Recurrence of carboxylic acid-pyridine supramolecular synthon in the crystal structures of some pyrazinecarboxylic acids.

X-ray crystal structures of pyrazinic acid 1 and isomeric methylpyrazine carboxylic acids 2-4 are analyzed to examine the occurrence of carboxylic acid-pyridine supramolecular synthon V in these heterocyclic acids. Synthon V, assembled by (carboxyl)O-H...N(pyridine) and (pyridine)C-H...O(carbonyl) hydrogen bonds, controls self-assembly in the crystal structures of pyridine and pyrazine monocarboxylic acids. The recurrence of acid-pyridine heterodimer V compared to the more common acid-acid homodimer I in the crystal structures of pyridine and pyrazine monocarboxylic acids is explained by energy computations in the RHF 6-31G* basis set. Both the O-H.N and the C-H...O hydrogen bonds in synthon V result from activated acidic donor and basic acceptor atoms in 1-4. Pyrazine 2,3- and 2,5-dicarboxylic acids 10 and 11 crystallize as dihydrates with a (carboxyl)O-H...O(water) hydrogen bond in synthon VII, a recurring pattern in the diacid structures. In summary, the carboxylic acid group forms an O-H...N hydrogen bond in pyrazine monocarboxylic acids and an O-H...O hydrogen bond in pyrazine dicarboxylic acids. This structural analysis correlates molecular features with supramolecular synthons in pyridine and pyrazine carboxylic acids for future crystal engineering strategies.     

Very strong C-H...O, N-H...O, and O-H...O hydrogen bonds involving a cyclic phosphate.

Very short C-H...O, N-H...O, and O-H...O hydrogen bonds have been generated utilizing the cyclic phosphate [CH2(6-t-Bu-4-Me-C6H2O)2]P(O)OH (1). X-ray structures of (i) 1 (unsolvated, two polymorphs), 1...EtOH, and 1...MeOH, (ii) [imidazolium](+)[CH2(6-t-Bu-4-Me-C6H2O)2PO2](-)...MeOH [2], (iii) [HNC5H4-N=N-C5H4NH](2+)[(CH2(6-t-Bu-4-Me-C6H2O)2PO2)2](2-)...4CH3CN...H2O [3], (v) [K, 18-crown-6](+)[(CH2(6-t-Bu-4-Me-C6H2O)2P(O)OH)(CH2(6-t-Bu-4-Me-C6H2O)2PO2)](-)...2THF [4], (vi) 1...cytosine...MeOH [5], (vii) 1...adenine...1/2MeOH [6], and (viii) 1...S-(-)-proline [7] have been determined. The phosphate 1 in both its forms is a hydrogen-bonded dimer with a short O-H...O distance of 2.481(2) [triclinic form] or 2.507(3) A [monoclinic form]. Compound 2 has a helical structure with a very short C-H...O hydrogen bond involving an imidazolyl C-H and methanol in addition to N-H...O hydrogen bonds. A helical motif is also seen in 5. In 3, an extremely short N-H...O hydrogen bond [N...O 2.558(4) A] is observed. Compounds 6 and 7 also exhibit short N-H...O hydrogen bonds. In 1...EtOH, a 12-membered hydrogen-bonded ring motif, with one of the shortest known O-H...O hydrogen bonds [O...O 2.368(4) A], is present. 1...MeOH is a similar dimer with a very short O(-H)...O bond [2.429(3) A]. In 4, the deprotonated phosphate (anion) and the parent acid are held together by a hydrogen bond on one side and a coordinate/covalent bond to potassium on the other; the O-H...O bond is symmetrical and very strong [O...O 2.397(3) A].     

