Configurational assignment in alkyl‐branched sugars via the geminal C,H coupling constants

The measurement of the magnitude and sign of ²J(C,H) couplings offers a reliable way to determine the absolute configuration at a carbon center in a fixed cyclic system. A decrease of the dihedral angle ? in the O—C_A—C_B—H fragment always leads to a change of the ²J(C_A,H_B) coupling to more negative values, independent of the type and position of substituents at the two carbon centers. The orientations of the two substituents at C‐3 of the epimeric pair 1 and 2 were determined unambiguously through the measurement of the geminal coupling constants between C‐3 and the hydrogen atoms at C‐2 and C‐4. In particular, ²J(C‐3,H‐2ax) with ?1.5 Hz, ? = 174° in 1 and ?6.6 Hz, ? = 47° in 2 , and ²J(C‐3,H‐4) with +1.5 Hz, ? = 175° in 1 and ?4.7 Hz, ? = 49° in 2 showed the greatest differences between the two epimers. Both couplings therefore allow the determination of the absolute configuration at C‐3. It should be noted, however, that the size of the coupling constants can be different for dihedral angles of nearly identical size, when there are different numbers of electronegative substituents on the two coupling pathways, i.e. no O‐substituent at C‐2, but one axial O‐substituent at C‐4. It becomes clear that it is not sufficient to measure the magnitude of ²J coupling constants only, but that the sign of the geminal coupling is needed to identify the absolute configuration at a chiral center. The coupling of C‐3 with H‐2eq is not useful for the determination of the configuration at C‐3, as the similarity of the dihedral angles ? (O—C‐3—C‐2—H‐2eq) (57° in 1 and 70° in 2 ) leads to identical coupling constants (?6.1 Hz) for both epimers. Copyright © 2003 John Wiley & Sons, Ltd.     

NMR spectra of 1,3-butadienes: VII—the conformation of 2,4-dimethyl-2,4-pentadiene

Proton magnetic resonance spectra of 2,4-dimethyl-2,4-pentadiene have been examined for a number of solution conditions and (for a CF₂Cl₂ solution) over a range of temperatures. Coupling constants for the ethylenic protons were obtained from double resonance (methyl decoupled) spectra. Further double resonance experiments established the sign of ⁴J_c. It is found that ⁴J_c = ?1·20 Hz and [⁴J_t] = 0·18 Hz. It is concluded that the compound exists in a single nonplanar conformation with an average dihedral angle (from the s-cis form) ? = 50° ± 15°. Some long range coupling constants involving the methyl group were found to be appreciable in spite of the lack of planarity of the diene chain.     

13C, 1H spin coupling constants of dimethylacetylene

¹³C, ¹H spin coupling constants of dimethylacetylene have been determined by the complete analysis of the proton coupled ¹³C NMR spectrum. For the methyl carbon ¹J(CH) = + 130.6₄ Hz and ⁴J(CH) = + 1.5₈ Hz, and for the acetylenic carbon ²J(CH) = ? 10.34 Hz and ³J(CH) = +4.30 Hz. The ⁵J(HH) long-range coupling constant (+2.79 Hz) between the methyl protons was also determined.     

The crystal and molecular structure of phenothiazine-10-propionitrile. The effect of crystal packing on the dihedral angle of phenothiazine heterocycles

Phenothiazine-10-propionitrile, C₁₂H₈SNC₂H₄CN, crystallizes in the centrosymmetric monoclinic space group P2₁/n, with a = 5.785(1)Å, b = 15.427(3)Å, c = 14.497(4)Å, β = 92.50(1)°, Z = 4, D_meas = 1.29(1) g cm³ and D_calc = 1.28 g cm³ at 23°. Three dimensional X-ray data were collected with a manual diffractometer using MoKα (λ 0.71069Å) radiation and by multiple film Weissenberg techniques using CuKα (λ 1.5418Å) radiation. The structure was determined by Patterson and Fourier methods and refined with 519 observed reflections by full matrix least-squares methods to an R of 0.077. The dihedral angle between the two planes of the o-phenylene rings is 135.4(3)°. In the folded heterocyclic ring the C-S-C angle is 97.8(7)° and the average C? S bond is 1.76(1)Å. A comparison of this structure to that of phenothiazine-10-propionic acid shows the two chemically similar molecules have the same dihedral angles in spite of completely different solid state packing patterns.     

