On interpretation of a missing spectral band; IR spectra of acidic salts of benzohydroxamic acid

Using solid benzohydroxamic acid (BHA) as a model compound for its salts, a broad absorption at 2730 cm−1 was generally agreed to represent the ν(OH) band of the OH⋯ON bonds. The absence of ν(OH) band in the IR region 3600-1700 cm−1 was taken to indicate the absence of OH groups in the solid salts. The same argument and conclusion was erroneously accepted for lithium (LiBHA) and potassium (KBHA) salts of BHA which are not “normal” but acid salts and do contain OH groups. X-ray diffraction results allow explanation of this discrepancy. The lithium and potassium acid salts have much shorter OH⋯ON bridges (O⋯O distances of 247 and 245 pm, respectively) than the model compound (258 pm). Accordingly, the ν(OH⋯O) bands in the acidic salts are shifted into the crowded 1600-700 cm−1 interval. Obviously, interpretation of a missing spectral band in terms of molecular structure can be misleading when the models do not include all possible structural variations.