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Showing papers on "Intramolecular force published in 1996"


Journal ArticleDOI
TL;DR: In this paper, various aspects of molecular solvation are reviewed from the perspective provided by gas-phase aromatic solute-(solvent)n clusters, including hydrogen bonding to aromatic alcohols, water complexation to indole and its derivatives, and the hydrogen-bonded networks of benzene-(H2O)n and benzene(CH3OH)n).
Abstract: ▪ Abstract Various aspects of molecular solvation are reviewed from the perspective provided by gas-phase aromatic solute-(solvent)n clusters. Particular emphasis is placed on hydrogen-bonded clusters, varying from 1:1 aromatic-H2O complexes up to clusters containing several water or methanol molecules. Recent advances in experimental methods for obtaining accurate structures, binding energies, and intermolecular and intramolecular vibrational spectra are highlighted. Many of these methods provide size and conformation selectivity and can be readily extended to both ground and electronically excited neutral states. The π hydrogen bond, hydrogen bonding to aromatic alcohols, water complexation to indole and its derivatives, and the hydrogen-bonded networks of benzene-(H2O)n and benzene-(CH3OH)n clusters are reviewed in special detail.

470 citations


Journal ArticleDOI
TL;DR: Double-mutant cycle analysis can be used to measure the strength of intramolecular and intermolecular pairwise interactions in proteins or protein-ligand complexes with known structure and to characterize structures that are inaccessible to NMR and X-ray crystallography.

295 citations


Journal ArticleDOI
TL;DR: Experimental studies discussed here are consistent and compelling: hydrogen bonding stabilizes globular proteins.

290 citations


Journal ArticleDOI
TL;DR: A family of novel oligomers based on the anthranilamide nucleus has been prepared and shown to form well-defined secondary structural features as mentioned in this paper, and 1H NMR and X-ray crystallographic techniques have demonstrated that intramolecular hydrogen bonds play a key role in stabilizing linear sheet and helical conformational forms.
Abstract: A family of novel oligomers based on the anthranilamide nucleus has been prepared and shown to form well-defined secondary structural features. 1H NMR and X-ray crystallographic techniques have demonstrated that intramolecular hydrogen bonds play a key role in stabilizing both linear sheet and helical conformational forms.

246 citations


Journal ArticleDOI
TL;DR: The first asymmetric intramolecular Stetter reaction using the chiral triazolium salt as catalyst was reported in this paper, starting from the easily accessible 4-(2-formylphenoxy)but-2-enoates 2.
Abstract: The first asymmetric intramolecular Stetter reaction is reported, using the chiral triazolium salt 1 as catalyst. Starting from the easily accessible 4-(2-formylphenoxy)but-2-enoates 2, this protocol opens up an enantioselective pathway to the benzo-annulated pyran-4-ones (chroman-4-ones) 3a–h with good yields and enantiomeric excesses of up to 74%.

235 citations


Journal ArticleDOI
TL;DR: In this article, aryl halides with pendant amides or sulfonamides are cyclized to form five and six-membered rings, and a similar process can be applied to a ring with pendants.

228 citations


Journal ArticleDOI
TL;DR: The relative energies of H and D bonds are due to differences in zero-point vibrational energy (ZPVE) as discussed by the authors, which can be traced to one particular vibrational mode, the one which displaces the bridging atom away from the O-O axis.
Abstract: The relative energies of H and D bonds are due to differences in zero-point vibrational energy (ZPVE). Ab initio calculations are used to assess the changes in this quantity that accompany all possible substitutions of protium by deuterium in a number of complexes. The ZPVE of the D bond is lower than that of the H bond in the neutral dimer and trimer of water. This difference can be traced to one particular vibrational mode, the one which displaces the bridging atom away from the O···O axis. The heavier mass of D lowers the frequency, and hence the ZPVE associated with it. The situation reverses itself in ionic H bonds. The total ZPVE of the (H2O··H··OH2)+ complex is higher when a D occupies the bridging position, as compared to a terminal site. This difference is attributed to the intramolecular modes. Although replacement of the central H by D reduces the intermolecular ZPVE, the reduction of the intramolecular ZPVE is even larger when the substitution is made at a peripheral atom, so a D would tend to...

