DFT study on the dissolution mechanisms of α-cyclodextrin and chitobiose in ionic liquid

Accepted Manuscript Title: DFT study on the dissolution mechanisms of ␣-cyclodextrin and chitobiose in ionic liquid Authors: Bobo Cao, Jiuyao Du, Ziping Cao, Xuejun Sun, Haitao Sun, Hui Fu PII: DOI: Reference:

S0144-8617(17)30385-5 http://dx.doi.org/doi:10.1016/j.carbpol.2017.04.012 CARP 12200

To appear in: Received date: Revised date: Accepted date:

17-2-2017 1-4-2017 6-4-2017

Please cite this article as: Cao, Bobo., Du, Jiuyao., Cao, Ziping., Sun, Xuejun., Sun, Haitao., & Fu, Hui., DFT study on the dissolution mechanisms of ␣-cyclodextrin and chitobiose in ionic liquid.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.04.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

DFT study on the dissolution mechanisms of α-cyclodextrin and chitobiose in ionic liquid Bobo Cao1, Jiuyao Du1, Ziping Cao1, Xuejun Sun1*, Haitao Sun1*, Hui Fu2* 1

School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, Shandong,

273165, China 2College

of Science, China University of Petroleum, Qingdao, Shandong, 266580, China

Graphical abstract

As the potential substitute of fossil fuels, cyclodextrin and chitosan dissolved in [Emim][OAc] were investigated using DFT method, in which non-covalent interactions were identified as the driving force. Both anion and cation in ionic liquid interacting with cyclodextrin and chitosan contributed to the dissolution reaction. AIM, RDG and NBO analyses were employed to characterize the nature of the intermolecular interactions.

1

Highlights 

Dissolution mechanism of cyclodextrin and chitosan in [Emim][OAc] is studied.



[Emim]+ and [OAc]- interacting with carbohydrate contribute to the dissolution.



Non-covalent interactions are identified as the driving force of the dissolution.



Non-covalent interaction in the dissolution is dominated by hydrogen bonding.

Abstract Density functional theory (DFT) was employed to study the dissolution mechanisms of

α-cyclodextrin

and

chitobiose

in

1-ethyl-3-methyl-imidazolium

acetate

([Emim][OAc]). Geometrical analysis of the studied complexes indicated that both anion and cation in ionic liquid interacting withα-cyclodextrin and chitobiose contributed to the dissolution reaction. Intermolecular interactions in the complexes were identified as non-covalent interactions, such as hydrogen bonds, van der Waals interactions and repulsions, which were considered as the driving force of dissolution. Among them, hydrogen bonding interactions played a dominant role, which was further visualized in the real space by combination of atoms in molecules (AIM) and reduced density gradient (RDG) techniques. The nature of intermolecular orbital interactions was characterized using natural bond orbital (NBO) theory.

Keywords: Density functional theory; Ionic liquid; Chitobiose; α-Cyclodextrin; Dissolution mechanism

2

1. Introduction The increasing consumption of fossil fuels has caused severe disasters such as environmental pollution, resources exhaustion and greenhouse effect. It severely affects human survival and development. Polysaccharides are polymeric carbohydrate molecules constructed from monosaccharide unit, which are widely distributed renewable resources in nature and used as substitute of fossil fuels (West, Tucker, Braden, & Dumesic, 2009; Yao & Tang, 2013). Cyclodextrins (CDs), chitosan and cellulose are made up of thousands of D-glucosamine or D-glucose units linked by β-(1-4) glucosidic bonds (Buschmann, Knittel, & Schollmeyer, 2001; Feng, Yu, Hu, & Zhao, 2003; Streubel, Huber, Wöhrer, Chott, & Raderer, 2015). Hexagonal boron nitride nanosheets (hBN) functionalized by the monomer of chitosan has been studied to explore the possible technological applications of nanomaterial (Anota, Rodríguez, & Cocoletzi, 2013; Rodríguez-Juárez, Hernández-Cocoletzi, & Chigo-Anota, 2015). Their properties, including high biocompatibility, renewability and low toxicity, make them widely utilized in papermaking, biomedicine, water-quality improvement, drug deliveries and other light industries (Glisoni et al., 2015; Senra, Khoukh, & Desbrières, 2017; X. F. Sun, Chi, & Mu, 2014; Vyas, Saraf, & Saraf, 2008; Xue et al., 2016). Unfortunately, the application of carbohydrate polymers is suffered greatly from poor solubility in neutral or basic solvents, which is attributed to the large amounts of hydrogen bond between molecules (Amarasekara, Williams, & Ebede, 2008; Qian & Zhang, 2010). Many organic/inorganic acid systems have been developed for carbohydrate dissolution. However, drawbacks in those systems are also found, such as volatile, pollution, corrosive, toxic (Geng, Kwon, & Jang, 2005; Rinaudo, Pavlov, & Desbrières, 1999; Sashiwa, Kawasaki, Nakayama, Muraki, & Aiba, 2002). Thus, it is critical to seek and develop highly effective and sustainable solvents for carbohydrate polymers. Ionic liquid (IL), referred to “green” solvent, is currently regarded as a potential and attractive replacement for conventional solvent due to its tunable properties by the choice of ion pairs, including negligible vapor pressure, designability and high 3

