Direct determination of the permanent dipole moments and structures of Al-CH₃CN and Al-NH₃ by using a 2-m electrostatic hexapole field

The supersonic beams of the (1-1) metal-ligand complexes of Al-CH₃CN and Al-NH₃ were produced by a laser evaporation method. Nondestructive structure selection of the complexes and the dipole moment determination were performed by using a 2-m electrostatic hexapole field. The experimentally determined permanent dipole moments are 1.2 ± 0.1 D for Al-CH₃CN and 2.7 ± 0.2 D for Al-NH₃. We find that the dipole moment of Al-NH₃ becomes larger than that of neat NH₃, while the formation of the Al-CH₃CN complex produces a smaller dipole moment than that of neat CH₃CN on the other hand. We performed the ab initio calculations to draw out plausible complex structures and to clarify the bonding character after formation of the complex, and we made comparisons with the computational results done by several groups. The Mulliken population analysis suggests the Al→CH₃CN charge flow, but on the other hand the Natural population analysis indicates very little charge flow. For the Al-NH₃ complex, the polarization effect of NH₃ and the N→Al σ donation would enhance the dipole moment strength. However, there still remains a controversial disagreement between the theoretical predictions and the experimental results. Further experimental determination using the hexapole method for various metal-ligand complexes and clusters could reveal the basic nature of interaction in the complex systems in general, and this method would complement theoretical calculations.