Hydrogen Bonding in Ionic Liquids
It has been observed in the previous chapter that the hydrogen bonding plays an important role in COSMO-SAC predictions. One of the definitions of hydrogen bonding was formulated by Pauling who stated that (Grabowski, 2011) '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.' Pauling also pointed out that the hydrogen bond is situated only between most electronegative atoms and it usually interacts strongly with one of them. The later interaction is a typical covalent bond (A-H), where A is an electronegative atom like oxygen, nitrogen and fluorine and H is hydrogen. The interaction between the hydrogen and the other electronegative atom is much weaker and mostly electrostatic in nature; it is a nonbonding interaction (H - • • B). This system is often designated as A-H—B where B-centre (acceptor of proton) should possess at least one lone electron pair. A-H is called the proton donating bond. Pauling stated that sometimes the H— B interaction possesses characteristics of a covalent bond. In addition to the conventional H-bond, there are blueshifted H-bonds, dihydrogen A-H—H-B H-bonds, inverse H-bonds, resonance-assisted H-bonds, charge-assisted H-bonds, ionic H-bonds and C-H—H, X-H—n, n—H—n and even H—e—H H-bonds (Hunt, Ashworth, & Matthews, 2015). Thus Steiner proposed the definition of the H-bond as follows: an X-H—Y interaction is called a H-bond if it constitutes a local bond and X-H acts as a proton donor to Y (Steiner, 2002). Hydrogen bonding can be defined in terms of polarization of charges. For a strong acid, the polarization of charge will leave the donor H-atom that is electron deficient and highly positively charged. This donor H-atom will pull electron density from base and form the H-bond. A strong base will similarly push its electron density to the donor and will form the H-bond. This push and pull will depend on electronegativity, hardness/softness and polarizability of acid and base. The [FHF]- ion is an example where the proton is inserted between two negative fluorine ions, accurately in the middle of the F---F distance. Hence, both H - • • F interactions are equivalent. But the O-H - • • O and N-H— N hydrogen bonds are not equivalent because fluorine is the most electronegative atom. In case of O-H—O and N-H---N, stronger covalent bonds as well as weaker hydrogen bonds are observed. Although weaker, these hydrogen bonds contribute considerably in the physical properties like boiling point of water.
With the basic introduction of H-bonding, we concentrate our discussion on formation of hydrogen bonding in ILs. A significant overlap over these two topics has been established in recent years and predictive models are thus improved. It is possible to partition H-bonding into electrostatics, charge transfer, dispersion, polarization and exchange-repulsion terms. Thus the predictive models like COSMO-SAC are also upgraded and include structural and electronic characterization of H-bonding. Within an IL, a significantly different type of H-bonding can be formed which is unlikely for other molecular solvents. The H-bonding of IL is referred as doubly ionic hydrogen bond; primarily to differentiate from other ionic H-bonding. Based on the ability of forming H-bonding, ILs are divided into two broad classes, namely, protic and aprotic ILs. Protic ILs typically consist of Bronsted acid and Bronsted base where a proton is transferred from acid to base. Ammonium-based cations are an example of protic ILs where hydrogen is covalently bonded with cationic charge (either P or N atoms). In aprotic ILs, cations have a C-H unit attached which is the primary H-bond donor unit. Thus the indirect attachment of the H-bond donor unit makes aprotic ILs weaker H-bond forming materials than protic ILs. Despite having a tendency of forming strong H-bonding, protic ILs are less studied. Typical H-bonding formation for an ammonium-based IL is N-H—Y, whereas for aprotic ILs, the representation is C-H—Y. It is believed that proton transfer has a substantial impact on the physical and chemical properties of protic ILs. Networking is another feature of ILs due to very high density of H-bonding among ionic species. If the numbers of acceptor and donor sites are equal, that is, perfectly matched, rigid networks are formed. When ILs are mixed with other H-bonding species, additional donor/acceptors make a substantial impact on overall H-bonding. Imidazolium cations are the most widely studied IL cations. For imidazolium cations, the C-H unit is considered as the most probable donor unit. Intermediate carbene is formed by abstracting a proton from methyl and methylene groups. By increasing structural complexity, H-bonding characteristics of anions are increased. Bis-trifuoromethyl imide (Tf2N) is considered among most complex anions in IL family because of electron-rich central N atom and pendant groups, containing O and N atoms (Hunt et al., 2015). In this chapter, we will discuss extraction of biooil-derived chemicals by IL where hydrogen bonding of imidazolium-[Tf2N]- based ILs will be modelled by COSMO-SAC theory.