Showing papers by "José Elguero published in 2003"
TL;DR: The chiral recognition in the formation of hydrogen-bonded (HB) dimers of 1,6a-dihydropyrrolo[2,3-b]pyrrole derivatives as well as in their proton-transfer processes have been studied by means of ab initio calculations.
Abstract: The chiral recognition in the formation of hydrogen-bonded (HB) dimers of 1,6a-dihydropyrrolo[2,3-b]pyrrole derivatives as well as in their proton-transfer processes have been studied by means of ab initio calculations. The heterochiral dimers are in general the most stable ones, but amphiprotic substituents that are able to form attactive interactions with twin groups revert this tendency. Energy differences up to 4.0 kcal/mol have been found favoring the homo- or heterochiral complexes. Two possible proton-transfer processes have been studied, the concerted one and the nonconcerted one. The compresion of the systems in the transition structures produce an increase in the energetic differences when compared to the corresponding minima complexes. A Steiner-Limbach relationship has been found for the geometrical properties of the HB in the minima and transition states calculated here. The electron density and its Laplacian at the bond critical point have been found to correlate with the HB distance.
63 citations
TL;DR: In this article, the structures of all new compounds have been established by extensive NMR spectroscopic studies, including bis(pyrazoles) 5e,f bearing hydroxyphenyl substituents and bis(chromones) 6c,d with hydrazine hydrate.
59 citations
TL;DR: In this paper, a theoretical approach using MP2(full)/6-31++G** and MP(full/6-311++G++ levels was used to characterize the interaction of the π-cloud of C6F6 with cations (Y+ = Li+, Na+, and K+).
Abstract: A theoretical approach, using ab initio MP2(full)/6-31++G** and MP2(full)/6-311++G** levels, has been used to characterize the interaction of the π-cloud of C6F6 with cations (Y+ = Li+, Na+, and K+). In addition, the situation where C6H6 and C6F6 simultaneously interact with an anion (X- = F-, Cl-, and Br-) and a cation in opposite faces of the aromatic ring has been studied. For comparative purposes, other dispositions, such as those of the isolated cations and the anions and the complexes between neutral XY salts and the aromatic systems, have been considered. Complexes where the π-cloud of the aromatic ring interacts with a cation or, simultaneously, with a cation and an anion have been found to be mimima structures. However, these complexes show high relative energies when compared to other minima of the potential hypersurface. The interaction energy has been decomposed into MEP, polarization and charge−charge interaction terms.
50 citations
TL;DR: The Pd-Cu-catalyzed cross-couplings of 1-alkynes with pyrazolyl halides is the most convenient method for synthesizing the above mentioned compounds as mentioned in this paper.
Abstract: – N -Unsubstituted 4-iodopyrazole ( 1) is easily converted into 4-alkynylderivatives ( 5 ) in moderate to good overall yields by using intermediateprotection of the nitrogen atom of the pyrazole ring by ethyl vinyl ether. INTRODUCTION Acetylenic derivatives of pyrazoles are attractive synthons as well as promising biologically activecompounds. 1,2 Special interest is shown by pyrazolylacetylenes bearing a formyl group, which gives thepossibility to create stable radicals, such as nitronylnitroxides derived from 2-imidazolines. 3,4 We havereported the synthesis of some acetylenyl- N -alkylpyrazolylnitronylnitroxides, which showed interestingoptical and magnetic properties. 3,4 However, more promising for the design of high dimension molecularferromagnetics are N- unsubstituted pyrazolyl halides due to the possibility to use the NH-fragment, forinstance to form N–H···N aggregates in the solid state. From the various papers dealing with the synthesisof alkynylpyrazoles it may be concluded that the Pd-Cu-catalyzed cross-couplings of 1-alkynes withpyrazolyl halides is the most convenient method for synthesizing the above mentioned compounds.
36 citations
TL;DR: The nuclear magnetic resonance triplet wave function model (NMRTWM) as mentioned in this paper was proposed to understand the sign of the spin-spin coupling constant between a pair of atoms, which is based on the phases of excited triplet state wavefunctions and the response of magnetic nuclei to these phases.
