Showing papers by "José Elguero published in 2004"

Do coupling constants and chemical shifts provide evidence for the existence of resonance-assisted hydrogen bonds?

Abstract: Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to determine two-bond 17O–17O and 15N–15N spin–spin coupling constants (2h J O–O and 2h J N–N) for ten neutral structures which may exhibit intramolecular O–H–O or N–H–N hydrogen bonds. MP2 chemical shifts of hydrogen-bonded protons have also been evaluated. The molecules include malonaldehyde and its diaza counterpart, and the corresponding saturated analogues. The aim of this study is to investigate whether the magnetic properties of the two-bond spin–spin coupling constants and the chemical shifts of the hydrogen-bonded protons provide evidence for the existence of resonance-assisted hydrogen bonds (RAHBs). The two-bond coupling constant for the equilibrium structures is greater for the hydrogen-bonded unsaturated molecule than for the saturated molecule, a result of the shorter O–O distance and stronger hydrogen bond in the former. However, when the O–O or N–N distances are forced to be the same in corre...

Fluorine–Fluorine Interactions: NMR and AIM Analysis

Abstract: The structure of a number of compounds that show experimental F⋅⋅⋅sF coupling constants across the space has been studied using HF-DFT methods (B3LYP) and Atoms in Molecules (AIM) methodologies For all the cases with strong coupling constants a bond critical point and the corresponding bond path between the fluorine atoms involved has been found in the electron density map In an attempt to predict NMR properties, new compounds for which no experimental F-F coupling constants are available, but with the same characteristics in the electron density maps, have been calculated

Dynamic NMR study of the mechanisms of double, triple, and quadruple proton and deuteron transfer in cyclic hydrogen bonded solids of pyrazole derivatives.

Abstract: Using dynamic solid state 15N CPMAS NMR spectroscopy (CP = cross polarization, MAS = magic angle spinning), the kinetics of the degenerate intermolecular double and quadruple proton and deuteron transfers in the cyclic dimer of 15N labeled polycrystalline 3,5-diphenyl-4-bromopyrazole (DPBrP) and in the cyclic tetramer of 15N labeled polycrystalline 3,5-diphenylpyrazole (DPP) have been studied in a wide temperature range at different deuterium fractions in the mobile proton sites. Rate constants were measured on a millisecond time scale by line shape analysis of the doubly 15N labeled compounds, and by magnetization transfer experiments on a second timescale of the singly 15N labeled compounds in order to minimize the effects of proton-driven 15N spin diffusion. For DPBrP the multiple kinetic HH/HD/DD isotope effects could be directly obtained. By contrast, four rate constants k1 to k4 were obtained for DPP at different deuterium fractions. Whereas k1 corresponds to the rate constant kHHHH of the HHHH isot...

A theoretical study of the tautomerism and ionization of 5-substituted NH-tetrazoles

Abstract: The energies, geometries and charge distributions of the five more stable forms (anion, 1H- and 2H-neutral tautomers, 1,3-H,H+- and 1,4-H,H+-cation tautomers) of a series of 5-R tetrazoles [R=H, CH3, C(CH3)3, Ph, Cl, CF3, NO2] in the gas phase have been calculated with the DFT/B3LYP method at the 6-31G* level. For tetrazolate anions, a shortening of the 1–2 bond and a simultaneous elongation of the 2–3 bond is observed when the value of the σp constant of the substituent increases. A considerable ring aromaticity is exhibited by tetrazolate anions, and also by the 2H-form of neutral tetrazoles and 1,3-H,H+-form of cations and it somewhat depends on the nature of the substituent. The calculated values of the dipole moments of 2H- and 1H-forms of neutral tetrazoles correlate well with the σp constants of substituents. The relative thermodynamic stability of 2H-forms as compared with 1H-ones does not practically depend on the nature of the substituent R. In contrast, in the case of tetrazolium cations, the most stable form is the 1,4-H,H+—for the electron-donating substituents and the 1,3-H,H+—for the electron-withdrawing ones. Good correlations have been observed between the energies of protonation of anions and neutral tetrazoles and the experimental pKa and pKBH+ values.

A GIAO/DFT study of 1H, 13C and 15N shieldings in amines and its relevance in conformational analysis

Abstract: The 1H, 13C and 15N absolute shieldings of 13 amines were calculated at the GIAO/B3LYP/6-311++G** level. For some compounds (ethylamine, piperidine and 1-methylpiperidine) two conformations were calculated. The 13C and 15N data could be correctly correlated with experimental chemical shifts, allowing the conformation of 1-methylpiperidine to be established. The 1H NMR absolute shieldings, although less well correlated with delta values, were used to account for the anisotropy effects of the N lone pair.

