The inner-hydrogen migration and ground-state structure of porphycene

Abstract

Following on from previous work on free-base porphyrin, we present the results of a comprehensive study on the structure and inner-hydrogen migration in porphycene, a structural isomer of porphyrin. We used density functional theory with the hybrid B3-LYP exchange-correlation functional, and both the 6-31G(d) and a triple-zeta double-polarization (TZ2P) basis set (the latter containing 726 contracted basis functions). Full geometry optimizations were carried out and all stationary points were characterized by vibrational analysis. A scaled quantum mechanical (SQM) treatment of the theoretical force constants shows convincingly that the trans-isomer is the ground state, with trans–trans inner-hydrogen migration taking place—as is the case with porphyrin—in a two-step process via a (highly unstable) cis intermediate. With the TZ2P basis, excluding zero-point effects, the trans–cis barrier height is 4.9 kcal/mol, the cis–trans energy difference is 2.4 kcal/mol and the reverse cis–trans barrier height is only 2.5 kcal/mol. We also map out and fully characterize an alternative, high-energy migration path involving a second, nonplanar cis isomer.