RESUMEN

The high content of hydride-hydride contacts Hδ-···Î´-H in hydrogen storage materials appears to be relevant for hydrogen formation. At present time there is no consensus whether these contacts are attractive or repulsive. Accordingly, the main goal of this article is to shed light on physical factors which constitute homopolar hydride-hydride interactions Hδ-···Î´-H in selected transition metal complexes i.e. HCoL4, L = CO,PPh3,PH3. In order to achieve this goal, the charge and energy decomposition ETS-NOCV approach along with the Interacting Quantum Atoms (IQA) and reduced density gradient (NCI) are applied for the bonded adducts L4CoH···HCoL4. Based on DFT and correlated methods it has been shown, that hydride-hydride interactions might be attractive and even far stronger than classical hydrogen bonds. The stability of the adducts is increased by phosphine ligand installation: overall Hδ-···Î´-H bonding energy changes in the order: CO << PPh3 ~ PH3. It has been revealed that depending on monomer's conformations Hδ-···Î´-H bonds are dominated by charge delocalization or London dispersion forces and the electrostatic term is also relevant. It is finally determined by IQA energy decomposition, that diatomic hydride-hydride interaction CoH···HCo is chameleon-like, namely, it is attractive in CO4CoH···HCoCO4, whereas the repulsion is unveiled in (CO)3(PPh3)CoH···HCo(CO)3(PPh3).

RESUMEN

In the present work the origin of highly varied acidity of hydroxycoumarins (pKa values) has been for the first time investigated by joint experimental and computational studies. The structurally simple regio-isomers differing in the location of hydroxyl group, 3-hydroxycoumarin (3-HC), 4-hydroxycoumarin (4-HC), 6-hydroxycoumarin (6-HC), 7-hydroxycoumarin (7-HC), as well as 4,7-dihydroxycoumarin (4,7-HC) and the larger 4-hydroxycoumarin-based derivatives: warfarin (WAR), 7-hydroxywarfarin (W7), coumatetralyl (CT), and 10-hydroxywarfarin (W10), have been compared in terms of enthalpy-entropy relationships accounting for the observed pKa values. We have revealed that in the case of large molecules the acidic proton is stabilized by the following noncovalent interactions OH···O (WAR and W7), OH···π (CT), and OH···OH···O (W10), this effect leads to a compensatory enthalpy-entropy relation and yields a moderate pKa increase. On the other hand, different location of the hydroxyl group in the regio-isomers (3-HC, 4-HC, 6-HC, and 7-HC) leads to the massive changes in acidity due to a lack of enthalpy-entropy compensation. Our results suggest that the solvent-solute interactions and electron delocalization degree in anions contribute to the observed behaviors. Such knowledge can be useful in the future to design novel systems exhibiting desired acid-base properties, and to elucidate enthalpy-entropy compensation phenomena.

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RESUMEN

In the present account, the real space fragment attributed molecular system energy change (FAMSEC) approach, interacting quantum atoms energy decomposition scheme as well as molecular orbitals based the extended transition state scheme coupled with natural orbitals for chemical valence (ETS-NOCV) have been, for the first time, successfully used to delineate factors of importance for stability of the 2-butene conformers (cis-eq, cis-TS, trans-eq, trans-TS). Our results demonstrate that atoms of the controversial H-H contact in cis-eq (i) are involved in attractive interaction dominated by the exchange-correlation term, (ii) are weekly stabilized, (iii) show trends in several descriptors found in other typical H-bonds, and (iv) are part of most stabilized CH-HC fragment (loc-FAMSEC = -3.6 kcal/mol) with most favourably changed intrafragment interactions on trans-eq→cis-eq. Moreover, lower stability of cis-eq vs. trans-eq is linked with the entire HCCH (ethylenic) fragment which destabilized cis-eq (mol-FAMSEC, +3.9 kcal/mol) the most. Although the H-H contact can be linked with smaller, relative to trans-, rotational energy barrier in cis-2-butene, we have proven that to rationalize this phenomenon one must account for changes in interactions between various fragments that constitute the entire molecule. Importantly, we discovered a number of comparable trends in fundamental properties of equivalent molecular fragments on a methyl group rotation; for example, interaction between BP-free H-atoms in trans-eq (involving CH bonds of the methyl and ethylenic units) and BP-linked H-atoms in cis-eq. Clearly, rotational energy barrier cannot be entirely (i) rationalized by the properties of or (ii) attributed to the H-H contact in cis-eq. © 2016 Wiley Periodicals, Inc.

