Theoretical study of proton encircling modes in proton sponges with tetraamido/diamino quaternized macrocycles: the role of π-conjugated and aliphatic bridges

Theoretical studies of the proton encapsulation within a series of tetraamido/diamino quaternized macrocycles consisting of –NH–CO– functionalities linked by different π-conjugated (phenyl, pyridine, furan, thiophene, butadienyl, and ethylene) or aliphatic (propyl and butyl) bridges are carried out. In macrocycles containing π-conjugated spacers, the protons are held by asymmetrical hydrogen bonds O–H⋯O ⇌ O⋯H–O with very small barriers for the proton transfer. In the case of macrocycles with aliphatic spacers, a symmetrical hydrogen bond (O⋯H⋯O) is formed in the propyl analogue, in contrast to the asymmetrical hydrogen bond in the butyl analogue. The energy-based fragmentation quantum chemistry method is employed to calculate the hydrogen bond energies of macrocycles. Taking advantage of the fragment-based calculations, contributions from individual bridges to the strength of hydrogen bonds are also revealed. A qualitative agreement is shown between fragmentation MP2/6-31G(d,p) calculation results, and the hydrogen bond energies estimated from the empirical equation on the basis of O–H stretching frequency. The close correlation between the hydrogen bond energy with electrostatic interactions within the hydrogen bond framework, and the proton affinity of macrocycles, indicate that the intramolecular hydrogen bonds are mostly electrostatic. The calculations show that the modifications of the bridges linking the –NH–CO– functionalities and hence the O⋯O distance between the proton acceptor and donor determine the proton binding modes.