Molecular complexes between sodium and carbonyl compounds: Photoionization and ab initio molecular orbital studies

The molecular complexes formed between sodium and three carbonyl compounds, formaldehyde (fd), acetaldehyde (ad), and acetone (ac), were studied by the ab initio molecular orbital method and photoionization mass spectrometry. In the case of the monosodium complexes, by a search over the global electronic ground state at the MP2/6-31+G(d,p) level, three locally stable minima were located for each of the three complex systems. One of them has the character of a van der Waals interaction and is designated as the complexation-pair state (cp state). The other two are in ion-pair form and are denoted as the ipl and ip2 states. The nature of the sodium cp bonding with a carbonyl group was compared to the traditional alkali metal atom bonding with a simple Lewis base molecule. At the CCSD(T)/6-311,G(2d,p) level, except for the formaldehyde complex, whose most stable ip state has a small bonding energy, the energies of the other ion-pair states are all found to be slightly higher than the dissociation limit of the neutral dissociation channel. The energies of these three locally stable isomers are actually quite close to each other,with the largest energy separation being only 0.617 eV in Na(ac). For the disodium complexes, only the cp states were considered. The photoionization threshold energies of Na(ad) (cp), Na(ac) (cp), and Na-2(ac) were determined to be 4.23 +/- 0.04, 4.12 +/- 0.05, and 4.34 +/ 0.04 eV, respectively. The photoionization threshold energy of Na-2(ad) was estimated to be 4.5 eV. These experimental values are in good agreement with those calculated at the B3PW91/ 6-311+G(2d,p) level. From the photoionization efficiency spectra of Na(ac) and Na-2(ac), it is suggested that the autoionization process originating from the pi*<- n electronic transition of acetone could contribute to the ionization signal around and beyond the threshold region. The related cationic complexes were also studied theoretically. A unified view of the chemical and physical properties of these three systems is emphasized and discussed.