ABX Spin System Research Articles

Analysis of ABX spin systems in some 1,4‐dihydro‐1λ5, 4λ5‐diphosphinines: Signs of 31P13C coupling constants and isotope effects

AbstractThe ABX spin systems of the [mono‐13C]isotopomers with vinylic and phenyl ipso‐13C atoms of a number of symmetrical 1,4‐dihydro‐1,4‐diphenyl‐1λ5,4λ5‐diphosphinines were analysed. The substituent dependence of the absolute signs and magnitudes of 2J(PC) between the two 31P nuclei and two vinylic 13C atoms and of the 4J(PC) and 3J(PP) are reported. 13C isotope effects on δ(31P) and 3J(PP) are discussed.

Cross-polarization magic-angle spinning nuclear magnetic resonance study of platinum complexes containing the cis-P 2PtC 2 fragment

31P and 195Pt cross-polarization magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectra of three platinum complexes of formal oxidation state Pt(0) and Pt(II), respectively, are reported. All three complexes, (Ph 3P) 2Pt(C 2H 4) ( 1), (Et 2PCH 2CH 2PEt 2)Pt(CCH) 2 ( 2) and (Ph 2PCH 2CH 2 PPh 2)Pt(CCC(CH 3) CH 2) 2 ( 3) contain the square-planar cis-P 2PtC 2 fragment and show unusual NMR spectroscopic properties insofar that the 195Pt shielding patterns are fairly narrow in relation to what one would generally have to expect for 195Pt in square-planar coordination. Another unexpected NMR property of the cis-P 2PtC 2 fragment in 1–3 is the absence of spinning frequency-dependent second-order effects in this solid-state ABX spin system.

Direct analysis of the ABX spin system in two symmetrical diphosphonates: Comparison with two‐dimensional methods

AbstractThe results of the direct analysis of the ABX spin system present in two symmetrical diphosphonates containing a single vinylic 13C nucleus are presented. This allows a comparison to be made with the two‐dimensional methods used by Beckmann et al. Significant differences are found in the results obtained: the need to recognize the existence of isotopic chemical shifts is emphasized.

Separation of Longitudinal Multispin Order by Frequency Cycling

Longitudinal multispin orders provide an effective way for measurement of scalar couplings and also to probe molecular interactions and dynamics. Analysis of longitudinal orders has been made in strongly coupled AB and ABX spin systems to determine the dependence of strong coupling parameter on these orders. Experimental and simulated spectra of various longitudinal orders are illustrated for these spin systems. This general procedure can be extended to broad range of spin systems to understand the influence of strong coupling on longitudinal orders.

A NEW FORMALISM FOR DESCRIBING THE EVOLUTION OF DENSITY OPERATORS

A new formalism for describing the evolution of density operators was proposed. It can evaluate the evolution of density operators of strongly and weakly coupled spin systems in a u-nified way, and describe the experiments involving weak RF pluse. Some problems were explicitly treated including spin tickling, isotropic mixing in systems of two coupled nuclei (I= 1/2), dipolar evolution and strongly coupled AB and ABX spin systems.

Amended heteronuclear correlations by multiple quantum evolution. Theoretical aspects of an ABX spin system

AbstractThe results of a theoretical investigation, using a modified SMART program, on the behaviour of an ABX spin system during the basic INEPT and multiple quantum evolution pulse sequences are presented. We found, in agreement with the experimental results, that the constant time version of the multiple quantum experiment provides an almost complete suppression of spurious signals, even in the case of the most strongly coupled spin system.

Dipolar cross-correlation effects in the nuclear overhauser experiments of weakly and strongly coupled spins

The effect of dipolar cross correlation in 1H 1H nuclear Overhauser effect experiments is investigated by detailed calculation in an ABX spin system. It is found that in weakly coupled spin systems, the cross-correlation effects are limited to single-quantum transition probabilities and decrease in magnitude as ωτ c increases. Strong coupling, however, mixes the states and the cross correlations affect the zero-quantum and double-quantum transition probabilities as well. The effect of cross correlation in steady-state and transient NOE experiments is studied as a function of strong coupling and ωτ c . The results for steady-state NOE experiments are calculated analytically and those for transient NOE experiments are calculated numerically. The NOE values for the A and B spins have been calculated by assuming nonselective perturbation of all the transitions of the X spin. A significant effect of cross correlation is found in transient NOE experiments of weakly as well as strongly coupled spins when the multiplets are resolved. Cross correlation manifests itself largely as a multiplet effect in the transient NOE of weakly coupled spins for nonselective perturbation of all X transitions. This effect disappears for a measuring pulse of 90° or when the multiplets are not resolved. For steady-state experiments, the effect of cross correlation is analytically zero for weakly coupled spins and small for strongly coupled spins.

