Showing papers on "Conformational isomerism published in 1983"

Solution conformation of asparagine-linked oligosaccharides: alpha(1-6)-linked moiety.

Abstract: The solution conformation is presented for representatives of each of the major classes of asparaginyl oligosaccharides. In this report the conformation of the alpha(1-6)-linked moiety is described. The conformational properties of these glycopeptides were determined by high-resolution 1H nuclear magnetic resonance in conjunction with potential energy calculations. The NMR parameters that were used in this analysis were chemical shifts and nuclear Overhauser enhancements. Potential energy calculations were used to evaluate the preferred conformers available for the different linkages in glycopeptides and to draw conclusions and to draw conclusions about the behavior in solution of these molecules. For all classes, identical conformations were found for the 6-arm except for the torsional angle, omega, about the C5-C6 bond of the alpha 1-6 linkage. For high mannose and hybrid structures omega was found to be -60 degrees, for bisected biantennary complex structures omega was 180 degrees, and for complex biantennary structures averaging between -60 degrees and 180 degrees occurs.

Molecular force fields of s-trans-1,3-butadiene and the second stable conformer.

Abstract: Normal-coordinate analyses of s-trans-1,3-butadiene and its various deuterated and 13C-substituted analogs were performed using the group-coordinate force field, and a set of force constants was determined. The infrared spectra of the second stable conformer of 1,3-butadiene-d0, -1,1,4,4-d4, and -d6 were analyzed on the basis of normalcoordinate calculations of the planar s-cis form and a nonplanar gauche form. Such an analysis suggests that the second stable conformer takes a gauche form rather than the s-cis. To increase the number of reliable data to be used in this study, the infrared and Raman spectra of the d0, 1,1,4,4-d4, and d6 species were observed in low-temperature Ar matrices. The infrared spectra of the second stable conformer were also observed using a combination of the high-temperature nozzle and low-temperature matrix techniques.

Solution Conformations, Photophysics, and Photochemistry of Bile Pigments; Bilirubin and Biliverdin, Dimethyl Esters and Related Linear Tetrapyrroles

Abstract: Bilirubin and biliverdin dimethyl esters (BRE and BVE, respectively) and related linear tetrapyrroles have been studied using a combination of photochemical and spectroscopic techniques, the latter including absorption, fluorescence fluorescence excitation, medium-induced circular dichroism, and proton magnetic resonance. Both types of tetrapyrroles form mixtures of different topological isomers in very dilute solutions. In the case of the bilirubins the heterogeneity of the solutions is caused by two coexisting conformers with different orientations of the A/B and C/D pyrromethenone moieties with repect to each other. The spectral properties of one conformer resemble the isolated parent pyrromethenone, whereas those of the other result from electronic coupling of the two subchromophores presumably held in a “ridge tile” -like orientation. CC rotations at the C-5 and C-15 bridges substantially compete in both components with the photochemical channels (EZ isomerization and lumirubin formation) for the radiationless deactivation of the excited singlet state. The more rigid “ridge tile” component additionally undergoes hydrogen bond-mediated deactivation, and it photoisomerizes more efficiently. The situation is markedly more complex with the biliverdins. In order to obtain a more detailed insight into the mechanisms of the radiationless excited-state processes, time-resolved optoacoustic spectroscopy and ultrafast absorption (pump-probe) and fluorescence detection (single-photon-timing) techniques were used to supplement the stationary methods. The solution mixtures are composed of a (family of) helically coiled all-Z, all-syn species, and of species differing from the former by stretched arrangements of the rings B and C around the central C-10 bridge (E-anti, E-syn, and Z-anti). Two excited singlet states with picosecond lifetimes are attributed to either one or two coiled ground-state forms, and two remarkably long-lived nanosecond excited states arise each from a stretched ground state. The radiationless deactivation of the shorter-lived of the picosecond states is brought about by ultrafast intramolecular proton transfer between the B/C nitrogen atoms, in addition to the CC rotational modes operative in both. ZE photoisomerization is also an appreciable deactivation channel of excited biliverdin dimethyl ester. It is confined to the central C-10 double bond and selectively affords a stretched isomer (10E-anti), which thermally reforms the coiled starting meterial at room temperature via a sequence of tautomerization and CC rotation. Heating or ultrasonic treatment can reverse this sequence and drive it farther to populate another stretched isomer (10E-syn) which is thermally stable at room temperature. This stretched form aggregates (presumably to dimers) already at concentrations at which the coiled species still appears to be fully monomeric.