Cation-anion hydrogen bonds: a new class of hydrogen bonds that extends their strength beyond the covalent limit. A theoretical characterization

The existence of O-H···O hydrogen bonds having a strength within the -80 to -210 kcal/mol range, that is, in the range of strength of covalent bonds and well beyond the so-called covalent limit (-50 kcal/mol), is reported on complexes where the O-H proton donor and O acceptor groups are located in ions of opposite sign. A complete analysis of short distance O-H···O hydrogen bonds between charged fragments was performed for cases where the OH and O groups are both located on charged molecules. It shows that these interactions (a) are nonsymmetrical for the O-H and H···O distances, (b) have a noncovalent H···O bond critical point, and (c) have a strong and energetically stable electrostatic component when the OH and O groups are located in oppositely charged molecules. These cation-anion O-H···O interactions are energetically stable, satisfy the usual topology for hydrogen bonds, HBs, and also have the same directionality found in other HBs. Therefore, they should be considered as a new class of HBs, the cation-anion hydrogen bonds.     

Phonon-driven proton transfer in 3,5-pyridine dicarboxylic acid studied by 2H, 14N, and 17O nuclear quadrupole resonance

Hydrogen bonding in crystalline 3,5-pyridine dicarboxylic acid has been studied by (2)H, (14)N, and (17)O nuclear quadrupole resonance. The (2)H and (17)O data show the presence of two distinct hydrogen bonds, a "normal" O-H···O bond and a short, strong N···H···O bond, with significantly different NQR parameters. In the latter, the temperature variation of the (14)N nuclear quadrupole resonance (NQR) parameters is related to the phonon-driven proton transfer in the N···H···O hydrogen bond. The temperature dependence of the N···H and H···O distances in the N···H···O hydrogen bond is extracted from the (14)N NQR data.     

Unusual electron charge density in carboxylic acid. 17O quadrupole coupling in cis-cyclobutane-1,2-dicarboxylic acid

The (17)O NQR frequencies have been measured in cis-cyclobutane-1,2-dicarboxylic acid and the quadrupole coupling tensors have been determined at various temperatures. Two O···H oxygen positions and two O-H oxygen positions are observed, showing the presence of two different types of O-H···O hydrogen bonds in the unit cell. The quadrupole coupling constants at the O-H oxygen positions are approximately 30% lower than the lowest quadrupole coupling constants experimentally observed at the C-O-H positions in other carboxylic acids with either ordered or disordered hydrogen bonds. The O-H distances have been calculated from the (17)O-(1)H dipole-dipole interaction at the O-H oxygen positions. The obtained values are longer than the O-H distances usually found in O-H···O hydrogen bonds with comparable O···O distance, in agreement with the proposed proton exchange O-H···O ? O···H-O, which partially averages the dipole-dipole interaction. The energy difference of the two proton configurations, O-H···O and O···H-O, is calculated from the O-H distances determined by NQR. The temperature dependence of the (17)O quadrupole coupling tensors at the (17)O···H-O oxygen positions is analyzed in the model of proton exchange and the energy differences of the two proton configurations obtained by this analysis agree with the values obtained from the O-H distances. The quadrupole coupling tensors are analyzed in a model based on the Townes and Dailey model. The model shows that the population of an oxygen lone pair orbital is at this oxygen position reduced from 2 to approximately 1.3. The electron electric charge is most probably transferred to the oxygen σ and π electron orbitals. This may be associated with the structure of the cyclobutane ring, where the X-ray data show the presence of two unusually short C-C bonds.     

Proton transfer in hydrogen-bonded pyridine/acid systems: the role of higher aggregation

In this work the role of higher molecular aggregation in the proton transfer processes within hydrogen bond (H-bond) is investigated. The H-bonded complex consisting of 4-cyanopyridine (CyPy) with trichloroacetic acid (TCA) has been studied in the solutions of acetonitrile, carbon tetrachloride, chloroform and dichloroethane as solvent by FTIR spectroscopy and quantum chemical DFT calculations. In order to illustrate the effect of increasing H-bond strength FTIR investigations have also been performed on solutions of CyPy with H(2)O, acetic-, trifluoroacetic- and methanesulfonic acids. Proton states in the H-bond have been monitored using vibrational CyPy ring modes in FTIR spectra. The stabilization of the CyPy/TCA complex in its protonated form upon increasing polarity of the solvent has been evidenced. It was shown that formation of the CyPy/(TCA)(2) aggregates in the solutions favors the proton transfer process. An X-ray diffraction study has been performed on a single 1 : 2 co-crystal of pyridine/3,5-dinitrobenzoic acid. The H-bond motif found in this system exhibits the same connectivity by strong hydrogen bonds N-H(+)[dot dot dot]O(-) and O-H[dot dot dot]O as that in the CyPy/(TCA)(2) complex predicted by DFT calculation. Certain discrepancies are observed in C-H[dot dot dot]O connectivity only. The networks of H-bonds in both assemblies differ from those usually pictured for 1 : 2 base/carboxylic acid complexes in the literature.     

