This letter is devoted to the explanation of the differences between the IR spectral properties of hydrogen bond systems found in crystals of two isomeric thioamides, thioacetanilide (TACN) and N-methylthiobenzamide (NMTB). The magnitude of the temperature-dependent Davydov-splitting effects, the HID isotopic 'self-organization' effects in the crystalline polarized IR spectra depend on the electronic properties of the two different associating molecular systems, which are affected by the methyl and phenyl substituent groups linked to the H-N-C=S common structural fragments. (C) 2015 Elsevier B.V. All rights reserved.