Exploring the lower limit in hydrogen bonds: analysis of weak C-H...O and C-H...pi interactions in substituted coumarins from charge density analysis

Experimental electron densities in coumarin, 1-thiocoumarin, and 3-acetylcoumarin have been analyzed based on the X-ray diffraction data at 90 K. These compounds pack in the crystal lattice with weak C-H...O and C-H...pi interactions, and variations in charge density properties and derived local energy densities have been investigated in the regions of intermolecular interactions. Theoretical charge density calculations on crystals using the B3LYP/6-31G* method show remarkable agreement with the derived properties and energy densities from the experiment. The intermolecular interactions follow an exponential dependence of electron density and energy densities at the bond critical points. The Laplacian follows a "Morse-like" dependence on the length of the interaction line. Based on the set of criteria defined using the theory of "atoms in molecules", it has become possible to distinguish between a hydrogen bond (C-H...O) and a van der Waals interaction (C-H...pi). This has resulted in the identification of a "region of overlap" in terms of electron densities, energy densities, and mutual penetration of the hydrogen and acceptor atoms with respect to the interaction length. This approach suggests a possible tool to distinguish between the two types of interactions.     

Copper(II) diamino acid complexes: quantum chemical computations regarding diastereomeric effects on the energy of complexation

[reaction: see text] Quantum chemical calculations were used to rationalize the observed enantiodifferentiation in the complexation of alpha-amino acids to chiral Cu(II) complexes. Apart from Cu(II)[bond]pi interactions and steric repulsions between the anchoring cholesteryl-Glu moiety and an aromatic amino acid R group, hydrogen bonding also plays a role. In fact, in the case of tryptophan, C[double bond]O...H[bond]N hydrogen bonding between the glutamate moiety and the tryptophan N[bond]H group compensates for the loss of intramolecular hydrogen-bonding and diminished Cu(II)[bond]pi interactions.     

The structure and chemical bonding in the N(2)-CuX and N(2)...XCu (X = F, Cl, Br) systems studied by means of the molecular orbital and Quantum Chemical Topology methods

Ab initio studies carried out at the MP2(full)/6-311+G(2df) and MP2(full)/aug-cc-pVTZ-PP computational levels reveals that dinitrogen (N(2)) and cuprous halides (CuX, X = F, Cl, Br) form three types of systems with the side-on and end-on coordination of N(2): N[triple bond]N-CuX (C(infinity v)), N(2)-CuX (C(2v)) stabilized by the donor-acceptor bonds and weak van der Waals complexes N(2)...XCu (C(2v)) with dominant dispersive forces. An electron density transfer between the N(2) and CuX depends on type of the N(2) coordination and a comparison of the NPA charges yields the [N[triple bond]N](delta+)-[CuX](delta-) and [N(2)](delta-)-[CuX](delta+) formula. According to the NBO analysis, the Cu-N coordinate bonds are governed by predominant LP(N2)-->sigma*(Cu-X) "2e-delocalization" in the most stable N[triple bond]N-CuX systems, meanwhile back donation LP(Cu)-->pi*(N-N) prevails in less stable N(2)-CuX molecules. A topological analysis of the electron density (AIM) presents single BCP between the Cu and N nuclei in the N[triple bond]N-CuX, two BCPs corresponding to two donor-acceptor Cu-N bonds in the N(2)-CuX and single BCP between electron density maximum of the N[triple bond]N bond and halogen nucleus in the van der Waals complexes N(2)...XCu. In all systems values of the Laplacian nabla(2)rho(r)(r(BCP)) are positive and they decrease following a trend of the complex stability i.e. N[triple bond]N-CuX (C(infinity v)) > N(2)-CuX (C(2v)) > N(2)...XCu (C(2v)). A topological analysis of the electron localization function (ELF) reveals strongly ionic bond in isolated CuF and a contribution of covalent character in the Cu-Cl and Cu-Br bonds. The donor-acceptor bonds Cu-N are characterized by bonding disynaptic basins V(Cu,N) with attractors localized at positions corresponding to slightly distorted lone pairs V(N) in isolated N(2). In the N[triple bond]N-CuX systems, there were no creation of any new bonding attractors in regions where classically the donor-acceptor bonds are expected and there is no sign of typical covalent bond Cu-N with the bonding pair. Calculations carried out for the N[triple bond]N-CuX reveal small polarization of the electron density in the N[triple bond]N bond, which is reflected by the bond polarity index being in range of 0.14 (F) to 0.11 (Cl).     