221 citations


Journal ArticleDOI
TL;DR: In this paper, a practical procedure (FUERZA) to obtain internal force constants from Cartesian second derivatives (Hessians) is presented and discussed, which allows a systematic analysis of pair atomic interactions in a molecular system, and it is fully invariant to the choice of internal coordinates of the molecule.
Abstract: A practical procedure (FUERZA) to obtain internal force constants from Cartesian second derivatives (Hessians) is presented and discussed. It allows a systematic analysis of pair atomic interactions in a molecular system, and it is fully invariant to the choice of internal coordinates of the molecule. Force constants for bonds or for any pair of atoms in general are defined by means of the eigenanalysis of their pair interaction matrix. Force constants for the angles are obtained from their corresponding two-pair interaction matrices of the two bonds or distances forming the angle, and the dihedral force constants are similarly obtained using their corresponding three-pair interaction matrices. © 1996 John Wiley & Sons, Inc.

210 citations


Journal ArticleDOI
TL;DR: In this paper, a series of 6 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, P3C to P8C, was studied by using photostationary and time-resolved fluorescence measurements.
Abstract: The fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles is discussed. The magnitude of the energy gap between the two lowest excited states is shown to determine the occurrence or absence of ICT. The excited state behaviour of a series of six 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, P3C to P8C, is studied by using photostationary and time-resolved fluorescence measurements. The ICT rate constant strongly decreases with decreasing ring size. ICT does not occur with P3C and P4C in diethyl ether. This is attributed to the increase of the amino nitrogen inversion barrier with decreasing ring size. The configurational change of the amino nitrogen from pyramidal to planar is considered to be an important reaction coordinate in the ICT process. The photophysics of the 4-aminobenzonitriles is different from that of other systems such as donor/acceptor-substituted stilbenes and 9,9′-bianthryl, which are governed by the charge distribution and macroscopic Coulombic interaction in their CT states.

207 citations


Journal ArticleDOI
TL;DR: In this paper, a series of monoesters of dicarboxylic acids was used to calculate the probability of NAC formation and the mole fraction (probability, P) of each ester.
Abstract: Intramolecular and enzymatic reactions require conformations in which the orbitals of reactants are properly aligned and van der Waals surfaces are in a near-attack conformation (NAC). Much of the changes in steric and electrostatic energies that take place in moving from the favored ground state conformation (FGSC) to the transition state are accounted for when the NAC has been assumed. Upon assessing the steric, electrostatic, and zero point energies, one is in position to calculate the probability of NAC formation. Using a series of monoesters of dicarboxylic acids, we have (i) created, via stochastic search, 10 000−40 000 minimized conformations per ester; (ii) chosen only the unique local minima conformations for each ester; (iii) determined the energies of each minimum conformation using MM3(92); (iv) identified the geometry of the NACs where the van der Waals overlap has not begun for the approach of carboxyl anion and ester carbonyl; and (v) calculated the mole fraction (probability, P) of each es...

188 citations


Journal ArticleDOI
TL;DR: In this article, the authors argue for turnover-limiting C⋮C/CC insertion into a Ln−N bond, followed by protonolysis of the resulting ln−C bond.

Journal ArticleDOI
TL;DR: The results of experiments designed to probe the mechanism by which the bis(μ-oxo)dicopper complexes decompose to products arising from macrocyclic ligand N-dealkylation are described in this paper.
Abstract: The results of experiments designed to probe the mechanism by which the bis(μ-oxo)dicopper complexes [(LCu)2(μ-O)2](ClO4)2 (L = 1,4,7-tribenzyl-, 1,4,7-triisopropyl-, and 1,4-diisopropyl-7-benzyl-1,4,7-triazacyclononane ligands; LBn3, LiPr3, and LiPr2Bn, respectively) decompose to products arising from macrocyclic ligand N-dealkylation are described. After removal of copper from the decomposed solutions, analysis of the organic products revealed N-dealkylated ligands and aldehyde or ketone, the oxygen atoms in the latter being derived from the bis(μ-oxo)dicopper core as shown by 18O-isotope labeling experiments. Thus, the overall N-dealkylation is akin to monooxygenase reactions carried out by various metalloenzymes such as cytochrome P450, dopamine β-monooxygenase, and peptidyl glycine α-amidating monooxygenase. Direct, intramolecular attack of the bis(μ-oxo)dicopper core at a ligand substituent C−H bond during the rate-determining step was indicated by the observed first-order kinetics, the results of H...