trans‐4‐[4‐(Dimethylamino)phenyl­iminomethyl]‐N‐phenylpyridinium hexafluorophosphate

The crystal structure of the dipolar chromophoric title compound, C₂₀H₂₀N₃⁺·PF₆^?, is described. The phenyl­ene and pyridyl rings are almost coplanar [dihedral angle 7.5 (2)°], but the phenyl substituent forms a dihedral angle of 56.6 (1)° with the pyridyl ring. The compound crystallizes in the non‐centrosymmetric space group Cc and is a likely candidate for the display of quadratic non‐linear optical effects.     

Stéréochimie en série aziridine: VI—Etude en Résonance Magnétique Nucléaire d'alcools aziridiniques primaires (configuration à l'azote,isoméries de rotation)

The PMR spectra of six primary aziridinyl carbinols are studied over a temperature range of ?30°C to +100°C. Nitrogen configuration is determined. When the inversion process is effective, kinetic parameters are evaluated. Rotational isomerism about the ‘ring? CH₂OH’ bond is studied from vicinal coupling constants associated with the two diastereotopic protons on the ? CH₂OH group. From the J(HOCH) coupling constant (in CCL₄) rotamer populations of the hydroxyl group are determined in some cases and the overall conformational distribution can be established.     

Ethyl β‐{2‐[4‐(dimethylamino)phenyliminomethyl]‐1‐(phenylsulfonyl)indol‐3‐yl}acrylate

The title compound, C₂₈H₂₇N₃O₄S, crystallizes in the centrosymmetric space group P2₁/n, with one mol­ecule in the asymmetric unit. In the indole ring, the dihedral angle between the fused rings is 3.6 (1)°. The phenyl ring of the sulfonyl substituent makes a dihedral angle of 79.2 (1)° with the best plane of the indole moiety. The phenyl ring of the di­methyl­amino­phenyl group is orthogonal to the phenyl ring of the phenyl­sulfonyl group. The dihedral angle formed by the weighted least‐squares planes through the pyrrole ring and the phenyl ring of the di­methyl­amino­phenyl group is 7.8 (1)°. The molecular structure is stabilized by C—H?O and C—H?N interactions.     

N‐Benzylnicotinamide and N‐benzyl‐1,4‐dihydronicotinamide: useful models for NAD+ and NADH

3‐Aminocarbonyl‐1‐benzylpyridinium bromide (N‐benzylnicotinamide, BNA), C₁₃H₁₃N₂O⁺·Br⁻, (I), and 1‐benzyl‐1,4‐dihydropyridine‐3‐carboxamide (N‐benzyl‐1,4‐dihydronicotinamide, rBNA), C₁₃H₁₄N₂O, (II), are valuable model compounds used to study the enzymatic cofactors NAD(P)⁺ and NAD(P)H. BNA was crystallized successfully and its structure determined for the first time, while a low‐temperature high‐resolution structure of rBNA was obtained. Together, these structures provide the most detailed view of the reactive portions of NAD(P)⁺ and NAD(P)H. The amide group in BNA is rotated 8.4 (4)° out of the plane of the pyridine ring, while the two rings display a dihedral angle of 70.48 (17)°. In the rBNA structure, the dihydropyridine ring is essentially planar, indicating significant delocalization of the formal double bonds, and the amide group is coplanar with the ring [dihedral angle = 4.35 (9)°]. This rBNA conformation may lower the transition‐state energy of an ene reaction between a substrate double bond and the dihydropyridine ring. The transition state would involve one atom of the double bond binding to the carbon ortho to both the ring N atom and the amide substituent of the dihydropyridine ring, while the other end of the double bond accepts an H atom from the methylene group para to the N atom.     

