Density functional theory PBE/TZVP calculations of the geometric structures in the ground singlet state S₀ and the first excited triplet state T₁ were carried out for Mg pheophorbide a (MgPhe) (i.e., chlorophyll a devoid of the phytol "tail"), Mg bacteriopheophorbide a (MgBPhe), Mg chlorin (MgC), and Mg bacteriochlorin (MgBC). A comparison of the bond lengths for the pairs MgC–MgPhe and MgBC–MgBPhe found that the central macrocycle of the first pair is more unstable to splittings of equivalent bond lengths as a result of the S₀ → T₁ transition and as the molecular structure becomes more complex (mainly due to cyclopentanone ring V formation). The symmetry of MgC in the T₁ state is lowered as compared to the C_2v symmetry in the S₀ state while MgBC has D_2h symmetry in both states. These peculiarities are related to the central π-system of the MgC–MgPhe pair being antiaromatic (containing 24 electrons corresponding to the Huckel 4n rule) and the central π-system of the MgBC–MgBPhe pair being aromatic (containing 22 electrons corresponding to the Huckel 4n + 2 rule). The energies of the T₁ states of the studied molecules are calculated. The computed \( {E}_{T_1} \) values for MgPhe are 11,400, 10,850, and 10,200 cm^–1 for the vertical S₀ → T₁ transition taking into account optimization of the geometry in the T₁ state and changes of the zero-point vibrations at the S₀ → T₁ transition, respectively, and agree well with the experimental value of 10,310 cm^–1 (for chlorophyll a). The computed \( {E}_{T_1} \) values for MgBPhe are 8350, 8100, and 7700 cm^–1 while the experimental value is 8190 cm^–1 (for bacteriochlorophyll a).

密度泛函理论 PBE/TZVP 计算了基态单线态 S ₀和第一激发三线态 T ₁的几何结构，用于镁脱镁叶绿素 a (MgPhe)（即叶绿素 a 没有植醇“尾部”）， Mg 细菌脱镁叶绿素 a (MgBPhe)、Mg 二氢卟酚 (MgC) 和 Mg 菌绿素 (MgBC)。对 MgC-MgPhe 和 MgBC-MgBPhe 键长的比较发现，由于 S ₀ → T ₁转变和分子结构，第一对的中心大环对等效键长的分裂更不稳定变得更复杂（主要是由于环戊酮环 V 的形成）。MgC 在 T _{1 中}的对称性与S ₀状态下的 C _2v对称性相比，MgBC状态降低了，而 MgBC在两种状态下都具有 D _2h对称性。这些特性与 MgC-MgPhe 对的中心 π 系统是反芳香性的（包含 24 个电子对应于 Huckel 4n 规则）和 MgBC-MgBPhe 对的中心 π 系统是芳香性的（包含 22 个电子对应于休克尔 4n + 2 规则）。计算所研究分子的 T ₁态的能量。对于垂直 S ₀ → T ₁，MgPhe的计算\( {E}_{T_1} \)值为 11,400、10,850 和 10,200 cm ^–1过渡分别考虑到 T ₁状态下的几何优化和 S ₀ → T ₁过渡处零点振动的变化，并且与实验值 10,310 cm ^–1（对于叶绿素 a）非常吻合。MgBPhe的计算\( {E}_{T_1} \)值为 8350、8100 和 7700 cm ^–1，而实验值为 8190 cm ^–1（对于细菌叶绿素 a）。