Molecular complexes between sodium and carbonyl compounds: Photoionizationand 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 th e ab initio molecular orbital method and photoionization mass spectrometry. In the case of the monosodium complexes, by a search over the global elect ronic ground state at the MP2/6-31+G(d,p) level, three locally stable minim a were located for each of the three complex systems. One of them has the c haracter of a van der Waals interaction and is designated as the complexati on-pair state (cp state). The other two are in ion-pair form and are denote d as the ipl and ip2 states. The nature of the sodium cp bonding with a car bonyl 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 fo r 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 sligh tly higher than the dissociation limit of the neutral dissociation channel. The energies of these three locally stable isomers are actually quite clos e to each other,with the largest energy separation being only 0.617 eV in N a(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 estimate d 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 eff iciency spectra of Na(ac) and Na-2(ac), it is suggested that the autoioniza tion process originating from the pi*<- n electronic transition of acetone could contribute to the ionization signal around and beyond the threshold r egion. The related cationic complexes were also studied theoretically. A un ified view of the chemical and physical properties of these three systems i s emphasized and discussed.