[η6‐1‐Chloro‐2‐(pyrrolidin‐1‐yl)benzene](η5‐cyclopentadienyl)iron(II) hexafluoridophosphate and (η5‐cyclopentadienyl){2‐[η6‐2‐(pyrrolidin‐1‐yl)phenyl]phenol}iron(II) hexafluoridophosphate

In the complex salt [η⁶‐1‐chloro‐2‐(pyrrolidin‐1‐yl)benzene](η⁵‐cyclopentadienyl)iron(II) hexafluoridophosphate, [Fe(C₅H₅)(C₁₀H₁₂ClN)]PF₆, (I), the complexed cyclopentadienyl and benzene rings are almost parallel, with a dihedral angle between their planes of 2.3 (3)°. In a related complex salt, (η⁵‐cyclopentadienyl){2‐[η⁶‐2‐(pyrrolidin‐1‐yl)phenyl]phenol}iron(II) hexafluoridophosphate, [Fe(C₅H₅)(C₁₆H₁₇NO)]PF₆, (II), the analogous angle is 5.4 (1)°. In both complexes, the aromatic C atom bound to the pyrrolidine N atom is located out of the plane defined by the remaining five ring C atoms. The dihedral angles between the plane of these five ring atoms and a plane defined by the N‐bound aromatic C atom and two neighboring C atoms are 9.7 (8) and 5.6 (2)° for (I) and (II), respectively.     

The crystal and molecular structure of phenothiazine-10-propionic acid

The crystal structure of phenothiazine-10-propionic acid, C₁₂H₈SNC₂H₄COOH, was determined from three-dimensional X-ray diffraction data collected with a manual diffractomer using MoKα (λ 0.71069 Å) radiation. The space group is P2₁/c with a = 7.888 (2)Å, b = 8.703 (2)Å, c = 19.700 (8)Å, β = 101.42 (1)°, Z = 4, D_meas = 1.35(2) g. cm^?3 and D_calc = 1.36 g. cm^?3 at 23°. The structure was determined by the direct method and refined with 500 observed reflections by full-matrix least squares to an R of 0.072. The molecule is folded along the S-N axis and the dihedral angle is 136.5°. The C-S-C angle is 98.5(7)° and the average C-S bond is 1.77(2)Å. The shortening of the C-S bond, the small value of the C-S-C angle and the folding of the molecule are typical of the phenothiazine class of compounds and are assumed to be due to sulfur d orbital participation in ring bonding.     

2,2′‐Anhydro‐1‐(3′,5′‐di‐O‐acetyl‐β‐D‐arabinofuranosyl)uracil,a cyclouridine nucleoside with a C4′‐endo furanosyl conformation

2,2′‐Anhydro‐1‐(3′,5′‐di‐O‐acetyl‐β‐D‐arabinofuranosyl)uracil, C₁₃H₁₄N₂O₇, was obtained by refluxing 2′,3′‐O‐(methoxymethylene)uridine in acetic anhydride. The structure exhibits a nearly perfect C4′‐endo (⁴E) conformation. The best four‐atom plane of the five‐membered furanose ring is O—C—C—C, involving the C atoms of the fused five‐membered oxazolidine ring, and the torsion angle is only −0.4 (2)°. The oxazolidine ring is essentially coplanar with the six‐membered uracil ring [r.m.s. deviation = 0.012 (5) Å and dihedral angle = −3.2 (3)°]. The conformation at the exocyclic C—C bond is gauche–trans which is stabilized by various C—H...π and C—O...π interactions.     

