Pyramidane 2. Further computational studies: potential energy surface, basicity and acidity, electron-withdrawing and electron-donating power, ionization energy and electron affinity, heat of formation and strain energy, and NMR chemical shifts

The novel cycloalkane pyramidane (tetracyclo[2.1.0.0^1,30^2,5]pentane, [3.3.3.3]fenestrane), C₅H₄, with a pyramidal carbon atom, was investigated further. Calculations at the B3LYP/6-31G^* and G2(MP2) levels supported earlier conclusions from QCISD(T)/6-31G^*//MP2(FC)/6-31G^* energies that pyramidane lies in a deep well (ca. 100 kJ mol⁻¹) on the potential energy surface. The pyramidal carbon is predicted to have a lone electron pair, and calculations (CBS-4) indicate that pyramidane is remarkably basic for a saturated hydrocarbon (proton affinity 976, cf. 922 and 915 kJ mol⁻¹ for pyridine and aniline, respectively). The calculated (CBS-4) acidity is similar to that of tetrahedrane and toluene; the pyramidyl group (C₅H₃) attached to an atom bearing a lone electron pair appears to be much more strongly electron-withdrawing than the phenyl group. The infrared CO stretching frequency and C–CHO rotational barriers of pyramCHO, PhCHO and cyclopropylCHO indicate that the pyramidyl group is comparable to phenyl and cyclopropyl in its ability to donate electrons to an electron-deficient carbon. The adiabatic ionization energy of pyramidane is ca. 9.0 eV (MP2/6-31G^*, energy differences and Koopmans’ theorem), similar to that of typical cycloalkanes. The heat of formation of pyramidane was calculated by the G2(MP2) method and isodesmic reactions to be to be 585 kJ mol⁻¹ and the strain energy was estimated to be 622 kJ mol⁻¹; pyramidane is 122 kJ mol⁻¹ more strained than its isomer spiropentadiene. Application of the NMR NICS method, varying the position of the probe nucleus, gave no evidence for benzenoid-type aromaticity in the potentially cyclobutadiene cation-like base of pyramidane.