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.     

1,2‐, 1,7‐ and 1,12‐Dicarba‐closo‐dodecaborane(12) Derivatives Revisited by 13C NMR Spectroscopy and DFT Calculations. First Observation of Isotope‐Induced Chemical Shifts 1Δ10/11B(13C), and the Signs and Magnitudes of Coupling Constants 1J(13C,13C) and 1J(13C,11B)

The origin of broadening of ¹³C(carborane) NMR signals of 1,2‐, 1,7‐ and 1,12‐dicarba‐closo‐dodecaboranes(12) and several diphenylsilyl derivatives has been examined in detail and could be traced only partially to unresolved ¹³C–¹¹B spin‐spin coupling. Other contributions to the line widths arise from ¹³C–¹H dipole‐dipole interactions and, in particular, from isotope‐induced chemical shifts ¹Δ^10/11B(¹³C), observed here for carboranes for the first time. In the case of 1‐diphenylsilyl‐1,2‐dicarba‐closo‐dodecaborane(12), the coupling constant ¹J(¹³C,¹³C) = 9.3 Hz was measured in natural abundance of ¹³C. The small value of this coupling constant and its negative sign is predicted by calculations based on optimised structures [B3LYP/6‐311+G(d,p) level of theory] of the parent carboranes and 1‐silyl‐1,2‐dicarba‐closo‐dodecaborane(12) as a model compound [calcd. ¹J(¹³C,¹³C) = –10.5 Hz]. Calculated coupling constants ¹J(¹³C,¹¹B) are small (<7 Hz), in contrast to published assumptions, and of either sign, whereas ¹J(¹¹B,¹¹B) are all positive and range up to 15 Hz.     

13C, 1H and 31P dynamic NMR studies of tetraalkyldiphosphanes: Barrier to internal rotation about the P?P Bond in tetraisopropyldiphosphane

The ¹H, ³¹P and ¹³C NMR spectra of tetramethyldiphosphane (1), tetraethyldiphosphane (2) and tetraisopropyldiphosphane (3) have been studied in the temperature range 30 to ?150°C and at magnetic induction up to 5.87 T. In the range ?100 to ?135 °C, the ¹H and ¹³C spectra of 3 show important changes which can be attributed to freezing the interconversion between two equivalent non-trans conformations. The line shape analysis of the ¹³C signals leads to ΔG^≠ = 29.4 kJ mol^?1 at ?113 °C for the dynamic process involved. The spin coupling parameters ¹J(PP) and N(PC) = ¹J(PC) + ²J(PC) observed for 1 in the temperature range 30 to ?120 °C show variations which could be due to a preference for the trans conformation in this diphosphane.     

CH,CD, CC and HH coupling constants in isotopically enriched cyclobutene

¹H, ²H and ¹³C NMR studies of cyclobutene and a series of isotopically enriched species have led to a determination of the ¹H? ¹H, ¹³C? ¹H, ¹³C? ²H and ¹³C? ¹³C coupling constants in these compounds. In agreement with general observations, ¹J(CH) is found to depend on the hybridization of the carbon atoms. Likewise, ²J(HH), ²J(CC), ³J(HH) and ³J(CH), but not ²J(CH), depend on the angles between the bonds connecting the coupled nuclei. When comparing cyclobutene with thiete 1,1-dioxide (thiete sulphone) an increase of almost 20 Hz is observed for ¹J(C-2, H-2) in the latter compound. All but one of the observed deuterium isotope effects on chemical shifts are negative. In the case of isotope effects upon the one-bond coupling constants, the obtained values support the results of the theoretical calculations of Sergeev and Solkan.     

Signs of proximate 31P, 19F and 19F, 19F spin–spin coupling constants in 2-trifluoromethylphenyldifluorophosphine

In 2-trifluoromethylphenyldifluorophosphine the proximate couplings ⁴J(¹⁹F³¹P) and ⁵J(¹⁹F¹⁹F) are + 68.3 and + 8.3 Hz, respectively. ¹J(¹³C³¹P) is ?57.0 Hz, ²J(¹³C-1, ¹⁰F) is + 9.9 Hz and ²J(¹³C-6, ¹³C-6, ³¹P) is + 10.1 Hz. The trifluoromethyl substituent induces substantial changes in some coupling constants, particularly those between the ³¹P and ring ¹³C nuclei.     

