Showing papers by "José Elguero published in 2001"
TL;DR: In this paper, the local kinetic G( r ), potential V( r ), and total energy densities, calculated at the critical points of 37 H⋯F closed-shell interactions by quantum mechanical methods, have been compared to their estimated values obtained by using an approximate evaluation of G(r ) and the local form of the virial theorem.
367 citations
TL;DR: In this paper, theoretical calculations on a series of SiXY3-ZW complexes, where X and Y are H, F, and Cl, and Z corresponds to an electron donor atom (ZW ) NH3, NCH, CNH, OH2, FH), were performed.
Abstract: Theoretical calculations on a series of SiXY3‚‚‚ZW complexes, where X and Y are H, F, and Cl, and Z corresponds to an electron donor atom (ZW ) NH3, NCH, CNH, OH2, FH), were performed. The calculations were carried out using B3LYP/6-311++G**, MP2/6-311++G** and MP2/6-311++G(2d,2p) computational methods. The electron density was characterized by means of the atoms in molecules (AIM) methodology, and the interaction nature was studied with the NBO method. Finally, the effect of the complexation on the nuclear chemical shieldings was evaluated with the GIAO method. The results display a wide range of interaction distances that vary from 2.1 to 4.1 A. The complexes with shorter interaction distances (2.1 A) show important distortion effects and large dipole moment enhancements. The NBO analysis indicates that in those complexes an ionic interaction is formed between the Si and Z atoms. Comparison of the chemical shieldings of the complexes and the monomers indicates that these interactions could be detected experimentally using 29 Si NMR. In addition, in the case of the complexes with NH3 and OH2, the use of 15 N NMR and 17 O NMR could be adequate to check the potential formation of the corresponding complexes.
179 citations
TL;DR: In this paper, it was shown that the cis isomer of the o-cresol radical cation shows a low-frequency shift of the OH stretching attributed to an intramolecular hydrogen bond.
Abstract: Considering the findings of Fujii et al. showing that the cis isomer of the o-cresol radical cation shows a low-frequency shift of the OH stretching attributed to an intramolecular hydrogen bond wi...
122 citations
TL;DR: In this article, the gas phase and aqueous basicities of six 1,2,3-triazoles have been determined, the former by FT-ICR and the latter by spectrophotometry and 1H NMR.
121 citations
TL;DR: Using high-resolution solid-state (15)N CMAS NMR, X-ray crystallography, and ab initio calculations, the structure of solid pyrazole-4-carboxylic acid is studied, which shows a proton disorder-order transition where the protons are preferentially localized on given nitrogen and oxygen atoms.
Abstract: Using high-resolution solid-state 15N CMAS NMR, X-ray crystallography, and ab initio calculations, we have studied the structure of solid pyrazole-4-carboxylic acid (1). The crystal structure was determined at 295 and 150 K. Molecules of 1 are located on a two-fold axis, implying proton disorder of the NH and OH groups; no phase transition was observed between these two temperatures. The compound forms quasi-linear ribbons in which the molecules are linked by cyclic hydrogen bonds between pyrazole and carboxylic acid groups with disordered hydrogen-bonded protons. Crystallography is unable to decide whether the disorder is dynamic or static. NMR shows that this disorder is dynamic, that is, consisting of very fast degenerate double proton transfers between two rapidly interconverting O−H···N and O···H−N hydrogen bridges. However, at low temperature, NMR shows a proton disorder−order transition where the protons are preferentially localized on given nitrogen and oxygen atoms. An amorphous phase exhibiting ...
77 citations
TL;DR: A convenient synthesis of 5alpha,8alpha,14alpha,17alpha,5,17:8,14-dimethano-5,7,8,13,14,17,18-octahydrodibenzo derivatives is described, and the compounds have been fully characterized by NMR.
