Hexatrienes structure

Thin films have been prepared from relatively simple versions of the benzothiophene hexatriene structure decorated with ferf-butyl groups (the fcrf-butyl isomer of 4, for example), which can be used to photomodulate the refractive index [37]. However, photoactivity is reduced in this example and while irradiation of solutions produce as much as 45 % of the ring-closed isomer, only 10% can be generated in the spin-cast film. Another limitation with this example is the relatively low glass transition temperature (Eg), which was measured to be 47 °C. For practical applications, a Tg above 100 is desirable to avoid spontaneous crystallization. Compounds 18 [38], 19 and 20 [39] were prepared to address this limitation (Chart 21.3). 

Spectrum and the cyclization product is thermally converted back to its ring-closed counterpart. In either case, only one pair of thiophene rings can donate their C=C double bond to the hexatriene at any one time. As a result, the derivatives cannot really be considered to contain two different hexatriene structures. 

To be photochromic, fulgides should have at least one aromatic ring or heteroaromatic ring (Ar) on the exo-methylene carbon atom, so that they form a 1,3,5-hexatriene structure that may undergo 6jt-electrocyclization. 

Aromaticity has been deeply rooted in chemical literature, even before the structural and bonding principles have been clearly established [1-6]. It has been one of the most ubiquitous concepts and its origin dates back to the isolation of benzene by Faraday in 1825 illuminating the gas from whale oil [1]. Historically, the molecule benzene is closely associated with aromaticity. The first known use of the word aromatic as a chemical term is by Hofmann in 1855 [2]. However, even before that in 1833, Mitscherlich distilled benzene from benzoic acid and lime [3]. The cyclo-hexatriene structure for benzene was first proposed by Kekule in 1865 [4] while the cyclic nature of benzene was finally confirmed by the crystallographer Kathleen Lonsdale [5]. The term aromaticity owes its name to the pleasant aroma that some members of this class have and later on it was denoting exceptional stability that this family of compounds exhibit. An explanation for the exceptional stability of benzene is attributed to Sir Robert Robinson, who coined the term aromatic sextet... 

The most stable conformation of both hexatriene and octatetraene is the al -s-trans one. Figure 2 represents these structures schematically. 

Vibrational frequencies of hexatriene and octatetraene have been reported by several authors21,24-26,36. The increase in the size of these molecules with respect to butadiene limits the use of highly accurate levels of calculation, so that a good choice of scaling factors is necessary to obtain useful results. Kofraneck and coworkers21 have shown that employing scale factors determined from vibrational data for trans structures alone does not give a balanced description of cis and trans structures. 

Liu and Zhou29 have computed the quadratic force field of cis-hexatriene by a systematic scaling of ah initio force constants calculated at the planar C2V structure. Their results reproduce satisfactorily the observed spectral features of this molecule. 

The amount of high precision experimental structural data on conjugated polyenes is limited. Some structure results are presented in Table 5. In gas electron diffraction studies it is difficult to determine closely spaced bond distances accurately, because these parameters are highly correlated with the corresponding vibrational amplitudes. Today it is possible to calculate the vibrational amplitudes accurately, if the vibrational frequencies are known. This was, however, not the case when the GED studies presented in Table 5 were carried out. The observed differences between the terminal and central C=C bonds in the GED studies of traw.s-l,3,5-hexatriene and c/s-l,3,5-hexatricne are probably too large29. A very accurate X-ray study of traw.s-l,3,5-hexatriene has, however, been carried out also in connection with the preparation of this chapter4. Figure 4 shows the molecular structures of trans-1,3-butadiene and trans-l,3,5-hexatriene as found in the crystal lattice. 

FIGURE 4. Molecular structures of 1,3-butadiene and trans-l, 3,5-hexatriene presentation with thermal probability plots of 50%... 

A gas-phase electron diffraction study95 has shown that cis -hexatriene has a slightly twisted structure (torsional angle of 10° around the central C=C bond) and the lengths of the terminal C=C, central C=C and C—C bonds are 1.336, 1.362 and... 

A, respectively. The infrared and Raman spectra of m-hexatriene84- 86,96 and its deuterated analogs87,88 have been reported. The structures and vibrational frequencies have been calculated by means of MO methods87,88,91,93,94. According to ab initio MO calculations91,93,94 (HF/6-31G, HF/6-31G and MP2/6-31G levels), cis-hexatriene has a planar structure. The observed vibrational spectra have been reasonably explained by the... 

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 structural variety increases if the second (and further) substituent(s) is (are) not bound to the allene nucleus. For vinylallene (2), the additional vinyl group can be introduced at C-5, leading to 1,2,4,6-heptatetraene (22 only. E-isomer shown) or at C-4, providing 4-methylene-l,2,5-hexatriene (23), the former being an important substrate for cyclization reactions, as will be discussed in Section 5.5 (Scheme 5.2). 

Chen and coworkers26 reported the structures of spiral hexatrienes and the NMR data. 

In order to put the structure of benzene into better perspective, we give a similar calculation of 1,3,5-hexatriene for comparison. Table 15.5 shows the energies for a set of calculations parallel to those in Table 15.2 for benzene. The most obvious difference is the smaller total spread in the energies, about 3. 3 eV rather than the 5.5 eV for benzene, and the SCVB energy is closer to the full n than in benzene. Standard arguments say that there is only a rather small amormt of resonance... 

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