Instituto de Química Física Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain.
J Phys Chem A. 2010 Mar 18;114(10):3583-90. doi: 10.1021/jp910778j.
This paper reports an experimental and computational thermochemical study on 5,5-dimethylbarbituric acid and the solid-phase structure of the compound. The value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -(706.4 +/- 2.2) kJ x mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated NB(2) stream, and a value of the enthalpy of sublimation at T = 298.15 K was derived as (115.8 +/- 0.5) kJ x mol(-1). From these results a value of -(590.6 +/- 2.3) kJ x mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 level were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in reasonable agreement with the experimental value. 5,5-Dimethylbarbituric acid was characterized by single crystal X-ray diffraction analysis. In the crystal structure, N-H...O=C hydrogen bonds lead to the formation of ribbons connected further by weak C-H...O=C hydrogen bonds into a three-dimensional network. The molecular and supramolecular structures observed in the solid state were also investigated in the gas phase by DFT calculations.
本文报道了 5,5-二甲基巴比妥酸的实验和计算热化学研究以及该化合物的固相结构。在 T = 298.15 K 时，测定了标准（p(o) = 0.1 MPa）摩尔生成焓在气相中的值。通过静态弹燃烧量热法测量了燃烧能，根据所得结果，计算了在 T = 298.15 K 时晶相标准摩尔生成焓为-(706.4 +/- 2.2) kJ x mol(-1)。利用饱和 NB(2)流中的传递（蒸发）法测定了升华焓，并得出 T = 298.15 K 时的升华焓值为(115.8 +/- 0.5) kJ x mol(-1)。由此得出在 T = 298.15 K 时气相生成焓的-(590.6 +/- 2.3) kJ x mol(-1)值。进行了 G3 水平的理论计算，并对化合物的分子和电子结构进行了研究。计算的生成焓与实验值吻合良好。5,5-二甲基巴比妥酸通过单晶 X 射线衍射分析进行了表征。在晶体结构中，N-H...O=C 氢键导致形成进一步由弱 C-H...O=C 氢键连接的带状物，形成三维网络。在固态中观察到的分子和超分子结构也通过 DFT 计算在气相中进行了研究。