Abstract: A convenient synthesis of 5α,8α,14α,17α-5,17:8,14-dimethano-5,8,14,17-tetraaza-5,6,7,8,13,14,17,18-octahydrodibenzo[e,e‘]benzo[1,2-a:3,4-a‘]dicyclooctene derivatives is described, and the compounds have been fully characterized by NMR; in some cases, the molecular structure has been determined by X-ray crystallography. These compounds represent the first examples of a new class of molecular tweezers.
73 citations
TL;DR: In this article, the isomer with the least steric hindrance was found for the derivatives PdClR(N-N′), R-Me or COMe, while the two possible isomers were found for similar derivatives with pz*pm, with the thermodynamically stable isomer being cis,trans-RuCl2(PPh3)2(bpzpm).
Abstract: New complexes of palladium(II), silver(I), copper(I) and ruthenium(II) containing the ligands 2-(pyrazol-1-yl)pyridine
(pzpy), 2-(pyrazol-1-yl)pyrimidine (pzpm), 2-(3,5-dimethylpyrazol-1-yl)pyrimidine (pz*pm) and 4,6-bis(pyrazol-1-yl)pyrimidine (bpzpm) have been prepared and characterised. Only the isomer with the least steric hindrance was
found for the derivatives PdClR(N–N′), R = Me or COMe, N–N′
= pzpy or pzpm, while the two possible isomers were found for
similar derivatives with pz*pm. In the reaction of [RuCl2(cod)]n with pzpy, pzpm and bpzpm, the isomer trans-RuCl2(cod)(N–N′) was always formed. Three different isomers were consecutively formed after the reaction of RuCl2(PPh3)3
with bpzpm, with the thermodynamically stable isomer being cis,trans-RuCl2(PPh3)2(bpzpm). A fluxional
process involving interchange of the H4/H6 pyrimidine protons, which implies M–N(pm) bond rupture, was found in the silver, copper and some palladium derivatives. Such an interchange was not observed in the ruthenium complexes. The trans influence of the group situated trans to the pyrimidine ring was found to be extremely important
for Pd–N bond rupture. The isomer interconversion for the derivatives PdClR(pz*pm), R = Me or COMe, was studied
by variable temperature 1H NMR studies. The process is of higher energy than
the H4/H6
interchange and is facilitated by the presence of the acyl group.
45 citations
TL;DR: In this article, the coupling constants of N,N′-diphenyl-6-aminopentafulvene-1-aldimine-15N2 (1) and its asymmetric analog N-phenyl-N′-(1,3,4-triazol)- 6-amino-enzyme-1,aldimines type were studied.
Abstract: Multinuclear liquid state magnetic resonance experiments have been performed on two seven-membered 15N-labeled H-chelates of the 6-aminofulvene-1-aldimines type in order to characterize the strong intramolecular NHN hydrogen bonds as a function of the molecular symmetry. In particular, the symmetrically substituted N,N′-diphenyl-6-aminopentafulvene-1-aldimine-15N2 (1) and its asymmetric analog N-phenyl-N′-(1,3,4-triazol)- 6-aminopentafulvene-1-aldimine-15N5 (2) have been studied. For 1, an NN coupling constant across the hydrogen bridge of 2hJ(15N,15N) = 10.6 Hz was determined indirectly by 13C NMR at two different Larmor frequencies, 125.76 and 67.93 MHz; this coupling constant is characteristically enhanced compared with the value of 8.6 Hz obtained previously for 2. Because of a fast degenerate proton tautomerism the hydrogen bond proton in 1 is coupled with both nitrogen atoms with a coupling constant of −40.8 Hz. {15N} tickling experiments were performed on 2 in order to determine the relative signs of the coupling constants of the NHN hydrogen bridge. We find that 2hJ(15N,15N) and 1hJ(1H,15N) = +4.4 Hz exhibit the same sign, i.e. the opposite sign compared with 1J(15N,1H) = −88.6 Hz. This finding proves that 1hJ(1H,15N) corresponds to an intrinsic coupling, which is not induced by a tautomerism absent in 2 because of the large difference in basicities of the aniline and the amino-1,3,4-triazole substituents. Therefore, these observations indicate a sign change of J(15N,1H) when the proton is transferred successively from one nitrogen to the other, as observed previously for FHF hydrogen bonds. The relation between the values of the coupling constants and the hydrogen bond geometries is discussed in terms of the valence bond order model, as are the implications for obtaining equilibrium constants of tautomerism from coupling constant data. Copyright © 2001 John Wiley & Sons, Ltd.