thermal and chemical stability (B et al., 2017; Dr & Prof, 2000; Jordan & Gathergood, 2015; Kudłak, Owczarek, & Namieśnik, 2015; Rogers & Seddon, 2003; H. Sun, Cao, Du, et al., 2016; Yan & Mu, 2015). Although many efforts have been devoted to study the recyclability of ILs in recent years, their recycling is still a severe problem in the industrial application (Cao et al., 2017; Li, Xue, Cao, Yan, & Mu, 2016). It was reported that imidazolium-based ILs could readily dissolve cellulose without any byproducts, which led to an increasing and continuing exploration on carbohydrate polymers dissolution in ILs (Cao et al., 2016; Pinkert, Marsh, Pang, & Staiger, 2009; X. Sun, Xue, & Mu, 2014; X. F. Sun et al., 2014; Swatloski, Spear, And, & Rogers, 2002). Recently, people have been working on the dissolution of chitosan in ILs. However, systematically theoretical and computational investigations on the dissolution mechanism of cyclodextrin and chitosan in ILs are rare (Liu, Janssen, Rantwijk, & Sheldon, 2005; X. Sun, Tian, Xue, Zhang, & Mu, 2014). The solubility of α-cyclodextrin was tested in [Bmim]Cl, [Bmim][BF4] and [Bmim][PF6], respectively (Armstrong, He, & Liu, 1999). It was reported that α-cyclodextrin can be dissolved in 1-methoxymethyl-3-methylimidazolium bromide with the high concentrations (And & Nakashima, 2001). Previous works suggested that [Bmim][OAc] showed a higher carbohydrate dissolving capacity than [Amim]Cl and [Bmim]Cl (Wu, Sasaki, Irie, & Sakurai, 2008). Wang et al. reported that anion of IL played an important role in dissolving process by disrupting native hydrogen bonds in carbohydrate polymers (Chen, Xu, Li, Wang, & Zhang, 2011). Mu et al. studied the tail effect of cation on the solubility of chitosan in acetate-based ILs within the temperature range of 40-140℃ (X. Sun, Q. Tian, et al., 2014). The solubility and modification of carbohydrate polymers in ILs have gained tremendous interest in many fields (Le, Viau, & Vioux, 2011; Rogers & Seddon, 2003; Yang, Wang, & Yu, 2014; Zakrzewska, BogelŁukasik, & BogelŁukasik, 2010). It is imperative to understand their dissolution mechanism in ILs to develop highly efficient and “green” solvent systems for carbohydrate polymers. Herein, density functional theory (DFT) method is employed to reveal the dissolution

mechanisms

of

α-cyclodextrin 4

and

chitosan

in

1-ethyl-3-methylimidazolium acetate ([Emim][OAc]), respectively. To overcome the limitations in biomacromolecule calculations, chitosan and cyclodextrin are modeled by chitobiose and α-cyclodextrin for stimulating their electronic natures and chemical environments, respectively (Fig. S1). DFT computations are originally carried out to optimize structures and understand the fundamental bonding concepts in the studied system. Possible electrophilic/nucleophilic sites, dominated by electrostatic interactions, are predicted by electrostatic potential (ESP) method. Inter- and intramolecular non-covalent interactions are studied in atoms in molecules (AIM), natural bond orbital (NBO) and reduced density gradient (RDG).

