Photofragmentation reactions

This chapter has to do with reactions wherein the photochemical event is the breaking of a bond in a molecule. For a single bond this results in the formation of a pair of radicals or a diradical. For a double bond as in diazo compounds or in azides a carbene or a nitrene and nitrogen are formed. All these intermediates will then undergo further mono- or bi-molecular dark reactions or eventually recombine to ground state starting materials. 

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The Mg+ complexes of cytosine, thymine and uracil are the most complex system studied via photodissociation spectroscopy to date . A complication for these systems is that these nucleobases can exist in various tautomeric forms and that complexation of a metal can change the stability order of the tautomers. DFT calculations located four tautomeric Mg(cytosine)+ complexes, and three of these (29, 30, and 31) were suggested to be responsible for the four reactive photofragment ions 32-35 observed at a wavelength of 360 nm (Scheme 4) . Related photofragmentation reactions were observed for the Mg(thymine)+" and Mg(uracil)+" complexes . ... 

The direct determination of the rates of elementary photofragmentation reactions. These studies have provided information concerning the... 

Direct Determination of Rates of Elementary Photofragmentation Reactions. These studies have provided information concerning the molecular features that determine, for example, how energy is distributed among the products of a reaction, and they provide hints concerning when it is possible to achieve mode-selective dynamics in large molecules. 

Alternative Probes. Even though C02 possesses a broad range of special virtues, other small molecules could also be used as reporter probes. Once the technique has been refined using C02, photofragmentation reactions that yield N2, CO, HCN, S02, 02, NO, N02 or any of a host of other small molecules should be amenable to similar mechanistic studies using Raman or FTIR spectroscopy. 

Exciplex emission and photofragmentation reactions of contact ion pairs generated via quenching of cyanoaromatic singlet states by aminoalcohols is an example of a detailed study of an electron transfer reaction involving chemical change. ... 

In this review, we explain the SAC-CI applications to molecular spectroscopy with some examples. In Section 2, we briefly explain the theoretical and computational aspects of the SAC-CI method. Then, we show some SAC-CI applications to molecular spectroscopy the excitation and ionization spectra of tt-conjugated organic molecules (Section 3), collision-induced absorption spectra of van der Waals complex (Section 4), excitation spectra and NMR chemical shifts of transition metal complexes (Section 5), photofragmentation reaction of Ni(CO)4 (Section 6), absorption spectrum of free-base phthalocyanine (FBPc) and bacterial photosynthetic reaction center... 

Transition metal carbonyl is important in laser chemistry as sources of metal atoms or as precursors for chemical vapor deposition (CVD) [104]. Ni(CO)4 shows a typical photofragmentation reaction initiated by the XeCl laser (308 nm, 4.03 eV), and the knowledge on the mechanism is valuable for the design and control of the laser-induced CVD. The SAC-Cl method was applied to the excitation spectrum and the potential energy curves relevant to the photofragmentation reaction [105]. 

Fig. 39.15. Overall energy diagram and the pathway for the photofragmentation reaction of Ni(CO)4 [105].

Various kinds of the theoretical spectroscopies for the transition metal complexes were also reviewed. For the excitation spectrum of Cr02Cl2, the SAC-Cl method simulated accurate spectrum. For tetraoxo metal complexes, the systematic studies explained the spectral differences when the central metal was substituted. In the analysis of the NMR chemical shift, not only the optically allowed states but also the magnetically allowed states are important. In the molybdenum complexes, the inverse of the d-d excitation energy is proportional to the experimental chemical shift. The photofragmentation reaction of Ni(CO)4 was also studied and the reaction mechanism was clarified. 

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