Kalescky Robert, Kraka Elfi, Cremer Dieter
Computational and Theoretical Chemistry Group (CATCO), Department of Chemistry, Southern Methodist University , 3215 Daniel Avenue, Dallas, Texas 75275-0314, United States.
J Phys Chem A. 2014 Jan 9;118(1):223-37. doi: 10.1021/jp4092514. Epub 2013 Dec 24.
A new approach is presented to determine π-delocalization and the degree of aromaticity utilizing measured vibrational frequencies. For this purpose, a perturbation approach is used to derive vibrational force constants from experimental frequencies and calculated normal mode vectors. The latter are used to determine the local counterparts of the vibrational modes. Next, relative bond strength orders (RBSO) are obtained from the local stretching force constants, which provide reliable descriptors of CC and CH bond strengths. Finally, the RBSO values for CC bonds are used to establish a modified harmonic oscillator model and an aromatic delocalization index AI, which is split into a bond weakening (strengthening) and bond alternation part. In this way, benzene, naphthalene, anthracene, and phenanthrene are described with the help of vibrational spectroscopy as aromatic systems with a slight tendency of peripheral π-delocalization. The 6.8 kcal/mol larger stability of phenanthrene relative to anthracene predominantly (84%) results from its higher resonance energy, which is a direct consequence of the topology of ring annelation. Previous attempts to explain the higher stability of phenanthrene via a maximum electron density path between the bay H atoms are misleading in view of the properties of the electron density distribution in the bay region.
本文提出了一种利用测量的振动频率来确定π离域和芳香性程度的新方法。为此，采用微扰方法从实验频率和计算得到的简正模式向量中导出振动力常数。后者用于确定振动模式的局域对应物。接下来，从局域拉伸力常数中获得相对键强度顺序（RBSO），它提供了C-C键和C-H键强度的可靠描述符。最后，利用C-C键的RBSO值建立一个修正的谐振子模型和一个芳香离域指数AI，该指数可分为键弱化（强化）和键交替部分。通过这种方式，借助振动光谱将苯、萘、蒽和菲描述为具有轻微外围π离域趋势的芳香体系。菲相对于蒽具有6.8 kcal/mol更高的稳定性，这主要（84%）源于其更高的共振能，这是环稠合拓扑结构的直接结果。鉴于湾区区域电子密度分布的性质，以往通过湾区H原子之间的最大电子密度路径来解释菲更高稳定性的尝试具有误导性。
