Abstract
It is well known that singlet state aromaticity is quite insensitive to substituent effects, in the case of monosubstitution. In this work, we use density functional theory (DFT) calculations to examine the sensitivity of triplet state aromaticity to substituent effects. For this purpose, we chose the singlet state antiaromatic cyclopentadienyl cation, antiaromaticity of which reverses to triplet state aromaticity, conforming to Baird’s rule. The extent of (anti)aromaticity was evaluated by using structural (HOMA), magnetic (NICS), energetic (ISE), and electronic (EDDBp) criteria. We find that the extent of triplet state aromaticity of monosubstituted cyclopentadienyl cations is weaker than the singlet state aromaticity of benzene and is, thus, slightly more sensitive to substituent effects. As an addition to the existing literature data, we also discuss substituent effects on singlet state antiaromaticity of cyclopentadienyl cation.
Highlights
Aromaticity and antiaromaticity are important concepts in science since they explain the special physical and chemical properties of cyclically delocalized systems
We find that the extent of triplet state aromaticity of monosubstituted cyclopentadienyl cations is weaker than the singlet state aromaticity of benzene and is, slightly more sensitive to substituent effects
We answer the question posed in the Introduction: How similar are singlet and triplet state aromaticity with respect to their sensitivity to substituent effects? The results presented in Section 3.3 show that the structural aromaticity index, HOMA, span a rather narrow range of 0.147 for the triplet state of substituted cyclopentadienyl cation, compared with 0.625 for the closed-shell singlet state
Summary
Aromaticity and antiaromaticity are important concepts in science since they explain the special physical and chemical properties of cyclically delocalized systems. Compounds showing aromaticity delocalize (4n + 2), usually π–electrons, as stated by Hückel’s rule [1,2,3]. They show enhanced thermodynamic stability, develop diamagnetic ring currents when exposed to an external magnetic field, and tend to equalize bond lengths and retain their cyclic delocalization during a chemical reaction [4]. Compounds that delocalize 4n π–electrons have been named antiaromatic by Breslow et al since they are unstable and very reactive [5,6] They develop paramagnetic ring currents when placed in a magnetic field and are characterized by alternating single and double bonds [4]. The extent and type of electron delocalization [7], molecular geometry [8], energy [9,10], and magnetic properties [11] are often taken into account when defining a system as aromatic or antiaromatic
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