Experimental Evidence of Synergistic Interactions in Pyrrole–Phenol Complexes at Low Temperatures under Isolated Conditions

The simultaneous possession of π-electron clouds and acidic hydrogen atoms in pyrrole (C4H5N) and phenol (C6H5OH) framework opens the potentiality in exploring the synergistic interactions in their weakly bonded complexes. In this work, the synergistic hydrogen bonding in C4H5N–C6H5OH complexes is therefore investigated using FTIR spectroscopy under isolated conditions at low temperatures. Computations performed at DFT, DFT-GD3, M06, and MP2 level of theories employing aug-cc-pVDZ basis set yielded three minima on the potential energy surface for the 1:1 complex of C4H5N–C6H5OH. All three optimized structures showed synergistic interactions, where both C6H5OH and C4H5N simultaneously act as a proton donor and acceptor at MP2/aug-cc-pVDZ level of theory. In the global minimum complex A, the hydroxyl proton and the C–H group of C6H5OH interact with the π-cloud of C4H5N. The first local minimum corresponds to complex B, where the N–H and π-electrons of C4H5N interact with π-electrons of C6H5OH. In complex C, the N–H and C–H groups of C4H5N interact with O–H and π-cloud of C6H5OH, respectively. Complex A was the lowest energy structure at all levels of theory, whereas the stabilization energies of complexes B and C varied depending upon the levels of theory used. Interestingly, the stabilization energies as predicted by the DFT method are in accordance with Etter’s and Legon–Millen rules; however, a deviation in the Legon–Millen rule was discerned with empirical (DFT-GD3, M06) and dispersion corrected (MP2) methods. On comparing the experimental vibrational wavenumber shifts in the N–H stretching and bending modes of C4H5N and O–H stretching mode of C6H5OH submolecules with the computed shifts, all three complexes were identified in the N2 matrix. Natural Bond Orbital and Energy Decomposition analyses were performed to characterize the nature of the synergistic interaction in these complexes