Van Hoozen Brian L, Petersen Poul B
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA.
J Chem Phys. 2015 Mar 14;142(10):104308. doi: 10.1063/1.4914147.
Medium and strong hydrogen bonds are common in biological systems. Here, they provide structural support and can act as proton transfer relays to drive electron and/or energy transfer. Infrared spectroscopy is a sensitive probe of molecular structure and hydrogen bond strength but strongly hydrogen-bonded structures often exhibit very broad and complex vibrational bands. As an example, strong hydrogen bonds between carboxylic acids and nitrogen-containing aromatic bases commonly display a 900 cm(-1) broad feature with a remarkable double-hump structure. Although previous studies have assigned this feature to the OH, the exact origin of the shape and width of this unusual feature is not well understood. In this study, we present ab initio calculations of the contributions of the OH stretch and bend vibrational modes to the vibrational spectrum of strongly hydrogen-bonded heterodimers of carboxylic acids and nitrogen-containing aromatic bases, taking the 7-azaindole--acetic acid and pyridine--acetic acid dimers as examples. Our calculations take into account coupling between the OH stretch and bend modes as well as how both of these modes are affected by lower frequency dimer stretch modes, which modulate the distance between the monomers. Our calculations reproduce the broadness and the double-hump structure of the OH vibrational feature. Where the spectral broadness is primarily caused by the dimer stretch modes strongly modulating the frequency of the OH stretch mode, the double-hump structure results from a Fermi resonance between the out of the plane OH bend and the OH stretch modes.
中等强度和强氢键在生物系统中很常见。在此，它们提供结构支撑，并可作为质子转移中继来驱动电子和/或能量转移。红外光谱是分子结构和氢键强度的灵敏探针，但强氢键结构通常表现出非常宽且复杂的振动带。例如，羧酸与含氮芳香碱之间的强氢键通常呈现出一个900 cm⁻¹的宽峰，具有显著的双峰结构。尽管先前的研究已将此特征归因于OH，但这种不寻常特征的形状和宽度的确切起源尚不清楚。在本研究中，我们以7-氮杂吲哚 - 乙酸和吡啶 - 乙酸二聚体为例，对OH伸缩和弯曲振动模式对羧酸与含氮芳香碱强氢键异二聚体振动光谱的贡献进行了从头算。我们的计算考虑了OH伸缩和弯曲模式之间的耦合，以及这两种模式如何受到低频二聚体伸缩模式的影响，低频二聚体伸缩模式会调节单体之间的距离。我们的计算重现了OH振动特征的宽度和双峰结构。光谱宽度主要由二聚体伸缩模式强烈调制OH伸缩模式的频率引起，而双峰结构则是由面外OH弯曲与OH伸缩模式之间的费米共振导致的。
