Relative Bond Strengths Research Articles

Quantitative oscillator analysis of IR-optical spectra on spin-cast chitosan films

Infrared optical properties of spin-cast chitosan films have been determined using spectroscopic ellipsometry. Infrared index of refraction and extinction coefficients from 750 cm–1to 4000 cm–1were determined using ellipsometric data fits to dispersion models based on Gaussian shaped oscillators. The free electron contribution was analyzed using a Drude model. This modeling determined that optical anisotropy was present over the entire infrared region. Line shape and oscillator strength analysis was performed to determine oscillator strengths, abundance, and relative bond strength.

Shear viscosities of CaO-Al 2O 3-SiO 2 and MgO-Al 2O 3-SiO 2 liquids: Implications for the structural role of aluminium and the degree of polymerisation of synthetic and natural aluminosilicate melts

The shear viscosity of 66 liquids in the systems CaO-Al 2O 3-SiO 2 (CAS) and MgO-Al 2O 3-SiO 2 (MAS) have been measured in the ranges 1–10 4 Pa s and 10 8–10 12 Pa s. Liquids belong to series, nominally at 50, 67, and 75 mol.% SiO 2, with atomic M 2+ /(M 2+ + 2Al) typically in the range 0.60 to 0.40 for each isopleth. In the system CAS at 1600°C, viscosity passes through a maximum at all silica contents. The maxima are clearly centered in the peraluminous field, but the exact composition at which viscosity is a maximum is poorly defined. Similar features are observed at 900°C. In contrast, data for the system MAS at 1600°C show that viscosity decreases with decreasing Mg/(Mg + 2Al) at all silica contents, but that a maximum in viscosity must occur in the field where Mg/2Al >1. On the other hand, the viscosity at 850°C increases with decreasing Mg/(Mg + 2Al) and shows no sign of reaching a maximum, even for the most peraluminous composition studied. The data from both systems at 1600°C have been analysed assuming that shear viscosity is proportional to average bond strength and considering the equilibrium: Al [ 4 ] - ( Mg , Ca ) 0.5 ⇔ ( Mg,Ca ) 0.5 -NBO + Al XS where Al [4]-(Mg,Ca) 0.5 represents a charge-balanced tetrahedrally coordinated Al; (Mg, Ca) 0.5–NBO represents a nonbridging oxygen (NBO) associated with Ca or Mg, and Al XS represents any structural role of Al that does not require a charge-balancing cation. The viscosity data were fitted using two adjustable variables: i) the equilibrium constant of the above reaction, and ii) the relative bond strength of Al XS. The values of these parameters in the system CAS suggest that Al XS remains in tetrahedral coordination, its charge deficit being satisfied by association with a three-coordinate oxygen in a structure called a tricluster. In contrast, fits to the MAS data at 1600°C infer the presence of high-coordinate Al. These interpretations are found to be consistent with independent spectroscopic and theoretical data. Furthermore, the fitted value of the equilibrium constant in the system MAS is close to zero, implying that Al has no thermodynamic preference to be charge-balanced by Mg. In light of these results, it is shown that values of the ratio ‘NBO/T’ (the ratio of NBO to tetrahedrally coordinated network formers) will not be a true reflection of the polymerisation state of the liquid if it is assumed that all Al is charge-balanced by metal cations. This will be important to take into account when considering the compositional dependence of physical and thermodynamic parameters and may have direct relevance to certain liquids produced during partial melting of the mantle.