35 citations
TL;DR: The polydentate N-donor ligand 2,4,6-tris(3,5-dimethylpyrazol-1-yl)-1,3, 5-triazine (Me(2)-TpzT) has been used to synthesize the new palladium derivatives and exhibits a rich dynamic behavior, including three metallotropic processes (metal-hurdling, 1,4-metallotropic shifts, and an intermolecular process
Abstract: The polydentate N-donor ligand 2,4,6-tris(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine (Me2-TpzT) has been used to synthesize the new palladium derivatives Pd(R)2(Me2-TpzT), R = C6F5, 1; R = m-C6ClF4, ...
33 citations
TL;DR: In this paper, a series of 2-chloro-4,6-bis(pyrazolylamino)-1,3,5-triazines with applications in crystal engineering have been prepared.
Abstract: A series of 2-chloro-4,6-bis(pyrazolylamino)-1,3,5-triazines with applications in crystal engineering have been prepared. At low temperature, the presence of two or three isomers has been detected and these assigned to 4,6-diamino-1,3,5-triazine structures on the basis of comparison with model compounds. 2D-Exchange spectroscopy studies in various solvents and at different temperatures have been used to determine the equilibrium constants and the activation free energies of the restricted rotation about the amino–triazine bond. A plot of the activation free energy versus temperature showed a good linear correlation and confirmed that the same process is present in all of the compounds under investigation. Comparison with model compounds also confirmed both the occurrence of the restricted rotation and the 4,6-diamino-1,3,5-triazine tautomerism for triazines 1–4 in solution. The structure of compound 1 has been determined in the solid state by X-ray crystallography and consists of a 4,6-diamino-1,3,5-triazine structure stabilized by intra and intermolecular hydrogen bonds.
33 citations
TL;DR: In this article, the structure of the bases of these DNA triplexes in terms of their interaction energies and electron densities for the different hydrogen bonds was studied by means of ab initio calculations, and a good agreement has been found with experimental results and previous calculations.
Abstract: Since their discovery, DNA triplexes have received a large amount of interest because of their potential as viable strategy for control of gene expression and their formation with some specific drugs. For that reason, we have studied, by means of ab initio calculations, the structure of the bases of these DNA triplexes in terms of their interaction energies and electron densities for the different hydrogen bonds, which is one of the interactions that determine the structure and dynamics of nucleic acid molecules. A good agreement has been found with experimental results and previous calculations. In general, when a third base is added to a dimer, the strength of the hydrogen bonds of the dimer becomes modified provoking changes in their structures by increasing the distance in the major groove and decreasing the space in the minor groove.
22 citations
TL;DR: In this paper, high level density functional theory calculations have been carried out for a benchmark set of benzene derivatives, including methyl, ethyl, n-propyl, i-Propyl, tert-butyl, phenyl and benzyl groups as substituents.
Abstract: High level density functional theory calculations have been carried out for a benchmark set of benzene derivatives, including methyl, ethyl, n-propyl, i-propyl, tert-butyl, phenyl, and benzyl groups as substituents. Geometries were obtained using the B3LYP method and three basis set expansions, namely 6-31G(d), 6-311G(d,p), and 6-311++G(d,p). Final energies were calculated in B3LYP/6-311+G(3df,2p) single-point calculations. Based on these calculations the performance of different theoretical schemes aiming at reproducing substituent effects on enthalpies of formation has been assessed. The poorest performance is obtained when atomization energies or isodesmic reactions are used. No significant improvement is found when using homodesmotic processes. A significant improvement is achieved when the isodesmic processes used involve the unsubstituted parent compound. That means that this procedure can be a good alternative to explore substituent effects on the enthalpies of formation, although the absolute valu...
22 citations
TL;DR: Two-, three-, and four-bond spin-spin coupling constants for a set of small fluoro-substituted organic molecules have been computed using ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) theory as mentioned in this paper.