2. Computational details Chitobiose is widely used as the model compound of chitosan in computational chemistry and the applicability of the results has been verified by previous works. (Deka & Bhattacharyya, 2015; Mu, Yang, Zhang, & Liu, 2016; Terreux, Domard, Viton, & Domard, 2006) The M06-2X density functional and the 6-311++G** basis set are employed in the structure optimization processes. The M06-2X method is a highly parameterized empirical exchange-correlation functional, which is widely used to describe long-range effects, such as hydrogen/halogen bonding and van der Waals interaction (Walker, Harvey, Sen, & Dessent, 2013). Dispersion forces included in the M06-2X functional are essential, especially in studying the weak interactions of the IL systems (H. Sun, Cao, Tian, et al., 2016; Zhao, Schultz, & Truhlar, 2005; Zhao & Truhlar, 2008). The 6-311++G** basis set contains diffuse functions and polarizable functions, which is widely used in DFT studies of carbohydrate polymers system (Bobo Cao et al., 2016; Loerbroks, Rinaldi, & Thiel, 2013). The multiplicity study indicates that the optimized states (multiplicity=1) correspond to the true minima on the potential energy surface, which is shown in Table S1 (Supporting information). The stability of structures and interactions has been confirmed following the minimum energy criterion and no imaginary frequency (See Fig S2 and S3) is found in present work. The basis set’s superposition error (BSSE) is corrected in 5

counterpoise method (BOYS & BERNARDI, 2002). Electrostatic potential (ESP) is performed as a useful method to predict possible sites of electrostatically-driven interaction (Fu, Lu, & Chen, 2014; T. Lu & F. Chen, 2012). AIM (R. F. W. Bader, 1985) and RDG (Johnson et al., 2010) methods are performed to study the nature of interactions between ILs with chitobiose and α-cyclodextrin by employing Multiwfn program (Tian Lu & Feiwu Chen, 2012). NBO (Glendening, Landis, & Weinhold, 2012) method is performed to study orbital interactions supported by Gaussian09 program (Frisch et al., 2003). The localized orbital interactions, involved in non-covalent interactions, are quantified using second order perturbation theory, in which the second-order energy (E(2)) was used to measure the interaction strengths. It is worth mentioning that all the wavefunctions used in this work are generated by Gaussian09 program. Geometric positions of the studied structures are given in the Supporting information. ESP technique is commonly applied for the prediction possible reactive sites of electrostatically-driven interaction in quantum chemistry (B. Cao et al., 2015; T. Lu & F. Chen, 2012; Murray & Peter, 2002; Politzer & Murray, 2007), which is defined as: 𝑉Total (𝑟) = 𝑉𝑁𝑢𝑐 (𝑟) + 𝑉𝐸𝑙𝑒𝑐 (𝑟) = ∑𝐴

𝑍𝐴 ︱r−𝑅𝐴 ︱

−∫

𝜌(𝑟 ′ ) ︱r−𝑟 ′ ︱

𝑑𝑟 ′

(1)

where 𝑅𝐴 and 𝑍𝐴 are radius vector and nuclear charge of atom A, respectively. The positive and negative values correspond to the positions in blue and red regions, which imply that the current positions are dominated by nuclear or electronic charges, respectively.

3．Results and discussions 3.1 Reactive site predictions based on ESP analyses Computed ESPs of reactants are depicted in Fig. 1, which are performed on molecular van der Waals (vdW) surface (Richard F. W. Bader, Carroll, Cheeseman, & Chang, 2002; Politzer & Murray, 2002). The ESP plots of chitobiose/[Emim][OAc] and α-cyclodextrin /[Emim][OAc] complexes are depicted in Fig. S4, respectively.

