Stable Simple Enols. 10.^1a 500-MHz ¹H NMR of Trimesitylethenol and Deuterated Analogues: Complete Signal Assignments and Direct Determination of Equilibrium Steric Isotope Effects from the NMR Integrals of the Isotopomeric Enols

E/Z mixtures of the isotopomers Mes*C(Mes)=C(OH)Mes (7a/7b) and Mes*C(Mes)=C(OH)Mes* (8a/8b) [Mes = 2,4,6-(CH₃)₃C₆H₂; Mes* = 2,4,6-(CD₃)₃C₆H₂] were prepared by addition of MesMgBr and Mes*Br to Mes*C-(Mes)=C=0. High resolution ¹H NMR at 500 MHz of trimesitylethenol (5) and the labeled isotopomers together with NOE data, saturation transfer, and T₁ relaxation times have now led to a complete assignment of the signals for all 15 nonequivalent proton sites in these triarylvinyl propellers. Two OH signals were observed for each isotopomeric pair. The relative integration of the two OH signals and of the methyl signals of the β-ring (cis to α-Mes) and the β'-ring (cis to OH) gave the isotopomeric ratio. Lorentz-Gaussian line-shape transformations, base-line corrections, and several repeated integrations at 500 MHz were used in order to obtain reliable ratios. The E isomers 7b and 8b (β-ring = Mes*) were present in excess, and the best values of the isotope effects at 302 K are 1.026 ± 0.007 for the ratio 7b/7a and 1.048 ± 0.027 for 8b/8a. A statistical analysis showed that the values are significantly different from unity. The isotopomer ratios were ascribed to an equilibrium steric isotope effect where the isomer with the smaller Mes* group cis to the a-mesityl ring is present in a slight excess. The appearance of a separate OH signal for each isotopomer results from slightly different conformations of the labeled derivatives due to the steric isotope effect. This is the first direct determination of a steric isotope effect by simple integration of NMR signals.