Hexatrienes techniques

These compounds have been the subject of several theoretical [7,11,13,20)] and experimental[21] studies. Ward and Elliott [20] measured the dynamic y hyperpolarizability of butadiene and hexatriene in the vapour phase by means of the dc-SHG technique. Waite and Papadopoulos[7,ll] computed static y values, using a Mac Weeny type Coupled Hartree-Fock Perturbation Theory (CHFPT) in the CNDO approximation, and an extended basis set. Kurtz [15] evaluated by means of a finite perturbation technique at the MNDO level [17] and using the AMI [22] and PM3[23] parametrizations, the mean y values of a series of polyenes containing from 2 to 11 unit cells. At the ab initio level, Hurst et al. [13] and Chopra et al. [20] studied basis sets effects on and y. It appeared that diffuse orbitals must be included in the basis set in order to describe correctly the external part of the molecules which is the most sensitive to the electrical perturbation and to ensure the obtention of accurate values of the calculated properties. 

Unstable conformers of trans- and cis-hexatriene have been detected by means of the combination of matrix-isolation infrared spectroscopy and photoexcitation (or the high-temperature nozzle technique)84. Ab initio MO calculations at the HF/6-31G level have been performed for several conformers of 1,3,5-hexatriene93. The observed infrared bands of unstable conformers have been attributed to the gTt (major species) and gTg (minor species) conformers of /raw.s -hexalricne and the gCt conformer of cw-hexatriene93. It is noted that, in the previous paper93, the notation c is used for twisted structures for the sake of simplicity. The calculated torsional angles around C—C bonds for the gTt, gTg and gCt conformers are in the range between 32° and 45°. The observed and calculated vibrational frequencies of gTt and gCt are reported in Reference 93. 

The particle in a box model can be used to illustrate many of the techniques of quantum mechanics in chemistry. It is also of some use in predicting the absorption spectra of delocalised systems such as hexatriene. For the particle in a box model the true ground state energy is given by... 

The conrotatory electrocyclic ring opening of 1,3-cyclohexadiene can be observed by several ultrafast techniques. It is believed that the first step after excitation is attainment of a planar geometry, which occurs within 10 s. " The observations are consistent with an excitation followed by return to the ground state via a Cl that permits formation of either reactant or Z-l,3,5-hexatriene. The lifetimes of the two excited states are both shorter than 100 fs, and the product is formed within 200 fs. These results are summarized in Figure 12.21. 

As stated in the previous section, polyacetylene is quite insoluble as synthesized and not particularly stable upon long exposure to air. The lack of stability parallels the behavior of small polyenes such as 1,3,5-hexatriene and 1,3,5,7-octatetraene which polymerize in air at room temperature quite readily. However, it has been known for at least the past 60 years that a, co-diphenylpolyenes of equivalent length are both thermally stable and relatively insensitive to ambient oxygen. Diphenylpolyenes up to n = 8 have been synthesized and fully characterized by a wide range of spectroscopic techniques, and although polyenes longer than n = 8 have been reported, they are insoluble in... 

Let s now use this technique for analyzing electrocyclic reactions to make some predictions about the l,3,5-hexatriene-l,3-cyclohexadiene system (Fig. 20.16). 

---