6

Fig. 1 shows that the [Emim]+ cation is an electron-deficient group, partially delocalized heteroaromatic ring. On the contrary, [OAc]- anion is an electron-rich group, partially delocalized oxygen atoms of carbonyl. The ESP plot of [Emim]+/[OAc]- suggests that an interacting electron-rich/ electron-deficient group is more favorably reside at the increased positive/negative potential region (deep blue/red). Distinctly, ESP distribution on the surface of chitobiose is relative uniform. The positive potential regions (deep blue) are associated on hydrogen atoms of hydroxyl and amino. However, the negative potential regions are associated with the lone pairs of oxygen in hydroxyl. As expected, these correspond to the front and side sites for [OAc]- and [Emim]+ location around chitobiose, respectively (Fig. S4). This analysis can be extended to predict possible reactive sites on the molecule surface of α-cyclodextrin. Comparing with sole reactant and ion, both of positive regions (deep blue) and negative regions (deep red) reduced on formation of the complexes and [Emim][OAc] ion pair. 3.2 Geometrical analyses Herein, mutual penetration distance (𝑑𝑝 ) of vdW surfaces is introduced to describe the structural features and the strength of non-covalent interactions. [58-60] 𝑑𝑝 is defined as the difference between the distance of X-Y and the sum of their non-bonded radii in a non-covalently interacting atom pair XY. The structures of [Emim][OAc], chitobiose and α-cyclodextrin, obtained by DFT global optimization, are

depicted

in

Fig.

S1,

respectively.

The

complex

structures

of

chitobiose/[Emim][OAc] and α-cyclodextrin/[Emim][OAc] are shown in Fig. 2, respectively. Fig. S1 shows that the computed H···O distances (connected by dotted line) in [Emim][OAc] are 2.01, 2.17 and 2.21 Å, respectively, which are distinctly shorter than the summation (2.72 Å) of vdW radii of oxygen and hydrogen atoms (R. F. W. Bader, 1985). Their corresponding bond angles are 127.6, 143.5 and 111.2 degree, respectively. These parameters indicate that the formation of hydrogen bonds between cation and anion, and the driving structural feature of ILs is Coulomb force (Matthews, Welton, & Hunt, 2015). 7

In an endeavor to understand the significance of non-covalent interactions for the structure

variations

of

carbohydrate

polymers

in

[Emim][OAc],

different

conformations of chitobiose/[Emim][OAc] complex are originally designed and optimized. Structures A and B are chosen as stable conformations of chitobiose/[Emim][OAc] complex, which are given in Fig. 2, respectively. The following discussions are mainly focused on structure A because of its lower-energy conformer. Structure A contains many inter- and intramolecular hydrogen bonds. Intermolecular hydrogen bonds, O(68)···H(23)-O(22) and O(67)···H(20)-N(19), are formed between chitobiose and [OAc]- anion with the bond lengths are 1.70 and 2.14 Å, respectively. Distinctly, 𝑑𝑝 of the former (1.02 Å) is larger than the latter (0.58 Å). Generally, the larger 𝑑𝑝 corresponds to the stronger hydrogen bonding interaction strength. Simultaneously, the intramolecular hydrogen bonds, O(34)···H(23)-O(22) and O(18)···H(23)-O(22), are disrupted, in which the O···H distance increase from 2.39 Å in chitobiose to 3.03 and 3.16 Å in structure A, respectively. They are larger than the sum (2.72 Å) of non-bonded radii of oxygen and hydrogen atoms. Electron-deficient hydrogens in imidazole ring are favorably to interact with electron-rich oxygens in chitobiose by hydrogen bonding interactions, such as O(16)···H(58)-C(55), O(16)···H(54)-C(53) and O(12)···H(54)-O(53). It indicates that chitobiose dissolution process in [Emim][OAc] is influenced by not only anion but also cation. Interestingly, an intramolecular hydrogen, N(44)···H(17)-O(16) bond length decreases from 2.10 to 2.06 Å, which is attributed to the decrease of electron cloud density between O(16) and H(17) atoms by formatting intermolecular hydrogen bonds. The bond elongation and weakening of O-H (i.e. O(16)-H(17)) are well known features of the hydrogen bonds. These analyses indicate that chitobiose dissolution process in [Emim][OAc] is influenced by not only anion but also cation. Structure

C

in

Fig.