Hydrogen bonds with pi and sigma electrons as the multicenter proton acceptors: high level ab initio calculations

Pi-electrons of acetylene and sigma-electrons of molecular hydrogen were investigated as Lewis bases in different complexes. Hence high level ab initio calculations were performed up to the MP2/6-311++G(3df,3pd) level of approximation. It was found that species analyzed possess characteristics typical for H-bonded systems. The Bader theory was additionally applied; bond paths between proton and pi-electrons of acetylene or sigma-electrons of molecular hydrogen were detected with the corresponding bond critical points attributed to the proton-acceptor interactions. Numerous correlations between topological, geometrical and energetic parameters were also found. For example, the H...pi or H...sigma interaction is stronger for the shorter corresponding distance between the proton and the middle of C[triple bond]C or H-H bond. It is connected with the greater elongation of C[triple bond]C or H-H bonds and the greater transfer of electron charge from the Lewis base to the Lewis acid.     

1‐Benzoyl‐3‐[(2‐benzylsulfanyl)phenyl]thiourea

In the title compound, C₂₁H₁₈N₂OS₂, a strong intramolecular N—H...O hydrogen bond [N...O = 2.642 (3) Å] between the amide N atom and the benzoyl O atom forms an almost planar six‐membered ring in the central part of the molecule. In the crystal, molecules are packed through weak N—H...S interactions. Intra‐ and intermolecular hydrogen bonds and van der Waals interactions are the stabilizing forces for the crystal structure.     

An ionic 2D inorganic–organic hybrid of tris[((1H-tetrazol-5-yl)methyl) morpholine] dodecatungstophosphate(V) pentahydrate: Synthesis X-ray crystal structure, and spectroscopic characterizations

A unique ionic hybrid material [C6H12N5O]3[(PO4)W12O36]•5H2O has been synthesized from the reaction of ((1H-tetrazole-5-yl)methyl)morpholine and α-H3[(PO4)W12O36].21H2O. It has successfully been characterized by elemental analysis, IR and 1H NMR spectroscopies, TGA and single-crystal X-ray diffraction method. The title compound is constructed from the three [C6H12N5O]+ cations and α-Keggin [(PO4)W12O36]3- polyoxoanion. The most remarkable structural feature of this hybrid can be described as two-dimensional inorganic infinite plane-like (2D/∞[(PO4)W12O36]3-) which forming via van der Waals interactions along the x axis.     

Probing ruthenium-acetylide bonding interactions: synthesis, electrochemistry, and spectroscopic studies of acetylide-ruthenium complexes supported by tetradentate macrocyclic amine and diphosphine ligands

The synthesis and spectroscopic properties of trans-[RuL4(C[triple bond]CAr)2] (L4 = two 1,2-bis(dimethylphosphino)ethane, (dmpe)2; 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane, 16-TMC; 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane, N2O2) are described. Investigations into the effects of varying the [RuL4] core, acetylide ligands, and acetylide chain length for the [(-)C[triple bond]C(C6H4C[triple bond]C)(n-1)Ph] and [(-)C[triple bond]C(C6H4)(n-1)Ph] (n = 1-3) series upon the electronic and electrochemical characteristics of trans-[RuL4(C[triple bond]CAr)2](0/+) are presented. DFT and TD-DFT calculations have been performed on trans-[Ru(L')4(C[triple bond]CAr)2](0/+) (L' = PH3 and NH3) to examine the metal-acetylide pi-interaction and the nature of the associated electronic transition(s). It was observed that (1) the relationship between the transition energy and 1/n for trans-[Ru(dmpe)2{C[triple bond]C(C6H4C[triple bond]C)(n-1)Ph}2] (n = 1-3) is linear, and (2) the sum of the d(pi)(Ru(II)) --> pi*(C[triple bond]CAr) MLCT energy for trans-[Ru(16-TMC or N2O2)(C[triple bond]CAr)2] and the pi(C[triple bond]CAr) --> d(pi)(Ru(III)) LMCT energy for trans-[Ru(16-TMC or N2O2)(C[triple bond]CAr)2]+ corresponds to the intraligand pi pi* absorption energy for trans-[Ru(16-TMC or N2O2)(C[triple bond]CAr)2]. The crystal structure of trans-[Ru(dmpe)2{C[triple bond]C(C6H4C[triple bond]C)2Ph}2] shows that the two edges of the molecule are separated by 41.7 A. The electrochemical and spectroscopic properties of these complexes can be systematically tuned by modifying L4 and Ar to give E(1/2) values for oxidation of trans-[RuL4(C[triple bond]CAr)2] that span over 870 mV and lambda(max) values of trans-[RuL4(C[triple bond]CAr)2] that range from 19,230 to 31,750 cm(-1). The overall experimental findings suggest that the pi-back-bonding interaction in trans-[RuL4(C[triple bond]CAr)2] is weak and the [RuL4] moiety in these molecules may be considered to be playing a "dopant" role in a linear rigid pi-conjugated rod.     

