Showing papers on "Intramolecular force published in 1997"
TL;DR: Dextran filaments linked to a gold surface were probed with the atomic force microscope tip by vertical stretching and the conformational change was found to be reversible and was corroborated by molecular dynamics calculations.
Abstract: Recent developments in piconewton instrumentation allow the manipulation of single molecules and measurements of intermolecular as well as intramolecular forces. Dextran filaments linked to a gold surface were probed with the atomic force microscope tip by vertical stretching. At low forces the deformation of dextran was found to be dominated by entropic forces and can be described by the Langevin function with a 6 angstrom Kuhn length. At elevated forces the strand elongation was governed by a twist of bond angles. At higher forces the dextran filaments underwent a distinct conformational change. The polymer stiffened and the segment elasticity was dominated by the bending of bond angles. The conformational change was found to be reversible and was corroborated by molecular dynamics calculations.
1,079 citations
TL;DR: In this article, single-molecule fluorescence spectroscopy of a multichromophoric conjugated polymer (molecular weight ∼20,000) revealed surprising single-step photobleaching kinetics and acute jumps in fluorescence intensity.
Abstract: Single-molecule fluorescence spectroscopy of a multichromophoric conjugated polymer (molecular weight ∼20,000) revealed surprising single-step photobleaching kinetics and acute jumps in fluorescence intensity. These jumps were shown not to result from spectral diffusion and were attributed to fluctuations in the quantum yield of emission for the molecules. The data indicate efficient intramolecular electronic energy transfer along the polymer chain to a localized fluorescence-quenching polymer defect. The defects are created by reversible photochemistry of the polymer. These findings have implications for the use of conjugated polymers in light-emitting diode displays and sensors.
508 citations
TL;DR: In this paper, the synthesis, characterization and spectroscopic properties of the compound Eu(thenoyltrifluoroacetonate), 2(dibenzyl sulfoxide) which is highly luminescent under UV excitation are described.
389 citations
TL;DR: In this paper, the vibrational cooling of azobenzene after photoisomerization was investigated by time resolved IR spectroscopy with femtosecond time resolution, and the experimental data were discussed in terms of a simple theoretical model which was derived in order to account for the off-diagonal anharmonicity between the investigated highfrequency modes and the bath of the remaining low-frequency modes in polyatomic molecules.
Abstract: The vibrational cooling of azobenzene after photoisomerization is investigated by time resolved IR spectroscopy with femtosecond time resolution. Transient difference spectra were obtained in a frequency range where phenyl ring modes and the central N=N-stretching mode absorbs. The experimental data are discussed in terms of a simple theoretical model which was derived in order to account for the off-diagonal anharmonicity between the investigated high-frequency modes and the bath of the remaining low-frequency modes in a polyatomic molecule. It is shown that these off-diagonal anharmonic constants dominate the observed transient absorbance changes while the anharmonicity of the high-frequency modes themselves (diagonal anharmonicity) causes only minor effects. Based on the transient IR spectra, the energy flow in the azobenzene molecule can be described as follows: After an initial ultrafast intramolecular energy redistribution process, the decay of the related intramolecular temperature occurs via intermolecular energy transfer to the solvent on a time scale of ca. 20 ps.
342 citations
TL;DR: In this paper, the authors developed a method for determining experimentally the charges of BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene, also ET) molecules.
298 citations
TL;DR: In this paper, the preferred geometry and interaction strength for O-H, N-H and S-H interactions with phenyl rings was investigated through searches of the Cambridge Structural Database, combined with semi-empirical and abinitio molecular orbital calculations.
Abstract: X–H···π (phenyl) is the interaction of a hydrogen atom with the π-system of an aromatic ring in either an intermolecular or an intramolecular fashion. Although the existence of these types of interaction has gained recent attention in the literature, the geometry is poorly understood. This paper attempts to investigate the preferred geometry and interaction strength for O–H, N–H, N–H+, S–H, s C–H and s C–H interactions with phenyl rings. This has been done through searches of the Cambridge Structural Database, combined with semi-empirical and abinitio molecular orbital calculations. It is found that the classical image of T-shaped geometry is rarely adopted and that the preferred geometry involves direct interaction of X–H with the carbon atoms of the phenyl ring. The binding energy associated with the interaction decreases with the electronegativity of the donor atom X.