Elucidating heteroatom influence on homonuclear 4J(H,H) coupling constants by DFT/NMR approach

We report the structural dependency of long range scalar J-coupling constant across four bonds as function of the dihedral angles Φ1 and Φ3. The calculated homonuclear coupling constants ⁴J(_H,H), obtained at a density functional theory level, were measured between C(1)─X(2) and X(2)─C(3) bonds in three-term models, where C, N, O, and S were systematically used as the second atom of the alkyl structures ( 1 - 4 ). The ⁴J_(H,H) calculated values, tabulated for variation of 30° for both Φ1 and Φ3, have disclosed an unexpected detectable coupling constant (⁴J(_H,H) ≥ 1 Hz) across heteroatoms, useful to provide valuable structural information. A 2-methyl-1,3-dithiane sulfide ( 5 ) was used as a case study to prove the applicability and reliability of the calculated values to real issues. The ⁴J(_H,H) values obtained at density functional theory for the system 4 have reproduced with good accuracy an unexpected experimental ⁴J(_H2ax-H4ax) = 1.01 Hz of sulfide molecule ( 5 ), suggesting these calculated coupling constant values as a new powerful tool for the organic synthesis and stereochemical analysis.     

Transition-metal complexes with bidentate ligands spanning trans-position. Part XVI. X-ray structural and 31P-NMR solution studies of 2,11-Bis(diethylphosphinomethyl)benzo[c]phenanthrenesilver(I) perchlorate

The preparation of complexes {AgX(1c)} (X ? Cl, Br, I, NO₃ and ClO₄; 1c = 2,11-bis(diethylphosphinomethyl)benzo[c]phenanthrene) is reported. The ³¹P-NMR spectra of the above complexes were recorded and the ¹J(¹⁰⁷Ag, ³¹P) values are compared with the corresponding data for related complexes. The X-ray crystal structure of [Ag(1c)](ClO₄) was determined. There are two crystallographically independent molecules in the unit cell each containing two-coordinate silver, the O-atoms of the perchlorate anions being outside bonding range from the central atom. The two molecules, however, show different bonding parameters: Thus for ‘molecule 1’ P(1)? Ag(1)? P(2) = 167.6(1)°, Ag(1)? P(1) = 2.389(3) and Ag(1)? P(2) = 2.393(3) Å, while for ‘molecule 2’ P(3)? Ag(2)? P(4) = 164.8(1)°, Ag(2)? P(3) = 2.377(3), and Ag(2)? P(4) = 2.378(3) Å. These differences are probably due to packing forces in the crystal lattice.     

Weitreichende Kopplung zwischen Protonen und Fluor in 2-Fluorbenzamiden

Large ‘through space’ hydrogen-fluorine coupling has been observed in 2-fluorobenzamides (⁵J_HF = 10 to 16 Hz). The solvent dependence of the coupling shows that nuclear spin information is transmitted via hydrogen bonds.     

Refinement of the angular dependence of the peptide vicinal NHCαH coupling constant

The refined dependence of the peptide NHCαH vicinal coupling constant on the dihedral angle θ have been derived on the basis of the accumulated experimental data. The mean permissible values (in Hz) are approximated by ³J_NHCH = 9·4 cos² θ - 1·1 cos θ + 0·4 An analogous relationship for the sum of two vicinal NHCαH₂ coupling constants in the glycyl residue have been calculated from the above dependence. Measurements on N-methylacetamide in various solvents and in the presence of an alkali salt showed the vicinal constant NHCH to vary by not more than ± 3%. Some of the other proposed ³J_NHCH(θ) dependencies give too low values for the cis-oriented NH and CαH bonds. This may be due to the fact that in these correlations the data for compounds with cis-amide bonds have been used for 0° ? θ ? 90° region of the dependence.     

The crystal and molecular structure of 1-(p-iodobenzenesulfonyl)-1-azaspiro[2,5] octane

The structure of 1-(p-iodobenzenesulfonyl)-1-azaspiro[2.5]octane was determined by a single-crystal x-ray diffraction study. The compound crystallizes in the orthorhombic space group P2₁ 2₁ 2₁ with cell dimensions: a = 11.65₅, b = 11.82₅, c = 11.06₈ ± 0.002 Å. The aziridine ring is spiro-fused to the cyclohexane ring with the nitrogen atom occupying the equatorial position. The cyclohexane ring is in an undistorted chair conformation and forms a dihedral angle of 97.1° with the aziridine ring. The structure was refined to a final value of R = 0.07₄ for the 590 independent reflections.     