Transformations of griseofulvin in strong acidic conditions--crystal structures of 2'-demethylgriseofulvin and dimerized griseofulvin

The structure of griseofulvic acid, C16H15ClO6, at 100 K has orthorhombic (P2(1)2(1)2) symmetry. It is of interest with respect to biological activity. The structure displays intermolecular O-H...O, C-H...O hydrogen bonding as well as week C-H...pi and pi...pi interactions. In strong acidic conditions the griseofulvin undergoes dimerization. The structure of dimerized griseofulvin, C34H32C12O12 x C2H6O x H2O, at 100 K has monoclinic (P2(1)) symmetry. The molecule crystallized as a solvate with one ethanol and one water molecule. The dimeric molecules form intermolecular O-H...O hydrogen bonds to solvents molecules only but they interact via week C-H...O, C-H...pi, C-Cl...pi and pi...pi interactions with other dimerized molecules.     

Three-dimensional supramolecular architecture based on 4,4′-methylene-bis(benzenamine) and aromatic carboxylic acid guests: Synthons cooperation, robust motifs and structural diversity

The two-component solid forms involving 4,4??-methylene-bis(benzenamine) included both salts and co-crystals, while 4,4??-methylene-bis(benzenamine) crystallized exclusively as a salt, in agreement with the differences in the pK _a values. Many of the crystal structures displayed either the neutral or the ionic form of the carboxylic acid-amino heterosynthon, and the similarity in crystal structures between the neutral and the ionized molecules makes the visual distinction between a salt and co-crystal dependent on the experimental location of the acidic proton. A variety of supramolecular hydrogen bonded motifs involving interactions between the aza molecules and carboxylic acid groups are observed rather than just the O-H??N/O-H??O motif. The motifs are identical in all the two compounds analyzed showing the robustness of these supramolecular synthons. In all adducts, recognition between the constituents is established through either N-H??O and/or O-H??O/O-H??N pairwise hydrogen bonds. In all adducts, COOH functional groups available on 1 and 2 interact with the N-donor compounds. The COOH moieties in 1 forms only single N-H??O hydrogen bonds, whereas in 2, it forms pairwise O-H??N/N-H??O hydrogen bonds. The supramolecular architectures are elegant and simple, with stacking of networks in 2, but a rather complex network with a threefold interpenetration pattern was found in 1. Thermal stability of these compounds has been investigated by thermogravimetric analysis (TGA) of mass loss.     

Thermochemical properties of HxNO molecules and ions from ab initio electronic structure theory

Coupled-cluster calculations through noniterative triple excitations were used to compute optimized structures, atomization energies at 0 K, and heats of formation at 0 and 298 K for NH2O, HNOH, NH2O-, NH2OH+, NH3OH+, HNO-, and HON. These molecules are important in the gas-phase oxidation of NH3, as well as its solution-phase chemistry. The O-H, N-H, and N-O bond energies of these molecules are given and compared. The N-H and O-H bond energies are quite low, and, for NH2OH, the O-H bond is weaker than the N-H bond (by 7.5 kcal/mol). The energetics for a variety of ionic chemical processes in the gas phase, including the electron affinities of NH2O and HNO, the proton affinities of NH2O and NH2OH, and the acidities of NH2OH and NH2O, are given. The compounds are weak bases and weak acids in the gas phase. Solvation effects were included at the PCM and COSMO levels. The COSMO model gave better values than the PCM model. The relative values for pKa for NH2O and NH2OH are in good agreement with the experimental values, showing both compounds to be very strong bases in aqueous solution with NH2OH being the stronger base by 1.8 pK units at the COSMO level, compared to the experimental pK difference of 1.1+/-0.3 pK units. We predict that NH2OH+ will not be formed in aqueous solution, because it is a very strong acid. Based on the known acidity of NH3OH+, we predict pKa(NH2OH+)=-5.4 at the COSMO level, which is in good agreement with the experimental estimate of pKa(NH2OH+)=-7+/-2.     

