Showing papers by "José Elguero published in 1997"
TL;DR: In this article, the authors present theoretical proof that the nature of the interactions existent in the complexes formed between hydrogen fluoride and a series of π-systems (acetylene, ethylene, cyclopropene, cyclobutadiene, and benzene) and three-memebered-ring derivatives (cyclopropane and tetrahedrane) is that of a hydrogen bond between the hydrogen atom and the π cloud.
Abstract: We present theoretical proof that the nature of the interactions existent in the complexes formed between hydrogen fluoride and a series of π-systems (acetylene, ethylene, cyclopropene, cyclobutadiene, and benzene) and three-memebered-ring derivatives (cyclopropane and tetrahedrane) is that of a hydrogen bond between the hydrogen atom and the π-cloud. Geometric and energetic data and mainly the study of the topology of the electronic density within the frame of the theory of atoms in molecules established by Bader have been used for this analysis.
172 citations
TL;DR: A theoretical study of the possible interaction of the π-cloud of hexafluorobenzene (C6F6) with several small electron-donor molecules (FH, HLi, :CH2, NCH, and CNH) has been carried out as mentioned in this paper.
Abstract: A theoretical study of the possible interaction of the π-cloud of hexafluorobenzene (C6F6) with several small electron-donor molecules (FH, HLi, :CH2, NCH, and CNH) has been carried out. The calculations have been performed using HF, MP2, and hybrid HF/DFT methods (B3LYP) with the 6-31G** and 6-311++G** basis sets. The topology of the electron density of the complexes has been characterized using the AIM methodology. The characteristics of the electron density and molecular electrostatic potential maps of benzene and hexafluorobenzene have been compared. Finally, the results obtained from a search in the Cambridge Structural Database system of this kind of interaction are shown.
168 citations
TL;DR: In this article, the effect of an external electric field on three different proton equilibrium systems has been studied with ab initio methods, and it has been found that by applying an external field the transfer of H atoms between two heavy atoms is possible.
62 citations
TL;DR: The crystal and molecular structures of five 3(5)-amino-5(3)-aryl pyrazoles differing in the nature of the substituent at the para position of the phenyl ring (1: X = H; 3.H2O) have been determined by X-ray analysis as discussed by the authors.
44 citations
TL;DR: In this article, theoretical calculations have been performed on the pyrazole cyclic dimer, trimer and tetramers as well as on linear stabilizers for intermolecular proton transfer.
Abstract: Ab initio theoretical calculations have been performed on
the pyrazole cyclic dimer, trimer and tetramers as well as on linear
oligomers, assuming that there is no relaxation of the geometry during the
proton transfer. The ground state and a wide variety of transition states,
corresponding to different pathways for intermolecular proton transfer,
have been explored and the results compared with experimental data from
crystallography and solid state NMR spectroscopy. For the simplest case of
the dimer, the reaction path corresponding to a double proton transfer has
been explored as well as the effect of relaxing the geometry.
44 citations
TL;DR: In this paper, a general post-synthetic labeling method for the preparation of fully deuterated pyrazoles and quinoxalines by base-induced isotope exchange in superheated deuterium oxide is described.
36 citations
TL;DR: In this article, the X-ray molecular structure of 7b has been determined and the Troger's base analogues with pentagonal aromatic frameworks, 6b and 7b, have been synthesized for the first time.
34 citations
01 Jun 1997
TL;DR: In this article, the authors studied the kinetics of degenerate intermolecular triple proton and deuteron transfer in the cyclic trimers of 15 N-labeled polycrystalline 3,5-dimethylpyrazole (DMP) in a wide temperature range.
Abstract: Using dynamic solid state 15 N CPMAS NMR spectroscopy (CP?cross polarization, MAs?magic angle spinning) the kinetics of the degenerate intermolecular triple proton and deuteron transfer in the cyclic trimers of 15 N-labeled polycrystalline 3,5-dimethylpyrazole (DMP) have been studied in a wide temperature range. At high temperatures, rate constants of the various isotopic HHH, HHD, HDD, and DDD transfer reactions are obtained in the millisecond timescale by lineshape analysis of partially deuterated doubly 15 N-labeled DMP. At low temperatures, the kinetics were followed by magnetization transfer methods in the laboratory frame. In order to suppress artifacts arising from 15 N-spin diffusion these experiments were performed on singly 15 N labeled DMP, partially diluted in the non-labeled material. As multiple kinetic hydrogen/deuterium isotope effects on triple proton transfer reactions have not yet been studied these effects are modeled theoretically for the single barrier case involving a concerted proton motion and for the triple barrier case where the three protons are transferred stepwise. The experimental kinetic isotope effects obey the rule of the geometric mean, i.e. k HHH /k HHD = k HHD / k HDD =k HDD / k DDD =3.6 i.e. = k HHH /k DDD =47 at 300K, which is indicative of a single barrier where all hydrons loose zero-point energy in the transition state. At low temperatures, strong deviations from an Arrhenius behavior are observed for all isotopic reactions, indicating incoherent triple hydron tunneling processes, which are described in terms of a modified Bell tunneling model.
