Showing papers on "Conformational isomerism published in 1982"
TL;DR: Hard-sphere exo-anomeric (HSEA) calculations have been used to predict conformers for the linear tetrasaccharide (αLRha(1.7 and 0.2 kcal/mol) as discussed by the authors, which constitutes a part of the Shigellaflexneri O-antigen.
Abstract: Hard-sphere (HS) calculations predict three conformers for the branched B human blood group antigenic determinant (α LFuc(1 → 2)[α DGal(1 → 3)]βDGal) and two conformers for the linear tetrasaccharide (αLRha(1 → 2)αLRha(1 → 3)αLRha(1 → 3)βDGlcNAc), which constitutes a part of the repeating unit of the Shigellaflexneri O-antigen, which differ in conformational energy by less than 0.7 and 0.2 kcal/mol, respectively. However, a detailed 1H nmr study of specific interunit deshielding effects, quantitative treatment of spin-lattice relaxation times, and nuclear Overhauser enhancements requires that only one of the conformers thus predicted be, in fact, conformationally preferred in solution. These conformers are those predicted by hard-sphere exo-anomeric (HSEA) calculations.
326 citations
TL;DR: The rotameric state model developed by Flory was used to describe the conformations adopted by the alkyl chain in this paper, and it was also used to predict the conformational distributions for both the isotropic and nematic phases of these mesogens; the calculated distributions are contrasted with those assumed by other models.
Abstract: The constituent molecules of thermotropic liquid crystals invariably contain an aromatic core to which is attached one and often two n -alkyl chains. Nuclear magnetic resonance investigations have revealed that the chain does not exist entirely in an all- trans configuration and that the orientational order of the methylene groups varies markedly along the chain. In this paper we develop a theory to explain these variations for a uniaxial phase at a given temperature and pressure. Our theory contains two distinct parts: one is a model for the conformational distribution of the alkyl chains while the other is for the potential of mean torque experienced by a conformer and which is responsible for its alignment with respect to the director. We use the rotameric state model developed by Flory to describe the conformations adopted by the alkyl chain. The potential of mean torque presents a more difficult problem and we assume that it may be represented by a sum of contributions from each rigid group in the molecule. The order parameters for the methylene and methyl groups in the alkyl chains of the 4- n -pentyl and 4- n -octyl-49-cyanobiphenyls predicted by the theory are found to be in good accord with those obtained experimentally. We have also used the theory to predict the conformational distributions for both the isotropic and nematic phases of these mesogens; the calculated distributions are contrasted with those assumed by other models. In addition we have obtained the ordering matrices for the various conformers adopted by the two 4- n -alkyl-49-cyanobiphenyls. The results of these calculations do not support the notion that the orientational order of such molecules can be described by a single, cylindrically symmetric ordering matrix.
255 citations
TL;DR: In this paper, the main photoreactions (isomerization and cycliza- tion) and luminescence of stilbene analogues where polynuclear aryl groups or pyridyl groups are joined through an ethylene bridge have been investigated.
Abstract: The main photoreactions (isomerization and cycliza— tion) and luminescence of stilbene analogues where polynuclear aryl groups or pyridyl groups are joined through an ethylene bridge have been investigated. More details are given about two series of compounds, the styrylnaphthalenes and the styryl— pyridines. The naphthyl analogues of stilbene may have a lowest excited state of a naphthalene character which induces a reduction of the radiative rate parameters, possibly opening a triplet path to isomerization. The low-lying n,W states of the azastilbenes contribute to open non—radiative paths for the decay from l' especially for ortho and para aza—compounds. Internal conversion is important in fluid media whilst intersystem crossing may be operative in a rigid matrix at low temperatures. Bimolecular processes of the excited diarylethylenes and their aza—analogues with suitable additives (energy, proton and electron donors or acceptors) are also described. It is shown how they can selectively populate the singlet or triplet manifolds of the olefins. Finally, conformational equilibria involving rotation of the aromatic groups about the quasi—single bonds are described. A fluorimetric analysis of the mixture of the two conformers postulated allows the photophysical parameters of the single components to be evaluated.