Effect of hydrogen bonds on pKa values: importance of networking

The pK(a) of an acyclic aliphatic heptaol ((HOCH(2)CH(2)CH(OH)CH(2))(3)COH) was measured in DMSO, and its gas-phase acidity is reported as well. This tertiary alcohol was found to be 10(21) times more acidic than tert-butyl alcohol in DMSO and an order of magnitude more acidic than acetic acid (i.e., pK(a) = 11.4 vs 12.3). This can be attributed to a 21.9 kcal mol(-1) stabilization of the charged oxygen center in the conjugate base by three hydrogen bonds and another 6.3 kcal mol(-1) stabilization resulting from an additional three hydrogen bonds between the uncharged primary and secondary hydroxyl groups. Charge delocalization by both the first and second solvation shells may be used to facilitate enzymatic reactions. Acidity constants of a series of polyols were also computed, and the combination of hydrogen-bonding and electron-withdrawing substituents was found to afford acids that are predicted to be extremely acidic in DMSO (i.e., pK(a) < 0). These hydrogen bond enhanced acids represent an attractive class of Br?nsted acid catalysts.     

A partial proton transfer in hydrogen bond O-H···O in crystals of anhydrous potassium and rubidium complex chloranilates

Hydrogen bonding and proton transfer in the solid state are studied on the crystals of isostructural anhydrous potassium and rubidium complex chloranilates by variable-temperature single crystal X-ray diffraction, solid state (1)H NMR and IR spectroscopies, and periodic DFT calculations of equilibrium geometries, proton potentials, and NMR chemical shifts. Their crystal structures reveal neutral molecules of chloranilic acid and its dianions connected into a chain by O-H···O hydrogen bond. A strong hydrogen bond with a large-amplitude movement of the proton with NMR shift of 13-17 ppm and a broad continuum in IR spectra between 1000 and 500 cm(-1) were observed. Periodic DFT calculations suggest that proton transfer is energetically more favorable if it occurs within a single pair of chloranilate dianion and chloranilic acid molecule but not continuously along the chains of long periodicity. The calculated chemical shifts confirm the assumption that the weak resonance signals observed at lower magnetic fields pertain to the case when the proton migrates to the acceptor side of the hydrogen bond. The detected situation can be described by a partial proton transfer.     

Modeling energy landscapes of proton motion in nonaqueous, tethered proton wires

We have modeled structures and energetics of anhydrous proton-conducting wires: tethered hydrogen-bonded chains of the form ···HX···HX···HX···, with functional groups HX = imidazole, triazole, and formamidine; formic, sulfonic, and phosphonic acids. We have applied density functional theory (DFT) to model proton wires up to 19 units long, where each proton carrier is linked to an effective backbone to mimic polymer tethering. This approach allows the direct calculation of hydrogen bond strengths. The proton wires were found to be stabilized by strong hydrogen bonds (up to 50 kJ/mol) whose strength correlates with the proton affinity of HX [related to pK(b)(HX)] and not to pK(a)(HX) as is often assumed. Geometry optimizations and ab initio molecular dynamics near 400 K on imidazole-based proton wires both predict that adding a proton to the end of such wires causes the excess charge to embed into the interior segments of these wires. Proton translocation energy landscapes for imidazole-based wires are sensitive to the imidazole attachment point (head or feet) and to wire architecture (linear or interdigitated). Linear imidazole wires with head-attachment exhibit low barriers for intrawire proton motion, rivaling proton diffusion in liquid imidazole. Excess charge relaxation from the edge of wires is found to be dominated by long-range Grotthuss shuttling for distances as long as 42 ?, especially for interdigitated wires. For imidazole, we predict that proton translocation is controlled by the energetics of desorption from the proton wire, even for relatively long wires (600 imidazole units). Proton desorption energies show no correlation with functional group properties, suggesting that proton desorption is a collective process in proton wires.     

