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A theoretical study of the formation of carbamic acids of pyrazole and indazole has been carried out using DFT computational methods. The effects of the substituents and the solvent (using explicit and implicit solvent models) have been considered. In addition, the deprotonation of the carbamic acid and its influence on the stability of the system has been calculated. In the neutral systems, only the formation of indazole-1-carbamic acid derivatives is favored vs. the non-covalent complexes between pyrazole or indazole with CO2. The deprotonation of the carbamic acid highly stabilizes the system preventing its dissociation. [ABSTRACT FROM AUTHOR]

The reactivity of a B3P3-doped hexa-cata-hexabenzocoronene, as a model of nanographene (B3P3-NG), towards carbon dioxide was studied at the DFT M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G* level of theory. This compound can be classified as a poly-cyclic poly-Frustrated Lewis Pair (FLP) system, as it presents more than one Lewis Acid/Lewis Base pair on its surface, making the capture of several carbon dioxide molecules possible. Two scenarios were considered to fully characterize the capture of CO2 by this multi-FLP system: (i) fixation of three CO2 molecules sequentially one by one; and (ii) simultaneous contact of three CO2 molecules with the B3P3-NG surface. The resulting adducts were analyzed as function of activation barriers and the relative stability of the CO2 capture. A cooperativity effect due to the π-delocalization of the hexa-cata-hexabenzocoronene is observed. The fixation of a CO2 molecule modifies the electronic properties. It enhances the capture of additional CO2 molecules by changing the acidy and basicity of the rest of the boron and phosphorus atoms in the B3P3-NG system. [ABSTRACT FROM AUTHOR]

The topological and energetic properties of the electron density distribution ρ(r) of the isolated pairwise H···F interaction have been theoretically calculated at several geometries (0.8

The title compound, C11H12N2, is not planar due to the folding of the seven-membered ring. In the crystal, molecules are packed opposite each other to minimize the electronic repulsion but the long intermolecular distances indicate that no directional contacts are found. [ABSTRACT FROM AUTHOR]

A search of the Cambridge Structural Database (CSD) was carried out for phosphine-water and arsine-water complexes in which water is either the proton donor in hydrogen-bonded complexes, or the electron-pair donor in pnicogen-bonded complexes. The range of experimental P-O distances in the phosphine complexes is consistent with the results of ab initio MP2/aug'-cc-pVTZ calculations carried out on complexes H2XP: OH2, for X = NC, F, Cl, CN, OH, CCH, H, and CH3. Only hydrogen-bonded complexes are found on the H2(CH3)P: HOH and H3P: HOH potential surfaces, while only pnicogen-bonded complexes exist on H2(NC)P: OH2, H2FP: OH2, H2(CN)P: OH2, and H2(OH)P: OH2 surfaces. Both hydrogen-bonded and pnicogen-bonded complexes are found on the H2ClP: OH2 and H2(CCH)P: OH2 surfaces, with the pnicogen-bonded complexes more stable than the corresponding hydrogen-bonded complexes. The more electronegative substituents prefer to form pnicogen-bonded complexes, while the more electropositive substituents form hydrogen-bonded complexes. The H2XP: OH2 complexes are characterized in terms of their structures, binding energies, charge-transfer energies, and spin-spin coupling constants 2hJ(O-P), 1hJ(H-P), and ¹J(O-H) across hydrogen bonds, and ¹pJ(P-O) across pnicogen bonds. [ABSTRACT FROM AUTHOR]

Structures of selected polycyclic conjugated hydrocarbons with -B=B- and -BH-BH- moieties inserted in different places were calculated at the B3LYP/6-311++G** level and their aromatic properties evaluated. HOMA, NICS(0), NICS(1)zz, Λ and PDI indices were used for studying their aromatic properties. Both optimized planar (as in parent hydrocarbons) and non-planar structures were taken into account. It is shown that insertion of both types of boron groups disturbs and decreases the aromaticity of the corresponding hydrocarbons. The decreasing effect of the -BH-BH- group is much stronger. What is quite intriguing is that it appears that non-planar structures of the studied compounds have a little higher aromaticity than the strictly planar ones. Mutual correlations between results obtained by different aromaticity indices are calculated and thoroughly discussed. [ABSTRACT FROM AUTHOR]

