Showing papers on "Intramolecular force published in 2000"
TL;DR: The resonance-assisted hydrogen bond (RAHB) is a model of synergistic interplay between π-delocalization and hydrogen-bond strengthening originally introduced by Gilli, G. as discussed by the authors.
Abstract: The resonance-assisted hydrogen bond (RAHB) is a model of synergistic interplay between π-delocalization and hydrogen-bond (H-bond) strengthening originally introduced (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023; Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1991, 113, 4917) for explaining the abnormally strong intramolecular O−H···O bonds formed by the ···OC−CC−OH··· β-enolone fragment I which are typical of β-diketone enols. The applicability of this model to the intramolecular N−H···O hydrogen bonds formed by a number of heteroconjugated systems (···OC−CC−NH···, β-enaminones II; ···OC−CN−NH···, ketohydrazones III; and ···ON−CC−NH···, nitrosoenamines IV) is investigated. The X-ray crystal structures of five molecules which close a six-membered ring by an intramolecular N−H···O bond through the resonant ···OX−CX−NH··· (X = C, N) fragments II−IV are compared to those of two other molecules closing the same ring through the nonresonant ···OC...
444 citations
TL;DR: The intention of this review article is to present a critical evaluation of the actual development of static force spectroscopy, and mainly focuses on experiments dealing with inter- and intramolecular forces-starting with "simple" electrostatic forces, then ligand-receptor systems, and finally the stretching of individual molecules.
Abstract: How do molecules interact with each other? What happens if a neurotransmitter binds to a ligand-operated ion channel? How do antibodies recognize their antigens? Molecular recognition events play a pivotal role in nature: in enzymatic catalysis and during the replication and transcription of the genome; it is also important for the cohesion of cellular structures and in numerous metabolic reactions that molecules interact with each other in a specific manner. Conventional methods such as calorimetry provide very precise values of binding enthalpies; these are, however, average values obtained from a large ensemble of molecules without knowledge of the dynamics of the molecular recognition event. Which forces occur when a single molecular couple meets and forms a bond? Since the development of the scanning force microscope and force spectroscopy a couple of years ago, tools have now become available for measuring the forces between interfaces with high precision—starting from colloidal forces to the interaction of single molecules. The manipulation of individual molecules using force spectroscopy is also possible. In this way, the mechanical properties on a molecular scale are measurable. The study of single molecules is not an exclusive domain of force spectroscopy; it can also be performed with a surface force apparatus, laser tweezers, or the micropipette technique. Regardless of these techniques, force spectroscopy has been proven as an extraordinary versatile tool. The intention of this review article is to present a critical evaluation of the actual development of static force spectroscopy. The article mainly focuses on experiments dealing with inter- and intramolecular forces—starting with “simple” electrostatic forces, then ligand–receptor systems, and finally the stretching of individual molecules.
436 citations
TL;DR: In this paper, the lifetime of terphenyl-based lanthanide complexes with a triphenylene antenna chromophore (Ln) has been investigated and the processes preceding the luminescence in the sensitization process have been studied in detail.
Abstract: m-Terphenyl-based lanthanide complexes functionalized with a triphenylene antenna chromophore ((Ln)1) exhibit sensitized visible and near-infrared emission upon photoexcitation of the triphenylene antenna at 310 nm. Luminescence lifetime measurements of the (Eu)1 and (Tb)1 complexes in methanol-h1 and methanol-d1 revealed that one methanol molecule is coordinated to the lanthanide ion, indicating that all eight donor atoms provided by the ligand are involved in the encapsulation of the lanthanide ion. The luminescence lifetimes of the near-IR-emitting complexes (Er)1, (Nd)1, and (Yb)1 in DMSO-h6 and DMSO-d6 are in the microsecond range, and are dominated by nonradiative deactivation of the luminescent state. The processes preceding the lanthanide luminescence in the sensitization process have been studied in detail. The complexed lanthanide ion reduces the antenna fluorescence and increases the intersystem crossing rate via an external heavy atom effect. The subsequent energy-transfer process was found to take place via the antenna triplet state in all complexes. Luminescence quantum yield measurements and transient absorption spectroscopy indicated that in solution two conformational isomers of the complexes exist: one in which no energy transfer takes place, and one in which the energy transfer does take place, resulting in the lanthanide luminescence. The intramolecular energy-transfer rate is higher in the (Eu)1 and (Tb)1 complexes than in the near-infrared-emitting complexes. In methanol the energy-transfer rate is 3.8 × 107 s-1 for (Eu)1 and (Tb)1. In DMSO-d6 the intramolecular energy-transfer rate is higher in the (Nd)1 complex (1.3 × 107 s-1) than in the (Er)1 (3.8 × 106 s-1) and (Yb)1 (4.9 × 106 s-1) complexes.
