Cellulose
flexible and ductile materials. Both aspects are rele- vant to cellulose.
interaction and London dispersion (van der Waal’s) interaction contribute to the intermolecular forces in cellulose materials. Notably, even if the dispersion force is the weakest and most short-ranged of them when considering just a pair of atoms, their additivity makes the total contribution to the force between supra-molecular objects both considerably larger in magnitude and also significantly more long-ranged (Hamaker 1937), to the end that they can become the dominating interaction between two (uncharged) molecular surfaces.
The physical chemistry of hydrogen bonds
Intermolecular interactions are complex and often – even in leading textbooks – their description is heavily simplified and sometimes even incorrect (Truhlar 2019). Hydrogen bonding is a simple name for a complex situation, even in the case of two isolated molecules. Its contributing molecular elements are conventionally named as ‘‘donor’’ (D) and ‘‘acceptor’’ (A) where, provided that both A and D are sufficiently electronegative, D supposedly donates a proton that A accepts, forming the H-bond system D-H A. For two isolated molecules, the experimental standpoint is simple: in this system there is (i) a positive charge density on H, negative charge density on A and therefore there will be an electrostatic term contribut- ing, and (ii) a nonzero electron density between H and A as unequivocally shown by the non-zero hyperfine coupling between D and A (Grzesiek et al. 2004; Dracˇ´ınsky´ and Hodgkinson 2015). Compton scattering experiments concur (Isaacs et al. 1999). This latter feature suggests that there is charge transfer or, if one wishes, covalency over the H A bond. In addition, induction and dispersion terms are contributing as well. There is some argument going on over the relative importance of the different contributions, and the extent of the covalent nature of the bond (Grabowski 2011). However, the clear correlation between the electron density between H and A and the bond strength suggests that the covalency cannot be entirely negligible (Shahi and Arunan 2014). Even with the difficulties of uniquely defining H-bonding (the IUPAC definition from 2011 is quite vague (Arunuan et al. 2011)) it undoubtedly exists, and its energies spread from weak (about 1 k B T in C-H C bonds), through moderate (5-15 k B T , ‘‘normal’’ O- H O bonds) to very strong ( [ 50 k B T , in HF). A final complication is that H-bonding is cooperative (Ma- hadevi and Sastry 2016). This means that a bond which is part of an extended H-bond network may be different in strength from the isolated bond. Specifi- cally, for the case of cellulose the intermolecular H-bond strength was shown to increase with the number of glucose units, and to plateau at a degree of polymerization of four (Qian 2008). In addition to H-bonding, ionic interaction, electrostatic multi-pole
Modeling and simulation of hydrogen bonds
In quantum chemistry (QC), some profound issues concerning hydrogen bonds remain unsettled and controversies seem to prevail about methodology, specifically the choice of base functions and its consequences for covalency, namely that some models predict significant charge transfer, whereas some predict much less (Stobe 2017; Weinhold and Glen- dening 2018; Stone and Szalewicz 2018). A related problem is that QC estimates of H-bond strengths remain uncertain, and so does the relative importance of the electrostatic, charge transfer, induction and dispersion terms. Moreover, QC cannot tell exactly what the angular dependence of the bond strength is, that is how directional are the H-bonds (Gilli and Gilli 2009). Probably, this depends on the exact D-H A system. Molecular dynamics (MD) simulations are rou- tinely used to study molecular-scale structure and dynamics in most fields of material science, and there are several optimized parameter sets (force fields) specifically designed for simulations of carbohydrates, such as GLYCAM06 (Kirshner and Woods 2001), CHARMM 36 (Guvench et al. 2008, 2009), and GROMOS56 CARBO (Hansen and Hu¨nenberger 2010). Most contemporary empirical biomolecular force fields do not employ any special H-bonding potential. The non-bonded interactions are usually limited to the Coulomb potential, which acts pairwise between fractional charges that are distributed over the atoms with a distance dependence of r 1 , and the Lennard- Jones potential, which describes repulsion with an r 12 , term, and dispersion attraction with a r 6 , term. Nevertheless, H-bond configurations form anyway as a consequence of a favorable combination of Coulomb
123
Made with FlippingBook Online document maker