143 citations
TL;DR: Findings provide strong support for the presence of four conformers of significantly different structure at the heme ligand-binding site and for rapid interconversions among these structures.
104 citations
TL;DR: In this article, the authors measured the terminal conformer populations in supersonic molecular beams by using matrix IR spectroscopy, which is based on trapping of the beam molecules in to a cryogenic matrix.
90 citations
TL;DR: It is suggested that base sequence, extent of modification, and external conditions such as salt concentration determine which of a number of possible conformational effects is actually induced by AAF.
Abstract: Minimized conformational potential energy calculations were performed for dCpdG modified with the carcinogen 2-(acetylamino)fluorene (AAF) The major adduct, linked via a covalent bond between guanine C-8 and N-2 of AAP, was investigated The 12 variable torsion angles and both dexoyribose puckers were independent flexible parameters in the energy minimizations Three categories of low-energy conformers were calculated in which the guanine was syn and nearly perpendicular to the plane of the fluorene: (1) forms in which fluorene is stacked with cytidine (included among these is the global minimum energy conformation); (2) conformers which preserve guanine-cytidine stacking while placing the fluorene in a base-pair obstructing position; (3) conformers which maintain guanine-cytidine stacking and place the fluorene at the helix exterior, without interfering with base pairing The Z form is important in this group In addition, a low-energy conformation with guanine anti, but still nearly perpendicular to fluorene, was computed Molecular models were constructed for the most important conformations incorporated into larger polymers These indicated that the fluorene-cytidine stacked forms induce a severe kink in the B helix Conformers with guanine-cytidine stacking and AAF in a base-pair obstructing position place the AAF at the B-type helix interior with little distortion in the helixmore » direction Conformers with the guanine-cytidine stack in which AAF does not affect base pairing place the fluorene at the Z or alternate helix exterior It is suggested that base sequence, extent of modification, and external conditions such as salt concentration determine which of a number of possible conformational effects is actually induced by AAF The variety of observed experimental results with AAF-modified DNA may reflect these various conformational possibilities« less
56 citations
TL;DR: In this article, the asymmetric potential function for internal rotation of the O = C-F top has been determined from far infrared data, and the following potential constants have been evaluated: V1 = 257±4; V2 = 1582±6; and V3 = 173±3 cm−1.
Abstract: The infrared (3500–40 cm−1) and Raman (3500–10 cm−1) spectra have been recorded for the gaseous and solid phases of methacryloyl fluoride. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. The asymmetric potential function for internal rotation of the O = C–F top has been determined from far infrared data, and the following potential constants have been evaluated: V1 = 257±4; V2 = 1582±6; and V3 = 173±3 cm−1. From these data, the s‐trans conformer was determined to be the predominant rotamer at ambient temperature with the other conformer being the s‐cis; the enthalpy difference between the s‐trans and s‐cis conformers is 430 cm−1 (1230 cal/mol). This value compares well with the value of 444 cm−1 (1270 cal/mol) obtained for the liquid from a variable temperature study of the Raman spectrum. The methyl torsional barrier was also calculated from the far infrared data to be 1.77 kcal/mol. A vibrational assignment and values for the the...
54 citations
TL;DR: In this paper, the conformational equilibria of four β-maltose conformers have been studied theoretically in 12 solvents and the stability of the conformers in dilute solution has been compared by using the continuum reaction field method.
Abstract: The conformational equilibria of four β-maltose conformers have been studied theoretically in 12 solvents. The stability of the conformers in dilute solution has been compared by using the continuum reaction field method. The solvation energy consists of electrostatic, dispersion, and cavity terms, which have been determined from the calculated properties of β-maltose and physicochemical properties of solvents. The calculated population of four conformers significantly depends on the solvent (e.g., in dioxane, M1:M2:M3:M4 = 53.3: 20.3:17.7:8.7; in dimethyl sulfoxide, 40.1:21.8:22.8:15.3; and in water, 25.7:17.5:26.3:30.5) and was found to correlate with experimentally observed data. The results obtained indicate that the conformation adopted by β-maltose in the crystalline form is not the one preferred in solution. The roles of the individual contributions to the solvation energy have been analyzed. Based on the determined abundance of conformers, averaged residual optical activity and vicinal carbon–proton coupling constants have been calculated and discussed from the point of view of the solution behavior of β-maltose.
