Hydrogen Bonding (OpenStax Chemistry 2e)
Nitrosyl fluoride (ONF, molecular mass 49 amu) is a gas at room temperature. Water (H2O, molecular mass 18 amu) is a liquid, even though it has a lower molecular mass. We clearly cannot attribute this difference between the two compounds to dispersion forces. Both molecules have about the same shape and ONF is the heavier and larger molecule. It is, therefore, expected to experience more significant dispersion forces. Additionally, we cannot attribute this difference in boiling points to differences in the dipole moments of the molecules. Both molecules are polar and exhibit comparable dipole moments. The large difference between the boiling points is due to a particularly strong dipole-dipole attraction that may occur when a molecule contains a hydrogen atom bonded to a fluorine, oxygen, or nitrogen atom (the three most electronegative elements). The very large difference in electronegativity between the H atom (2.1) and the atom to which it is bonded (4.0 for an F atom, 3.5 for an O atom, or 3.0 for a N atom), combined with the very small size of a H atom and the relatively small sizes of F, O, or N atoms, leads to highly concentrated partial charges with these atoms. Molecules with F-H, O-H, or N-H moieties are very strongly attracted to similar moieties in nearby molecules, a particularly strong type of dipole-dipole attraction called hydrogen bonding. Examples of hydrogen bonds include HF⋯HF, H2O⋯HOH, and H3N⋯HNH2, in which the hydrogen bonds are denoted by dots. Figure 1 illustrates hydrogen bonding between water molecules.
Despite use of the word “bond,” keep in mind that hydrogen bonds are intermolecular attractive forces, not intramolecular attractive forces (covalent bonds). Hydrogen bonds are much weaker than covalent bonds, only about 5 to 10% as strong, but are generally much stronger than other dipole-dipole attractions and dispersion forces.
Hydrogen bonds have a pronounced effect on the properties of condensed phases (liquids and solids). For example, consider the trends in boiling points for the binary hydrides of group 15 (NH3, PH3, AsH3, and SbH3), group 16 hydrides (H2O, H2S, H2Se, and H2Te), and group 17 hydrides (HF, HCl, HBr, and HI). The boiling points of the heaviest three hydrides for each group are plotted in Figure 2. As we progress down any of these groups, the polarities of the molecules decrease slightly, whereas the sizes of the molecules increase substantially. The effect of increasingly stronger dispersion forces dominates that of increasingly weaker dipole-dipole attractions, and the boiling points are observed to increase steadily.
If we use this trend to predict the boiling points for the lightest hydride for each group, we would expect NH3 to boil at about −120 °C, H2O to boil at about −80 °C, and HF to boil at about −110 °C. However, when we measure the boiling points for these compounds, we find that they are dramatically higher than the trends would predict, as shown in Figure 3. The stark contrast between our naïve predictions and reality provides compelling evidence for the strength of hydrogen bonding.
Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019, February 14). Chemistry 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/books/chemistry-2e
Research Article: Structure, hydrogen bonding and thermal expansion of ammonium carbonate monohydrate
Date Published: December 01, 2014 Publisher: International Union of Crystallography Author(s): A. Dominic Fortes, Ian G. Wood, Dario Alfè, Eduardo R. Hernández, Matthias J. Gutmann, Hazel A. Sparkes. http://doi.org/10.1107/S205252061402126X Abstract: Single-crystal neutron diffraction, ab initio calculations and Raman spectroscopy are applied to understand the structure and hydrogen bonding of ammonium carbonate monohydrate, a hitherto poorly … Continue reading
Date Published: June 17, 2011 Publisher: Public Library of Science Author(s): Hongtao Zhao, Danzhi Huang, Peter Butko. http://doi.org/10.1371/journal.pone.0019923 Abstract: Ligand binding involves breakage of hydrogen bonds with water molecules and formation of new hydrogen bonds between protein and ligand. In this work, the change of hydrogen bonding energy in the binding process, namely hydrogen bonding penalty, … Continue reading
Research Article: Deciphering the hydrogen-bonding scheme in the crystal structure of triphenylmethanol: a tribute to George Ferguson and co-workers
Date Published: September 01, 2019 Publisher: International Union of Crystallography Author(s): Tomasa Rodríguez Tzompantzi, Aldo Guillermo Amaro Hernández, Rosa Luisa Meza-León, Sylvain Bernès. http://doi.org/10.1107/S2053229619010714 Abstract: The disordered crystal structure of triphenylmethanol features tetrahedral chiral clusters formed through weak hydrogen bonds, leading to the formation of three-dimensional supramolecular motifs having left or right handedness. Partial Text … Continue reading