42 citations
TL;DR: The structure was determined by X-ray diffraction and shown to correspond to tautomer 1 a (both in the solid state and in solution) and open up a new field of hydrogen-bond research by NMR spectroscopic methods.
Abstract: An unusually substantial coupling is observed across the hydrogen bond of fully 15 N-labeled compound 1 when it is studied by 1 H and 15 N NMR spectroscopy. The structure was determined by X-ray diffraction and shown to correspond to tautomer 1 a (both in the solid state and in solution). These results open up a new field of hydrogen-bond research by NMR spectroscopic methods.
37 citations
TL;DR: In this article, the 1 H-, 13 C-, 15 N- and 29 Si-NMR spectra of 3-aminopropyltrimethoxysilane and N,N-dimethylamino-propyl trimethoxide syilane have been analyzed and it was shown that these compounds exist as open-chain structures and not as ring-closed silatranes.
33 citations
TL;DR: In this article, the 13 C and 15 N chemical shifts of twelve azoles and benzazoles have been determined in the solid state at room temperature and in methanol solution at 178 K.
Abstract: The 13 C and 15 N chemical shifts of twelve azoles and benzazoles have been determined in the solid state at room temperature and in methanol solution at 178 K. The experimental values were compared with the absolute shieldings calculated at the GIAO/B3LYP/6-31G* level. These comparisons show that some signals, 13 C NMR but especially 15 N NMR, have to be corrected for hydrogen bonds present in certain solvents and in the solid state. The linear regressions are good enough to be used to predict some missing values.
TL;DR: A spontaneous self-ionization and concomitant proton transfer can occur in dimers involving phosphinic acid derivatives, even when they present the same functional group.
Abstract: Proton transfer between dimers is associated with strong bases and very good hydrogen bond donors, namely, between very different chemical species. A spontaneous self-ionization and concomitant proton transfer can occur in dimers involving phosphinic acid derivatives (see picture), even when they present the same functional group. It is also possible, by choosing the appropriate substituents, to modulate the degree of the interaction, from a hydrogen bonded complex to an ion pair, without changing the nature of the functional groups involved.
TL;DR: Both B3LYP/6-31G(d) and QCISD(T) calculations were used to determine the reaction path which connects the initial tetrahedrane-ammonium complex with the final products, protonated cyclobutadiene (CBDH+) and ammonia.
Abstract: The gas-phase basicity (GB) of tetra-tert-butyltetrahedrane (tBu4THD) was determined by FT-ICR mass spectrometry and comparison with reference compounds of known basicity. Its GB, 1035+/-10 kJ x mol(-1), makes tetra-tert-butyltetrahedrane one of the strongest bases reported so far. Ab initio calculations [B3LYP/6-31G(d) and B3LYP/6-311 + G(d,p)//6-31G(d)] have been carried out in order to compare the high experimental basicity of tBu4THD with that estimated theoretically. Both B3LYP/6-31G(d) and QCISD(T) calculations were used to determine the reaction path which connects the initial tetrahedrane-ammonium complex with the final products, protonated cyclobutadiene (CBDH+) and ammonia.
TL;DR: In this paper, the NMR effects produced on the nitrogen absolute shieldings in a series of electron donors when they interact with hexafluorobenzene, C6F6, have been theoretically studied.
