title: X-ray Crystal Structures and ab Initio Calculations on the Photochemically Formed Dewar Isomers of the 4(3H)-Pyrimidinone Derivatives.
abstract: The structures of the 5-oxo-2,6-diazabicyclo[2.2.0]hex-2-enes (imine Dewar pyrimidinones) 2a-h have been studied by X-ray diffraction analysis and ab initio calculations at the HF/6-31G(d,p) and MP2/6-31G(d,p) levels of theory. The crystal structures of 1-alkyl-3-tert-butyl-6-methyl imine Dewar pyrimidinones 2a-c have been determined at low temperature. The X-ray diffraction studies revealed that both the 2-azetidinone and dihydroazete rings are almost planar, and their eight bond angles are nearly 90 degrees (81-100 degrees ). The C1-N2(=C3) bond distance in the dihydroazete ring is longer due to the bond angle strain by ca. 0.06 Å than that of a typical straight-chain imine molecule. Full geometry optimizations on the imine Dewar isomers (2a and 2d-h) show deviations between the HF and MP2 geometries. The MP2 structure of the 3-tert-butyl-1,6-dimethyl imine Dewar pyrimidinone 2a is compared with those of the X-ray diffraction. The results of the calculations are found to be in good agreement with the X-ray diffraction data. The full geometry optimizations are necessary with the inclusion of electron correlation for the highly strained molecules. The ab initio calculations of the 2-azetidinone 9, 3,4-dihydroazete 11, and N-ethylidenemethylamine 12 have been carried out at the HF and MP2/6-31G(d,p) levels of theory to compare with the structures, orbital hybridization, bond orders, and charge distributions of the Dewar pyrimidinones 2. The theoretical results reveal that the MP2 C1-N2 bond distance of the Dewar pyrimidinone 2a is consistent with the abnormally elongated X-ray C1-N2 bond distance. The electronegative N2 and N6 atoms in the Dewar pyrimidinones 2 give the great positive charge on the C1 atom. The smaller bond angles (ca. 90 degrees ) in the Dewars 2 than typical sp(2) and sp(3) bond angles (ca. 120 and 109 degrees ) increase p character in the endocyclic bonds and decrease p character in the exocyclic bonds.
authors.affiliation: Department of Advanced Chemical Technology, National Institute of Materials and Chemical Research, 1-1, Higashi, Tsukuba, Ibaraki 305, Japan.
authors.forename: Shun-ichi
authors.lastname: Hirokami Si
authors.forename: Akihiro
authors.initials: A
authors.lastname: Murao
authors.forename: Hiroko
authors.initials: H
authors.lastname: Kakuda
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Shinoda
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Koga