2

shows

the

optimized

structure

of

α-cyclodextrin/[Emim][OAc] complex, in which intermolecular hydrogen bonding interactions are also found between α-cyclodextrin with both anion and cation. The analyses above can be expanded to this system. 8

3.3 AIM analyses To understand the dissolution mechanisms, AIM theory are performed based on the wavefunctions based on the optimized structures. Poplier’s criteria, based on electron density topology, are employed in this section (Koch & Popelier, 1995). The molecular graphs of [Emim][OAc], chitobiose and α-cyclodextrin are given in Fig. S5, respectively.

The

molecular

graphs

of

the

chitobiose/[Emim][OAc]

and

α-cyclodextrin/[Emim][OAc] complexes are depicted in Fig. S6, respectively. Topological parameters mainly discussed in this section are presented in Table 1, where 𝜌, 𝛻 2 𝜌 and 𝐻𝐵𝐶𝑃 are electron density, the Laplacian of electron density and total electron energy density at bond critical point (BCP), respectively. AIM calculations shows that the BCPs and the linear bond paths exist between oxygen/nitrogen and hydrogen system for studied system (Fig. S6), implying the presence of hydrogen bonding interactions, which is the first criterion. Generally, the bond strength is measured by the magnitude of the 𝜌 value. Table 1 suggests all the values of 𝜌 are within the range of 0.0119-0.0673 a.u. and it can be compared to the range of 0.002-0.0274 recommended by Koch and Popelier. It is obviously the larger values are found in both the upper and bottom limits in present work because topological parameters do depend on computational method and basis set (Shahi & Arunan, 2014). The values of 𝜌 in α-cyclodextrin/[Emim][OAc] are larger than those in chitobiose/[Emim][OAc], except for O(68)···H(23)-O(22). The sign of 𝛻 2 𝜌 is generally used to differentiate bonding types from closed-shell (hydrogen bond, ionic bond and van der Waal interaction) to shared-shell (covalent bond) interactions. For the details, the negative (positive) 𝛻 2 𝜌 value reveals electron accumulation (charge depletion) in interatomic interaction. Additionally, 𝐻𝐵𝐶𝑃 < 0 and 𝛻 2 𝜌 > 0 at BCPs are commonly considered as partly covalent in nature is included in the interaction (Cremer & Kraka, 1984; Shahi & Arunan, 2014). The positive 𝐻𝐵𝐶𝑃 and 𝛻 2 𝜌 values in Table 1 indicate that the O(N)···H interactions, between [Emim][OAc] with chitobiose and α-cyclodextrin, are hydrogen bonding (closed-shell) interactions dominated by electrostatic force. In contrast, all the O(N)···H bonds with negative 𝐻𝐵𝐶𝑃 are corresponding to larger 𝜌 9

(𝜌 > 0.031) and positive 𝛻 2 𝜌, which reveals covalent bonding is included in the intermolecular interactions. Obviously, partial hydrogen bonding interactions in α-Cyclodextrin/[Emim][OAc] are stronger than those in chitobiose/[Emim][OAc]. This result is well consistent with geometric analyses. Laplacian bond order (LBO) is a novel definition of covalent bond order based on the Laplacian of electron density in fuzzy overlap space (Lu & Chen, 2013). LBOs are calculated to distinguish the relative magnitude of covalent contribution in the hydrogen bonding interactions, which are shown in Table S2. It can be found that the values of Laplacian bond orders are very small and even negligible. This further confirmed the previous conclusion that electrostatic interactions are the major contribution for hydrogen bonding in this work. 3.4 RDG analyses The sign of 𝛻 2 𝜌, dominated by the principal axis of variation, is positive for all the closed-shell interactions, which can’t be used to differentiate between different types of weak interactions within AIM theory. RDG method is employed to distinguish the type of multiple non-covalent interactions by color mapped isosurfaces according to the values of sign(𝜆2 )𝜌. 𝜆2 is the second Hessian eigenvalue, which is an indicator of bonded (𝜆2 < 0) or nonbonded (𝜆2 > 0) interactions, whereas the magnitude of 𝜌 at BCP is used to measure the bonding strength. Larger positive (negative) values of sign(𝜆2 )𝜌 are indicative of steric repulsions (hydrogen bonds), displayed in deep red (blue), and values near zero indicate weak vdW interactions colored in green. Fig. 3 shows the color mapped RDG isosurfaces and scatter diagrams of chitobiose/[Emim][OAc] complex (structure A in Fig. 2). It can be seen hydrogen bonds, O22-H23···O68 and N19-H20···O67, are formed between chitobiose and [OAc]- anion according to the blue isosurfaces and the spikes located at -0.045 and -0.017 a.u., respectively. Clearly, the former is stronger than the latter corresponding to the deep blue isosurfaces and larger negative value of sign(𝜆2 )𝜌. Three weaker hydrogen bonds are also observed the chitobiose and [Emim]+ cation according to the light blue isosurface and spikes with 0.012-0.019a.u., respectively. Additionally, the 10