C-H bond activation by a ferric methoxide complex: modeling the rate-determining step in the mechanism of lipoxygenase.

Lipoxygenases are mononuclear non-heme iron enzymes that regio- and stereospecifcally convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the pentadiene subunit of the substrate by an active ferric hydroxide species to give a ferrous water species and an organic radical. Reported here are the synthesis and characterization of a ferric model complex, [Fe(III)(PY5)(OMe)](OTf)(2), that reacts with organic substrates in a manner similar to the proposed enzymatic mechanism. The ligand PY5 (2,6-bis(bis(2-pyridyl)methoxymethane)pyridine) was developed to simulate the histidine-dominated coordination sphere of mammalian lipoxygenases. The overall monoanionic coordination provided by the endogenous ligands of lipoxygenase confers a strong Lewis acidic character to the active ferric site with an accordingly positive reduction potential. Incorporation of ferrous iron into PY5 and subsequent oxidation yields a stable ferric methoxide species that structurally and chemically resembles the proposed enzymatic ferric hydroxide species. Reactivity with a number of hydrocarbons possessing weak C-H bonds, including a derivative of the enzymatic substrate linoleic acid, scales best with the substrates' bond dissociation energies, rather than pK(a)'s, suggesting a hydrogen atom abstraction mechanism. Thermodynamic analysis of [Fe(III)(PY5)(OMe)](OTf)(2) and the ferrous end-product [Fe(II)(PY5)(MeOH)](OTf)(2) estimates the strength of the O-H bond in the metal bound methanol in the latter to be 83.5 +/- 2.0 kcal mol(-1). The attenuation of this bond relative to free methanol is largely due to the high reduction potential of the ferric site, suggesting that the analogously high reduction potential of the ferric site in LO is what allows the enzyme to perform its unique oxidation chemistry. Comparison of [Fe(III)(PY5)(OMe)](OTf)(2) to other coordination complexes capable of hydrogen atom abstraction shows that, although a strong correlation exists between the thermodynamic driving force of reaction and the rate of reaction, other factors appear to further modulate the reactivity.     

Theoretical study of the symmetry of the (OH...O)- hydrogen bonds in vinyl alcohol-vinyl alcoholate systems

The interactions between substituted vinyl alcohols and vinyl alcoholates (X = NH(2), H, F, Cl, CN) are studied at the B3LYP/6-311++G(d,p) level of theory. In a first step, the conformation of the monomers is investigated and the proton affinities (PA(A(-))) of the enolates are calculated. The enols and enolates are held together by strong (OH...O)(-) hydrogen bonds, the hydrogen bond energies ranging from 19.1 to 34.6 kcal mol(-1). The optimized O...O distances are between 2.414 and 2.549 A and the corresponding OH distances from 1.134 and 1.023 A. The other geometry parameters such as C[double bond]C or CO distances also indicate that, in the minimum energy configuration, the hydrogen bonds are characterized by a double well potential. The Mulliken charges on the different atoms of the proton donors and proton acceptors and the frequencies of the nu(OH) stretching vibrations agree with this statement. All the data indicate that the hydrogen bonds are the strongest in the homomolecular complexes. The transition state for hydrogen transfer is located with the transition barrier estimated to be about zero. Upon addition of the zero-point vibration energies to the total potential energy, the barrier vanishes. This is a characteristic feature of low-barrier hydrogen bonds (LBHBs). The hydrogen bond energies are correlated to the difference 1.5 PA(AH) - PA(A(-)). The correlation predicts different energies for homomolecular hydrogen bonds, in agreement with the theoretical calculations. Our results suggest that a PA (or pK(a)) match is not a necessary condition for forming LBHBs in agreement with recent data on the intramolecular hydrogen bond in the enol form of benzoylacetone (J. Am. Chem. Soc. 1998, 120, 12117).     