Intramolecular photochemical electron transfer. 2. Fluorescence studies of linked porphyrin-quinone compounds

Abstract: Systematic studies of absorption spectra and fluorescence spectra and lifetimes have been carried out on a series of meso-tetratolylporphyrins to which various molecular entities have been covalently attached via diamide linkages with the two amides being separated by n methylene groups (n=2, 3, or 4). The attached end groups include p-benzoquinone, hydroquinone, and dimethoxybenzene. These studies reveal the existence of at least two more or less distinct forms: a family of ''complexed'' conformers in which the end group is likely folded so as to interact with the porphyrin, and one or more ''extended'' conformers in which the porphyrin moiety is relatively unperturbed by the end group. The complexed conformers exhibit perturbations of spectra and diminished fluorescence lifetimes and quantum yields as compared with the extended conformer(s). Oxidation of the porphyrin-linked hydroquinone form to the quinone form does not significantly affect the absorption or fluorescence spectra but causes strong quenching of fluorescence and diminution of the fluorescence lifetimes. This quenching is interpreted primarily in terms of electron transfer from the lowest excited singlet state of the porphyrin to the quinone moiety. On the basis of the assumption that these shorter fluorescence lifetimes of the quinone relative to the hydroquinone are duemore » entirely to electron transfer, apparent electron-transfer rate constants k/sup et/ at room temperature range from <1 x 10/sup 8/ to <7 x 10/sup 9/ s-/sup 1/, depending on solvent and probably the specific geometry of the conformers. Quenching in both sets of conformers appears to be thermally activated and is strongly inhibited in frozen matrices. Parallel studies of porphyrin-quinone molecules with various methylene chains (n=2, 3, and 4) indicate that the geometry of the linkage is critical to the rate of electron transfer. A methylene chain with n = 3 appears to be optimum. 4 figures, 6 tables.« less

Infrared-induced conformational isomerization of ethylene glycol in a low-temperature argon matrix

Abstract: The infrared spectra of ethylene glycol and its three deuterated species trapped in low-temperature Ar matrices show irreversible changes upon exposure to the radiation from the light source of an infrared spectrophotometer. Spectral changes in the O-H stretching region demonstrate the existence of two intramolecularly hydrogen-bonded conformers, namely, TGG′ and GGG′, which are predominant immediately after deposition but diminish under irradiation through isomerization to other conformers. Analysis of the spectral changes in the C-O stretching and OH bending regions suggests that the extended all-trans conformer, TTT, is the main product of isomerization. Therefore, the main pathways of conformational isomerization under study are those which go from both TGG′ and GGG′ to TTT. This conclusion is consistent with more detailed vibrational assignments based on normal-coordinate calculations. Further, the intensities of bands assigned to TGG′ are found to decrease faster than those of bands assigned to GGG′. Experiments with filter show that the infrared light between 3950 and 3600 cm−1, especially in the 3700-3600 cm−1 region, is responsible for the conformational isomerization. A possible isomerization mechanism is proposed.

Synthesis and conformation of procyanidin diastereoisomers

Abstract: The four theoretically possible diastereoisomeric 4 → 8 linked procyanidin dimers with two 2,3-cis-flavanoid units, epicatechin-(4β→ 8)-epicatechin (6), epicatechin-(4β→ 8)-ent-epicatechin (8), ent-epicatechin-(4α→ 8)-epicatechin (7), and ent-epicatechin-(4α→ 8)-ent-epicatechin (2), have been synthesized together with the 2,3-trans diastereoisomers catechin-(4α→ 8)-catechin (14) and catechin-(4α→ 8)-ent-catechin (15). The preferred rotamer conformations of the deca-acetate derivatives of (6), (8), (14), and (15) in chloroform were deduced from their 13C n.m.r. and high-field 1H n.m.r. spectra. The 1H n.m.r. spectra of the preferred conformers of the pairs of acetate derivatives of (6) and (15), and (8) and (14), were qualitatively similar as the relative configuration about the interflavanoid bond of one pair has a meso-relationship to the other.

A theoretical investigation of the conformation of polychlorinated biphenyls (PCB's)

Abstract: The rotational conformations of BP*, 2,2′-DCBP, 2,3′-DCBP, 3,3′-DCBP, 3,3′,4,4′-TCBP and 3,3′,4,4′,5′5′-HCBP are investigated by the GAUSSIAN-80 ab initio technique. All molecules that do not have ortho -substitution have an energy minimum at 42° and barriers of approx. 3.6 and 2.3 kcal mol −1 occur at 0° (and 180°) and 90°, respectively. Ortho -substitution results in an extremely high barrier at 0° (and 180°) and a shift of the energy minimum toward 90°. These results may account for the diminished physiological activity of ortho substituted PCB's; the planar (0° or 180°) rotamers are not energetically accessible.