297 citations
TL;DR: The intermolecular interactions between X−H groups (X = C, N, and O) and metal atoms in crystalline organometallic complexes have been investigated in this article.
292 citations
TL;DR: In this article, photoinduced formation of a nonfluorescent tautomeric form 8-HQ(T*) was investigated in both water and alkane solvents, and it was shown that biprotonic concerted proton transfers are then expected to occur within the dimer upon excitation.
Abstract: 8-Hydroxyquinoline (8-HQ), referred as to oxine in analytical chemistry, is a fluorogenic ligand. Its lack of fluorescence in water and alkanes, and its low quantum yield in many other organic solvents, are rationalized in the present study in terms of photoinduced formation of a nonfluorescent tautomeric form 8-HQ(T*). In water, intermolecular proton transfers with surrounding water molecules are expected, but intrinsic intramolecular proton transfer between the −OH and ⩾N functions cannot be ruled out because the presence of a weak internal H bond can be inferred from the ground-state properties of 8-HQ such as pKa values or solubility. In organic solvents, vapor pressure osmometry measurements in conjunction with infrared spectra allow us to show that (i) in alkane solvents, a very stable dimer is formed in the ground state (Kdim = 7 × 107 at 25 °C); biprotonic concerted proton transfers are then expected to occur within the dimer upon excitation, as was previously reported for 7-azaindole; (ii) in chl...
231 citations
TL;DR: In this article, a two-dimensional fifth-order Raman spectroscopy has been used to probe nonlinear interactions between well defined vibrational motions in liquids, revealing the nonlinear dependence of the molecular polarizability on vibrational coordinates, intermolecular interaction-induced effects, and anharmonic couplings between modes.
Abstract: Two-dimensional fifth-order Raman spectroscopy has the ability to probe nonlinear interactions between well defined vibrational motions in liquids. It can reveal the nonlinear dependence of the molecular polarizability on vibrational coordinates, intermolecular interaction-induced effects, and anharmonic couplings between modes. We use this technique to probe these interactions at an intramolecular level in liquid CC${\mathrm{l}}_{4}$ and CHC${\mathrm{l}}_{3}$, and at an intermolecular level in a mixture of these two liquids.
187 citations
TL;DR: In this article, the vibrational spectra of mixed valence systems that contained one carbon monoxide ligand on each triruthenium cluster were used to characterize the infrared spectral band shape.
Abstract: Single-electron reductions of linked triruthenium clusters of the general type Ru3-pyrazine-Ru3 produced mixed valence systems showing spectroscopic characteristics of rapid intramolecular electron transfer. Reflectance infrared spectroelectrochemistry was used to characterize the vibrational spectra of mixed valence systems that contained one carbon monoxide ligand on each Ru3 cluster. Infrared spectra in the CO stretching region showed two resolved, partially coalesced, and coalesced ν(CO) bands for clusters with rate constants for intramolecular electron transfer k e increasing from = 1 × 109 s– 1 up to 5 × 1011 and 9 × 1011s– 1, respectively. These data provide a strong correlation between rates of intramolecular electron transfer and infrared spectral bandshape.
181 citations
TL;DR: In this article, a discussion is presented of the fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles.