RESUMEN

In the present study, the inorganic analogues of alkanes as well as their isoelectronic BN/CC counterparts that bridge the gap between organic and inorganic chemistry are comparatively studied on the grounds of static DFT and Car-Parrinello molecular dynamics simulations. The BN/CC butanes CH3 CH2 BH2 NH3 , BH3 CH2 NH2 CH3 , and NH3 CH2 BH2 CH3 were considered and compared with their isoelectronic counterparts NH3 BH2 NH2 BH3 and CH3 CH2 CH2 CH3 . In addition, systematical replacement of the NH2 BH2 fragment by the isoelectronic CH2 CH2 moiety is studied in the molecules H3 N(NH2 BH2 )3-m (CH2 CH2 )m BH3 (for m=0, 1, 2, or 3) and H3 N(NH2 BH2 )2-m (CH2 CH2 )m BH3 (for m=0, 1, or 2). The DFT and Car-Parrinello simulations show that the isosteres of the BN/CC butanes CH3 CH2 BH2 NH3 , BH3 CH2 NH2 CH3 , and NH3 CH2 BH2 CH3 and of larger oligomers of the type (BN)k (CC)l where k≥l are stable compounds. The BN/CC butane H3 NCH2 CH2 BH3 spontaneously produces molecular hydrogen at room temperature. The reaction, prompted by very strong dihydrogen bonding NH⋅⋅⋅HB, undergoes through the neutral, hypervalent, pentacoordinated boron dihydrogen complex RBH2 (H2 ) [R=(CH2 CH2 )n NH2 ]. The calculations suggest that such intermediate and the other BN/CC butanes CH3 CH2 BH2 NH3 , BH3 CH2 NH2 CH3 , and NH3 CH2 BH2 CH3 as well as larger BN/CC oligomers are viable experimentally. A simple recipe for the synthesis of CH3 CH2 BH2 NH3 is proposed. The strength of the dihydrogen bonding appeared to be crucial for the overall stability of the saturated BN/CC derivatives.

RESUMEN

Several distinct analytical issues have been addressed by performing capillary electrophoresis-based separations of the warfarin, 7-hydroxywarfarin and 10-hydroxywarfarin in an achiral and cyclodextrin-containing media. The measurements were conducted across a range of pH in order to find optimum conditions for achiral and chiral separations. The values of acid dissociation constant (pKa) have been determined and compared. Subsequently, after performing a series of mobility shift assays at different pH and cyclodextrin concentration, the pKa values ascribed to diastereomeric complexes with methyl-ß-cyclodextrin have been estimated. The significant pKa shifts upon complexation have been noticed for warfarin - up to 1.5 pH units, and only subtle for 10-hydroxywarfarin. A new approach that allows the estimation of association percentage based on the electrophoretic mobility curves has been also demonstrated. The complex mechanism of chiral separation has been found to be responsible for the observed migration profile, relying on a combined equilibrium between complexation/partition and protonation/deprotonation phenomena. The occurrence of the pKa-related migration order reversal has been demonstrated in achiral medium between warfarin and 7-hydroxywarfarin, and in chiral medium between enantiomers, causing a drop in enantioselectivity at specific pH. In parallel, the density functional theory-based calculations have been performed in order to obtain the structures of warfarin and its derivatives as well as to rationalize the shifts in pKa values.

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RESUMEN

Accurate gas-phase and solution-phase valence bond calculations reveal that protonation of the hydroxyl group of aliphatic alcohols transforms the C-O bond from a principally covalent bond to a complete charge-shift bond with principally "no-bond" character. All bonding in this charge-shift bond is due to resonance between covalent and ionic structures, which is a different bonding mechanism from that of traditional covalent bonds. Until now, charge-shift bonds have been previously identified in inorganic compounds or in exotic organic compounds. This work showcases that charge-shift bonds can occur in common organic species.

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In this study ab initio Car-Parrinello molecular dynamics simulations, extended transition state (ETS)-natural orbitals for chemical valence (NOCV) and QTAIM bonding analyses, were performed to characterize the ansa-bridged molybdocene complexes [(C(5)H(4))(2)XMe(2)MoH(3)](+) for X = C, Si, Ge, Sn, Pb, and nonbridged Cp(2)MoH(3)(+) system. The results have shown that the [(C(5)H(4))(2)CMe(2)MoH(H(2))](+) complex exhibits nonclassical dihydrogen/hydride (H(2)/H) conformation (97.6% of time of simulation), contrary to trihydride (H(3)) structure noted for nonbridged Cp(2)MoH(3)(+) (86.9%) and ansa-bridged [(C(5)H(4))(2)SnMe(2)MoH(3)](+) (84.8%), [(C(5) H(4))(2)PbMe(2)MoH(3)](+) (84.9%) systems. Further, [(C(5)H(4))(2)SiMe(2)MoH(3)](+) and [(C(5)H(4))(2)GeMe(2)MoH(3)](+) complexes, appeared to exist in both conformations (H(2)/H--55.4%, H(3)--44.6% for Si-based system and H(2)/H--36.2%, H(3)--63.8 % for germanium congener). It has been proven that the "steric availability" of the metal center, measured by the changes in the Cp-Mo-Cp angle (α), determines the existence of a given conformation--namely, the smaller value of the angle (molybdenum is sterically more accessible) the larger preference for the formation of dihydrogen/hydride structure. ETS-NOCV method allowed to conclude that increase in the Cp-Mo-Cp angle (from α ca. 120° to α ca. 150°) leads to the enhancement of donation from H(2) and back-donation from Mo to the σ*(H-H), what consequently leads to breaking of the H-H bond and formation of the trihydride structure. Systematical increase in the charge depletion from the σ-bonding orbital of H(2) can be related to the reduction of the energy gap between the major orbitals involved in this contribution, namely highest occupied molecular orbital (HOMO) of H(2) with lowest unoccupied molecular orbital (LUMO) of [MoHL](+); ΔE = 0.0868 a.u. [for L =(C(5)H(4))(2)C], ΔE = 0.0827a.u. [for L = (C(5)H(4))(2)Si] ΔE = 0.0638 a.u. [for L = Cp(2)]. Further, the relatively low energetic barrier to hydrogen exchange (ΔE(#) = 3.3 kcal/mol) for carbon-bridged complex, [(C(5)H(4))(2)CMe(2)MoH(c)(H(a)H(b))](+) → [(C(5)H(4)) (2)CMe(2)MoH(a)(H(b)H(c))](+), is related to strengthening of the Mo-H bonds when going from the substrate to the transition state (TS). Notably higher barrier to hydrogen rotation (ΔE(#) = 10.1 kcal/mol) in [(C(5)H(4))(2) CMe(2)MoH(H(2))](+) is due to lowering in the electrostatic stabilization as well as weakening of the donation (H(2) → Mo charge transfer) and practically lack-of back-donation (Mo → H(2)) in the rotated TS.