Abstract: Two-, three-, and four-bond 1 9 F- 1 9 F spin-spin coupling constants ( n J F F ) for a set of small fluoro-substituted organic molecules have been computed using ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) theory. The computed values reproduce the experimental signs and magnitudes of n J F F . The straight line that relates the experimental and computed coupling constants has a slope of approximately 1 and passes through the point (0,0 Hz) within the uncertainties of the fit. Hence, EOM-CCSD values of n J F F should be excellent predictors of experimental values when these are not available. All of the components of n J F F except for the diamagnetic spin-orbit term may be large and must be evaluated if agreement between theory and experiment is to be obtained.
19 citations
TL;DR: In this article, the difficulties to obtain convenient monocrystals of the important fungicide RPA 406194 have been overcome by a combination of solid state 13C NMR, X-ray powder diffraction and molecular modeling.
TL;DR: In this article, a theoretical study of the hydrogen-bonded (HB) complexes between a protein model and nucleic acid bases (NAB) has been carried out, and the electron density of the complexes has been analyzed using the atoms in molecules methodology.
Abstract: A theoretical study of the hydrogen-bonded (HB) complexes between a protein model and nucleic acid bases (NAB) has been carried out. As protein models, N-formylglycinamide (For-Gly-NH 2 , 2-formylaminoacetamide), 1, in β- and γ-conformations and as NABs, the isolated ones, and the AU, GC dimers in the Watson-Crick (WC) disposition have been considered. Only those dispositions with a double HB between the protein model and the nucleic acid bases have been studied. The aromatic CH groups of the nucleic acids have been included as HB donor. The results indicate that the strongest HBs between the individual NAB and the protein models involve the atoms that participate in the formation of the WC dimers. In the trimeric complexes, no significant preference is obtained for the 1-AU trimers studied while in the 1-GC ones the complex where formylglycinamide interacts simultaneously with the carbonyl group of guanine and the amino of cytosine is favored. The electron density of the complexes has been analyzed using the atoms in molecules methodology, finding exponential relationships between the electron density and its Laplacian vs the bond distance. Finally, the effect in the nuclear chemical shielding due to the complexation has been explored. Exponential relationships have been found for the variation of the chemical shift of the 1 H signal for the NH...O and NH...N interactions with the HB distance.
TL;DR: In this paper, the structures of six new tetrazines have been determined and their molecular packing has been compared to the supermolecular architecture observed in related carboxylic acid dimers.
Abstract: The structures of six new tetrazines have been determined and their molecular packing has been compared to the supermolecular architecture observed in related carboxylic acid dimers. In the tetrazines, covalent N-N bonds are considered to replace the intermolecular O-(HO)-O-... hydrogen bonds of the carboxylic acids. In the systems investigated, it is apparent that, in the majority of cases, the covalent six-membered ring of the tetrazine is an appropriate replacement for the carboxylic acid synthon. This apparent interplay between molecular and supramolecular units may have applications in the crystal engineering of new materials.
TL;DR: In this article, the effect of the insertion of a krypton atom in the C-H bond of acetylene and hydrogen cyanide in their electronic properties and as hydrogen bond acceptor and donor has been evaluated by means of ab initio calculations, up to MP2/6-311++G(2d,2p) level.
TL;DR: In this article, the reaction of the polydentate N-donor ligand hexakis(pyrazol-1-yl)benzene (hpzb) with AgSbF6/PPh3 or AgClO4PPhPh3 in a 1:1 ratio leads to the complexes [Ag(PPh 3)(hpzb)]SbFs 6 (1) or [AgClO 4PPh4(hpzb)) ClO4 (3).
TL;DR: The oxalate salts and free bases of fentanyl and N-[1-(2-phenylethyl)-4-piperidyl]-N-(1-phenyl-4-pyrazolyl)propanamide, a new lead compound for long-acting analgesia, have been characterized by (1)H- and (13)C-NMR spectroscopy.