34 citations
TL;DR: Hybrid HF-DFT calculations have been carried out on pairs of neutral molecules and their conjugated bases in order to determine their gas-phase acidities (ΔH values) as mentioned in this paper.
33 citations
TL;DR: In this article, the molecular structures of 2,6-bis-(2′-methoxyphenyl)pyridine 1, the tetrafluoroborate salt of 2.6-bis-(2.6)-methoxide-mithoxy-polymorphic pyridine 2 and 3,5-bimodal pyrazole 4 have been determined by X-ray analysis.
31 citations
TL;DR: In this article, the Diels-Alder reaction of pyrimidine and pyrimidone o -quinodimethanes with C 60 derivatives having hydrophobic or hydrophilic chains were obtained by acylation of one of the adducts A porphyrin-C 60 hybrid was also prepared.
TL;DR: In this paper, the enol/enol tautomerism of 25 β-diketones and 8 β-ketoaldehydes was investigated using AM1 semi-empirical calculations.
TL;DR: In this article, the gas-phase infrared spectrum of N -methylpyrazole was measured in the range 5000-500 cm −1 and with a resolution of 0.5 cm − 1.
TL;DR: Theoretical calculations on a wide variety of NH-pyrazoles clearly show a relationship between ring strain and annular tautomerism, i.e. having the largest singleond character in the C3-C4 bond as discussed by the authors.
Abstract: Theoretical calculations on a wide variety of NH-pyrazoles clearly show a relationship between ring strain and annular tautomerism, i.e., the most stable tautomer is that having the largest singleond character in the C3-C4 bond.
TL;DR: In this article, the European Community for financial support, Project No. CHRX-CT94-0582, and Dr. H.H. Marfisi gratefully acknowledges a grant from the same project for the C CPMAS NMR spectrum.
Abstract: The authors thank the European Community for financial
support, Project No. CHRX-CT94-0582. 0.H. gratefully acknowledges a
grant from the same project. We are indebted to Dr. Marfisi (Bruker Es-
pabola) for the "C CPMAS NMR spectrum.
TL;DR: The influence of the 1-adamantyl group on the structure and the proton transfer dynamics of N-unsubstituted pyrazoles has been determined.
Abstract: The influence of the 1-adamantyl group on the structure and the
proton transfer dynamics of N-unsubstituted pyrazoles has been
determined. Four compounds have been labelled with 15
N and
studied by variable temperature 15
N CP MAS NMR spectroscopy:
3(5)-(1-adamantyl)pyrazole 2, 4-(1-adamantyl)pyrazole 3,
3,5-dimethyl-4-(1-adamantyl)pyrazole 4 and 3,5-di(1-adamantyl)pyrazole
5. Compound 2 (a 1∶1 mixture of both tautomers) is a long chain of
hydrogen bonded molecules (‘catemer’) and as in most
catemers there is no proton transfer since it would imply an
‘infinite’ number of proton jumps. Compound 3, although also
a ‘catemer’, is possibly an exception to this rule, in that
it seems to show proton transfer. In the solid state, compounds 4 and 5
should be cyclic hydrogen-bonded structures, dimers or trimers, but the
activation energies for proton transfer, about 39 kJ
mol
-1
, are quite low compared with those of
3,5-dimethylpyrazole. It appears that the quasi-spherical shape of the
1-adamantyl substituent and its solid-state plasticity may play a role
in lowering these barriers. The crystal structure of 2 has been
determined by X-ray analysis. Individual molecules of 2 form chains
through
N–H · · ·
N hydrogen bonds (‘catemers’) very similar
to those already described for 4-(1-adamantyl)pyrazole and for pyrazole
itself; however, the packing of these catemers is different. Tautomers
2a and 2b are present in the crystal in a 1∶1 ratio, forming
alternating chains of hydrogen-bonded molecules
(2a · · ·
2b · · ·
2a · ·
· 2b · ·
·); the NH hydrogen atoms are linked to both nitrogen
positions (N1 and N2) and show a 1∶1 disorder.
TL;DR: The molecular and crystal structures of 3(5)-methyl-5(3)-phenylpyrazole hydrochloride (3.HCl) and hydrobromide ( 3.HBr) have been determined by X-ray analysis as mentioned in this paper.
TL;DR: In this paper, the photophysics of perimidine derivatives have been characterized and shown to exhibit high photostability against the UV light with small photochemical quantum yields in the range of 10 −5 −10 −6, implying that non-radiative processes different from the excited state intramolecular proton transfer (ESIPT) mechanism are responsible for their photophys.