317 citations
TL;DR: In this paper, the UV-visible spectra of 2-hydroxy Schiff bases are investigated in different solvents, acidic and basic media, and the structure of compound (2) was examined crystallographically.
239 citations
TL;DR: Advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of metal-metal bonding, which offers attractive perspectives in stoichiometric or even catalytic transformations.
Abstract: Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations.
231 citations
TL;DR: A possible mechanism involving ethynyl chelation-assisted electrophilic metalation of aromatic C-H bonds by in-situ generated cationic Pd(II) species has been discussed and the involvement of vinylcationic species have been suggested.
Abstract: A new and general method has been developed for preparation of coumarins and quinolinones by intramolecular hydroarylation of alkynes. Various aryl alkynoates and alkynanilides undergo fast intramolecular reaction at room temperature in the presence of a catalytic amount of Pd(OAc)(2) in a mixed solvent containing trifluoroacetic acid (TFA), affording coumarins and quinolinones in moderate to excellent yields with more than 1000 turnover numbers (TON) to Pd. The methodology proved to tolerate a number of functional groups such as Br and CHO. On the basis of isotope experiments, a possible mechanism involving ethynyl chelation-assisted electrophilic metalation of aromatic C-H bonds by in-situ generated cationic Pd(II) species has been discussed. Also the involvement of vinylcationic species has been suggested.
209 citations
TL;DR: A review of the metal complex forming coordination polymers can be found in this paper, where a polymer-metal complex is composed of synthetic polymer and metal ions bound to the polymer ligand by a coordinate bond.
193 citations
193 citations
TL;DR: The EKG structure suggests a molecular mechanism for stabilization at neutral pH values, where both Glu and Lys are ionized, but suggest that this occurs because of the effects of ionization on the individual residues, rather than ion pair formation.
193 citations
TL;DR: In this article, a hydrogen-bonded molecular duplex (3·4) was constructed with two oligoamide molecular strands, 3 and 4, with the complementary hydrogen bonding sequences ADAADA and DADDAD, respectively.
Abstract: This paper describes the design, synthesis, and characterization of a hydrogen-bonded molecular duplex (3·4). Two oligoamide molecular strands, 3 and 4, with the complementary hydrogen-bonding sequences ADAADA and DADDAD, respectively, were found to form an extremely stable (Ka = (1.3 ± 0.7) × 109 M-1) molecular duplex (3·4) in chloroform. Evidence from 1D and 2D 1H NMR spectroscopy, isothermal titration calorimetry, and thin-layer chromatography confirmed the formation and the high stability of the duplex. The exceptional stability is explained by positive cooperativity among the numerous hydrogen-bonding and van der Waals interactions and the preorganization of the individual strands by intramolecular hydrogen bonds. This design has opened a new avenue to supramolecular recognition units with programmable specificities and stabilities.
TL;DR: In this paper, it was shown that linear oligomers and cyclic complexes are strongly emissive at room temperature in solution and in the solid state, respectively, when n = 1, 3, 4, 5, and 6.
Abstract: The complexes [{–Ph2P(CH2)nPPh2AuNC5H4C5H4NAu–}x]2x+ [CF3CO2−]2x (n = 1–6) were prepared as colourless, air-stable solids by reaction of silver trifluoroacetate with the corresponding precursor complex [(CH2)n(Ph2PAuCl)2], and subsequent treatment of the products [(CH2)n(Ph2PAuO2CCF3)2] with 4,4′-bipyridyl. The complexes are suggested to exist in solution as an equilibrium mixture of linear oligomers and cyclic complexes. When n = 1, 3 or 5 the cationic complexes were shown to exist as 26-, 30- and 34-membered macrocyclic rings respectively; only when n = 1 are there significant intramolecular Au⋯Au contacts of 3.106(1) and 3.084(1) A. Some of the complexes are strongly emissive at room temperature in solution and in the solid state.
TL;DR: In this article, the Gibbs energy change ΔGHB for the formation of 1:1 hydrogen bonding complexes between hydrogen bond acceptors (bases) and a reference hydrogen bond donor (4-fluorophenol) in tetrachloromethane at 298 K was measured.
