Showing papers on "Conformational isomerism published in 2002"
TL;DR: In this article, the enthalpy difference between the hydrogen-bonded (HB) form and the lowest energy conformer in which the OH is rotated into the “away” position was defined.
Abstract: For a series of 61 2-substituted phenols, 2-X−PhOH, forming a total of 73 intramolecular hydrogen bonds, the intramolecular hydrogen bond enthalpy, ΔHintra-HB, has been determined by density functional theory (DFT) calculations on the B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) level. The ΔHintra-HB was defined as the enthalpy difference between the hydrogen-bonded (HB) form and the lowest-energy conformer in which the OH is rotated into the “away” position. The correlation of ΔHintra-HB with geometrical factors such as r(O−H), or r(OH···A), with A as the hydrogen bond accepting atom, was generally very poor, showing that none of these parameters can be used as an universal descriptor for the hydrogen bond strength. The relation between ΔHintra-HB and ν(O−H) intra-HB is also insignificant, in contrast with previous estimates. The data clearly demonstrate that the genuine ΔHintra-HB of a phenolic compound cannot be unequivocally derived by simple rotation of the OH group into the “away” orientation, because additio...
195 citations
TL;DR: These investigations probe the conformational order of bulk octadecane and low molecular weight polyethylene as they undergo solid/liquid phase transitions and demonstrate that Raman spectroscopy is sensitive to very subtle changes in alkane chain structure and conformation.
Abstract: The correlation of Raman spectral indicators for the determination of alkyl chain interactions and conformational order is presented. These investigations probe the conformational order of bulk octadecane and low molecular weight polyethylene as they undergo solid/liquid phase transitions. Spectral indicators are quantitatively correlated to the I[νa(CH2)]/I[νs(CH2)], as this is the primary indicator of rotational and conformational order obtained empirically from Raman spectra. These indicators are interpreted in terms of alkane intramolecular motion, intermolecular interactions between alkyl chains, crystal structure of these solid materials, and the presence of methylene conformers. Results demonstrate that Raman spectroscopy is sensitive to very subtle changes in alkane chain structure and conformation. These results can be used to understand molecular interactions and structure−function relationships in alkane-based materials.
161 citations
TL;DR: In this paper, the Gibbs free energy of a given carbon atom in each linear stereoisomer was considered as the average chemical shift value of the same atom in the different conformers.
Abstract: Ab initio calculations at the Hartree-Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) 1 3 C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an openchain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental 1 3 C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8-13 relevant conformers per stereoisomer. Thermochemical calculations at the ab initio level in the harmonic approximation of the vibrational modes, allowed the evaluation, at 298.15 K, of the standard Gibbs free energy of the conformers. The 1 3 C NMR chemical shift of a given carbon atom in each stereoisomer was considered as the average chemical shift value of the same atom in the different conformers. The averages were obtained by the Boltzmann distribution, using the relative standard free energies as weighting factors. Computed parameters related to linear correlation plots of experimental 1 3 C chemical shifts versus the corresponding computed average data allowed us to distinguish among the four stereoisomers.
158 citations
TL;DR: Force-ramp atomic force microscopy readily captures the ring transitions under conditions where the entropic elasticity of the molecule is separated from its conformational transitions, enabling a quantitative analysis of the data with a simple two-state model.
Abstract: Under a stretching force, the sugar ring of polysaccharide molecules switches from the chair to the boat-like or inverted chair conformation. This conformational change can be observed by stretching single polysaccharide molecules with an atomic force microscope. In those early experiments, the molecules were stretched at a constant rate while the resulting force changed over wide ranges. However, because the rings undergo force-dependent transitions, an experimental arrangement where the force is the free variable introduces an undesirable level of complexity in the results. Here we demonstrate the use of force-ramp atomic force microscopy to capture the conformational changes in single polysaccharide molecules. Force-ramp atomic force microscopy readily captures the ring transitions under conditions where the entropic elasticity of the molecule is separated from its conformational transitions, enabling a quantitative analysis of the data with a simple two-state model. This analysis directly provides the physico-chemical characteristics of the ring transitions such as the width of the energy barrier, the relative energy of the conformers, and their enthalpic elasticity. Our experiments enhance the ability of single-molecule force spectroscopy to make high-resolution measurements of the conformations of single polysaccharide molecules under a stretching force, making an important addition to polysaccharide spectroscopy.