Theoretical Determination of Molecular Structure and Conformation. XI. The Puckering of Oxolanes

Abstract: Structural, conformational and energetic properties of cyclopentane (1) and the seven oxolanes monoxolane (2), 1,3-dioxolane (3), 1,2-dioxolane (4), 1,2,4-trioxolane (5), 1,2,3-trioxolane (6), tetroxolane (7), and pentoxolane (8) are investigated employing the polarized 6-31G* basis set at the Hartree-Fock level of theory. Extensive geometry optimization is carried out within the model of the semirigid pseudorotor. The conformational potentials V of compounds 1–8 are evaluated as a function of the puckering amplitude q and the pseudorotation phase angle Φ. Ring molecules 1 and 8 are free pseudorotors, while pseudorotation is hindered by barriers ≤ 3.3 kcal/mol for oxolanes 2–7. Puckering and inversion barriers increase with the number of O-O bonds but decrease with the number of ether bridges. Puckered C2-symmetrical twist forms are the most stable conformations for compounds 2–7 but 6, where highest stability is found for the C2-symmetrical envelope forms. At room temperature a multitude of conformers of 1–8 coexists either because of free pseudorotation (barriers < RT) or large amplitudes of pseudolibration. These results are rationalized in terms of the rotor potentials of appropriate reference compounds (C2H6, CH3OH, H2O2). A more elegant approach leads to a simple π electron count and an analysis of bonding and antibonding overlap in the π-type HOMO's. In this way the effects of substituents can be predicted and conformational preferences of the furanose ring in nucleotides, nucleosides and carbohydrates explained. The relative stability of the oxolanes is analyzed by calculating O-O bond energies, bond-bond interactions and ring strain for each compound. The lability of the higher oxolanes is traced back to increased ring strain. A new method of the conformational analysis of ring compounds is outlined.

Multiple conformational states of a Pro-Pro peptide. Solid-state and solution conformations of Boc-Aib-Pro-Pro-NHMe

Abstract: The solid-state and solution conformations of the model peptide Boc-Aib-Pro-Pro-NHMe have been studied by X-ray diffraction and NMR. The peptide adopts a poly(pro1ine 11) conformation in the solid state. Two molecules are observed in the asymmetric unit differing in the geometry (cisltrans) of the urethane group. The molecules are held together in the crystal by a complex network of hydrogen bonds involving three molecules of water, which cocrystallize. Dissolution of single crystals at low temperature (\sim 233 K) permits NMR observation of the solid-state conformer. In solution, the peptide undergoes a trans-cis isomerization of the Pro-Pro bond. Low- temperature NMR measurements allow the detection of three conformational states of the Pro-Pro segment. Both cis’ and trans’ rotational isomers about the $C^\alpha -CO$ $(\psi)$ bond of Pro-3 are detectable at low temperatures. Theoretical calculations suggest an appreciable activation barrier to $\psi$ rotation. Temperature and solvent dependence of NH chemical shifts provide evidence for an intramolecular hydrogen bond, involving the NHMe group in the cis Pro-Pro conformer. Energy calculations suggest the possibility of a type VI $\beta$-turn conformation stabilized by a 4 $\rightarrow$ 1 hydrogen bond between the Aib-1 CO and NHMe groups.

Effects of pressure on conformer equilibria in liquid n‐hexane

Abstract: From a study of the dependence of the polarized Raman spectra of liquid n‐hexane (30 °C) on pressure (0.26–7.38 kbar), we have confirmed the findings of Schoen et al. [Nature 270, 412 (1977)] that pressure is effective in perturbing the distribution of conformers. However, our results on the behavior of the gauche forms differ from these authors. We find that increased pressure decreases the concentration of the ttt conformer and increases the concentrations of both the single‐ and double‐gauche forms. Surprisingly, the pressure dependence of the single‐ and double‐gauche conformers is nearly the same. Thus while the volume change for ttt→gtt is found to be about −1.1±0.0 cm3/mol, that for the single‐gauche to double‐gauche conversion is about 0.0±0.0 cm3/mol. These results appear to rule out a simple additivity model for the n‐alkanes in which CC bonds respond independently to pressure. Our measurements also suggest that the concentration of the all‐trans conformer in liquid n‐hexane is significantly hig...

