Showing papers by "José Elguero published in 1999"
TL;DR: In this article, a theoretical study of the ability of amine oxides and phosphine oxide as hydrogen bond acceptors has been carried out using ammonium oxide, trimethylamine oxide, and PHO as model systems.
Abstract: A theoretical study of the ability of amine oxides and phosphine oxides as hydrogen bond (HB) acceptors has been carried out using ammonium oxide, trimethylamine oxide, and phosphine oxide as model systems. The analysis of the energetic results indicate that only small spatial preferences are observed in the HB interaction. The value of the interaction energies are in several cases within the range of strong HB (>12 kcal/mol), and in complexes between amine oxides and strong acids in the gas phase, a spontaneous proton transfer is obtained. A logarithmic correlation between the electron density at the HB critical points and the HB distance that is able to fit not only calculated data but also experimental ones has been obtained. Finally, a linear relationship has been found between the number of HBs and the 31P NMR shielding in the H3PO···(HF)n series, in good agreement with experimental reports.
151 citations
TL;DR: In this article, the enthalpies of formation measured using static bomb combustion calorimetry, the “vacuum sublimation” drop-calorimetric method, and the Knudsen-effusion method are reported for the first time for four azoles: 1-methylimidazole (1MeIMI), 1-methylpyrazole(1MePYR), 1-(1-benzyl pyrazole) (1BnPYR).
Abstract: Accurate experimental enthalpies of formation measured using static bomb combustion calorimetry, the “vacuum sublimation” drop calorimetry method, and the Knudsen-effusion method are reported for the first time for four azoles: 1-methylimidazole (1MeIMI), 1-methylpyrazole (1MePYR), 1-benzylimidazole (1BnIMI), and 1-benzylpyrazole (1BnPYR). These values and those corresponding to imidazole (1HIMI), pyrazole (1HPYR), 1-ethylimidazole (1EtIMI), 1-ethylpyrazole (1EtPYR), 1-phenylimidazole (1PhIMI), and 1-phenylpyrazole (1PhPYR) are compared with theoretical values using the G2(MP2) and the B3LYP/6-311*G(3df,2p)//6-31G(d) approaches. In general, there is a very good agreement between calculated and experimental values for the series of N-substituted imidazoles, while the agreement is less good for the series of the N-substituted pyrazoles. Experimentally, the gap between the enthalpies of formation of imidazoles and pyrazoles decreases significantly upon N-substitution, while the theoretical estimates indicat...
60 citations
TL;DR: In this article, the crystal and molecular structure of two 3-(2-benzyloxy-6-hydroxyphenyl)-5-styryl pyrazoles were determined by X-ray analysis.
53 citations
TL;DR: The X-ray molecular structure of the important molecule 3,5-bis(trifluoromethyl)pyrazole has been determined at 120 K and gave crystals belonging to the triclinic Pspace group as mentioned in this paper.
Abstract: The X-ray molecular structure of the important molecule 3,5-bis(trifluoromethyl)pyrazole has been determined at 120 K and gave crystals belonging to the triclinic Pspace group. The compound forms tetramers through N–H···N hydrogen bonds and some proton disorder is necessary to explain the geometric features of the monomers. The IR spectra have been recorded in the gas phase (monomers) and in the solid state (tetramers) and analyzed by comparison with the calculated normal frequencies. The use of solid-state NMR spectroscopy combined with abinitio GIAO calculations suggests that a certain amount (about 40±10%) of dynamic disorder involving intramolecular proton transfers could be present in the crystal.
32 citations
TL;DR: The X-ray molecular structure of 4-methyl pyrazole (3) at 100 K has been determined in this article, where the compound crystallizes as a trimer with the N-H protons localized.
Abstract: The X-ray molecular structure of 4-methylpyrazole (3) at 100 K has been determined. The compound crystallizes as a trimer with the N–H protons localized. 4-Methylpyrazole belongs to the series of eight NH-pyrazoles bearing only H and CH3 substituents on the carbon atoms. There are two pairs of tautomers so the number of pyrazoles existing in solution is only six. The thermal behavior of the six pyrazoles was investigated by differential scanning calorimetry and their melting points analyzed on the basis of their solid state structures and number and position of C-methyl substituents.
27 citations
TL;DR: The characteristics of low-valence derivatives (monohydrides and monofluorides) of boron, aluminum, nitrogen, and phosphorus (group 15 of the periodic system) have been investigated in this article.
