Electronic states 17, conjugated molecules

Ultraviolet visible (UV VIS) spectroscopy (Section 13 21) An alytical method based on transitions between electronic en ergy states in molecules Useful in studying conjugated systems such as polyenes... 

The new orbital of the TT-electron in the 7T -state is larger than it was in the unexcited state. The two adjacent atoms with which the electron was associated in the ground state may be partially held by the electron in its expanded 7r -orbital. The atoms, in adjusting to the new binding condition, must move farther apart. They absorb the energy necessary to do this from the electron in its 7r -orbital. An additional vibrational ampHtude is attained by the two atoms as a system. Some added energy is transmitted to other atoms of the conjugated molecule. These atomic vibrational adjustments take place very quickly, in 10 to 10 s. 

Many early studies in ESR spectroscopy were concerned with anions, cations and triplet states derived from conjugated molecules. The unpaired electron(s)... 

Huckel s rule (Section 15.3) A rule stating that monocyclic conjugated molecules having 4n + 2 tt electrons n = an integer) are aromatic. 

The orbital phase theory includes the importance of orbital symmetry in chanical reactions pointed out by Fukui [11] in 1964 and estabhshed by Woodward and Holiimann [12,13] in 1965 as the stereoselection rule of the pericyclic reactions via cyclic transition states, and the 4n + 2n electron rule for the aromaticity by Hueckel. The pericyclic reactions and the cyclic conjugated molecules have a conunon feature or cychc geometries at the transition states and at the equihbrium structures, respectively. 

Now we assume the complete a — n separation for a conjugated molecule, which states that the total Hamiltonian can be written as the sum of the Hamiltonian for the u-core and that for the 7c-electron system ... 

Glowka et al. studied two tetrahydro[l,2,4]triazolo[4,3- ]pyrimidine derivatives 66a and 66b by X-ray diffraction <1994JCX375>. In the solid state, both molecules had the same conformation and were found to be packed in similar manner. The triazole rings proved to be planar, and the conjugation of the Jt-electrons with the lone pair of the N-8 atom was assumed to be the reason of the sofa conformation of the saturated six-membered ring. 

These simple molecular orbital pictures provide useful descriptions of the structures and spectroscopic properties of planar conjugated molecules such as benzene and naphthalene, and heterocychc species such as pyridine. Heats of combustion or hydrogenation reflect the resonance stabilization of the ground states of these systems. Spectroscopic properties in the visible and near-ultraviolet depend on the nature and distribution of low-lying excited electronic states. The success of the simple molecular orbital description in rationalizing these experimental data speaks for the importance of symmetry in determining the basic characteristics of the molecular energy levels. 

Since thermal deactivation from high-lying electronically excited states occurs on the picosecond timescale [70, 71] for conjugated molecules, excitation energy... 

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