Pyrrole multimers and pyrrole-acetylene hydrogen bonded complexes studied in N2 and para-H2 matrixes using matrix isolation infrared spectroscopy and ab initio computations

Hydrogen bonded interaction of pyrrole multimer and acetylene-pyrrole complexes were studied in N2 and p-H2 matrixes. DFT computations showed T-shaped geometry for the pyrrole dimer and cyclic complex for the trimer and tetramer were the most stable structures, stabilized by NH⋯π interactions. The experimental vibrational wavenumbers observed in N2 and p-H2 matrixes for the pyrrole multimers were correlated with the computed wavenumbers. Computations performed at MP2/aug-cc-pVDZ level of theory showed that C2H2 and C4H5N forms 1:1 hydrogen-bonded complexes stabilized by CH⋯π interaction (Complex A), NH⋯π interaction (Complex B) and π⋯π interaction (Complex C), where the former complex is the global minimum and latter two complexes were the first and second local minima, respectively. Experimentally, 1:1 C2H2C4H5N complexes A (global minimum) and B (first local minimum) were identified from the shifts in the NH stretching, NH bending, CH bending region of pyrrole and CH asymmetric stretching and bending region of C2H2 in N2 and p-H2 matrixes. Computations were also performed for the higher complexes and found two minima corresponding to the 1:2 C2H2C4H5N and three minima for the 2:1 C2H2C4H5N complexes. Experimentally the global minimum 1:2 and 2:1 C2H2C4H5N complexes were identified in N2 and p-H2 matrixes.