138 citations
TL;DR: This work demonstrates that the mechanism of the chiral induction by counterions involves specific anion-cation recognition and the induction of conformationally labile chirality in the cations and provides a general framework for designing new tunable membrane systems.
Abstract: In membranes, the chirality of the amphiphile constituents is sometimes expressed at a supramolecular scale of nanometers or micrometers. We have recently reported that membranes of nonchiral dicationic n−2−n amphiphiles can also be chirally twisted upon interacting with chiral tartrate counterions. Here, we demonstrate that the mechanism of the chiral induction by counterions involves specific anion−cation recognition and the induction of conformationally labile chirality in the cations. Single-crystal X-ray diffraction shows that the amphiphilic cations exist as a mixture of chiral conformers. 1H NMR data establish a specific recognition between tartrate and n−2−n cations and show that chiral conformers also exist in solution. Circular dichroism (CD) in the UV−vis shows a sharp conformational change of tartrate ions from anti to gauche when bound to the chiral cationic membranes. This is confirmed by CD in the infrared region which also shows concomitant induced CD bands in the vibrations of the n−2−n a...
133 citations
TL;DR: Substitution of phenylalanine by lysine on either side of tryptophan has no effect on fluorescence quantum yield or lifetime, indicating that intramolecular excited-state proton transfer catalyzed by the epsilon-ammonium does not occur in these peptides.
Abstract: Intramolecular quenching of tryptophan fluorescence by protein functional groups was studied in a series of rigid cyclic hexapeptides containing a single tryptophan. The solution structure of the canonical peptide c[d-PpYTFWF] (pY, phosphotyrosine) was determined in aqueous solution by 1D- and 2D-1H NMR techniques. The peptide backbone has a single predominant conformation. The tryptophan side chain has three χ1 rotamers: a major χ1 = −60° rotamer with a population of 0.67, and two minor rotamers of equal population. The peptides have three fluorescence lifetimes of about 3.8, 1.8, and 0.3 ns with relative amplitudes that agree with the χ1 rotamer populations determined by NMR. The major 3.8-ns lifetime component is assigned to the χ1 = −60° rotamer. The multiple fluorescence lifetimes are attributed to differences among rotamers in the rate of excited-state electron transfer to peptide bonds. Electron-transfer rates were calculated for the six preferred side chain rotamers using Marcus theory. A simple ...
128 citations
TL;DR: A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy, and an almost ideal eta1 coordination is shown by the anti conformer of the C(ortho)-substituted complex due steric effects.
Abstract: A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy. The compound [(C16H19)PdCl]2*CH2Cl2 with ortho and para methyl substituents at the arene has been characterized by means of X-ray diffraction techniques. The C(ipso) atom of the arene lies almost at the fourth planar coordination site of the metal [Pd-C(ipso) = 2.22(1) A (average)], and due to the arene's tilting, the substituted C(ortho) atom is relatively close to the metal atom [2.54(1) A (average)]. The coordinated C(ipso)-C(ortho) linkage, in a seemingly dihapto coordination, is anti with respect to the CH2 bridge of the bicycloheptyl unit. Variable-temperature NMR experiments for the para-substituted dimer 9 reveal restricted rotation of the two aryl groups about the corresponding C-C(ipso) bonds (DeltaE < or =17 kcal x mol(-1)). DFT-B3LYP calculations have been carried out on the known and similar monomer (phenylbicycloheptenyl)Pd(PPh3)I (4) and its related substituted derivatives. The essential results are as follows: (i) The potential energy surface for twisting the phenyl ring away from the symmetric eta(1) coordination in 4 is very flat (DeltaE < or = 1 kcal x mol(-1)) whereas an Atoms in Molecules analysis excludes the existence of an actual Pd-C(ortho) bond in the seemingly eta2-type conformer. (ii) Complete rotation of the unsubstituted phenyl ring is not facile but feasible. A significant strain affects the transition-state structure featuring a Pd-HC(aryl) agostic-type bond. The calculated destabilization of 10.3 kcal x mol(-1), with respect to the ground state, can be compared to the experimental barrier of the dimer 9. (iii) Various methyl-substituted derivatives of 4 have been optimized, and their structural and energetic trends are discussed. An almost ideal eta1 coordination is shown by the anti conformer of the C(ortho)-substituted complex due steric effects. For all of the other cases, a slipped eta2 coordination may be described. As a general conclusion, the unsaturated metal center receives pi electron density of the arene mainly through its C(ipso) atom. The effect may be slightly improved if the C(ortho) atom also gets closer to the metal, but in no case, does the slipped eta2 coordination seem to be crucial for the stability of the system.
