Further analysing the first and second derivatives at the critical points validate the assumption of ionic bonding in the silatrane.

Si(1) O(2) | bcp (3,-1) | ||

coordinates | X = 4.24113875 Å | Y = -1.48450393 Å | Z = -0.45686378 Å |

distance from | Si 1.27 Å | O 1.87 Å | |

0.1203 e/a_{0}^{3} |
|||

0.964 e/a_{0}^{5} |

Si(1) N(5) | bcp (3,-1) | ||

coordinates | X = 3.15303269 Å | Y = -0.135561143 Å | Z = 0.158368394 Å |

distance from | Si 1.56 | N 2.54 | |

0.0497 | |||

0.148 |

The low electron density at the critical points and the positive Laplacian
(
)
are clear evidence that the bonds are more ionic than
covalent. As one compares these results to those obtained by *Gordon*
[12] there is a good agreement with results obtained on 6-31G(d)
optimised structures, showing that the X-ray structure is a valid geometry for
doing ab-initio calculations.

To connect the analysis to the concept of the bond order (which is more familiar to the
preparative chemist),
Bader has derived an expression to calculate the bond order *n*
from the electron density
at the bond critical point
[16]:

Comparing the bond orders derived by Mulliken population analysis and calculated using equation (1) one finds that the silicon-oxygen bond is attributed a much more ionic character in the Bader analysis (table 6).

1998-06-18