Computational studies on five-membered heterocycles with single heteroatom and their isomeric dehydro-borole 1a–1c, cyclopentadiene 2a–2c, pyrrole 3a–3c, furan 4b–4c, phosphole 5a–5c, and thiophene 6b–6c radicals have been carried out. Geometrical aspects through ground state electronic structures and stability aspects using bond dissociation energies (BDE) and radical stabilization energies (RSE) have been envisaged in this regard. Spin densities, electrostatic potentials (ESP), and natural bond orbital (NBO) analysis unveiled the extent of spin delocalization. The estimated nucleus-independent chemical shifts (NICS) values revealed the difference in aromaticity characteristics of radicals. Particularly the heteroatom centered radicals exhibit odd electron π-delocalized systems with a quasi-antiaromatic character. Various factors such as, the relative position of the radical center with respect to heteroatoms, resonance, ring strain and orbital interactions influence the stability that follows the order: heteroatom centered > β-centered > α-centered radicals. Among the influences of various factors, we confirmed the existence of a competition between delocalization and the ring strain, and the interplay of both decides the overall stability order.

中文翻译：

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五元杂环自由基的破译稳定性：离域和环应变之间的平衡作用

具有单个杂原子的五元杂环及其异构体脱氢硼烷1a - 1c，环戊二烯2a - 2c，吡咯3a - 3c，呋喃4b - 4c，磷5a - 5c和噻吩6b - 6c的计算研究激进分子已经进行了。在这方面，已经设想了通过基态电子结构的几何方面以及使用键离解能（BDE）和自由基稳定能（RSE）的稳定性方面。自旋密度，静电势（ESP）和自然键轨道（NBO）分析揭示了自旋离域化的程度。估计的独立于核的化学位移（NICS）值揭示了自由基的芳香性特征的差异。特别是，以杂原子为中心的自由基表现出具有准电子的奇数π-离域系统-抗芳香族字符。各种因素（例如，自由基中心相对于杂原子的相对位置，共振，环应变和轨道相互作用）会影响以下顺序的稳定性：杂原子中心>β-中心>α-中心。在各种因素的影响下，我们确认了离域和环应变之间存在竞争，并且两者的相互作用决定了整体的稳定性顺序。