Extensive ab initio investigations of the ground and electronic excited states of the AlCH₂ free radical have been carried out in order to predict the spectroscopic properties of this, as yet, undetected species. Difficulties with erratic predictions of the ground state vibrational frequencies, both in the literature and in the present work, have been traced to serious broken-symmetry instabilities in the unrestricted Hartree-Fock orbitals at the ground state equilibrium geometry. The use of restricted open-shell Hartree–Fock or complete active space self consistent field orbitals avoids these problems and leads to consistent and realistic sets of vibrational frequencies for the ground state. Using the internally contracted multireference configuration interaction method with aug-cc-pV(T+d)Z basis sets, we have calculated the geometries, energies, dipole moments, and vibrational frequencies of eight electronic states of AlCH₂ and AlCD₂. In addition, we have generated Franck–Condon simulations of the expected vibronic structure of the $\tilde{\text{A}}-\tilde{\text{X}}$, $\tilde{\text{B}}-\tilde{\text{X}}$, $\tilde{\text{C}}-\tilde{\text{X}}$, and $\tilde{\text{C}}-\tilde{\text{A}}$ band systems, which will be useful in searches for the electronic spectra of the radical. We have also simulated the expected rotational structure of the 0–0 absorption bands of these transitions at modest resolution under supersonic expansion cooled conditions. Our conclusion is that if AlCH₂ can be generated in sufficient concentrations in the gas phase, it is most likely detectable through the ${\tilde{B}}^2{A}_2$–${\tilde{X}}^2{B}_1$ or ${\tilde{C}}^2{A}_1$–${\tilde{X}}^2{B}_1$ electronic transitions at 515 nm and 372 nm, respectively. Both band systems have vibrational and rotational signatures, even at modest resolution, that are diagnostic of the aluminum methylene free radical.

中文翻译：

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从头开始对亚甲基铝（AlCH2）自由基进行光谱分析。

从头开始对AlCH _2的基态和电子激发态进行广泛的研究为了预测该尚未发现的物种的光谱性质，已经进行了自由基处理。无论是在文献中还是在目前的工作中，都难以对基态振动频率进行预测，这归因于基态平衡几何条件下无限制的Hartree-Fock轨道中的严重破碎对称不稳定性。使用受限制的开壳式Hartree-Fock或完整的活动空间自洽场轨道可避免这些问题，并导致基态的振动频率集一致且逼真。使用具有aug-cc-pV（T + d）Z基集的内部收缩多参考配置相互作用方法，我们计算了AlCH八个电子态的几何形状，能量，偶极矩和振动频率₂和AlCD ₂。此外，我们还生成了预期的振动结构的Franck-Condon模拟。$\widetilde{\text{一种}}-\tilde{\text{X}}$， $\widetilde{\text{乙}}-\tilde{\text{X}}$， $\tilde{\text{C}}-\tilde{\text{X}}$和 $\tilde{\text{C}}-\widetilde{\text{一种}}$带系统，这对于搜索自由基的电子光谱将很有用。我们还在超音速膨胀冷却条件下以中等分辨率模拟了这些跃迁的0-0吸收带的预期旋转结构。我们的结论是，如果可以在气相中以足够的浓度生成AlCH ₂，则很可能通过气相色谱法检测到。${\widetilde{乙}}^2{\mathrm{一种}}_2$–${\tilde{X}}^2{乙}_{\mathrm{1个}}$ 要么 ${\tilde{C}}^2{\mathrm{一种}}_{\mathrm{1个}}$–${\tilde{X}}^2{乙}_{\mathrm{1个}}$电子跃迁分别在515 nm和372 nm。即使在中等分辨率下，两个谱带系统都具有振动和旋转特征，可以诊断亚甲基铝自由基。