Theoretical study of the structure and torsional potential of pyrrole oligomers

Abstract

The molecular structure and conformational behaviour of 2,2′-bipyrrole (α-BPy), 2,2′:5′,2″-terpyrrole (α-TPy) and 2,2′:5′,2″:5″,2‴-quaterpyrrole (α-QPy) have been determined by abinitio HF, MP2 and density functional theory methods, using the 6-31G* basis set. The syn↔anti interconversion process through internal rotation about the C–C inter-ring bond generates in all calculations a three-barrier, four-fold potential. Minima are found in the anti-gauche and syn-gauche regions, maxima for the planar and perpendicular conformations, the anti-gauche structure being the global minimum. The energetics and the location of these critical points significantly depend on the theoretical method, the electron correlation and zero-point vibrational energies beng important factors. The relaxed rotor approximation must be used to obtain quantitative results, especially in the syn region where strong NH–NH dipole interactions are present which induce some loss of planarity of the pyrrole ring and tilting of the NH bond with respect to the ring plane. HF and B3LYP potential-energy curves are rather flat, particularly around the planar anti conformation, suggesting conformational flexibility. By contrast MP2 calculations strongly favour the anti-gauche form, indicating hindered rotation. The energetics and conformational behaviour of α-oligopyrroles are closely related to the torsional potential of the parent α-BPy. The minimum-energy conformations of α-TPy and α-QPy were all found to be anti-gauche (helix-like) structures. There is evidence that the π-electron system strengthens, the geometrical parameters of the pyrrole ring rapidly converge and the torsional potential around the planar anti conformation decreases as the α-oligomerization increases, suggesting that in the polymer limit planar, very conformationally flexible structures are highly probable, in agreement with the X-ray results in the solid. The potential-energy curves were analyzed in terms of conjugative and nonbonding interactions through a Fourier decomposition procedure. This highlights the predominant role of nonbonding interactions over conjugative ones and the peculiar behaviour of the MP2 method which favours hyperconjugative and probably NH–π hydrogen-bonding interactions which stabilize the perpendicular conformation.

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