Does the recent IUPAC definition on hydrogen bonding lead to new intermolecular interactions?

Hydrogen bonding interaction is one of the most important intermolecular interactions. This interaction has been known almost for a century, but it is popular only after the work of Pauling, who was the first to give a definition of the hydrogen bond. According to Pauling, ‘under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms, instead of only one, so that it may be considered to be acting as a bond between them. This is called the hydrogen bond’. This interaction may be designated as X–HY. X–H is the hydrogen bond donor in which X and H are covalently bound and Y is the acceptor. Pauling considered both X and Y to be electronegative atoms mainly O, N, F. There are many systems which would not be considered as hydrogen bonds according to Pauling’s definition. One example is the C–HO interaction. This is now accepted as weak hydrogen bond, as the interaction plays an important role in many systems. After the work of Pauling, numerous experimental and theoretical studies on these interactions have been made, from which it is now well established that X can be any atom or a fragment whose overall electronegativity is more than that of hydrogen and Y can be any electron-rich region in a molecule, such as a lone pair,  pair, unpaired and  electrons. A recent report has recognized the diverse nature of hydrogen bond donors and acceptors. The study of hydrogen bonding is of special interest for a variety of reasons. It is one of the most common interactions and is found in many systems. The relatively high boiling point of water is a classic example of hydrogen bonds. Hydrogen bonding interaction is found in biological systems. It plays a role in determining the shapes of folded proteins and in linking base pairs in strands of DNA. According to the new IUPAC definition, the hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation. From this definition, this interaction may be designated as X–HY–Z. With following criteria given in the IUPAC report this interaction it is now believed to be fully understood. (a) The forces involved in the formation of a hydrogen bond include those of an electrostatic origin, those arising from charge transfer between the donor and acceptor leading to partial covalent bond formation between H and Y, and those originating from dispersion. (b) The atoms X and H are covalently bonded to one another and the X–H bond is polarized, the H–Y bond strength increasing with the increase in electronegativity of X. (c) The X–H—Y angle is usually linear (180), and closer the angle is to 180, stronger is the hydrogen bond and shorter is the HY distance. (d) The length of the X–H bond usually increases on hydrogen bond formation leading to a red shift in the infrared X–H stretching frequency and an increase in the infrared absorption crosssection for the X–H stretching vibration. The greater the lengthening of the X–H bond in X–H—Y, the stronger is the HY bond. Simultaneously, new vibrational modes associated with the formation of the HY bond are generated. (e) The X–H—Y–Z hydrogen bond leads to characteristic NMR signatures that typically include pronounced proton deshielding for H in X–H, through hydrogen bond spin–spin couplings between X and Y, and nuclear Overhauser enhancements. (f) The Gibbs energy of formation for the hydrogen bond should be greater than the thermal energy of the system for the hydrogen bond to be detected experimentally. After the recent definition of hydrogen bonding given by IUPAC, many researchers around the world have studied and characterized different types of intermolecular interactions. According to a Google Scholar search on 18 September 2015, 387 researchers had cited the recent definition of hydrogen bond given by IUPAC till that day. We found that these records were journal articles, thesis and scientific papers in different languages. When same was searched in Web of Science (WoS) on the same date and around the same time, the number of citations recorded was 274. However, in the WoS records we found that only journal articles published in English cited this recent definition. Moreover, the works related with various types of intermolecular interactions also cited this new IUPAC definition of hydrogen bonding. These different of intermolecular interactions are discussed briefly here.