Neutral and ionic hydrogen bonding in Schiff bases

Low-temperature, high-resolution X-ray studies of charge distributions in the three Schiff bases, the dianil of 2-hydroxy-5-methylisophthaldehyde, 3,5-dinitro-N-salicylidenoethylamine and 3-nitro-N-salicylidenocyclohexylamine, have been carried out. These structures exhibit interesting weak interactions, including two extreme cases of intramolecular hydrogen bonds that are ionic N(+)-H...O- and neutral O-H...N in nature. These two types of hydrogen bond reflect differences in geometrical parameters and electron density distribution. At the level of geometry, the neutral O-H...N hydrogen bond is accompanied by an increase in the length of the C(1)-O(1) bond, opening of the ipso-C(1) angle, elongation of the aromatic C-C bonds, shortening of the C(7)-N(2) bond and increased length of the C(1)-C(7) bond, relative to the ionic hydrogen bond type. According to the geometrical and critical point parameters, the neutral O-H...N hydrogen bond seems to be stronger than the ionic ones. There are also differences between charge density parameters of the aromatic rings consistent with the neutral hydrogen bond being stronger than the ionic ones, with a concomitant reduction in the aromaticity of the ring. Compounds with the ionic hydrogen bonds show a larger double-bond character in the C-O bond than appears in the compound containing a neutral hydrogen bond; this suggests that the electronic structure of the former pair of compounds includes a contribution from a zwitterionic canonical form. Furthermore, in the case of ionic hydrogen bonds, the corresponding interaction lines appear to be curved in the vicinity of the hydrogen atoms. In the 3-nitro-N-salicylidenocyclohexylamine crystal there exists, in addition to the intramolecular hydrogen bond, a pair of intermolecular O...H interactions in a centrosymmetric dimer unit.     

The pressure tunning Raman and IR spectral studies on the multinuclear metal carbyne complexes

The Raman and infrared (IR) spectra of four tungsten metal carbyne complexes I, II, IV and V [Cl(CO)2(L)W[triple bond]CC6H4[triple bond](C[triple bond]CC6H4)n[triple bond]N[triple bond]C[triple bond]]2M (L = TMEDA, n = 0, M = PdI2 or ReCl(CO)3; L = DPPE, n = 1, M = PdI2 or ReCl(CO)3) were studied at high external pressure. Their pressure-induced phase transitions were observed near 20kbar (complexes I), 15 kbar (complexes II), 25 kbar (complex IV) and 30 kbar (complex V). The pressure-induced phase transition likely is first order in complex I and the pressure-induced phase transitions of complexes II, IV and V are mostly second order. The pressure sensitivities d nu/dp of nu(W[triple bond]C) are high in the low-pressure phase area and very low in the high-pressure phase area due to the pressure strengthening pi back-bonding from metal W to pi* orbital of C[triple bond]O in fragment Cl(CO)2(L)W[triple bond]C. The pressure strengthening metal pi back-bonding from metal Re or Pd to pi* orbital of C[triple bond]O or C[triple bond]N also happened to both of central metal centers of NCPd(I2)CN in complex I and NCReCl(CO)3CN in complex II.     

