Ab Initio Study of the Rotation of the C≡C Group in Benzvalyne

For benzvalyne, we have characterized the minima and transition states of the molecule using the B3LYP, MP2, and M06-2X methods and aug-cc-pVTZ basis set. This was done by calculating the energy of the molecule while rotating the alkyne bond contained within it. After the initial rotation, optimized parameters for the molecule are found. Following this, imaginary frequencies were attempted to be found at the local maximum produced by the graph of the Energy v. rotation angle. Should the negative frequencies be found, the energy of the molecule will be characterized by the CCSD (T) method and aug-cc-pVQZ basis sets. This energy was then corrected using the zero-point correction in order to give us the most accurate ΔE value. This information has not been identified to my previous knowledge, and the ΔE of the molecule was found to be 47.061 kcal*mol-1 for B3LYP, 54.218 kcal*mol-1 for MP2, 37.498 kcal*mol-1 for M06-2X, 52.698 kcal*mol-1 for the CCSD (T) calculation in MP2, and 51.993 kcal*mol-1 for the CCSD (T) calculation in M06-2X. The CCSD (T) calculations for both MP2 and M06-2X gave nearly identical numbers. In addition, the base MP2 calculation gave a ΔE that was very close to those done by higher quality calculations. The B3LYP calculation as well as the M06-2X calculation seemed to underestimate the value of the ΔE. This could be due to a variety of reasons, but likely stems from the fact that both methods are hybrid and as such, will use both Hartree-Fock methods as well as DFT methods to obtain the estimate of ΔE