Experimental and theoretical analysis of vicinal and long-range proton-proton coupling constants for anthracene derivatives

We studied vicinal and long-range coupling constants for 9-anthracene derivatives, e.g., Br, CN, CHO, NO2, CH2Cl, CH2OH, and OCH3. We performed the accurate measurements using modified J doubling in the frequency domain, even for the smallest couplings immersed within the line width. Density functional theory allowed us to reproduce and exhaustively analyze the physical contributions to the values of these spectroscopic parameters. The theory of atoms in molecules defines a delocalization index that correlates linearly with vicinal and long-range coupling constants when they are grouped in terms of the number of bonds between the coupled nuclei. An exception to this behavior is obtained for 4J(H4,H10) values, which have a negative Fermi contact and the largest delocalization index for each molecule. This observation can be explained by a characteristic "gable roof" arrangement formed by the five nuclei involved in the coupling.     

A Karplus-type relationship for the vicinal coupling 3J(CCCP) in phosphonates

The complete assignment of the ¹³C NMR spectrum of a conformationally rigid camphane phosphonate derivative leads to a Karplus-type function for the vicinal ³J(CCCP) coupling constants in phosphonates. The general applicability of this relationship is demonstrated.     

Hydroxymethyl group conformation in saccharides: structural dependencies of (2)J(HH), (3)J(HH), and (1)J(CH) spin-spin coupling constants

Experimental and theoretical methods have been used to correlate (2)J(HH) and (3)J(HH) values within the exocyclic hydroxymethyl groups (CH(2)OH) of saccharides with specific molecular parameters, and new equations are proposed to assist in the structural interpretation of these couplings. (3)J(HH) depends mainly on the C-C torsion angle (omega) as expected, and new Karplus equations derived from J-couplings computed from density functional theory (DFT) in a model aldopyranosyl ring are in excellent agreement with experimental values and with couplings predicted from a previously reported general Karplus equation. These results confirm the reliability of DFT-calculated (1)H-(1)H couplings in saccharides. (2)J(HH) values depend on both the C-C (omega) and C-O (theta) torsions. Knowledge of the former, which may be derived from other parameters (e.g., (3)J(HH)), allows theta to be evaluated indirectly from (2)J(HH). This latter approach complements more direct determinations of theta from (3)J(HCOH) and potentially extends these more conventional analyses to O-substituted systems lacking the hydroxyl proton. (1)J(CH) values within hydroxymethyl fragments were also examined and found to depend on r(CH), which is modulated by specific bond orientation and stereoelectronic factors. These latter factors could be largely, but not completely, accounted for by C-C and C-O torsional variables, leading to only semiquantitative treatments of these couplings (details discussed in the Supporting Information). New equations pertaining to (2)J(HH) and (3)J(HH) have been applied to the analysis of hydroxymethyl group J-couplings in several mono- and oligosaccharides, yielding information on C5-C6 and/or C6-O6 rotamer populations.     

Prediction of anti and gauche vicinal proton-proton coupling constants in carbohydrates: A simple additivity rule for pyranose rings

The effect of the relative orientation and electronegativity of substituents on the magnitude of ³J(aa), ³J(ae) and ³J(ee) is well predicated by a simple set of additivity constants, valid for pyranose rings in carbohydrates. The proposed set of parameters is used to calculate 327 coupling constants [³J(HH)] in a variety of pyranosides and related compounds. A comparison with experimental values taken from the literature shows that couplings in molecules which are conformationally pure and underformed can be predicted with a surprising accuracy. An overall root-mean-square agreement of 0.29 Hz is attained for a selected group of 305 coupling values. A statistical breakdown of ΔJ(aa) and ΔJ(ae) [ΔJ=J(exp)-J(calc)] along each carbon-carbon bond in the pyranose systems reveals an unexpected degree of geometrical homogeneity.     

A computational study of 2J(HH)(gem) indirect spin-spin coupling constants in simple hydrides of the second and third periods

Several theoretical methods have been used to compute (2)J(HH) in neutral, anionic and cationic HXH hydrides, X being the 14 nuclei from Li to Cl (28 molecules). Since the calculations also provide (1)J(XH) spin-spin coupling constants (SSCC), these have also been analyzed. The best results were obtained using Second-order polarization propagator approximation (SOPPA)/sadJ. The geminal coupling constants appear to be dependent on the electronegativity of the X-atom.     

Additivity of substituent effects on the proton-proton coupling constants of disubstituted pyridines

From the spin-spin coupling constants of pyridine and monosubstituted pyridines the effects of several substituents have been calculated- Assuming an additivity relationship when two of these substituents are present in the same molecule, the spin-spin coupling constants for 11 disubstituted pyridines have been calculated.The NMR spectra of 13 disubstituted pyridines have been studied to obtain accurate values of their coupling constants. The experimental values of these constants are in very good agreement with those calculated using the additivity relationships.     