Hydrogen Bonding Assembled 3D Supramolecular Structures Formed by 5-Amino-2,4,6-triiodoisophthalic Acid and N-Heterocyclic Aromatic Ligands

Three salts constructed by 5-amino-2,4,6-triiodoisophthalic acid(ATIPA) with N-heterocycles aromatic coformers such as pyridine tetrazolium, tetramethylpyraziiie and cyanuric acid were synthesized by slowing evaporation of solvent. X-Ray single crystal analysis shows that hydrogen protons of the carboxyl groups transfer to nitrogen atoms of the N-heterocyclic coformers to form N-H…0 hydrogen bonds in all the three compounds. A huge amount of H-bonds play significant role in tlie construction of these compounds and all of them generate 3D structures through strong O-H…N, O-H…O, N-H…O and weak C-H…O hydrogen bonds. Moreover, solvent water molecules are indispensable in the formation of compounds 1 and 3, which constitutes different supramolecular synthons to bridge individual molecules and chains to form stable structures. In addition, these crystal structures were further characterized by themiogravimetric analysis and infrared spectroscopy.     

Hydrogen-bonded complexes of nitrous and nitric acids with ethene Ab initio and DFT studies

The complexes formed by ethene with nitric and nitrous (trans and cis) acids have been investigated by ab initio (SCF and MP2) and B3LYP calculations with 6-311++G(d,p) basis set. Full geometry optimisation has been performed for the complexes studied. The most stable structures of the complexes are established. Bearing in mind the corrected values of the dissociation energy the studied hydrogen-bonded complexes can be ordered as follows: C(2)H(4)...HONO(2)>C(2)H(4)...HONO-trans>C(2)H(4)...HONO-cis. In the complexes the acids act as proton donors forming the pi-type of hydrogen bond with ethene. The predicted changes in the vibrational characteristics (vibrational frequencies and infrared intensities) arising from the hydrogen bonding are in good agreement with the experimentally measured. The predicted frequency shift of the stretching OH vibration in the nitric acid is largest (-210 cm(-1)), followed by the shifts in the trans-HONO (-141 cm(-1)) and cis-HONO (-109 cm(-1)). The calculations predict an increase of the IR intensity of the stretching O-H vibration in the complexes from 6 to 10 times.     

H-bonding cooperativity and energetics of alpha-helix formation of five 17-amino acid peptides

Five peptides, each containing 17 amino acids, have been completely geometrically optimized in their alpha-helical and beta-strand forms using a mixed DFT/AM1 procedure. B3LYP/D95** was used for the entire helical structures, while AM1 was initially used to optimize the side chains, followed by reoptimization at the DFT level. The energetic and structural results show (1) that the helices are favored over the strands by 29.5 to 37.4 kcal/mol; (2) that alkyl groups on the amino acid side chains favor helix formation even in the absence of solvent; (3) that C-H...O hydrogen bonds contribute to the relative stability of the helices that contain amino acids (val, leu and ile) with beta-hydrogens in their alkyl side chains; (4) that formation of these helices entails approximately 6.6 kcal/mol of strain within the backbone per hydrogen bond; and (5) that H-bond cooperativity is essential for the alpha-helix to become more stable than a corresponding beta-strand. This last observation strongly suggests that pairwise potentials are inadequate for modeling of peptides and proteins.     

