The pentacyanocyclopentadienyl system: structures and energetics

Area of Science:

• Computational chemistry
• Theoretical inorganic chemistry
• Quantum chemistry

Background:

• Recent advances in silylation routes have enabled synthesis of protonated pentacyanocyclopentadienyl (PCCp) anion.
• Pentacyanocyclopentadienyl (PCCp) species, including the anion, radical, and protonated forms, are of significant interest.

Purpose of the Study:

• To theoretically investigate the electronic structure and properties of PCCp anion, radical, and protonated species.
• To elucidate the Jahn-Teller distortion in the PCCp radical.
• To explore the protonation pathways and stability of PCCp acid dimers.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• Zero-point vibrational energy (ZPVE) corrections were applied.
• Analysis of adiabatic electron affinity, Jahn-Teller distortion, protonation energies, and dimer structures was performed.

Main Results:

• An exceptionally high adiabatic electron affinity of 5.56 (5.47) eV was predicted for PCCp.
• The PCCp radical exhibits Jahn-Teller distortion from D(5h) symmetry, leading to five equivalent (2)B(2) minima.
• Protonation preferentially occurs at a nitrile nitrogen over a ring carbon (6.5 kcal/mol difference) due to aromaticity retention.
• Both the previously suggested polymeric acid structure and a novel self-contained dimer were found to be competitive.

Conclusions:

• The theoretical study provides comprehensive insights into the properties of PCCp species.
• Computational predictions can guide experimental efforts for identifying these compounds.
• The findings support and expand upon previous experimental observations regarding PCCp structures.