Chemistry of the phenoxathiins and isosterically related heterocycles. XVI. The synthesis and molecular structure of 3-azaphenoxathiin: Evidence in support of factors responsible for control of the dihedral angle

The synthesis of the 3-azaphenoxathiin ring system and its molecular structure are reported. Based on ¹³C-nmr chemical shift additivities associated with the insertion of an annular nitrogen atom and the observed ¹³C-nmr shift of Cα, the title compound was predicted to have a dihedral angle θ = 160.2°. The observed dihedral angle from the crystal structure was found to be θ = 167.07° which is in reasonably good agreement with the predicted value. It is proposed that the position of the annular nitrogen atom is solely in-control of the observed dihedral angle.     

The molecular structure of tetrathiofulvalene-13C (TTF) from proton magnetic resonance studies using nematic solvents

The molecular structure of TTF dissolved in nematic liquid crystalline solvents has been determined from the proton magnetic resonance including couplings due to ¹³C in natural abundance. The molecule is puckered in a boat conformation with the SCHCHS planes making a dihedral angle of 13 ± 2° with the S₂C  CS₂ plane. The other structural parameters obtained are r_CH = 1.085 ± 0.014 Å and the angel CCH = 123.7 ± 1.5°.     

Carbon-13 NMR studies of twenty-four methyl-substituted morpholines

Carbon-13 NMR spectra of all the isomers of monomethyl-, 2,3-, 2,5-, 2,6-, 3,5-dimethyl-, 2,3,5-, 2,3,6-trimethyl- and 2,3,5,6-tetramethylmorpholine have been obtained at both ambient (25 °C) and low temperature (～ ?100 to ?120 °C). The ring carbon shifts appear to be additive with respect to the position of the methyl groups. A good correlation between predicted and experimental shift values was obtained (r = 0.998₉). The values were used in an attempt to assign, conformationally, the ‘all cis’ isomer 2,3,5,6-tetramethylmorpholine, which from ¹HNMR spin–spin coupling studies has been unsuccessful. Methyl carbon shifts to high field were found for axially oriented carbons. The extracted ‘steric shift’ values for such carbons were compared to their corresponding proton shift data.     

Bonding and singlet–triplet gap of silicon trimer: Effects of protonation and attachment of alkali metal cations

We revisit the singlet–triplet energy gap (ΔE_ST) of silicon trimer and evaluate the gaps of its derivatives by attachment of a cation (H⁺, Li⁺, Na⁺, and K⁺) using the wavefunction‐based methods including the composite G4, coupled‐cluster theory CCSD(T)/CBS, CCSDT and CCSDTQ, and CASSCF/CASPT2 (for Si₃) computations. Both ¹A₁ and ³ states of Si₃ are determined to be degenerate. An intersystem crossing between both states appears to be possible at a point having an apex bond angle of around α = 68 ± 2° which is 16 ± 4 kJ/mol above the ground state. The proton, Li⁺ and Na⁺ cations tend to favor the low‐spin state, whereas the K⁺ cation favors the high‐spin state. However, they do not modify significantly the ΔE_ST. The proton affinity of silicon trimer is determined as PA(Si₃) = 830 ± 4 kJ/mol at 298 K. The metal cation affinities are also predicted to be LiCA(Si₃) = 108 ± 8 kJ/mol, NaCA(Si₃) = 79 ± 8 kJ/mol and KCA(Si₃) = 44 ± 8 kJ/mol. The chemical bonding is probed using the electron localization function, and ring current analyses show that the singlet three‐membered ring Si₃ is, at most, nonaromatic. Attachment of the proton and Li⁺ cation renders it anti‐aromatic. © 2015 Wiley Periodicals, Inc.