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the behavior of 2-(2'-hydroxyphenyl)benzimidazole (HBI) in aqueous and ethanolic solutions of HBI by means of UV-vis absorption and fluorescence spectroscopy.
Abstract: Excited-state proton transfer in aqueous and ethanolic solutions of 2-(2‘-hydroxyphenyl)benzimidazole (HBI) was investigated by means of UV−vis absorption and fluorescence spectroscopy. The behavior of HBI in water differed from its behavior in ethanol, and in both solvents fluorescence behavior depended on acidity. In both neutral water and neutral ethanol, ground-state HBI exhibits conformational equilibrium between a cis-enol form with an intramolecular hydrogen bond and a trans-enol form that is hydrogen-bonded to the solvent; the ground-state keto tautomer is also present in water but was not detected in ethanol. The excited cis-enol conformer always undergoes ultrafast intramolecular proton transfer to afford the excited keto tautomer. The excited trans-enol form fluoresces in both solvents, and in water it also loses its hydroxyl proton to the solvent, leaving the excited anion. In both acidic aqueous solution and acidic ethanol, excited protonated HBI loses its hydroxyl proton to give the excited ...

Journal ArticleDOI
TL;DR: A protein fluorescence probe system, coupling excited-state intermolecular Förster energy transfer and intramolecular proton transfer (PT), is presented and it is demonstrated that two binding sites are involved in the human serum albumin-3-HF interaction.
Abstract: A protein fluorescence probe system, coupling excited-state intermolecular Forster energy transfer and intramolecular proton transfer (PT), is presented. As an energy donor for this system, we used tryptophan, which transfers its excitation energy to 3-hydroxyflavone (3-HF) as a flavonol prototype, an acceptor exhibiting excited-state intramolecular PT. We demonstrate such a coupling in human serum albumin–3-HF complexes, excited via the single intrinsic tryptophan (Trp-214). Besides the PT tautomer fluorescence (λmax = 526 nm), these protein–probe complexes exhibit a 3-HF anion emission (λmax = 500 nm). Analysis of spectroscopic data leads to the conclusion that two binding sites are involved in the human serum albumin–3-HF interaction. The 3-HF molecule bound in the higher affinity binding site, located in the IIIA subdomain, has the association constant (k1) of 7.2 × 105 M−1 and predominantly exists as an anion. The lower affinity site (k2 = 2.5 × 105 M−1), situated in the IIA subdomain, is occupied by the neutral form of 3-HF (normal tautomer). Since Trp-214 is situated in the immediate vicinity of the 3-HF normal tautomer bound in the IIA subdomain, the intermolecular energy transfer for this donor/acceptor pair has a 100% efficiency and is followed by the PT tautomer fluorescence. Intermolecular energy transfer from the Trp-214 to the 3-HF anion bound in the IIIA subdomain is less efficient and has the rate of 1.61 × 108 s−1, thus giving for the donor/acceptor distance a value of 25.5 Å.

