HP4- - THE MONOCONJUGATE BASE FROM TETRAPHOSPHABICYCLOBUTANE - EVIDENCE FOR AN EXO GROUND-STATE CONFORMATION IN THE GAS-PHASE

Ab initio theoretical studies reconsider recent experimental findings concerning the ground state conformation of the HP4-anion. Basis sets as large as triple zeta plus double polarization plus f functions plus diffuse functions (TZ2Pf+diffuse) were used in connection with method s that explicitly include the effects of electron correlation. At the highest level of theory, the exo structure is predicted to lie about t wo kcal/mol below the endo structure, in apparent disagreement with ex periment. The Li+HP4-system has also been examined to approximate more closely the system synthesized in the laboratory. The energetics are significantly shifted by the appendage of the Li+ cation, and the agre ement between theory and experiment deteriorates further.