Synthesis, structure and luminescent properties of yttrium benzene dicarboxylates with one- and three-dimensional structure

The synthesis and structure of two yttrium benzene dicarboxylates, 1 is proportional to [[Y2(C12N2H8)2(C8H4O4)3].H2O], I and 3 is proportional to [[Y2(C12N2H8)2(C8H4O4)3]], II with one- and three-dimensional structure has been accomplished employing hydrothermal methods in the presence of 1,10-phenanthroline. While I is formed with phthalic aid (1,2-BDC), II is formed using isophthalic acid (1,3-BDC). Both the structures appear to have comparable building units, an eight-membered ring and a paddle-wheel arrangement, connected through the carboxylic acid. The 1,10-phenanthroline, connected to Y as a secondary ligand, occupies the inter-chain spaces in I, and projects into the channels in II. The channels in II are inter-connected. Photoluminescence studies indicate that both I and II exhibit a bathochromic shift with respect to the acids (1,2-BDC and 1,3-BDC) and a hypsochromic shift with respect to 1,10-phenanthroline. Both the compounds exhibit reasonable pi...pi interactions.     

pi-H...O hydrogen bonds: multicenter covalent pi-H interaction acts as the proton-donating system

The MP2 method and the Pople-style basis sets 6-311++G(d,p), 6-311++G(2df,2pd), and 6-311++G(3df,3pd) were used to perform calculations on H3O+...C2H2 and C2H3+...C2H2 complexes and related species. Hydrogen bonds existing for the analyzed complexes were investigated as well as related pi-H...O --> pi...H-O and pi-H...pi --> pi...H-pi proton-transfer processes. For some of the complexes analyzed the multicenter pi-H interaction possessing the properties of a covalent bond acts as a proton donor; more generally it is classified as the Lewis acid. The quantum theory of "atoms in molecules" (QTAIM) was also applied to deepen the nature of these interactions in terms of characteristics of bond critical points. The pi-H...O, O-H...pi, and pi-H...pi interactions analyzed here may be classified as hydrogen bonds since their characteristics are the same as or at least similar to those of typical hydrogen bonds. H...pi interactions are common in crystal structures of organic and organometallic compounds. The analyses performed here show a continuum of such interactions since there are H...pi contacts possessing the characteristics of weak intermolecular interactions on the one hand and pi-H multicenter covalent bonds on the other. Ab initio and QTAIM results support the latter statements.     

Infrared intensity analysis of hydroxyl stretching modes in carboxylic acid dimers by means of the charge–charge flux–dipole flux model

The Charge‑Charge Flux‑Dipole Flux (CCFDF) model in terms of multipoles from the quantum theory of atoms in molecules (QTAIM) was used to investigate the variations in infrared intensities of hydroxyl (O H) stretching modes during the dimerization of carboxylic acids. The hydrogen bond formation in these systems results into bathochromic shifts of vibrational frequencies for all the O H stretching modes along with huge infrared intensity increments for some of them. These bands become more intense on dimerization due mainly to changes in the cross-term contribution between charge and charge flux. In addition, interaction energies for the pair of atoms directly involved in individual hydrogen bonds (O…H) are linearly correlated to electron densities at their bond critical points (BCPs). Therefore, the hydrogen bonds between the carbonyl group (CO) of acetic acid and the hydroxyl group of halogenated monomers show the largest electron density values at their BCPs. The formation of these intermolecular interactions is also accompanied by ionic character enhancements of O H bonds and electron density decrements at their BCPs. We finally noticed that the hydrogen atom belonging to the hydroxyl group loses electronic charge, while the oxygen from the CO end becomes more negatively charged during dimerization. © 2019 Wiley Periodicals, Inc.     

Cocrystallization of adamantane‐1,3‐dicarboxylic acid and 4,4′‐bipyridine

The cocrystallization of adamantane‐1,3‐dicarboxylic acid (adc) and 4,4′‐bipyridine (4,4′‐bpy) yields a unique 1:1 cocrystal, C₁₂H₁₆O₄·C₁₀H₈N₂, in the C2/c space group, with half of each molecule in the asymmetric unit. The mid‐point of the central C—C bond of the 4,4′‐bpy molecule rests on a center of inversion, while the adc molecule straddles a twofold rotation axis that passes through two of the adamantyl C atoms. The constituents of this cocrystal are joined by hydrogen bonds, the stronger of which are O—H...N hydrogen bonds [O...N = 2.6801 (17) Å] and the weaker of which are C—H...O hydrogen bonds [C...O = 3.367 (2) Å]. Alternate adc and 4,4′‐bpy molecules engage in these hydrogen bonds to form zigzag chains. In turn, these chains are linked through π–π interactions along the c axis to generate two‐dimensional layers. These layers are neatly packed into a stable crystalline three‐dimensional form via weak C—H...O hydrogen bonds [C...O = 3.2744 (19) Å] and van der Waals attractions.     

