![]() Importantly, the types of atoms involved in both the H-bridge donor and acceptor have been amplified. ![]() Evidence can be experimental ( e.g., Infrared spectra, Nuclear Magnetic Resonance spectra, neutron diffraction crystallography, determination of Gibbs energy of formation) or calculational, or (preferably) both ( Arunan et al., 2011b). These recommendations both broadened the definition of an H-bridge as well as insisting on evidence before claiming its presence. Y forms an angle of ~180°Ĥ) The average bond length X-H increases as the H-bond forms X-H.The main criteria are paraphrased below ( Arunan et al., 2011b).ġ) H-bridges are formed by inter-species forces, which might contain various componentsĢ) X-H before H-bridge formation is a polar covalent bond, with a polarity indicated by δ- X-H δ+ģ) The equilibrium geometry of X-H Y, Y-Z) are important to define the H-bridge ( Desiraju, 2011).įor the definition of an H-bridge, the IUPAC task group provides several criteria.Thus, the formation of an H-bridge requires that X-H and Y-Z (separated in space initially) approach each other to form X-H Y-Z, where the three dots indicate the non-covalent interaction.The interaction between X-H and Y-Z forms a new species X-H Perhaps the most clarifying proposal by IUPAC was the insistence that H-bridges exist between an H-bridge donor made up of X-H, where X is an abbreviation of an atom or group of atoms connected to a hydrogen atom, and an H-bridge acceptor, made up of Y-Z, where Y is the site of interaction with H and Z is an atom or group of atoms ( Arunan et al., 2011a). For these reasons, and to provide a twenty-first century update after 100 years of the first publication on the water dimer ( Arunan, 2020 Scheiner, 2020), we will use the term ‘H-bridge’ throughout this article. More importantly, for teaching purposes in English-speaking chemistry courses, the differentiation between ‘bond’, which most students associate with a covalent bond ( Niaz, 2001), and the non-covalent ‘bridge’ is important to remove confusion due to polysemy. ‘Bridges’ are also used frequently for interactions in biological structures ( Jeffrey & Saenger, 1991). Today, the term ‘bridge’ is still extant in Spanish-speaking (puente de hidrógeno) and German-speaking (Wasserstoffbrücke) chemistry courses. Both H-bond and H-bridge were in common use until the 1930s ( Huggins, 1936), at which point Pauling’s influence tipped the balance toward ‘bond’ ( Quane, 1990). There are several reasons to agree with this recommendation. One of the members of the IUPAC task group, Desiraju, recommends the term H-bridge be used to describe this interaction ( Desiraju, 2002 Desiraju, 2011). For most of the twentieth century, both instructors and students of introductory chemistry courses have been left uncertain by the many concepts involved in hydrogen bonding, without a clear guide to organize these concepts. Within a group (family) of elements, atoms form ions of a certain charge.In 2011, the International Union of Pure and Applied Chemistry (IUPAC) published recommendations for the definition and determination of hydrogen bonds (H-bonds or H-bridges) ( Arunan et al., 2011b). The charge that an atom acquires when it becomes an ion is related to the structure of the periodic table.
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