Aromaticity energetic measures

As the best available measure of classical aromaticity, Katritzky et al. commended the Bird 4 parameter, which is a new index of aromatic character based on a statistical evaluation of the deviations in peripheral bond orders <8511409, 86T89>, and is later improved to /a <92T335>. Unlike the resonance and aromatic energetical order, the Bird D or 7a estimates that pyrazine has a higher stability than pyridine and other diazines. 

The numerous aromaticity measures proposed in recent decades have widened the number of descriptors in the literature considerably [2,8,9,44,45], Aromaticity descriptors can be classified as magnetic, energetic, electronic, and structural [46]. 

Energy-resolved rate constant measurements near the threshold for diplet methylene formation from ketene have been used to provide confirmation of the fundamental hypothesis of statistical transition state theory (that rates are controlled by the number of energetically accessible vibrational states at the transition state).6 The electronic structure and aromaticity of planar singlet n2-carbenes has been studied by re-election coupling perturbation theory.7 The heats of formation of three ground-state triplet carbenes have been determined by collision-induced dissociation threshold analysis.8 The heats of formation of methylene, vinylcarbene (H2C=CHCH), and phenylcarbene were found to be 92.2 3.7, 93.3 3.4, and 102.8 33.5 kcal mol-1, respectively. 

Since aromaticity is a central issue in the present report, it is of importance to summarize briefly the measures and attributes of aromaticity, while for more detailed discussion the reader is recommended recent reviews [47-49], Although the use of quantitative measures for the description of aromaticity is questioned from time to time [52], and their direct use to rank aromaticity in different compounds might be misleading due to the multidimensional nature [53-56] of this phenomenon, the usefulness of different quantitative measures cannot be questioned. Four main criteria can be selected in the discussion of aromaticity. These are the energetic, geometric, magnetic, and reactivity criteria. Among these the latter is usually more qualitative, therefore only the first three will be mentioned briefly here. 

An MO multicenter bond index involving the cr + 7t electron population is proposed as a measure of aromaticity. It is related both to the energetic and to the magnetic criteria <2000PGP3381>. 

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