An intramolecular ionic hydrogen bond stabilizes a cis amide bond rotamer of a ring‐opened rapamycin‐degradation product

Rapamycin (1), a macrolide immunosuppressant, undergoes degradation into ring-opened acid products 2 and 3 under physiologically relevant conditions. The unsaturated product (3) was isolated and studied in this work. Unlike 1, which has its amide primarily in a trans conformation in solution, 3 has both cis and trans conformations in approximately a 1:1 ratio in dimethyl sulfoxide (DMSO). The amount of cis rotamer was increased dramatically in the presence of an organic base such as triethylamine. The detailed NMR results indicate that the cis rotamer is stabilized through an intramolecular ionic hydrogen bond of the carboxylate anion with the tertiary alcohol as part of a nine-membered ring system. This hydrogen bond was characterized further in organic media and the trans-cis rotamer equilibria were used to estimate the relative bond strengths in several solvents. The additional stabilization arising from this ionic hydrogen bond in the cis rotamer was determined to be 1.4 kcal mol(-1) in DMSO-d6, 2.0 kcal mol(-1) in CD3CN and 1.1 kcal mol(-1) in CD3OD.

Frequencies of the NH vibrations and macrocycle structure of sterically distorted porphyrins

The behavior of the bands of the NH stretching vibrations of sterically distorted porphyrins whose macrocycles are undergoing different types of deformation (saddling, elongation, waving, etc.) is studied in CHCl3 and CCl4 solutions. The geometry of sterically distorted porphyrins and the frequencies of their normal vibrations are calculated in terms of the density functional theory. The relative strength of intramolecular hydrogen bonds is estimated in terms of the theory of natural (localized) orbitals. The frequency of the stretching NH vibrations of porphyrins with saddling, elongation, or waving deformation of their macrocycles is shown to be inversely proportional to the energy of stabilization E (2), arising due to the charge transfer from the orbitals of unshared electron pairs of the nitrogen atoms of the pyrrolenyne rings to the antibonding σ*NH orbitals. It is shown that this frequency can serve as a measure of the strength of intramolecular hydrogen bonds. A specific feature of the ruffling deformation of the porphyrin macrocycle is that the strengthening of the hydrogen bonds (due to a decrease in the size of the internal porphyrin window) is accompanied by a shortening of the NH bonds, whereas, depending on the magnitude of the ruffling, the frequency of the stretching NH vibrations decreases or increases. This phenomenon is associated with the fact that the shortening of the NH bond facilitates a decrease in its effective van der Waals radius and weakening of destabilizing interactions of the σNH orbitals.

Effect of ligand radicals on vibrational IR, Raman and vibronic spectra of europium β-diketonates

Vibrational IR, Raman spectra and vibronic sidebands of Eu 3+ electronic transitions of europium tris-β-diketonates Eu(β) 3·Ph (β-dipyvaloylmethane (DPM), acetylacetone (AA), benzoylacetone (BA), thenoyltrifluoroacetone (TTFA) and other β-diketones; Ph-methyl-, phenyl-, and nitro-derivatives of 1,10-phenanthroline (Phen)) as well as Eu(β) 3·Bpy and Eu(β) 3·D-Bpy (Bpy- and D-Bpy–H- and D-2,2′-bipyridine) were studied. Effect of ligand radical properties on spectra and manifestation of the reciprocal influence of non-equivalent ligands in spectra are discussed. Dependence of the spectra on electronic density distribution in both ligands as well as on the strength of MO and MN bonds at the variation of radicals of one of the ligands, β or Ph, was examined. Shape of vibronic sidebands was analysed. Behaviour of bands in the middle and far regions of IR spectra of the series Eu(β) 3·Phen and Eu(TTFA) 3·Ph was investigated. Increase of the polarising influence of Eu 3+ ions on Phen and Bpy molecules and strengthening the EuN bonds in TTFA compounds in comparison with DPM compounds were disclosed from the Raman spectra of Eu(β) 3·Phen and Eu(β) 3·Bpy, that is in accordance with properties of β-diketone radicals. Conclusion about weaker EuN bonds in europium β-diketonates with heterocyclic diimines in comparison with corresponding nitrates was derived from the spectra. Spectral data concerning the relative strength of Euligand bonds are in agreement with available X-ray data.

Electrospray ionization mass spectrometry in the structural characterization of some diphenyllead(IV) thiosemicarbazonates.

