Hydrogen molecule, photoinduced

MacGillivray s group has employed rigid bifunctional molecules [42], such as 1,3-dihydroxybenzene and 1,8-naphthalenedicarboxyHc acid [42a], as linear templates to organize reactants such as fra s-l,2-bis(4-pyridyl)ethylene via hydrogen bonds for single and multiple photoinduced [2+2] cycloadditions, for the template-controlled synthesis of a l,2,3,4-(4-pyridyl)cyclobutane] [42b] and also of paracyclophanes [42d]. Very recently, [ ]ladderanes ( =2,3) have been synthesized in the solid state by UV irradiation of 2(5-methoxyresorci-nol) 2(4-pyr-poly-m-ene) (m=2,3) [42e]. 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

Photoinduced Hydrogen Transfer to Solvent Molecules in Clusters... 

The photoinduced electron transfer (PET) is especially important in the case of large or giant molecules (supermolecules), ie systems made up of molecular components in the same way as molecules are made up of atoms [11-19], As the systems are made up of a number of discrete components held together by different but not always exactly specified forces (covalent bonds, electrostatic interactions, hydrogen bonds, or other intermolecular interactions), the photoinduced electron transfer or energy transfer in these systems may be formally treated as intermolecular [20],... 

Diffusion coefficients (D) of various radicals created by the photoinduced hydrogen abstraction reactions from alcohols (ethanol and 2-propanol) as well as those of the parent molecules are measured by using the transient grating (TG) method. Dependence of D on the viscosity, molecular size, and temperature are investigated, and the results are interpreted in terms of microscopic aggregation of the radicals with solvents or solutes. 

Since translational diffusion process is sensitive to the microscopic structure in the solution, understanding the diffusion provides an important insight into the structure as well as the intermolecular interaction. Therefore, dynamics of molecules in solution have been one of the main topics in physical chemistry for a long time. 1 Recently we have studied the diffusion process of transient radicals in solution by the TG method aiming to understand the microscopic structure around the chemically active molecules. This kind of study will be also important in a view of chemical reaction because movement of radicals plays an essential role in the reactions. Here we present anomalous diffusion of the radicals created by the photoinduced hydrogen abstraction reaction. The origin of the anomality is discussed based on the measurments of the solvent, solute size, and temperature dependences. 

So far a hydrogen chloride molecule and a methyl free radical have been formed. Notice that the carbon radical was not formed originally, but only after the reaction with the more reactive chlorine radical, which in turn had been formed by the photoinduced homolytic fission. The hydrogen chloride takes no further part in the reaction. The methyl radical may react further with, for example, a chlorine molecule. Write down the equation and mechanism for this step. 

Figure 5.56 Number of molecules of oxygen accumulated at the surface of zirconia as a function of irradiation time during the (1) photoinduced adsorption (reduction) of oxygen, and (2) the photooxidation of hydrogen. Reprinted with permisison from Emeline et al. (2005a). Copyright (2005) American Chemical Society.

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