Strenalyuk Tatyana, Samdal Svein, Volden Hans Vidar
Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway.
J Phys Chem A. 2008 May 29;112(21):4853-60. doi: 10.1021/jp801284c. Epub 2008 May 6.
The molecular structure of the trans isomer of metal-free phthalocyanine (H2Pc) is determined using the gas electron diffraction (GED) method and high-level quantum chemical calculations. B3LYP calculations employing the basis sets 6-31G**, 6-311++G**, and cc-pVTZ give two tautomeric isomers for the inner H atoms, a trans isomer having D2h symmetry and a cis isomer having C2v symmetry. The trans isomer is calculated to be 41.6 (B3LYP/6-311++G**, zero-point corrected) and 37.3 kJ/mol (B3LYP/cc-pVTZ, not zero-point corrected) more stable than the cis isomer. However, Hartree-Fock (HF) calculations using different basis sets predict that cis is preferred and that trans does not exist as a stable form of the molecule. The equilibrium composition in the gas phase at 471 degrees C (the temperature of the GED experiment) calculated at the B3LYP/6-311++G** level is 99.8% trans and 0.2% cis. This is in very good agreement with the GED data, which indicate that the mole fraction of the cis isomer is close to zero. The transition states for two mechanisms of the NH tautomerization have been characterized. A concerted mechanism where the two H atoms move simultaneously yields a transition state of D2h symmetry and an energy barrier of 95.8 kJ/mol. A two-step mechanism where a trans isomer is converted to a cis isomer, which is converted into another trans isomer, proceeds via two transition states of C(s) symmetry and an energy barrier of 64.2 kJ/mol according to the B3LYP/6-311++G** calculation. The molecular geometry determined from GED is in very good agreement with the geometry obtained from the quantum chemical calculations. Vibrational frequencies, IR, and Raman intensities have been calculated using B3LYP/6-311++G**. These calculations indicate that the molecule is rather flexible with six vibrational frequencies in the range of 20-84 cm(-1) for the trans isomer. The cis isomer might be detected by infrared matrix spectroscopy since the N-H stretching frequencies are very different for the two isomers.
采用气体电子衍射（GED）方法和高水平量子化学计算确定了无金属酞菁（H2Pc）反式异构体的分子结构。使用6 - 31G**、6 - 311++G和cc - pVTZ基组的B3LYP计算给出了内氢原子的两种互变异构体，一种是具有D2h对称性的反式异构体，另一种是具有C2v对称性的顺式异构体。计算得出反式异构体比顺式异构体稳定41.6（B3LYP/6 - 311++G，零点校正）和37.3 kJ/mol（B3LYP/cc - pVTZ，未零点校正）。然而，使用不同基组的Hartree - Fock（HF）计算预测顺式异构体更优，且反式异构体不是分子的稳定形式。在B3LYP/6 - 311++G水平计算得出在471℃（GED实验温度）气相中的平衡组成为99.8%反式和0.2%顺式。这与GED数据非常吻合，GED数据表明顺式异构体的摩尔分数接近零。已对NH互变异构化的两种机制的过渡态进行了表征。两个氢原子同时移动的协同机制产生了一个具有D2h对称性的过渡态和95.8 kJ/mol的能垒。根据B3LYP/6 - 311++G计算，反式异构体先转化为顺式异构体，再转化为另一种反式异构体的两步机制通过两个具有C(s)对称性的过渡态进行，能垒为64.2 kJ/mol。由GED确定的分子几何结构与量子化学计算得到的几何结构非常吻合。使用B3LYP/6 - 311++G**计算了振动频率、红外和拉曼强度。这些计算表明，对于反式异构体，该分子相当灵活，有六个振动频率在20 - 84 cm(-1)范围内。由于两种异构体的N - H伸缩频率差异很大，顺式异构体可能会通过红外矩阵光谱检测到。
