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The results of a molecular orbital study for the ion pair Li+NO3−, alone and hydrated, have been related to the vibrational data for the same species matrix isolated in argon. Initially it was established that a plot of the N–O bond overlap population vs empirical FNO force constants, for fourteen molecules, is impressively linear. Subsequently this plot has been used to convert the Gaussian 70-derived N–O overlap populations for the N–O bonds of Li+NO3−, alone or hydrated, to FNO values for the particular species. In the simple Li+NO−3 pair, for example, the minimum energy bidentate structure has a significantly smaller overlap population in the two metal-coordinated N–O bonds. These overlap populations indicate an N–O stretching force constant difference of 3.94×102 N m−1 which can be converted to a 245 cm−1 splitting of the otherwise degenerate ν3 antisymmetric stretching mode. The latter value compares well with the experimental value of 265 cm−1 and, together with equally encouraging comparisons for the hydrated ion pair, suggests that the Gaussian 70 results for these systems, even using the STO-3G minimum basis set, realistically depict the bonding for Li+NO3− as well as the influence of hydration on that bonding.

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