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Reactions in gaseous metal-organic complexes induced by the photoexcitation of the metal chromaphores

Authors Liu, Haichuan
Issue Date 2004
Summary Gas phase photodissociation spectroscopy has been employed to study photo-induced reactions of a series of mass-selected M⋅+-L complexes (M⋅+= Mg⋅+, Ca⋅+) and Mg⋅+-Ln, clusters. Molecules with various functional groups are chosen for these studies, which include fluorinated benzene, toluene, and pyridine, nucleobases, bidentate ligands, amines, amides, and acetonitrile, etc. A rich variety of photo-induced reaction patterns have been found in the photodissociation of the Mg⋅+-L systems mentioned above. Benzyne radical cations [(CH3)yC6H4-x-yFx⋅+] of highly chemical interest are readily generated in the photodissociation of the complexes Mg⋅+[C6H4-x-yF2+x(CH3)y] (when y=0, x=0-2; when y=1, x=0 or 1) by loss of very stable MgF2. Given enough photon energies, the benzyne radical cations will undergo fragmentation in manner of C4 + C2 pattern. It's found that the fluorine and methyl substitution respectively increases and lowers the energy needed for the decomposition of the benzyne radical cations. Comparative studies on photodissociation of the complexes Mg⋅+-pyridine or fluoropyridine have been performed. While the photodissociation of Mg⋅+-pyridine leads primarily to the evaporative dissociation, abundant photo-induced reaction patterns are found in the photodissociation of Mg⋅+(2-fluoropyridine). Unusual photochemistry of Mg⋅+(2-fluoropyridine) is resulted from a crucial fluorine shift from C to Mg⋅+ in the initial step, forming a key intermediate FMg+-C5H4N, followed by the loss of CN-Mg-F, ⋅MgF, C5H4N⋅, HCN, and HF. Although similar intermediate [FMg+-C5H4-xFxN (x=l or 4)] can also be formed in the photodissociation of Mg⋅+(2,6-and penta-fluoropyridine), subsequent photochemistry is very different, characterized by the formation of pyridyne radical cations C5H3-xFxN⋅+ (x=0 or 3) as the dominant reaction channel. Photo-induced reaction patterns of the complexes Mg⋅+- or Ca⋅+-nucleobases (uracil, thymine, cytosine, guanine, and adenine) have been studied and compared. Abundant photofragments with low yield are found in the photodissociation of Mg⋅+- pyrimidine bases (uracil, thymine, and cytosine). In comparison, fewer and even no photoreaction channels are observed in the photodissociation of Mg⋅+-guanine and Mg⋅+-adenine, respectively, which are purine bases. Association of different metal cation (Ca⋅+) toward nucleobases has some influence on both dissociation manner of the complexes and yield of the photofragments. In the photodissociation of Mg⋅+-bidentate molecules (CH3OCH2CH2OH, CH3OCH2CH2OCH3, and H2NCH2CH2NH2), an initial hydrogen-shift at end C or N is found to play a crucial role for the subsequent photoreactions involving C-O, C-N, or C-C bond activations. In another case, the photoproducts of Mg⋅+[HCON(CH3)2] are generated through (1) H-abstraction next to the carbonyl group by the photoexcited Mg⋅+* (to form (CH3)2NCO+) and/or (2) the subsequent CO loss (to form CH3NH+=CH2). Microsolvation of Mg⋅+(NCCH3)n (n=1-4) clusters has been investigated in the spectral region of 230-560 nm. The photodissociation action spectra of Mg⋅+(NCCH3)n(n=1-4) consist of two or three broad peaks with different magnitude of blue-shift or red-shift towards the atomic transition of Mg⋅+ (3 2P←3 2S) due to different interactions of Mg⋅+ with bonding or antibonding orbitals of the N≡C- group.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2004
Language English
Format Thesis
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