Ab initio calculation of the equilibrium geometry and the ¹³C, ¹⁷O and ²⁷A1 NMR chemical shielding tensors of the system Al³⁺/CO

The complex Al³⁺/CO was investigated with respect to the equilibrium geometry using ab initio SCF calculations. Two minima have been found, both with linear configurations. The minimum corresponding to CO ... Al³⁺ is slightly deeper (ΔE_O = -0.206062 au) and reveals an elongation of the equilibrium bond length CO (0.1195 nm) with respect to that of isolated CO (0.1102 nm) and a shorter equilibrium distance R_O = 0.1743 nm between Al³⁺ and O. The other minimum corresponding to Al³⁺...CO amounts -0.198951 au and reveals a shortening of the equilibrium bond length CO (0.1075 nm) and an equilibrium distance R_C = 0.2062 nm between Al³⁺ and C. Using the equilibrium distances from these calculations the ¹³C, ¹⁷O and ²⁷Al NMR chemical shielding tensors were investigated using coupled Hartree-Fock calculations with large gaussian basis sets. The results are compared with ¹³C NMR investigations of carbon monoxide adsorbed on decationated zeolites.