The red shift of the X-H stretching frequency, with a significant increase in intensity of the corresponding spectral band and a downfield chemical shift of hydrogen (deshielding) in nuclear magnetic resonance (NMR) spectroscopy, has traditionally been used as a criterion for identifying X-H···Y hydrogen bonds (HBs) where X is the hydrogen donor and Y is the acceptor. However, over the past two decades, it has become evident that certain HBs can exhibit a blue shift in the X-H stretching frequency and may also show a decrease in IR intensity, diverging from classical expectations. In this study, we investigate a wide array of HBs, encompassing both red-shifted and blue-shifted systems, as well as protonic and hydridic HB systems. We focus on understanding the underlying electronic conditions behind the reverse chemical shift effects─upfield shifts (shielding) upon HB formation, challenging the view that hydrogen bonding (H-bonding) typically leads to deshielding. We employ state-of-the-art quantum chemical methods, integrating computed NMR shielding tensors and electron deformation density, in combination with experimental NMR, to probe that phenomenon. The computational findings are thoroughly validated against experimental results. Our research confirms that shielding is also possible upon HB formation, thereby broadening the conceptual framework of H-bonding.

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Previously studied complexes with protonic and hydridic hydrogen bonds exhibit significant similarities. The present study provides a detailed investigation of the structure, stabilization, electronic properties, and spectral characteristics of protonic and hydridic hydrogen bonds using low-temperature infrared (IR) spectroscopy and computational methods. Complexes of pentafluorobenzene with ammonia (C₆F₅H⋯NH₃) and triethylgermane with trifluoroiodomethane (Et₃GeH⋯ICF₃) were analyzed using both experimental and computational tools. Additionally, 30 complexes with protonic hydrogen bonds and 30 complexes with hydridic hydrogen bonds were studied computationally. Our findings reveal that, despite the opposite atomic charges on the hydrogens in these hydrogen bonds, and consequently the opposite directions of electron transfer in protonic and hydridic hydrogen bonds, their spectral manifestations - specifically, the red shifts in the X-H stretching frequency and the increase in intensity - are remarkably similar. The study also discusses the limitations of the current IUPAC definition of hydrogen bonding in covering both types of H-bonds and suggests a way to overcome these limitations.

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• časopisecké články MeSH