127 citations
TL;DR: A novel picture of the mechanism provides a sound basis for the future development of chiral sulfides for enantioselective epoxide synthesis.
Abstract: Sulfur ylides R2S+−C-HR‘ react with aldehydes R‘ ‘−CHO to form epoxides, predominantly as the trans isomers, in a synthetically useful reaction which is increasingly used in its asymmetric variant with chiral sulfides. The mechanisms of the “model” reaction (R = Me, R‘ = R‘ ‘ = H) and the reaction forming stilbene oxide (R = Me, R‘ = R‘ ‘ = Ph) have been studied in detail using density functional theory, the B3LYP density functional, and flexible basis sets. It has been shown that for this reaction involving highly polar intermediates, continuum solvation models need to be used throughout to obtain reasonable results. For the reaction of benzaldehyde with dimethylsulfonium benzylide, the key steps are shown to be quasi [2 + 2] addition of the ylide to the aldehyde to form a betaine R‘−CH(S+Me2)−CH(O-)−R‘ ‘ in which the charged groups are gauche to one another, and torsional rotation around the C−C single bond of the betaine to form its rotamer with the two charged groups anti. The final step, elimination ...
120 citations
TL;DR: It is shown that the kinetic energy released during the photodissociation of 1-iodopropane ions depends strongly on the conformation of the ions, suggesting that the well-known concept of conformation selection forms the basis of a simple scheme for reaction control.
Abstract: Many molecules can rotate freely around single bonds and thereby interconvert between different conformations, such as gauche and anti 1,2-disubstituted ethane, a classic example of conformational isomerism1,2,3. Even though rotation occurs rapidly at room temperature, the product selectivity seen in some reactions has been explained by conformation-dependent reaction mechanisms4,5: if reactant molecules differing only in their conformation are located at different positions on the reaction path, they may undergo different reactions. But a direct verification of this effect is difficult, because the energy barrier separating conformational isomers is so low that under ambient conditions reactants with more than one conformation will be present1. But by using temperatures low enough to suppress the interconversion between different conformations, gauche-1-iodopropane ions and anti-1-iodopropane ions have been selectively generated6. Here we show that the kinetic energy released during the photodissociation of 1-iodopropane ions depends strongly on the conformation of the ions. Thermodynamic arguments and ab initio calculations indicate that this difference in kinetic energy release results from differences in the reaction mechanism, with gauche-1-iodopropane ions forming 2-propyl ions and anti-1-iodopropane ions forming protonated cyclopropane ions. These findings suggest that the well-known concept of conformation selection forms the basis of a simple scheme for reaction control, thus providing in some cases an attractive alternative for more involved schemes that utilize the phase and pulse shape of laser beams to control chemical reactions7,8,9,10.
119 citations
TL;DR: In this paper, a combination of methods, including laser-induced fluorescence excitation, fluorescence-dip infrared (FDIR) spectroscopy, and UV-UV hole burning (UVHV) spectrograms, have been used to study the infrared and ultraviolet spectra of single conformations of two methyl-capped dipeptides: N-acetyl tryptophan amide (NATA) and NATMA).
Abstract: A combination of methods, including laser-induced fluorescence excitation, fluorescence-dip infrared (FDIR) spectroscopy, and UV-UV hole-burning spectroscopy, have been used to study the infrared and ultraviolet spectra of single conformations of two methyl-capped dipeptides: N-acetyl tryptophan amide (NATA) and N-acetyl tryptophan methyl amide (NATMA). Density functional theory calculations predict that all low-energy conformers of NATA and NATMA belong to one of two conformational families: C5, with its extended dipeptide backbone, or C7eq, in which the dipeptide backbone forms a seven-membered ring joined by a H bond between the ψ-amide NH and the φ-amide carbonyl groups. In NATA (NATMA), the LIF spectrum has contributions from two (three) conformers. FDIR spectroscopy has been used to record infrared spectra of the individual conformers over the 2800–3600 cm−1 region, free from interference from one another. The NH stretch region provides unequivocal evidence that one of the conformers of NATA is C5, ...