43 citations
TL;DR: Observations of conformational dynamics of lasalocid A suggest a scenario for carboxylic ionophore mediated transmembrane monovalent cation transport at the molecular level.
Abstract: The conformational dynamics of lasalocid A have been studied in a series of solvents of graded polarity by means of circular dichroism (CD) and computer-generated molecular models. In high polarity solvents, the uncomplexed anionic ionophore assumes as acyclic conformation minimizing intrinsic molecular strain energy. In this state, the dipoles of the liganding oxygens in the carbon backbone and the terminal carboxylate are stabilized by a high degree of solvent association. As the solvent polarity decreases, the dynamic conformational equilibrium progressively shifts toward a cyclic conformation which predominates at low polarity. Cyclization proceeds by rotation about three carbon-carbon hinge bonds. The resulting twist of the backbone introduces torsional strain which is offset at low polarity by electrostatic stabilization gained through intramolecular hydrogen bonding. Formation of a cation inclusion complex also stabilizes the cyclic conformer, even in relatively polar solvents. These observations suggest a scenario for carboxylic ionophore mediated transmembrane monovalent cation transport at the molecular level. The cation encounters an acyclic ionophore at the membrane interface where it ion pairs to the terminal carboxylate moiety, initiating formation of a lipophilic, cyclic cation inclusion complex. The complex, no longer constrained to the polar interface, diffuses across the membrane interior to the opposite face. There it reequilibrates with the polar environment, the ionophore reassuming the low energy, acyclic conformation and concomitantly releasing the enclosed cation. The free, acyclic ionophore is now confined to the opposite polar interface where it awaits the capture of a new cation to complete its catalytic transport cycle.
40 citations
TL;DR: In this article, an equilibrium mixture of gauche and cis conformers for (bromomethyl)cyclopropane and epibromohydrin has been found for the Raman spectra.
Abstract: Infrared and Raman spectra of (bromomethyl)cyclopropane and epibromohydrin have been recorded. Reversible spectral changes which accompany changes in temperature indicate the existence of an equilibrium mixture of gauche and cis conformers for (bromomethyl)cyclopropane. The gauche form is more stable, and from the variable temperature studies an enthalpy difference of 2.0±0.2 kcal mol−1 has been obtained for the liquid phase. For epibromohydrin, two non-equivalent gauche conformers have been identified, and the gauche-1 form is more stable than the gauche-2 by 0.80±0.20 kcal mol−1 in the liquid phase. Additionally there is evidence for a third conformer, presumably the cis form, but characterization of this conformer has been limited by the complexity of the spectra associated with the gauche-1 and gauche-2 forms. Consequently, only a lower limit can be given for the ΔH separating the cis and gauche-1 conformers. The results are consistent with previous observations for similar molecules.
38 citations
TL;DR: A conformational study of various 4-phenylpiperidine analgesics has been performed, and phenyl axial conformers were unexpectedly found to be especially destabilized by a 3-methyl group in the beta configuration due to the steric crowding of the three piperidine substituents.
Abstract: A conformational study of various 4-phenylpiperidine analgesics (the prodines, ketobemidone, meperidine, and 1,3,4-trimethyl-4-phenylpiperidines) has been performed with Allinger's Molecular Mechanics II (MM2) program. Phenyl equatorial conformations were found to be preferred for the prodines, ketobemidone, and meperidine. For ketobemidone and meperidine, however, phenyl axial conformations were computed to be only 0.7 and 0.6 kcal/mol higher in energy. It was suggested that phenyl axial conformers can explain the potency-enhancing effect of a phenyl m-hydroxy group in these two compounds. In contrast, phenyl axial conformers were computed to be relatively unfavorable for the prodines, being 1.9, 2.8, and 3.4 kcal/mol higher in energy for 3-demethyl-, alpha-, and beta-prodine, respectively. In addition, relative concentrations of an analgesic conformation can be related to the potencies of the three prodines. A phenyl axial conformer was computed to be preferred by 0.7 kcal/mol for the 3-demethyl compound of 1,3,4-trimethyl-4-phenylpiperidine, with phenyl equatorial conformers preferred by 1.3 and 3.3 kcal/mol for the alpha and beta compounds. Phenyl axial conformers were unexpectedly found to be especially destabilized by a 3-methyl group in the beta configuration due to the steric crowding of the three piperidine substituents. Detailed comparisons were made between the computed structures and those observed by X-ray crystallography.
