Juan Carlos del Valle

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: a model for the secondary evolution of the classic C2h dimer and comparison of four mechanisms.

A mechanism is proposed for the formation in gas phase, during a short time, of the delicately symmetrical coplanar C2h classic 7-azaindole (7AI) doubly hydrogen-bonded dimer. Of the five card-pack or otherwise random geometry structures most likely to be formed in the supersonic jet expansion molecular beam, none would be an obvious precursor to the C2h dimer. One unstable dimer with dipole–dipole, van der Waals, and plane-to-plane hydrogen bonding is shown to be capable of unhinging about the hydrogen-bond pair as an axis, from 0° to 90° to 180°, yielding a deep minimum for the C2h structure with its delicate geometry and symmetry. This relaxation mechanism is feasible in the 3-μs interval between the nozzle escape and the first laser pulse interception of the molecular beam. In the second part of the paper four published mechanisms are compared for concerted vs. two-step biprotonic phototransfer for the 7AI dimers. The dependence of the latter two models on H-atom instead of proton-transfer as an intermediate step negates the mechanism in a singlet (π,π*) electronic state by the valency repulsion, in the 3-electron orbital that would be generated. The concerted mechanism for biprotonic phototransfer is reaffirmed by the analysis of the quantum mechanical conditions set on the biprotonic transfer in the photo-excited molecular 7AI pair.

The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles

Six stable dimer models for 7-azaindole (including the classic C2h doubly hydrogen-bonded, coplanar, centrosymmetric dimer) are considered to be observable in adiabatic nozzle jet molecular beams. They are analyzed by hybrid density functional theory (DFT), the MP2 ab initio method for the ground electronic state, and the single-excitation configuration interaction (CIS) (over frozen ground state optimized geometries obtained from DFT) excited state calculations, for global potential minima and proton-transfer potential energy curves. Three simultaneity principles are stated: (i) intermolecular coherent excitation molecular exciton simultaneity, (ii) intramolecular acid–base change simultaneity at the pyrrolo-N-H and aza-N proton-donor, proton-acceptor sites, and (iii) intermolecular simultaneity of catalytic proton-donor, proton-acceptor action. It is suggested that the formation of the classic C2h dimer of 7-azaindole, which is considered exclusively by previous researchers, can be formed from at least one of the several card-pack hydrogen-bonded dimers in a secondary slower step approaching a microsecond scale, instead of the picosecond events at the supersonic nozzle. It is proposed that the complexity of dimerization modes is the basis of the postexcitation, postionization diverse kinetic isotope results.

Conformity of the 7-azaindole dimer cationic potential with photoionization/Coulomb-explosion MS observations and the concerted biprotonic transfer mechanism

Abstract In this Letter we consider the second laser-pulse time delay experiments of Castleman and co-workers [D.E. Folmer, L. Poth, E.S. Wisniewski, A.W. Castleman, Jr., Chem. Phys. Lett. 287 (1998) 1; D.E. Folmer, E.S. Wisniewski, S.M. Hurley, A.W. Castleman, Jr., Proc. Natl. Acad. Sci. USA 96 (1999) 12980] as interrogating the (density functional theory) biprotonic phototautomerization as correlated with the cationic doublet ground electronic state potential D 0 (+) for the species produced in the photoionization step. We demonstrate that the 7-AI(H + ) cation can result as a product of this photoionization, and is then revealed a Coulomb-explosion time-of-flight mass spectrum (TOF–MS) species. We demonstrate that the mass-119/mass-118 ratios as a function of photoionization, and Coulomb-explosion laser pulse delay can conform to the concerted biprotonic mechanism. The interpretation emphasizes the invasive nature of the TOF–MS technique requirement in studying the excited state of the doubly-H-bonded 7-azaindole dimer.

The singular coincidence of fluorescence spectra of the anionic and cationic species formed by the respective deprotonated and protonated pyrido–pyrrolo bases

Study of the chemically produced anionic and cationic species of the heterocyclics 7-azaindole (7-AI) and 1-H-pyrrolo[3,2-h]quinoline (1-HPQ) in 298 K solution spectroscopy displays a striking coincidence of the anion/cation spectra for each. A hybrid density functional calculation revealed that, in spite of a very large electrostatic skeletal perturbation for the respective anion and cation pair, the lowest S0S1 transition energy was largely unperturbed. Generalization to other anion/cation pairs suggests the following theorem: In a diheterocyclic aromatic molecule with proton-acceptor and proton-donor heteroatom sites, a coincidence or near coincidence of the corresponding cation and anion fluorescence bands will be manifested as a result of (+) and (−) electrostatic skeletal perturbations on the intact π-electron system. An analysis of the detailed electronic mechanistic steps involved in the sequential laser pulse (1) excitation and (2) photoionization steps indicate that an observed (7-AI)(H+) mass-119 cation observed in femtosecond time-of-flight mass spectrometry is produced in the invasive photoionization step and is not a likely intermediate in the biprotonic phototransfer in the 7-AI H-bonded dimer.

