Potential energy surface excited isomers

In the following sections, studies of isomeric ions are reported in which the ions are reactively probed. Where calculations are available, information on potential energy surfaces is given. This is usually the structure of the stable isomeric forms and transition states and their relative energies thus only points on the potential surface are known. The detailed form of the potential surface is almost never available nor is the connectivity between the various states usually established theoretically (chemical intuition is often used to connect the states). Pertinent experimental data on CID and metastable ions, isomers produced in binary reactions, and potential surfaces probed by binary reactions (with the excited isomeric ion as the reaction intermediate) are also given. 

The fluorenone-sensitized irradiation of the all rrawj-triene (140) gives a photostationary mixture composed of the trans-cis-trans, cis-trans-trans and cis-cis-trans isomers. The potential energy surfaces of the ground and excited state of the triene (140) have been mapped,and a study of the fluorescence from this triene in lipid bilayers and isotropic solvents has been carried out. The photophysical properties of the dithienylpolyenes (141) and (142) have been measured. The photoisomerism of all-tranj-retinal to the 11-m isomer brought about by honeybee retina enzyme photoisomerase has been described. ... 

In 2-styrylanthracene (2c) the rotational isomerization does not occur in the singlet excited state, but occurs in the triplet state in a one-way manner from the s-trans isomer to the s-cis isomer as revealed by the observation of the change of the T-T absorption spectra [109]. Thus, the potential energy surface is proposed as shown in Fig. 10. Furthermore, in fV-methoxy-l-(2-anthryl)ethanimine (20) the rotational isomerization took place in the excited singlet state as well as in the excited triplet state [110]. The former behavior is followed by the change of the fluorescence spectra from the s-trans to the s-cis isomer, whereas the latter is studied by the spectral change of the T-T absorption spectra. 

As the complexity (atomicity) of the ions increases, a greater number of structural isomers are possible. Theoretical calculations are sometimes available for small molecular ions that indicate which structural isomers have minima on the potential energy surface. Experimentally, in SIFT and SIFDT studies, the existence of two or more structural isomers at the same molecular weight is often manifest by curvature on ion decay plots as a result of the isomers different reactivities with specific monitor gases. Then one has to determine if the different reactivity is due to the presence of excited states of a given structural isomer or to... 

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