Conjugative overlap

Three dimensional X-ray diffraction data point to a largely planar structure in all of the molecules studied this ensures optimum conjugation (overlapping of the TT-electrons). Intramolecular hydrogen bonds probably contribute considerably towards supporting the planar conformation. 

Cyclopropyl ketones (29) behave as weakly conjugated compounds, with significant orbital overlap between one ty-bond (or sometimes two) of the cyclopropane ring and the jr-orbital of the carbonyl group (p. 242). The three-membered ring is reduced by lithium in liquid ammonia, with rupture of whichever cr-bond most effectively overlaps the carbonyl tt-orbital [280]. The reduction probably involves transfer of an electron to the jS -carbon atom of this cr-bond, so that conjugative overlap can be maintained during a smooth transition to the species (30) which is both a carbanion and a resonance-stabilised enolate radical. A second electron trans-... 

Two further interesting aspects deserve mention. In spite of the apparent presence of several conjugated chromophores, the alkaloid displays only end absorption in the u.v. This is because the ring stereochemistry allows little conjugative overlap between the various components of the apparently extended chromophore. 

FIGURE 10 5 (a) Isolated double bonds are separated from one another by one or more sp hybridized carbons and cannot overlap to give an extended it orbital (b) In a conjugated di ene overlap of two it orbitals gives an extended it system encompassing four carbon atoms... 

Section 10 7 Conjugated dienes are stabilized by electron delocalization to the extent of 12-16 kJ/mol (3 kcal/mol) Overlap of the p orbitals of four adja cent sp hybridized carbons in a conjugated diene gives an extended tt system through which the electrons are delocalized... 

The two most stable conformations of conjugated dienes are the s cis and s trans The s trans conformation is normally more stable than the s cis Both conformations are planar which allows the p orbitals to overlap to give an extended tt system... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

The deviation from planarity that is present in a structure such as 1 raises the question of how severely a conjugated system can be distorted from the ideal coplanar alignment of p orbitals and still retain aromaticity. This problem has been analyzed by determining the degree of rehybridization necessary to maximize p orbital overlap in 1. It is found that rehybridization to incorporate fractional amounts of s character can improve orbital alignment substantially. Orbitals with about 6% s character are suggested to be involved... 

Homoaromaticity is a term used to describe systems in which a stabilized cyclic conjugated system is formed by bypassing one saturated atom. The resulting stabilization would, in general, be expected to be reduced because of poorer overlap of the orbitals. The properties of several such cationic species, however, suggest that substantial stabilization does exist. The cyclooctatrienyl cation is an example ... 

In the NMR spectrum of the N-methylaniline enamine of cyclohexanone (135), the vinylic proton appears at a much lower field, i.e., at 324 Hz (75). Here the electron pair on nitrogen tends to conjugate with the phenyl group thus exhibiting a very small degree of overlap with the enamine double bond. 

Although I used the example of ethene, where n =2, the same consideration, apply to ZDO calculations on all conjugated molecules. All overlap matrices are real symmetric, positive definite and so have eigenvalues > 0. 

The UV spectra of —I—M 2-substituted thiophenes show two almost overlapping high-intensity bands both of which are displaced, with increasing conjugating power of the substituent, toward longer wavelengths. The extinction increases in the same order as in the... 

What accounts for the stability of conjugated dienes According to valence bond theory (Sections 1.5 and 1.8), the stability is due to orbital hybridization. Typical C—C bonds like those in alkanes result from a overlap of 5p3 orbitals on both carbons. In a conjugated diene, however, the central C—C bond results from conjugated diene results in part from the greater amount of s character in the orbitals forming the C-C bond. 

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

Conjugation (Chapter 14 introduction) A series of overlapping p orbitals, usually in alternating single and multiple bonds. For example, 1,3-butadiene is a conjugated diene, 3-buten-2-one is a conjugated enone, and benzene is a cyclic conjugated triene. 

In the following section, step by step a qualitative picture is formed describing the impact of intcrmolecular interactions oil the absorption and luminescence of organic conjugated chains. The present calculations do not distinguish between dimers and aggregates (for which the wavefunctions of adjacent chains interact in the ground state, due to, for instance, solid-state effects) and excimers (where overlap occurs only upon photoexcitation) [29]. 

The systems discussed in this chapter give some examples using different theoretical models for the interpretation of, primarily, UPS valence band data, both for pristine and doped systems as well as for the initial stages of interface formation between metals and conjugated systems. Among the various methods used in the examples are the following semiempirical Hartree-Fock methods such as the Modified Neglect of Diatomic Overlap (MNDO) [31, 32) and Austin Model 1 (AMI) [33] the non-empirical Valence Effective Hamiltonian (VEH) pseudopotential method [3, 34J and ab initio Hartree-Fock techniques. 

It is known that the overlap of wave functions of -electrons of conjugated bonds gives rise to a common multicenter system of electrons delocalized along the... 

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