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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.
ARKIVOC: archive for organic chemistry 05/2013; 1:843. · 1.25 Impact Factor
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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.
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ABSTRACT: A computational study of the intramolecular pnicogen bond in PHF-(CH2 )n -PHF (n=2-6) systems was carried out. For each compound, two different conformations, (R,R) and (R,S), were considered on the basis of the chirality of the phosphine groups. The characteristics of the closed conformers, in which the pnicogen interaction occurs, were compared with those of the extended conformer. In several cases, the closed conformations are more stable than the extended conformations. The calculated interaction energies of the pnicogen contact, by means of isodesmic reactions, provide values between -3.4 and -26.0 kJ mol(-1) . Atoms in molecules and electron localization function analysis of the electron density showed that the systems in the closed conformations with short P⋅⋅⋅P distances have a partial covalent character in this interaction. The calculated absolute chemical shieldings of the P atoms showed an exponential relationship with the P⋅⋅⋅P distance. In addition, a search in the Cambridge crystallographic database was carried out to detect those compounds with a potential intramolecular pnicogen bond in the solid phase.
ChemPhysChem 03/2013; · 3.41 Impact Factor
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ABSTRACT: Ab initio MP2/aug'-cc-pVTZ calculations have been performed to investigate the structures and energies of binary complexes LA:PH2F and LA:PH3 and of ternary complexes LA:H2FP:NFH2 and LA:H3P:NH3 in which the pnicogen-bonded P atom also acts as an electron-pair donor to a Lewis acid (LA), for LA = BH3, NCH, ClH, FH, FCl, and HLi. Hydrogen bonds, halogen bonds, and dative covalent bonds are found at P in some cases, depending on the nature of the Lewis acid. HLi forms a lithium bond with P only in the binary complex HLi:PH3. The binding energies of ternary complexes exhibit a classical synergistic effect, although the computed cooperativity may be overestimated due to neglect of the interaction of the Lewis acid with NH2F or NH3 in some cases. The hydrogen-bonding Lewis acids appear to have little effect on the strength of the P···N bond, while the remaining Lewis acids strengthen the pnicogen bond. (31)P absolute chemical shieldings increase in LA:H2FP:NFH2 complexes relative to the corresponding LA:PH2F complexes as the positive charge on P decreases, while chemical shieldings decrease in LA:H3P:NH3 relative to the corresponding LA:PH3 complexes as the positive charge increases. Absolute values of (1p)J(P-N) spin-spin coupling constants in complexes LA:H2FP:NFH2 decrease as the P-N distance decreases. It appears that this behavior is associated with the presence of a second intermolecular interaction, whether electron-donation by P or hydrogen bond formation at P-F.
The Journal of Physical Chemistry A 03/2013; 117(14):3133. · 2.95 Impact Factor
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Zeitschrift f?r Physikalische Chemie 03/2013; · 1.57 Impact Factor
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ABSTRACT: A theoretical study of the complexes between dihydrogen, H2, and a series of amine, phosphine and arsine derivatives (ZH3 and ZH2X, with Z = N, P or As and X = F, Cl, CN or CH3) has been carried out using ab initio methods (MP2/aug-cc-pVTZ). Three energetic minima configurations have been characterized for each case with the H2 molecule in the proximity of the pnictogen atom (Z). In configuration A, the σ-electrons of H2 interact with σ-hole region of the pnictogen atom generated by the of X-Z bond. These complexes can be ascribed as pnictogen bonded. In configuration C the lone electron pair of Z acts as the Lewis base and H2 plays the role of the Lewis acid. Finally, configuration B presents a variety of non-covalent interactions depending on the binary complex considered. The Atoms in Molecules theory (AIM), Natural Bond Orbitals (NBO) method as well as the DFT-SAPT approach were used in this study to deepen the nature of the interactions considered.
The Journal of Physical Chemistry A 03/2013; · 2.95 Impact Factor
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ABSTRACT: Achiral compounds 4-methoxy-4-(p-methoxyphenyl)cyclohexanoneethylene ketal (2), 4-hydroxy-4-(p-methoxy phenyl)cyclohexanoneethylene ketal (3), and 3,5-dimethyl-4-nitropyrazole (4) crystallized in chiral structures and the samples showed an enantiomeric excess. We have determined the absolute structures of these compounds by using X-ray diffraction with copper radiation at low temperatures. Moreover, we have also established the prevalent absolute structures in these samples, by comparing their calculated and solid-state vibrational circular dichroism (VCD) spectra. The consistency of this method was confirmed by using (R,R)-2,8-diiodo-4,10-dimethyl-6 H,12H-5,11-methano-dibenzo[b,f][1,5]diazocine, Tröger's base, (R,R)-1, as a chiral compound of known absolute configuration.
