Showing papers by "José Elguero published in 2000"

Behavior of Ylides Containing N, O, and C Atoms as Hydrogen Bond Acceptors

Abstract: The hydrogen bond (HB) basicity of a series of ylides containing nitrogen, oxygen, or carbon as heavy atoms, as well as the influence of the formation of the HB complexes on their structure, has been studied. In addition, in this paper we propose the formation of some rather strong HBs (that could be considered low-barrier hydrogen bonds, LBHBs) between ylides and different neutral molecules. The ylides chosen for the study were H3N+−N-H, Me3N+−N-H, H2O+−N-H, Me2O+−N-H, H2O+−O-, Me2O+−O-, and Me3N+−C-H2. As HB donors, classical donors such as HF, HCN, and HCCH were used. The analysis of the protonation energies of the ylides and the optimized geometries, interaction energies, and characteristics of the electron density of the complexes shows that these ylides are very good HB acceptors, forming stable complexes even with weak HB donors. With strong donors, when the proton transfer did not take place, very strong HBs were formed with quite large interaction energies and very short HB distances which could ...

Water Clusters: Towards an Understanding Based on First Principles of Their Static and Dynamic Properties

Abstract: If a single molecule were to be selected as the most important chemical entity for life, most people would agree that this is water. Moreover, water has been used to find a common definition for the Fahrenheit, Celsius, and Kelvin scales through its melting, boiling, and triple points. It is clear that these points are bulk properties and that a single H2O molecule has neither a melting nor a boiling point. Therefore, water offers a paradigmatic case for studying the transition from quantum reality to classical physics: How many water molecules are necessary for the bulk properties to appear? Water clusters, discrete or polymeric (ice), can be seen as one of the simplest models connecting molecular to supramolecular chemistry. Consider, for instance, one of the possible water hexamers, I, and cyclohexane (II), both undergoing boat ‐ chair equilibria (Scheme 1). The study by

Comparison of models to correlate electron density at the bond critical point and bond distance

Abstract: Two models proposed in the literature to correlate the electron density at the bond critical point with the bond distance have been compared. The first one, proposed independently by Knop–Boyd and Destro, considers an implicit double logarithmic relationship between these two properties. We propose a second model, which uses a logarithmic relationship. Van der Waals and hydrogen bond interactions as well as traditional covalent bonds have been considered in order to have a broader range of the compared properties. The results indicate that both models provide good results when individual bond types are correlated, although the logarithmic model seems to be slightly better than the double logarithmic one. Finally, a unique equation corresponding to model 2 has been devised to correlate all the H–X or C–X bonds.

Supramolecular structure of 1H-pyrazoles in the solid state: a crystallographic and ab initio study

Abstract: The secondary structure of 1H-unsubstituted pyrazole derivatives bearing only one hydrogen donor group and one or more acceptor groups has been analyzed in terms of some descriptors representing the substituents at C3 and C5. The substituent at C4 appears to affect mainly the tertiary or quaternary structure of these compounds. The proposed semi-quantitative model, which explains most hydrogen-bonded motifs as a combination of the effects of substituents at C3 and C5, has also been examined as a function of the steric and polarizability effects of these substituents represented by molar refractivity. The model also applies to other five-membered rings (1,2,4-triazoles, 1,2,4-diazaphospholes and 1,2,4-diazaarsoles). Furthermore, ab initio calculations at RHF/6-31G* have been performed to discover the relative stability of three of the four hydrogen-bond patterns displayed by several symmetrical pyrazoles (dimers, trimers, tetramers). The fourth motif, catemers, has only been discussed geometrically.

The reaction between hydrazines and β-dicarbonyl compounds: proposal for a mechanism

Abstract: The reaction between aryl or heteroarylhydrazines with fluorinated β-diketones (CF3COCH2COR) yields a variety of 3-, 5-, and 3,5-trifluoromethylpyrazoles and 5-trifluoromethyl-5-hydroxy-Δ2-pyrazoli...

Pd(II) complexes with polydentate nitrogen ligands. Molecular recognition and dynamic behavior involving Pd-N bond rupture. X-ray molecular structures of [[Pd(C6HF4)2](bpzpm)] and [[Pd(eta 3-C4H7)]2(bpzpm)] (CF3SO3)2 [bpzpm = 4,6-bis(pyrazol-1-yl)pyrimidine].

