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).