Photodissociation pathways

There are a variety of potential photodissociation pathways, including the following ... 

An issue of considerable interest is whether it would be possible to control or manipulate the photodissociation pathways of a particular molecule by initial vibrational preexcitation. A main concern in achieving this control is the difficulty in preparing states with vibrational energy that is localized in a specific mode that resembles the reaction coordinate. This is due to intramolecular vibrational... 

Although, relevant information about ferrous hemeproteins kinetics, dynamics and ligand photodissociation pathways has been obtain, less is known about ferric hemeproteins photophysic processes. Recent studies performed with Hbl-CN and Mb-CN at ultrafast time scale, have suggested that some of the transients intermediaries observed after ferrous complexes ligand photodissociation are observed in ferric Mb and Hbl [7], However, time-resolved infrared data shows that the complex remained six coordinated after photoexcitation. In this work we present ultrafast data on ferric Hbl-NO, HM-N3, HM-H2S and metHbl complexes that suggest a mechanism for the photoinduced reduction of Hbl species. 

N—[2—(5,5-Diphenyl-2,4-imidazolidinedion-3-yl)ethyl]-7-acetoxy-1-napthalene sulfonamide This substituted sulfonamide has a molecular weight of 543 daltons, and in the positive LD mass spectrum two ions at m/z 566 and m/z 582, corresponding to attachment of sodium and potassium ions, respectively, were observed. No significant photodissociation pathway other than loss of the attached cation was seen for either of these two ions. In the negative ion LD mass spectrum, an ion at m/z 500 originating from loss of C HjO was observed. This ion photodissociated when irradiated by the (gated) cw laser to produce an ion at m/z 251 (see Figure 9) (Equation 6). 

The second example of the application of TDDFT to the electronic spectroscopy of metalloporphyrins concerns a CO-ligated iron porphyrin, a system that models the active centers of hemoproteins, recently investigated by Head-Gordon et al [146, 147] in the context of a theoretical study of the initial step of the photodissociation pathway of CO-ligated heme. 

Since the last reviews of carbonyl photochemistry (61,191), there have been several studies of the primary photodecomposition processes- The earlier studies have shown that there are two main primary photodissociation pathways, radical (I) and molecular (II) ... 

Sanov A, Nandi S and Lineberger WC 1998 Transient soivent dynamics and incoherent controi of photodissociation pathways in f-Tciuster ions J. Chem. Phys. 108 5155... 

The donor—acceptor nature of co-frgands that comprise the primary coordination sphere, ultimately modulating the electroruc structures and transitions that dictate the photodissociation pathways of Ru—NO complexes. 

The photodissociative pathway was confirmed by Meyer and Hammond, who foimd that essentially no o- or p-hydroxyacetophenone was formed in the photolysis of phenyl acetate in the gas phase. Instead, all products could be rationalized by recombination of phenoxy and methyl radicals (formed from decarbonylation of acyl radicals). Similarly, a photodissociative mechanism for the photo-Claisen reaction was supported by observation of products expected from the recombination of radicals produced by photodissociation of 3-methyl-l-phenoxybut-2-ene (113, Table 12.6). In addition to phenol, products of the reaction are the rearranged ether 114, the two y,y-dimethylallyl phenols 115 and 116, and the two rearranged allyl phenols... 

Dixon RN, Hwang DW, Yang XF et al (1999) Chemical double slits dynamical interference of photodissociation pathways in water. Science 285 1249-1253... 

In another study, Mebel et al. examined the photodissociation path involving a spin-forbidden channel in methane. Their MRCI and equation of motion (EOM) CCSD calculations, including MEXP, indicated that the photodissociation pathway is likely to proceed via intersystem crossing from to repulsive Ti( A ), instead of the speculated So <— 5i internal conversion. Similar studies have been done by Hwang and Mebel on spin-forbidden reactions in N2O N2 + O ( P) and N2O N2 + O... 

Study of Vibronic Spectra and Photodissociation Pathways of Methane. 

CASSCF calculations have found Jahn-Teller geometries present in the photodissociation pathways for these systems, with tetrahedral symmetry for singlet Fe (CO)4, and a Dsh trigonal bipyramid for singlet Cr(CO)s and doublet Mn(CO)5. The topology on the lower surface around the point of intersection has a number of symmetry equivalent minima separated by equivalent transition states (see Fig. 3). In the case of Cr(CO)s and Mn(CO)5 the surfaces are very similar involving three... 

When a certain wavelength light excites a molecule, photodissociation can occur in energetically permissible multi-pathways, and the photolytic quantum yield for each process has to be determined experimentally. For example, the photodissociation pathways of ozone (O3) by solar ultraviolet radiation. 

On the theoretical side, Jaffe and Morokuma have reported small basis set (4-31G) MCSCF calculations on the photodissociation pathways of formaldehyde. More recently, Goddard and Schaeferhave completed an extensive Cl study of the H2CO surface including transition structures leading to both H2 + CO and hydroxycarbene. In this section we compare the results of M ller-Plesset calculations to the Cl calculations of Goddard and Schaefer. 

Waschewsky et al. (1994) used crossed laser-molecular beam experiments to monitor the competition between photodissociation pathways in chloroacetone. Their studies proved that C—C bond fission, process (II), competes with C—Cl bond fission, process (1), in C1CH2C(0)CH3 photolysis at 308 nm, although the latter process is favored. Several kJ of translational energy were evident in the translation of the initial products of the two competing dissociation pathways. The authors point out that this may indicate that, for both channels, dissociation proceeds via a reaction coordinate that has a significant exit barrier (barrier to reverse reaction), so fragments exert a repulsive force on each other as they separate. The pathway to dissociation probably does not involve... 

Some other photodissociation pathways have been suggested (Calvert et al., 2008) in which the alkyl nitrates (C H2 +i0N02) form either 1) HONO and an aldehyde... 

Primary process (I) is the dominant photodissociation pathway for hydrogen peroxide at all wavelengths of sunlight available within the troposphere (Vaghjiani and Ravishankara, 1990 Vaghjiani et al., 1992 Schiffman et al., 1993). 

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