Charge-enhanced C–H–O interactions of a self-assembled triple helical spine probed by high-pressure

Abstract

C–H–O interactions of a self-assembled triple helix based on the 1-acetamido-3(2-pyrazinyl)-imidazolium cation has been probed by high pressure. The infrared spectroscopic profiles and ab initio calculations allow us to make a vibrational assignment of this compound. The C–H bonds forming C–H–O interactions shorten as the pressure was elevated, while free C–H vibration modes show low sensitivity to high pressure. The pressure-dependent results can be attributed to the strengthening of C–H–O electrostatic–dispersion interactions upon increasing pressure. The appearance of the free-NH infrared absorption indicates that the conventional N–H–O hydrogen bond does not dominate the inter-strand packing in the compound. It is proposed that the charge-enhanced C–H–O interactions, forming a helical hydrogen-bonding network, disturb the formation of inter-strand N–H–O hydrogen-bonding in order to form a maximum number of hydrogen bonds. Applying high-pressure seems not to change the C=O bond length in contrast to the trend of blue-shift in frequency of C–H vibrations. London dispersion energy is suggested to be required for understanding the pressure-dependent results, although more additional terms, such as the effect in the presence of charge, are needed for the correct explanation. This work demonstrates that high-pressure studies may have the potential to provide insight into the C–H–O structural properties of biological related systems.

Magnitude and Mechanism of Charge Enhancement of CH··O Hydrogen Bonds

“…This result was later confirmed by Schmiedekamp and Nanda, who extended the concept of the positive charge to the proximate positioning of a cationic metal. Along a similar vein, the effects of charge were manifest when the cationic imidazole donor + CH··O H-bond supplanted a neutral NH··O bond as it led to a triple helical structure of 1-acetamido-3-(2-pyrazinyl)imidazolium.…”

Ionic Liquid Crystals: Versatile Materials

“…In the strictest sense, they only occur between N, O, and F atoms, with the link being a shared H atom. However, a special class of hydrogen bonds are “ionic” or “charge-assisted” hydrogen bonds between ions and molecules, with bond strengths of 20–160 kJ mol –1 . ,, “Nonclassical” hydrogen bonds such as the C–H···X – and C–H···X interactions found in many ILs and ILCs can be considered to belong to this class. − ] Introduction of charges is a very efficient way to increase the degree of amphiphilicity of LCs , and to reinforce nanosegregation, which in turn can lead to an enhanced mesophase stability. A similar strategy has been adopted in the design of LC dendrimers. , Smectic ordering in ILCs is stabilized to a large extent by nanosegregation, with orientational ordering typically being less important than in conventional smectic LCs (see also section ).…”

Pressure-Enhanced C−H···O Interactions in Aqueous tert-Butyl Alcohol

“…The importance of understanding the nature of intermolecular interactions, such as hydrogen bonding, has been recognized for some time. , A characteristic feature of hydrogen-bond formation in an X−H···Y system is X−H bond lengthening with a concomitant red shift of the X−H stretching frequency. , A number of experimental and theoretical studies have reported, however, the existence of an unusual class of blue-shifted hydrogen bonds in which H-bond formation leads to C−H bond shortening and to a blue shift of the C−H unit's IR stretching frequency. , Because C−H···O hydrogen bonds are typically weaker than those of the more conventional O−H···O and N−H···O hydrogen bonds, direct measurements of their strength are very difficult to achieve. Nevertheless, the widespread occurrence of C−H···O interactions has made them an active topic of research from both theoretical and experimental perspectives. − It has been suggested that C−H···O interactions play important functional and structural stabilization roles in biological systems. , Conclusive experimental evidence for the existence of C−H···O hydrogen bonding in biological macromolecules is notoriously difficult to obtain because C−H···O hydrogen bonds, which are typically weak in biological systems, usually coexist with many other types of strong interactions. − Because alcohol molecules are amphiphilic, that is, they possess both alkyl and hydroxyl groups, their hydrophobic and hydrophilic interactions are subtly balanced in aqueous alcohol solutions. Therefore, a detailed investigation of aqueous alcohol solutions may provide information necessary for understanding hydrophobic hydration mechanisms in biological systems.…”