Abstract: The characteristics of low-valence derivatives (monohydrides and monofluorides) of boron, aluminum (group 13 of the periodic system), nitrogen, and phosphorus (group 15 of the periodic system) have been investigated. Several aspects of these derivatives have been studied such as the energy gap between their singlet and triplet configuration, their proton affinity in the different parts of the molecule, and their ability as hydrogen bond acceptors. The geometries and energies of all the monomers and complexes have been fully optimized using a hybrid method (B3LYP) and the second-order Moller−Plesset (MP2) levels with the 6-311++G** basis set. In addition, the G2 and MCSCF methodologies were also used. The natural population analysis and the natural bond orbital analysis have been used to evaluate the charge transfer and second-order interaction energies, respectively. Topological properties of the electron density have been characterized using the atoms in molecules methodology. Our results show surprising...
26 citations
TL;DR: In this paper, the problem of magnetic vs geometric criteria of aromaticity was approached using NICS/GIAO/B3LYP/6-31G* calculations on benzene and distorted benzenes, the conclusion being that a relationship exists between these criteria but only for specific subsets.
Abstract: The problem of magnetic vs. geometric criteria of aromaticity is approached using NICS/GIAO/B3LYP/6-31G* calculations on benzene and distorted benzenes, the conclusion being that a relationship exists between these criteria but only for specific subsets.
25 citations
TL;DR: In this article, the kinetics of degenerate inter- molecular triple proton and deuteron transfers in the cyclic trimers of 15 N-labeled polycrystalline 4-nitropyrazole (4NO 2 P) and 4-bromopyrazoles (4BrP) have been studied as a function of temperature.
Abstract: Using dynamic solid state 15 N CPMAS NMR spectroscopy (CP ≡ cross polarization, MAS ≡ magic-angle spinning), the kinetics of degenerate inter- molecular triple proton and deuteron transfers in the cyclic trimers of 15 N-labeled polycrystalline 4-nitropyrazole (4NO 2 P) and 4-bromopyrazole (4BrP) have been studied as a function of temperature and are compared to the kinetics of triple proton transfer in bulk solid 3,5-dimethylpyrazole (DMP) studied previously. The results show that the transfer kinetics in the new trimers are much faster than in DMP. However, the kinetic HHH/HHD/HDD/DDD isotope effects of 4NO 2 P are similar to those of DMP. These effects indicate a single barrier for the triple proton transfers where all three protons lose zero-point energy in the transition state, as expected for a structure with three compressed hydrogen bonds. At low temperatures, strong deviations from an Arrhenius-behavior are observed which are described in terms of a modified Bell tunneling model and a concerted proton motion. The barrier for the triple proton transfer in 4NO 2 P and 4BrP is substantially smaller than in DMP. As there is no correlation with the electronic properties of the substituents, we assign this finding to steric effects where the bulky methyl groups of DMP in the 3- and 5- positions hinder the hydrogen bond compression, in contrast to 4NO 2 P and 4BrP exhibiting substitutents in the 4-position. These results lead to a minimum energy pathway of the proton transfer following in the absence of steric hindering the hydrogen bond correlation line q 1 = f(q 2 ), established previously, where q 1 represents the deviation of the proton from the hydrogen bond center and q 2 the N...N distance. Tunneling occurs at constant N...N distances.
24 citations
TL;DR: In this article, the standard (p °=0.1 MPa) molar enthalpies of formation for liquid 1-ethylimidazole and 1-ethylpyrazole were derived from the standard molar entropy of combustion Δ c H m °, in oxygen, at T =298.15 K, measured by static bomb combustion calorimetry.
22 citations
TL;DR: The crystal structure of 3(5)-phenyl-4-bromo-5(3)-methyl pyrazole, 3, has been determined by X-ray crystallography as mentioned in this paper.
20 citations
TL;DR: In this article, the synthesis and spectroscopic properties of six new 1-adamantylmethylene azines were reported, which exist in the solid state in the E,E-configuration.
TL;DR: In this paper, flash vacuum pyrolysis experiments were carried out between 500 and 800°C on 3(5)-phenyl-3-methyl-pyrazolinone and 3-(5)-methoxy-5(3)-methyl pyrazole and showed that the percentage of the CH tautomer increases with temperature.
Abstract: Flash vacuum pyrolysis experiments were carried out between 500 and 800 °C on 3(5)-phenyl- and 3(5)-methyl-pyrazolinones and on 3(5)-methoxy-5(3)-phenylpyrazole. The origin of the isolated products (mainly indanone, hydroxyalkynes and α,β-unsaturated aldehydes) can be explained as arising from the hydroxy tautomers of pyrazolinones. Temperature effects on the tautomeric equilibrium of 1-phenyl-3-methylpyrazolinone in solution show that the percentage of the CH tautomer increases with the temperature. MP2 ab initio calculations on the model compound, pyrazolinone itself, have been used to rationalize these findings. The problem of the aromaticity of the four tautomers of pyrazolinone has been examined through Schleyer’s NICS (nuclear independent chemical shifts) calculations.
