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

Abstract

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.