No abstract

The experimental and theoretical bases for a synchronous or concerted double-proton transfer in centrosymmetric H-bonded electronically excited molecular dimers are presented. The prototype model is the 7-azaindole dimer. New research offers confirmation of a concerted mechanism for excited-state biprotonic transfer. Recent femtosecond photoionization and coulombic explosion techniques have given rise to time-of-f light MS observations suggesting sequential two-step biprotonic transfer for the same dimer. We interpret the overall species observed in the time-of-f light experiments as explicable without conf lict with the concerted mechanism of proton transfer.The doubly H-bonded dimer of 7-azaindole (7-AI) has been studied exhaustively as a model prototype for DNA base-pair tautomerization, as it is recognized to undergo a biprotonic transfer (1) on photoexcitation. A central issue in doubleproton transfer (PT) reactions is whether a sequential (stepwise) or a concerted mechanism is applicable at the two proton-donor, proton-acceptor sites (2). A stepwise mechanism requires a reaction potential energy curve having an intermediate minimum between the potential minimum for the normal tautomer species, and that of the PT tautomer species, so that a finite lifetime for the transient intermediate species could be observed (3). Recent femtosecond MS results on 7-AI dimer produced in adiabatic expansion (4, 5) had been reported in supercooled molecular beams produced by adiabatic jet expansion, claiming to have established by fast transient kinetics a two-step PT mechanism for photo-excited 7-AI, with apparent theoretical calculation corroboration (6). Another laboratory (7) has made a claim of arresting the intermediate involving a one-PT in 7-AI dimer via a coulomb explosion technique, also in supercooled molecular beam experiments. We believe these results have been misinterpreted, and because they already are being accepted in some quarters (2, 8) as proven, we present refined calculations and experimental results to verify the concerted biprotonic transfer mechanism for 7-AI. Both of the jet expansion molecular beam experiments use severely invasive techniques (photoionization, coulomb explosion) to produce cationic species as required in the time-of-flight (TOF)-MS detection. We show that these laser photoionization procedures introduce a major perturbation on the electronic states of the molecular systems involved, and in effect, to create the cationic molecular species assumed to be an intermediate.The development of femtosecond laser techniques now permits dynamic chemical events to be clocked at the picosecond and subpicosecond time scale. Consequently, a novel duality of criteria for molecular reaction mechanisms seems to have arisen: (a) the classical requirement with demonstration of an intermediate valley in a reaction potential energy curve, or (b) a (sub)picosecond observation of passage through an intermediate molecular configuration. Both of these could satisfy the Bridgman operational criteri...

M. Kasha (Discuss. Faraday Soc., 1950, 9, 14–19): “The emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity”.

Several ultra‐compact accurate wave functions in the form of generalized Hylleraas‐Kinoshita functions and Guevara‐Harris‐Turbiner functions, which describe the domain of applicability of the Quantum Mechanics of Coulomb Charges (QMCC), for the ground state energies (4‐5 significant digits [s.d.]) of He‐like and Li‐like iso‐electronic sequences in the static approximation with point‐like, infinitely heavy nuclei are constructed. It is shown that for both sequences the obtained parameters can be fitted in Z by simple smooth functions: in general, these parameters differ from the ones emerging in variational calculations. For the He‐like two‐electron sequence the approximate expression for the ground state function, which provides absolute accuracy for the energy ∼10−3 a.u. and the same relative accuracies ∼10−2 to 10−3 for both the cusp parameters and the six expectation values, is found. For the Li‐like three‐electron sequence the most accurate ultra‐compact function taken as the variational trial function provides absolute accuracy for energy ∼10−3 a.u., 2 to 3 s.d. for the electron‐nuclear cusp parameter for Z ≤ 20 and 3 s.d. for the two expectation values for Z = 3.

No abstract

The substituted naphthalene compounds investigated in this paper, i.e., methyl 2-hydroxy-3-naphthoate (MHN23), methyl 1-hydroxy-2-naphthoate (MHN12), and methyl 2-hydroxy-1-naphthoate (MHN21), show a strong intramolecular hydrogen bond (IMHB) in their ground electronic state. The relative position of the IMHB in the naphthalene skeleton acts as a switch and controls the yield of an excited state intramolecular proton transfer (ESIPT) process. As a matter of fact, only MHN23 exhibits a proton transfer (PT) emission and possesses a theoretically proved ESIPT mechanism. The role that the ESIPT mechanism plays on the photostability of the molecule MHN23 is unravelled by comparison with the model compounds methyl salicylate (MS), MHN12, and MHN21. On one hand, the low photoreaction quantum yield, Φr = 0.00015, and therefore the high photostability of MS, under direct ultraviolet (UV) irradiation, has been explained due to the photophysics of its proton transfer tautomer. On the other hand, (a) the two benzene-fused ring derivatives of methyl salicylate, MHN12 and MHN21, also possess a great photostability to UV radiation, and they do not support an ESIPT mechanism; and (b) although MHN23 exhibits an excited state proton transfer, its efficiency is only of 1.8%, and the photostability is 5 times larger than that of MS. As a result, the photostability of MHN23, MHN12, and MHN21 does not rely on the photophysics of their proton transfer tautomers but on the nonradiative dynamics of their respective normal tautomers. We present experimental evidence which supports the above-mentioned statement on the existence of distinctive nonradiative channels for the molecules MHN23, MHN12, and MHN21.

This work reviews several properties of liquid water, including the dielectric constant and the proton-spin lattice relaxation, and draws attention to a bilinear behaviour defining a crossover in the temperature range 50 ± 10°C between two possible states in liquid water. The existence of these two states in liquid water plays an important role in nanometric and biological systems. For example, the optical properties of metallic (gold and silver) nanoparticles dispersed in water, used as nanoprobes, and the emission properties of CdTe quantum dots (QDs), used for fluorescence bioimaging and tumour targeting, show a singular behaviour in this temperature range. In addition, the structural changes in liquid water may be associated with the behaviour of biological macromolecules in aqueous solutions and in particular with protein denaturation.

No abstract