A stable phosphorus ylide is obtained in high yield by a four component reaction between triphenylphosphine and dimethyl acetylendicarboxylate in the presence of the intermediate product formed from phenyl isocyanate and substituted phenols in dichloromethane at room temperature. This stable phosphorus ylide exists in solution as a mixture of two geometrical isomers resulting from restricted rotation around the carbon-carbon partial double bond due to conjugation of the ylide moiety with the adjacent carbonyl group. Dynamic effects are observed in 1 H NMR spectra that are attributed to restricted rotation around the carboncarbon double bond and the carbon-nitrogen single bond. These effects are used to calculate the free activation energy (ΔG ‡ ) and other activation parameters such as ΔH ‡ , ΔS ‡ and E a .

The hindered internal rotations around the partial carbon-carbon double bond (MeO 2 C=CPPh 3 ), nitrogen-carbon (N-C-CH) and carbon-carbon single (MeO 2 CH-C-C=PPh 3 ) bonds within the dimethyl-2-(6-aza thiothymin-1-yl)-3-(triphenyl-phosphoran ylidene) butandioate has been experimentally studied by the dynamic 1 H nuclear magnetic resonance (NMR) spectroscopic in variable temperatures. The activation (DH à , DS à , DG à ) and kinetic (k c , E a ) parameters were calculated in a series of separate dynamic 1 H NMR spectra. The energy of activation parameters obtained from classic, Eyring and Arrhenius methods were also compared. The results from Eyring and Arrhenius plots were in good agreement; however, were slightly different from the classic method.

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The internal rotations around the some within a ylide involving theophylline has been studied using the dynamic 1 H NMR spectroscopic technique in different temperatures. The kinetic and activation quantities were calculated. Arrhenius, Eyring and classic methods were applied for isomers. compared for carbon-carbon double, carbon-carbon single bond for Z isomer. The results obtained by classic method was different with data calculated from Arrhenius and Eyring methods while these methods were consistent together.