Phosphorus-31, deshielding

The reverse mesomeric effect (p -p conjugation) is believed to be very favourable in the A -phospholen system (5). Compared with the corresponding A -phospholens (6), the conjugated system (5) shows both Y and the phosphorus atom to be deshielded, and the vinyl proton is shielded. The cyclic nature of the molecule is important because the analogous... 

It is not clear why the sp2-hybridized carbon atom of a fluorenylidene germene (entry 3, Table II) is so shielded (8 79.8), unless this chemical shift is from the spectrum of the THF adduct.28 The chemical shift of the silicon atom in the germasilene (entry 4, Table II) and the phosphorus atom in germaphosphenes (entries 6-10, Table II) is deshielded compared to the corresponding silicon analogs. For example, for Mes2M=SiMes2... 

The configurations at phosphorus in compounds 73-76 were assigned on the basis of II-NMR data assuming that in phosphorus-containing heterocycles, the protons in a 1,3-m-relation to a P=0 group are deshielded. Thus, since H4 and H5 resonate at lower field in compounds 73a-76a than in compounds 73b-76b, the P=0 group must be cis to H4 and H5 in the a series [32]. These assignments were fully supported by the crystal structure studies of 75a, which showed that this... 

The acceptor number, AN, of a solvent is a measure of the power of the solvent to accept a pair of electrons [18], Experimental evaluation of AN involves observing the frequency changes induced by a solvent on the 31P NMR spectrum when triethylphosphine oxide, Et3P=0, is dissolved in the solvent. Donation of an electron pair from the oxygen atom of Et3P=0, as shown in Scheme 1.2, reduces the electron density around the phosphorus, causing a deshielding effect which leads to an increase in chemical shift. Hexane (AN = 0) and SbCls (AN = 100) were used as fixed points to define this scale. 

The 13C NMR spectra of 192 and 194 show characteristic deshielded sp carbons at 243.5 ppm175 and 235.1 ppm.174 A similar value was found for the analogous silicon compound (233.6 ppm).180 This chemical shift lies in the normal range and as expected, about 45 ppm upheld of that for germaphosphaallene Mes2Ge = C = PAr (280.9 ppm)173 because of the presence of the more electronegative carbon than phosphorus (see Section X,B). 

The deshielded 31P and 13C NMR chemical shifts observed for 41-43 ( 31P +130, 13C +76, /PC 85 Hz) are consistent with the presence of a P = C double bond and positive charge development at phosphorus these spectroscopic data are in fact very similar to those observed for the methylenephosphonium salt 40.59,6° This similarity is reinforced by the X-ray analysis of the gallium adduct 42 (Fig. 4) (i) The phosphorus and carbon atoms adopt a trigonal planar geometry, (ii) there is a twist angle between the two planes of 34.1°, and (iii) the phosphorus-carbon bond distance is rather short (1.61 A). 

In vinylic phosphonium salts, the jft-carbon is found to be substantially deshielded (A<5 = 22.4 ppm for the vinyltriphenylphosphonium bromide as compared with ethylene), although only a small inductive effect was expected. The extent of this was interpreted to be a consequence of pn-dn bonding between phosphorus and carbon39. This argument was used to identify structures when or 31PNMR spectra failed96. 

Similar upfield shifts for C-3/5 are observed with an axial R = alkyl, aryl, O or S. An axial methyl at the P atom in the 1-oxide, however, is shielded compared with an equatorial one. 31P resonances of phosphorus oxides and sulfides are strongly deshielded. Some examples show the differences (5 values in p.p.m.) Me3P, -62 Me4P+ I-, 25 Me3P=0, 36 Me3P=S, 59 MesP, unknown. Other physical methods, such asP=0 or P=S stretching frequencies and dipole moments, are sometimes very useful in conformational and structural determinations. 

Hydroxy(alkoxy)phosphonium Ions. Olah and McFarland560 studied the protonation in HS03F or HS03F-SbF5 solution of varied phosphorus oxyacids and derivatives. Treatment of tetravalent phosphoms compounds (phosphorus, phosphonic, and phosphinic acid and their trialkyl and triaryl derivatives) results in (9-protonation and the formation of hydroxyphosphonium ions. Trivalent phosphites, in turn, are protonated at the phosphorus atom. The 31P shifts observed for the latter ions are significantly deshielded, which was attributed to significant oxonium ion character. 

Complexes of type 3 with four-membered chelate rings (not included in the data of Figure 2) are a special case the observed, substantial deshielding of the 103Rh nucleus (around ca. 1100 ppm) cannot be explained by the Rh-P bond lengths (15). According to the model calculations for 5, it is the tilt of the lone pairs at phosphorus away from the Rh-P axis that is responsible for the deshielding in this case. 

Finally, in the quite unusual -C=P group, the carbon is deshielded to 8 185, while metal carbyne complexes (M=C-R) are even further deshielded into the region 8 230-365. This indicates not only less shielding by the magnetic anisotropy of the CP triple bond (suggesting that there is less efficient circulation of the C=P triple bond electrons around the internuclear axis) but also paramagnetic deshielding by the phosphorus or metal atom. 

Complexes of type A with phosphane are determined by the tr-donor bond of the phosphorus atom. On the contrary, the (d-d)n back donation of the phosphorus is poor, due to much lower x-acceptor ability, and the balance of electrons cannot take place within the phosphorus metal bond. With respect to 31P NMR the phosphorus molecule is deshielded, exhibiting a low-field shift for the i 1-coordinated P atom of about 30-60 ppm compared to the uncoordinated phosphane phosphorus. 

A selection of AN values has already been given in Table 2-5 of Section 2.2.6 cf. also Table 6-6 in Section 6.5.1. The observed solvent-dependent P chemical shifts result mainly from the polarization of the dipolar P=0 group, induced by the interaction with electrophilic solvents A, particularly HBD solvents. The decrease in electron density at the phosphorus atom results in a deshielding proportional to the strength of the probe/solvent interaction. In solutions of protic acids, the P chemical shift of the 0-protonated triethyl hydroxyphosphonium salt is observed. Since Et3PO is very hygroscopic and therefore not very suitable from an experimental point of view, the use of (n-Bu)3PO instead of Et3PO as probe molecule has been recommended [250]. 

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