113 citations
TL;DR: The calculated gg, gt, and tg rotamer populations of the hydroxymethyl and methoxymethyl groups in 9-16 agreed well with experimental data, and it was concluded that the preference of rotamers in 1-16 is due to the hydrogen bonding and solvent effects.
TL;DR: The parameters for the OPLS–AA potential energy function have been extended to include some functional groups that are present in macrocyclic polyketides and it is proposed that the exact chemical makeup of the effector domain has little influence on the conformational preference of tetraketides.
Abstract: The parameters for the OPLS-AA potential energy function have been extended to include some functional groups that are present in macrocyclic polyketides. Existing OPLS-AA torsional parameters for alkanes, alcohols, ethers, hemiacetals, esters, and ketoamides were improved based on MP2/aug-cc-pVTZ and MP2/aug-cc-pVDZ calculations. Nonbonded parameters for the sp(3) carbon and oxygen atoms were refined using Monte Carlo simulations of bulk liquids. The resulting force field predicts conformer energies and torsional barriers of alkanes, alcohols, ethers, and hemiacetals with an overall RMS deviation of 0.40 kcal/mol as compared to reference data. Densities of 19 bulk liquids are predicted with an average error of 1.1%, and heats of vaporization are reproduced within 2.4% of experimental values. The force field was used to perform conformational analysis of smaller analogs of the macrocyclic polyketide drug FK506. Structures that adopted low-energy conformations similar to that of bound FK506 were identified. The results show that a linker of four ketide units constitutes the shortest effector domain that allows binding of the ketide drugs to FKBP proteins. It is proposed that the exact chemical makeup of the effector domain has little influence on the conformational preference of tetraketides.
TL;DR: Vicinal, equatorial OH groups in the (4)C(1) conformer of glucopyranose are, however, able to form strong bidentate hydrogen bonds with water molecules in a cooperative manner, each water molecule acting simultaneously as both hydrogen bond donor and acceptor.
Abstract: Topological analysis of the electron density profiles and the atomic basin integration data for the most energetically favorable 4C1 and 1C4 conformers of β-d-glucopyranose, calculated at the B3LYP/6-31+G(d), MPWlPW91/6-311+G(2d,p), and MP2/6-31+G(d) levels, demonstrates that intramolecular hydrogen bonding between adjacent ring OH groups does not occur in glucopyranose, given the need to demonstrate a bond critical point (BCP) of correct (3,−1) topology for such an interaction to be termed a hydrogen bond. On the other hand, pyranose ring OH groups separated by three, rather than just two, carbon atoms are able to form an intramolecular hydrogen bond similar in topological properties and geometry to that found for propane-1,3-diol. Vicinal, equatorial OH groups in the 4C1 conformer of glucopyranose are, however, able to form strong bidentate hydrogen bonds with water molecules in a cooperative manner, each water molecule acting simultaneously as both hydrogen bond donor and acceptor, and characterized by...
TL;DR: In this article, an ab initio theoretical study of five low energy isomers of the water hexamer {Chair, Cage(du), Book, Prism, and Boat}, their intramolecular vibrations, binding energies De and dissociation energies D0.
Abstract: We present an ab initio theoretical study of five low-energy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies De and dissociation energies D0. Moller–Plesset second order perturbation calculations using the aug-cc-pVTZ basis set at aug-cc-pVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the De and D0 levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O–H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets, liquid-helium droplets and solid para-hydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The (H2O...
TL;DR: Observation of cis-amide melatonin conformers in the molecular beam, despite the large energy gap between trans- and cis-amides, is striking because there are at least nine lower-energy trans-amide minima that are not detected.
Abstract: The hormone melatonin (N-acetyl-5-methoxytryptamine) is an indole derivative with a flexible peptide-like side chain attached at the C3 position. Using a combination of two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence excitation (LIF), resonant ion-dip infrared spectroscopy (RIDIRS), fluorescence-dip infrared spectroscopy (FDIRS), and UV−UV hole-burning spectroscopy, the conformational preferences of melatonin in a molecular beam have been determined. Three major trans-amide conformers and two minor cis-amide conformers have been identified in the R2PI spectrum and characterized with RIDIRS and FDIRS. Structural assignments are made using the infrared spectra in concert with density functional theory and localized MP2 calculations. Observation of cis-amide melatonin conformers in the molecular beam, despite the large energy gap (∼3 kcal/mol) between trans- and cis-amides, is striking because there are at least nine lower-energy trans-amide minima that are not detected. The im...
