Experimental Evidence of Synergistic Interactions in Pyrrole-Phenol Complexes at Low Temperatures under Isolated...

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.