The structures and energies of phosphaalkyne trimers, (HCP)(3)

(CH)(3)P-3 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 m ain factor determining the realtive energy of isomers; the substitution pat tern is less important. The thermodynamic stability order of heteroatom-sub stituted benzenes > benzvalenes > Dewar benzenes > prismanes > bicycloprope nyls 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 comput ed activation barriers are low: Delta H*(298) = 2.2 and 0.8 kcal mol(-1), r espectively, at MP4/6-31+G**//MP2(fu)/6-31G* (2.0 and 1.5 kcal mol(-1) at M C-QDPT2/6-31G*//CAS-SCF(6,6)/6-31G*). Mono- as well as ortho-, meta-, and p ara-diphosphabenzenes and the triphosphabenzenes are found to be as aromati c as benzene according to geometric and energetic criteria. NICS calculatio ns (a magnetic criterion) suggest only slightly reduced aromaticity.