Abstract: The ligands 4,6-bis(pyrazol-1-yl)pyrimidine (bpzpm) and 4,6-bis(4-methylpyrazol-1-yl)pyrimidine (Me−bpzpm) were synthesized and their reactions with some palladium derivatives explored. Mononuclear or dinuclear neutral or cationic complexes were obtained by reaction of the ligands with 1 or 2 equiv of Pd(C6XF4)2(cod) (cod = 1,5-cyclooctadiene; X = F, H) or the palladium fragment [Pd(η3-2-Me-C3H4)(S)2]+ (S = acetone). The reaction of the dinuclear derivatives with 1 equiv of the respective free ligand immediately led to the regeneration of the mononuclear complexes. Except in the case of the synthesis of [{Pd(C6HF4)2}{Pd(C6F5)2}(bpzpm)], where two similar metallic groups are present, all attempts to obtain dinuclear asymmetric complexes with two different palladium fragments failed. Instead, the dinuclear symmetric complexes were formed. This result could be considered as an example of molecular recognition with the ligand acting as a ditopic receptor. This behavior is comparable to chemical symbiosis but ...

Solution and solid state (CPMAS) NMR studies of the tautomerism of six-membered heterocyclic compounds related to 2-pyridones

Abstract: Several 13 Ca nd 15 N chemical shifts of 2-pyridone (1), 4(3H)-pyrimidone (2), uracil (3) and cytosine (4) have been measured in solution and in the solid state. These data have been discussed in relation with the tautomerism of the four hete- rocycles. GIAO ab initio calculations of absolute shieldings have been carried out to identify the predominant tautomers in the case of compounds (1 )a nd (2).

Wassercluster: auf dem Weg zu einem Verständnis anhand von Grundprinzipien ihrer statischen und dynamischen Eigenschaften

Abstract: Wenn ein einziges Molekl als die fr das Leben wichtigste chemische Spezies benannt werden sollte, wrden die meisten Leute zustimmen, dass dies das Wassermolekl ist Wasser wurde auserdem herangezogen, um anhand seines Schmelz-, Siede- und Tripelpunkts gemeinsame Definitionen fr die Fahrenheit-, Celsius- und Kelvin-Temperaturskala zu erhal- ten Selbstverst‰ndlich sind diese Punkte Volumeneigen- schaften - ein einzelnes H2O-Molekl hat weder einen Schmelz- noch einen Siedepunkt Wasser ist also auch ein Paradebeispiel fr das Studium des ‹bergangs von der Quantenwelt zur klassischen Physik: Wie viele Wassermole- kle braucht man, damit die Volumeneigenschaften auf- treten? Isolierte und polymere Wassercluster (Eis) knnen als eines der einfachsten Modelle betrachtet werden, das Molekl- und supramolekulare Chemie verbindet Nehmen Sie beispiels- weise eines der mglichen Wasser-Hexamere, I, und Cyclo- hexan II Beide durchlaufen Boot-Sessel-Gleichgewichte (Schema 1) Bartons und Hassels Studie des Boot-Sessel- Gleichgewichts von Cyclohexan war einer der Meilensteine

Basicity and Proton Transfer in Proton Sponges and Related Compounds: An Ab Initio Study

Abstract: Theoretical calculations at the B3LYP/6-31G* level were carried out on a family of 1,8-diR-naphthalenes, which include the proton sponge (1,8-bisdimethylaminonaphthalene, R = NMe2) and other substituents (R = NH2, R = OH, R = CH3, R = F). Their basicity was compared with that of the corresponding monosubstituted benzenes. The dianion of 1,8-dihydroxynaphthalene should be a compound of extraordinary high basicity. The barriers to proton transfer, geometry, and density at the bond critical point of the hydrogen bond have been calculated and compared with experimental data when available.

Some Considerations about the Structure of 3(5)-Methylpyrazole

Abstract: The behavior of neat 3(5)-methylpyrazole has been studied at low temperatures by DSC and by NMR (13C and 15N). The main conclusion is that the supercooled liquid is a mixture of four trimers formed by 3-methyl and 5-methyl tautomers.

Weak C-H...O and C-H...N interactions in nitropyrazoles

Abstract: The structures of 1-methyl-3-nitro­pyrazole and 1-methyl-4-nitro­pyrazole, C4H5N3O2, have been determined. The 3-nitro derivative has crystallographic m-symmetry while the 4-nitro compound has no imposed symmetry. The significant differences in bond distances and angles between the structures are ascribable to the electron-withdrawing effects of the nitro group attached to C3 or C4, respectively. In both structures, the mol­ecules are organized into layers by an extensive network of C—H⋯O or C—H⋯N hydrogen interactions. Within a layer, the mol­ecules are arranged in a similar way, although differences of up to 0.3 A in the analogous H⋯O or H⋯N intermolecular distances are observed. The cohesion of the layers is due to van der Waals and C—H⋯O contacts.

FAB mass spectrometry of 6-aminofulvene-2-aldimines: non classical aromatic compounds?

Abstract: The use of meta-nitrobenzyl alcohol (NBA) as matrix allows to obtain better spectra than those recorded with the more popular matrix glycerol (G) in the analysis of fulvene derivatives by FAB mass spectrometry. The influence of NBA on the mass spectra corresponds to its oxidative properties. Finally, the necessary structural information was easily obtained through tandem mass spectrometry experiments.