The behavior in electrospray ionization mass spectrometry (ESI-MS) conditions of some complexes formed by Pb(C6H5)2(OAc)2 with salicylaldehyde, 2-ketobutyric acid, pyridine-2-carbaldehyde, 2-acetylpyridine, and 2-benzoylpyridine thiosemicarbazones, was studied in detail with the aid of collisional experiments performed by ion trap. The homoleptic complexes of tridentate thiosemicarbazonate dianions (TSC2-) lose the phenyl groups first, destabilizing the high oxidation state of the metal and leading to Pb(II) complexes in which the TSC2- ligand or a part of it remains coordinated to the metal. The main difference among the complexes derives from the presence of a carboxylate group on the 2-ketobutyric acid thiosemicarbazonato ligand, which probably interacts with a Na+ cation leading to ESI-generated [M+Na]+ species. In the absence of the carboxylate group, the production of abundant protonated molecules is observed. These different behaviors have been rationalized from the structural point of view. The heteroleptic mononuclear complexes, including thiosemicarbazonate and AcO- monoanions in the coordination sphere, do not yield intact ionized molecular species, and the most abundant ESI-generated ion is due to AcO- loss. The spectrum of the binuclear heteroleptic complex formed by the salicylaldehyde thiosemicarbazonate monoanion is conditioned by the weak bonding interaction displayed by the acetate bridge. MSn experiments yield important information on the relative ligand-Pb bond strengths. This work forms part of the search for chelating agents that can be used for effective chemotherapy of organolead poisoning.

Floc strength characterization technique. An insight into silica aggregation.

This paper tests an approach to the estimation of relative particle bond strength based on the nondimensional floc and aggregation factors. The strength of flocs formed by aggregating nanosized silica particles with the addition of potassium chloride (KCl) or cationic surfactants, alkyltrimethylammonium bromide (mixture of CTAB, DTAB, and MTAB) was analyzed. The bonding force of the flocs formed in surfactant compared to that formed in the KCl system was estimated using the new dimensional analysis approach. This force ratio was then compared to that obtained by atomic force microscopy.

Low-coordinate iron(II) amido complexes of beta-diketiminates: synthesis, structure, and reactivity.

The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented. Complexes L(R)FeNHAr (R = methyl, tert-butyl; Ar = para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and (1)H NMR evidence supports the existence of agostic interactions in solid and solution states. Complexes L(R)FeNHAr may be oxidized using AgOTf, and the products L(R)Fe(NHAr)(OTf) are characterized with (19)F NMR spectroscopy, UV/vis spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography. In the structures of the iron(III) complexes L(R)Fe(NHAr)(OTf) and L(R)Fe(OtBu)(OTf), the angles at nitrogen and oxygen result from steric effects and not pi-bonding. The reactions of the amido group of L(R)FeNHAr with weak acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by terminal alkynes and alcohols to give free H(2)NAr and three-coordinate acetylide and alkoxide complexes. The trends in complex stability give insight into the relative strength of bonds from three-coordinate iron to anionic C-, N-, and O-donor ligands.

Resonance Raman studies of HOO-Co(III)bleomycin and Co(III)bleomycin: identification of two important vibrational modes, nu(Co-OOH) and nu(O-OH).

Bleomycin is an antitumor agent whose cytotoxicity is dependent on its ability to bind DNA in the nucleus and effect double-stranded DNA cleavage, which is difficult for the cell to repair. In order for this DNA cleavage to occur, bleomycin must, through a series of reactions, form a low-spin Fe(III) complex, the putative "activated" form of the drug, HOO-Fe(III)bleomycin. The relative strengths of the bonds in the Fe(III)-OOH linkage have not been determined due to the weakness of the hydroperoxo-to-iron(III) charge-transfer transition. The much more stable HOO-Co(III)bleomycin has often been studied as a structural analogue of HOO-Fe(III)bleomycin, and hence, an understanding of the relative bond strengths in the Co-OOH linkage may serve to enhance our understanding of the analogous Fe-OOH linkage. In this report, we present resonance Raman data that identify two important vibrational modes in the Co-OOH linkage, the stretching modes, nu(Co-OOH) and nu(O-OH). Both of these vibrational modes were found to be unperturbed by complexation of the drug with calf thymus DNA. Advantage was also taken of the isostructural realtionship between Fe-bleomycin and Co-bleomycin to analyze and assign the high-frequency modes for HOO-Co(III)bleomycin and Co(III)bleomycin (A(2) and B(2)). These data could be useful for future studies of photoactivated Co-bleomycin and Co-bleomycin analogues in an attempt to characterize oxygen-independent DNA damage pathways.

