The temperature dependence of the 1H NMR resonance of the C-4 olefinic proton in vinylcyclopropane was investigated through a combination of ab initio calculations and Boltzmann statistics. A torsional energy profile as a function of the (φ) dihedral angle was obtained using HF methodology with a 6-311G** basis set, while the corresponding 1H chemical shift profiles for the C-4 proton were computed using the GIAO approach and either HF, DFT (B3LYP) or MP2 methods at the 6-311G** level of theory. Chemical shifts at different temperatures calculated as canonical ensemble averages in which the different ab initio 1H chemical shift profiles and a Boltzmann factor defined by the HF/6-311G** energy function are employed reproduce remarkably well the temperature dependence observed experimentally. Attempts to perform a similar study using only the GIAO-MP2 1H chemical shift profile and (φ) dihedral angle trajectories obtained from molecular dynamics simulations at different temperatures failed to reproduce the experimental trends. This shortcoming was attributed to the inability of the force fields employed, Tripos 6.0 and MMFF94, to reproduce properly the three-well torsional potential of vinylcyclopropane. The application of both methodologies to the calculation of population-dependent chemical shifts in other systems is discussed.
All Science Journal Classification (ASJC) codes
- Materials Science(all)