From linear inorganic chains to helices: chirality in the M(pyz)(H(2)O)(2)MoO(2)F(4) (M = Zn,Cd) compounds

Cd(C(4)H(4)N(2))(H(2)O)(2)MoO(2)F(4) (C(4)H(4)N(2) = pyrazine, pyz) was synthesized via hydro(solvato)thermal methods and characterized by single-crystal X-ray diffraction methods (P3(2)()21, no. 154, Z = 3, a = 7.4328(7) A, c = 16.376(2) A). Both of the known M(pyz)(H(2)O)(2)MoO(2)F(4) (M = Zn, Cd) compounds are comprised of trans-M(pyz)(2)(OH(2))(2)F(2) and cis-MoO(2)F(4) octahedra that share fluoride vertices to form helical chains along the 3-fold screw axes. Individual chains are bridged to six symmetry-equivalent helices through metal-pyrazine and OH(2)...F and OH(2)...O hydrogen bonds. Structural comparisons of similar oxyfluoride chains demonstrate that they can be varied from linear to helical through (1) the replacement of pyridine or pyrazine by H(2)O molecules and (2) the substitution of cis-directing MoO(2)F(4)(2-) anions in place of trans-directing WO(2)F(4)(2-) or TiF(6)(2-) anions. Infrared absorption (IR) measurements for M = Cd show two distinct O-H stretches corresponding to hydrogen-bonded O-H...F and O-H...O groups. Contrastingly for M = Zn, IR measurements exhibit O-H stretches for averaged hydrogen-bonded O-H...(O/F) groups, free (unbound) O-H groups, and higher energy Mo-F stretches. The IR data suggest a small fraction of the O-H...F hydrogen bonds are broken in the M = Zn analogue as a result of the racemic twinning. Both compounds exhibit nonlinear optical behavior, with second harmonic generation (SHG) intensities, relative to SiO(2), of approximately 0.25 ( = 0.28 pm/V) for the racemically twinned Zn(pyz)(H(2)O)(2)MoO(2)F(4) and approximately 1.0 ( = 0.55 pm/V) for the enantiopure Cd(pyz)(H(2)O)(2)MoO(2)F(4).     

Boronic acids in the design and synthesis of supramolecular assemblies

Supramolecular assemblies of phenylboronic and 4-methoxyphenylboronic acids with 4,4^′-bipyridine and an assembly of phenylboronic acid with 1,2-bis(4-pyridyl)ethene, which were obtained due to the formation of O-H?N hydrogen bonds between hetero N-atoms and -B(OH)₂ are reported. Further, a centrosymmetric cyclic C-H?O hydrogen bonding dimer is identified in the crystal structure of 4-methoxyphenylboronic acid.     

(4R)-4-Hydroxy-1-nitroso-l-proline: synthesis, X-ray structure, ab initio and conformational calculations

4-Hydroxy-l-proline, an amino acid, an important component of collagen, was transformed into its N-nitroso-derivative, (4R)-4-hydroxy-1-nitroso-l-proline, 1 by butylnitrite in the acidic medium. The structure is a cyclic hydroxy-N-nitrosoacid with the carboxyl and hydroxyl groups trans to each other. The carboxyl group is in the syn-conformation. In the structure, the neutral molecules are connected via classical intermolecular O-H?O hydrogen bonds involving the hydroxyl and carboxyl groups [O?O=2.6251(14) Å], and form chains along the a-axis direction. The chains are linked into sheets via O-H?O hydrogen bond, [O?O=2.6813(15) Å] with participation of oxygen atom of nitroso group. Ab initio calculations based on density functional theory at the B3LYP/6-311++G(d, p) level of theory were performed to analyze the influence of 4-hydroxy-l-proline (Hyp) nitrosation on the conformation of the synthesized N-nitroso-compound. The geometry optimization of 1 and initial 4-hydroxy-l-proline was carried out in the gas phase and in solution using the polarizable continuum model. The single-point calculation was performed for the crystal structure of 1. The most stable conformer of 1 is observed in an aqueous solution. In this state, the pyrrolidine ring adopts an envelope conformation, which is also maintained in the gas phase. The twisted conformation of the pyrrolidine ring is present in all states of Hyp and in the crystal structure of 1. In 1 the interchange of five-membered ring conformation in solution and in the gas phase in comparison with the crystal is accompanied by an increase of the dipole moment of the molecule, which is maximal in solution.