Abstract: Both proton transfer and hydrogen bonding play important roles in biological systems. In order to measure hydrogen bond basicity, we are building a new scale that differs significantly from the pKa scale of proton transfer basicity. The strength of hydrogen bond acceptors (HBAs) is measured from the Gibbs energy change ΔGHB for the formation of 1:1 hydrogen bonding complexes between hydrogen bond acceptors (bases) and a reference hydrogen bond donor (4-fluorophenol) in tetrachloromethane at 298 K. The pKHB database (1.364 pKHB =–ΔGHB (kcal mol-1)) comprises ca. 1000 hydrogen bond acceptors. The HBA strength depends on (i) the position of the acceptor atom in the periodic table, (ii) polarizability, field/inductive and resonance effects of substituents around the acceptor atom, and (iii) proximity effects including steric hindrance of the acceptor site, intramolecular hydrogen bonding and lone-pair–lone-pair repulsions. The ranking of oxygen and sp nitrogen bases does not depend very much on the solvent and the reference hydrogen bond donor, but sp2 and sp3 nitrogen bases gain strength in solvents of higher reaction field than CCl4 and lose strength toward CH and weak NH donors. The complete scatter pattern exhibited by the pKa versus pKHB plot demonstrates the non-equivalence of the two scales. The HBA strength scale is applied to the prediction of the hydrogen bonding site in polybasic drugs (e.g strychnine and carbimazole), and to the calculation of octanol–water partition coefficients. A possible relationship between HBA strength and antihistaminic activity is studied for the `push–pull' drugs cimetidine, ranitidine and famotidine.
TL;DR: The catalytic properties of the transition-metal complexes depend on the appropriate choice of ligand, solvent, temperature, and counteranion as mentioned in this paper, and the X-ray crystal structure of one catalyst, [PdCl(triphos)](CF3SO3).
TL;DR: In this paper, small molecules such as malonaldehyde containing such a H-bond are examined, and the transfer barrier is linearly related to the equilibrium length of the Hbond in various states of each system.
Abstract: Ab initio calculations that address the problem of excited-state proton transfer across an intramolecular hydrogen bond are reviewed. Small molecules, such as malonaldehyde, containing such a H-bond are first examined. This work reveals that in comparison to the ground state, the H-bond is strengthened and the transfer barrier reduced in the1ππ* state; opposite trends are noted in the triplet ππ* as well as nπ* states. Replacement of the H-bonding O atoms of malonaldehyde by N has only a small effect upon these results, as does enlargement or reduction of the malonaldehyde ring, coupled with anionic charge. The transfer barrier is linearly related to the equilibrium length of the H-bond in the various states of each system. Attachment of a phenyl ring to malonaldehyde introduces a fundamental asymmetry into the proton transfer potential, as the enol and keto tautomers are inequivalent. Whereas the enol is more stable in the ground and nπ* states, a reversal occurs in the ππ* states, which may be understoo...
TL;DR: The bond lifetime is calculated as a function of an applied force and unbinding forces under an increasing applied load and the relationship between both quantities is determined.
TL;DR: Aqueous hydrogen sulfite reacts with Hg2+ to form, in the absence of excess HSO3-, the HgSO3 complex, observed here for the first time.
Abstract: Aqueous hydrogen sulfite reacts with Hg2+ to form, in the absence of excess HSO3-, the HgSO3 complex, observed here for the first time. Its UV spectrum is described by e(234 nm) = (1.57 ± 0.05) × 104 M-1 cm-1. HgSO3 decomposes in an intramolecular redox reaction which is kinetically first-order. The rate constant is independent of [Hg2+], [HSO3-], [O2(aq)], and ionic strength. An acid-assisted pathway becomes significant at pH ≤ 1, attributed to the contribution of HgSO3H+. The rate of the intramolecular reaction of HgSO3 was measured by trapping the Hg0 product as Hg22+; the value of the rate constant is k0 = (0.0106 ± 0.0009) s-1 at 25.0 °C, pH 3. The activation parameters for pH 3, ΔH‡ and ΔS‡, are (105 ± 2) kJ/mol and (68 ± 6) J/mol·K, respectively, consistent with a unimolecular bond cleavage mechanism. A pathway involving H2O-induced concerted 2e- transfer is proposed.
Journal Article•
TL;DR: In this article, the intramolecular proton transfer pathways for the passage from the neutral form of NH 2 -CH 2 -COOH (GN) to the zwitterionic form + NH 3 −CH 2 −COO − (GZ) of glycine hydrated by three water molecules are computed using DFT and ab initio methods at high levels of theory.