Multiple internal reflectance infrared spectra of variably hydrated hemoglobin and myoglobin films: effects of globin hydration on ligand conformer dynamics and reactivity at the heme.

Abstract: Multiple internal reflectance infrared (IR) spectra are reported for variably hydrated films (1.2-0.1 g of H2O/g of protein) of the carbon monoxy and oxy forms of human Hb and sperm whale Mb. The spectra show that even the limited removal of liquid and icelike hydration constraints at the globin surface is sufficient to cause a dramatic, but completely reversible, shift toward a normally minute population of sterically unhindered, linear-perpendicular, Fe-CO conformer modes (nu CO = 1968-1967 cm-1), and the destabilization of distally hindered, tilted (or bent), Fe-CO modes (nu CO = 1951, 1944-1933 cm-1). Corroborative evidence from IR band broadening trends [delta delta nu 1/2 (1968, 1967 cm-1) approximately 2-4 cm-1], corresponding changes in the visible, and H-D exchange kinetics confirm that the shift toward 1968-1967 cm-1 results in a more open distal heme pocket configuration and that it is also accompanied by a buildup of deoxy-like steric hindrance proximal to the heme. Denaturation effects are eliminated as a potential cause of the shifts, as are specific protein-protein, ion-protein, intersubunit, and MIR crystal-film surface interactions. The hydration effect exhibits globin-dependent and ligand-dependent differences, which highlight the intrinsic importance of distal steric effects within the heme pocket and their dynamic coupling with exterior solvent constraints. CO-photodissociation and O2-exchange experiments conducted on rapidly interconverting (coupled and fully hydrated) and noninterconverting (uncoupled and partially hydrated) Fe-CO conformers also suggest that the open linear-perpendicular mode corresponds to a more tightly bound form of CO than the axially distorted Fe-CO species; similar differences are not evident in Fe-O2, which already prefers a bent end-on geometry within the heme pocket. Control IR spectra aimed at monitoring the progressive effects of various denaturants on HbCO further indicate that this same open mode serves as a common precursor to any of a number of more highly disordered folding modes. The overall properties of the 1968-1967-cm-1 conformer are discussed in terms of (1) the possibility of its corresponding to an available relaxation mode capable of facilitating the dynamics of ligand entry-release events and (2) its potential additional significance as a native folding mode that exhibits a marked tendency to be destabilized by hydration.

Theoretical conformational studies on some dopamine antagonistic benzamide drugs: 3-pyrrolidyl- and 4-piperidyl derivatives.

Abstract: Model derivatives of 3-pyrrolidyl- and 4-piperidyl-o-methoxybenzamides, as representatives of neuroleptic substituted benzamide drugs, have been investigated by theoretical conformational analysis. Folded conformers of 2-methoxy-N-(1-methyl-3-pyrrolidyl)benzamide have the lowest energy, but extended conformers are only a few kilocalories per mole less stable. As regards to piperidyl derivative, it has been found that folded conformers are of much higher energy than extended ones. These and previous results are discussed in terms of the pharmacologically active conformers of substituted benzamide drugs and of possible modes of interaction with the dopamine receptor.

Structural properties of some monosubstituted cyclobutanes, C4H7X, determined by ab initio calculation II. X = OH, NH2 and CH3

Abstract: Complete equilibrium geometries have been determined for all stable conformations of cyclobutanol, C4H7OH, cyclobutylamine, C4H7NH2, and methyl cyclobutane C4H7CH3, by ab initio Hartree—Fock gradient calculation with a 4–21 basis augmented by d functions on O and N. Conformations differ in having the substituent equatorial (eq) or axial (ax) on the puckered ring and by gauche (g) or anti (a) orientation of the substituent hydrogen with respect to the ring hydrogen. For C4H7NH2 four stable conformers exist, a—g—eq being the most stable with ΔE for the others being: g—g—eq, 3.2; g—g—ax, 4.2; and a—g—ax, 6.5 kJ mol−1. For C4H7OH only three conformers are found with a—eq most stable, ΔE for g—eq, 2.1, ΔE for g—ax, 8.1 kJ mol−1, and no minimum found for a—ax. For C4H7CH3, only two conformers can exist when the CH3 group staggers the adjacent CCH group. The eq form is 1.7 kJ mol−1 more stable than the ax. These results extend our earlier series and show that the equatorial position is most favored by the most electronegative substituent. The axial conformer becomes increasingly more stable in the series F < OH < NH2 < Cl < CH3 < Li. Ring structural parameters show only small differences, but these vary quite systematically with substituent electronegativity. Trends are established for differences in adjacent vs. opposite CC bond lengths, ring puckering angle, distortions of the CH2 groups, ∠HCX and ∠XC2C1C4.

