The molecular orbital constrained electron diffraction (moced) structural model of quadricyclane determined by electron diffraction combined with ab initio calculations of potential and geometrical parameters

Abstract

The gas electron diffraction (GED) data of quadicyclane (previously recorded by the University of Tokyo group) were reinvestigated using constraints taken from ab inito (4–21G) gradient geometry and force field calculations. It was the purpose of this study to determine the MOCED structural model for this compound that is obtained when the differences between some unrasolved bond distances and angles in the GED data analysis are constrained to the calculated values. In addition, a novel procedure was tested, in which a scale factor for the ab initio calculated vibrational amplitudes was refined from the diffraction data together with the variable structural parameters. This novel procedure greatly reduces parameter correlation in the least-squares analysis and is expected to be useful whenever extraneous constraints of unresolved parameters are needed for fitting molecular models to the diffraction data. Subject to the ab initio constraints, the analysis yields the following model ( r g-distances, r α-angles; numbers in parentheses are six times the standard deviations of the least-squares intensitv refinements): (CH) = 1.109(1) A, (CC) = 1.526(2) A 1 C 1C 2 = 1.525(24) A, C 2C 6 = 1.525(12) A 1 C 2C 3 = 1.543(24) A 1 C 1C 7 = 1.514(42) A 1 and C 1C 7C 4 = 98.7(6)°. With these parameters, the dependent angles are C 1C 2C 3 = 104.3°, C 1C 2- C 4 = 60°, C 2C 1C 6 = 60 °, and C 2C 1C 7 = 110.6°.