Charge density analysis of two polymorphs of antimony(III) oxide.

High-resolution X-ray diffraction data have been collected on the cubic polymorph of antimony(III) oxide (senarmontite) to determine the charge distribution in the crystal. The results are in quantitative agreement with crystal Hartree-Fock calculations for this polymorph, and have been compared with theoretical calculations on the orthorhombic polymorph (valentinite). Information about the nature of bonding and relative bond strengths in the two polymorphs has been extracted in a straightforward manner via topological analysis of the electron density. All the close contacts in both polymorphs are found to be similar in nature based on the value of the Laplacian, the magnitude of the electron density and the local energy density at the bond critical points, and these characterise the observed interactions as substantially polar covalent, similar to molecular calculation results on Si-O and Ge-O. Electrostatic potential isosurfaces reveal the octopolar nature of this function for senarmontite, and shed light on the observed packing arrangement of Sb4O6 molecules in the crystal.

Classical versus Nonclassical Covalent Bonding Between the Metal Hydride Radicals MH and M′Hj (MH: HBe, HMg, HCa; M′Hj: Li, BeH, BH2, Na, MgH, AlH2, K, CaH, GaH2)

AbstractFor Abstract see ChemInform Abstract in Full Text.

Classical versus Nonclassical Covalent Bonding between the Metal Hydride Radicals MH and M‘Hj(MH = HBe, HMg, HCa; M‘Hj= Li, BeH, BH2, Na, MgH, AlH2, K, CaH, GaH2)

We report structures, bond strengths, and relative energies for all apparent minima on the B3LYP/6-311+G** potential energy surfaces of the main group compounds [MHj+1M‘] formed between the first three metal hydride radicals of group 2 (MH) and the first three metal hydride radicals of groups 1, 2, and 13 (M‘Hj). Relative energies and bond strengths at the G2(MP2)thaw level of theory, using the B3-LYP/6-311+G** optimized geometries, have also been obtained: the G2(MP2)thaw values conform closely to the B3-LYP results. Significant structural trends identified include the characterization of at least one competing “nonclassical” isomer for each of the 24 classical HM−M‘Hj structural formulas, an increasing preference for nonclassical rather than classical bonding as M is switched from Be to Mg to Ca, and an increased tendency toward stability of unorthodox structures as M‘ progresses from group 1 to 2 to 13. An exploration of the factors underpinning bond formation between M and M‘, focusing particularly on the three [CaHNa] isomers we have located, has been undertaken using the atoms-in-molecules (AIM) computational method. Although our AIM calculations confirm that the metal−metal bonds are largely covalent in character, some aspects of the bonding remain mysterious. Prospects for experimental investigation of polymorphism, via matrix isolation or other techniques, appear to be particularly promising for [MgH2Ca], for which four distinct structural isomers are separated by only approximately 30 kJ mol-1.

