Periodic Trends for Transition Metal Dihydrides MH₂, Dihydride Dihydrogen Complexes MH₂·H₂, and Tetrahydrides MH₄ (M = Ti, V, and Cr)

Ab initio quantum mechanical methods were employed to study the periodic trends of transition metal (M = Ti, V, and Cr) hydrides MH2, dihydride dihydrogen complexes MH2·H2, and tetrahydrides MH4. The configuration interaction with single and double excitations (CISD), coupled cluster including all single and double substitutions (CCSD) methods, and CCSD with the effects of connected triple excitation added perturbatively [CCSD(T)] were used with the TZP, TZP+f, and TZP(f,d) basis sets. The ground electronic states for TiH2 and VH2 were found to be 3B1 and 4B2, respectively. The bond angles for the TiH2 and VH2 molecules are predicted to be 142° and 139°, respectively, at the TZP(f,d) CISD level of theory. On the low-spin potential energy surfaces, the lowest lying electronic states for the TiH2, VH2, and CrH2 molecules are 1A1, 2A1, and 3B2, respectively. The energy separations between the ground state and the lowest lying low-spin state were found to be 33, 40, and 59 kcal mol-1 for the TiH2, VH2, and CrH2 molecules, respectively, at the TZP CCSD level of theory. The binding energies of the dihydride dihydrogen complexes decrease with increasing atomic number. The d → σ* back donation dominates the periodic trend for the formation of low-spin MH2·H2 complexes. All three MH2·H2 complexes are in the high-spin ground state, primarily due to the fact that the corresponding parent dihydrides have high-spin ground states. The low-spin dihydrides interact with the H2 moiety more strongly than do the high-spin species. The d → σ* back donation was so strong for the low-spin TiH2 that H2 dissociates without barrier upon contact with singlet TiH2 to form TiH4. Due to the Jahn−Teller distortion the ground state of VH4 is the 2A1 electronic state having D2d symmetry. TiH4 is predicted to lie 9 kcal mol-1 lower in energy than its ground state MH2·H2 isomer, whereas VH4 and CrH4 are higher in energy by 22 and 39 kcal mol-1, respectively, at the TZP CCSD level of theory. However, comparing MH4 and MH2·H2 in the same spin state, MH4 is always lower in energy than its dihydrogen complex isomer, MH2·H2, on the low-spin potential energy surface. Comparison between the present work and experimental IR spectra from the matrix isolation of the cocondesation of transition metal atoms (Ti, V, and Cr) with H2 molecules confirmed the existence of CrH2·H2 by identifying a strong unique absorption at 1510 cm-1. It was found TiH2·H2 rather than TiH2 may be observed experimentally, and that VH2·H2 may be formed concomitantly with the VH2 molecule.本研究采用从头算量子力学方法，探究过渡金属（M=钛（Ti）、钒（V）、铬（Cr））氢化物MH₂、二氢化物-二氢配合物MH₂·H₂以及四氢化物MH₄的周期性变化规律。所采用的计算方法包括：单双激发组态相互作用（Configuration Interaction with Single and Double Excitations, CISD）、包含全部单双取代的耦合簇（Coupled Cluster with Single and Double Substitutions, CCSD）方法，以及通过微扰校正连接性三重激发效应的CCSD[CCSD(T)]；基组则选用了TZP、TZP+f及TZP(f,d)基组。研究发现，TiH₂与VH₂的基电子态分别为³B₁和⁴B₂。在TZP(f,d)-CISD理论水平下，TiH₂与VH₂分子的键角分别被预测为142°与139°。在低自旋势能面上，TiH₂、VH₂与CrH₂分子的最低能量电子态分别为¹A₁、²A₁与³B₂。在TZP-CCSD理论水平下，TiH₂、VH₂与CrH₂分子的基态与最低能量低自旋态之间的能量差分别为33、40与59 kcal·mol⁻¹。二氢化物-二氢配合物的结合能随原子序数的增大而降低。d轨道→σ*反键轨道的反馈作用主导了低自旋MH₂·H₂配合物生成过程的周期性变化规律。三种MH₂·H₂配合物均以高自旋态为基态，这主要是因为其对应的母体二氢化物本身即为高自旋基态。低自旋二氢化物与H₂片段的相互作用强度高于高自旋物种。对于低自旋TiH₂而言，其d→σ*反馈作用极强，以至于单重态TiH₂与H₂接触时无需能垒即可使H₂解离并生成TiH₄。由于姜-泰勒（Jahn-Teller）畸变，VH₄的基态为具有D₂d点群对称性的²A₁电子态。在TZP-CCSD理论水平下，TiH₄的能量比其基态MH₂·H₂异构体低9 kcal·mol⁻¹；而VH₄与CrH₄的能量则分别比对应异构体高22与39 kcal·mol⁻¹。但在相同自旋态下，于低自旋势能面上比较MH₄与MH₂·H₂配合物时，MH₄的能量始终低于其二氢配合物异构体MH₂·H₂。将本研究结果与过渡金属原子（Ti、V、Cr）与H₂分子共冷凝后的基质隔离实验红外光谱进行对比后，通过在1510 cm⁻¹处识别出一处强且独特的吸收峰，证实了CrH₂·H₂的存在。研究表明，实验中可观测到的物种应为TiH₂·H₂而非TiH₂；而VH₂·H₂可与VH₂分子同时生成。