Multiple hydrogen bonds and tautomerism in naphthyridine derivatives

Abstract: The behaviour of three 2,7-disubstituted 1,8-naphthyridines able to exhibit tautomerism has been studied by NMR in solution and in two cases in the solid state. The three derivatives studied are 2,7-dihydroxy- (1), 2-acetamido-7-amino- (3) and 2,7-diacetamido-1,8-naphthyridine (4). To explore the problem of secondary interactions, a series of complexes, with up to four simultaneous hydrogen bonds, where the monomers are generated using pyridine and 4-pyridone as building blocks, have been theoretically studied. The calculated interaction energies have been correlated with the number of hydrogen bonds and with attractive and repulsive secondary interactions. Further analysis of the electron density and orbital interactions shows that the secondary interactions, both attractive and repulsive, have a purely electrostatic origin. The X-ray structure of compounds 3 and 4 have been determined. In the solid state these compounds exist in the “diamino” tautomers with the N–H proton of the amido groups pointing towards the naphthyridine nitrogen. DFT and GIAO calculations have been essential to disentangle the problem of the structure of these compounds.

One-Bond Spin−Spin Coupling Constants of X−1H Proton Donors in Complexes with X−H−Y Hydrogen Bonds, for X = 13C, 15N, 17O, and 19F: Predictions, Comparisons, and Relationships among 1JX-H, 1KX-H, and X−H Distances

Abstract: Ab initio calculations at the equation-of-motion coupled cluster (EOM-CCSD) level of theory have been carried out to investigate one-bond (13)C-(1)H, (15)N-(1)H, (17)O-(1)H, and (19)F-(1)H coupling constants in a systematic study of monomers and hydrogen-bonded complexes. Computed coupling constants ((1)J(X-H)) for monomers are in good agreement with available experimental data. All reduced Fermi-contact terms and reduced coupling constants ((1)K(X-H)) for monomers and complexes are positive. Plots of (1)K(X-H) versus the X-H distance for the 16 monomers and the 64 complexes in which these monomers are proton donors exhibit significant scatter. However, a linear relationship has been demonstrated for the first time between coupling constants and X-H distances for different X atoms by plotting the ratios of the coupling constants for complexes and corresponding monomers versus the ratios of distances for complexes and corresponding monomers times the square of the Pauling electronegativity. Since the ratio removes the dependence of coupling constants on the magnetogyric ratios of X, this relationship holds for both (1)K(X-H) and (1)J(X-H). The decrease in reduced coupling constants ((1)K(X-H)) as the X-H distance increases is due primarily to the increased proton-shared character of the hydrogen bond.

Modeling protein-RNA interactions: An electron-density study of the formamide and formic acid complexes with RNA bases

Abstract: The complexes formed by the double interaction established between the four RNA bases (adenine, cytosine, guanidine, and uracil) and formamide and formic acid as a model for the interacting groups of certain amino acid side chains have been theoretically studied. Density functional theory (B3LYP/6-31+G**) methods have been used for this study. The interaction energies obtained range between 10 and 19 kcal mol - 1 . The analysis of the electron density and the natural bond orbital analysis show that these complexes are bound by medium strength double hydrogen bonds established between the donor and acceptor groups of formamide and formic acid and those of the RNA bases. Comparisons are made with the results obtained in some experimental studies and the analysis of protein-RNA interactions databases.

Predicted signs of reduced two-bond spin-spin coupling constants (2hKX-Y) across X-H-Y hydrogen bonds.

Abstract: The reduced two-bond Fermi-contact terms and the reduced spin-spin coupling constants (2h)K(X-Y) across X-H-Y hydrogen bonds for complexes stabilized by C-H-N, N-H-N, O-H-N, F-H-N, C-H-O, O-H-O, F-H-O and C-H-F hydrogen bonds are positive. The NMR Triplet Wavefunction Model (NMRTWM) indicates that the signs of the reduced FC terms and (2h)K(X-Y) are determined by excited triplet states that have an odd number of nodes intersecting the X-Y axis between X and Y, thereby leading to an antiparallel alignment of the nuclear magnetic moments of atoms X and Y.

