The enthalpies of combustion and sublimation of 2,5-thiophenedicarboxylic acid [CASRN 4282-31-9] were measured by rotary-bomb combustion calorimetry and the method of transference in a saturated stream of nitrogen, and the gas-phase enthalpy of formation was determined, Delta(f)H(o)(m)(g) = -(632.6 +/- 2.2) kJ x mol(-1). Standard ab initio molecular orbital calculations at the G2(MP2) and G3(MP2) levels were performed, and a theoretical study on the molecular and electronic structure of the compound has been carried out. The three most stable conformers have been explicitly taken into account. The calculated enthalpy of formation averaged using three different isodesmic reactions, -631.1 kJ x mol(-1), is in very good agreement with the experimental value. A comparison of the substituent effect of the carboxylic groups in benzene and thiophene ring has been made. The relative stability obtained for the substitution of two H atoms by COOH in position 2,5- for thiophene and 1,4- for benzene involve the same energetic effects, DeltaDelta(f)H(o)(m)= -747.6 +/- 2.4 and -748.2 +/- 2.7 kJ x mol(-1), respectively.

中文翻译：

---

2,5-噻吩二甲酸的热化学。

通过旋转炸弹燃烧量热法和在饱和氮气流中转移的方法，测定了2,5-噻吩二甲酸[CASRN 4282-31-9]的燃烧和升华焓，气相生成焓为确定，Delta（f）H（o）（m）（g）=-（632.6 +/- 2.2）kJ x mol（-1）。进行了标准的从头算分子轨道在G2（MP2）和G3（MP2）的计算，并且对该化合物的分子和电子结构进行了理论研究。明确考虑了三个最稳定的构象体。使用三个不同的等渗反应-631.1 kJ x mol（-1）求出的平均形成焓，与实验值非常吻合。比较了苯和噻吩环中羧基的取代作用。噻吩2,5-位置和苯1,4-位置被COOH取代两个H原子获得的相对稳定性涉及相同的能量效应，DeltaDelta（f）H（o）（m）= -747.6 + / -2.4和-748.2 +/- 2.7 kJ x mol（-1）。