A Study Of The Nitrogen Inversion Barrier In Quisqualic Acid And Its Analogs

The geometry and inversion barrier of the ring substituted nitrogen in a quisqualic acid model compound 5, and the corresponding carbon (hydantoin) and nitrogen (dioxotriazolidine) analogues (6 and 7) has been investigated using the three MOPAC hamiltonians, PM3, AM1 and MNDO. In the case of the quisqualic acid and the nitrogen analogue model compounds, all three methods predict essentially pyramidal nitrogen configurations in agreement with X-ray crystallographic data for related compounds (1 and 8, respectively). For the carbon analogue model compound (6), only MNDO predicts a planar nitrogen geometry, in agreement with that found in the crystal structure of 3-(2,4-dioxo-2,3,4,5-tetrahydroimidazol-1-yl)alanine 3, with the other two methods suggesting a considerably more pyramidal structure. A search of the Cambridge crystallographic database indicates that nitrogen planarity is a general feature of this carbon containing hydantoin ring system, further emphasising the value of the MNDO hamiltonian when studying this type of heterocyclic structure. Our results indicate that different potential energy surfaces may be obtained from identical reaction coordinate calculations on different computer platforms, when PRECISE convergence criteria are used.