TL;DR: In this article, a vibrational assignment for both s-cis and s-trans conformers of pyrrole-2-carboxylic acid conformers is proposed.
Abstract: The infrared and Raman spectroscopic study of pyrrole-2-carboxylic acid (PCA) confirms the formation of the cyclic acid dimer species in the solid state. The molecular structure, vibrational frequencies, and binding energies of cyclic dimers have been also examined using the density functional theory (DFT) at the B3LYP/6-311+G(d) level. In addition, a complete vibrational assignment is proposed for the both s-cis and s-trans PCA conformers. The vibrational assignments are supported by normal coordinate calculations utilizing force constants predicted using the DFT method. The “atoms in molecules” theory of Bader is also used to characterize hydrogen bonds.
TL;DR: A strategy to design face discriminated glycosyl donors that exist predominantly in only one family of conformers is proposed based on observations of high beta-stereoselectivity of mannopyranosyl donors and high alpha-stirring of glucopyranoyl donors with the 4,6-O-benzylidene protecting groups.
TL;DR: The conformational behavior of the title compounds has been investigated by Hartree–Fock, MP2, and DFT computations on the most significant structures related to variations of the backbone dihedral angles, cis/trans isomerism around the peptide bond, and diastereoisomeric puckering of the pyrrolidine ring.
Abstract: The conformational behavior of the title compounds has been investigated by Hartree–Fock, MP2, and DFT computations on the most significant structures related to variations of the backbone dihedral angles, cis/trans isomerism around the peptide bond, and diastereoisomeric puckering of the pyrrolidine ring. In vacuum the reversed γ turn (γl), characterized by an intramolecular hydrogen bridge, corresponds to the absolute energy minimum for both puckerings (up and down) of the pyrrolidine ring. An additional energy minimum is found in the helix region, but only for an up puckering of the pyrrolidine ring. When solvent effects are included by means of the polarizable continuum model the conformer observed experimentally in condensed phases becomes the absolute minimum. The down puckering is always favored over its up counterpart, albeit by different amounts (0.4–0.5 kcal/mol for helical structures and about 2 kcal/mol for γl structures). In helical structures cis arrangements of the peptide bond are only slightly less stable than their trans counterparts. This is no longer true for γl structures, because the formation of an intramolecular hydrogen bond is possible only for trans peptide bonds. In most cases, proline and hydroxyproline show the same general trends; however, the electronegative 4(R) substituent of hydroxyproline leads to a strong preference for up puckerings irrespective of the backbone conformation. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 341–350, 2002
TL;DR: Aliphatic amines, incorporating one or three (branched) acylated beta-D-glucopyranosyl residues, were coupled with the acid chloride of ferrocenecarboxylic acid to afford four dendrimer-type, carbohydrate-coated ferrocene derivatives in good yields (54-92%).
Abstract: Aliphatic amines, incorporating one or three (branched) acylated beta-D-glucopyranosyl residues, were coupled with the acid chloride of ferrocenecarboxylic acid and with the diacid chloride of 1,1'-ferrocenedicarboxylic acid to afford four dendrimer-type, carbohydrate-coated ferrocene derivatives in good yields (54-92%). Deprotection of the peracylated beta-D-glucopyranosyl residues was achieved quantitatively by using Zemplen conditions, affording four water-soluble ferrocene derivatives. When only one of the two cyclopentadienyl rings of the ferrocene unit is substituted, strong complexes are formed with beta-cyclodextrin in H2O, as demonstrated by liquid secondary ion mass spectrometry (LSIMS), 1H NMR spectroscopy, electrochemical measurements, and circular dichroism spectroscopy. Molecular dynamics calculations showed that the unsubstituted cyclopentadienyl ring is inserted through the cavity of the toroidal host in these complexes. The electrochemical behavior of the protected and deprotected ferrocene-containing dendrimers was investigated in acetonitrile and water, respectively. The diffusion coefficient decreases with increasing molecular weight of the compound. The potential for oxidation of the ferrocene core, the rate constant of heterogeneous electron transfer, and the rate constant for the energy-transfer reaction with the luminescent excited state of the [Ru(bpy)3]2+ complex (bpy = 2,2'-bipyridine) are strongly affected by the number (one or two) of substituents and by the number (one or three) of carbohydrate branches present in the substituents. These effects are assigned to shielding of the ferrocene core by the dendritic branches. Electrochemical evidence for the existence of different conformers for one of the dendrimers in aqueous solution was obtained.
