31P chemical shift

All of the soluble polymers (1 and 3-6) give high resolution NMR spectra (1H, 13C, and 31P) that are completely consistent with their proposed structures. As observed for other types of poly(phosphazenes), the 31P chemical shifts of these alkyl/aryl substituted polymers are consistently ca. 15-30 ppm upfield from those of the analogous cyclic trimers and tetramers. Some important structural information is provided by 13C NMR spectroscopy, particularly for the phenyl/alkyl derivatives 3 and 4. These polymers are rare examples of phos-phazenes that contain two different substituents at each phosphorus atom in the chain. Thus, they have the possibility of being stereoregular. The fact that the structures are completely atactic, however, is confirmed by the observation of three doublets in the P-Me region of the 13C NMR spectrum (ca. 22 ppm) in a 1 2 1 intensity ratio. 

Proton and Carbon Spectra. Proton and carbon NMR data, including 31P chemical shift and P—C coupling constants for the above compounds are given in Scheme 3.27. 

P nuclear magnetic resonance (NMR) spectroscopy has been of great use in determining the coordination state and stereochemistry of the phosphorus atom at the spiro position in spirophosphonia compounds, spirophosphoranes and spiroperphosporanides. The 31P chemical shift is also sensitive to the nature of the atoms directly bonded to the spiro phosphorus center and the size of rings of the spirocyclic system. 

Treatment of all four diasteromers of 72 with sodium or potassium bases yielded stilbenes with high stereoselectivity (Scheme 15) <2003T255>. Two of the diastereomers gave rise to some retro-aldol products. In two cases, hexacoordinate species were identified by the upheld 31P chemical shift (5—112 ppm). It was noted that phosphoranes that ring-closed to form hexacoordinate tricyclic species were the ones that did not undergo the retro-aldol reaction to produce aldehydes. The ring closure was disfavored for intermediates with steric repulsion between trifluoromethyl and phenyl groups. 

Thus, DFT-IGLO calculations of the 31P chemical shift without spin-orbital corrections yielded a value of S = 274, whereas additional spin-... 

Remarkably, the calculation of the 31P chemical shift of the parent compound of 20d, H2P2Sn3Cl2, gave a value of S = 52 with a spin-orbital... 

An excellent example of the use of 31P chemical shifts, in combination with H-NMR and 103Rh-NMR data as well as coupling constants information, can be found in the report by Duckett et al. [21] on the activation of... 

The resulting product showed a single 31P chemical shift at 40 ppm with a XJ coupling to rhodium of 118 Hz, indicative of a rhodium (III) center. As this rhodium atom was shown to couple to two phosphorus atoms, it could be concluded that the phosphorus atoms are in a symmetrical position, as shown in Figure 11.3. The other Rh atom couples only to a phosphoms atom with a jphp of 196 Hz, indicative of a rhodium (I) center. 

The DD-CSA cross-correlated relaxation, namely that between 13C-1H dipole and 31P-CSA, can also be used to determine backbone a and C angles in RNA [65]. The experiment requires oligonucleotides that are 13C-labeled in the sugar moiety. First, 1H-coupled, / - DQ//Q-II CP spectra are measured. DQ and ZQ spectra are obtained by linear combinations of four subspectra recorded for each q-increment. Then, the cross-relaxation rates are calculated from the peak intensity ratios of the doublets in the DQ and ZQ spectra. The observed cross-correlation rates depend on the relative orientations of CH dipoles with respect to the components of the 31P chemical shift tensor. As the components of the 31P chemical shift tensor in RNA are not known, the barium salt of diethyl phosphate was used as a model compound with the principal components values of -76 ppm, -16 ppm and 103 ppm, respectively [106]. Since the measured cross-correlation rates are a function of the angles / and e as well, these angles need to be determined independently using 3/(H, P) and 3/(C, P) coupling constants. 

As an example of the measurement of cross-correlated relaxation between CSA and dipolar couplings, we choose the J-resolved constant time experiment [30] (Fig. 7.26 a) that measures the cross-correlated relaxation of 1H,13C-dipolar coupling and 31P-chemical shift anisotropy to determine the phosphodiester backbone angles a and in RNA. Since 31P is not bound to NMR-active nuclei, NOE information for the backbone of RNA is sparse, and vicinal scalar coupling constants cannot be exploited. The cross-correlated relaxation rates can be obtained from the relative scaling (shown schematically in Fig. 7.19d) of the two submultiplet intensities derived from an H-coupled constant time spectrum of 13C,31P double- and zero-quantum coherence [DQC (double-quantum coherence) and ZQC (zero-quantum coherence), respectively]. These traces are shown in Fig. 7.26c. The desired cross-correlated relaxation rate can be extracted from the intensities of the cross peaks according to ... 

Fig. 8.10 31P chemical shift anisotropy (CSA) tensor orientation of a nucleotide from a DNA dode-camer with its helix axis pointing vertically. The DNA structure has been calculated using CSA data obtained under anisotropic conditions... 

The major absorption in the 31P n.m.r. spectrum of an equimolar solution of penta-phenoxyphosphorane and sodium phenoxide in DMF-acetonitrile is due to the hexaphenoxyphosphate anion, as predicted from the low equilibrium constant estimated for equation (2) (page 35).27 Catechol and phosphorus oxychloride in refluxing benzene gave the spirophosphorane(108), which with triethylamine gave the salt (109).45 On the basis of its 31P chemical shift in DMF solution, (108) was formulated86 as the free six-co-ordinate acid (110), but it seems probable that DMF is... 

Another interesting point is to be presented in relation to chemical shift evaluations. From the directly-measured spin-rotation constant of POF3, diamagnetic and paramagnetic contributions to phosphorus chemical shifts were estimated (ca. +1000 and —640 p.p.m.). Extended to H3P04, which is the commonly accepted reference compound, this gives about 320 p.p.m. for the absolute 31P chemical shift.a999,9)... 

NMR spectroscopy of phosphaalkenes and other compounds containing C—P double bonds has been reviewed309. Considerable differences in the 31P chemical shifts have been found for diastereomeric pairs of phosphaalkenes, for example310,311 ... 

TABLE 2. 31P NMR chemical shifts (ppm) of phosphonium and aminophosphonium halides35,37 (also steric compression was shown to give upfield 31P chemical shifts in cycloalkyltriphenylphosphonium salts)... 

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