Abstract: A discussion is presented of the fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles. The occurrence or absence of this ICT reaction is shown to be determined by the magnitude of the energy gap ΔE (S 1 ,S 2 ) between the two lowest excited states. The strong decrease of the ICT efficiency in the series P8C to P3C of 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, is attributed to the increase with decreasing ring size of the barrier for the configurational change of the amino nitrogen from pyramidal to planar, which is considered to be an important reaction coordinate in the ICT process. Dual fluorescence does not occur with P3C nor with NC in diethyl ether. Application of a pressure of 6000 bar leads to a threefold increase of the CT/LE fluorescence quantum yield ratio for the 4(dialkylamino)benzonitrile DMABN (methyl) in diethyl ether, whereas this ratio only slightly increases for DDABN ( n -decyl) in n -hexane. This difference is attributed to the increase of the solvent dielectric constant with pressure, which effect is much larger for diethyl ether than for n-hexane. These results mean that a large amplitude motion does not occur during the ICT reaction of DMABN and DDABN, in accordance with the model of planar intramolecular charge transfer for dual fluorescence. Dual fluorescence is observed with the rigidized aminobenzonitrile NMC7, showing that a full perpendicular twist of the amino group is not essential for the CT state. I -(Dimethylamino) naphthalene ( I DMAN) undergoes a fast thermally activated internal conversion (IC), for which the activation energy increases with solvent polarity. This IC reaction depends on the magnitude of the ΔE (S 1 ,S 2 ) energy gap and is similar in this respect to the ICT reaction of DMABN.
TL;DR: The current status of absolute asymmetric synthesis in a chiral crystalline environment has been reviewed in this article, where a number of topochemically controlled four-center type photocycloadditions are described for a series of unsymmetrically substituted diolefin crystals and CT crystals.
Abstract: The current status of absolute asymmetric synthesis in a chiral crystalline environment has been reviewed. A number of topochemically controlled four-center type photocycloadditions are described for a series of unsymmetrically substituted diolefin crystals and CT crystals. This concept has been applied to intramolecular photoreactions, and several successful absolute asymmetric syntheses have been achieved, involving Norrish Type II reaction, di-π-methane rearrangement, electrocyclization, thietane formation, oxetane formation, hydrogen abstraction by thiocarbonyl and alkenyl groups, and radical-pair intermediates.
TL;DR: In this paper, the response kernel of some organic molecules including pyrazine, pyrazinyl radical, acetone, and 2-hydroxypropyl radical were calculated along the present formulation.
Abstract: We performed ab initio molecular orbital (MO) calculations of the response kernel (∂Qa/∂Vb), which represents the response of the intramolecular charge polarization by external electrostatic field, on the basis of the coupled perturbed Hartree−Fock equation. The response kernels of some organic molecules including pyrazine, pyrazinyl radical, acetone, and 2-hydroxypropyl radical were calculated along the present formulation. The results revealed that the hydrogen abstraction of pyrazine causes the product radical to be remarkably deformable in the partial charge distribution, while the hydrogen abstraction of acetone does not induce such enhancement of the charge sensitivity. The augmented sensitivity does not appear in the usual polarizability for a uniform field but emerges for a local fluctuated field. To elucidate the remarkable difference, we performed the normal mode analysis and decomposition based on the intrinsic soft MO pairs or localized orbitals. As a result, the enhancement in the aromatic sp...
TL;DR: In this paper, the relative strengths of normal and low-barrier hydrogen bonds (LBHBs) in the gas phase were analyzed by means of quantum-mechanical and thermodynamic calculations on the mesaconic/citraconic and several maleic/fumaric cis/trans isomerization equilibria.
Abstract: Relative strengths of normal and low-barrier hydrogen bonds (LBHBs) in the gas phase were analyzed by means of quantum-mechanical and thermodynamic calculations on the mesaconic/citraconic and several maleic/fumaric cis/trans isomerization equilibria. All geometries were fully optimized with correlation effects included via second-order Moller−Plesset perturbation theory. The cis isomer of the maleic monoanion (also known as hydrogen maleate) is greatly stabilized in the gas phase owing to the formation of an intramolecular low-barrier hydrogen bond more than 20 kcal/mol stronger, in free energy terms, than the corresponding normal intramolecular hydrogen bond in maleic diacid. The very short internuclear distance (2.41 A) obtained at the MP2 level between the hydrogen donor and the hydrogen acceptor in hydrogen maleate, as well as the high value of the NMR chemical shift for the participating proton, are two other characteristics experimentally attributed to the formation of an LBHB. The transition state...
TL;DR: In this paper, two phases of HQNN (α- and β-HQNN) were obtained and the crystal structure was characterized by the intramolecular hydrogen bond between the o-hydroxy group and the nitronyl nitroxide and by the intermolecular one-dimensional hydrogen-bonded chain between the O- and p-hydrox groups of the neighboring molecules, respectively.
