Restricted Rotation and Pseudorotation

4 Studies of Equilibria, Configuration and Conformation. - Alcoholic solutions of acylphosphonates have been shown to contain considerable amounts of hemiketals by examination by P NMR spectroscopy. Because of the great difference between the P chemical shifts of acylphosphonates (831 0) and their hemiketals (63ip= 17-21), P NMR spectroscopy was shown to be a suitable method for studying the rates and equilibrium of hemiketal formation of acylphosphonates with different alcohols. 

3 Restricted Rotation and Pseudorotation. - Dynamic NMR spectroscopy of lithium bis(diphenylphosphino) amide gave an 8.1 kcal/mol rotation barrier around the PN bonds. The CP/MAS spectrum of the solvate has a single Li line, whereas the P CP/MAS spectrum reveals the chemical non-equivalence of the phosphorus sites. The appearance of two P signals indicates a minimum activation barrier for the P,P-exchange process of AG 13.4 kcal/mol. Li and P NMR spectroscopy indicated that the monomeric structure in THF solution is similar to the X-ray structure of a solid 5.3 THF solvate. 

4 Studies of Equilibria, Configuration and Conformation. - Conformational analysis of trihalo- triphospholanes (29) by low-temperature P NMR spectroscopy revealed the presence of two conformers. The ylidyl substituent of the chlorophosphine (94) exerts a strong influence on the P-Cl bond. The ylidyl chlorophosphines (94 R = R2N) are covalent in benzene but become more or less ionic in dichloromethane solution. The solvent dependent dissociation equilibrium can be followed by P NMR spectroscopy. In the case of an enamine-derived ylidyl chlorophosphine, the equilibrium shifts almost completely from the covalent to the ionic side within a rather narrow range of solvent composition, from 20 to 70% (vol) of dichloromethane.  

The absolute configurations of amines and alcohols can be readily determined by H and P NMR spectroscopy of the cyclophosphamides formed by reaction with diazacyclo- phosphamides (87, 88) and oxaazacyclophosphamidic chlorides 

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