Polyene 1,3-butadiene

Linear polyenes (butadiene, hexatriene, etc.) absorb ultraviolet radiation. They have absorption maxima at the approximate wavelengths given in Table 6-1. 

The principal feature in the optical spectrum of linear polyenes is the strongly allowed absorption that lies in the near ultraviolet for the shortest polyene butadiene and shifts to... 

The most easily obtained information from such calculations is the relative orderings of the eneigy levels and the atomic coefficients. Solutions are readily available for a number of frequently encountered delocalized systems, which we will illustrate by referring to some typical examples. Consider, first, linear polyenes of formula C H 2 such as 1,3-butadiene, 1,3,5-hexatriene, and so forth. The energy levels for such compounds are given by the expression... 

The rectangular structure is calculated to be strongly destabilized (antiaromatic) with respect to a polyene model. With 6-3IG calculations, for example, cyclobutadiene is found to have a negative resonance energy of—54.7 kcal/mol, relative to 1,3-butadiene. In addition, 30.7 kcal of strain is found, giving a total destabilization of 85.4 kcal/mol. G2 and MP4/G-31(d,p) calculations arrive at an antiaromatic destabilization energy of about 42kcal/mol. ... 

The Tz orbitals of butadiene (Scheme 18) qualitatively obtained from the orbitals of ethylenes are also supported by the electronic spectra of polyenes. The HOMO of butadiene is higher that the HOMO of ethylene since the former is the out-of-phase combination of the latter. The LUMO of butadiene is the in-phase combination of the LUMOs of ethylene and lies lower than the LUMO of ethylene. The energy gap between the HOMO and the LUMO is smaller in butadiene. In fact, the wavelength ( m ) is longer for butadiene (217 nm) than for ethylene (165 nm). The wavelength increases with the chain length of the polyenes. 

A. Miyake, H. Kondo, M. Nishino, and S. Tokizane Catalytic formation of macrocyclic polyenes from butadiene, pp. 201-211 (13). 

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. 

Many examples of photocycloaddition reactions of olefins and polyenes yielding cyclobutane derivatives have been reported. Irradiation of mixtures of butadiene and 1,1-dichloroethylene in the presence of a... 

We will first discuss results corresponding to 1,3-butadiene. This molecule is the simplest of the series, so that several levels of calculation have been used, thus permitting one to establish the minimum requirements of the theoretical treatment. The extension to trienes, tetraenes and longer polyenes will be discussed in further subsections. 

Kofraneck and coworkers24 have used the geometries and harmonic force constants calculated for tram- and gauche-butadiene and for traws-hexatriene, using the ACPF (Average Coupled Pair Functional) method to include electron correlation, to compute scaled force fields and vibrational frequencies for trans-polyenes up to 18 carbon atoms and for the infinite chain. 

Because butadiene is the smallest polyene, its low-lying electronic states have been extensively studied theoretically49,51-62. Most of the studies have been performed for the most stable tram isomer. 

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. 

We naturally exclude here the cyclopropenyl, cyclopentadienyl and cycloheptatrienyl radicals, all of which can also be recognized as cyclic C H species much as we did not include in our discussion the enthalpies of formation of allyl and pentadienyl radical as part of our analysis of polyenes such as butadiene and hexatriene. 

Very powerful tools for the study of dienes and, to some extent, polyenes (in particular annular polyenes) are both H and 13 C NMR spectroscopies, which will be discussed in a separate section. As previously mentioned 1,3-butadiene is more stable in the s-trans conformation and in the H NMR spectrum both butadiene (1) and 2,3,6,7-tetramethyl-2,4,6-octatriene (3) display the vinyl proton at a low chemical shift value. In these simple examples the S value can be predicted theoretically. The 111 NMR spectrum of a C25-branched isoprenoid was examined as part of the structural determination for biomarkers and is shown in Figure l6. The other spectral and structure assignments are described later in this review. 

From the advent of organic chemistry, dienes (and polyenes) have played a very important role in both the theoretical and synthetic aspects. For example, 1,4-addition of bromine to 1,3-butadiene to form l,4-dibromo-2-butene rather than 3,4-dibromo-l-butene as the major product was a challenging problem for theoretical chemists, who interpreted the phenomenon in terms of resonance or delocalization of jr-electrons1. 

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