Nitrogen-15 coupling constants: Geminal coupling, 2J(15NH), in cis- and trans-aldonitrones and vicinal coupling, 3J(15NH), in cis-and trans-ketimines,oxaziridines and nitrones

Nitrogen-15-hydrogen ²J(¹⁵NH), ¹⁵N, ¹³C ¹J(¹⁵N¹³C) and ¹³C, H ¹J(¹³CH) coupling constants have been measured and their signs determined for cis-and trans-[9-anthryl(¹³C)methylene](²H₃)methylamine (¹⁵N)-oxide. Values of ²J(¹⁵NH) were of similar magnitude (～2 Hz) but were of opposite sign. The results are compared and contrasted with those reported for related imino and quaternary imino systems. Vicinal ³J(¹⁵NH) coupling constants have been measured in cis- and trans-¹⁵N-[1-(α-naphthyl)ethylidene]benzylamine and ¹⁵N-[1-(p-nitrophenyl)ethylidene]-t-butylamine and were found to be larger when the imino methyl group was cis to the nitrogen lone pair. The corresponding cis and trans ketonitrones formed by photoisomerization of the derived oxaziridines had ³J(¹⁵NH) values of c. 3.3 Hz. A study of the signs and magnitudes of observed and calculated ¹⁵N,H coupling constants for all of the ¹⁵N labelled imines, oxaziridines, imine N-oxides (nitrones) and similar model systems which were synthesized is described.     

Simultaneous measurement of J(HH) and two different nJ(CH) coupling constants from a single multiply edited 2D cross‐peak

Three different J‐editing methods (IPAP, E.COSY and J‐resolved) are implemented in a single NMR experiment to provide spin‐state‐edited 2D cross‐peaks from which a simultaneous measurement of different homonuclear and heteronuclear coupling constants can be performed. A new J‐selHSQMBC‐IPAP experiment is proposed for the independent measurement of two different ⁿJ(CH) coupling constants along the F2 and F1 dimensions of the same 2D cross‐peak. In addition, the E.COSY pattern provides additional information about the magnitude and relative sign between J(HH) and ⁿJ(CH) coupling constants. Copyright © 2013 John Wiley & Sons, Ltd.     

Long-range J(CH) heteronuclear coupling constants in cyclopentane derivatives

Detailed measurements of long-range heteronuclear spin-spin coupling constants, especially (2, 3)J(CH) spin-spin couplings for various cyclopentane derivatives, are reported. The measurements are based on a 2D heteronuclear correlation experiment named G-BIRD(R, X)-CPMG-HSQMBC.     

Structural assignments in isomeric pyrazoles based on 3J(15NH) coupling constants

Vicinal ¹⁵N–H coupling constants are used to characterize the isomeric pyrazoles obtained from the condensation of phenylhydrazine, ¹⁵N-enriched at the amino nitrogen atom, with various 1,3-diketones.     

2J(29Si-O-29Si) coupling constants in oligosiloxanes

The absolute values of spin-spin couplings between (29)Si nuclei in siloxanes, (2)J((29)Si-O-(29)Si), were determined. The couplings are small and cover a narrow range of values (0-5 Hz). The coupling constants depend on the branching or on the number of electronegative substituents on the Si-O-Si moiety.     

BIRD-J-resolved HMBC and BIRD-high-resolution HMBC pulse sequences for measuring heteronuclear long-range coupling constants and proton-proton spin coupling constants in complicated spin systems

Efficient pulse sequences for measuring long-range C-H coupling constants (J(C-H)) and proton-proton spin coupling constants (J(H-H)), named BIRD-J-resolved HMBC and BIRD-high-resolution HMBC, respectively, have been developed. In spin systems possessing a secondary methyl group positioned between protonated carbons (e.g. -CH(2)-CH(CH(3))-CH(2)-), the methine proton splits in a complicated fashion, resulting in difficulty in the determination of its spin coupling constants. For easy and accurate measurements of the long-range J(C-H) and J(H-H) in such a spin system, the BIRD pulse [90°x(H)-180°x (H/C)- 90° (-x)(H)] or [90°x(H)-180°x(H/C)-90° (-x)(H)180°x(C)] is incorporated into the J-resolved portion of the pulse sequence. As a result, the above secondary methyl group can be selectively decoupled, providing simplified cross-peak patterns, which are suitable for the accurate measurements of the long-range J(C-H) and J(H-H).     

Finite perturbation configuration interaction calculations of nuclear spin-spin coupling constants: Vicinal proton-proton coupling in ethane

Non-empirical finite perturbation calculations at the Hartree-Fock and configuration interaction levels of approximation are reported for the vicinal proton-proton coupling constant in ethane for different values of the dihedral HCCH' angle. The correlation effects are found to be of importance. The results fit the Karplus relation well. The calculated values are in good agreement with available experimental data when the correlation contribution is included. Possibilities of using approximations in evaluating the correlation energy are discussed.     

Theoretical prediction and assignment of vicinal 1H-1H coupling constants of diastereomeric 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes

Spin-spin coupling constants between nuclei in NMR spectroscopy reflect their spatial arrangement. A number of calculation methods, applying different levels of theory, have been developed to support the stereochemical assignment of novel compounds. Nevertheless, revisions of the assignment of structures in the literature are not rare. In the present work, the reliability of the calculation methods amenable for a theoretical prediction of spin-spin coupling constants of vicinal protons to support correct stereochemical assignment of substitution at five-membered rings of 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes was studied. Experimental (3)J(H,H) coupling constants were compared with the coupling constants calculated for all possible diastereomers. The fully quantum chemical approach provided theoretical (3)J(H,H) coupling constants with an absolute deviation of no more than 1.1 Hz for 91% of the experimentally studied coupled spins, whereas the methods without quantum chemical geometry optimization resulted in completely unreliable predictions. Consequently, for a reliable stereochemical assignment of small and medium size molecules, the protocol for calculating the coupling constants based on the results of the quantum chemical geometry optimization is recommended.