J. L. G. De Paz

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.

On the ordering of the first two excited electronic states in all-trans linear polyenes

Reported experimental evidence of the relative position of the first two excited electronic states in linear polyenes was carefully examined and compared with that derived from time dependent density functional theory (TDDFT) theoretical calculations performed at the B3LYP level on optimized geometries. The energy values for the first two triplet states 3Bu and 3Ag, obtained from TDDFT calculations, were found to be highly strongly correlated with the experimental values. Also, the theoretical calculations for the electronic transition 1 1Ag --> 1 1Bu were also extremely well correlated with their experimental counterparts; even more important, the three reported experimental data for 1 1Ag --> 2 1Ag transitions in these systems conformed to the correlation for the TDDFT 1 1Ag --> 1 1Bu transition. The first excited electronic state in the linear polyenes studied (from ethene to the compound consisting of 40 ethene units, P40) was found to be 1Bu. The energy gap between the excited states 2 1Ag and 1 1Bu decreased with increasing length of the polyene chain, but not to the extent required to cause inversion, at least up to P40. In the all-trans linear polyenes studied, the widely analyzed energy gap from the ground electronic state to the first excited singlet state for infinitely long chains may be meaningless as, even in P40, it is uncertain whether the ground electronic state continues to be a singlet.

On the photophysics of all-trans polyenes: Hexatriene versus octatetraene

The disparate photophysical behavior of trans-1,3,5-hexatriene (nonfluorescent) and trans-1,3,5,7-octatetraene (with two fluorescence emissions) in the gas phase is explained in terms of the tendency of their 1B(u) excited states to rotate about their terminal carbon-carbon single bonds in order to adopt a quasiplanar molecular form of lower energy than the 1B(u) state in the parent all-trans structure. The origin of their disparate photophysical behavior is that such a transformation is subject to a small energy barrier in octatetraene; the barrier produces two minima (two fluorescence emissions) in the corresponding potential-energy curve. Instead of an energy barrier, hexatriene gives a 1,3-diene species which falls to the ground state so rapidly that no emission is produced.

Double- or single-well potential for GSIPT in 1-hydroxy-2-acetonaphthone?

Abstract Potential energy surfaces for intramolecular proton transfer of ground state (GSIPT) of 2-hydroxyacetophenone and 1-hydroxy-2-acetonaphthone have been calculated by using MP2 and B3LYP methods. The main results are as follows. (a) Intramolecular proton transfer shows an strong coupling with the distances between the oxygen atoms which support the intramolecular hydrogen-bonding. (b) The correlated GSIPT curves of both compounds have a single minimum in the ground state, corresponding to the enol form. (c) This previous coclusion does not support the assumption made by Douhal et al. on the HAN GSIPT curve is a double well potential curve.

On the Inoperativeness of the ESIPT Process in the Emission of 1-Hydroxy-2-acetonaphthone: A Reappraisal

For a molecule which contains an intramolecular hydrogen bond (IMHB) in its chemical structure to undergo an excited singlet intramolecular proton transfer (ESIPT) process, on photoexcitation, there must occur a simultaneous increase, in a substantial manner, in the acidity of the proton donor and the basicity of the proton acceptor forming the IMHB [J. Am. Chem. Soc. 2001, 123, 11940]. For the reason that those changes occur on photoexcitation of the 2-hydroxyacetophenone but not for 1-hydroxy-acetonaphthone, one draws the conclusion that, while ESIPT is operative in the 1(pi,pi*)(1) electronic state of the monocyclic compound 2-hydroxyacetophenone, it is not operative in its bicyclic homolog 1-hydroxy-2-acetonaphthone. We have shown the photophysics of 1-hydroxy-2-acetonaphthone in its first excited electronic state to be governed by two stable, easily reconverted enol structures, the presence of which causes the peaks in the free-jet fluorescence excitation spectrum for the compound to split into two of similar strength. In this paper, we rationalize photophysical evidence for 1-hydroxy-2-acetonaphthone obtained by femtosecond spectroscopy over the past 13 years in the light of existing photophysical patterns based on steady-state spectra for the compound [J. Am. Chem. Soc. 1993, 115, 4321].

On the acidity and basicity of azoles: the Taft scheme for electrostatic proximity effects

Abstract A theoretical study of the acid-base behavior of azole systems was carried out using density functional theory (B3LYP). The results were consistent with experimental measurements of the systems in the gas phase. A two-way linear relationship between theoretical and experimental data is established that links the acid and base ranges for the neutral forms. The applicability of the Taft model for electrostatic proximity effects to this type of compounds is demonstrated.

Photoinduced intramolecular proton transfer in methyl salicylates: a theoretical study with spectroscopic support

Abstract Intramolecular proton transfer processes in the ground and excited state of methyl calicylate, methyl 4-hydroxysalicylate and methyl 5-hydroxysalicylate have been studied by CNDO-CI and INDO semiempirical methods in order to determine the existence of a single or a double energy minimum in the corresponding potential curves, as well as to interpret thier photophysical behaviour. Hydrogen-bonding strenghts of ground state and local energy-minized geometries of ground and excited states are also discussed. These studies, combinded with IR and 1 H HMR spectral data from methyl 4-methoxysalicylate and methyl 5-methoxysalicylate, support the existence of two different intramolecular hydrogen-bonded forms in the ground state and, at the same time, show that the photphysical behaviour of all these salicylic acid derivatives closely depends on the distance between the two oxygen atoms supporting the bond.

A theoretical study of the acidity of nitrogen heterocycles

Abstract We perform an INDO and AM1 study of C-H and N-H deprotonation (acidity) of 20 nitrogen heterocycles. Acidity is explained by taking into account lone pair repulsion, aza, and annelation effects. The AM1 method has to be corrected by adding 9 kcal mol −1 for each adjacent lone pair-lone pair interaction.

Acidity and basicity of azoles: Solvent effects

Abstract Fully optimized INDO geometries have been calculated for twelve azoles, their corresponding cations and anions, both isolated and specifically solvated by five water molecules. The protonation and deprotonation energies of the isolated molecules do not show any characteristic trend but those of the solvated species show a behaviour similar to that found experimentally in solution. From these calculations it is possible to explain the particular behaviour of 1,2-diazoles compared with all other azoles with regard to acid-base equilibria in water.