Solvent impact on the planarity and aromaticity of free and monohydrated zinc phthalocyanine: a theoretical study

Abstract

A theoretical investigation on the planarity of molecular structure of zinc phthalocyanine (ZnPc) and its aromaticity has been performed using B3LYP and M06-2X density functionals combined with selected Pople-type basis sets. The effect of the applied calculation method on the optimized structure of ZnPc and ZnPc•••H 2 O, both in the gas phase and in the polar solvent, was analyzed. To quantify the aromaticity of the ZnPc and ZnPc•••H 2 O complexes, both the geometric and magnetic criteria, i.e., Harmonic Oscillator Model of Aromaticity (HOMA) index and the nucleus-independent chemical shift (NICS) values at the centers or 1 Å above the centers of structural subunits, were calculated. The energies of highest energy occupied molecular orbital (HOMO) and lowest energy unoccupied molecular orbital (LUMO) and energy gaps were also estimated. The results show that the free ZnPC molecule is flat in the gas phase and nonplanar in the polar environments (DMSO and water). ZnPC•••H 2 O is nonpolar in the gas phase and polar solvent which is in agreement with recent X-ray reports. Both HOMA and NICS indexes indicate the presence of highly aromatic macrocycle and benzene rings while these parameters for pyrrolic ring are significantly smaller than in free pyrrole. The presence of polar solvents practically does not change aromaticity of the ring subunits of the studied compounds.

Local aromaticity mapping in the vicinity of planar and nonplanar molecules

“…MPc are formed from conjugated benzene and pyrrole rings, and therefore, their local aromaticity could differ from their free “building blocks.” Indeed, our NICS(R) and NICS(R)zz scan experiments nicely confirm such predictions and are similar to earlier reports. Graphs showing distance dependence of the studied magnetic indexes NICS(R) and NICS(R)zz for the five‐membered ring in ZnPc are gathered in Figure .…”

“…Thus, we observe a lowering of pyrrole aromatic character by a factor of 2–3 upon its “insertion” into the MPc macrocycle . Similar behavior of NICS(R)zz scans is apparent from Figures b and b showing the corresponding dependencies for free pyrrole and ZnPc.…”

“…In the first step, we used benzene and pyrrole molecules as two planar reference systems for subsequent analysis of six‐ and five‐membered rings in MPc and nonplanar rings at fullerene C60 wall. The corresponding NICS(R) curve (black solid line) with a profile resembling Morse or log‐normal curve is shown in Figure a.…”

“…Benzene, pyrrole, zinc, aluminum, and free phthalocyanine (ZnPc, Al(OH)Pc, and H 2 Pc) geometries were optimized at B3LYP/6‐31+G* level of theory . Copper phthalocyanine (CuPc) was fully optimized at UB3LYP/6‐31+G* level of theory (for details, see our earlier study). “NICS‐scan” calculations were performed with 6‐31+G* basis sets for C, N, Zn, Al, Cu, and H atoms and aug‐pcS‐3 basis set for He and Li (pcS‐2 for helium in fullerenes).…”

Local aromaticity in polyacenes manifested by individual proton and carbon shieldings: DFT mapping of aromaticity

“…Calculation of NICS(1) and NICS(−1) parameters with respect to planar rings produce the same value of aromaticity index. However, in case of distorted planes, an asymmetric distribution of magnetic field was calculated (NICS(1) ≠ NICS(−1)) . Besides, a significant decrease of free pyrrole aromaticity upon its “insertion” into phthalocyanine macrocycle and conjugation with other rings was calculated …”