Potential energy curves isomerization

A kinetic scheme and a potential energy curve picture ia the ground state and the first excited state have been developed to explain photochemical trans—cis isomerization (80). Further iavestigations have concluded that the activation energy of photoisomerization amounts to about 20 kj / mol (4.8 kcal/mol) or less, and the potential barrier of the reaction back to the most stable trans-isomer is about 50—60 kJ/mol (3). 

Figure 2. CS INDO Cl potential energy curves describing irans-cis isomerization of BMPC about the...

Figure 5. Effects of an highly polar solvent (e=40) on the potential energy curves describing trans-cis isomerizations at the 2-3 and 3-4 C-C bonds of BMPC when using charge distributions obtained by...

Fig. 2. Potential energy curves for cis-lranQ isomerization. Curve I—ground-state singlet II—first-excited triplet state III—first-excited singlet state.

Figure 10. Schematic potential energy curves accounting for the primary event in vision. I. Excited state of rhodopsin and bathorhodopsin. II. Ground state of rhodopsin and bathorhodop-sin. III. Ground state of the isolated PRSB chromophore. E] and E2 are estimated from the equilibrium constants IC = [rhodopsin]/ [11-cis retinal][opsin] < 10 and K2 + [bathorhodopsin]/[all-trans retinal][opsin] < 10"2, respectively. With E3 = 1 kcal, this leads to E4 = E2 + Ei - E3 > 13 kcal. The value estimated from thermal noise data in photoreceptors is E4 = 30 kcal. Note that according to the isomerization model (Section III-B-l-c),...

The use of Walsh diagrams, based on one-electron molecular orbitals, shows that on n —> 7t excitation the azobenzene molecule is stretched, which is the beginning of inversion. All calculations and suggestions for an inversion mechanism agree that the potential energy curve for inversion has a relatively steep slope at the E- and the Z- geometries. This is corroborated by the experimental evidence of a continuous n tc absorption band in both isomers. In fact, a structured n band in an azo compound that can isomerize has never been observed. 

FIGURE 1.15 (A) The calculated singlet states relevant for isomerization by rotation according to Cattaneo and Persico. (Adapted from reference 184, by permission.) (B)The potential energy curve system for rotation and inversion according to Rau (From reference 34, by permission.)... 

Triplet state and Z/E isomerization of p-styrylstilbene induced by various sensitizers and the T, potential energy curves and one way photoisomerization (c -> t only) of styryl aromatics " are the subject of two papers on the behaviour of triplets of this class of compound. Time resolved resonance Raman spectra of the triplet state and radical cation of 5-dibenzosuberenol has been used to study and examine the mechanism of photoisomerization of this compound. ... 

Olson was the first to postulate that optical excitation of the ethylenic double bond involves rotation around a double bond in its excited state and that this rotation leads to an observable photoisomerization process [8-10]. Olson dealt with this aspect in terms of potential energy curves and mentioned the possibility of adiabatic photoisomerization process. Later, Lewis and co-workers [11] studied the photoisomerization process of tronj-stilbene with great interest but could not detect the cw-stilbene fluorescence. More recently, more detailed fluorescence studies carried out by Saltiel and co-workers [12-15] revealed that cw-stilbene fluoresces very weakly (Oa 0.(X)01) and shows an inefficient adiabatic isomerization process. The singlet mechanism currently proposed by Saltiel [16] is supported by quenching studies [17-20]. The extensive studies carried out on stil-bene and its analogs have already been reviewed [21-23]. Here the nature of the singlet excited state involved in the trans-cis isomerization process is dealt with. 

Strutinsky developed an extension of the liquid drop model which satisfactorily explains the fission isomers and asymmetric fission. For such short half-lives the barrier must be only 2-3 MeV. Noting the manner in which the shell model levels vary with deformation ( 11.5, the "Nilsson levels"), Strutinsky added shell corrections to the basic liquid-drop model and obtained the "double-well" potential energy curve in Figure 14.14b. In the first well the nucleus is a spheroid with the major axis about 25 % larger than the minor. In the second well, the deformation is much larger, the axis ratio being about 1.8. A nucleus in the second well is metastable (i.e. in isomeric state) as it is unstable to y-decay to the first well or to fission. Fission from the second well is hindered by a 2 - 3 MeV barrier, while from the first well the barrier is 5 - 6 MeV, accounting for the difference in half-lives. 

Figure 3.5 depicts the three distinct shapes of reaction path potential energy curves for unimolecular reactions. Figure 3.5(a), shows the reaction path potential energy curve for an isomerization reaction such as... 

Fig. 1. Evolution of the quantum state

FIGURE 8.17 Cluster molecules Potential energy curve of a heavy nucleus showing schematically the location of ground state, shape- and fission-isomeric states and of tri-molecular states. 

Calculations of the Potential Energy Curves for the trans-cis Photo-Isomerization of Protonated Schiff Base of Retinal. 

Figure 5.4 sketches the potential energy curves for dissociation and isomerization in one dimension. In both cases the molecule has to gain sufficient energy to undergo reaction. The Lindemann-Christiansen hypothesis, formulated independently by both scientists around 1921, says the molecule acquires the necessary energy by collisions with other molecules, after which it can either lose its energy in a subsequent collision, or cross the transition barrier to form products. The process is represented schematically by... 

Scheme 8.1 Representation of the chemical equilibria and photochemical reactions involving components IH and 2 and simplified potential energy curves (free energy versus ring-axle distance) for the threading-dethreading of the ring and the axle in its EE left) and ZZ (right) isomeric forms...

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