Spectroscopic characterization and <Emphasis Type="Italic">Ab initio</Emphasis> calculations of new diazaphosphole and diazaphosphorinane

Phosphoryl chloride is used as a starting material to synthesize new diazaphosphole, (1) and diazaphosphorinane, (2). The products are characterized by ¹H, ¹³C, ³¹P NMR, and IR spectroscopy. A high value ² J(PNH) = 17.0 Hz, 17.2 Hz is measured for two non-equivalent NH protons of endocyclic nitrogen atoms in compound 1, while it greatly decreases to 4.5 Hz in 2. Also, great amounts are obtained for two ² J(P,C) as well as two ³ J(P,C) in the ¹³C NMR spectrum of 1, but they are zero in 2. Here, the effect of ring strain and ring size on the structural and spectroscopic parameters is observed. The ³¹P NMR spectra reveal that δ(³¹P) of compound 1 is far much more downfield (12.63 ppm) relative to that of compound 2 (−10.39 ppm). Furthermore, ab initio quantum chemical calculations are performed to optimize the structures of these molecules by density functional theory (B3LYP) and Hartree-Fock (HF) methods, using the standard 6−31+G** basis set. The stabilization energies are calculated by the equation ΔE _{stabilization} = E _molecule − ΣE _i , where i = atom. To obtain the atomic hybridizations, NBO computations are made at the B3LYP/6−31+G** level. Also, by NMR calculations the ¹H, ¹³C, ³¹P chemical shifts are obtained and compared with the experimental ones.     

Substituent effects on nuclear spin–spin carbon–carbon coupling constants in derivatives of acetylene

¹J(¹³C?¹³C) nuclear spin–spin coupling constants in derivatives of acetylene have been measured from natural abundance ¹³C NMR spectra and in one case (triethylsilyllithiumacetylene) from the ¹³C NMR spectrum of a ¹³C-enriched sample. It has been found that the magnitude of J(C?C) depends on the electronegativity of the substituents at the triple bond. The equation ¹J(¹³C?¹³C) = 43.38 E_x + 17.33 has been derived for one particular series of the compounds Alk₃SiC?CX, where X denotes Li, R₃Sn, R₃Si, R₃C, I, Br or Cl. The ¹J(C?C) values found in this work cover a range from 56.8 Hz (in Et₃SiC?Li) to 216.0 Hz (in PhC?CCI). However, the ¹J(C?C) vs E_x equation combined with the Egli–von Philipsborn relationship allows the calculation of the coupling constants in Li₂C₂ (32 Hz) and in F₂C₂ (356 Hz). These are probably the lowest and the highest values, respectively, which can be attained for ¹J(CC) across a triple bond. The unusually large changes of the ¹J(C?C) values are explained in terms of substituent effects followed by a re-hybridization of the carbons involved in the triple bond. INDO FPT calculations performed for two series of acetylene derivatives, with substituents varied along the first row of the Periodic Table, corroborate the conclusions drawn from the experimental data.     

Carbon-13 nuclear magnetic resonance spectra of exo- and endo-2-norbornyltrimethyl-stannanes: Corrected assignments

The ¹³C NMR spectra of pure exo-2-norbornyltrimethylstannane and a mixture of the exo- and endo-isomers have been recorded. ¹H–¹³C polarization transfer spectra have been obtained and require the previously reported assignments for C-3 and C-4 in the exo-isomer to be reversed. The reported assignments for the endo-isomer are correct. The new assignment for C-4-exo [with J(¹¹⁹Sn,¹³C) vic=12 Hz, instead of the previously assigned J(vic)=23 Hz], has a very minor effect on the nature of the Karplus curve [for ³J(¹¹⁹Sn,¹³C)] generated previously.     

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.     