The effects of xyloglucan on the properties of paper made from bleached kraft pulp

Xyloglucan was adsorbed onto bleached soft-wood kraft pulp followed by preparation and analysis of handsheets with respect to sheet formation as well as sheet mechanical and optical properties. Adsorption of xyloglucan was found to be slow. After more than 20 hrs adsorption, equilibrium had not been reached. The amount of xyloglucan adsorbed increased with beating, but neither the rate of adsorption nor the quantity adsorbed was significantly affected by temperature. Xyloglucan was found to be practically irreversibly adsorbed onto the fibres and the effects of xyloglucan on paper sheet properties were investigated after thorough washing of the pulp. The adsorption characteristics of xyloglucan confirm observations by other authors on other cellulose substrates. Tensile index values for handsheets formed with the xyloglucan-containing pulps were higher than those measured for control pulps with a comparable beating degree. The light scattering coefficient was, however, not affected by xyloglucan adsorption. Hence, the increase in tensile strength is attributed to an increased relative bond strength between the fibres. Tensile strength versus tear strength relationship was similar for pulps with and without xyloglucan, but water retention value and dewatering resistance were lower for the xyloglucan treated pulps than for the reference pulps at the same tensile strength. In addition, formation was improved for pulps with adsorbed xyloglucan. The conclusion is that xyloglucan is a promising wet end additive that decreases the necessity for beating of the pulp and improves the formation of paper.

Evidence for the rapid incorporation of hopanoids into kerogen

Hopanoids bound into the insoluble organic matter (kerogen) of Recent sediments from a freshwater lake (Priest Pot) and an anoxic sulphidic fjord (Framvaren) were released by hydropyrolysis and examined by gas chromatography-mass spectrometry. Bound hopanoids are present in high concentration (190–1400 μg/g TOC) and represent 22 to 86% of the total analysable hopanoids (i.e., bound and solvent-soluble), this proportion increasing with depth in Framvaren Fjord. The hopanes generated by hydropyrolysis contain higher amounts of the C 35, C 32, and C 30 homologues, reflecting the carbon number distribution of the bound hopanoids and indicating that both biohopanoids (C 30 and C 35) and their diagenetic products (dominated by C 32) are incorporated into the kerogen on a timescale of only 0 to 350 years. Sequential (multiple temperature) hydropyrolysis experiments gave an indication of the relative strengths of bonds being cleaved in association with hopane generation: The hopanoids of a sediment from Priest Pot are almost entirely bound by strong covalent bonds, interpreted to be mainly ether linkages, whilst a Framvaren sediment contains hopanoids that are bound by a mixture of weak di-/polysulphide linkages and stronger ether bonds. Labelling with deuterium indicated that the strong covalent linkages dominate, even for the Framvaren sediment.

Theoretical studies on C-heteroatom bond formation via reductive elimination from group 10 M(PH3)2(CH3)(X) species (X = CH3, NH2, OH, SH) and the determination of metal-X bond strengths using density functional theory.

Density functional calculations have been used to investigate C-C, C-N and C-O bond forming reactions via reductive elimination from Group 10 cis-M(PH3)2(CH3)(X) species (X = C-I3, NH2, OH). Both direct reaction from the four-coordinate species and a three-coordinate mechanism involving initial PH3 loss have been considered. For the four-coordinate pathway the ease of reductive elimination to give M(PH3)2 and CH3-X follows the trend M = Pd < Pt < Ni. The reaction of the cis-M(PH3)2(CH3)(NH2) species is promoted by the formation of methylamine adducts. Non-planar transition states are located and the C-heteroatom bond forming processes are characterised by migration of CH3 onto the cis-heteroatom ligand. For a given ligand, X, activation energies follow the trend M = Ni < Pd < Pt. Formation of the three-coordinate M(PH3)(CH3)(X) species is promoted by a labilisation of the cis-PH3 ligand in the four-coordinate reactants when X = NH2 or OH. For the three-coordinate pathway the energy change for reductive elimination to give M(PH3) and CH3-X again follows the trend M = Pd < Pt < Ni and in all cases the initial product is an M(PH3)(XCH3) adduct. The three-coordinate transition states again involve migration of the CH3 ligand onto the cis-X ligand and for X = NH2 or OH activation energies follow the trend Ni > Pd < Pt. For a given metal activation energies in both the four- and three-coordinate pathways increase along the series CH3 < NH2 < OH. These trends in activation energy can be rationalised in terms of the strength of M-CH3/M-X bonding as long as the extent of geometrical distortion required to obtain the transition state geometry is taken into account. Further calculations on cis-Pd(PH3)2(CH3)(SH) suggest that the more common experimental observation of C(sp3)-S compared to C(sp3)-O reductive elimination arises from the greater kinetic accessibility of the former process rather than an intrinsic thermodynamic preference for C-S bond formation. By comparison, the calculations indicate that C(sp3)-N reductive elimination should be feasible from Ni and Pd systems. DF calculations are shown to reproduce the relative homolytic bond strengths determined experimentally for Pt-X bonds. In the cis-M(PH3)2(CH3)(X) systems the M-CH3 homolytic bond strength increases down the group while for M-NH2 and M-OH bonds the trend is M = N approximately equal to Pd < Pt. M-NH2 and M-OH are considerably stronger than M-CH3 bonds and the presence of a heteroatom ligand serves to weaken M-CH3 bonds even further.