RESUMEN

In the present study the natural orbitals for chemical valence (NOCVs) combined with the energy decomposition scheme (ETS) were used to characterize bonding in various clusters of ammonia borane (borazane): dimer D, trimer TR, tetramer TE, and the crystal based models: nonamer N and tetrakaidecamer TD. ETS-NOCV results have shown that shortening of the B-N bond (by ~0.1 Å) in ammonia borane crystal (as compared to isolated borazane molecule) is related to the enhancement of donation (by 6.5 kcal/mol) and electrostatic (by 11.3 kcal/mol) contributions. This, in turn, is caused solely by the electrostatic dipole-dipole interaction between ammonia borane units; dihydrogen bonding, BH···HN, formed between borazane units exhibits no direct impact on B-N bond contraction. On the other hand, formation of dihydrogen bonding appeared to be very important in the total stabilization of single borazane unit, namely, ETS-based data indicated that it leads to significant electronic stabilization ΔE(orb) = -17.5 kcal/mol, which is only slightly less important than the electrostatic term, ΔE(elstat) = -19.4 kcal/mol. Thus, both factors contribute to relatively high melting point of the borazane crystal. Deformation density contributions (Δρ(i)) obtained from NOCVs allowed to conclude that dihydrogen bonding is primarily based on outflow of electron density from B-H bonding orbitals to the empty σ*(N-H) (charge transfer component). Equally important is the covalent contribution resulting from the shift of the electron density from hydrogen atoms of both NH and BH groups to the interatomic regions of NH···HB. Quantitatively, averaged electronic strength of dihydrogen bond per one BH···HN link varies from 1.95 kcal/mol (for the crystal structure model, N), 2.47 kcal/mol (for trimer TR), through 2.65 kcal/mol (for tetramer TE), up to 3.95 kcal/mol (for dimer D).

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The present study characterizes changes in the electronic structure of reactants during chemical reactions based on the combined charge and energy decomposition scheme, ETS-NOCV (extended transition state-natural orbitals for chemical valence). Decomposition of the activation barrier, ΔE (#), into stabilizing (orbital interaction, ΔE (orb), and electrostatic, ΔE (elstat)) and destabilizing (Pauli repulsion, ΔE (Pauli), and geometry distortion energy, ΔE (dist)) factors is discussed in detail for the following reactions: (I) hydrogen cyanide to hydrogen isocyanide, HCN → CNH isomerization; (II) Diels-Alder cycloaddition of ethene to 1,3-butadiene; and two catalytic processes, i.e., (III) insertion of ethylene into the metal-alkyl bond using half-titanocene with phenyl-phenoxy ligand catalyst; and (IV) B-H bond activation catalyzed by an Ir-containing catalyst. Various reference states for fragments were applied in ETS-NOCV analysis. We found that NOCV-based deformation densities (Δρ (i)) and the corresponding energies ΔE (orb)(i) obtained from the ETS-NOCV scheme provide a very useful picture, both qualitatively and quantitatively, of electronic density reorganization along the considered reaction pathways. Decomposition of the barrier ΔE(#) into stabilizing and destabilizing contributions allowed us to conclude that the main factor responsible for the existence of positive values of ΔE (#) for all processes (I, II, III and IV) is Pauli interaction, which is the origin of steric repulsion. In addition, in the case of reactions II, III and IV, a significant degree of structural deformation of the reactants, as measured by the geometry distortion energy, plays an important role. Depending on the reaction type, stabilization of the transition state (relatively to the reactants) originating either from the orbital interaction term or from electrostatic attraction can be of vital importance. Finally, use of the ETS-NOCV method to describe catalytic reactions allows extraction of information on the role of catalysts in determination of ΔE (#).

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