Chemistry 03/2013; · 5.93 Impact Factor
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ABSTRACT: The structures of three azines derived from 2-formylimidazole, 4(5)-formylimidazole, and 4(5)-formyl-5(4)-methylimidazole have been determined in solution and in the solid state. Density Functional Theory (DFT) Polarizable Continuum Model (PCM) calculations (geometries, energies, and chemical shifts), NMR [solution and cross polarization magic-angle spinning (CPMAS)], and X-ray crystallography [azine of 4(5)-formylimidazole] have been used. The configuration around the central C = N bonds has been determined and some insights about prototropic tautomerism and conformation have been gained. Copyright © 2013 John Wiley & Sons, Ltd.
Magnetic Resonance in Chemistry 02/2013; · 1.44 Impact Factor
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Mario Alvarado,
Juan Decara,
María Jesús Luque,
Laura Hernandez-Folgado,
María Gómez-Cañas,
María Gómez-Ruiz,
Javier Fernández-Ruiz, José Elguero,
Nadine Jagerovic,
Antonia Serrano,
Pilar Goya,
Fernando Rodríguez de Fonseca
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ABSTRACT: Searching for novel antiobesity agents, a series of cannabinoid LH21 and of Rimonabant-fatty acid amide analogues have been prepared. Synthesis of pyrazoles 2a-2c was achieved by a two steps simple methodology via α,β-unsaturated ketones. Carboxamides 8a-8h were obtained in good yields from esters 7a-7c by a one-pot procedure which takes place under mild conditions. New compounds have been evaluated in vivo as anorectic agents. Some of them showed interesting properties reducing food intake in rats by a mechanism which does not involve the endocannabinoid system.
Bioorganic & medicinal chemistry 02/2013; · 2.82 Impact Factor
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ABSTRACT: The acid catalysis of the mutarotation mechanism in the two aldotetroses, d-erythrose and d-threose, has been studied at B3LYP/6-311++G(d,p) computational level in gas phase and in solution employing the PCM-water model. The open-chain, the furanose and the connecting TS structures have been characterized. To study the enhancing effect of acid groups on the electrophilicity of the carbonyl carbon atom, four situations have been considered: (i) a classical Lewis acid as BH3; (ii) a classical hard-Pearson acid as Na(+); (iii) classical Brønsted acids as H(+) and H3O(+); and (iv) a combined strategy using H3O(+) and one bridge-H2O molecule as assistant in the proton transfer process. All the acidic groups reduce the activation energy with exception of the Na(+), which can act, in some cases, as inhibitor. It is greatly reduced by the presence of Brønsted acids (iii) and through the combined strategy (iv). For this last mechanism, the electronic activation energies are between 12 and 43kJmol(-1) in vacuum and between 13 and 25kJmol(-1) in water solution, through the use of the PCM model.
Carbohydrate research 01/2013; 372C:1-8. · 2.03 Impact Factor
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ABSTRACT: The structure and electronic properties of the complexes formed by the interaction of imidazole and pyrazole with different BeXH(BeX(2)) (X = H, Me, F, Cl) derivatives have been investigated via B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) calculations. The formation of these azole:BeXH(BeX(2)) complexes is accompanied by a dramatic enhancement of the intrinsic acidity of the azole, as the deprotonated azole is much more stable after the aforementioned interaction. Most importantly, the increase in acidity is so large that the azole:BeXH or azole:BeX(2) complexes behave as NH acids, which are stronger than typical oxyacids such as phosphoric acid and oxalic acid. Interestingly, the increase in acidity can be tuned through appropriate selection of the substituents attached to the Be atom, permitting us to modulate the electron-accepting ability of the BeXH or BeX(2) molecule.
