Dependence of the substituent energy on the level of theory

Abstract

Most often, the substituent effects are described using rather troublesome Hammett constants. Quite recently, it has been proposed to use the so‐called substituent energy, which is based on total energies of the X‐substituted polycyclic aromatic hydrocarbon and phenyl. This article concerns the influence of the applied level of theory (i.e., both the basis set and the method) on the determined values of the substituent energies. For this purpose, the energies of the NH2 and NO2 groups in 16 unique positions of naphthalene, anthracene, tetracene, phenanthrene, and pyrene were calculated using 10 different basis sets and 23 various exchange‐correlation functionals representing the entire Jacob's Ladder, from local, through gradient‐ and meta‐gradient‐corrected, to hybrid and double‐hybrid ones. Additionally, using even larger 6‐311++G(2df,2p) basis set, the energies of NH2, NO2, CN, and Cl were also computed. Both the basis set and the method used have little effect on the substituent energy if the substituent is in the benzene‐like position. On the contrary, the effect of the level of theory is pronounced especially in the case of the most spatially crowded 4‐substituted phenanthrene. Substituent energies have been shown to be very useful theoretical parameters describing the proximity effect in the substituted derivatives of polycyclic aromatic hydrocarbons.

The physical nature of the ultrashort spike–ring interaction in iron maiden molecules

“…It has recently been shown 14 that the X substituent energy is a convenient parameter to describe the proximity effect. For example, this parameter had a particularly high positive value for the nitro group substituted at the sterically unfavorable 4‐position of phenanthrene 14 .…”

“…It has recently been shown 14 that the X substituent energy is a convenient parameter to describe the proximity effect. For example, this parameter had a particularly high positive value for the nitro group substituted at the sterically unfavorable 4‐position of phenanthrene 14 . Therefore, it should be particularly helpful in describing the influence of confinement in the in forms of the nX ‘iron maiden’ molecules on the energy of X.…”

“…This result perfectly confirms the previous conclusions based on structural changes, namely that for a given X (i) shortening of the side chain leads to a greater steric hindrance, (ii) replacing the carbon atom with a sulfur atom in the side chains leads to a reduction of steric effects exerted on the apical X. When recalling the energy of the nitro group in the sterically most crowded position 4 of phenanthrene (+10.0 kcal/mol; B3LYP/6–311++G(2df,2p)), 14 it becomes obvious that the obtained values are very large, of course especially for systems with three‐atom chains (i.e. CSC or CCC) and either Cl or Br atoms as X.…”

“…In order to investigate the influence of the H → X (X = F, Cl, and Br) substitution, the energy of the X atom in a given MX in/out molecule, , was computed (in kcal/mol) according to the following Equations 14 : where the symbol without a subscript is the total energy of the molecule shown in parantheses. For example, E (MeX) is the total energy of the halomethane and E (MX in/out ) is the total energy of either the in or out form of the MX ‘iron maiden’ molecule.…”

Energetic and Geometric Characteristics of the Substituents: Part 2: The Case of NO2, Cl, and NH2 Groups in Their Mono-Substituted Derivatives of Simple Nitrogen Heterocycles

“…The substituents in mono-substituted polycyclic aromatic hydrocarbons can only interact with adjacent hydrogen atoms. For the nitro and amino groups, it has been shown that their energies strongly depend on the effects of proximity [14,15]. Moreover, it was found that, in these systems, the energetic and geometric properties of the substituents are highly interdependent.…”