AB-INITIO THEORETICAL INVESTIGATION OF THE FORMATION OF PHOSPHIRENYLIUM IONS FROM 1-HALO-1H-PHOSPHIRENES

The formation of phosphirenylium ions from fluoro- and chlorophosphire nes was studied by ab initio calculations at various theoretical level s up to CCSD(T)/6-31+G. To study the substituent effects, 13 sets of differently substituted halophosphirenes and phosphirenylium ions were optimized using HF and MP2 methods and the 6-31+G basis set. The ene rgies were improved by MP4-(SDTQ) and CCSD(T) single-point calculation s using the same basis set. The obtained reaction energies vary betwee n 114 and 209 kcal/mol depending on the halogen at phosphorus and the substitution at the ring carbons. The effect of liquid SO2 as a solven t was approximated by calculations of discrete complexes of solvent mo lecules and solutes. It is found that this solvent stabilizes the phos phirenylium cation by 18 kcal/mol. The electronic structures of the mo lecules and the ions were studied by NBO analyses. The P-X bonds are f ound to be of highly ionic character and can be described as interacti ons between phosphirenylium and halide ions. The aromatic character of the phosphirenylium ion is shown to be based on a three-center two-el ectron bond of pi-type and the resonance energy is assessed by calcula tion of a homodesmotic reaction and is estimated to be 38 kcal/mol.