Probing the flexibility of internal rotation in silylated phenols with the NMR scalar spin-spin coupling constants.

The rotation of a trimethylsiloxy (TMSO) group in three silylated phenols (with three different ortho substituents -H, -CH 3 , and -C(CH 3 ) 3 ) was studied with the NMR n J(Si,C), n = 2, 3, 4, 5, scalar spin-spin coupling between the 29 Si nucleus of the TMSO group and the 13 C nuclei of the phenyl ring. The internal rotation potential calculated with the B3LYP and MP2 calculation methods including the effect of a solvent environment (gas phase, chloroform, and water) was used for the calculation of the dynamical averages of the scalar coupling constants in the framework of the rigid-bender formalism. Solvent effects, the quality of the rotational potential, and the applicability of the classical molecular dynamic to the problem is discussed. Quantum effects have a sizable impact on scalar couplings, particularly for the internal rotational states well localized within the wells of the potential surfaces for the TMSO group. The overall difference between the experimental and theoretical scalar couplings calculated for the global energy-minima structures (static model) decreases substantially for both model potentials (B3LYP, MP2) when the molecular motion of the TMSO group is taken into account. The calculated data indicate that the inclusion of molecular motion is necessary for the accurate calculation of the scalar coupling constants and their reliable structural interpretation for any system which possesses a large-amplitude motion.