The potential energy surfaces of the H₂S binding to iron-porphyrin (FeP) with the imidazole (Im) ligand via intersystem crossings are investigated by using density functional theory. The minimum energy intersystem crossing point (MEISCP) between the quintet and triplet states (MEISCP_TQ) for the Fe(II)P(Im)-H₂S complex is located at a FeS distance of 3.39 Å with only 1.1 kcal/mol above the quintet state minimum. The second spin-crossover point, where a change from the triplet to the singlet state occurs, comes at a much shorter FeS distance of 2.79 Å, and the MEISCP_ST is located at 3.7 kcal/mol above the triplet state minimum. The nature of the chemical bonding along the FeS reaction coordinate from the ground state singlet to the quintet state along the path to the separated species is analyzed. An inspection of the vibrational modes reveals that the largest contribution to the triplet-quintet transition around the quintet and triplet state minimum comes from the symmetric shrinking of the pyrrole units of the porphyrin ring, indicating that the related reaction coordinate plays a main role in the intersystem crossing. The fully optimized structures of the Fe(II)P(Im)-HS⁻ complex corresponding to three different spin multiplicities (M = 1, 3, 5) are characterized by a bent Fe-H-S conformation. The binding of the hydrosulfide anion to Fe(II)P(Im) in the quintet state induces a 0.2 Å displacement of the Fe atom out of the nitrogen porphyrin (N_pyr) plane. The fully optimized structure of the ground state of Fe(II)P(Im)-HS⁻ agrees well with experimental data for the corresponding heme models.

利用密度泛函理论研究了H ₂ S与咪唑（Im）配体通过系统间交叉与铁卟啉（FeP）结合的势能面。Fe（II）P（Im）-H ₂ S络合物的五重态和三重态（MEISCP _TQ）之间的最小能量系统间交点（MEISCP）位于FeS距离为3.39Å且仅1.1 kcal / mol高于五重奏状态下限。第二个自旋交越点发生了从三重态到单重态的转变，它的Fe S距离要短得多，为2.79Å，而MEISCP _ST位于三重态最低状态下的3.7 kcal / mol。沿着铁的化学键的性质分析了从基态单重态到五重态的S反应坐标，以及到达分离物种的路径。对振动模态的检查表明，对五重态周围的三重态-五重态过渡和最小的三重态的最大贡献来自于卟啉环的吡咯单元的对称收缩，表明相关的反应坐标起着主要作用。系统间交叉。的Fe的完全优化的结构（II）P（虚部）-HS ^-络合物对应于三个不同的自旋多重（中号 = 1，3,5）的特征在于弯曲的Fe-HS构象。在五重态下氢硫化物阴离子与Fe（II）P（Im）的结合导致Fe原子从氮卟啉中移出0.2Å（N_pyr）平面。Fe的基态的完全优化的结构（II）P（虚部）-HS ^-与用于相应的血红素模型的实验数据符合得很好。