High-level ab initio Calculation of the Dissociation and Ionization Energies of PH, PH₂ and PH₃ Nautrals and Cations

Comparative ab initio calculations at the single reference QCISD（T） and CCSD（T）levels of thoery in both Frozen-Core（FC） and full electronic correlation（Full） schemes were carried out on the PH_n（n=1～3）neutrals and cations involved in some of the dissociation and ionization reactions by using the Gaussian-94 packages of programs Relatively larger built-in standard basis sets of 6-311++G（3df, 3pd） and aug-cc-pVTZ from the Gaussian-94 were used The results of these calculations were compared with those of the earlier¹⁵ multi-reference CASSCF/SOCI and SOCI plus Davidson corrections as well as those of the experiments³. Molecular geometries were optimized at the MP2（Full）/6-31G（d, p）level of theory and compared with those of the experiments¹². Our results show that experimental thermochemistry data are all reliable, and the theoretically size-consistent with non-iterative triple excitation QCISD（T） and CCSD（T）calculations reproduce them very well by very small diviations of ± 7kJ/mol for dissociation （D₀）and ± 5kJ/mol for ionization （IE）energies.Over all, the results of our calculations are closer to the experiments than those of the even larger calculations of SOCI and SOCI+Davidson, where slightly smaller basis sets, e.g.（12s9p3dlf/6s5p3d lf）on P and （10s2p/8s2p）on H, were practically used.In this paper the inversion barrier（in kJ/mol, no ZPE）of PH₃（144 5）and PH₃⁺ （13.3）, the singlet-triplet splitting （4. 516 and 3. 394eV） of PH₃（¹A₁～³A₂"（D_3h） and ¹A₁～³B₁（C_2v）, respectively）were also calculated at the QCISD（T）（fu）/aug-cc-pVTZ level and compared with earlier work It is interesting that frequency calculation resulted in two imaginary frequencies for the D_3h and the analytical gradient MP2（fu）/6-31G（d, p） optimization resulted in C_2v geometry for PH₃（³B₁） other than either D_3h¹⁵ or pure repulsive³⁰. The results of CCSD（T）for both FC and Full calculations were compared with those of the QCISD（T） to be almost identical although CCSD（T）needs larger amount of CPU time In this study, the results of FC and Full calculations at QCISD（T）were compared for the two basis sets The FC by 6-311++G（3df, 3pd）reproduce the experimental D₀ better than by Dunning’s basis sets while it reproduce the IE values very slightly worse.