The internal rotational barrier heights of biphenyl were calculated with the Dunning correlation consisted basis sets (up to cc-pVQZ, 960 basis functions) and the electron correlation correction by the second order Møller-Plesset method (MP2). Although previous Hartree-Fock (HF) and MP2 calculations showed that the internal rotational barrier height at 0° (ΔE0) was substantially larger than that at 90° (ΔE90), our MP2/cc-pVQZ//MP2/6-31G* calculations showed that ΔE0 (2.28 kcal/mol) was close to ΔE90 (2.13 kcal/mol), which agreed with the estimation from experimental measurements. The calculations of benzene dimers suggested that the dispersion interaction increased the relative stability of the coplanar conformer. The basis sets employed in the previous calculations were not large enough to evaluate the attractive dispersion interaction. The underestimation of the stabilization of the coplanar conformer by the dispersion interaction would be one of the reasons for the overestimation of ΔE0 in the previous calculations.
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