Bond lengths changes with substitutions

The reduction of l,2,4,5-tetrakis(trimethylsilyl)benzene under aprotic conditions yielded the sodium salt of the radical anion, and the red needles obtained from DME were shown by an X-ray investigation to have an ionic structure with no short Na-C contacts. There is a small but significant change from the neutral molecule ArH to the radical anion and the dianion.125 As electrons are taken up by the aromatic system, the bond lengths between the substituted carbons are increased from 141 pm in ArH126 to 147 pm in ArH and 155 pm in ArH2. The reduction of 9,10-bistrimethylsilylanthracene is similar. The radical anion has been character-... 

Bond length changes which occur on substituting H by F, or vice versa, in HMF compounds with elements M of the first or second period have been considered [9]. 

The reason that does not change with isotopic substitution is that it refers to the bond length at the minimum of the potential energy curve (see Figure 1.13), and this curve, whether it refers to the harmonic oscillator approximation (Section 1.3.6) or an anharmonic oscillator (to be discussed in Section 6.1.3.2), does not change with isotopic substitution. Flowever, the vibrational energy levels within the potential energy curve, and therefore tq, are affected by isotopic substitution this is illustrated by the mass-dependence of the vibration frequency demonstrated by Equation (1.68). 

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