Abstract: The oxalate salts and free bases of fentanyl and N-[1-(2-phenylethyl)-4-piperidyl]-N-(1-phenyl-4-pyrazolyl)propanamide, a new lead compound for long-acting analgesia, have been characterized by (1)H- and (13)C-NMR spectroscopy. The crystal structure of the hydrochloride of N-[1-(2-phenylethyl)-4-piperidyl]-N-(1-phenyl-4-pyrazolyl)propanamide monohydrate has been determined. Two centrosymmetrically related cations, joined through C(phenyl)-H em leader pi contacts, encapsulate a large void that contains pairs of anions and bridged water molecules into a zero-dimensional (0D) supramolecular motif. The cations are linked to this framework via N(+)H em leader Cl(-) contacts. GIAO/B3LYP calculations have been carried out to compare the experimental (13)C chemical shifts with the absolute shieldings thus calculated. The protonation of both molecules takes place on the piperidine ring (axial protonation), as has been verified both in the solid state (X-ray) and in solution (NMR).
TL;DR: The structure of 3(5) -[(4-diphenylphosphinoyl)phenyl] pyrazole has been determined by X-ray crystallography.
Abstract: The structure of 3(5) -[(4-diphenylphosphinoyl)phenyl]pyrazole has been determined by X -ray crystallography. The compound crystallizes from DMF as the 1H-3-substituted tautomer (monoclinic, space group P21/c). The complete characterization by means of 13 C, 31 P and 15 N CPMAS NMR allowed us to confirm that at the solid state only such tautomer is found, with strong intermolecular hydrogen bonds between the pyrazole-NH donor and the phosphine oxide group acceptor. At 300 K in DMSO-d6 solution, the two tautomers were detected in a ratio 3tautomer versus 5-tautomer of 86:14. According to DFT calculations (B3LYP/6-31G** + ZPE), the 3-[(4-diphenylphosphinoyl)phenyl]-1H-pyrazole tautomer is more stable by about 3.8 kJ mol –1 .
TL;DR: The molecular structures of two aminophosphonium salts (bromide and tetrafluoroborate) have been determined by X-ray analysis as discussed by the authors, and a coupling of ∣1.5∣ Hz between the 15N and the 31P nuclei, separated by three bonds, was observed experimentally for the bromide in CDCl3 solution, which appears to be a classical 3JN-P across the covalent bonds and not a 3hJN -p across the IMHB.
Abstract: The molecular structures of two aminophosphonium salts (bromide and tetrafluoroborate) have been determined by X-ray analysis. They have similar conformations and hydrogen bond (HB) networks: the N–H acid proton is bonded to the anion and, in the case of the fluoroborate, to the oxygen atom of the phosphine oxide, forming a pseudo six-membered ring closed by a weak N–H⋅⋅⋅O intramolecular hydrogen bond (IMHB). These compounds have been studied by multinuclear NMR in solution, including the 15N-labeled derivatives, to determine a complete set of coupling constants. A coupling of ∣1.5∣ Hz between the 15N and the 31P nuclei, separated by three bonds, was observed experimentally for the bromide in CDCl3 solution, which appears to be a classical 3JN-P across the covalent bonds and not a 3hJN-P across the IMHB.
TL;DR: In this article, the analogs of the Troger's base, 7,17methano-6,7,16,17-tetrahydro-diacridino-[2,1-b:2',1'-f]-[1,5]-diazocine, have been fully characterized by Hand CNMR spectroscopies using monoand bi-dimensional techniques.
Abstract: The analogs of the Troger’s base, 7,17-methano-6,7,16,17-tetrahydrodiacridino-[3,4-b:3’,4’-f][1,5]-diazocine was prepared starting from 2-aminoacridine instead of 3-aminoacridine. The new compound, 7,17-methano-6,7,16,17-tetrahydro-diacridino-[2,1-b:2’,1’-f]-[1,5]-diazocine, has been fully characterized by Hand CNMR spectroscopies using monoand bi-dimensional techniques. Other aminoacridine derivatives failed to afford Troger’s bases.
TL;DR: The crystal structure of 1-hydroxy-2,4,5-triphenyl-1H-imidazole3-oxide has been determined fromlaboratory X-raypowder-diffraction data as discussed by the authors.