Dynamics of an interfacial methylene in dimyristoylphosphatidylcholine vesicles using carbon-13 spin relaxation.

Abstract: The dynamic and conformational properties of the 2-methylene on the sn-2 chain of dimyristoylphosphatidylcholine have been investigated in small unilamellar vesicles. An analysis of the spin relaxation of a proton-coupled 13C nucleus has been used to provide the additional information necessary to propose a specific geometry for motion. The results suggest a model with three motions in addition to vesicle tumbling: (1) a slow axial rotation of the entire molecule about the bilayer normal (tau congruent to 2 X 10(-8) s); (2) torsional oscillations about C-C bonds on a very fast time scale; and (3) rapid jumps (tau = 6 X 10(-10) s) between two conformers having approximate gauche+ and gauche- conformations about the C2-C3 bond of the sn-2 chain. The proposed conformations are compared to those previously predicted on the basis of crystal structures, spectroscopic data, and energy-minimization calculations.

Carbon-13 NMR in conformational analysis of nudeic add fragments. Heteronuclear chemical shift correlation spectroscopy of RNA constitutents

Abstract: The assignment of the non-quaternary 13C resonances by means of two-dimensional heteronuclear chemical shift correlation spectroscopy is presented for several oligoribonucleotides: The dimers m6(2)AU, m6(2)Am6(2)A and mpUm6(2)A and the trimers m6(2)AUm6(2)A and m4(2)Cm4(2)Cm6(2)A. The temperature and concentration dependency of the 13C chemical shifts are studied with emphasis on the behaviour of the dimer m6(2)AU. The present study shows that in the 5-50 mM range the concentration-dependent chemical shift changes of the ribose carbons are negligible compared to chemical shift changes due to intramolecular events. All compounds studied show a surprising correlation between the chemical shifts of the carbon atoms of the ribose ring and the sugar conformational equilibrium as expressed by the percentage N or S conformer. Thus the chemical shift data can be used to obtain the thermodynamical parameters of the two-state N/S equilibrium. Parameters deduced for m6(2)AU are Tm = 306 K and delta S = -25 cal mol-1 K-1, which values are in satisfactory agreement with results obtained earlier from 1H NMR and from Circular Dichroism.

Ursachen einer unerwarteten Rotationsbehinderung in phosphinosubstituierten Phosphor-Yliden R3PC(PR2)2. Kristallstruktur von (C6H5)3PC[P(C6H5)2]2

Abstract: Die 31P-NMR-Spektren der Ylide (C6H5)3PC[P(C6H5)2]2(1) und (C6H5)2PCH2(C6H5)2PC-[P(C6H5)2]2 (2) sowie der Alkali-phosphoniumbisylide Na⊕ und K⊕ [C6H5)2PCH⊖](C6H5) 2PC-[P(C6H5)2]2 (3 bzw. 4) sind stark temperaturabhangig. Oberhalb ca. + 50°C sind in den Solventien Toluol oder Chloroform (1, 2) bzw. Tetrahydrofuran (3, 4) die P-Atome der (C6H5)2P-Gruppen an der Ylid-Funktion chemisch aquivalent, wahrend sie bei ca. −50°C in ihren Werten fur die chemische Verschiebung und die Kopplungskonstanten drastisch verschieden sind. Das Phanomen wird durch eine Bevorzugung von unsymmetrischen Rotameren (z. B. 1b) gedeutet. Die Rontgenbeugungsanalyse von 1 bestatigt die Richtigkeit dieser Annahme auch fur den kristallinen Zustand. Das Molekul besitzt ein fast planares PCP2-Gerust, und die freien Elektronenpaare der beiden dreiwertigen P-Atome an der Ylid-Funktion besitzen in dieser PCP2-Ebene ihre Vorzugsrichtung. Die Aktivierungsenergie der Rotation um die PC-Einfachbindung betragt fur 1–3 ca. 50 kJ·mol−1. Origin of the Unexpected Restricted Rotation in Phosphino-substituted Phosphorus Ylides R3PC(PR2)2. Crystal Structure of (C6H5)3PC[P(C6H5)2]2 The 31 PNMR spectra of the ylides (C6H5)3PC[P(C6H5)2]2 (1) and (C6H5)2PCH2(C6H5)2PC [P(C6H5)2]2 (2) as well as of the alkali phosphoniumbisylides Na⊕ and K⊕ [C6H5)2PCH⊖]-(C6H5) 2PC[P(C6H5)2]2 (3 and 4, respectively) are strongly temperature dependent. Above + 50°C, in toluene, chloroform (1, 2) or tetrahydrofuran solvent (3, 4), the phosphorus atoms of the (C6H5)2P groups attached to the ylidic function are chemically equivalent while at −50°C they differ drastically both in their chemical shifts and in their coupling constants. The phenomenon is explained on the basis of a preference of unsymmetrical rotamers (e.g. 1b). This model is proven to be valid also in the crystalline state by a single crystal X-ray diffraction analysis of compound 1. The molecule has an almost planar PCP2 skeleton, with the lone pairs of electrons of the trivalent phosphorus atoms having their preferred orientation in this PCP2 plane. The activation energy for the hindered rotation around the P C single bond amounts to no less than 50 kJ·mol−1.

