Highest occupied molecular orbital 1,3-butadiene

Figure 14.12 Ultraviolet excitation of 1,3-butadiene results in the promotion of an electron from i / , the highest occupied molecular orbital (HOMO), to 1//3, the lowest unoccupied molecular orbital (LUMO).

The bonding orbitals and of ethylenes are combined in phase to be the lowest TZ molecular orbitals (tTj) of butadiene (Scheme 18). The out-of-phase combined orbital (jtj) is the highest occupied molecular orbital (HOMO). The in-phase combination of... 

The stereochemistry of the cyclobutene isomerizations and the reverse processes of this type, involving the formation of a bond between the ends of a linear system containing a number of 7i--electrons, has been discussed by Woodward and Hoffmann (1965). They term such processes electrocyclic and consider that their steric course is determined by the symmetry of the highest occupied molecular orbital of the open-chain isomer. In an open-chain system containing 4 7T-electrons (such as butadiene), the symmetry of the highest occupied ground-state orbital is such that bonding interaction between the ends of the chain must involve overlap between orbital envelopes on opposite faces of the system, and this can only occur in a conrotatory process ... 

These theories assert that the pathway of a chemical reaction accessible to a compound is controlled by its highest occupied molecular orbital (HOMO). For the thermal reaction of butadiene, which is commonly called ground-state chemistry, the HOMO is 2 and lowest unoccupied molecular orbital (LUMO) is photochemical reaction of butadiene, which is known to be excited-state chemistry, the HOMO is 1//3 (Fig. 3.5.6). 

Ultraviolet (UV) spectroscopy is a method cd structure determination applicable specifically to conjugated systems. When a coi jugated molecule is irradiated with ultraviolet light, ener gy absorption occurs and a it electron is promoted from the highest occupied molecular orbital tHOMOl to the lowest unoccupied molecular orbital (L.UMO1. For I,3< butadiene, radiation of 217 nm ia required. As a general rule, the... 

Photoelectron (PE) spectra have been measured for several (s-cis-diene)zirconocenes (5g, 51, 5s), for one example of the hafnium analogue (5n), and for (s-t/-ans-i7 -butadiene)Zr(C5Me5)2 (3c) (54). For both Cp and Cp series the highest occupied molecular orbital (HOMO) is separated from subsequent molecular orbitals (MO s) by a substantial energy difference. The observed PE spectra of 5 each exhibit a band around 6-6.5 eV (oi), well separated from several overlapping bands at 8-9 eV (6], f>2, fl2> bi). The PE spectrum of 3c shows a similar pattern [5.9 eV (aj) 7.1, 7.5, 7.5, 8.3 eV (fl2, i)]- The PE bands are in good accord with the... 

Let us begin with the thermal cyclization of a disubstituted butadiene, RCH=CH CH -CHR. As we have already seen (Fig. 29.6, p. 932), the highest occupied molecular orbital of a conjugated diene is 2 It is the electrons in this... 

On irradiation with ultraviolet light ihp), 1,3-butadiene absorbs energy and a tt electron is promoted from the highest occupied molecular orbital, or HOMO, to the lowest unoccupied molecular orbital, or LUMO. Since the electron is promoted from a bonding tt molecular orbital to an antibonding it molecular orbital, we call this a tt —> tt excitation (read as pi to pi star ). The energy gap between the HOMO and the LUMO... 

Electrocyclic transformations, in fact, proceed with high stereospecificity dictated by the number of n electrons in the open-chain it system (XIX). The reaction course taken by an electrocyclic transformation follows that in which the highest occupied molecular orbital in XIX has maximum bonding character throughout the transformation. The symmetry of this orbital, therefore, dictates the course of transformation and is the basis of the Woodward-HoiFmann selection rules (i). Consider, for example, the interconversion of butadiene (XXI) and cyclobutene (XXII). 

Analyze the reaction of two butadiene molecules by the Woodward-Hoffmann rules by examining the symmetry properties of the highest occupied molecular orbital (HOMO) of one diene and the lowest nnoccupied molecular orbital (LUMO) of the other. 

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