of stabilization for cyclopropane. In analogy with benzene, which is stabilized by aromaticity provided by six ji-electrons, Dewar argued for o-aromaticity provided by the six o-electrons of the C-C bonds of cyclopropane. Dewar s arguments lay with simple MO theory and energetic analysis. 

The classical Dewar-Chatt-Duncanson model for metal-alkene bonding has been revisited with a combination of X-ray structural data (see Diffraction Methods in Inorganic Chemistry) and DFT calculations (see Molecular Orbital Theory), particularly on complexes of the type (acac)Rh(alkene)2. These indicate the existence of distortions from idealized geometry involving a twist (127), where the axis of the double bond is no longer perpendicular to the molecular plane and a roll (128), where the line... 

On the basis of molecular orbital theory, Dewar (64) divided the metal-olefin bond into a a portion and a tt portion. The former type of bond can result from a filled bonding olefin tt orbital overlapped with a vacant 5s orbital of silver (III), while the latter type of bond can result from a filled... 

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