Infrared multiphoton excitation, isomerization

Selective isomerization of the ( )-alkene to (Z)-alkene has also been carried out via infrared multiphoton excitation [70]. Upon IR laser irradiation at MOOO/cm of 2-butene 19a and 2-pentene 19b, essentially quantitative contrathermodynamic conversion (i>to-Z) can be achieved (Sch. 9). It is concluded that small differences in cross-section may be amplified in the multiphoton up-pumping process. However, the quantum yield was too small to measure (<10-6). 

Different types of chemical reactions involve different types of vibrational modes, e.g. dissociation reactions may be controlled by stretching vibrations, isomerizations by skeletal modes, and so on. The argument that infrared quanta are relatively energy-poor and infrared transitions generally have low absorption cross sections, especially if multiphoton excitation is required, limits the choice of suitable molecular transitions. With respect to these constraints the type of reaction chosen and described below was dissociation, involving molecules with maximal transition dipole moments, comparatively weak bonds to be broken, and vibrational excitation in the mid-infrared spectral range. 

Farneth s group used infrared multiphoton irradiation of VCP with a CO2-TEA (transversely excited atmospheric) laser to form cyclopentene in low yield (-32%). Unfortunately, the remainder of the VCP isomerized to a mixture of cyclic and acyclic dienes. ... 

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