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The Structures and Energies of Phosphaalkyne Trimers, (HCP) 3
Author(s) -
Hofmann Matthias,
von Ragué Schleyer Paul,
Regitz Manfred
Publication year - 1999
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/(sici)1099-0690(199912)1999:12<3291::aid-ejoc3291>3.0.co;2-#
Subject(s) - chemistry , crystallography , computational chemistry
(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 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: Δ H * 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.