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Conformations of model compounds of proteins II. Infrared spectra of n‐acetyl‐amino acid methylamides

N‐Acetyl‐amino acid methylamides CH3CONHCHRCONHCH3 were prepared from L‐ and DL‐alanine, L and DL‐α‐amino‐n‐butyric acid, L‐ and DL‐norvaline, DL‐norleucine, L‐ and DL‐methionine, L‐ and DL‐leucine, L‐aspartic acid and DL‐phenylalanine. The deuterium homologs of the type CH3CONDCHRCONDCH3, CD3CONHCHRCONHCH3, and CH3CONHCHRCONHCD3 were also prepared. The infrared spectra of these compounds were measured down to 300 cm−1 in the crystalline state. The infrared spectra of N‐isopropylacetamide CH3CONHCH(CH3)2, N‐methylisobutyramide (CH3)2CHCONHCH3 and their deuterium homologs were also measured. The CO in‐plane and out‐of‐plane bending vibration bands of the CH3CONHCα group (amide IVa and VIa) and those of the –CαCONHCH3 group (amide IVb and VIb) were assigned from these data. Two crystalline modifications, form I and form II, were found for the compounds prepared from L‐alanine, DL‐leucine, L‐aspartic acid and DL‐phenylalanine. The two forms show quite different skeletal vibrations, which suggest, rotational isomerism. Two distinct patterns were found as to the positions of the amide IVa and VIa bands for the above compounds. The two amide bands were found near 630 and 600 cm−1 in form I, whereas they were found near 600 cm−1 in form II. The crystals of the remaining compounds were also classified into form I or form II on the basis of the arrangement of the amide bands. The X‐ray structure analyses suggest that these two forms have different hydrogen‐bond structures.