Journal ArticleDOI
TL;DR: In this paper, the C−C triple bond was cleaved in the presence of the carbonyl complex with the formation of the toluene and the 1-alkyne to vinylidene tautomerism.
Abstract: The hydration of phenylacetylene in the presence of the complex mer,trans-(PNP)RuCl2(PPh3) in THF at 60 °C leads to the cleavage of the C−C triple bond with formation of the carbonyl complex fac,cis-(PNP)RuCl2(CO) and toluene [PNP = CH3CH2CH2N(CH2CH2PPh2)2]. A study under different experimental conditions, the use of model and isotope labeling experiments, and the detection of several intermediates, taken altogether, show that the C−C bond cleavage reaction comprises a number of steps, among which the most relevant to the mechanism are 1-alkyne to vinylidene tautomerism, conversion of a vinylidene ligand to hydroxycarbene by intramolecular attack of water, deprotonation of hydroxycarbene to σ-acyl, deinsertion of CO from the acyl ligand, and hydrocarbon elimination by protonation of the metal-alkyl moiety. The following intermediate species have been isolated and characterized: the vinylidene fac,cis-(PNP)RuCl2{CC(H)Ph}, the (aquo)(σ-alkynyl) complex fac-(PNP)RuCl(C⋮CPh)(OH2), and the (benzyl)carbonyl me...

Journal ArticleDOI
TL;DR: In this paper, an ab initio study of specific and non-specific solvent effects on the glycine zwitterion energetics and formation mechanism is presented, which is carried out at the HF an MP2 levels with the 6-31 + G ∗∗ basis set.

Journal ArticleDOI
TL;DR: In this article, an intramolecular charge transfer (CT) was used to inhibit the fluorescence and laser emission in the derivatives with an amino terminal group in the side chain, and a high lasing efficiency in the green-blue zone of the spectrum was obtained with a N 2 pulsed laser.

Journal ArticleDOI
TL;DR: In this paper, a rod coil copolymer containing 2-(2'- hydroxyphenyl)benzothiazole in the backbone was used to induce a reversible intramolecular proton transfer.
Abstract: : Application of a voltage across a thin film of a rod coil copolymer containing 2-(2'- hydroxyphenyl)benzothiazole in the backbone induces a reversible intramolecular proton transfer. The observed electroluminescence occurs from the excited keto form, produced electrically by a chemical reaction termed electrically generated intramolecular proton transfer, EGIPT. Stimulated emission between 540-640 nm was observed by Ps transient absorption spectroscopy. EGIPT and related proton transfer electroluminescence have implications for understanding proton transfer reactions and organic light emitting devices.

Journal ArticleDOI
TL;DR: In this article, the equilibrium for formation of the intramolecular hydrogen bond (KHB) in a series of substituted salicylate monoanions was investigated as a function of delta pKa, the difference between the pKa values of the hydrogen bond donor and acceptor, in both water and dimethyl sulfoxide.
Abstract: The equilibrium for formation of the intramolecular hydrogen bond (KHB) in a series of substituted salicylate monoanions was investigated as a function of delta pKa, the difference between the pKa values of the hydrogen bond donor and acceptor, in both water and dimethyl sulfoxide. The dependence of log KHB upon delta pKa is linear in both solvents, but is steeper in dimethyl sulfoxide (slope = 0.73) than in water (slope = 0.05). Thus, hydrogen bond strength can undergo substantially larger increases in nonaqueous media than aqueous solutions as the charge density on the donor or acceptor atom increases. These results support a general mechanism for enzymatic catalysis, in which hydrogen bonding to a substrate is strengthened as charge rearranges in going from the ground state to the transition state; the strengthening of the hydrogen bond would be greater in a nonaqueous enzymatic active site than in water, thus providing a rate enhancement for an enzymatic reaction relative to the solution reaction. We suggest that binding energy of an enzyme is used to fix the substrate in the low-dielectric active site, where the strengthening of the hydrogen bond in the course of a reaction is increased.