Several bis(triazolium)-based receptors have been synthesized as chemosensors for anion recognition. The central naphthalene core features two aryltriazolium side-arms. NMR experiments revealed differences between the binding modes of the two triazolium rings: one triazolium ring acts as a hydrogen-bond donor, the other as an anion-π receptor. Receptors 92+⋅2BF4 − (C6H5), 112+⋅2BF4 − (4-NO2C6H4), and 132+⋅2BF4− (ferrocenyl) bind HP2O73− anions in a mixed-binding mode that features a combination of hydrogen-bonding and anion-π interactions and results in strong binding. On the other hand, receptor 102+⋅2 BF4 − (4-CH3OC6H4) only displays combined Csp2H/anion-π interactions between the two arms of the receptors and the bound anion rather than triazolium (CH)+⋅⋅⋅anion hydrogen bonding. All receptors undergo a downfield shift of the triazolium protons, as well as the inner naphthalene protons, in the presence of H2PO4− anions. That suggests that only hydrogen-bonding interactions exist between the binding site and the bound anion, and involve a combination of cationic (triazolium) and neutral (naphthalene) CH donor interactions. Theoretical calculations relate the electronic structure of the substituent on the aromatic group with the interaction energies and provide a minimum-energy conformation for all the complexes that explains their measured properties. [ABSTRACT FROM AUTHOR]

The X-ray molecular structure of two benzimidazoles unsubstituted on the nitrogen atoms (2-ethyl-1 H-benzimidazole and 2-propyl-1 H-benzimidazole) has been determined and analysed in the context of crystallographic and solid-state NMR data of seven substituted benzimidazoles. The first compound presents polymorphism, and two polymorphs [forms 3a and 3c] were obtained from different solutions of dichloromethane/hexane. Polymorph ( 3a) contains two independent molecules differing in the conformation of the ethyl group. Polymorph ( 3c) contains only one molecule in the asymmetric unit, with similar configuration as the ordered molecule of polymorph ( 3a). Another polymorphic form ( 3b) is obtained by cooling form ( 3a) below 179 K. The transition is related to an order-disorder process and involves a change in the hydrogen-bonding scheme. The second compound ( 4) presents enantiomorphism and its asymmetric unit contains four independent molecules that differ in the conformation of the 2-propyl group. For form ( 3a), the splitting of NMR signals is almost perfectly consistent with the features of the crystal structure, whereas not splitting is observed for compound ( 4). Agreements between NMR and X-ray crystallography techniques are also observed in other simple 2-R-1 H-benzimidazoles (R = C4H9, CH2C6H5 and C6H5), but disagreements are found for R = H, CH3. [ABSTRACT FROM AUTHOR]

An extension of the harmonic oscillator model of aromaticity (HOMA) model for systems with boron-boron bonds is presented. For the first time, the parameters of the HOMA model are estimated using only theoretically calculated bond lengths. The HOMA parameters obtained make geometric aromaticity studies possible for a large number of compounds containing the boron-boron bond. The derived HOMA parameters have been used to investigate how the introduction of the boron-boron moiety in the structure of selected hydrocarbons modifies their aromaticity. The conclusion is that the insertion of a boron-boron bond usually strongly decreases the aromaticity of the boron-containing compounds in comparison to their parent hydrocarbons. [ABSTRACT FROM AUTHOR]

An extension of the harmonic oscillator model of aromaticity (HOMA) model to systems with carbon-boron bonds is presented. Model parameters were estimated using experimental and theoretical bond lengths. It is shown that both approaches produce very similar HOMA models. In the second part of the article, the aromaticity levels of several model compounds containing carbon-boron bonds are calculated using the previously obtained parameters. The results of these calculations are compared with those provided by other aromaticity indices. The aromaticity of boron-containing compounds is also compared with the aromaticity of analogous compounds containing carbon and nitrogen. [ABSTRACT FROM AUTHOR]

The chiral discrimination in the self-association of 2,5-diketopiperazines derivatives has been studied using density functional theory (DFT) methods. Hence, clusters from dimers to tetramers have been considered. We have found a variety of linear and cyclic structures depending on the geometry of the monomers. In general, the heterochiral dimers ( RR:SS or SS:RR) are more stable than the homochiral ones ( RR:RR or SS:SS) with slight energetic differences. Nevertheless, most of the larger clusters (trimers and tetramers) show changes in stability, the homochiral cyclic structures being favored due to the better geometry of the hydrogen bond and/or the higher number of interactions. Some interesting correlations between the hydrogen bond geometrical descriptors and theoretical parameters obtained from the Natural Bond Orbital analysis (NBO), Natural energy decomposition analysis (NEDA), and the Atoms in Molecules theory (AIM) have been found. The chiral discrimination in the recognition process of these self-complexes has been evaluated. In addition, evidence of cooperative effects has been observed. Copyright © 2010 John Wiley & Sons, Ltd. The chiral discrimination in the self-association of 2,5-diketopiperazines derivatives has been studied using density functional theory methods. [ABSTRACT FROM AUTHOR]

