A post-Hartree-Fock quantum chemical investigation of biuret planarity

Abstract

The structures and energies of planar and nonplanar biuret were studied 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 predicted at all applied levels of theory for nonplanar conformations. Planar 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.