Journal of Molecular Modeling 01/2013; · 1.80 Impact Factor
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Journal of Physical Organic Chemistry 01/2013; · 1.96 Impact Factor
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ABSTRACT: An ab initio MP2/aug'-cc-pVTZ study has been carried out to identify local minima on the (NH(2)F)(2), H(2)FP:NFH(2), and (PH(2)F)(2) potential surfaces, to characterize the types of interactions which stabilize the complexes found at these minima, and to evaluate their binding energies. With one exception, (NH(2)F)(2) complexes are stabilized by N-H···N or N-H···F hydrogen bonds. Only one complex, that with the smallest binding energy, has a pnicogen N···N bond. In contrast, (PH(2)F)(2) complexes are stabilized by P···P or P···F pnicogen bonds or by an antiparallel alignment of the dipole moment vectors of the two monomers, but not by hydrogen bonds. The most stable complex has an F-P···P-F alignment which approaches linearity. Both hydrogen-bonded and pnicogen-bonded complexes exist on the H(2)FP:NFH(2) surface, with the most stable being the pnicogen-bonded complex with F-P···N-F approaching a linear arrangement. Charge transfer transitions from a lone pair on a P, N, or F atom in one molecule to an antibonding σ* orbital of the other stabilize these complexes. These transitions are most important for complexes with pnicogen bonds. Although net charge transfer occurs in complexes in which the two monomers are inequivalent, charges on N and P do not correlate with N and P absolute chemical shieldings. Rather, these shieldings also reflect charge distributions and overall bonding patterns. EOM-CCSD two-bond spin-spin coupling constants (2h)J(X-Y) across X-H···Y hydrogen bonds tend to be small, due in part to the nonlinearity of many of the hydrogen bonds. (1p)J values across a particular kind of pnicogen bond are relatively large and vary significantly but do not correlate with corresponding distances.
The Journal of Physical Chemistry A 12/2012; · 2.95 Impact Factor
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ABSTRACT: A theoretical study of FCCF:(HF)n complexes, with n = 1 and 2, has been carried out by means of ab initio
computational methods. Two types of complexes are formed: those with FH� � �p interactions and those
with FH� � �FC hydrogen bonds. The indirect spin–spin coupling constants have been calculated at the
CCSD/aug-cc-pVTZ-J computational level. Special attention has been paid to the dependence of the different intramolecular coupling constants in FCCF on the distance between the coupled nuclei and the presence or absence of the hydrogen fluoride molecule. The sensitivity shown by these coupling constants to
the presence of hydrogen fluoride is quite notorious
Computational and Theoretical Chemistry. 10/2012; 998:98-105.
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ABSTRACT: Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the influence of F-H···F hydrogen bonds on the P···N pnicogen bond in complexes nFH:(H(2)FP:NFH(2)) for n = 1-2, and a selected complex with n = 3. The NBO analysis indicates that the N(lp) → P-Fσ* charge-transfer transition has a much greater stabilizing effect than the P(lp) → N-Fσ* transition. When hydrogen bonding occurs at P-F, charge transfer associated with the pnicogen bond and the hydrogen bond are in the same direction but are in opposite directions when hydrogen bonding occurs at N-F. As a result, the formation of F-H···F hydrogen bonds at P-F leads to shorter P···N distances, increased strength of P···N bonds, and synergistic energetic effects; hydrogen bonding at N-F has opposite effects. (31)P and (15)N chemical shieldings do not correlate with charges on P and N, respectively, but (31)P shieldings correlate quadratically with the P-N distance. (1p)J(P-N) coupling constants do not correlate with the intermolecular P-N distance. However, when hydrogen bonding occurs only at P-F, (1p)J(P-N) decreases in absolute value as the P-N distance decreases, thereby approaching (1)J(P-N) for H(2)P-NH(2). However, the P···N bond in 3FH:(H(2)FP:NFH(2)) has little covalent character, unlike the P···P bond in the corresponding complex 3FH:(PH(2)F)(2).
The Journal of Physical Chemistry A 08/2012; · 2.95 Impact Factor
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ABSTRACT: The complexes formed by CuF with CC double and triple bonds have been studied at the MP2 and CCSD(T) computational levels. The interaction of CuF with acetylene, ethylene and their fluoro derivatives is very strong, with interaction energies close to those of conventional covalent bonds. Hence, these complexes could be actually viewed as a new kind of metallocycles, with significantly strong Cu-C linkages. All electronic indexes analyzed by means of the AIM, ELF and NBO formalisms, indicate that the strength of the interaction should increase with the number of fluorine substituents in both series of compounds. Surprisingly, however, although both series of compounds exhibit the same bonding arrangements, they follow opposite stability trends and the expected increase of the interaction energies with the number of fluorine substituents is only observed in the acetylene series. The reason for this unexpected behavior is once more associated with the effects triggered by the distortion of the interacting subunits. Deformation not only has a direct energetic cost but dramatically affects the intrinsic properties of the interacting systems.