Abstract: The NMR effects produced on the nitrogen absolute shieldings in a series of electron donors when they interact with hexafluorobenzene, C6F6, have been theoretically studied. The complexes have been optimized at the B3LYP/6-311++G** level and the NMR shieldings have been calculated using the GIAO method. The results obtained have allowed devising an experiment (C6F6···NCCH3 complex) that is compatible with the theoretical calculations.
TL;DR: In this article, a proposal for the relationships between the different compounds and a possible mechanism is presented, and the products have been characterised by MS, NMR (1H and 13C) and X-ray crystallography (in the case of 6a).
TL;DR: In this article, the tautomerism of pyridones and 1,2,4-triazoles related to two crown ethers and two crown esters derived from these heterocycles was studied theoretically.
TL;DR: In this paper, the ability of the (CO) 4 Co − organometallic complex as a HB acceptor has been studied, considering the importance of the formal negative charge of Co in the interaction with other systems.
Abstract: The ability of the (CO) 4 Co − organometallic complex as a HB acceptor has been studied. For this theoretical study, several standard HB donors such as HF, HCN and HNC have been used. The inclusion of the HN(CH3) 3 + cation as HB donor, allows the comparison of theoretical and experimental results. In order to consider the importance of the formal negative charge of (CO) 4 Co − in the interaction with other systems, the isoelectronic neutral (CO) 4 Ni organometallic complex has been considered as HB acceptor. The electronic changes, within the Atoms in Molecules (AIM) and Natural Bond Orbitals (NBO) methodologies, have been evaluated. Geometry, protonation energy, interaction energy, and electron density results confirm that the stability as a protonated species of the organometallic complexes (i.e. their gas-phase basicity) studied here gives an indication of their ability to act as a HB acceptor. It can be concluded that the Co organometallic anion seems to be a better HB acceptor than the Ni derivative, due to the formal charge of the former, more than the corresponding electronic distribution.
TL;DR: In this paper, the absolute shieldings for the different nuclei present in azolides derived from pyrrole, imidazole, pyrazole, both triazoles, tetrazoles, pentazole and carbazole bearing at the nitrogen atom are compared with experimental ones, when available, and also internally compared.
TL;DR: The behavior of strained π-systems with regard to their interaction with hydrogen bond donors was studied in this article, where the interaction of a model system, strained acetylene, and ortho-benzyne with hydrogen fluoride in their singlet and triplet electronic configuration was explored at the B3LYP/6-311++ G ∗∗ and MP2/6 -311++G ∗ ∗ levels.
TL;DR: In this paper, the solid state 13C and 15N CPMAS NMR spectra of 3,5-di-tert-butyl-1,2,4-diazaphosphole 4 and 3, 5-diphenyl- 1, 2, 4-diamantropophosphole 5 have been recorded.
Abstract: The solid state 13C and 15N CPMAS NMR spectra of 3,5-di-tert-butyl-1,2,4-diazaphosphole 4 and 3,5-diphenyl-1,2,4-diazaphosphole 5 have been recorded. The X-ray structure of the first compound was already known (it is a cyclic dimer with localized N–H protons) while the structure of the second cannot be determined due to the difficulty to grow suitable single crystals. NMR results pointed out that 4 is a “classical” compound while 5 is probably a tetramer showing Intermolecular Solid-State Proton Transfer (ISSPT). GIAO/ab initio calculations have been carried out to estimate the absolute 1H, 13C and 15N shieldings. The agreement with the experimental chemical shifts is good enough to assign the signals of carbons C-3 and C-5.
TL;DR: The molecular structures of three 4-tert-butylpyrazoles have been determined at 173 K as discussed by the authors, which is the first example of a planar planar tetramer.