Investigating CN- cleavage by three-coordinate M[N(R)Ar]3 complexes

Three-coordinate Mo[N((t)Bu)Ar]3 binds cyanide to form the intermediate [Ar((t)Bu)N]3Mo-CN-Mo[N((t)Bu)Ar]3 but, unlike its N2 analogue which spontaneously cleaves dinitrogen, the C-N bond remains intact. DFT calculations on the model [NH2]3Mo/CN- system show that while the overall reaction is significantly exothermic, the final cleavage step is endothermic by at least 90 kJ mol(-1), accounting for why C-N bond cleavage is not observed experimentally. The situation is improved for the [H2N]3W/CN- system where the intermediate and products are closer in energy but not enough for CN- cleavage to be facile at room temperature. Additional calculations were undertaken on the mixed-metal [H2N]3Re+/CN- /W[NH2]3 and [H2N]3Re+/CN-/Ta[NH2]3 systems in which the metals ions were chosen to maximise the stability of the products on the basis of an earlier bond energy study. Although the reaction energetics for the [H2N]3Re+/CN /W[NH2]3 system are more favourable than those for the [H2N]3W/CN- system, the final C-N cleavage step is still endothermic by 32 kJ mol(-1) when symmetry constraints are relaxed. The resistance of these systems to C-N cleavage was examined by a bond decomposition analysis of [H2N]M-L1[triple bond]L2-M[NH2]3 intermediates for L1[triple bond]L2 = N2, CO and CN which showed that backbonding from the metal into the L1[triple bond]L2 pi* orbitals is significantly less for CN than for N2 or CO due to the negative charge on CN- which results in a large energy gap between the metal d(pi), and the pi* orbitals of CN-. This, combined with the very strong M-CN- interaction which stabilises the CN intermediate, makes C-N bond cleavage in these systems unfavourable even though the C[triple bond]N triple bond is not as strong as the bond in N2 or CO.     

Characterizing the properties of the N7,N9-dimethylguaninium chloride ion pairs: prospecting for the design of a novel ionic liquid

The structures, infrared spectra, and electronic properties of the N7,N9-dimethylguaninium chloride have been studied. The interaction of one cation with one to four Cl anions and one Cl anion with two cations were investigated. Fifteen stable conformers are obtained. It is found that there are four acidic regions in the vicinity of the guaninium cations. In these regions, the cation could H-bond with one to three Cl anions but no more than three nearest anions. One Cl anion could H-bond with two cations. Additionally, evidence of a Cl...pi interaction between the anion and cation is observed. Among these structures, one cation interaction with two anions and two cations interaction with one anion have the larger interaction energies than the other series. Natural bond orbital analyses and molecular orbitals reveal that the charge transfer from anion(s) to the cation(s) occurs mainly through either the Cllp --> sigma C-H, Cllp --> sigma N-H, or Cllp --> pi C8-N7 interactions. The interaction between Cl and sigma (C/N-H) or pi C-N produces a small bond order. This indicates that the Cl...H (Cl...pi) interaction exhibits a weak covalent character and suggests a strong ionic H-bond (Cl...pi bond). What's more, formation of Cl...H/Cl...pi bond decreases the bond order of the associated C/N-H bond or C8-N7 bond. In addition, examination of vibrational spectrum of each conformer explains the origin of H-bonding character.     