Abstract: The intramolecular proton transfer pathways for the passage from the neutral form of NH 2 –CH 2 –COOH (GN) to the zwitterionic form + NH 3 –CH 2 –COO − (GZ) of glycine hydrated by three water molecules are computed using DFT and ab initio methods at high levels of theory. The three water molecule cluster yields a zwitterion minimum of about the same energy as the neutral form. The transfer barrier and the GZ–GN energy difference are strongly sensitive to the correlation effects. The solvent effect on the unhydrated and the trihydrated proton transfer surfaces are treated using a continuum model. As modeled in water, the solvent stabilizes the zwitterionic cis conformation of glycine with regard to the neutral cis form. The free energy stabilization of GZ( cis ) over the GN( cis ) form is 5.4 kcal mol −1 for the solvated trihydrated complex compared to an experimental value of 7 kcal mol −1 . Also computed is the small free energy barrier of 2.2 kcal mol −1 for the conversion of GN( cis ) to GZ( cis ). Rationalization of why this barrier persists at all levels of calculation is found in the fact that the solvent effect only becomes important when the structure is close to the zwitterionic configuration.
TL;DR: In this paper, the results of ab initio calculations for cyclic clusters of methanol, ethanol, 1-propanol, and methanethiol are presented.
Abstract: The results of ab initio calculations for cyclic clusters of methanol, ethanol, 1-propanol, and methanethiol are presented. Dimer, trimer, and tetramer clusters of all four compounds are studied, as are pentamer and hexamer clusters of methanol. From optimized clusters at HG/6--31G**, total energies and binding energies were calculated with both the HF and MP2 theories using the aug-cc-pVDZ basis set. Accurate binding energies were also calculated for the dimer and trimer of methanol using symmetry-adapted perturbation theory with the same basis set. Intermolecular and intramolecular distances, charge distribution of binding sites, binding energies, and equilibrium constants were computed to determine the hydrogen bond cooperativity effect for each species. The cooperativity effect, exclusive to hydrogen bonding systems, results form specific forces among the molecules, in particular charge-transfer processes and the greater importance of interactions between molecules not directly hydrogen bonded because of the longer range of the interactions. The ratios of equilibrium constants for forming multimer hydrogen bonds to that for dimer hydrogen bond formation increase rapidly with the cluster size, in contrast to the constant value commonly used in thermodynamic models for hydrogen bonding liquids.
TL;DR: In this paper, the dynamics and mechanisms of radiationless transitions from higher excited electronic states as well as the ultrafast intramolecular vibronic relaxation in porphyrin derivatives were elucidated.
Abstract: To elucidate the dynamics and mechanisms of radiationless transitions from higher excited electronic states as well as the ultrafast intramolecular vibronic relaxation in porphyrin derivatives, we ...
TL;DR: In this article, the intramolecular cyclization in hyperbranching polycondensation reactions of the type AB2/Bf was investigated both theoretically and experimentally.
Abstract: The intramolecular cyclization in hyperbranching polycondensation reactions of the type AB2/Bf was investigated both theoretically and experimentally. Hyperbranched aliphatic polyesters prepared from 2,2-bis(hydroxymethyl)propionic acid (BisMPA) and 1,1,1-tris(hydroxymethyl)propane (TMP) as B3 core molecule as well as commercially available samples with ethoxylated pentaerythrol as B4 core (Perstorp, Boltorn®) have been investigated with a combination of methods (NMR, vapor pressure osmometry (VPO), size exclusion chromatography (SEC), MALDI-TOF-MS) with respect to the formation of intramolecular cycles during polymerization. Absolute molecular weights were determined for the first time and found to be in the range of 632 (G2) to 1046 g/mol (G5) and 827 (PG2) to 1974 g/mol (PG5) for the commercial samples, respectively, i. e. considerably lower than reported previously. Theoretical considerations as well as experimental data clearly demonstrate cyclization to be an important reaction regardless of the presence of a core. The fraction if cyclized molecules was found to increase with molecular weights from 32% to 48%, limiting molecular weights drastically. In summary, previously reported results could not be confirmed. Incremental addition of the BisMPA monomer to the TMP-core did not permit to control molecular weights and the term “pseudogeneration” is inappropriate for the low molecular weight materials formed.
TL;DR: A variety of 2-aryl-2H-indazoles were synthesized by the palladium-catalyzed intramolecular amination of the corresponding N- Daryl-N(o-bromobenzyl)hydrazines using the combination of Pd(OAc)2/dppf/tBuONa.