The un-ionised carboxyl group

The un-ionised carboxyl absorption. The normal carbonyl absorption of the COOH group appears only in hydrochlorides in the amino-acid series, except in the case of dicarboxylic mono-amino-acids in which both the ionic and non-ionic forms appear. 

This correlation should also be applicable to the hydrochlorides of NH2 type acids, but, as in previous cases, not much information is available. Sarcosine hydrochloride again appears to be slightly abnormal in showing its carbonyl absorption at 1757 cm , but both proUne and A-phenylglycine hydrochloride absorb at 1730 cm . Histidine hydrochloride absorbs at 1706 cm and ornithine dihydrochloride [13] at 1739 cm . As far as the limited data available allow, therefore, it seems reasonable to assume that the correlation is obeyed by this type of compound. 

Thompson et al [19] have drawn attention to the presence of a weak, but definite band near 2100 cm in the spectra of the simple 

Hydrochlorides of amino-acids, along with other types, exhibit an almost continuous series of band of moderate intensity between 3030 cm and 2500 cm [ 17]. The series is highly characteristic and is readily identified. No interpretation of the great multiplicity of bands has been attempted in detail, but like the somewhat related series in carboxylic acid dimers it is probable that it originates in overtone and combination bands superimposed on a very strong and very broad NH band. 

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