title: On the participation of photoinduced N-H bond fission in aqueous adenine at 266 and 220 nm: a combined ultrafast transient electronic and vibrational absorption spectroscopy study.
abstract: A combination of ultrafast transient electronic absorption spectroscopy (TEAS) and transient vibrational absorption spectroscopy (TVAS) is used to investigate whether photoinduced N–H bond fission, mediated by a dissociative 1πσ(*) state, is active in aqueous adenine (Ade) at 266 and 220 nm. In order to isolate UV/visible and IR spectral signatures of the adeninyl radical (Ade[-H]), formed as a result of N–H bond fission, TEAS and TVAS are performed on Ade in D2O under basic conditions (pD = 12.5), which forms Ade[-H](-) anions via deprotonation at the N7 or N9 sites of Ade's 7H and 9H tautomers. At 220 nm we observe one-photon detachment of an electron from Ade[-H](-), which generates solvated electrons (eaq(-)) together with Ade[-H] radicals, with clear signatures in both TEAS and TVAS. Additional wavelength dependent TEAS measurements between 240–260 nm identify a threshold of 4.9 ± 0.1 eV (∼250 nm) for this photodetachment process in D2O. Analogous TEAS experiments on aqueous Ade at pD = 7.4 generate a similar photoproduct signal together with eaq(-) after excitation at 266 and 220 nm. These eaq(-) are born from ionization of Ade, together with Ade(+) cations, which are indistinguishable from Ade[-H] radicals in TEAS. Ade(+) and Ade[-H] are found to have different signatures in TVAS and we verify that the pD = 7.4 photoproduct signal observed in TEAS following 220 nm excitation is solely due to Ade(+) cations. Based on these observations, we conclude that: (i) N–H bond fission in aqueous Ade is inactive at wavelengths ≥220 nm; and (ii) if such a channel exists in aqueous solution, its threshold is strongly blue-shifted relative to the onset of the same process in gas phase 9H-Ade (≤233 nm). In addition, we extract excited state lifetimes and vibrational cooling dynamics for 9H-Ade and Ade[-H](-). In both cases, excited state lifetimes of <500 fs are identified, while vibrational cooling occurs within a time frame of 4–5 ps. In contrast, 7H-Ade is confirmed to have a longer excited state lifetime of ∼5–10 ps through both TEAS and TVAS.
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol BS8 1TS, United Kingdom.
authors.forename: Gareth M
authors.initials: GM
authors.lastname: Roberts
authors.forename: Hugo J B
authors.initials: HJ
authors.lastname: Marroux
authors.forename: Michael P
authors.initials: MP
authors.lastname: Grubb
authors.forename: Michael N R
authors.initials: MN
authors.lastname: Ashfold
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Orr-Ewing
mesh_terms: D065966:Absorption, Physicochemical; D000225:Adenine / Q000737:chemistry*; D004583:Electrons; D008958:Models, Molecular; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D055668:Photochemical Processes; D011522:Protons; D013057:Spectrum Analysis; D014732:Vibration; D014867:Water / Q000737:chemistry*
chemical_list: D011522:Protons; D014867:Water; D000225:Adenine; D009584:Nitrogen