Predicted Signs of One-Bond Spin−Spin Coupling Constants (1hJH-Y) across X−H−Y Hydrogen Bonds for Complexes with Y = 15N, 17O, and 19F

Abstract: Ab initio equation-of-motion coupled cluster (EOM-CCSD) calculations have been performed on a set of 44 complexes to obtain one-bond H-Y spin-spin coupling constants ( 1 h J H - Y ) across X-H-Y hydrogen bonds, with Y as the second-period elements 1 5 N, 1 7 O, and 1 9 F For complexes with traditional hydrogen bonds, the reduced Fermi-contact terms and the reduced one-bond spin-spin coupling constants ( 1 h K H - Y ) are negative Since 1 K X - H has been shown previously to be positive, a change of sign of these two coupling constants must occur along the proton-transfer coordinate For complexes with symmetric X-H-X hydrogen bonds, the two reduced X-H coupling constants are equal and positive at equilibrium For complexes stabilized by hydrogen bonds that have some proton-shared character, both 1 h K H - Y and 1 K X - H are also positive The signs of all three reduced coupling constants ( 1 h K H - Y , 2 h K X - Y , and 1 K X - H ) that can arise between pairs of hydrogen-bonded atoms are interpreted in terms of the nuclear magnetic resonance triplet wave function model (NMRTWM) Determination of the signs of 1 h K H - Y and 1 K X - H could be useful for confirming the presence or absence of a proton-shared hydrogen bond

19F–19F spin–spin coupling constant surfaces for (HF)2 clusters: The orientation and distance dependence of the sign and magnitude of JF–F

Abstract: Ab initio calculations using the equation-of-motion coupled cluster method have been carried out to investigate 19F–19F spin–spin coupling constants for a pair of HF molecules. The overall features of the JF–F coupling surface with respect to the F–F distance and the orientation of the pair of HF molecules reflect those of the Fermi-contact (FC) surface, although the FC term may not be a good quantitative estimate of JF–F. The hydrogen-bonded HF dimer exhibits unusual behavior compared to other hydrogen-bonded complexes, since both the FC term and 2hJF–F exhibit variations in sign and magnitude as the F–F distance changes and the linearity of the hydrogen bond is destroyed. The FC term for F–F coupling is relative small and negative for the equilibrium dimer. At the dimer F–F distance, the maximum negative value for the FC term is found for the linear arrangement F–H⋯H–F, while the maximum positive value is found for the linear H–F⋯F–H arrangement, despite the fact that neither of these structures is bound. Changes in the sign and magnitude of the FC term are analyzed using the nuclear magnetic resonance triplet wave function model, which relates the orientation of magnetic nuclei to the phases of the wave functions for excited triplet states that couple to the ground state. The FC term for a particular orientation is a result of competing positive and negative contributions from different triplet states, the sign of each contribution being determined by the alignment of the nuclear magnetic moments in that state. Factors are identified which must play a role in determining which types of wave functions dominate.

Homoheteroaromaticity: the case study of azepine and dibenzazepine

Abstract: Geometrical and energetic DFT calculations as well as GIAO and NICS chemical shifts have been calculated for 1H-azepine and 5H-dibenz[b,f]azepine and their cations. The last compound has been studied experimentally by 1H and 13C NMR in neutral and acidic conditions establishing that the cation corresponds to an N-protonated structure. The conclusion is that the neutral molecules are antiaromatic while the cations are aromatic (homoheteroaromaticity).

Apparent Allyl Rotation in New Allylpalladium(II) Complexes with Pyrazolyl N-Donor Ligands

Abstract: The ligands 2,4,6-tris(pyrazol-1-yl)-1,3,5-triazine (TPzT), 2,4,6-tris(3,5-dimethyl-pyrazol-1-yl)-1,3,5-triazine (Me2-TPzT), and 2,4,6-tris(3,4,5-trimethylpyrazol-1-yl)-1,3,5-triazine (Me3-TPzT) have been synthesised and their reaction with [Pd(η3-C4H7)(THF)2]X was explored. The reaction in a 1:3 molar ratio (ligand/solvate) leads to the new derivatives [{Pd(η3-C4H7)}2(BPz′TO)]X (Pz′ = pyrazole, X = BF4, 2a; Pz′ = 3,5-dimethylpyrazole, X = PF6, 2c; Pz′ = 3,4,5-trimethylpyrazole, X = PF6, 2d) in which the N-donor ligand has been partially hydrolysed. The complexes exist in the form of two isomers, a meso form and a d,l pair. An apparent allyl rotation process leads to a syn-syn/anti-anti interconversion and also to an interchange between the two isomers. The values of ΔGc‡ at the coalescence temperature have been calculated. The results, which have also been compared with those previously obtained for the complex with the 4-methylpyrazole ligand, indicate that the activation barrier for the process is not affected by the change in the pyrazole group. Appreciable effects were not observed on changing the concentration or on the addition of water to the acetone solutions of 2c. However, the influence of the solvent (acetone versus chloroform) is worth noting. A negative activation entropy has been found and a mechanistic proposal for the process is included. The molecular structure of 2c has been determined by X-ray diffraction. The meso isomer, in which the two C−Me axes of the allylic groups are oriented in the same direction, is found in the solid state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Kinetic Hydrogen/Deuterium Isotope Effects in Multiple Proton Transfer Reactions

