We have measured the electron energy dependence for production of a great variety of anion fragments, induced by resonant attachment of subionization electrons to thymine (T) and cytosine (C) within femto-second time scales. At the lowest electron energies we also observe stable molecular anions of these bases, viz., T− and C−. Our measurements suggest that this resonant mechanism may relate to critical damage of irradiated cellular DNA by subionization electrons prior to thermalization.

An excess proton can migrate from a solute to solvent molecules upon asymmetric solvation. The migration depends sensitively on solvation number, solvation structure, and proton affinity differences between solute and solvent molecules. The present study demonstrates this intriguing solvation-induced effect using protonated dimethyl ether-water clusters as the benchmark system. An integrated examination of H + [(CH 3 ) 2 O]-(H 2 O) n by vibrational predissociation spectroscopy and ab initio calculations indicates that the excess proton is (1) localized on (CH 3 ) 2 O at n ) 1, (2) equally shared by (CH 3 ) 2 O and (H 2 O) 2 at n ) 2, and (3) completely transferred to (H 2 O) n at n g 3. The dynamics of proton transfer is revealed by the characteristic free-and hydrogen-bonded-OH stretching vibrations of the water molecules in direct contact with the excess proton. Both hydrogen bond cooperativity and zero-point vibrations have crucial influences on the final position of the proton in the clusters. Further insight into this remarkable phenomenon of intracluster proton transfer is provided by a comparison between H + [(CH 3 ) 2 O](H 2 O) n and its structural analogues, H + (H 2 O) n+1 and H + [(C 2 H 5 ) 2 O](H 2 O) n .

Characterization of protonated formamide clusters by vibrational predissociation spectroscopy confirms theoretical predictions that O-protonation occurs in preference to N-protonation in formamide. The confirmation is made from a close comparison of the infrared spectra of H + [HC(O)NH 2 ] 3 and NH 4 + [HC(O)NH 2 ] 3 produced by a supersonic expansion with the spectra produced by ab initio calculations. For NH 4 + [HC(O)NH 2 ] 3 , prominent and well-resolved vibrational features are observed at 3436 and 3554 cm -1 . They derive, respectively, from the symmetric and asymmetric NH 2 stretching motions of the three formamide molecules linked separately to the NH 4 + ion core via three N-H + ‚‚‚O hydrogen bonds. Similarly distinct absorption features are also found for H + [HC(O)NH 2 ] 3 ; moreover, they differ in frequency from the corresponding vibrational modes of NH 4 + [HC(O)NH 2 ] 3 by less than 10 cm -1 . The result is consistent with a picture of proton attachment to the oxygen atom, rather than the nitrogen atom in H + [HC(O)NH 2 ] 3 . We provide in this work both spectroscopic and computational evidence for the O-protonation of formamide and its clusters in the gas phase.

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