TL;DR: In this paper, six 1,8-diarylnaphthalenes, each having closely held parallel aromatic rings, were synthesized by a Kharash-type Grignard cross-coupling of arylmagnesium iodide and 1-8-diiodonaphthalene in the presence of N,N′-bis(1-methyl-3-oxobutylidene)ethylenediaminatonickel(II) as a catalyst.
Abstract: Six new 1,8-diarylnaphthalenes, each having closely held parallel aromatic rings, were synthesized by a Kharash-type Grignard cross-coupling of arylmagnesium iodide and 1,8-diiodonaphthalene in the presence of N,N′-bis(1-methyl-3-oxobutylidene)ethylenediaminatonickel(II) as a catalyst. Among them, 1,8-di(1-naphthyl) naphthalene was obtained as cis and trans rotamers, caused by the restricted rotation at the 1,8-position. Interconversion between the rotamers on heating above their melting points was confirmed by differential scanning calorimetry. The proton magnetic resonance spectral studies of the 1,8-diarylnaphthalenes indicated the presence of a face-to-face arrangement of aromatic rings in the peri-position. Two new 8,8′-diaryl-1,1′-binaphthyls, each having two sets of closely held parallel aromatic rings, were isolated as minor coupling products. Trans-trans conformations were proposed for them on the basis of their thermal behavior. Several nickel(II) complexes were found to be effective catalysts f...
TL;DR: In this paper, the synthesis and resonance Raman spectra of several new N-acyl glycine ethyl dithioesters RC(=O)NH-CH2-C(=S)SR are reported.
Abstract: The synthesis and resonance Raman spectra of several new N-acyl glycine ethyl dithioesters RC(=O)NH—CH2—C(=S)SR are reported. The resonance Raman spectra of these compounds contain at least two peaks in the "C=S stretching region" between 1050 and 1200 cm−1. The solvent, concentration, temperature, and excitation wavelength dependencies of these features have been investigated. The results obtained are consistent with two conformations of the dithioesters co-existing in solution with the conformational differences being controlled by rotational isomerism about one or both of the NH—CH2 and CH2—CS bonds. Strong support for the existence of more than one conformer in solution comes from the Raman spectrum of a single crystal of N-acetyl glycine ethyl dithioester. In the spectrum of the single crystal several Raman bands are absent compared to the spectrum of the dithioester in solution indicating that only one of the conformers found in the solution phase is retained in the crystal. In one of the conformers...
TL;DR: The results of a conformational study by 1H and 13C high-resolution NMR at 270 and 500 MHz on the peptide hormone somatostatin have been compared with a series of conformers generated by semi-empirical energy calculations to conclude that this set of conformations forms a major contribution to the conformational equilibrium at room temperature.
TL;DR: In this article, force field calculations and dynamic NMR measurements of symmetrical 1,2-dialkyl-1,1,2,2tetramethylethanes where the alkyl group is methyl, ethyl, isobutyl, neopentyl, cyclohexyl, or t-butyl are reported.
TL;DR: In this paper, a model for 13C-shift calculation is proposed in which shift increments for the different torsional arrangements are obtained by linear interpolation between corresponding cyclohexane values.
TL;DR: In this article, a rigorous error analysis of LIS is described which avoids the use of any normalising atom and is used to investigate the LIS of benzaldehyde and thiophen- and furan-2-aldehyde.
TL;DR: In this paper, a conformational isomerism investigation of meta-nitrobenzaldehyde and furan- and thiophene-3-aldehydes, and of the conformation of acetophenone was carried out.
