PCCP and its isomers: a theoretical study

Structural and electronic features of the linear 1,4-diphosphabutadiyne P=C -C=P and its rhombic isomer were explored using ab initio quantum chemical methods. Portions of the lower-lying [C2P2] potential energy surfaces of bo th neutral and ionized states were constructed. Geometries, rotational cons tants, 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 evaluat ed using the equation of motion EOM-CCSD. Atomic basis sets of different qu alities ranging from 6-311 ++ G(d,p) to aug-cc-pV6Z, were employed. Some th ermochemical parameters were evaluated as follows: heat of formation at 0 K Delta H-f(0)(PCCP) = 413 +/- 8 kJ mol(-1), adiabatic ionization energy IEa (PCCP) = 9.6 +/- 0.2 eV, proton affinity PA(PCCP) = 715 +/- 8 kJ mol(-1). T he heats of formation at 0 K of relevant species are: Delta H-f(0)(CP) = 51 0 kJ mol(-1) and Delta H-f(0)(HCP) = 225 kJ mol(-1), and the dissociation e nergies, D-e(H-CP) = 501 kJ mol(-1) and D-e(PC-CP) = 606 kJ mol(-1), with e xpected errors of +/- 10 kJ mol(-1).