TL;DR: In this article, L-phenylalanine (Phe), one of the aromatic amino acids, and its hydrated clusters were generated in supersonic expansion and investigated by resonant two-photon ionization.
Abstract: L-phenylalanine (Phe), one of the aromatic amino acids, and its hydrated clusters were generated in supersonic expansion and investigated by resonant two-photon ionization. Excitation spectra of Phe and Phe–(H2O)1 were obtained near their S0–S1 origins. We found that, by comparing the experimental results with the density functional theory and ab initio calculations, the water in Phe–(H2O)1 tends to form a cyclic hydrogen bond at the carboxyl group while inducing little change in the corresponding monomer structure. No sign of water making bridged hydrogen bonds with both polar groups was found. In order to form the cyclic hydrogen bond, hydration takes place only with the conformers whose carboxyl hydrogen is free, i.e., not occupied in the intramolecular hydrogen bonding with the amino nitrogen in the monomer.
TL;DR: In this article, the color polymorphism is explained by the conformational differences between polymorphs, which cause varying degrees of π-conjugation between the o-nitroaniline chromophore and the thiophene group.
Abstract: 5-Methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (1) crystallizes as seven conformational polymorphs of red, orange, and yellow colors. Comparable thermodynamic stability of the polymorphs enables the solution of six crystal structures, a number unmatched by current entries in the Cambridge Structural Database, making 1 well suited for structure−property studies. The different crystal colors of 1 have been studied using optical crystallography, polarized single-crystal absorption spectroscopy and computational methods. Different crystal colors (“color polymorphism”) are well explained by the conformational differences between polymorphs, which cause varying degrees of π-conjugation between the o-nitroaniline chromophore and the thiophene group. The isolation of different conformers in crystal polymorphs permits an evaluation of computational models of electronic structures and transitions through observed spectral properties. The newly matured Time Dependent Density Functional Theory gave the bes...
TL;DR: In this paper, the authors compare the conformational partition functions of poly(ethylene terephthalate) and poly(methylene 2,6-naphthalate) polyesters.
TL;DR: In this article, a density functional theory analysis of trifluoromethyliodine dihalides was performed to investigate the equilibrium molecular structures, rotational barriers, vibrational spectra of (trifluorsomethryl)iodine (TDI) dihalide, CF 3 IX 2 (X=F, Cl, Cl), with the several basis sets.
Abstract: Ab initio and density functional theory calculations have been performed to investigate the equilibrium molecular structures, rotational barriers, vibrational spectra of (trifluoromethyl)iodine dihalides, CF 3 IX 2 (X=F, Cl), with the several basis sets. For each molecule the eclipsed form is found to be more stable than the staggered form. The staggered conformer corresponds to the transition state structure of which energy barrier is less than ∼0.5 kcal/mol. The natural bond orbital analysis at the restricted Hartree–Fock level shows that the energy preference of an eclipsed form over a staggered form is primarily due to the σ IX →σ CF ∗ interactions.
TL;DR: In this paper, the conformational peculiarities of macromolecules of cellulose of various structural modifications have been studied by the methods of Fourier transform infrared and Raman spectroscopy and theoretical conformational analysis.
TL;DR: In this article, steady-state and time-resolved fluorescence spectroscopy was employed to provide evidence that the emission bands originate from different conformers, and the ground-state geometry of the majority species is identical to that found in the crystalline state.
Abstract: Electron donor−acceptor systems, in which phenothiazine is tethered to pyrene by means of a phenyl bridge, exhibit a dual emission in moderately and very polar solvents. Employing steady-state and time-resolved fluorescence spectroscopy, we were able to provide evidence that the “blue” and “red” emission bands originate from different conformers. The ground-state geometry of the majority species is identical to that found in the crystalline state (the quasi-equatorial conformer). This conformation executes a fast electron-transfer process accompanied by significant structural relaxation. Consequently, its fluorescence exhibits a large solvatochromic shift typical for charge-transfer states. The photophysical properties of the minority species (the quasi-axial conformer) vary significantly with the substitution pattern of the bridging phenyl ring. In part, this difference is related to the orientational factor, κ, governing the rate of energy transfer between the pyrene and phenothiazine moieties. In the p...