TL;DR: The potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V1=−618±6, V2=1970±27, V3=399±6 and V4=−48±10 cm−1.
Abstract: The infrared (3500 to 50 cm−1) and Raman (3200 to 10 cm−1) spectra have been recorded for the gaseous and solid states of cyclopropylcarbonyl fluoride, c‐C3H5CFO. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. The asymmetric torsion for the conformer which has the carbonyl group cis to the cyclopropane ring has been observed at 88.5 cm−1 in the far infrared spectrum of the gas with five accompanying hot bands, and the corresponding torsion of the trans conformer was observed at 91.0 cm−1 with two additional hot bands occurring at 90.6 and 90.2 cm−1. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V1=−618±6, V2=1970±27, V3=399±6, and V4=−48±10 cm−1. It has been determined that the cis conformer is the predominant form at ambient temperature in the gas phase and the enthalpy difference between the cis and the trans confo...
TL;DR: In this article, a possible conformation of the complex formed by one calcium ion and two molecules of the ionophore A23187 at a simulated lipid-water interface was predicted by a variant method for conformational analysis.
Abstract: A possible conformation of the complex formed by one calcium ion and two molecules of the ionophore A23187 at a simulated lipid--water interface was predicted by a variant method for conformational analysis. This method takes into account, in addition to the Van der Waals energy, electrostatic interaction, and torsional potential, the alteration of electrostatic forces attributable to changes in dielectric constant at the interface and the transfer energy for each part of the complex as it moves through the lipid-water interface. The most probable conformer was characterized by a two-fold axial symmetry that was maintained during transition to the hydrophobic bulk conformation. Minor changes in the interfacial structure were sufficient to achieve the configuration characteristic of the hydrophobic bulk phase.
TL;DR: It is shown that the unmethylated dimers can be arranged in two groups according to their melting temperatures and the postulation of a right-handed helical S-S stacked state on the basis of NMR and circular dichroic data is rationalized.
Abstract: Proton NMR studies at 360 MHz are reported on the adenine dinucleoside monophosphates N6-dimethyladenyly(3'-5')-N6-dimethyladenosine (m(6)(2)Apm(6)(2)A), ApA, rApdA, dAprA and on the methyl phosphate esters of the monomers m(6)(2)Ap, pm(6)(2)A, Ap and pA. Complete 1H-NMR spectral assignments are given. The dimers were also investigated by means of circular dichroism to obtain accurate thermodynamic parameters of the stacking equilibrium. With the aid of the thermodynamic data NMR coupling constants are extrapolated to values appropriate to the stacked conformers. A modernized version of pseudorotation analysis is used to delineate the conformational behaviour of the ribose and 2'-deoxyribose rings. It is shown that the unmethylated dimers can be arranged in two groups (dApdA/dAprA vs ApA/rApdA) according to their melting temperatures. ApA and the fully N6-methylated dimer m(6)(2)Apm(6)(2)A prefer to adopt the classical right-handed N-N stacked conformation. Both dimers with a 2'-deoxyribose ring at the 5'-OH end (dApdA and dAprA) behave similarly and occur in solution as a 75:25 mixture of S-S and S-N stacked states. The fully stacked hybrid dimer rApdA displays an unexpectedly large amount of S conformers (greater than 40%) in both sugar rings. This finding is rationalized by the postulation of a right-handed helical S-S stacked state on the basis of NMR and circular dichroic data.
TL;DR: In this article, general procedures have been developed for preparing series of thiazole-4- and -5-carboxylates containing alkyl and halogeno substituents.
Abstract: Convenient general procedures have been developed for preparing series of thiazole-4- and -5-carboxylates containing alkyl and halogeno substituents. While both series of esters show i.r. carbonyl doublets caused by rotational isomerism, the more intense absorptions of the 4-carboxylates are the lower wavenumber components, whereas those of the 5-carboxylates are the higher wavenumber components. In both series the stronger bands arise from the thermochemically more stable forms; identification of these forms as the carbonyl O,S-syn-s-trans rotamers is more certain with the 4-carboxylates than with the 5-carboxylates.
