The prototype aluminum–carbon single, double, and triple bonds: Al–CH3, Al=CH2, and Al≡CH

Abstract

Nonempirical quantum mechanical methods have been used to investigate the AlCH3, AlCH2, and AlCH molecules, which may be considered to represent the simplest aluminum–carbon single, double, and triple bonds. Equilibrium geometries and vibrational frequencies were determined at the self-consistent-field level of theory using a double zeta basis set: Al(11s7p/6s4p), C(9s5p/4s2p), H(4s/2s). The 1A1 ground state of AlCH3 has a reasonably conventional Al–C single bond of length 2.013 Å, compared to 1.96 Å in the known molecule Al(CH3)3. The CH equilibrium distance is 1.093 Å and the Al–C–H angle 111.9 °. The structures of three electronic states each of AlCH2 and AlCH were similarly predicted. The interesting result is that the ground state of AlCH2 does not contain an Al–C double bond, and the ground state of AlCH is not characterized by an Al≡C bond. The multiply bonded electronic states do exist but they lie 21 kcal (AlCH2) and 86 kcal (AlCH) above the respective ground states. The dissociation energies of the three ground electronic states are predicted to be 68 kcal (AlCH3), 77 kcal (AlCH2), and 88 kcal (AlCH). Vibrational frequencies are also predicted for the three molecules, and their electronic structures are discussed with reference to Mulliken populations and dipole moments.