Probing backbone hydrogen bonds in the hydrophobic core of GCN4.

Backbone amide hydrogen bonds play a central role in protein secondary and tertiary structure. Previous studies have shown that substitution of a backbone ester (-COO-) in place of a backbone amide (-CONH-) can selectively destabilize backbone hydrogen bonds in a protein while maintaining a similar conformation to the native backbone structure. The majority of these studies have focused on backbone substitutions that were accessible to solvent. The GCN4 coiled coil domain is an example of a stable alpha-helical dimer that possesses a well-packed hydrophobic core. Amino acids in the a and d positions of the GCN4 helix, which pack the hydrophobic core, were replaced with the corresponding alpha-hydroxy acids in the context of a chemoselectively ligated heterodimer. While the overall structure and oligomerization state of the heterodimer were maintained, the overall destabilization of the ester analogues was greater (average DeltaDeltaG of 3+ kcal mol(-1)) and more variable than previous studies. Since burial of the more hydrophobic ester should stabilize the backbone and reduce the DeltaDeltaG, the increased destabilization must come from another source. However, the observed destabilization is correlated with the protection factors for individual amide hydrogens from previous hydrogen exchange experiments. Therefore, our results suggest that backbone engineering through ester substitution is a useful approach for probing the relative strength of backbone hydrogen bonds.

Recent developments in the MOVPE growth of low H content ZnSe-based compounds and heterostructures

The origin of unintentional hydrogen (H) incorporation during metalorganic vapour phase epitaxy (MOVPE) of ZnSe-based compounds is reviewed and discussed. Hydrogen enters in MOVPE-grown ZnSe as a result of alkyls surface reactions, effectively passivating intentional nitrogen (N) acceptors in p-doped ZnSe during the fabrication of blue-light emitting diodes and laser diodes. The existence of a marked trade-off between the proclivity of common Se alkyls to incorporate H and their thermal stability is pointed out. Current strategies to overcome this process limitation are then described along with results achieved and technological drawbacks. The use of a novel class of VI-group alkyl precursors of the form R 2X 2 [where X=Se, S and R is an ethyl (Et) or methyl (Me) radical] is proposed as an alternative solution. These alkyls allow a reduction of H incorporation in ZnSe-based materials, whilst retaining the low temperatures required for the growth of device quality wide band-gap II–VI compounds. Dimethyldiselenide (Me 2Se 2) and diethyldisulphide (Et 2S 2) allow the pyrolytic MOVPE growth of Zn(S)Se compounds below 400 °C. Mass spectrometry fragmentation experiments performed on the alkyl molecular ions allowed to investigate their relative bond strengths and likely decomposition paths. The reduced thermal stability of these alkyls is attributed to a weakening of the XC bonds in the R 2X 2 molecule induced by the stronger XX bond. Secondary ion mass spectrometry (SIMS) analysis showed that as-grown ZnSe have [H]≈(1–3)×10 17 cm −3, i.e. among the lowest ever reported for MOVPE-grown layers. The functional validation of the new S and Se alkyls is completed by the structural and optical characterisation of Zn(S)Se-based heterostructures grown on (100)GaAs. High-resolution X-ray diffraction studies are presented along with cathodoluminescence (CL) measurements and compared to what reported in the literature. The epilayer structural properties compare well with that of molecular beam epitaxy and MOVPE grown Zn(S)Se heterostructures. CL spectra of ZnSe epilayers appear of good quality, with pronounced band-edge emissions and reduced deep level contributions. Specific emissions in the spectra of ZnS and ZnSe confirm the occurrence of several impurities in the layers, whose origin can be in part attributed to the yet insufficient purity of the novel alkyls.

