Dissociation of cellulose is of great significance for the application of cellulose in the field of cosmetic, pharmaceutical, food, and textile industries, etc. [[1], [2], [3], [4]]. In the past four decades, several cellulose solvents have been developed for both industrial and laboratory use [5]. These reported cellulose solvents can be divided into two categories, derivatization (e.g., in the viscose process) and dissociation of cellulose in direct solvents. For the derivatizing solvents a series of cellulose derivatives, such as cellulose nitrate [6], cellulose xanthate [7], cellulose carbamate [8], and cellulose formate [9], etc., was successfully synthesized and found to be dissolved in diethyl ether/ethanol, alkali, NaOH, and DMSO/DMF/pyridine systems, respectively. To date, the traditional commercialized viscose process (cellulose xanthate) is the dominating method for cellulose shaping [10]. However, for the viscose process the toxicity of CS2 and the byproducts lead to serious environmental pollution [[11], [12], [13]]. Therefore, development of environmental-friendly solvents for dissolving cellulose have attracted considerable attentions in the past five decades. Several direct solvents such as ionic liquids, aqueous alkali solution, N-methylmorpholine-N-oxide, and N,N-dimethylacetamide/lithium chloride (DMAc/LiCl) have been demonstrated to be effective for breakage of hydrogen bonds between cellulose chains [[14], [15], [16], [17]] . Among them, DMAc/LiCl is a typical system for direct dissociation of cellulose and plays a significance role for development of cellulose science and technology [[18], [19], [20]]. However, the dissociation mechanisms of cellulose in DMAc/LiCl system are still debated.

According to 13C [21] and 7Li NMR, infrared spectroscopy, X-ray crystallography [22], electrospray ionization mass spectrometry [23], thermochemistry [24], and density functional theory (DFT) calculations, the Li+ ion has been found to be attached to the carbonyl oxygen atom of DMAc molecule to form a [Li(DMAc)x]+ cationic complex in cellulose/DMAc/LiCl system [25]. And the Cl− ions were observed to replace the hydrogen bonds between cellulose chains with the O–H‧‧‧Cl hydrogen bonds in the DMAc/LiCl solution [[26], [27], [28]]. According to these findings, three possible dissociation mechanisms of cellulose in DMAc/LiCl solution have been proposed. McCormick and co-workers found that the Li+ ion did not directly attach to the oxygen atoms of cellulose. Alternatively, [Li(DMAc)x]+ cationic complex is relative to Cl− ions via a strong electrostatic force in cellulose/DMAc/LiCl solutions [29] . Morgenstern et al. proposed that Li+ ion is attached to the oxygen atoms of cellulose chain because they found that the 7Li NMR chemical shift is closely associated with the cellulose concentration [30]. And one DMAc molecule coordinated to the Li+ was replaced by one hydroxyl group of cellulose when dissolving cellulose in DMAc/LiCl. The molecular dynamics simulations also indicated that the close interaction between Li+ ions and glucan chains in DMAc derived cellulose dissociation [31]. Liu and Huang found that splitting of Li+–Cl− ion pairs is induced by the formation of strong hydrogen bonds between Cl− ion and the hydroxyl protons of cellulose. And the Li+ ions are further solvated by the free DMAc molecules, which associated with the hydrogen-bonded Cl− ion to meet electric balance [26]. Apart from the mechanisms above, Isogai suggested that the solution-state structures of cellulose in DMAc/LiCl system contain similar interactions between the cellulose hydroxyl groups and Li+ and Cl− based on the observations from model molecule of pullulan in DMAc/LiCl solution [28].

In fact, the bonding interaction of Li and solvent and ligand molecules has been well studied by using various technologies in the past six decades. Referring to H bond theory, the existence of Li bond was proposed by Shigorin in 1959 because both Li and H ions are monovalent electropositive particle and display some analogous behavior as interaction with molecules [32]. Although the Li bond is analogous to the H bond theory, they have significant differences [[33], [34], [35]]. Firstly, the saturation and directionality of Li bond are not restricted to di-coordination case because of the large radius and strong metallic nature of Li atom relevant to H atom. Such as, X-ray and neutron scattering measurements on Li+ solution in liquid water showed that the Li+ ion has six near-neighbor water molecule partners [36,37]. However, that results have not been entirely uniform when compared with the studies of similar aqueous solution containing Li+ ions. The spectroscopic studies provided a tetrahedral coordination sphere around the Li+ ion in water [38,39]. Meanwhile, theoretical studies suggested that the Li+ ion with six water molecules has a low-energy structure with four inner shell and two outer shell water molecules relative to the structures with six water molecules in the innermost shell [40]. Furthermore, the coordination number of Li+ ion in liquid water was investigated by Pratt and co-workers based on their DFT and ab initio molecular dynamics calculations. The results indicated that Li+ center has four inner shell water ligands in liquid water [36]. Secondly, the H bonding interaction have been found to have a partial covalent character based on 1H NMR measure [41]. By contrast, the Li bond possesses more electrostatic character due to the metallic nature of Li elements. Thirdly, the bonding energy of Li bond is found to be 0.64 eV in a Li2S6@pyridinic nitrogen in graphene, which is significantly larger than that of H bond (0.04–0.22 eV) [42]. And the bonding energy of Li bond is significantly smaller than that of typical LiO (3.53 eV) and LiF bonds (5.94 eV) [43]. Fourthly, the barrier of the Li transfer process is lower than that of H because the Li atom in Li bond complexes is more shared between the coordination atoms [44]. Moreover, the barrier of the Li transfer is less sensitive to the chemical environment around the Li center, which is consistent with the greater ionic nature of Li bond.

In this study, the dissociation mechanism of cellulose in DMAc/LiCl solution was systematically explored by using DFT calculations, Fourier transform infrared spectroscopy, and 7Li NMR spectroscopy. The molecular geometries and electronic structure of the series of Lix(DMAc)yClz complexes have been discussed based on Li bond theory. Several cellulose-base complexes corresponding to the breakage of hydrogen bonds between cellulose chains by these Lix(DMAc)yClz complexes were also characterized by using both experimental and theoretical methods.