Phosphorus-31 chemical-shift parameters

I mentioned this in my chapter in Annual Review. That was when I found the isotopic shift due to on the NMR phosphorus chemical shifts of ATP because I had come full circle. I was now showing an isotopic mass effect in a spectroscopic parameter and that s where I started in my graduate work. It isn t a very important finding, but it gave me great pleasure to discover yet another effect of isotopic mass of a stable isotope with a spectroscopic manifestation. 

Letcher and Van Wazer applying LCAO theory first developed by Pople and Karplus and Das, have expressed the shielding (which was dominated by the magnitude of (7 ), in terms of localized bond parameters, e.g., ionic character, hybridization and double bonding. Using expressions for (7 obtained by Jameson and Gutowsky and taking account of both phosphorus p and d orbital occupation, the phosphorus chemical shift, <5 —was related to four parameters and/by equation (11)—... 

The iminophosphenium cation was also of interest for more physical investigations. It was noted to be a stable entity in gas phase experiments [52], the parameters were investigated in detail [53] and a systematic study of the nucleophilic addition of CH, NH and OH bonds was performed [54] with a concomitant interpretation of the chemical shifts (at the dicoordinate phosphorus centres). The latter authors also confirmed the loose interaction of a triphenylphosphine with the iminophosphenium cation (PP = 2.625 A). 

The chemical shifts, Sp, of substituted arylphosphonic acids (9) have been found to be linearly related to the Hammett a and Taft ojt and cti parameters. The shielding of the phosphorus nucleus increases with the electron-withdrawing properties of the substituents, which is analogous... 

The iminotriphenylphosphorane series (1) gave a plot with a positive slope (3.19) showing that the groups which increase electron availability at the ylidic nitrogen cause shielding of the phosphorus nucleus. The statistical parent chemical shift 6P° was 3.30. This parameter relates to the general shielding of the specified series and is usually close to the chemical shift of the unsubstituted compound. 

Up to this point, we have summarized the NMR parameters of some selected crystalline compounds. Model crystalline compounds of calcium phosphates are important structural reference for the study of biominerals because the highly complex hierarchical structures of bones and teeth contain different phosphorus species such as unprotonated orthophosphate (PC>43 ) and protonated orthophosphate (HP042 ).70,77 It is beyond doubt that 31P chemical shifts are the most important spectroscopic parameters for the characterization of calcium phosphates. While the 31P chemical shift can reflect the protonation state of a phosphate species, 31P CSA is a sensitive measure of the symmetry of the electronic... 

Also examined were the chemical shifts of substituted nickel carbonyls, X—Ni(CO)3 . The ligands, X, had the form MZ3 where M is a group 15 element, usually phosphorus. It can be seen on inspection of the equations we have used to estimate electrical-effect parameters for Group 15 tricoordinate substituents that we can use these parameters as a measure of the electrical effect of the X groups in this data set. The best regression equation obtained is ... 

Most of the reported results are given as evidence for structures in solution of compounds whose crystal structures are known from X-ray data. Some of them are completed by and P NMR spectra. We will give only the NMR parameters of these compounds using the chemical formulae. For more structural details, the reader can refer to the original publications. Platinum chemical shifts are referred to S = 21.4 MHz. In [Pt2Fe(dppm)2(CO)4], 5( Pt) = + 790 for the platinum atom bound to one phosphorus atom, and (i95pt)=+497 for the platinum bound to two phosphorus atoms, V(i 5pt,i95pt)= 1730 Hz [100]. Other results are presented in Table 20. 

Direct relationships between the P NMR parameters and structure are complicated by the fact that the chemical shifts are determined in phosphorus compounds predominantly by the paramagnetic rather than the diamagnetic contribution to the shielding tensor. This paramagnetic term is not necessarily directly related to the bonding electron... 

The principal values, 6n, 622, 633, of the chemical shift tensor were obtained for each form by CP/MAS NMR experiments. The orientation of the principal axes with respect to the molecular frame was investigated by P CP/single-crystal NMR for the complex with propan-2-ol. The principal axis 1 of both chemically equivalent phosphorus atoms is nearly parallel to the P-S bond and the principal axis 3 is very close to the P=S bond. The comparison of experimental P 6 parameters with theoretical data calculated by the DFT GIAO approach provided complementary information about the most sensitive NMR parameters, which best characterize the nature of the C-H -S contacts. 

Compounds. Theoretical estimates of 5p by CNDO/2 calculations required the inclusion of phosphorus adjustable parameter which depends on the type of compound, e.g. phosphine or phosphite. The chemical shifts of a series of cyano-compounds (5 X, Y=hal or CN) agree with those predicted by Letcher and van Wazer s quantum-mechanical interpretation. The sensitivity of 5p to stereochemical changes often leads to quite large differences of chemical shift between various conformers or isomers, e.g. the axial conformer (6) has <5p upheld... 

Chemical Shifts and Shielding ElSects.—Phosphorus-31. In this section, positive P chemical shifts (dp) are upheld from 85% phosphoric acid. A linear relationship between dp and vapour pressure for elemental phosphorus in the gas phase has been reported. Solvent effects have also been examined. dp compounds. The n.m.r. parameters of various protic and deuteriated... 

We wonld expect a considerable downfield shift npon coordination of the phosphane, and we are therefore not snrprised to observe it in the ensning adducts. However, why do we observe an npfield shift upon coordination to the borane with the phosphanes in the lower part of the table The difference mnst lie in the behavior of the snbstituents on phosphorns, as this is the one parameter that changes as we look down the list. In the top part, the snbstituents are H, methyl, and phenyl, whereas in the lower part, the snbstitnents are flnoride, amide, and methoxide. The latter three (F, NMe and MeO) are capable of a 7r-bonding interaction toward phosphorus that increases as the electron density on phosphorus diminishes upon coordination. Since the P-NMR chemical shifts are more sensitive toward. ir-interactions than cr-interactions, the net result can very well be an upfield shift upon coordination of the phosphane, if substituents capable of . r-backbonding are present on phosphorus. 

Following Knight s discovery of P chemical shifts, ° attempts have been made to correlate changes in the observed shift parameters with the inductive effects of neighbouring atoms or groups, and the much lower field shifts observed for trivalent phosphorus compounds... 

---