The keto-enol tautomerisation in homophthalic anhydride (HA) is investigated in the ground (S₀) and excited (S₁) electronic states. The keto form with a dicarbonyl structure is found to be the most stable form in S₀ and enol form with a monocarbonyl structure in S₁ indicating an excited state intramolecular proton transfer (ESIPT) process. The computed results show consistency with the change in basis sets and methods of calculations. Apart from the two tautomers, transition states are also identified. The barrier to interconversion is found to reduce substantially in S₁. Internal reaction coordinate (IRC) calculations confirm the pathway of interconversion between the two forms in S₀ and S₁. The observed FT-IR spectra corroborate well with our computed spectra. The appearance of two strong lines around 1800 cm⁻¹ confirms the lowest energy structure to be the keto tautomer with a dicarbonyl form in S₀. Our computations corroborate well with the crystal structure data for an analogous molecule. Electron distribution in HOMO and LUMO indicate the excitation process as π → π^* in nature. The qualitative chemical concepts like hardness and electrophilicity are calculated to estimate the stability of the tautomers. The energy and hardness profiles with the variation of IRC are opposite to each other, verifying the principle of maximum hardness.

中文翻译：

---

高纯邻苯二甲酸酐的互变异构体在基态和激发电子态：通过能量，硬度和振动信号进行分析。

在基态（S ₀）和激发态（S ₁）电子状态下研究了高邻苯二甲酸酐（HA）中的酮-烯醇互变异构。发现在S _0中具有二羰基结构的酮形式是最稳定的形式，在S _1中具有单羰基结构的烯醇形式表明分子内的质子转移（ESIPT）处于激发态。计算结果表明与基础集和计算方法的变化一致。除了两个互变异构体之外，还鉴定了过渡态。发现互变的障碍在S _1中大大降低。内部反应坐标（IRC）的计算确定了S _0中两种形式之间的相互转化途径和S ₁。观察到的FT-IR光谱与我们的计算光谱很好地证实。在1800 cm ^-1附近出现两条强线，证实最低能级结构为S _0中具有二羰基形式的酮互变异构体。我们的计算结果很好地证明了类似分子的晶体结构数据。本质上，HOMO和LUMO中的电子分布表明激发过程为π→π ^*。计算定性化学概念，例如硬度和亲电性，以估计互变异构体的稳定性。IRC的能量和硬度曲线彼此相反，从而证明了最大硬度的原理。