Conformational studies on poly(ethacrylic acid) in aqueous salts by potentiometric, viscometric, optical and1H-NMR measurements

Abstract: The conformation of poly(ethacrylic acid) was studied in aqueous salts by means of potentiometric, viscometric, optical and1H-NMR measurements. The modified potentiometric titration curves in aqueous NaCl at ionic strengths 0.01–0.3 and 5–35 °C indicated the pH-induced conformational transition from the compact to extended coil. The standard free energy changes and the difference in molar heat capacity between the two conformers were estimated from the potentiometric data. The viscometric and optical behaviours did also show the existence of the compact form at acid pH's and the pH-induced conformational transition to the extended coil in aqueous NaCl and NaClO4. From comparison with the data for poly(methacrylic acid), the results were concluded to be consistent with the existence of the compact form stabilized by hydrophobic interaction between ethyl groups. The methyl widths in 400 MHz1H-NMR spectra indicated that the side chains in the polymer in the compact form are in a more restricted motional state than in the coil form. Intermolecular aggregations were found at very low degrees of ionization of carboxyl groups.

On the conformation of 5-substituted uridines as studied by proton magnetic resonance.

Abstract: The proton magnetic resonance (pmr) spectra of 10 basemodified uridine derivatives x5Urd have been measured at 3 degrees, 30 degrees, and 60 degrees C in order to correlate the electronic effects of different substituents with the molecular conformation of the respective nucleosides. The results presented demonstrate the close relation between conformational parameters and the electron-affinity of the substituents as reflected by their Hammett constants. Going from electron-donating to electron-accepting groups, the portion of N-conformer in the ribose N in equilibrium S equilibrium increases from 44% to about 90%. In addition the percentage of gauche-gauche rotamer as measured for the exocyclic groups changes from about 30% in nh52Urd to more than 80% in no52Urd.

The chemistry of vitamin B12. Part 20. Diaquocobinamide : pK values and evidence for conformational isomers

Abstract: An improved method for preparing aqueous solutions of diaquocobinamide without hydrolysis products is described. The pKa values have been determined in 0.2 mol dm–3 NaClO4 at 25 °C as pK1= 5.9 ± 0.1 and pK2= 10.3 ± 0.2 and are shown to involve one proton each. Evidence is presented that diaquo-, aquohydroxo-, and dihydroxo-cobinamide exist in solution as isomers depending on hydrogen bonding between the axial ligands and different amide side-chains.

Far infrared spectra, conformational potential function, and barrier to methyl rotation of propionyl fluoride

Abstract: The far infrared spectra of propionyl fluoride CH3CH2CFO in the gaseous and solid states have been recorded from 500 to 40 cm−1. A rather complex spectrum of the gas was observed and a substantial number of bands have been assigned to the asymmetric torsional modes for both the s‐cis (oxygen atom eclipsing the methyl group) and the high energy gauche conformers. Analysis of these bands permitted the calculation of the torsional potential function present in this molecule. The potential coefficients for the asymmetric torsional mode were calculated to be: V1=341±24, V2=236±20, V3=390±3, and V4=21±6 cm−1, with an enthalpy difference between the more stable s‐cis and the high energy gauche conformers of 434±20 cm−1 (1.24±0.06 kcal/mol). This function gives a potential barrier of 692 cm−1 (1.98 kcal/mol) separating the s‐cis from the gauche form and 283 cm−1 (0.81 kcal/mol) separating the two equivalent gauche forms. From a temperature study of the Raman spectrum of the gas, the enthalpy difference between th...