Theoretical study of cyanophosphapropyne (NCCP), isocyanophosphapropyne (CNCP) and their isomers: stability and properties

Exploration of portions of the (C2NP) potential energy surface using both B3LYP and CCSD(T) methods with the 6-311++G(d,p) basis set, indicates that cyanophosphapropyne NC–CP is the most stable isomer, followed by isocyanophosphapropyne CN–CP, the linear azaphosphadicarbon CCNP and the bent isocyanophosphavinylidene NC–PC. These higher-lying isomers are relatively stable with respect to unimolecular rearrangements and fragmentations. Their molecular properties including the geometries, rotational constants, vibrational wavenumbers, 13C and 31P NMR chemical shifts, heats of formation, excitation and ionisation energies, proton and electron affinities were determined. For the thermochemical quantities, CCSD(T) and EOM-CCSD computations with larger 6-311++G(3df,2p) and aug-cc-pVTZ basis sets were employed. It is remarkable that as a substituent, the phosphaethynyl –CP moiety exerts a remarkably strong electron donor effect which markedly enhances the electron density and basicity of the attaching moieties. Thus, the proton affinities at N in NC–CP and at C in CN–CP are significantly increased thanks to the CP effect whereas those at C of CP are, as a consequence, strongly reduced.