MENTHYL-SUBSTITUTED ORGANO-PHOSPHORUS-COMPOUNDS. VII1: FLUORINATED COMPOUNDS AND DERIVATIVES

Abstract The synthesis of a series of compounds, containing the (–)-menthyl group, bonded to tri-, tetra- and penta-coordinate phosphorus is described. Specifically, the formation of (–)-menthyltetrafluorophosphorane is described, based on the oxidative addition of (–)-menthyl fluoride to phosphorus trifluoride or by the oxidation-reduction reaction between (–)-menthyldichlorophosphine and arsenic trifluoride. The hydrolysis of (–)-menthyltetrafluorophosphorane was found to proceed stepwise, with formation of (–)-menthylphosphonofluoridic acid and, and ultimately, (–)-menthylphosphonic acid. 1H-, 13-, 19F- and 31P-NMR techniques were used to establish molecular structures of (–)-menthyl-substituted phosphorus compounds. Particular attention is drawn towards the chiral discrimination of geminal fluorine pairs in (–-)-menthyl-P(X)F2 (X=:, O, S) compounds, giving rise to ABX spin systems in the 19F(H)- and 31P{H}-NMR spectra.

A product operator description of AB and ABX spin systems

A simple product operator description of the strongly coupled AB and ABX spin systems is developed. The utility of this description in the investigation of the effects of strong coupling is illustrated in a number of examples.

Analysis of ‘virtual one-bond coupling’ effects in heteronuclear chemical shift correlation 2D N.M.R. spectra

Analytical and numerical calculations of the effects of ‘one-bond virtual couplings’ on heteronuclear proton-carbon-13 chemical shift correlation two-dimensional N.M.R. spectra are presented. Experimental results in good agreement with theory are reported for the two heteronuclear ABX spin systems of 2,3-dibromothiophene and the two ABCX systems of methyl acrylate.

Simulation of two-dimensional NMR experiments using numerical density matrix calculations

A computer program, SPHINX (simulation of pulse sequences in high-resolution NMR experiments) is described. SPHINX is capable of calculating homonuclear and heteronuclear two-dimensional NMR experiments using a wide variety of schemes of radiofrequency pulses and delays, and applying the common phase cycles and data handling routines. In the present implementation on a DEC 10 computer the program can handle strongly coupled spin systems with up to six spins I = 1 2 . An additional program LINSHA creates lineshapes and allows graphical presentation of the simulated spectra. As an illustration, a variety of 2D NMR experiments with an ABX spin system have been computed.

Conformation of the eosinophil chemotactic tetrapeptides and analogues in dimethyl sulfoxide. 1H n.m.r. studies.

Proton nuclear magnetic resonance parameters are reported for DMSO-d6 solutions of the eosinophil chemotactic tetrapeptides, Val1-Gly2-Ser3-Glu4 and Ala1-Gly2-Ser3-Glu4, as well as three analogues of the Val1 tetrapeptide, D-Val1, Ala2 and Ala3. The synthesis of Val-(S)-[alpha-2 H1] Gly-Ala-Glu, in which the glycine has been stereospecifically deuterated in the H alpha 3 position, has allowed the assignment of the 1H resonances belonging to individual H alpha 2 and H alpha 3 glycine methylene protons. Simulation of the glycine ABX spin system yields two vicinal coupling constants which are consistent with a highly preferred conformation about the glycine HN-C alpha bond. The chemical shifts, coupling constants, temperature coefficients of amide proton chemical shifts and calculated side chain rotamer populations are reported for all peptides. The coupling constant analysis and temperature coefficients of amide proton chemical shifts together suggest that a type I beta-turn conformation is preferred by the Ala3 analogue. The 1H n.m.r. parameters of the other peptides suggest that these can also adopt a beta-turn conformation in DMSO. There are, however, considerable differences in the extent of conformational averaging undergone by the various peptides.

Two-Dimensional spin-echo multiple-quantum transitions in strongly coupled spin systems. Calculation of spectra

A general method for computation of two-dimensional spin-echo multiple-quantum spectra of strongly coupled spin systems is presented. It has been demonstrated that the 180° pulse can severely modify the multiple-quantum spectra of strongly coupled spins and explicit spectral details are given for the spin systems of types AB, ABX, and AB 2. In addition to the modified frequencies and additional transitions, it is found that such spectra possess additional frequency symmetry with respect to the center of each multiple-quantum order, even though the multiple-quantum spectrum without the 180° pulse may not be symmetric. The ABX spin system is treated in detail and the behavior of the homonuclear and heteronuclear multiple-quantum transitions in the presence of selective or nonselective 180° pulses is discussed. It has been demonstrated that strong coupling allows transfer of magnetization from abundant spins to a coupled rare spin even in the absence of excitation pulses on abundant spins. The amplitudes of various coherences created by selective and nonselective pulses and the nature of spin-echo zero-quantum transitions of strongly coupled spins are discussed in appendices.

