THEORETICAL-STUDIES ON THE STRUCTURE, STABILITY, ABILITY TO UNDERGO INTERNAL TRANSFORMATIONS, AND TAUTOMERIZATION, AS WELL AS REACTIVITY, OF H2PPH2 AND HPPH3 MOLECULES

Semiempirical MNDO, AM1, and PM3, as well as ab initio, studies applyi ng the STO-3G, 4-31G, 6-31G*, DZV, and DZP basis sets and also includ ing the MP4 and QCISD(T) corrections were carried out on two tautomeri c phosphorus hydrides H2PPH2 and HPPH3 and uni- and bimolecular reacti ons. First, the geometry of the two molecules was optimized using the theoretical methods mentioned in the Hartree-Fock (HF) scheme. The ene rgies of the molecules at stationary points corresponding to HF/6-31G geometries were subsequently calculated including electron correlati on effects on the level of the fourth-order Moller-Plesset (MP4) pertu rbation theory or quadratic configuration interaction with single and double substitutions and triples contribution (QCISD(T)) calculated us ing the 6-311+G-(3df,2p) or 6-311++G(3df,3pd) basis set. Complete geom etry optimizations at the MP4/6-31G* level resulted in only slight ch anges in the geometry and energy of the molecules as compared with HF results. In order to compare ab initio results with the available ther mochemical data, the energies of formation of all the entities were fi rst calculated following Hess's Law. The partition function contributi ons were subsequently determined in a harmonic approximation, which al lowed for the calculation of entropies, heat capacities, and enthalpie s and free enthalpies of formation of gaseous hydrides. This also resu lted in vibrational frequencies that have to be scaled by a factor of 0.889 to suit available experimental data. Other physicochemical chara cteristics of the hydrides, such as dipole moments and energies of the lowest unoccupied (LUMO) and highest occupied (HOMO) molecular orbita ls were obtained from theoretical calculations. The results of our nb initio calculations indicate unambiguously that H-2-PPH2 is thermodyna mically more stable than HPPH3, while the AM1 and PM3 semiempirical me thods predict the reverse order of thermodynamic stability. Further, t heoretical calculations predict that the H2PPH2 molecule can exist equ ally well in both gauche and staggered forms. To gain insight into the stability of the hydrides, energy changes for internal transformation s (rotation around the P-P bond and inversion at P in H2PPH2), intramo lecular (unimolecular) and intermolecular (bimolecular) hydrogen trans fer, and decomposition processes were determined. Combination of these data with classical thermodynamic and kinetic (RRKM theory) considera tions revealed that increasing temperature should cause the decomposit ion of H2PPH2 rather than transformation to HPPH3. On the other hand, an isolated HPPH3 molecule once created would be stable at ambient tem perature since barriers for its unimolecular tautomerization or decomp osition are relatively high. However, bimolecular tautomerization, pro ceeding with a negligibly small kinetic barrier over the thermodynamic one, would bring the system to the lowest energy H2PPH2 structure.