The iron(II) salt [Fe(bpp)₂](isonicNO)₂·HisonicNO·5H₂O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T₁ = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T₂ = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase. 1 undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W⋯O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at T₁(LIESST) = 68 K and T₂(LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice.

中文翻译：

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自旋交叉和质子沿短强氢键迁移之间的相互作用

铁( II )盐 [Fe(bpp) ₂ ](isonicNO) ₂ ·HisonicNO·5H ₂ O ( 1 ) (bpp = 2,6-双(吡唑-3-基)吡啶；isonicNO = 异烟酸N-氧化物阴离子) 在T ₁ = 167 K处经历对称性破缺的部分自旋交叉 (SCO)，进入混合自旋相（50% 高自旋 (HS)、50% 低自旋 (LS)），该相在T ₂ =以下是亚稳态的116 K。在较低温度下对化合物进行退火会产生 100% LS 相，该相与初始 HS 相不同，在两个结晶水分子（O4W 和 O5W）之间形成氢键 (HB)。中子晶体学实验还证明了两个等离子NO阴离子之间的短强氢键（SSHB）内的质子位移。这两种现象也可以在混合自旋相中检测到。1经历了100% HS相的光致激发态自旋捕获（LIESST），伴随着O4W⋯O5W HB的断裂以及SSHB中质子静态无序的开始，表明存在光致活化能质子运动的势垒。这种激发态在T ₁ (LIESST) = 68 K 和T ₂ (LIESST) = 76 K 处显示出阶梯式弛豫。第一个弛豫步骤后的光晶体学测量揭示了具有中间几何形状的单个 Fe 位点，这是由 Fe 的随机分布造成的。 HS 和 LS 位点遍布整个晶格。