Wang Huan, Agmon Noam
The Fritz Haber Research Center, Institute of Chemistry, The Hebrew University of Jerusalem , Jerusalem 91904, Israel.
J Phys Chem A. 2017 Apr 27;121(16):3056-3070. doi: 10.1021/acs.jpca.7b01856. Epub 2017 Apr 14.
Gas-phase HO has been considered an archetypal Eigen cation, HO(HO). Yet ab initio molecular dynamics (AIMD) suggested that its infrared spectrum is explained by a linear-chain Zundel isomer, alone or in a mixture with the Eigen cation. Recently, hole-burning experiments suggested a single isomer, with a second-order vibrational perturbation theory (VPT2) spectrum agreeing with the Eigen cation. To resolve this discrepancy, we have extended both calculations to more advanced DFT functionals, better basis sets, and dispersion correction. For Zundel-isomers, we find VPT2 anharmonic frequencies for four low-frequency modes involving the excess proton unreliable, including the 1750 cm band that is pivotal for differentiating between Zundel and Eigen isomers. Because the analogous bands of the HO cation show little effect of anharmonicity, we utilize the harmonic frequencies for these modes. With this caveat, both AIMD and VPT2 agree on the spectrum as originating from a Zundel isomer. VPT2 also shows that both isomers have the same spectrum in the high frequency region, so that the hole burning experiments should be extended to lower frequencies.
气相HO一直被认为是一种典型的特征阳离子，即HO(HO)。然而，从头算分子动力学（AIMD）表明，其红外光谱可以由线性链状的尊德耳异构体单独或与特征阳离子的混合物来解释。最近，烧孔实验表明存在单一异构体，其二阶振动微扰理论（VPT2）光谱与特征阳离子相符。为了解决这一差异，我们将计算扩展到了更高级的密度泛函理论（DFT）泛函、更好的基组和色散校正。对于尊德耳异构体，我们发现涉及过量质子的四个低频模式的VPT2非谐频率不可靠，包括区分尊德耳异构体和特征异构体的关键的1750 cm波段。由于HO阳离子的类似波段显示出很小的非谐效应，我们使用这些模式的谐频率。在此前提下，AIMD和VPT2都认为光谱源自尊德耳异构体。VPT2还表明，两种异构体在高频区域具有相同的光谱，因此烧孔实验应扩展到更低频率。