TL;DR: In this article, quantum mechanics for pyrazole itself and seven N-nnsubstituted C-fluoropyrazoles were performed at the RHF and MP2 levels.
Abstract: Quantum mechanic calculations have been done at the RHF and MP2 levels with the STO-3G, 6-31G**, 6-31 1G** basis sets on pyrazole itself and seven N-nnsubstituted C-fluoropyrazoles. These calculations have been used to discuss the molecular structure of these compounds in relation to their aromaticity. The corresponding 'H, l3C 15N and '9F chemical shifts were calculated using the GIAO perturbation method.
TL;DR: In this paper, a unique equation able to correlate two geometrical parameters in hydrogen-bonded (HB) systems, the X −H covalent bond distance of the donor and the H•••Y distance of an acceptor, has been devised.
Abstract: A unique equation able to correlate two geometrical parameters in hydrogen-bonded (HB) systems, the X—H covalent bond distance of the donor and the H•••Y distance of the acceptor, has been devised. The equation is able to fit high-quality crystallographic data of different HB donors and several nitrogen and oxygen acceptors.
TL;DR: In this paper, the authors used abinitio methods, low-temperature X-ray crystallography, differential scanning calorimetry, IR spectroscopy, liquid state 1H NMR, broadband solid state 2H NMRI and high resolution solid state 15N NMR spectrograms to study the structure and dynamics of the amino group and the characteristics of hydrogen bond association.
Abstract: The title compound N-aminopyrazole (AMPZ) has been studied both theoretically and experimentally using abinitio methods, low-temperature X-ray crystallography, differential scanning calorimetry, IR spectroscopy, liquid state 1H NMR, broadband solid state 2H NMR and high resolution solid state 15N NMR spectroscopy, focussing on the structure and dynamics of the amino group and the characteristics of hydrogen bond (HB) association. The complete amino group rotation–inversion surface of AMPZ and, for comparison, also of the aniline monomer were calculated at the B3LYP/6-31G* level. The results indicate that monomeric AMPZ exhibits an s hybridized pyramidal amino group where the HH-distance vector is located perpendicular to the pyrazole ring and the hydrogen atoms bent towards the pyrazole ring nitrogen atom, i.e. with the lone electron pair anti to the latter. Both inversion at the nitrogen atom and the 180° rotation of the amino group lead to a metastable structure, where the inversion barrier is much larger than the rotational barrier. These results contrast with those obtained for aniline where the amino group is almost planar and subject to a degenerate 180° rotation. Further calculations of the hydrogen bonded cyclic AMPZ dimer indicate that hydrogen bond association does not substantially perturb the geometry of the amino group, where one of the amino group protons and the ring nitrogen atoms are involved in hydrogen bonding. The X-ray structure was determined at -173°C (100 K) on a monocrystal obtained by zone crystallization. DSC shows that on cooling, neat liquid AMPZ exhibits a solid–solid phase transition around -45°C to a low-temperature phase. In this phase AMPZ forms a weakly hydrogen bonded chain where the structure is similar to the one calculated for the monomer. Only one of the amino protons is involved in hydrogen bonding, where the NH stretch of the hydrogen bonded proton appears at 3197 cm-1 and of the non-hydrogen bonded proton at 3314 cm-1. Solid state 2H NMR spectroscopy of [ND2]AMPZ shows low-temperature structures with rigid ND vectors below and isotropically rotating vectors above the solid–solid phase transition. Hydrogen bond association was further studied by solid state 15N CPMAS (cross polarization magic angle spinning) NMR and by low-temperature liquid state 1H NMR of AMPZ specifically 15N labelled in the amino group. In contrast to the solid state, hydrogen bond exchange rendering the two amino group protons equivalent was found to be fast in the NMR timescale even at 100 K.
TL;DR: Ab initio calculations at HF and B3LYP/6-31G** levels indicate that the 3,4-dimethylpyrazole tautomer is more stable than the 4,5-dimethylamine tautomers by only approximately 0.5 kcal mol(-1).
Abstract: The crystal and molecular structures of 3(5),4-dimethylpyrazole, C5H8N2, (I), and of 3,4,5-trimethylpyrazole, C6H10N2, (II), have been determined at 200 K In (I) the 4,5-dimethylpyrazole tautomer is present in the solid state and the six independent molecules in the asymmetric unit form trimers via NH⋯N hydrogen bonds related by a pseudo centre of symmetry The asymmetric unit of (II) contains one and a half molecules: these exhibit NH proton disorder and are hydrogen bonded to each other via their respective NH groups to form chains Ab initio calculations at HF and B3LYP/6-31G** levels indicate that the 3,4-dimethylpyrazole tautomer is more stable than the 4,5-dimethylpyrazole tautomer by only approximately 05 kcal mol−1 (1 kcal mol−1 = 4184 kJ mol−1)
TL;DR: Virgili et al. as discussed by the authors presented the crystal structure of racemic 2,2'-dimethoxy-9,9'-biacridine-CHCl.