Abstract: The molecular structures of three 4-tert-butylpyrazoles have been determined at 173 K: 4-tert-butyl-3(5)-isopropylpyrazole
1, 4-tert-butyl-3(5)-neopentylpyrazole 2 and 4-tert-butyl-3(5)-p-tolylpyrazole 3. 1H, 13C and 15N NMR spectroscopies,
in solution and in the solid state (CPMAS), have been used to complement the structural information. The major
tautomers in solution correspond to the tautomers present in the crystal: 5-isopropyl 1b, 5-neopentyl 2b and 3-p-tolyl
3a. All these compounds crystallise as tetramers, formed by four identical tautomers, through N–H···N hydrogen bonds,
the tetramers corresponding to 1b and 2b
being folded
but that of 3a is the first example of a planar tetramer.
TL;DR: In this article, a 3-center N−H−·OC hydrogen bond connects the molecules in the crystal forming chains parallel to the c axis, which promotes coplanarity of both ring systems.
Abstract: 2-(1‘-Pyridin-2‘-one)benzimidazole was synthesized in order to determine its structure in the solid state, its NMR properties, and its behavior in absorption−emission electronic spectroscopy. To rationalize these properties, ab initio calculations have been carried out on its ground and excited states. A three-center N−H···OC hydrogen bond connects the molecules in the crystal forming chains parallel to the c axis. The intramolecular hydrogen bond promotes coplanarity of both ring systems. In the ground state, only the N−H tautomer exists as a local minimum of the potential energy surface, while in the excited-state, both tautomers are found to be stable. The main consequence is that the photochemical behavior of this compound is governed by the existence of a proton transfer in its excited state.
TL;DR: In this article, the X-ray molecular structures of three chiral pyrazoles bearing (4S)-4-benzyloxazolidin-2-ones or (2R)-bornane-10,2-sultams were studied by diffraction and by NMR.
Abstract: The X-ray molecular structures of three chiral pyrazoles bearing (4S)-4-benzyloxazolidin-2-ones or (2R)-bornane-10,2-sultams
at position 3(5) have been studied by single crystal X-ray diffraction and by NMR. In the solid state, the same pyrazole
tautomer a has been found for the three compounds, that with the chiral group in the 3-position. Their crystal structures
consist of infinite chains (catemers) formed by N–H···OX (X: C, S) hydrogen bonds. The use of solid-state
13C NMR spectroscopy demonstrated that the pyrazole is also an a tautomer. 13C NMR spectra in methanol solution yield average signals even at −80 °C; nevertheless, interpolation
allows one to estimate that in solution tautomer a also predominates.
TL;DR: In this article, collisional activation of protonated 3-halogenopyrazoles (X-Pz, X=Cl, Br and I) in the high or low translational energy regime induced an intense loss of X giving C 3 H 4 N 2 + radical cations whose structure depends on the nature of the halogen.
TL;DR: In this paper, the absorption and emissive properties of 9-acridinones, 9-thioacridins and 9-aminoacridines including six crown ether derivatives were investigated.
TL;DR: In this article, four derivatives of 1.4-dihydroxybenzene have been isolated and identified in the reaction between 3(5)-methylpyrazole and p-benzoquinone.
Abstract: Four derivatives have been isolated and identified in the reaction between 3(5)-methylpyrazole and p-benzoquinone. H and C NMR, including C CPMAS NMR have been used to characterize these 1-pyrazolyl derivatives of 1.4-dihydroxybenzene. A discussion of their structure and meso/d,l isomerism is provided.
TL;DR: In this article, the azolyl substituents at positions 2 or 2,3 are 1- pyrazolyl, 3,5-dimethyl-1-pyrazolyls and 2-methyl-1imidazolyl.
Abstract: Nine new compounds have been prepared by methylation of monoazolyl and bisazolyl derivatives of 1,4-dihydroxybenzene. The azolyl substituents at positions 2 or 2,3 are 1- pyrazolyl, 3,5-dimethyl-1-pyrazolyl and 2-methyl-1-imidazolyl. The resulting compounds with one or two methoxy groups have been characterized by 1 H and 13 C NMR, by IR and by mass spectrometry. In two cases, both dimethoxy derivatives, the meso and d,l isomers have been