Imaginary Vibrational Modes in Polycyclic Aromatic Hydrocarbons: A Challenging Test for the Hardness Profiles

Abstract

In a very recent article (J. Am. Chem. Soc. 2006, 128, 9342-9343) Moran et al. found that electron-correlated methodologies using popular Pople basis sets lead to spurious nonplanar polycyclic aromatic hydrocarbon (PAH) equilibrium structures. Furthermore, some of the present authors have shown that the hardness profiles along a reaction path can be a useful tool to find spurious stationary points in the potential energy surface. Herein, we test the performance of the hardness profiles to detect shortcomings in energy profiles for the challenging case of nonplanar PAHs. The results obtained show that in 41 of the 42 imaginary vibrational modes studied, the hardness profiles indicate the wrong number and type of the potential energy surface stationary points.

Intramolecular basis set superposition error effects on the planarity of benzene and other aromatic molecules: A solution to the problem

“…Therefore, the BSSE is not directly related to the number of electrons of the system. This also explains the fact that hardness profiles, which have been successfully applied to the present problem, 35 are much more robust to level of theory and basis set effects than energy profiles ͑the chemical hardness is computed as the difference between the vertical ionization potential and electron affinity͒. Indeed, invoking the CP philosophy, an energy difference-based measure will perform better than a single energy value.…”

“…Thus, the two-electron BSIE diagnostic seems indeed a valuable tool to detect possible spurious geometries induced by BSSE effects, but it is not definitive yet. A recent study 35 based on the use of chemical hardness profiles is also able to detect such spurious structures, but again, it does not offer a solution to the problem.…”

Overcoming the Failure of Correlation for Out-of-Plane Motions in a Simple Aromatic: Rovibrational Quantum Chemical Analysis ofc-C3H2

“…Cyclopropenylidene ( c -C 3 H 2 ) is often called the simplest aromatic hydrocarbon since it follows Hückel’s 4 n + 2 rule with n = 0, but standard wave function based quantum chemical treatments have been known to fall short in describing the out-of-plane bends (OPBs) for this simple molecule. − Such behavior is not limited to this molecule, but ethylene, acetylene, benzene, and even nucleobases have been documented as producing imaginary or too-low OPB vibrational frequencies. − In truth, most wave function-based correlation treatments of π-bonded carbon atoms engaging in OPBs likely exacerbate this problem especially for anharmonic corrections, and c -C 3 H 2 is no exception. This work will serve to delve more deeply into the problem for this molecule building upon that recently reported and to provide strong predictions for the vibrational frequencies of this molecule for application to combustion thermochemistry and astrochemical spectral characterization.…”

The B–H–B bridging interaction in B-substituted oxazaborolidine–borane complexes: a theoretical study

“…Since the aVDZ basis set is larger and more diffuse than the 6-31G* basis we believe that no significant deterioration is introduced in values of geometrical parameters in investigated complexes. In particular, contrary to medium-sized Pople's basis set, the Dunning's correlation-consistent basis set are not found to suffer from spurious stationary points in the potential energy surfaces [46][47][48]. All optimized structures correspond to minima on the potential energy surfaces.…”