Origin of Remarkably Different Acidity of Hydroxycoumarins-Joint Experimental and Theoretical Studies.

In the present work the origin of highly varied acidity of hydroxycoumarins (pK 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 pK 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 pK increase.

On the Stability of Cis- and Trans-2-Butene Isomers. An Insight Based on the FAMSEC, IQA, and ETS-NOCV Schemes.

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.

From Saturated BN Compounds to Isoelectronic BN/CC Counterparts: An Insight from Computational Perspective.

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).

Analytical aspects of achiral and cyclodextrin-mediated capillary electrophoresis of warfarin and its two main derivatives assisted by theoretical modeling.

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.

Protonated alcohols are examples of complete charge-shift bonds.

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.

Theoretical description of dihydrogen/hydride and trihydride molybdocene complexes: an insight from static and molecular dynamics simulations.

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.

Bonding in ammonia borane: an analysis based on the natural orbitals for chemical valence and the extended transition state method (ETS-NOCV).

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.