TL;DR: In this article, the effect of substituents on the electronic charge distribution of aliphatic aldoximes was studied in terms of calculated electronic properties, and the most stable conformer and the energy difference between the anti and syn conformers were determined by fully optimizing geometrical parameters.
Abstract: Results of ab initio SCF calculations using a double-zeta (DZ) basis set are reported for aliphatic aldoximes RCHNOH (R H, CH3, NH2, OH and F). The most stable conformer and the energy difference between the anti and syn conformers have been determined by fully optimizing geometrical parameters. The effect of substituents on the electronic charge distribution of these molecules is studied in terms of calculated electronic properties.
TL;DR: Dynamics 13 C NMR studies of 7-substituted 2-methoxy-methylenecyclohexanes indicate that the proportion of axial orientation decreases as the 7substituent becomes less electron donating, the equatorial conformer being largely predominant in the 7-cyano derivative as discussed by the authors.
TL;DR: Conformational analysis of the alpha-ethyl group inalpha-ethylphenethylamine showed that this group exists in two equally populated conformations in both solvents, and it is suggested that these conformations hinder the approach of amphetamine to the brain synaptosomal NE uptake sites.
Abstract: Amphetamine is a strong competitive antagonist of brain synaptosomal [3H]norepinephrine ([3H]NE) uptake. Its alpha-ethyl analogue is much less active, while 2-aminotetralin and 1,2-dihydro-2-aminonaphthalene, in which the alpha-ethyl group is tied to the aromatic ring, possess about the same inhibitory potency as amphetamine. The conformational properties of these compounds in solution were studied by 1H and 13C NMR methods. Only small differences between amphetamine and alpha-ethylphenethylamine hydrochlorides were observed in the relative rotamer populations due to rotation around the C alpha -C beta bond of the side chain. In D2O the gauche conformation is slightly favored, while in CDCl3 the trans conformation is the predominant one. Conformational analysis of the alpha-ethyl group in alpha-ethylphenethylamine showed that this group exists in two equally populated conformations in both solvents. It is suggested that these conformations hinder the approach of alpha-ethylphenethylamine to the brain synaptosomal NE uptake sites.
TL;DR: In this paper, the 1,2,3,4,4-tetrachloro-9-(2-oxopropyl)triptycene grown from tetrahydrofuran-hexane was found to contain 1:1 ap and ±sc rotational isomers about the C9-CPr bond by the X-ray diffraction method.
Abstract: Crystals of 1,2,3,4-tetrachloro-9-(2-oxopropyl)triptycene grown from tetrahydrofuran-hexane were found to contain 1:1 ap and ±sc rotational isomers about the C9–CPr bond by the X-ray diffraction method. The molecular structures had the following features. The acetyl group in the 9-substituent takes an O-inside, relative to the triptycene skeleton, conformation in both forms. The benzene rings are deformed by the steric effect to form boats. The 2-oxopropyl groups in both forms were planar within the experimental error but the C2–C1 bond in the 2-oxopropyl group was appreciably shorter in the ±sc conformation than that in the ap. The results are discussed on the ground of steric repulsion and the existence of the interaction between the C=O and the Cl groups which are located only 3.035 A apart in the ±sc form.
TL;DR: The analysis of the chemical shifts, line widths, coupling constants and rotamer populations around chi 1 and chi 2 suggests that the conformational properties of Met-enkephalin in the two solvents are quite different.
