The comparison of the dynamic 1H NMR measurements between two special ylides derived from triphenylphosphine and triethylphosphite: Experimental and theoretical calculations

GRAPHICAL ABSTRACT ABSTRACT Dynamic 1H NMR measurements have been experimentally performed in particular phosphorus ylides involving 2-mercaptobenzimidazole and 2-mercaptobenzoxazole for comparison around the C˭C double bond and two single bonds (C‒C and C‒N). Also, activation parameters including ΔG‡, ΔS‡ and ΔΗ‡ were calculated using the ab initio and DFT methods at the HF/6-31G (d,p) and B3LYP/6-31G(d,p) levels of theory for the rotational interchangeable process of the two Z and E isomers of the ylides in the presence of a solvent environment (CDCl3). The obtained results were consistent with the dynamic 1H NMR data. The rotational energy barrier around the carbon=carbon double bond in the ylide 4 derived from triphenylphosphine was 2 kcal⋅mol−1 higher in energy than the generated ylide 5 from triethylphosphite. In general, in both rotational Z and E isomers of ylide 5, the rotational energy barrier around the carbon–nitrogen single bond was higher for the carbon‒carbon single bond due to the inversion of a lone pair on nitrogen atom during a rotation. Although, the HF/6-31G(d,p) level was a desirable level for the rotation around the single bond, nevertheless, the B3LYP/6-31G(d,p) level was recognized as a suitable level for rotation around the double bond. Applying solvent media (chloroform) at the two levels of calculations increased consistency with the experimental results.