Abstract: Departament de Quimica, Facultat de Ciencies, Universitat Autonoma de Barcelona, E-08193Bellaterra, SpainFax: (internat.) +34(3)5811265 , E-mail : albert.virgili@uab.es*Author to whom correspondence should be addressed.Received: 16 March 1998 / Accepted: 5 May 1998 / Published: 16 April 1999Abstract: Three derivatives of 9,9'-biacridine, 2,2'-dihydroxy, 2,2'-dimethoxy and 2,2'-diacetoxy have been prepared. The crystal structure of racemic 2,2'-dimethoxy-9,9'-biacridineCHCl
TL;DR: In this article, the stabilities of the eight possible conformers of (imz) 6 bz and hexakis(2-methylimidazol-1-yl)benzene have been estimated by semi-empirical calculations at the AM1 level.
TL;DR: In this paper, the authors studied the interaction of the methane radical cation (C 2v ) with noble gas atoms (He, Ne, Ar, and Kr) and found that the transition states obtained indicate an easy transfer between the symmetrical and non-symmetrical complexes.
TL;DR: The crystal structures of the first two pyrazole N-substituted primary amides (3-methyl and 4-bromo) were determined in this article.
TL;DR: In this paper, the crystal and molecular structure of two 3-(2-benzyloxy-6-hydroxyphenyl)-5-styryl pyrazoles were determined by X-ray analysis.
Abstract: 3-(2-Benzyloxy-6-hydroxyphenyl)-5-styrylpyrazoles 7a-e were prepared from the reaction of 2-styrylchromones and hydrazine hydrate. 3-(2-Benzyloxy-6-hydroxyphenyl)-5-(2-phenylethyl)-pyrazoles 8a,d,e and 3-(2-benzyloxy-β,6-dihydroxystyryl)-5-aryl-2-pyrazolines 9a-e were also obtained as by-products. The crystal and molecular structure of two 3-(2-benzyloxy-6-hydroxyphenyl)-5-styrylpyrazoles 7a,b have been determined by X-Ray analysis. Although the substitution of an hydrogen by a methyl group on the double bond of the styryl moiety seems to be a minor perturbation, it produces drastic changes in the crystal packing where only one conformer is present. The OH group is involved as donor of an intramolecular hydrogen bond and the NH group is responsible for the formation of chains via intermolecular hydrogen bonds.
TL;DR: The structure of 1,4-bis(pyrazol-1'-yl)-2,3,5,6- tetrakis(3'',5''-dimethyl pyrazol 1''-yl) benzene 1, its monohydrate, 1a, four 1 : 2 host : guest complexes, 1b-1e (acetic, propionic, pentanoic, and (±)-2-methyl butyric acids) and a di-picrate salt, 1f, have been determined by X-Ray analysis.
Abstract: The structures of 1,4-bis(pyrazol-1'-yl)-2,3,5,6- tetrakis(3'',5''-dimethylpyrazol-1''-yl) benzene 1, its monohydrate, 1a, four 1 : 2 host : guest complexes, 1b–1e (acetic, propionic, pentanoic, and (±)-2-methyl butyric acids) and a di-picrate salt, 1f, have been determined by X-Ray analysis. In all complexes, in the salt and in the monohydrate compound, the host molecules are hydrogen bonded to two centrosymmetric related guests and to the water molecule which is disordered over two positions to mimic the inclusion complexes. In all compounds, the host exhibits Ci symmetry having the lone pair on the N(2) atom of each pyrazole pointing alternately upwards (u) and downwards (d) from the benzene ring. 1H and 13C NMR spectra of the free host 1 and of the complexes are consistent with the ududud conformation and the stoicheiometry of the inclusion compounds.
TL;DR: In this paper, a series of bis-derivatives was prepared, among them bridged bis-acridin-(10H)-ones 9−14 and bis-mercapto-(9H)-acridines 15−18.
Abstract: Starting from 4-hydroxyacridin-9(10H)-one (1) and 4-hydroxyacridine-9(10H)-thione (2), a series of bis-derivatives was prepared, among them the bridged bis-acridin-(10H)-ones 9–14 and the bis-mercapto-(9H)-acridines 15–18. The reactivity of the 2- and 4-hydroxy series was compared; it was found that it is harder to demethylate 4-methoxy derivatives than their 2-methoxy analogues.