**Preface to "Intramolecular Hydrogen Bonding 2021"**

Studied for over a hundred years, hydrogen bonds are one of the most described phenomena in chemistry. This is because they occupy an important position in the world of inter- and intramolecular interactions, which in turn is related to their position on the scale of the strength of interactions and chemical bonds. Particularly, on this scale, hydrogen bonds are between the weaker van der Waals interactions and the stronger covalent bonds. For this reason, intermolecular hydrogen bonds act as a glue that binds individual molecules into dimers, trimers, or larger molecular clusters. However, the network of hydrogen bonds is dynamic, old hydrogen bonds are broken and new ones are formed. This ability is at least in part due to the small size of the hydrogen atom, which allows the X and Y atoms in the X-H···Y bridge to come closer to each other without major steric effects. The positive atomic charge of H is also important here, which, being contrary to the atomic charges on X and Y, enables this contact. The most frequently mentioned symptom of the presence of intermolecular hydrogen bonds is the curiously high boiling point of liquid water compared to slightly heavier hydrogen sulfide. Another manifestation of the presence of intermolecular hydrogen bonds is the lower density of ice than that of liquid water, thanks to which it floats on its surface, making it easier for fish to survive during cold winters. The good solubility of polar and ionic substances in water is also due to the formation of hydrogen bonds.

However, hydrogen bonds are not all about water. Their role is equally important for many much larger molecules, including macromolecules. For example, the presence of intermolecular hydrogen bonds between complementary nitrogen base pairs (cytosine–guanine and adenine–thymine) binds two strands of DNA together, giving them a double-helical structure, which is fundamental in the replication of genetic information. Intermolecular hydrogen bonds also affect the structure of proteins and maintenance of cellulose or polymer chains.

It seems that intramolecular hydrogen bonds are not as often described as their intermolecular counterparts. This is due to the fact that their detection is generally based on indirect methods, and this is often associated with some problems, for example in the form of finding an appropriately reliable reference system. In spectroscopic methods, the presence of intramolecular hydrogen bond is ascertained, for example, on the basis of a change in a certain parameter (e.g., the stretching vibration frequency of X-H) describing the proton-donor group X-H, but determining the value of this change obviously requires establishing a certain reference value characterizing the unperturbed X-H group. Like their intermolecular counterparts, intramolecular hydrogen bonds are, of course, also of grea<sup>t</sup> importance. First of all, they can significantly affect the conformational equilibrium, giving preference to certain conformers over others. A known case is the keto-enol equilibrium, which is related to the alpha hydrogen atom movement. Such equilibria can, in turn, affect the crystallographic structure of the compound in the solid. A very important effect related to the movement of the hydrogen atom in the X-H···Y bridge is the proton transfer effect, which occurs especially in the excited state of a molecule.

In my opinion, the grea<sup>t</sup> advantage of this book is that it contains the results of both theoretical and experimental research, and with the use of many different research methods. Therefore, it is an excellent review of these methods, while showing their applicability to the current scientific issues regarding intramolecular hydrogen bonds. The experimental techniques used include X-ray diffraction, infrared and Raman spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), nuclear quadrupole resonance spectroscopy (NQR), incoherent inelastic neutron scattering (IINS), and differential scanning calorimetry (DSC). The solvatochromic and luminescent studies are also

described. On the other hand, theoretical research is based on ab initio calculations and the Car–Parrinello Molecular Dynamics (CPMD). In the latter case, a description of nuclear quantum effects (NQE) is also possible. This book also demonstrates the use of theoretical methods such as Quantum Theory of Atoms in Molecules (QTAIM), Interacting Quantum Atoms (IQA), Natural Bond Orbital (NBO), Non-Covalent Interactions (NCI) index, Molecular Tailoring Approach (MTA), and many others.

> **Mirosław Jabło ´nski** *Editor*