Amino nitrogen and carbonyl oxygen in competitive situations: which is the best hydrogen-bond acceptor site?

The relative hydrogen-bond acceptor abilities of amino nitrogen and carbonyl oxygen in various chemical environments have been investigated using data retrieved from the Cambridge Structural Database (CSD) and viaab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors and push–pull or saturated aminoketone and aminoester molecular fragments show that the hydrogen bonds are much more frequent on the oxygen than on the amino nitrogen. In the push–pull families, the hydrogen-bonding (HB) ability of the carbonyl oxygen is increased by conjugation between the lone pair of the amino substituent and the CO group, a behaviour similar to resonance-assisted hydrogen bonding. The unexpected behaviour of twisted amides, in which only the amino nitrogen is involved in hydrogen bonding, is pointed out. This unusual feature is explained by the disruption of conjugation between the amino lone pair and the carbonyl owing to the cyclisation involving the amino moiety. In aliphatic systems, the amino nitrogen retains its HB ability but is still involved in a rather low number of contacts. This behaviour reflects the sensitivity of the amino nitrogen to steric hindrance. The relative strengths of the hydrogen bonds on the carbonyl oxygen in ketones and esters follow the experimental order of hydrogen-bond basicity as observed in solution through the pKHB scale. Ab initio molecular orbital calculations (B3LYP/6-31+G** level) of electrostatic and charge transfer descriptors also confirm the experimental observations.

FT-ICR studies of polypyrazolyl-1-yl borates of europium(II) adducts

In this work we report the synthesis and characterization of europium(II) hydrotris(pyrazol-1-yl)borate complexes, Eu(HBPz 3) 2L 2 (L=bis(pentamethylene)urea, diphenylphosphineamide, 2-azacyclononanone, 2-azacycloheptanone). The characterization of the complexes was accomplished by Fourier transform ion cyclotron resonance mass spectrometry using electrospray ionization. For all the complexes, the Eu(HBPz 3) 2L 2 + ion was observed. To evaluate the relative bond strength of the ligand L to europium polypyrazolylborate, infrared multiphoton dissociation (IRMPD) experiments were performed using a 40-W continuous-wave CO 2 ( λ=10.46 μm) laser aligned on-axis with the ion cloud. The parent ion was dissociated to form the daughter ions, Eu(HBPz 3) 2L + and Eu(HBPz 3) 2 +. The kinetics for unimolecular dissociation showed the relative bond strengths of the different adducts of the europium complex to be: diphenylphosphineamide>2-azacyclononanone≈2-azacycloheptanone. Dissociation experiments could not be made for the adduct with bis(pentamethylene)urea due to the weak bond.

Molecular conformation and intramolecular hydrogen bonding in 4-amino-3-penten-2-one

Ab initio calculations at the post Hartree–Fock level (MP2) have been used for a complete study of molecular structures of all possible conformations of 4-amino-3-penten-2-one (APO) to obtain the order of stability and relative hydrogen bond strength in four of the conformers. The comparison of energies for different species indicates that non-H bonded ketoenamine conformers are about 47kJ/mol more stable than non-H bonded enolimine conformers. The stability of ketoenamine over enolimine reduces to 23.6kJ/mol for the H-bonded ketoenamine/enolimine pair due to the stronger H-bond in enolimine conformers. The contribution of resonance to the stability of hydrogen bonding in ketoenamine is estimated to be around 16kJ/mol.