Photochemical Reactions of Aminonaphthols Caused by Laser Desorption/Ionization

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The formation of monomeric and dimeric ions of seven different aminonaphthols (ANLs) has been studied by using laser desorption/ionization (LDI) with a nitrogen laser. The positive-ion data of all the ANLs merely showed molecular ion M^·+ without protonated molecule [M+H]⁺, while 1-amino-2-naphthol (1,2-ANL) and 2-amino-1-naphthol (2,1-ANL) showed an intense dimeric ion [2 M−2H₂O+H]⁺. The negative-ion data showed deprotonated molecule [M−H]⁻ in common, while the spectra of 1,2-ANL, 2,1-ANL and 8-amino-2-naphthol (8,2-ANL) accompanied an intense peak corresponding to negative molecular ion M^·− and the 8,2-ANL and 4-amino-1-naphthol (4,1-ANL) accompanied dehydrogenated anion [M−2H]^·−. The formation of monomeric ions was discussed from the standpoints of thermochemical properties such as ionization energy, gas-phase acidity, electron affinity, and bond dissociation energy. The formation of dimeric ions [2 M−2H₂O+H]⁺ observed in the 1,2-ANL and 2,1-ANL could be explained by the radical combination in the amino groups. An isomer 5-amino-1-naphthol (1-ANL) did not give any dimeric ions in the both positive- and negative-ion spectra. The influence of laser fluence upon the appearance of the monomeric ions such as M^·+, [M+H]⁺, [M−H]⁻ and [M−2H]^·− of the 5,1-ANL has been examined.