UV photoelectron and ab initio study of intramolecular interactions in α-diethoxyphosphoryl carbonyl derivatives

Information on the geometric and electronic structures of α-diethoxyphosphoryl carbonyl derivatives, (RO)2P(O)CH2C(O)X, (1–5, X=CH31, CH3CH2O 2, CH3CH2S 3, C6H54, (CH3CH2)2N 5), is obtained by UV photoelectron spectroscopy and ab initio calculations. At the HF level, the CH2–P bond is gauche to the carbonyl group except in 5. However, at the DFT and MP2 levels, the gauche conformation prevails for all the derivatives, in agreement with the spectroscopic data. This conformation is dictated by the interplay of the electronic interactions between the XC(O) and (RO)2P(O) groups with two short contacts between pairs of oppositely charged atoms. The negatively charged carbonyl oxygen and one of the oxygen atoms bonded to phosphorus lie at distances shorter than (or close to) the sum of the corresponding van der Waals radii from the positively charged phosphorus and carbonyl carbon atom, respectively. The presence of through-space interactions in the ground and ionised states is confirmed by eigenvector analysis and by the shifts in the MO energies with respect to those of reference compounds. In the ground state the non-bonded interactions are weaker than the corresponding interactions in β-carbonyl sulphones, RSO2CH2C(O)X, because of the less suitable geometry of the PO3 group, with respect to the SO2R group, for interaction with the β-carbonyl group. In the ionised state, however, the negative charge on the oxygens of the phosphoryl group has a greater capacity to relax toward the ionised XC(O) group thus reducing the ionisation energy values.