The Structures and Energies of Phosphaalkyne Trimers, (HCP) 3

Abstract

(CH)3P3 structures have been investigated by ab initio (MP4SDTQ/6-31+G**//MP2(fu)/6-31G*) and DFT (B3LYP/6-311+G**) methods. The framework is the main factor determining the realtive energy of isomers; the substitution pattern is less important. The thermodynamic stability order of heteroatom-substituted benzenes > benzvalenes > Dewar benzenes > prismanes > bicyclopropenyls parallels the situation in the (CH)6 hydrocarbons. The 3 HCP 1,2,3-triphosphabenzene trimerization energy is only –84 kcal mol–1; half as large as that for the 3 acetylene benzene conversion (–164 kcal mol–1). Head-to-tail additions are favored for the [4+2] cycloadditions of HCP to 1,2- and 1,3-diphosphete (to give triphospha Dewar benzenes); the computed activation barriers are low: ΔH*298 = 2.2 and 0.8 kcal mol–1, respectively, at MP4/6-31+G**//MP2(fu)/6-31G* (2.0 and 1.5 kcal mol–1 at MC-QDPT2/6-31G*//CAS-SCF(6,6)/6-31G*). Mono- as well as ortho-, meta-, and para-diphosphabenzenes and the triphosphabenzenes are found to be as aromatic as benzene according to geometric and energetic criteria. NICS calculations (a magnetic criterion) suggest only slightly reduced aromaticity.