Trinuclear silver(I) complexes of fluorinated pyrazolates

Silver pyrazolates [[3-(CF3)Pz]Ag]3, [[3-(CF3),5-(CH3)Pz]Ag]3, [[3-(CF3),5-(Ph)Pz]Ag]3, [[3-(CF3),5-(But)Pz]Ag]3, and [[3-(C3F7),5-(But)Pz]Ag]3 have been synthesized by treatment of the corresponding pyrazole with a slight molar excess of silver(I) oxide. This economical and convenient route gives silver pyrazolates in high (>80%) yields. X-ray crystal structures of [[3-(CF3),5-(CH3)Pz]Ag]3, [[3-(CF3),5-(But)Pz]Ag]3, and [[3-(C3F7),5-(But)Pz]Ag]3 show that these molecules have trinuclear structures with essentially planar to highly distorted Ag3N6 metallacycles. [[3-(CF3),5-(CH3)Pz]Ag]3 forms extended columns via intertrimer argentophilic contacts (the closest Ag...Ag separation between the neighboring trimers are 3.355 and 3.426 A). The trinuclear [[3-(CF3),5-(But)Pz]Ag]3 units crystallize in pairs, basically forming "dimers of trimers", with the six silver atom core of the adjacent trimers adopting a chair conformation. However, in these dimers of trimers, even the shortest intertrimer Ag...Ag distance (3.480 A) is slightly longer than the van der Waals contact of silver (3.44 A). [[3-(C3F7),5-(But)Pz]Ag]3, which has two bulky groups on each pyrazolyl ring, shows no close intertrimer Ag...Ag contacts (closest intertrimer Ag...Ag distance = 5.376 A). The Ag-N bond distances and the intratrimer Ag...Ag separations of the silver pyrazolates do not show much variation. However, their N-Ag-N angles are sensitive to the nature (especially, the size) of substituents on the pyrazolyl rings. The pi-acidic [[3,5-(CF3)2Pz]Ag]3 and [[3-(C3F7),5-(But)Pz]Ag]3 form adducts with the pi-base toluene. X-ray data show that they adopt extended columnar structures of the type [[Ag3]2.[toluene]]infinity and [[Ag3]'.[toluene]]infinity ([[3,5-(CF3)2Pz]Ag]3 = [Ag3],[[3-(C3F7),5-(But)Pz]Ag]3 = [Ag3]'), in which toluene interleaves and makes face-to-face contacts with [[3-(C3F7),5-(But)Pz]Ag]3 or dimers of [[3,5-(CF3)2Pz]Ag]3.     

C-H...N and C-H...S hydrogen bonds--influence of hybridization on their strength

Simple complexes connected through C-H...S and C-H...N interactions are investigated: CH4...NH3, C2H4...NH3, C2H2...NH3, CH4...SH2, C2H4...SH2, and C2H2...SH2. Ab initio and DFT calculations are performed (SCF, MP2, B3LYP) using different basis sets up to the MP2/aug-cc-pVQZ//MP2/aug-cc-pVDZ level of approximation. The Bader theory is applied since MP2/6-311++G(d,p) wave functions are used to find and to characterize bond critical points in terms of electron densities and their Laplacians. The influence of hybridization on the properties of C-H...S and C-H...N systems is also studied showing that the strength of such interactions increases in the following order: C(sp3)-H...Y, C(sp2)-H...Y, C(sp)-H...Y, where Y = S, N--it is in line with the previous findings on C-H...O hydrogen bonds. The results also show that CH4...SH2 and C2H4...SH2 complexes should be rather classified as van der Waals interactions and not as hydrogen bonds. The frequency associated with the C-H stretch of C(sp3)-H...S is blue-shifted.     

Oxygen-bridged borate anions from tris(pentafluorophenyl)borane: synthesis, NMR characterization, and reactivity

The previously known anion [(C6F5)3B(mu-OH)B(C6F5)3]- (2) has been prepared by a two-step procedure, involving deprotonation of (C6F5)3BOH2 to give [B(C6F5)3OH]- (1), followed by addition of B(C6F5)3. The solution structure and the dynamics of 2 have been investigated by 1H and 19F NMR spectroscopy. The reaction of [NHEt3]2 with NEt3 resulted in the formation of [NHEt3]+ [(C6F5)3BOH]-, [NHEt3]+ [(C6F5)3BH]-, and (C6F5)3B- (CH2CH=N+ Et2). This indicates that in the presence of a nucleophile anion 2 can dissociate to B(C6F5)3 and 1. The reaction of [HDMAN]2 with 1,8-bis(dimethylamino)naphthalene (DMAN) confirmed this trend. In the presence of water, 2 transformed into the adduct [(C6F5)3BO(H)H...O(H)B(C6F5)3]- (3), containing the borate 1 hydrogen-bonded to a water molecule coordinated to B(C6F5)3. The same compound is formed by treating (C6F5)3BOH2 with 0.5 equiv of a base. A competition study established that for 1 the Lewis acid-base interaction with B(C6F5)3 is about 5 times preferred over H-bonding to (C6F5)3BOH2. The X-ray single-crystal analysis of [2-methyl-3H-indolium]3 provided the first experimental observation of an asymmetric H-bond in the [H3O2]- moiety, the measured O-H and H...O bond distances being significantly different [1.14(2) vs 1.26(2) A]. The reaction of NEt3 with an equimolar mixture of B(C6F5)3 and bis(pentafluorophenyl)borinic acid, (C6F5)2BOH, afforded the novel borinatoborate salt [NHEt3]+ [(C6F5)3BOB(C6F5)2]- ([NHEt3]4). X-ray diffraction showed that the B-O bond distances are significantly shorter than in [(C6F5)3B(mu-OH)B(C6F5)3]-. Variable-temperature 19F NMR revealed high mobility of the five aryl rings, at variance with the more crowded anion 2. 2D NMR correlation experiments showed that in CD2Cl2 the two anions [(C6F5)3BOH]- and [(C6F5)3BH]- form tight ion pairs with [NHEt3]+, in which the NH proton establishes a conventional (BO...HN) or an unconventional (BH...HN), respectively, hydrogen bond with the anion. The diborate anions 2-4, on the contrary, gave loose ion pairs with the ammonium cation, due both to the delocalized anionic charge and to the more sterically encumbered position of the oxygen atoms that should act as H-bond acceptors.     

