Baudhuin Melissa A, Boopalachandran Praveenkumar, Rajan Srijay, Leopold Doreen Geller
Chemistry Department, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, Minnesota55455, United States.
J Phys Chem A. 2021 Nov 11;125(44):9658-9679. doi: 10.1021/acs.jpca.1c07669. Epub 2021 Nov 1.
Photoelectron spectra of the niobium-molybdenum diatomic anion, obtained at 488 and 514 nm, display vibrationally resolved transitions from the ground state and one excited electronic state of the anion to the ground state and one excited electronic state of the neutral molecule. The electron affinity of NbMo is measured to be 1.130 ± 0.005 eV. Its Δ spin-orbit component is observed to lie 870 ± 20 cm above its previously identified Δ ground state. For NbMo, vibrational energies measured for levels up to = 4 for the Δ and Δ states give harmonic frequency and anharmonicity constant values of ω = 492 ± 12 cm and ω = 8.0 ± 3.2 cm, the former value corresponding to a force constant of 6.80 ± 0.35 mdyn/Å. These two vibrational parameters suggest a bond dissociation energy that is too low by at least a factor of 3, indicating that the ground state potential energy curve of NbMo deviates markedly from a Morse potential at higher energies. An excited electronic state of NbMo, assigned as a Σ state, is observed at 2900 ± 25 cm (). Vibrational energies up to = 8 in this excited state give values of ω = 544 ± 8 cm and ω = 1.9 ± 1.2 cm for NbMo. The former value corresponds to a high vibrational force constant of 8.30 ± 0.25 mdyn/Å. Both doublet states of the neutral molecule are accessed from the anion ground state, which is assigned as Σ. For the NbMo anion, the fundamental vibrational frequency (Δ) is 484 ± 15 cm. Electron affinity data indicate that the bond dissociation energy of NbMo is 0.213 ± 0.005 eV greater than that of neutral NbMo, whose previously reported value then gives = 4.85 ± 0.27 eV for the anion. An excited state of the anion lying 3050 ± 25 cm () above its ground state is assigned as Δ, and the energies of its spin-orbit components above the Δ lowest energy level are measured to be 450 ± 20 cm (Δ) and 1100 ± 20 cm (Δ). Their uneven spacing suggests that the energy of the Δ level is lowered by interaction with a higher energy Ω = 2 anion state. The vibrational frequency (Δ) for the Δ and Δ states is measured to be 433 ± 20 cm. Bond length differences among the observed states are estimated from Franck-Condon fits to vibrational band intensity profiles. When combined with the previously reported NbMo bond length, these provide bond length estimates for the ground state of the anion (1.940 ± 0.025 Å) and for the observed excited states. These species offer extreme examples of multiple metal-metal bonding, with formal bond orders of 5/ for the Δ ground and Σ excited doublet states of NbMo, 6 for the singlet ground state of the anion, and 5 for its low-lying triplet state. The relationships among their bonding properties and those of related multiply bonded transition metal dimers are discussed.
在488纳米和514纳米波长下获得的铌 - 钼双原子阴离子的光电子能谱，展示了从该阴离子的基态和一个激发电子态到中性分子的基态和一个激发电子态的振动分辨跃迁。测得铌钼的电子亲和能为1.130±0.005电子伏特。观察到其Δ自旋 - 轨道分量位于先前确定的Δ基态上方870±20厘米处。对于铌钼，在Δ和Δ态中测量到的高达 = 4的能级的振动能量给出了谐频和非谐性常数的值，分别为ω = 492±12厘米和ω = 8.0±3.2厘米，前一个值对应于力常数为6.80±0.35毫达因/埃。这两个振动参数表明键解离能至少低了3倍，这表明铌钼的基态势能曲线在较高能量下明显偏离莫尔斯势。观察到铌钼的一个激发电子态，被指定为Σ态，位于2900±25厘米（）处。在这个激发态中高达 = 8的振动能量给出铌钼的ω = 544±8厘米和ω = 1.9±1.2厘米的值。前一个值对应于高振动力常数8.30±0.25毫达因/埃。中性分子的两个双重态都可从被指定为Σ的阴离子基态跃迁得到。对于铌钼阴离子，基本振动频率（Δ）为484±15厘米。电子亲和能数据表明铌钼的键解离能比中性铌钼的键解离能大0.213±0.005电子伏特，其先前报道的值使得阴离子的键解离能为 = 4.85±0.27电子伏特。阴离子的一个位于其基态上方3050±25厘米（）处的激发态被指定为Δ，并且测量到其自旋 - 轨道分量在Δ最低能级上方的能量为450±20厘米（Δ）和1100±20厘米（Δ）。它们不均匀的间距表明Δ能级的能量因与更高能量的Ω = 2阴离子态相互作用而降低。测量到Δ和Δ态的振动频率（Δ）为433±20厘米。根据对振动带强度分布的弗兰克 - 康登拟合估算观察到的各态之间的键长差异。当与先前报道的铌钼键长相结合时，这些数据给出了阴离子基态（1.940±0.025埃）和观察到的激发态的键长估计值。这些物种提供了多重金属 - 金属键合的极端例子，铌钼的Δ基态和Σ激发双重态的形式键级为5/，阴离子的单重基态为6，其低能级三重态为5。讨论了它们的键合性质与相关多重键合过渡金属二聚体的键合性质之间的关系。