light blue isosurface, corresponding to the spike at -0.024 a.u., indicates the existence of intramolecular interactions. The results in Fig. 4 suggest that partial intermolecular hydrogen bonds in α-Cyclodextrin/[Emim][OAc] complex are distinctly stronger than those in chitobiose/[Emim][OAc] complex. Three spikes are located at the range from -0.067 to -0.031 a.u., representing two typical strong and moderate hydrogen bonds. Besides, the large area of green isosurface are found between [Emim][OAc] and chitobiose/α-Cyclodextrin, indicating the presence of weak vdW interactions. 3.5 NBO analyses Based on NBO theory, intermolecular orbital interactions can be estimated by the magnitude of E(2): 𝐹2

𝐸(2) = ∆𝐸𝑖𝑗 = 𝑞𝑖 𝜀 (𝑖,𝑗) −𝜀 𝑗

𝑖

(2) where 𝑞𝑖 denotes the occupancy of donor orbital, 𝐹(𝑖,𝑗) represents the off-diagonal NBO Fock matrix element and 𝜀𝑖 and 𝜀𝑗 are orbital energies (diagonal elements), respectively. According to the NBO results, 3D images of intermolecular orbital interactions are depicted in Fig. 5, which shows the orbital overlaps between [Emim][OAc] with chitobiose and α-cyclodextrin. The detailed NBO parameters are given in Table 2. As seen in Table 2, the intermolecular orbital interactions are clearly observed in three different types between the lone pairs of O68 atom in [OAc]- anion and the anti-bonding orbital σ*(1)O22-H23 in chitobiose, respectively. Specifically, the corresponding

E(2)s

(orbital

energy

gaps)

of

LP(2)O68→σ*(1)O22-H23,

LP(1)O68→σ*(1)O22-H23 and LP(3)O68→σ*(1)O22-H23 are 24.71(0.91), 8.02 (1.27) and 1.45 kcal/mol (0.84 a.u.), respectively. Simultaneously, substantial overlaps (Fig. 5) of interaction orbitals are reduced along with the decrements of E(2)s. The intermolecular electron transfer occurs from the lone pairs of oxygen atom to the anti-bonding orbitals of O-H bond, which can well explain the formation of hydrogen bonds. Comparing with [OAc]- anion, partial orbital interactions between chitobiose and [Emim]+ cation is weaker. Overall, the E(2)s in Table 2 indicates that the 11

hydrogen bonds in α-cyclodextrin/[Emim][OAc] is stronger than those in chitobiose/[Emim][OAc] to some degree. This result is well consistent with BCP parameters obtained between corresponding oxygen and hydrogen atoms in above AIM analyses.

