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Abstract：
The structures and relative stabilities as well as the rotational and vibrational spectra of the three low-energy conformers of pyruvic acid (PA) have been characterized using a state-of-the-art quantum-mechanical approach designed for flexible molecules. By making use of the available experimental rotational constants for several isotopologues of the most stable PA conformer, Tc-PA, the semiexperimental equilibrium structure has been derived. The latter provides a reference for the pure theoretical determination of the equilibrium geometries for all conformers, thus confirming for these structures an accuracy of 0.001 Å and 0.1 deg for bond lengths and angles, respectively. Highly accurate relative energies of all conformers (Tc-, Tt-, and Ct-PA) and of the transition states connecting them are provided along with the thermodynamic properties at low and high temperatures, thus leading to conformational enthalpies accurate to 1 kJ mol(-1). Concerning microwave spectroscopy, rotational constants accurate to about 20 MHz are provided for the Tt- and Ct-PA conformers, together with the computed centrifugal-distortion constants and dipole moments required to simulate their rotational spectra. For Ct-PA, vibrational frequencies in the mid-infrared region accurate to 10 cm(-1) are reported along with theoretical estimates for the transitions in the near-infrared range, and the corresponding infrared spectrum including fundamental transitions, overtones, and combination bands has been simulated. In addition to the new data described above, theoretical results for the Tc- and Tt-PA conformers are compared with all available experimental data to further confirm the accuracy of the hybrid coupled-cluster/density functional theory (CC/DFT) protocol applied in the present study. Finally, we discuss in detail the accuracy of computational models fully based on double-hybrid DFT functionals (mainly at the B2PLYP/aug-cc-pVTZ level) that avoid the use of very expensive CC calculations.
摘要：
丙酮酸（PA）的三种低能构象异构体的结构和相对稳定性以及旋转和振动光谱已使用为柔性分子设计的最先进的量子力学方法进行了表征。通过利用最稳定的PA构象异构体的几种同位素的可用实验旋转常数，Tc-PA,推导了半实验平衡结构。后者为纯理论确定所有构象的平衡几何形状提供了参考，因此，对于这些结构，键长和角度的精度为0.001和0.1度，分别。所有构象的高精度相对能量(Tc-，T-,和Ct-PA）以及连接它们的过渡态以及低温和高温下的热力学性质，从而导致构象焓精确到1kJmol(-1)。关于微波光谱学，为T-和Ct-PA构象提供精确到约20MHz的旋转常数，以及模拟其旋转光谱所需的计算的离心畸变常数和偶极矩。对于Ct-PA,准确到10cm（-1）的中红外区域的振动频率与近红外范围内的跃迁的理论估计一起报告，和相应的红外光谱，包括基本跃迁，泛音,并模拟了组合波段。除了上述新数据之外，将Tc-和Tt-PA构象的理论结果与所有可用的实验数据进行比较，以进一步确认本研究中应用的混合耦合簇/密度泛函理论（CC/DFT）协议的准确性。最后,我们详细讨论了计算模型的准确性完全基于双混合DFT功能（主要是在B2PLYP/aug-cc-pVTZ水平），避免使用非常昂贵的CC计算。
