“…The question arises about the nature of hydrogen bonding, particularly about the observed blue shift of N CS H frequencies in the IR spectra upon H-bond formation. It has been stated that the νNH in the solid state of secondary amides and thioamides depends on the acidity (N CS H is more acidic than N CO H), the mass of the alkyl substituent, and the geometry of the molecule. , In the particular case of oxalyl derivatives, the NH stretching frequency is shifted to lower values as the weight of the entire (XC) 2 (NH) 2 (X = O, S) fragment increases. Thus, comparison among compounds of t Bu- and s Bu-triads, of identical molecular weight, allows estimating the effect of hydrogen bonding in the NH stretching frequency.…”