Abstract: Nitronyl nitroxide derivatives carrying a hydroquinone or a resorcinol moiety 2-(2‘,5‘-dihydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide (HQNN) and 2-(3‘,5‘-dihydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl 3-oxide (RSNN)) have been designed and prepared. These compounds were found to afford hydrogen-bonded crystals. In the case of HQNN, two phases of crystals (α- and β-HQNN) were obtained. Of these, the crystal structure of α-HQNN is characterized by the intramolecular hydrogen bond between the o-hydroxy group and the nitronyl nitroxide and by the intermolecular one-dimensional hydrogen-bonded chain between the o- and p-hydroxy groups of the neighboring molecules, respectively. Two of the hydrogen-bonded chains run in parallel, and they are connected by the bifurcated hydrogen bonds formed between the o-hydroxy groups of the facing molecules in the individual chain. The χT value of this crystal increased monotonously with lowering temperature (ST model, J =...
TL;DR: In this article, photoinduced intramolecular charge transfer in a series of N-bonded donor-acceptor derivatives of 3,6-di-tert-butylcarbazole containing benzonitrile, nicotinonitrile or various dicyanobenzenes as an electron acceptor has been studied in solutions.
Abstract: Photoinduced intramolecular charge transfer (ICT) in a series of N-bonded donor−acceptor derivatives of 3,6-di-tert-butylcarbazole containing benzonitrile, nicotinonitrile, or various dicyanobenzenes as an electron acceptor has been studied in solutions. The latter group of compounds, contrary to benzonitrile and nicotinonitrile derivatives, shows a well-separated low-energy CT absorption band which undergoes a distinct blue shift with increasing solvent polarity. Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra clearly indicate the CT character of the emitting singlet states of all of the compounds studied both in a polar and a nonpolar environment. An analysis of the CT fluorescence and absorption band shapes leads to the quantities relevant for the electron transfer in the Marcus inverted region. The values of the fluorescence rate constants (kf) and corresponding transition dipole moments (M) and their solvent polarity dependence indicate that the elec...
TL;DR: It was shown that the interior of compounds 1 a-c preferentially adopts a C3 symmetrical conformation owing to strong intramolecular H-bonding, which gives rise to an extended core that induces strong interactions between molecules, leading to mesophases of enhanced thermal stability.
Abstract: A new type of disc-shaped molecule (1 a-c) has been synthesised and characterised. The molecules were built up by linking three lipophilic, N-monoacylated 2,2'-bipyridine-3,3'-diamine wedges to a central 1,3,5-benzenetricarbonyl unit. They show liquid crystalline behaviour, as shown by DSC, polarisation microscopy and X-ray diffraction. In all cases the mesophase was characterised as a Dho phase. From (1) H NMR results it was shown that the interior of compounds 1 a-c preferentially adopts a C3 symmetrical conformation owing to strong intramolecular H-bonding, which gives rise to an extended core. This large core induces strong interactions between molecules, leading to mesophases of enhanced thermal stability.
TL;DR: The solid-state shape, size, and intermolecular packing of a fifth-generation dendritic macromolecule were determined by a combination of site-specific stable-isotope labeling, rotational-echo double-resonance (REDOR) NMR and distance-constrained molecular dynamics simulations as mentioned in this paper.
Abstract: The solid-state shape, size, and intermolecular packing of a fifth-generation dendritic macromolecule were determined by a combination of site-specific stable-isotope-labeling, rotational-echo double-resonance (REDOR) NMR and distance-constrained molecular dynamics simulations. REDOR experiments measured dipolar couplings between 13C atoms located near the chain ends and an 19F label placed at the core of benzyl ether dendrimers (generations 1−5) based on 3,5-dihydroxybenzyl alcohol as the monomeric repeat unit. Intramolecular 13C−19F coupling was distinguished from intermolecular coupling by dilution with nonlabeled dendrimer. The average intramolecular 13C−19F distances for generations 3−5 were each approximately 12 A, which indicates inward-folding of chain ends with increasing generation number. The average intermolecular 13C−19F dipolar coupling decreased with increasing generation number, consistent with decreased interpenetration for larger dendrimers. The measured intra- and intermolecular distanc...