TL;DR: A series of N,N′-linked 1,2-ethanediyl-poly(benzimidazolin-2-ones) that can be classified as podands, are described in this article.
TL;DR: In this article, the structure of bisazolium salts of the type CA2 (C++ 2A−) were revealed by the study of the FAB spectra of their positive and negative ions.
TL;DR: The 1H and 13C chemical shifts as well as the 1H-2H and 2H-13C coupling constants of perdeuterated 3, 5-dimethylpyrazole and 3,5-diphenyl pyrazole have been measured and the values compared with those of unlabeled compounds as discussed by the authors.
Abstract: The 1H and 13C chemical shifts as well as the 1H–2H and 2H–13C coupling constants of perdeuterated 3,5-dimethylpyrazole and 3,5-diphenylpyrazole have been measured and the values compared with those of the unlabelled compounds.
TL;DR: The 1H and 13C chemical shifts as well as the 1H-1H and 1H−13C coupling constants of sixteen pyrazolo[1,2-a] pyrazole derivatives have been measured using resonance forms and simple additive models.
Abstract: The 1H and 13C chemical shifts as well as the 1H–1H and 1H–13C coupling constants of sixteen pyrazolo[1,2-a]pyrazole derivatives have been measured. The most relevant features are discussed using resonance forms and simple additive models. AM1 semi-empirical calculations have been carried out to provide a rationale for the NMR results.
TL;DR: In this article, four betaines derived from imidazo[4,5-a], imidaze[5,4-a] and thiazolo [5, 4-a]-acridine have been prepared in a six-step procedure starting from 2-chlorobenzimidazoles and benzothiazoles.
TL;DR: The stability of all possible conformers of hexa(pyrazol-1-yl)benzene [(pz)6bz], hexakis(3,5-dimethyl pyrazol 1-yl), 1,2,4,5,6-tetra (pyrazola-1yl)-3, 6-bis (3, 5dimethylpyrazoline-1 -yl), and 1,4-di(pz)-2(dmpz)4bz] has been determined by semi-empirical AM1
Abstract: The stability of all possible conformers of hexa(pyrazol-1-yl)benzene [(pz)6bz], hexakis(3,5-dimethylpyrazol-1-yl)benzene [(dmpz)6bz], 1,2,4,5-tetra(pyrazol-1-yl)-3,6-bis(3,5-dimethylpyrazol-1-yl)benzene [(pz)4(dmpz)2bz] and 1,4-di(pyrazol-1-yl)-2,3,5,6-tetrakis(3,5-dimethylpyrazol-1-yl)benzene [(pz)2(dmpz)4bz] has been determined by semiempirical AM1 calculations. In all series the absolute minimum corresponds to the conformation where the nitrogen lone pair of consecutive pyrazole rings is alternatively oriented to opposite sides of the benzene plane. The case of (pz)2(dmpz)4bz has been studied by 1H NMR studies in solution. Three isomers have been identified and all the signals assigned. The slow evolution to other isomers from those found in the solid state has been followed by 1H NMR spectroscopy thus allowing determination of the barrier to rotation of a (dmpz) substituent (ΔG‡ = 21–22 kcal mol–1).
TL;DR: The crystal and molecular structures of five 3(5)-amino-5(3)-aryl pyrazoles differing in the nature of the substituent at the para position of the phenyl ring (1: X = H; 3.H2O) have been determined by X-ray analysis as mentioned in this paper.
Abstract: The crystal and molecular structures of five 3(5)-amino-5(3)-arylpyrazoles differing in the nature of the substituent at the para position of the phenyl ring (1: X = H; 3.H2O: X = OCH3; 4: X = Cl; 5: X = Br and 6: X = NO2) have been determined by X-ray analysis. Three situations were detected in the crystal structures: the 3-tautomer is present in 1, 3 and 4; the 5-tautomer is only found in 6 and both tautomers (1:1) are observed in 5. The crystal packings are governed by NH… N O hydrogen bonds and also by OH…N interactions in the monohydrate of 3. It is worth noting that in 1, 3, 4 and 5 there are N-H…π(arene) contacts that might play a role in stabilizing the packing. Solid state 13C NMR results are consistent with the above crystallographic conclusions, thus allowing to determine that the only compound for which no good crystals have been obtained, the p-methyl derivative 2 should be a 3-amino tautomer. NMR solution studies (1H and 13C) allow to determine the 3-amino/5-amino tautomeric equilibrium constant, KT, which obeys a Hammett relationship with σp. Geometry optimizations of the 3 and 5-tautomers at semi-empirical level (AM1) were performed. In all compounds, the 3-tautomer has been found to possess a relatively lower energy by approximately 2 kcal mol−1. The potential energy surface as a function of the hybridization of the amino group and its conformation have also been analyzed.
Journal Article•