Chemistry of the phenoxathiins and isosterically related heterocycles. XXVII. Two‐dimensional J‐resolved spectra of 1,4,9‐triazaphenoxathiin: The influence of frequency of the position and intensity of resonances due to strong coupling

AbstractThe synthesis of 1, 4, 9‐triazaphenoxathiin and the analysis of the two‐dimensional J‐resolved (2DJ) 1H‐nmr spectra of the molecule at 100 and 200 MHz are reported. Due to the strong coupling of the ABX and AB spin systems, additional resonances are observed in the 100 MHz 2DJ spectrum which are diminished in intensity and shifted further from F1 = O Hz in the case of the ABX system at 200 MHz, or which have disappeared in the 200 MHz spectrum in the case of the AB spin system. Calculated frequencies and intensities of the resonances due to strong coupling were found to be in excellent agreement with the observed data.

Asymmetric NMR spectra of weakly coupled nuclei: A common characteristic in the 13C NMR spectra of [U‐13C]‐labeled molecules and in natural abundance protoncoupled 13C spectra

AbstractTypically encountered proton‐decoupled spectra of [U‐13C]‐labeled molecules or proton‐coupled spectra of natural abundance or singly labeled molecules contain many nuclei which satisfy the weak coupling approximation. The spectra of such nuclei are frequently highly asymmetric and often appear to exhibit the skewed intensity distribution characteristic of strongly coupled spins. Analysis of typical cases indicates that such effects arise in the presence of at least one moderately strong coupling interaction in the spin system (Jδν ∼ 1/3), and the apparent intensity asymmetry reflects small differences in the spacing of unresolved components of the observed resonance. This effect is analogous to the case of ‘virtual coupling’ in which weakly coupled spins cannot be analyzed as first order spectra; however; the ABX spin system which generally serves as a model for such systems does not predict the existence of spectral asymmetry for the X resonances. Prediction of this asymmetry requires a spin system of at least four spins with at least ABMX complexity, and can be treated generally using the effective Hamiltonian approach of Poeple and Schaefer.

Conformation of some ribofuranosyl imidazole and uracil nucleosides: a 1H and 13C nmr study

100 and 220 MHz 1H NMR spectra, recorded in DMSO- d 6 solution and including some measurements at 343 K, of three anomeric pairs of novel 2', 3'- O-isopropylidene nucleosides (two ribofuranosyl imidazoles, 1 and 2, and one uridine 3, all related to intermediates in de novo nucleotide biosynthesis) have been analysed, mostly as overlapping ABX spin systems. Chemical shifts and couplings have enabled the α- and β-anomers to be identified; the sugar-ring vicinal H-H coupling constants indicate that the 2',3'-ketal blocking group locks the α and β anomers in predominantly S and approximately 1:1 N/S ribofuranoside conformations, respectively. Among the 13C shifts reported for 3 in DMSO- d 6, those of the 2', 3'- O-isopropylidene CH 3 in the α anomer (shift difference 1.4ppm) are around 1.6 ppm upfield of those for the β anomer (which have a shift difference of 1.8 ppm).

Conformational analysis of the products of 1,4 addition of Tri(phenylthio)methyllithium to α,β-unsaturated ketones. The X-ray structuredetermination of 4-Phenyl-5,5,5-tri(phenylthio)pentan-2-one

Iterative analysis of the second-order ABX spin system in the 1H n.m.r. of the title compound gave coupling constants which are consistent with a conformation involving a H-C-C-H dihedral angle of about 90�. The structure was confirmed by X-ray analysis. The 1H n.m.r. of the related compounds 2,2-dimethyl-5-pheny1-6,6,6-tri(phenylthio)hexan-3-one and 1,3-diphenyl-4,4,4-tri(pheny1-thio)butan-1-one were also analysed and found to have similar conformational characteristics.

Spin-lattice relaxation in the tightly coupled ABX spin system

Spin-lattice relaxation in the tightly coupled ABX spin system

Unexpected second‐order effects in proton‐coupled 13C NMR spectra

Abstract‘Unexpected’ second‐order effects encountered in proton‐coupled 13C NMR spectra where relative proton shifts are large compared to proton‐proton coupling constants are illustrated. The observed phenomenon is explained, with reference to the ABX spin system as a model, as being due to the near equality of the relative proton shift, in frequency units, to the difference between one‐bond and long‐range 13CH couplings. The effect may be removed by a change in operating field strength or addition of a shift reagent.

Predicting non-first-order behaviour in fully coupled 13C-1H spectra: a general rule

The proton-coupled carbon-13 n.m.r. spectra in ABCDX, ABCX and ABX spin systems are shown to be non-first order when the condition 1JCH/2 = Δδ is met, where Δδ is the proton chemical shift separation between two closely coupled protons associated with adjacent carbon atoms. Solvent and temperature effects are shown to be useful in removing the spectral complexity and thus assist in complete analysis. Examples in heteroaromatic and hydrocarbon compounds are presented. For a chosen solvent system the non-first-order behaviour may be predicted from the 1H n.m.r. spectrum and used as an aid in 13C n.m.r. spectral assignment.