Abstract: In this paper, a description of multiple kinetic isotope effects (MKIE) of degenerate double, triple and quadruple proton transfer reactions in terms of formal kinetics is developped. Both single and multiple barrier processes are considered, corresponding to so-called "concerted" and "stepwise" reaction pathways. Each step is characterized by a rate constant and the transfer of a given H isotopes or ensemble of H isotopes. The MKIE are expanded in terms of kinetic H/D isotope effects P contributed by single sites in which a proton is transferred in the rate limiting steps. By combination with a modified Bell tunneling model the formalism developed can be used in order to take into account tunneling effects of both concerted and stepwise multiple proton transfers at low temperatures.

Substituent and ring effects on enthalpies of formation: 2-methyl- and 2-ethylbenzimidazoles versus benzene- and imidazole-derivatives

Abstract: The enthalpies of combustion, heat capacities, enthalpies of sublimation and enthalpies of formation of 2-methylbenzimidazole (2MeBIM) and 2-ethylbenzimidazole (2EtBIM) are reported and the results compared with those of benzimidazole itself (BIM). Theoretical estimates of the enthalpies of formation were obtained through the use of atom equivalent schemes. The necessary energies were obtained in single-point calculations at the B3LYP/6-311+G(d,p) on B3LYP/6-31G* optimized geometries. The comparison of experimental and calculated values of benzenes, imidazoles and benzimidazoles bearing H (unsubstituted), methyl and ethyl groups shows remarkable homogeneity. The energetic group contribution transferability is not followed, but either using it or adding an empirical interaction term, it is possible to generate an enormous collection of reasonably accurate data for different substituted heterocycles (pyrazole-derivatives, pyridine-derivatives, etc.) from the large amount of values available for substituted ...

The Structure of Two 1-(Nitrophenyl)-Δ2-pyrazolines: a Crystallographic and Theoretical Study

Abstract: The crystal and molecular structures of two nitrophenylpyrazolines have been determined. The geometries have been used as starting geometries for density functional theory (DFT) calculations. The differences in conformation between both molecules and between the solid state and gas phase are explained in terms of steric effects. An attractive intramolecular N···N interaction between the nitro group and the pyrazoline N2 nitrogens has been found. Absolute shieldings have been calculated (GIAO) and compared with experimental 1H and 13C chemical shifts.

Chiral discrimination and isomerization processes in monomers, dimers and trimers of sulfoxides and thioperoxides

Abstract: The chiral discrimination in cyclic dimers and trimers of mono-substituted sulfoxides and thioperoxides has been studied by means of DFT (B3LYP/6-31+G**) and ab initio (MP2/6-311+G**) calculations. In addition, the inter- and intramolecular proton transfer processes that interconvert these two classes of compounds have been considered for the isolated molecules and clusters. The thioperoxide clusters are more stable than the corresponding sulfoxides even though the strongest hydrogen bonds are found in the latter complexes. Correlations have been found between the relative energies of the sulfoxide versus the thioperoxide compounds and the transition state barriers. The geometry of the hydrogen bonds has been analyzed using a Steiner–Limbach relationship.

Karplus-type relationships between scalar coupling constants: 3 J HH molecular versus 4h J HH supramolecular coupling constants

Abstract: The 3JHH coupling constants in six H–X–Y–H systems (ethane, methylamine, methanol, hydrazine, hydroxylamine and hydrogen peroxide) and 4hJHH coupling constants in four H–...XH...Y–H, namely [H3NHNH3]+ (two arrangements), HOHNH3 and HOHOH2 have been calculated theoretically as a function of the torsion angle φ. For covalent situations, the corresponding Karplus equations have been fitted to calculated 3JHH=acos2 φ+bcos φ+c. The a, b and c terms have been analyzed as a function of the electronegativities of X and Y. In the case of ammonium/ammonia complexes (proton shared and not), water/ammonia, and water dimer the values are low (maximum 0.5 Hz) but follow closely a Karplus relationship. Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00214-003-0486-7

Pyrazoles as molecular probes to study the properties of co-crystals by solid state NMR spectroscopy

Abstract: Equimolar mixtures of 3,5-dimethylpyrazole (1) with four NH-imidazoles (2–5) have been studied by13C and 15N CPMAS NMR and by DSC. In three cases, the solid mixture behaves as the sum of the individual components [imidazole (2), 2-methylimidazole (3) and 2,4(5)-dimethylimidazole (5)]. In one case [4,5-dimethylimidazole (4)], the mixture corresponds to a new species in which the dynamic behavior of1 no longer exists.