High-Level ab Initio Predictions for the Ionization Energies, Bond Dissociation Energies, and Heats of Formation of Titanium Oxides and Their Cations (TiOn/TiOn+, n = 1 and 2)

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• 作者：Yi Pan ; Zhihong Luo ; Yih-Chung Chang ; Kai-Chung LauC. Y. Ng
• 刊名：Journal of Physical Chemistry A
• 出版年：2017
• 出版时间：January 26, 2017
• 年：2017
• 卷：121
• 期：3
• 页码：669-679
• 全文大小：440K
• ISSN：1520-5215

文摘

The ionization energies (IEs) of TiO and TiO₂ and the 0 K bond dissociation energies (D₀) and the heats of formation at 0 K (ΔH°_f0) and 298 K (ΔH°_f298) for TiO/TiO⁺ and TiO₂/TiO₂⁺ are predicted by the wave-function-based CCSDTQ/CBS approach. The CCSDTQ/CBS calculations involve the approximation to the complete basis set (CBS) limit at the coupled cluster level up to full quadruple excitations along with the zero-point vibrational energy (ZPVE), high-order correlation (HOC), core–valence (CV) electronic, spin–orbit (SO) coupling, and scalar relativistic (SR) effect corrections. The present calculations yield IE(TiO) = 6.815 eV and are in good agreement with the experimental IE value of 6.819 80 ± 0.000 10 eV determined in a two-color laser-pulsed field ionization-photoelectron (PFI-PE) study. The CCSDT and MRCI+Q methods give the best predictions to the harmonic frequencies: ω_e (ω_e⁺) = 1013 (1069) and 1027 (1059) cm^–1 and the bond lengths r_e (r_e⁺) = 1.625 (1.587) and 1.621 (1.588) Å, for TiO (TiO⁺) compared with the experimental values. Two nearly degenerate, stable structures are found for TiO₂ cation: TiO₂⁺(C_2v) structure has two equivalent TiO bonds, while the TiO₂⁺(C_s) structure features a long and a short TiO bond. The IEs for the TiO₂⁺(C_2v)←TiO₂ and TiO₂⁺(C_s)←TiO₂ ionization transitions are calculated to be 9.515 and 9.525 eV, respectively, giving the theoretical adiabatic IE value in good agreement with the experiment IE(TiO₂) = 9.573 55 ± 0.000 15 eV obtained in the previous vacuum ultraviolet (VUV)–PFI-PE study of TiO₂. The potential energy surface of TiO₂⁺ along the normal vibrational coordinates of asymmetric stretching mode (ω₃⁺) is nearly flat and exhibits a double-well potential with the well of TiO₂⁺ (C_s) situated around the central well of TiO₂⁺(C_2v). This makes the theoretical calculation of ω₃⁺ infeasible. For the symmetric stretching (ω₁⁺), the current theoretical predictions overestimate the experimental value of 829.1 ± 2.0 cm^–1 by more than 100 cm^–1. This work together with the previous experimental and theoretical investigations supports the conclusion that the CCSDTQ/CBS approach is capable of providing reliable IE and D₀ predictions for TiO/TiO⁺ and TiO₂/TiO₂⁺ with error limits less than or equal to 60 meV. The CCSDTQ/CBS calculations give the predictions of D₀(Ti⁺–O) – D₀(Ti–O) = 0.004 eV and D₀(O–TiO) – D₀(O–TiO⁺) = 2.699 eV, which are also consistent with the respective experimental determination of 0.008 32 ± 0.000 10 and 2.753 75 ± 0.000 18 eV.