TL;DR: In this paper, ten new hydrazino derivatives were synthesized, and their properties were studied by 1H and 13C NMR spectroscopy, including ring-chain tautomerism, cis-trans isomerism with respect to the substituents in the pyrazole ring, and (E)/(Z) rotamerism with relation to the nitrogen−carbon hydrazide bond with partial double-bond character.
TL;DR: In this article, the conformational state of the alkyl chains in their bound state was monitored via various conformation-sensitive vibrational bands, including symmetric and antisymmetric CH2 stretching bands.
Abstract: Silica gels, chemically modified with n-alkyl chains of various lengths (CnH2n+1, n = 8, 9, 10, 12, 16, 18, 22, and 30), in the dry state were examined over a broad temperature range (123−353 K) via infrared spectroscopy. Here, for the first time a comprehensive variable temperature FT-IR study is available dealing with the conformational order of alkyl modified silica gels. The conformational state of the alkyl chains in their bound state was monitored via various conformation-sensitive vibrational bands. A qualitative statement about the conformational order was possible based on the analysis of the symmetric and antisymmetric CH2 stretching bands. Likewise, CH2 wagging bands were used to identify and determine the relative amountsi.e., integral values over the whole chainsof gauche-trans-gauche, double gauche, and end gauche conformers. The amount of gauche conformers at a specific site in the alkyl chains was accessible via the analysis of CD2 rocking bands of samples bearing selectively deuterated al...
TL;DR: Two cyclic hexapeptides containing alternating all R and all S configured l-(4R/S)-hydroxyproline and 6-aminopicolinic acid subunits are presented, and the influence of the hydroxyl groups on the solubility, conformation, and receptor properties is investigated.
Abstract: Two cyclic hexapeptides containing alternating all R and all S configured l-(4R/S)-hydroxyproline and 6-aminopicolinic acid subunits are presented, and the influence of the hydroxyl groups on the solubility, conformation, and receptor properties is investigated. Cyclopeptide 2, containing the natural 4R configured hydroxyproline, adopts a conformation similar to that of the unsubstituted peptide 1, which is able to bind anions such as halides and sulfate in aqueous solution. 2 also interacts with these anions, but whereas 1 forms sandwich type 2:1 complexes, in which the anion is bound by two cyclopeptide moieties, 2 forms 1:1 complexes. The stabilities of the halide and sulfate complexes of 2 range between 100 and 102 M−1 in 80% D2O/CD3OD. Complex formation is detectable even in water, but with slightly smaller stability constants. Using this information a quantitative evaluation of the stability of the 2:1 complexes of 1, for which overall stability constants in the order 104 to 105 M−2 in 80% D2O/CD3OD were observed, was made. In contrast to 2, the conformation of 3, containing the non-natural 4S configured hydroxyproline, is strongly affected by the presence of the hydroxyl groups. In d6-DMSO and methanol/water mixtures a slow conformational equilibrium between two C3-symmetrical conformers is observed, and 3 is thus much less preorganized for anion binding than either 1 or 2.
TL;DR: In this paper, the first structural assignment of a model glycoside in the gas phase, namely phenyl β-D-glucopyranoside, was reported, based upon measurements of its resonant two photon ionisation and infra-red ion dip spectra together with supporting ab initio calculations.
Abstract: The first spectroscopic investigation and full structural assignment of a model glycoside in the gas phase, namely phenyl β-D-glucopyranoside, is reported, based upon measurements of its resonant two photon ionisation and infra-red ion dip spectra together with supporting ab initio calculations. Three conformers (two gauche and one trans) were identified and structurally assigned. A comparison with the predictions of earlier high-level ab initio calculations on the unsubstituted sugar, indicate that the addition of a phenyl substituent at position 1 on the pyranoside ring, has very little effect on the low energy region of its conformational landscape.
TL;DR: These data demonstrate that the photodissociation products can be efficiently steered with selective narrow-band infrared radiation promoting rotational isomerism, which makes a strong case of optically controlled chemical reactions.
Abstract: The trans and cis forms of formic acid (HCOOH) in solid argon favor essentially different photodissociation (193 nm) products, H2O + CO and H2 + CO2, respectively. The branching ratio of these channels differs between the two conformers by a factor of >10. The observed selective photodissociation features conformational memory when the transition state of a molecule is reached before torsional randomization. These data demonstrate that the photodissociation products can be efficiently steered with selective narrow-band infrared radiation promoting rotational isomerism, which makes a strong case of optically controlled chemical reactions