Continua of interactions between pairs of atoms in molecular crystals

The electron density distributions in crystals of five previously studied DMAN complexes and five Schiff bases (two new ones) have been analysed in terms of various properties of bond critical points (BCPs) found in the pair-wise interactions in their lattices. We analysed the continua of interactions including covalent/ionic bonds as well as hydrogen bonds and all other types of weak interactions for all pairs of interacting atoms. The charge density at BCPs and local kinetic and potential energy densities vary exponentially with internuclear distance (or other measures of separation). The parameters of the dependences appear to be characteristics of particular pairs of atom types. The Laplacian and the total (sum of kinetic and potential) energy density at BCPs show similar behaviour with the dependence being of the Morse type. The components lambda1, lambda2, lambda3 of the Laplacian at BCPs vary systematically with internuclear distance according to the type of atom pair. For lambda1 and lambda2 the distribution is of the exponential type, whereas lambda3 does not seem to follow any simple functional form, consistent with previous theoretical findings. Analytical nonlinear dependences of Laplacian on charge density have been found. They agree reasonably well with those obtained by least square fit of the Laplacian to charge density data. There are four distinct regions of the [symbol: see text]2rho(BCP)/rho(BCP) space, generated by E(BCP) = 0 and G(BCP)/rho(BCP) = 1 conditions. Two regions clearly correspond to the shared-shell and closed-shell interactions and the other two to some intermediate situation.     

The orientation of N-H...O=C and N-H...N hydrogen bonds in biological systems: How good is a point charge as a model for a hydrogen bonding atom?

In order to design new ligands for protein-binding sites of unknownstructure, it would be useful to predict the likely sites of hydrogenbonding of an unknown protein fragment to a known molecule. The positions ofmaxima and minima in the electrostatic potential at appropriate distancesfrom the van der Waals surface were calculated for various small molecules,nucleic acid bases, peptide units and amino acid side chains containinggroups which can form the biologically important N-H...O=C andN-H...N hydrogen bonds. Their ability to predict the positions of H andO/N in hydrogen bonded complexes, as predicted by optimising theelectrostatic interactions of pairs of such molecules constrained by themolecular shapes, was assessed. It is shown that extrema in theelectrostatic potential around the isolated molecules give worthwhilepredictions for the locations of hydrogen bonding partners. For moleculesbound by a single N-H...O=C hydrogen bond, the electrostatic maximumassociated with the H is usually less than 1 Å from an acceptor atom,while a C=O electrostatic minimum is generally less than 1.5 Å fromthe hydrogen bond proton. However, a significant number of hydrogen bondsform to the opposite lone pair from the electrostatic minimum, in which casethe separation is up to 3.3 Å. This reflects the broad electrostaticpotential well around a carbonyl oxygen between the lone pair directions.The model predicts when neighbouring atoms drastically change the hydrogenbonding characteristics of an N-H or C=O group. Although the geometries ofhydrogen bonded complexes are influenced by the other van der Waals contactsbetween the molecules, particularly multiple hydrogen bonds, theseinfluences are constant when considering hydrogen bonding to a specificuncharacterised binding site. Hence, the consideration of stericallyaccessible electrostatic extrema will be useful in the design of newligands.