TL;DR: In this article, the transformation of a rhodium(I) η2-alkyne model complex into the vinylidene form RhCl(PH3)2(CCH2) (E) has been examined by ab initio theoretical calculations using MP2 level geometry optimizations and localized molecular orbital analysis.
Abstract: The transformation of a rhodium(I) η2-alkyne model complex RhCl(PH3)2(HC⋮CH) (A) into the vinylidene form RhCl(PH3)2(CCH2) (E) has been examined by ab initio theoretical calculations using MP2 level geometry optimizations and localized molecular orbital (LMO) analysis. The vinylidene form E has been found to be 7.8 kcal/mol more stable than A. The previously found intraligand 1,2-hydrogen shift mechanism in the Ru(II)-coordinated alkyne−vinylidene isomerization is not relevant for the present Rh system. The reaction proceeds via the oxidative addition product RhCl(PH3)2(H)(C⋮CH) (C), followed by a bimolecular hydrogen shift from the metal to the terminal carbon of a second molecule rather than by intramolecular 1,3-hydrogen transfer. The LMO analysis of the transition state of the unimolecular 1,3-hydrogen shift indicates that the hydrogen moves as a proton while it interacts with the three centers simultaneously, i.e., Rh, Cα, and Cβ in the transition state. The hydrogen was analyzed to migrate also as a...
TL;DR: In this article, the authors showed that the product reverted to 1 upon heating in solution or in the solid state, with an enthalpy change of −29.8 (1.5) kcal mol-1.15) was unaffected by the presence of CCl4.
Abstract: Broad-band irradiation (λmax = 350 nm) of FvRu2(CO)4 (1, Fv = η5:η5-bicyclopentadienyl) resulted in rapid isomerization to colorless (μ2-η1:η5-cyclopentadienyl)2Ru2(CO)4 (2) in a novel process involving a formal dinuclear oxidative addition to a C−C bond. The product reverted to 1 upon heating in solution or in the solid state, under the latter conditions with an enthalpy change of −29.8 (1.5) kcal mol-1. Mechanistic studies with a mixture of 1 and 1-d8 revealed the absence of label scrambling, pointing to intramolecular pathways. The quantum yield (0.15) was unaffected by the presence of CCl4, and no chlorination products were observed under these conditions. Irradiation of solutions of 1 or 2 with 300 nm light provided Fv(μ2-η1:η5-cyclopentadienyl)2Ru4(CO)6 (6) or, in the presence of alkynes, the adducts FvRu2(CO)3(RCCR) (8−10, R = H, C6H5, CO2CH3). Heating 1 and PR3 (R = CH2CH3, CH3, or OCH3) yielded FvRu2(CO)3(PR3) (12−14), in which a fluxional process occurs characterized by intramolecular terminal t...
TL;DR: The reaction is most prominent in quadrupole ion trap and Fourier-transform ion cyclotron resonance experiments, is observed under some conditions in a triple quadrupoles, but is not seen in a sector instrument.
Abstract: Unique intramolecular rearrangement product ions have been observed in the product ion spectra of a number of peptides. Multiple stages of mass analysis (MSn), molecular modeling, and chemical modifications of peptides have been used to provide insight into the mechanism of this rearrangement reaction. The rearrangement process begins with a four-residue immonium ion that transfers a proton from the immonium nitrogen to the primary amine on the N-terminus. The proton transfer leads to the rearrangement of the peptide, exposing an internal amino acid on the terminus of the new ion. The internal amino acid that becomes the terminus of the new ion is then readily lost. The reaction seems to benefit from an extended experimental time frame available for reaction. The reaction is most prominent in quadrupole ion trap and Fourier-transform ion cyclotron resonance experiments, is observed under some conditions in a triple quadrupole, but is not seen in a sector instrument. Without previous knowledge of this proc...
TL;DR: In this paper, the photochemistry of ketoprofen (KPF), 2-(3-benzoylphenyl)propionic acid, has been studied in aqueous solutions by time resolved (picosecond and nanosecond) spectroscopic capabilities.