Conformational Dependence of One-Bond Cα,H Spin Coupling in Cyclic Peptides

Conformational effects on the one-bond C_α, H coupling constant have been studied in cyclic oligopeptides containing glycine, alanine, sarcosine and proline. The conformational contribution to ¹J (C_α, H) can be described as being composed of a positive hyperconjugative term from the neighbouring N-p_z orbital and a negative one from the carbonyl π-system. For glycyl units, a quantitative relation between ¹J (C_α,H) and the vicinal interorbital angles Φ′ and Ψ′ is derived with maximum values ΔJ (Ψ′) = + 14.0 Hz and ΔJ (Ψ′) = ?4.9 Hz. Applications to conformational analysis of cyclo (L -Pro-Gly)₃, cyclo (D -Ala-D -Ala-L -Ala-D -Ala-L -Ala-L -Ala) and cyclo (Sar)₈ are presented.     

Nuclear magnetic resonance spectroscopy. Analysis of the 1H and 13C spectra of Br13CH213CH2Br

The ¹H and ¹³C NMR spectra of 1,2-dibromoethane-¹³C₂ have been analyzed to determine the magnitude (38·9 Hz) and sign (positive) of ¹J(C? C) relative to those of ³J(H? H) (positive). This type of coupling appears to be rather insensitive to the presence of bromine or methyl as substituents on the carbons.     

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.     

Non‐empirical calculations of NMR indirect carbon–carbon coupling constants. Part 4: Bicycloalkanes

A systematic study of the one‐bond and long‐range J(C,C), J(C,H) and J(H,H) in the series of nine bicycloalkanes was performed at the SOPPA level with special emphasis on the coupling transmission mechanisms at bridgeheads. Many unknown couplings were predicted with high reliability. Further refinement of SOPPA computational scheme adjusted for better performance was carried out using bicyclo[1.1.1]pentane as a benchmark to investigate the influence of geometry, basis set and electronic correlation. The calculations performed demonstrated that classical ab initio SOPPA applied with the locally dense Dunning's sets augmented with inner core s‐functions used for coupled carbons and Sauer's sets augmented with tight s‐functions used for coupled hydrogens performs perfectly well in reproducing experimental values of different types of coupling constants (the estimated reliability is ca 1–2 Hz) in relatively large organic molecules of up to 11 carbon atoms. Additive coupling increments were derived for J(C,C), J(C,H) and J(H,H) based on the calculated values of coupling constants within SOPPA in the model bicycloalkanes, in reasonably good agreement with the known values obtained earlier on pure empirical grounds. Most of the bridgehead couplings in all but one bicycloalkane appeared to be essentially additive within ca 2–3 Hz while bicyclo[1.1.1]pentane demonstrated dramatic non‐additivity of ?14.5 Hz for J(C,C), +16.6 Hz for J(H,H) and ?5.5 Hz for J(C,H), in line with previous findings. Non‐additivity effects in the latter compound established at the SOPPA level should be attributed to the through‐space non‐bonded interactions at bridgeheads due to the essential overlapping of the bridgehead rear lobes which provides an additional and effective non‐bonding coupling path for the bridgehead carbons and their protons in the bicyclopentane framework. Copyright © 2003 John Wiley & Sons, Ltd.     

Carbon-proton coupling constants in α-chlorostyrene-α-13C

The two stereochemically distinct two-bond carbon-13- hydrogen coupling constants J(¹³C? CH), for α-chlorostyrene-α-¹³C have been shown to be of similar magnitude but opposite sign (?6.3 and +5.6 Hz). A simple additivity relationship which adequately reproduces all the reported J(¹³C? CH) values for chloroethylenes has been found.     

The 13C,H coupling constants in structural and conformational analysis: II The correlation of 3J(HCCH) and 3J(CH3CCH) in ethylenes and their methyl derivatives

The vicinal ³J(C H₃C?CH ) coupling constants were determined for a number of propylene derivatives and compared with the ³J(H C?CH ) couplings of the corresponding ethylenes. A linear regression analysis yielded the correlation ³J(CH) = 0.46 ׳ J(HH)+1.58 Hz, the correlation coefficient being 0.956.     