Journal ArticleDOI
TL;DR: In this article, an intermediate species formed by intramolecular proton transfer was used to obtain a low-quantum-yield fluorescence in copolymers of the MA-TIN copolymer.
Abstract: Various copolymers of MA-TIN 1, 2-[2-hydroxy-3-tert-butyl-5-(O-[2-hydroxy-3-(2-methylpropenoyloxy)propyl]-2-carbonyloxyethyl)phenyl]benzotriazole, and MA-TZ 1, 2,4-bis(2,4-dimethylphenyl)-6-[2-hydroxy-4-(2-hydroxy-3-[2-methylpropenoyloxy])propoxyphenyl]-1,3,5-triazine, with styrene, methyl methacrylate, and methacrylic acid have been synthesized by radical polymerization. Their absorption spectra in the long-wavelength UV region appear unchanged compared to those of the monomeric UV absorbers, indicating the stabilizer chromophore to remain unimpaired in the course of the polymerization. Both the monomeric and the polymeric stabilizers exhibit a strongly Stokes-shifted, temperature-dependent, low-quantum-yield fluorescence which arises from an intermediate species formed by intramolecular proton transfer. The intramolecular hydrogen bond which is essential for the photostability of this type of UV absorbers thus is still intact in the copolymers. Activation energies for the radiationless deactivation proc...

Journal ArticleDOI
TL;DR: In this article, the interaction of free electrons in the energy range between 0 and 15 eV with molecules is reviewed, including beam experiments with gas phase molecules under collision free conditions and at higher pressures, homogeneous and heterogeneous clusters generated in supersonic beams, and molecules adsorbed and condensed on a cold metallic surface.

Journal ArticleDOI
TL;DR: In this paper, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile solution was determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl−hemipinacol biradical.
Abstract: Nanosecond laser flash photolysis studies have been carried out of the kinetics of inter- and intramolecular phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone, and 4-alkoxybenzophenone triplets in acetonitrile and benzene solution. Information on the geometric requirements for abstraction by carbonyl n,π* and π,π* triplets is derived from the results for a series of ketones which contain a para-phenolic moiety attached via a para-oxyethyl linkage. For all of these compounds, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile solution indicates that triplet decay is determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl−hemipinacol biradical. The biradicals have also been detected, and are about an order of magnitude longer-lived than the triplet in each case. For three of the compounds, the rates of the intramolecular process follow the same trend as that observed in the rates of bimolecula...


Journal ArticleDOI
TL;DR: The first was constructed by intermolecular coupling of an indole anion with a 1,2-anhydrosugar derived from an endo-glycal by direct epoxidation as discussed by the authors.
Abstract: The total syntheses of staurosporine and ent-staurosporine have been achieved. Both glycosidic bonds were built from glycal precursors. The first was constructed by intermolecular coupling of an indole anion with a 1,2-anhydrosugar derived from an endo-glycal by direct epoxidation. The second bond was assembled from an exo-glycal by intramolecular iodoglycosylation.


Journal ArticleDOI
TL;DR: In this article, the authors quantitatively modeled the triiodide/solvent system using two intramolecular modes (the symmetric and antisymmetric stretches) and one low-frequency intermolecular or solvent mode.
Abstract: Resonance Raman spectra, including absolute cross sections, have been measured for the triiodide ion in ethanol, ethyl acetate, and acetonitrile solvents using eight excitation wavelengths that span the two lowest absorption bands. Preresonance Raman spectra have also been obtained with 488 nm excitation. The apparent vibrational line widths observed on resonance are approximately twice the preresonant line widths in all three solvents. By quantitatively modeling the triiodide/solvent system using two intramolecular modes (the symmetric and antisymmetric stretches) and one low-frequency intermolecular or solvent mode, we are able to reproduce the vibrational broadening on resonance, which, within the context of our model, is due to unresolved combination bands between the intramolecular degrees of freedom and the intermolecular mode. The Raman spectra in ethanol clearly show one-quantum transitions in the antisymmetric stretching mode (ν3) which should be symmetry forbidden in the linear D∞h geometry, ind...

