A POST-HARTREE-FOCK QUANTUM-CHEMICAL INVESTIGATION OF BIURET PLANARITY

The structures and energies of planar and nonplanar biuret were studie d using post Hartree-Fock MP2, DFT, and MP4(SDQ) levels of theory. The optimized planar conformations are each characterized by at least one imaginary harmonic vibrational frequency of large magnitude, i.e. one (198i cm(-1)) at the DFT/6-311G(df,pd) level, two (428i and 245i cm(- 1)) at the MP2/6- 311G(d,p) level and two (339i and 37i cm(-1)) at the MP4(SDQ)/6-31G (d,p) level of theory. Only real frequencies were pred icted at all applied levels of theory for nonplanar conformations. Pla nar biuret lies above its nonplanar isomer by 1.19 kcal mol(-1) (MP2/6 -311G(d,p)), 0.58 kcal mol(-1) (MP4(SDQ)/6-31G(d,p)), 0.74 kcal mol(-1 ) (CCSD[T]/6-31G(d,p)/MP4(SDQ)/6-31G(d,p)),and 0.08 kcal mol(-1) (DFT/ 6-311G(df,pd)) at the indicated levels of theory. (C) 1998 Elsevier Sc ience B.V.