1H- und 13C-NMR-Untersuchungen zur Struktur N-substituierter Tetrazole—Die Strukturen des N-Methoxycarbonyl-, N-Acetyl- und N-(Tri-n-butylstannyl)-tetrazols sowie Substituenteneffekte auf δ13C und 1J(CH)

The ¹H and ¹³C NMR spectra of several isomeric N-substituted tetrazoles have been investigated. ¹³C NMR is shown to be more useful for distinguishing between structural isomers of N-substituted tetrazoles except for those carrying electropositive substituents like SnBu₃. Correlations of δC-5 (inverse) and ¹J(C-5,H) with s?₁ found for 1-substituted tetrazole allowed the identification of the N SnBu₃ derivative as 1-(tri-n-butylstannyl)tetrazole. The phenyl carbon chemical shift difference ΔC′ = δC-3′-δC-2′ is insignificant for structure elucidation and conformational studies of N-substituted 5-phenyltetrazoles; ΔH′ from ¹H NMR spectra seems to be more useful.     

Temperature dependences of J(C,H) and J(C,D) in 13CH4 and some of its deuterated isotopomers

The nuclear spin—spin coupling constants J(C,H) and J(C,D) have been measured over the temperature range 200–370 K for the methane isotopomers ¹³CH₄, ¹³CH₃D, ¹³CHD₃ and ¹³CD₄. The coupling constants increase with increasing temperature for any one isotopomer and decrease with increasing secondary deuterium substitution at any one temperature. The results are entirely attributable to intramolecular effects and the data have been fitted by a weighted least-squares regression analysis to a spin—spin coupling surface thereby yielding a value for ¹J_e(C,H), the coupling constant at equilibrium geometry, and values for the bond length derivatives of the coupling. We find that ¹J_e(C,H) = 120.78 (±0.05) Hz which is about 4.5 Hz smaller than the observed value in ¹³CH₄ gas at room temperature. Results are also reported for J(H,D) in ¹³CH₃D and ¹³CHD₃ for which no temperature dependence was detected.     

Proton n.m.r. spin-tickling studies of 13C-labeled compounds. The correlation of J(C,H) and J(H,H)

A tabulation has been compiled for twenty ¹³C? H coupling constants of various carboxylic acids and includes ²J(C,H), ³J(C,H) and ⁴J(C,H) values of olefins (both cis and trans); ³J(C,H), ⁴J(C,H) and ⁵J(C,H) values of aromatics; ³J(C,H) and ⁴J(C,H) values of acetylenes; and ²J(C,H) and ³J(C,H) values of rigid aliphatics. This tabulation has been completed in the present study by the spin-tickling proton n.m.r. study of ¹³C-carboxyl-endo-1,2,3,4,7,7-hexachloronorbornene-5-carboxylic acid, which has established that the ²J(C,H) value is negative and the ³J(C,H) values (both cis and trans) are positive in this system. A plot of these twenty J(C,H) values vs the corresponding J(H,H) values of geometrically equivalent model systems (where there is a proton in place of a carboxyl group) gives a correlation coefficient of 0·975 (with a slope of 0·62), indicating that carbon–proton and proton–proton couplings operate by similar mechanisms throughout this broad series of structural types.     

Complete assignments of the 1H and 13C NMR spectra of 15 limonoids

Unambiguous and complete assignments of ¹H and ¹³C NMR chemical shifts for 15 limonoids, eight of them found in natural sources and seven other synthetic derivatives, are presented. The assignments are based on 2D shift‐correlated [¹H,¹H‐COSY, ¹H,¹³C‐gHSQC‐¹J(C,H), ¹H,¹³C‐gHMBC‐ⁿJ(C,H) (n = 2 and 3)] and NOE experiments. Copyright © 2003 John Wiley & Sons, Ltd.     

An 1H and 13C NMR study of pentane-2,4-dione tellurium(II) compounds; chemical shifts, 125Te?1H and 125Te?13C couplings

¹H and ¹³C NMR data are reported for pentane-2,4-dione tellurium(II) compounds which contain a 6-membered tellurane ring bearing methyl or ethyl substituents. The ¹²⁵TeCH coupling constants are particularly interesting in that they are highly stereospecific, viz. ～60 Hz and 0–8 Hz for equatorial and axial protons, respectively, in the chair conformation. ¹J(TeC) and ²J(TeCC) couplings lie in the ranges 118–146 Hz and 44–49 Hz, respectively. Geminal HCH couplings in these compounds (9–11 Hz) are lower than analogous values in tellurane or in cyclohexane-1,3-diones.