Van Hoozen Brian L, Petersen Poul B
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA.
J Chem Phys. 2015 Nov 14;143(18):184305. doi: 10.1063/1.4935062.
Medium and strong hydrogen bonds are well known to give rise to broad features in the vibrational spectrum often spanning several hundred wavenumbers. In some cases, these features can span over 1000 cm(-1) and even contain multiple broad peaks. One class of strongly hydrogen-bonded dimers that includes many different phosphinic, phosphoric, sulfinic, and selenic acid homodimers exhibits a three-peaked structure over 1500 cm(-1) broad. This unusual feature is often referred to as the Hadži ABC structure. The origin of this feature has been debated since its discovery in the 1950s. Only a couple of theoretical studies have attempted to interpret the origin of this feature; however, no previous study has been able to reproduce this feature from first principles. Here, we present the first ab initio calculation of the Hadži ABC structure. Using a reduced dimensionality calculation that includes four vibrational modes, we are able to reproduce the three-peak structure and much of the broadness of the feature. Our results indicate that Fermi resonances of the in-plane bend, out-of-plane bend, and combination of these bends play significant roles in explaining this feature. Much of the broadness of the feature and the ability of the OH stretch mode to couple with many overtone bending modes are captured by including an adiabatically separated dimer stretch mode in the model. This mode modulates the distance between the monomer units and accordingly the strength of the hydrogen-bonds causing the OH stretch frequency to shift from 2000 to 3000 cm(-1). Using this model, we were also able to reproduce the vibrational spectrum of the deuterated isotopologue which consists of a single 500 cm(-1) broad feature. Whereas previous empirical studies have asserted that Fermi resonances contribute very little to this feature, our study indicates that while not appearing as a separate peak, a Fermi resonance of the in-plane bend contributes substantially to the feature.
众所周知，中等强度和强氢键会在振动光谱中产生宽峰，这些宽峰通常跨越数百波数。在某些情况下，这些峰可跨越超过1000 cm⁻¹，甚至包含多个宽峰。一类强氢键二聚体，包括许多不同的次膦酸、磷酸、亚磺酸和亚硒酸同二聚体，在超过1500 cm⁻¹的范围内呈现出三峰结构。这一不寻常的特征通常被称为哈季ABC结构。自20世纪50年代发现这一特征以来，其起源一直存在争议。只有少数理论研究试图解释这一特征的起源；然而，此前没有研究能够从第一性原理出发重现这一特征。在此，我们展示了对哈季ABC结构的首次从头算计算。通过包含四种振动模式的降维计算，我们能够重现三峰结构以及该特征的大部分展宽。我们的结果表明，面内弯曲、面外弯曲以及这些弯曲的组合所产生的费米共振在解释这一特征中起着重要作用。通过在模型中包含一个绝热分离的二聚体伸缩模式，捕捉到了该特征的大部分展宽以及OH伸缩模式与许多泛音弯曲模式耦合的能力。该模式调节单体单元之间的距离，从而调节氢键的强度，导致OH伸缩频率从2000 cm⁻¹ 移动到3000 cm⁻¹。使用该模型，我们还能够重现氘代同位素体的振动光谱，其由一个500 cm⁻¹ 宽的单一特征组成。尽管此前的实证研究断言费米共振对这一特征的贡献很小，但我们的研究表明，虽然面内弯曲的费米共振没有以单独的峰出现，但它对该特征有很大贡献。
