The distance between atomic centers involved in a @[email protected] The notion of bond length is defined differently in various experimental methods of determination of molecular geometry; this leads to small (usually \(0.01-0.02\ \mathring{\text{A}}\)) differences in bond lengths obtained by different techniques. For example, in gas-phase [email protected]@ experiments, the bond length is the interatomic distance averaged over all occupied vibrational states at a given temperature. In an X-ray crystal structural method, the bond length is associated with the distance between the centroids of electron densities around the nuclei. In gas-phase microwave @[email protected], the bond length is an effective interatomic distance derived from measurements on a number of isotopic molecules, etc. A number of empirical relationships between bond lengths and bond orders in polyatomic molecules were suggested, see, for example, fractional @[email protected] (the Pauling's @[email protected]).