Abstract: 1H spectra at 270 MHz of zwitterionic Met-enkephalin pentapeptide (Tyr-Gly-Gly-Phe-Met) in (C2H3)2SO/water mixtures are reported and discussed in terms of solvent-induced conformational transitions. The analysis of the chemical shifts, line widths, coupling constants and rotamer populations around chi 1 and chi 2 suggests that the conformational properties of Met-enkephalin in the two solvents are quite different. In aqueous solution, the preferred structure, characterized by the absence of intramolecularly hydrogen-bonded NH groups and head-to-tail interactions, very likely is an equilibrium of unfolded conformations with approximately equal energy. In (C2H3)2SO, the preferred structure is folded, with the Met-5 NH intramolecularly bonded and the Gly-3 NH protected from the solvent, while the Gly-2 and Phe-4 amide protons are solvent exposed. A conformational transition of Met-enkephalin from the intramolecularly bonded to the unbonded one takes place at about 40 mol-% water in (C2H3)2SO, involving the Met-5 NH proton and the Tyr-Gly-Gly fragment. The Phe-4 and Met-5 phi angles do not change appreciably, which suggest that an inversion at the Gly-3 residue of the folded form, responsible for the conformational transition, does not affect the C-terminal moiety. At about 70 mol-% water in (C2H3)2SO a change in the solvent mixture properties affects the chi 1 rotamer populations and the ring dynamics of the aromatic side chains. The line broadening of the Tyr-1 delta and epsilon proton resonances indicates a specific interaction of the N-terminal ring with the solvent.
TL;DR: In this article, the 1H and 13C n.m. spectra of 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione and their Schiff's bases were studied in CDCl3 over a range of temperatures for elucidation of possible tautomerism.
Abstract: The 1H and 13C n.m.r. spectra of 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione (I) and 3,5-diacetyltetrahydropyran-2,4,6-trione (III) and their Schiff's bases were studied in CDCl3 over a range of temperatures for elucidation of possible tautomerism. Whereas compound (I) appears to exist in only one form, at least two forms have been detected for compound (III). The Schiff's bases exist predominantly in keto-amine forms and the rotational barrier due to substantial double bond character of the exocyclic carbon to carbon bonds allows ready detection of rotamers. Structures of the predominant rotamers are deduced from the 1Hand 13C spectra and the assignments of NH proton signals are based on the relative strengths of the intramolecular hydrogen-bonds. The chemical shifts of NH protons correlate linearly with the pKa of the amine precursors. The 15N spectra of some 15N-enriched Schiff's bases were obtained in CDCl3 and in (CD3)2SO and the considerable downfield shifts of the 15N resonances, compared with those of typical amides, are indicative of extensive electron delocalisation.
01 Nov 1982
TL;DR: In this paper, the fluorescence origins of ground-state conformers are split by 4850 cm−1 and the two naphthalene moieties are approximately perpendicular in the conformer with the higher-energy origin.
Abstract: Vibronic spectra of 1, 1′-binaphthyl in pentane Shpolskii matrices at 4 K are reported. Taken in conjunction with the IR and Raman spectra, it can be seen that these arise from two non-interconverting ground-state conformers. The fluorescence origins of these conformers are split by 4850 cm−1. A vibronic analysis suggest that the two naphthalene moieties are approximately perpendicular in the conformer with the higher-energy origin, while being approximately planar in the other. The absence of progressions in the torsional mode in the spectra of each conformer suggests that their conformations are unaltered in the excited state. Spectra in other media suggest that the two ground-state conformers also exist in many liquid solutions.
TL;DR: In this article, a ring-puckering potential function for 1-silacyclobutane-1-d/sub 1/ has been determined and its infrared spectra have been analyzed.
Abstract: 1-Silacyclobutane-1-d/sub 1/ has been prepared and its infrared spectra have been analyzed. Several ring-puckering transitions have been observed in the far-infrared region and sum and difference bands have been observed in the Si-H stretching region. A ring-puckering potential function which simultaneously fits the data for the D/sub 0/, d/sub 1/, and d/sub 2/ isotopic forms has been determined. Calculation of the wave functions for the d/sub 1/ molecule shows that each of the lowest two very nearly degenergy states (nu = 0 and nu = 1) corresponds to only one distinct molecular conformation. Similarly, the nearly degenerate nu = 2 and nu = 3 states are primarily restricted to specific conformations. Doublets observed for the Si-H and Si-D stretching bands, corresponding to stretches from equitorial or axial positions, confirm these calculations. Model calculations on related systems demonstrate that both a sufficiently large barrier height and an asymmetry of the kinetic energy expansions are necessary for the conformational separation according to vibrational states. Although the reduced masses of the two conformers of 1-silacyclobutane-1-d/sub 1/ differ by only 0.20%, this is enough to produce the observed conformational discrimination.