Abstract: The photochemistry of ketoprofen (KPF),
2-(3-benzoylphenyl)propionic acid, has been studied in aqueous
solutions by time resolved (picosecond and nanosecond) spectroscopic
techniques. Excited-state calculations were performed. The overall
reactivity of the molecule was rationalized within a scheme in which
the lowest triplet state can undergo intramolecular electron transfer
and photoionization. The main pathway for the formation of the
decarboxylated photoproducts was shown to involve intramolecular
electron transfer and to pass through triplet biradicals.
TL;DR: The first example of ionic [O-···H−N+] intramolecular hydrogen bonding in a structure of Schiff base was reported in this paper, where the authors showed that the title molecule is dominated by ionic canonical structures favored by through molecule conjugation between the nitro group and both groups of salicylidene fragment.
Abstract: Structural and spectroscopic properties of hydrogen bonding in solid 5-nitro-N-salicylideneethylamine have been investigated. This is the first example of ionic [O-···H−N+] intramolecular hydrogen bonding in a structure of Schiff base. Single-crystal X-ray diffraction and 13C magic-angle spinning NMR show that the title molecule is dominated by ionic canonical structures favored by through molecule conjugation between the nitro group and both groups of salicylidene fragment. Being involved in strong intramolecular [O-··· H−N+] hydrogen bonding, 5-nitro-N-salicylideneethylamine forms its crystal lattice by means of different types of weak intermolecular [O-···H−N+], N−H···O and C−H···O hydrogen bonds. On the basis of the solid-state NMR results, it has been suggested that the acidic proton can also stay at the oxygen atom, and this is qualitatively supported by X-ray diffraction.
TL;DR: In this article, the potential energy surfaces for the intramolecular proton transfer of ground (GSIPT) and excited (ESIPT), for 2-hydroxybenzoyl compounds were obtained.
Abstract: Potential energy surfaces for the intramolecular proton transfer of ground (GSIPT) and excited (ESIPT) states of 2-hydroxybenzoyl compounds were obtained. Based on the results, intramolecular proton transfer in this type of compound is strongly dependent on the distances between the oxygen atoms that bear the intramolecular hydrogen bond (IMHB). Also, the GSIPT curves for these compounds contain a single minimum that is located in the zone for the normal (enol) form. The ESIPT curves also contain a single minimum but lie in the zone for the keto form. There is no correlation between the strength of the IMHB and the proton transfer barrier through it. The energy for the excited singlet 1(n,π*) for these compounds is strongly dependent on the resonance effect of the substituent, −R, so this state is the first excited singlet only in derivatives with nearly nonresonating R. The ESIPT processes are of the proton transfer type, even though the final form possesses no zwitterionic connotations. Finally, these t...
TL;DR: In this article, the excited-state intramolecular proton transfer (ESIPT) of 2-(2'-hydroxyphenyl)benzimidazole (HBI) has been studied using absorbance, steady-state, and time-resolved emission spectroscopies in cyclodextrins (β-, γ-, and 2,6-di-O-methyl-β-cyclodextrin) and in binary aqueous solvent mixtures.
Abstract: The excited-state intramolecular proton transfer (ESIPT) of 2-(2‘-hydroxyphenyl)benzimidazole (HBI) has been studied using absorbance, steady-state, and time-resolved emission spectroscopies in cyclodextrins (β-, γ-, and 2,6-di-O-methyl-β-cyclodextrins) and in binary aqueous solvent mixtures. The spectral characteristics of HBI in cyclodextrins and binary solvent mixtures (dioxane:water, MeOH:water, and solutions of cyclohexane containing MeOH and trifluoroethanol) are compared to further study the effects of various microenvironments on the ground and excited-state properties of the molecule. The intermolecular interactions of HBI with cyclodextrins and various solvents appear to weaken intramolecular hydrogen bonding in HBI and facilitate the formation of strong intermolecular hydrogen bonds with the various cyclodextrins and solvent molecules. The acquired data are used to suggest the formation of a zwitterionic structure of HBI in cyclodextrins.