Physical Chemistry Chemical Physics 07/2012; 14(32):11468-77. · 3.57 Impact Factor
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ABSTRACT: The reaction mechanism of the hemiacetal formation from formaldehyde and methanol has been studied theoretically at the B3LYP/6-311++G(d,p) level. In addition to the study of the reaction between the isolated reactants, three different kinds of catalysis have been explored. The first one examines the use of assistants, especially bridging water molecules, in the proton transfer process. The second one attempts to increase the local electrophilicity of the carbon atom in formaldehyde with the presence of a Brønsted acid (H(+) or H(3)O(+)). The last one considers the combined effect of both catalytic strategies. The reaction force, the electronic chemical potential, and the reaction electronic flux have been characterized for the reaction path in each case. In general, it has been found that structural rearrangements represent an important energetic penalty during the activation process. The barriers for the reactions catalyzed by Brønsted acids show a high percentage of electronic reorganization contribution. The catalytic effects for the reactions assisted by water molecules are due to a reduction of the strain in the transition state structures. The reaction that includes both acid catalysis and proton assistance transfer shows the lowest energy barrier (25.0 kJ mol(-1)).
The Journal of Physical Chemistry A 07/2012; 116(31):8250-9. · 2.95 Impact Factor
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ABSTRACT: The potential energy surfaces for the different configurations of the D-erythrose and D-threose (open-chain, α- and β-furanoses) have been studied in order to find the most stable structures in the gas phase. For that purpose, a large number of initial structures were explored at B3LYP/6-31G(d) level. All the minima obtained at this level were compared and duplicates removed. A further reoptimization of the remaining structures was carried out at B3LYP/6-311++G(d,p) level. We characterized 174 and 170 minima for the open-chain structures of D-erythrose and D-threose, respectively, with relative energies that range over an interval of just over 50kJ/mol. In the case of the furanose configurations, the number of minima is smaller by approximately one to two dozen. G3B3 calculations on the most stable minima indicate that the α-furanose configuration is the most stable for both D-erythrose and D-threose. The intramolecular interactions of the minima have been analyzed with the Atoms in Molecules (AIM) and Natural Bond Orbital (NBO) methodologies. Hydrogen bonds were classified as 1-2, 1-3 or 1-4, based on the number of C-C bonds (1, 2 and 3, respectively) that separate the two moieties participating in the hydrogen bond. In general, the AIM and NBO methodologies agree in the designation of the moieties involved in hydrogen bond interactions, except in a few cases associated to 1-2 contact which have small OH···O angles.
Carbohydrate research 06/2012; 358:96-105. · 2.03 Impact Factor
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ABSTRACT: The complexes formed between dimethylchalcogens X(CH(3))(2) (X = S, Se, and Te) and hypohalous acids YOH (Y = F, Cl, Br, and I) have been studied at the MP2/aug'-cc-pVTZ computational level, five minima structures being located. Two of them correspond to hydrogen bonds (HB), another two to halogen bonds (XB) with the chalcogen acting as an electron donor, the last one showing a C-H···O contact. The most stable complexes of IOH and BrOH acids present halogen···chalcogen interactions with interaction energies, E(i), up to -49 kJ mol(-1). In the case of the ClOH and FOH molecules, the hydrogen bonded complexes are more stable with interaction energies between -27 and -34 kJ mol(-1). Linear correlations between the molecular electrostatic potential (MEP) stationary points at the van der Waals surface and the interaction energy have been found. The contribution of the different energy terms to the total interaction energy was analyzed by means of the DFT-SAPT theory finding that the electrostatic attractive term is dominant in the complexes with HB and XB, excepting a few cases in which the dispersion and induction terms become more important than the electrostatic one.
Physical Chemistry Chemical Physics 06/2012; 14(28):9880-9. · 3.57 Impact Factor
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ABSTRACT: A theoretical study of the possible protonation sites of simple molecules formed by C, N, Si, P, B and Al that present a triple
bond between those atoms has been carried out. The calculations performed include MP2 and CCSD(T) methods with the aug-cc-pVTZ
basis set. The nature of the protonated species has been analyzed with the Atoms In Molecules methodology.
Theoretical Chemistry Accounts 05/2012; 118(3):533-539. · 2.16 Impact Factor