Observation of a single proton transfer fluorescence in a biaxially symmetric dihydroxy diflavonol

Abstract The proton transfer (PT) emission of the diflavonol molecule 2,8-diphenyl-3,7-dihydroxy-4H,6H-pyrano[3,2-g]-chromene-4,6-dione (D3HF) is reported for the first time. The D3HF molecule possesses a chemical structure almost equivalent to two 3-hydroxyflavone molecules symmetrically condensed on one-benzene ring. Thus, D3HF contains two five-membered intramolecular hydrogen bonds (IMHB), and it may undergo double-PT. The discovery via experiments of the D3HF PT emission conforms to the theoretical finding, by using time-dependent (TD) density functional theory (DFT) (with the B3LYP hybrid functional), of a stable single PT tautomer either in the first excited electronic state (S 1 ) or in the ground electronic state. In the S 1 state, once the first single PT process occurs, the second one is hindered.

2-Arylperimidine derivatives. Part 1. Synthesis, NMR spectroscopy, X-ray crystal and molecular structures

The synthesis and NMR spectroscopy of seven 2-phenyl- 2-(2-hydroxyphenyl)- and 2-(2-methoxyphenyl)perimidines 1–7 are described. The X-ray structural characterization of four perimidine derivatives: 2-(2-hydroxyphenyl)perimidine (2), 2-(9-anthryl)perimidine (4), 1-methyl-2-phenylperimidine (5) and 1-methyl-2-(2-methoxyphenyl)perimidine (7) is reported. The conformation of the molecules in the crystal has been compared with the results of a quantum chemical analysis at the AM1 level. The agreement between the predicted and the experimental conformations depends on the presence of strong intramolecular hydrogen bonds.

Sulfonylurea effects on acid and pepsinogen secretion in isolated rabbit gastric glands

The influence of different sulfonylureas on the rate of acid and pepsinogen secretion was studied in isolated rabbit gastric glands. Neither tolbutamide (10-500 microM), chlorpropamide (10-500 microM), glibenclamide (1-50 microM) nor glipizide (1-50 microM) exerted a secretory effect. In contrast, gliquidone caused a marked and dose-dependent stimulation of acid production in gastric glands incubated under basal conditions and potentiated the stimulatory effect of both histamine and carbachol. Gliquidone also increased the rate of pepsinogen release in gastric glands incubated either under basal conditions or in the presence of cholecystokinin-octapeptide or isoproterenol. The secretory effects of gliquidone were associated with a significant increase in the glandular content of cyclic AMP, caused by a competitive inhibition of low-Km cyclic AMP phosphodiesterase. Our results indicate that, among the assayed sulfonylureas, only gliquidone, in the micromolar range, stimulates acid and pepsinogen secretion through a cyclic AMP-dependent mechanism.

CHEMICAL PHYSICS OF EXCITATION DYNAMICS VIA AMPLIFIED SPONTANEOUS EMISSION(ASE) LASER SPIKE SPECTROSCOPY IN SUBSTITUTED PHENYLOXAZOLES

Abstract A spectroscopic study of the fluorescence, amplified spontaneous emission (ASE), and gain spectra is made, testing for UV lasing from several phenyloxazoles. ASE laser spikes occur at 333 and 349 nm for 2-phenylbenzoxazole (PBO), 356 and 374 nm for 2,5-diphenyloxazole (PPO) and 365 and 385 nm for 2-(1-naphthyl)-5-phenyl-1,3,4-oxadiazole (α-NPD), all in hydrocarbon solution at 298 K. For DPOPOP, 1,4-bis(4-methyl-5-phenyloxazol-2-yl)benzene, the ASE laser spike occurs at 420 nm. The gain coefficients range from 5 to 10 cm −1 . Unorthodox features in the ASE spectroscopy serve as a clue to the electronic excitation dynamics of complex composite molecular systems.

On the existence of two states in liquid water: impact on biological and nanoscopic systems

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.

Understanding the solvatochromism of 10-hydroxybenzo[h]quinoline. An appraisal of a polarity calibrator

Abstract The solvatochromism of the 10-hydroxybenzo[h]quinoline molecule (HBQ) has been investigated for the S 0 →S 1 absorption of the normal tautomer, and for the S 1 ′ →S 0 ′ fluorescence emission of the proton-transfer (PT) tautomer. The wavelength shifts of the first absorption band depend principally on polarity of the solvent used with a minor contribution of acidity, contrary to the fluorescence band wavelength shifts, which exhibit a greater dependence on acidity than on polarity. These observations preclude the use of HBQ for probing polarity in cavities of biopolymers. Theoretical calculations show that the local minimum of electrostatic potential ( V min ) which is found at the carbonyl oxygen center in the S 1 ′ state of the PT tautomer is much deeper than that V min located at the hydroxy oxygen center in the S 1 state of the normal tautomer. This observation is interpreted as a significant increase in basicity of the carbonyl oxygen center, this being approximately twice more basic than the hydroxy oxygen center for the first singlet excited electronic state. Therefore, the oxygen center is the most subjected to be perturbed by solvent acidity. The steps followed in this paper for analyzing the response of HBQ to solvent effects can be applied to any molecular system to be used for probing polarity of the environment (e.g., in biopolymers).