The solvent effect for rotation about the conjugated C-N(CH3)(2) bond has been studied for N,N,N'-trimethylurea (TMU) and N,N,N'-trimethylthiourea (TMT) by dynamic NMR and theoretical calculations. The experimental part comprised the measurement of activation parameters (Delta H-double dagger, Delta S-double dagger, and Delta G(double dagger)) by DNMR in CD2Cl2, CD3OD, and D2O/CD3OD solutions. In methanol, TMU and TMT present similar rotational barriers, 11.3 +/- 0.6 and 10.5 +/- 0.3 kcal/mol, respectively. However, in D2O/CD3OD solution TMU has its barrier raised to 12.4 kcal/mol, whereas that of TMT remains unchanged. Molecular dynamics simulations combined to quantum chemical methods (HF, B3LYP, B3LYP-D, M06-2X, and MP2) showed that hydrogen bonding affects the rotational barriers of TMU and TMT in markedly different ways. For TMU, the rotational barrier increases due to hydrogen bonding whereas for TMT it decreases. This behavior is a consequence of the distinct ways the ground and the transition states for rotation experience hydrogen bonding. More specifically, the ground state of TMU can form strong hydrogen bonds at the carbonyl group, which stabilize the ground state relative to the transition state. On the other hand, the sulfur of TMT showed to be a poor proton acceptor. in such a way that only the nitrogen of twisted transition state is involved in hydrogen bonding.