A theoretical study of the preferred N-σ or π configuration of the mono- and bis-pyrrole and imidazole derivatives of alkaline earth metals has been carried by means of DFT and ab initio methods, up to G2 computational level. The energetic results show that the beryllium derivatives prefer the N-σ configuration while the calcium ones prefer the π one. In the case of magnesium, both configurations present similar stability. The calculated dissociation enthalpies correspond to the fact that the metallic atom is strongly bonded to the azoles in both configurations. The NBO analysis shows that the systems can be considered as formed by the azolates interacting with the charged metallic atom and thus the Natural Energy Decomposition Analysis (NEDA) indicates that the main attractive force is the electrostatic. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A theoretical study of the inverse hydrogen bonds complexes formed by the XeH2 molecule and hydride and fluoride derivatives of Li, Be, Na and Mg has been carried out by means of DFT (B3LYP/DGDZVP) and ab initio [MP2/DGDZVP and MP2/LJ18/6-311++G(2d,2p)] calculations. The complexes obtained present interaction energies up to −81 kJ/mol. The analysis of the electron density shows electron transfer from the XeH2 to the electron acceptor molecules. The calculated absolute chemical shieldings show the high sensitivity of the xenon atom upon complexation. [ABSTRACT FROM AUTHOR]

Ab intio EOM-CCSD calculations have been performed to obtain 17O-17O, 17O-1H, and 1H-1H coupling constants for small water clusters and complexes of OH3+ with one, two, and three H2O molecules. Coupling constants 2hJ(O-O) are quite small in water clusters, but increase significantly in complexes of OH3+ with H2O, and correlate with O-O distances. Protonation leads to a significant increase in the absolute value of 1J(O-H) and a decrease in 2J(H-H) for OH3+ compared to H2O. The variations in O-O, O-H, and H-H coupling constants are related to cluster size and structure for water clusters, and to the degree of hydration of OH3+. [ABSTRACT FROM AUTHOR]

A theoretical study of chiral recognition in the minimum and proton transfer transition state structures of 15 pairs of chiral phosphinic acid dimers was carried out using DFT and MP2 methods, up to the MP2/6–311++G(3df,2p) level. The proton transfer proceeds via a concerted pathway in all cases studied. Even though these complexes show high interaction energies, of the order of 120 kJ mol-1, and short interatomic HB distances, our results show small energy differences between the homochiral (RR or SS) and the heterochiral dimers (RS or SR) both in the equilibrium configuration and in the proton transfer transition state owing to the disposition of the nonoxygen substituents of the phosphorus atom as proved with additional model complexes. Copyright © 2005 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Studies the crystal and molecular structures of 3(5),4-dimethylpyrazole and 3,4,5-trimethylpyrazole. Trimer formation; Hydrogen bonding in the pyrazole derivatives; Cyclic features; Hydrogen bonding.

A theoretical study of the hydrogen bond (HB) and halogen bond (XB) complexes between 1-halo-closo-carboranes and hydrogen cyanide (NCH) as HB and XB probe has been carried out at the MP2 computational level. The energy results show that the HB complexes are more stable than the XBs for the same system, with the exception of the isoenergetic iodine derivatives. The analysis of the electron density with the quantum theory of atoms in molecules (QTAIM) shows the presence of a unique intermolecular bond critical point with the typical features of weak noncovalent interactions (small values of the electron density and positive Laplacian and total energy density). The natural energy decomposition analysis (NEDA) of the complexes shows that the HB and XB complexes are dominated by the charge-transfer and polarization terms, respectively. The work has been complemented with a search in the CSD database of analogous complexes and the comparison of the results, with those of the 1-halobenzene:NCH complexes showing smaller binding energies and larger intermolecular distances as compared to the 1-halo-closo-carboranes:NCH complexes. [ABSTRACT FROM AUTHOR]