PCCP and its isomers: a theoretical study

Abstract

Structural and electronic features of the linear 1,4-diphosphabutadiyne PC–CP and its rhombic isomer were explored using ab initio quantum chemical methods. Portions of the lower-lying [C₂P₂] potential energy surfaces of both neutral and ionized states were constructed. Geometries, rotational constants, vibrational frequencies, proton affinities and heats of formation of the ground state planar structures were determined using B3LYP and CCSD(T) computations. The vertical excitation and ionization energies were evaluated using the equation of motion EOM-CCSD. Atomic basis sets of different qualities ranging from 6-311++G(d,p) to aug-cc-pV6Z, were employed. Some thermochemical parameters were evaluated as follows: heat of formation at 0 K Δ_fH⁰(PCCP) = 413 ± 8 kJ/mol⁻¹, adiabatic ionization energy IE_a(PCCP) = 9.6 ± 0.2 eV, proton affinity PA(PCCP) = 715 ± 8 kJ mol⁻¹. The heats of formation at 0 K of relevant species are: Δ_fH⁰(CP) = 510 kJ mol⁻¹ and Δ_fH⁰(HCP) = 225 kJ mol⁻¹, and the dissociation energies, D_e(H–CP) = 501 kJ mol⁻¹ and D_e(PC–CP) = 606 kJ mol⁻¹, with expected errors of ±10 kJ mol⁻¹.