Journal ArticleDOI
TL;DR: Low-barrier hydrogen bonding of Tyr-14 to the intermediate, in conjunction with the previously demonstrated tunneling contribution to the proton transfer by Asp-38, provide a plausible and quantitative explanation for the high catalytic power of this isomerase.
Abstract: Delta 5-3-Ketosteroid isomerase (EC 5.3.3.1) promotes an allylic rearrangement involving intramolecular proton transfer via a dienolic intermediate. This enzyme enhances the catalytic rate by a factor of 10(10). Two residues, Tyr-14, the general acid that polarizes the steroid 3-carbonyl group and facilitates enolization, and Asp-38 the general base that abstracts and transfers the 4 beta-proton to the 6 beta-position, contribute 10(4.7) and 10(5.6) to the rate increase, respectively. A major mechanistic enigma is the huge disparity between the pKa values of the catalytic groups and their targets. Upon binding of an analog of the dienolate intermediate to isomerase, proton NMR detects a highly deshielded resonance at 18.15 ppm in proximity to aromatic protons, and with a 3-fold preference for protium over deuterium (fractionation factor, phi = 0.34), consistent with formation of a short, strong (low-barrier) hydrogen bond to Tyr-14. The strength of this hydrogen bond is estimated to be at least 7.1 kcal/mol. This bond is relatively inaccessible to bulk solvent and is pH insensitive. Low-barrier hydrogen bonding of Tyr-14 to the intermediate, in conjunction with the previously demonstrated tunneling contribution to the proton transfer by Asp-38, provide a plausible and quantitative explanation for the high catalytic power of this isomerase.

Journal ArticleDOI
TL;DR: In this paper, a series of 2-hydroxybenzoyl compounds (Ph(OH)(COY): Y = Cl, OH, SH, OCH3, SCH3, H, CH3, Ph, N(CH(CH3)CH2)2 CH2, NCH2CH2), NCH 2CH2, CH2CH 2, CH 2 CH 2, NH2, NH3, NHNH2, and NHNH 2) are reported and discussed.
Abstract: IR gas-phase and quantum chemical data that characterize the intramolecular hydrogen bonds in a series of 2-hydroxybenzoyl compounds (Ph(OH)(COY): Y = Cl, OH, SH, OCH3, SCH3, H, CH3, Ph, N(CH(CH3)CH2)2CH2, N(CH2CH2)2CH2, N(CH3)2, NHCH3, NH2, NHNH2) are reported and discussed. The sequences of ν(OH) and of ν(CO) frequencies calculated at HF/3-21G, HF/6-31G**, and B3LYP/6-31G** computational levels agree reasonably well with the corresponding experimental sequences. The consistency of the calculated data is demonstrated by exploiting several relations between calculated frequencies, force constants, and bond distances. It is shown that the sequence of hydrogen bond strengths, as defined either by ν(OH) frequency shifts or by R(O−H) bond distances, may qualitatively be interpreted in terms of two quantities, the R(H···O) hydrogen bond distances and the Q(O) oxygen partial charges, which characterize the intrinsic differences between the carbonyl groups.

Journal ArticleDOI
TL;DR: In this paper, the synthesis of three novel compounds of norfloxacin (nf) was reported and discussed, and their crystal structures are presented and discussed in the monoclinic space group P 21/c with a = 8.438(2), b = 14.281(3), c = 15.012(3).

Journal ArticleDOI
TL;DR: In this paper, the reaction mechanism for Cu[9]aneN3Cl2-catalyzed hydrolysis of ethyl 4-nitrophenyl phosphate was probed using kinetic isotope effects and isotope exchange experiments.
Abstract: The reaction mechanism for Cu[9]aneN3Cl2-catalyzed hydrolysis of ethyl 4-nitrophenyl phosphate was probed using kinetic isotope effects and isotope exchange experiments. The solvent deuterium isotope effect (Dk = 1.14), combined with the absence of 18O incorporation into 4-nitrophenol, suggests that hydrolysis proceeds through intramolecular attack of the metal-coordinated hydroxide at the phosphorus center. The secondary 15N isotope effect (15k = 1.0013 ± 0.0002) implies that loss of the leaving group occurs at the rate-limiting step with approximately 50% bond cleavage in the transition state. This study is one of the first applications of the secondary 15N isotope effect to simple metal-promoted hydrolysis reactions, and the result is consistent with concerted bond formation and cleavage. A mechanism consistent with the isotope studies is presented.