Abstract: )InstitutodeQuImicaMe ¬dica,CentrodeQuImicaOrganica−ManuelLoraTamayo×,CSIC,JuandelaCierva3,E-28006Madrid(e-mail:iqmbe17@iqm.csic.es)DedicatedtoProfessorJackD.Dunitzontheoccasionofhis80thbirthdayThe crystal structure of 1-hydroxy-2,4,5-triphenyl-1H-imidazole3-oxide(1) has been determined fromlaboratoryX-raypowder-diffractiondata.ThetwoindependentmoleculesintheasymmetricunitformchainsviaO H¥¥¥Ohydrogenbondsrelatedbyatwofoldscrewaxis.OneoftheO¥¥¥Odistancesisextremelyshort(2.32(1) and 2.43(1)a). Solid-state NMR spectroscopy (CPMAS) combined with calculation of absoluteshieldings(GIAO/B3LYP/6-31G*) allowedus to determine thatthe compoundbehaves as iftheO H¥¥¥Ohydrogenbondhastheprotoninthemiddle(single-wellpotential),resultinginthenearidentityofboth
TL;DR: The coupling constants of a series of acetylenic derivatives have been calculated using the finite perturbation method and a Karplus-type relationship has been obtained for coupling constant of hydrogen atoms separated up to 15 bonds.
Abstract: The coupling constants of a series of acetylenic derivatives have been calculated using the finite perturbation method. In the case of dimethylated derivatives a Karplus-type relationship has been obtained for coupling constants of hydrogen atoms separated up to 15 bonds. Additional relationships have been obtained between the interatomic distances and the coupling constants.
TL;DR: Two kinds of electron densities for several small molecules have been generated for a wide range of bond distances and the differences, associated to the bond formation, are found to be very important for most of the cases.
Abstract: Two kinds of electron densities for several small molecules (H(2), FH, CH(3)CH(3), CH(3)NH(2), CH(3)OH, and CH(3)F) have been generated for a wide range of bond distances. The first one, as the sum of the electron density of the isolated fragments, and the second one by optimizing the electron density at each given geometrical disposition. A number of properties of this two electronic distributions have been compared (position of the bond critical points, electron density, Laplacian, curvatures, and local energies). The differences, associated to the bond formation, are found to be very important for most of the cases.
TL;DR: In this paper, it was shown that DFT only works acceptably well for relatively strong HBs and that atomic hydrogen, although a much weaker hydrogen-bonded base, should form stable complexes with the ammonium cation.
Abstract: Two very significant, although for different reasons, radicals NO · and H · have been examined theoretically as hydrogen bond acceptors (HBA). Two acids have been studied as hydrogen bond donors (HBD), hydrogen fluoride and ammonium. Nitrogen oxide should be a nitrogen base towards both neutral and cationic acids as HBD. Atomic hydrogen, although a much weaker hydrogen-bonded base, should form stable complexes with the ammonium cation. A conclusion of this work is that DFT methods only work acceptably well for relatively strong HBs.
TL;DR: A series of aminotroponimine and aminotropic derivatives were studied by electrospray mass spectrometry and provided rapid structural assignments through inexpensive isotopic labeling MS/MS experiments.
Abstract: A series of aminotroponimine and aminotropone derivatives were studied by electrospray mass spectrometry. MS and MS/MS data were acquired according to automated procedures. In the case of mono- and di-halogenated compounds, their specific behavior upon collisionally-activated dissociation conditions provided rapid structural assignments through inexpensive isotopic labeling MS/MS experiments.
TL;DR: In this article, the isolation of pyridazinones and 6H-6,7- dihydropyrazolo(1,5-d)-1,2,4-triazine and determination of their structure by mass spectrometry and by 1 H and 13 C NMR.
Abstract: When 1,4-bis-ethoxymethylen-2,3-butanedione 2 reacts with hydrazine, following a slightly modified Effenberger's procedure, other compounds than the expected 3,5'-bipyrazole 1 are obtained. This paper describes the isolation, besides 1, of two pyridazinones and one 6H-6,7- dihydropyrazolo(1,5-d)-1,2,4-triazine and the determination of their structure by mass spectrometry and by 1 H and 13 C NMR.