4．Conclusions The dissolution mechanisms of cyclodextrin and chitosan in [Emim][OAc] are systematically studied with DFT method by using α-cyclodextrin and chitobiose as a model system. ESP method is performed to predict possible reactive sites at molecular surface, which is helpful in locating hydrogen bonding interactions and the most stable configurations. The geometric analyses well show the structural features of the complexes formed by [Emim][OAc] with chitobiose and α-cyclodextrin, in which intermolecular interactions are well depicted. In this process, not only anions but also cations in ILs play important roles in the formation of studied complexes. According to the results from AIM analyses, the feature at BCPs suggests the intermolecular interactions are closed-shell (non-covalent) interactions, dominated by electrostatic force, which are the driving force of the carbohydrates dissolution in [Emim][OAc]. Non-covalent interactions in the complex are characterized and visualized by employing RDG analysis combined with VMD program, in which hydrogen bonds, van der Waals interactions and repulsions are clearly depicted in the real space. The results show that non-covalent interactions in the dissolution progress are dominated by hydrogen bonding interactions. Additionally, NBO theory is applied to characterize the nature of the intermolecular orbital interactions in the complexes.

12

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21203250

and

21402106)

and

Shandong

(ZR2014BQ024).

13

Natural

Science

Foundation

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17

Fig. 1. Electronic potentials computed on van der Waals surface (0.001au) at M06-2X/6-311++G** level.

18

Fig.

2.

Selected

geometric

parameters

of

chitobiose/[Emim][OAc]

and

α-cyclodextrin/[Emim][OAc] complexes optimized at the M06-2X/6-311++G** level, respectively. 19

Fig.

3.

RDG

isosurface

plots

of

RDG

versus

sign(λ2 )ρ for

the

chitobiose/[Emim][OAc] complex. RDG isosurface plots are coloured according to a BGR scheme via the interaction strengths, blue (strong attraction), green (weak vdW interaction) and red (strong repulsion).

20

Fig.

4.

RDG

isosurface

plots

of

RDG

versus

sign(λ2 )ρ

for

the

α-cyclodextrin/[Emim][OAc] complex. RDG isosurface plots are coloured according to a BGR scheme via the interaction strengths, blue (strong attraction), green (weak vdW interaction) and red (strong repulsion).

Fig.

5.

Schematic

graphs

of

the

main

orbital

chitobiose/[Emim][OAc] complex based on NBO analyses.

21

interactions

for

the

Table

1

Topological

parameters

at

the

BCPs

for

the

complexes

chitobiose/[Emim][OAc] and α-cyclodextrin/[Emim][OAc], respectively. Species

Chitobiose/[Emim][OAc]

α-Cyclodextrin/[Emim][OAc]

Hydrogen bonds

𝜌

∇2 𝜌

𝐻𝐵𝐶𝑃

O68···H23-O22

0.045398

0.136208

-0.004829

O67···H20-N19

0.017290

0.062901

0.001863

O16···H58-C55

0.018836

0.072038

0.002422

O16···H54-C53

0.018561

0.068884

0.002253

O12···H54-C53

0.011989

0.040913

0.001375

N44···H17-O16

0.023689

0.072503

0.001229

O129···H51-O114

0.067284

0.184743

-0.008259

O128···H36-O120

0.038372

0.131765

-0.000905

O108···H141-C140

0.031258

0.096066

-0.001774

22

of

Table 2 NBO parameters for the complexes of chitobiose/[Emim][OAc] and α-cyclodextrin/[Emim][OAc], respectively. Species

Chitobiose/[Emim][OAc]

α-Cyclodextrin/[Emim][OAc]

Donor→Acceptor

E(2) (kcal/mol)

E(j)-E(i) (a.u.)

LP(2)O68→σ*(1)O22-H23

24.71

0.91

LP(1)N44→σ*(1)O16-H17

16.31

0.94

LP(1)O68→σ*(1)O22-H23

8.02

1.27

LP(1)O16→σ*(1)C55-H58

6.83

1.16

LP(1)O67→σ*(1)N19-H20

3.72

1.34

LP(2)O67→σ*(1)N19-H20

2.83

0.88

LP(1)O16→σ*(1)C53-H54

1.49

0.89

LP(3)O68→σ*(1)O22-H23

1.45

0.84

LP(1)O12→σ*(1)C53-H54

1.39

1.16

LP(2)O129→σ*(1)O114-H51

55.93

0.89

LP(1)O128→σ*(1)O120-H36

17.49

1.27

LP(2)O108→σ*(1)C140-H141

13.19

0.97

LP(1)O129→σ*(1)O114-H51

9.67

1.13

LP(1)O108→σ*(1)C140-H141

3.38

1.13

23