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Theoretical studies on the dissolution of chitosan in 1-butyl-3-methylimidazolium acetate ionic liquid
Carbohydrate Research 408 (2015) 107e113
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Theoretical studies on the dissolution of chitosan in 1-butyl-3methylimidazolium acetate ionic liquid Qingqing Tian a, Shuangyue Liu a, Xiaofu Sun b, Haitao Sun a, *, Zhimin Xue c, Tiancheng Mu b, * a b c
School of Chemistry, Qufu Normal University, Qufu 273165, China Department of Chemistry, Renmin University of China, Beijing 100872, China College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 December 2014 Received in revised form 13 February 2015 Accepted 20 February 2015 Available online 12 March 2015
In this work, the dissolution mechanism of chitosan in imidazolium acetic-based ionic liquid (IL) 1-butyl3-methylimidazolium acetate was investigated by density functional theory (DFT). Chitobiose is considered to symbolize chitosan during the DFT calculations. [Bmim]OAc is supposed to be the best suitable IL among the investigated ILs for the dissolution of chitosan since the complex formed between [Bmim]OAc and chitobiose has the lowest energy. The hydrogen bonds formed by IL and chitobiose were studied by discussing the geometric parameter variations and the vibration mode analyses. Four strong hydrogen-bond patterns C1eH1/O16, C2eH2/O16, O38eH39/O1 and O40eH41/O2 were found, which means the existence of strong interaction between chitosan and [Bmim]OAc. In addition, natural bond orbital (NBO) analysis was used to study the second order perturbation stabilization energies (E(2)) that denotes the intensity of the interactions between chitobiose with H2O and ILs. The E(2) of chitobiose with [Bmim]OAc is larger than that of chitobiose with other ILs and solvents studied, which proves that chitobiose can be dissolved in [Bmim]OAc but cannot in water and other solvents. Atom in molecules (AIM) theory shows that hydrogen bonds between chitobiose and [Bmim]OAc are stronger than that between chitobiose and other solvents. It means that the interactions between [Bmim]OAc and chitobiose interrupt the initial hydrogen bonds in the chitobiose due to the formation of new hydrogen bonds in the complexes. The calculation data provide the interaction mechanism of the dissolution of chitosan in [Bmim]OAc. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chitobiose Acetic-based ionic liquid Vibration mode analysis Natural bond orbital analysis Atom in molecules analysis
1. Introduction Chitosan is one of the most abundant biorenewable and biocompatible materials with a long and well-established technological base.1 It is a highly crystalline polymer with D-glucosamine units joined together in long chains by b-(1,4) glucosidic bonds.2 Chitosan and its derivatives are widely used in industry as drug carriers, water treatment additives, wound-healing agents and so on because they have antimicrobial activity.1,3,4 For these applications, chemical modifications of these biomacromolecules are sometimes needed. Thus, it is essential to form a stable
* Corresponding authors. Zhongguancun St. No 59, Haidian, Beijing 100872, China. Tel.: þ86 10 62514925; fax: þ86 10 62516444. E-mail addresses: [email protected] (Q. Tian), liushuangyue0919@163. com (S. Liu), [email protected] (X. Sun), [email protected] (H. Sun), [email protected] (Z. Xue), [email protected] (T. Mu). http://dx.doi.org/10.1016/j.carres.2015.02.015 0008-6215/© 2015 Elsevier Ltd. All rights reserved.
homogeneous solution to improve the efficiency of modification. However, owing to their stiff molecules and close chain packing via numerous intramolecular and intermolecular hydrogen bonds, they have very low solubility in water and conventional organic solvents, which limits their application in practical processes. Nevertheless, trifluoroacetic acid and various aqueous solutions of mineral and organic acids and hexafluoro-2-propanol have been found as solvents for chitosan.3e5 However, these solvent systems have disadvantages such as volatile, toxic or corrosive. In addition, the applications of chitosan as drug carriers limited for bioactive agents may be influenced by these solvents.6 Therefore, to propose new solvents for chitosan dissolution is important. Ionic liquids (ILs) have unique properties, such as nondetectable vapor pressure, low melting point, high chemical and thermal stability, non-flammability, and chemical tunabilities.7,8 They have attracted much attention as greener replacements for traditional volatile organic solvents in various fields such as
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catalytic reactions, organic synthesis, functional materials, life science and electrochemical reactions.9e16 The properties of ILs can be changed by altering the cation or anion, which dramatically increases their potential fields of application.17e19 ILs were first applied as solvents for cellulose.20 It led to continuing and increasing explosion of interest in the use of ILs for biomass treatment, mainly on cellulose.21e28 Xie et al. reported that chitosan could be efficiently dissolved in 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) with a solubility of about 10% at 110 C.29 Subsequently, 1-allyl-3-methylimidazolium chloride ([Amim]Cl) was studied as good solvent for chitosan.30 Then, 1-butyl-3methylimidazolium acetate ([Bmim]OAc) was viewed as a better solvent than [Bmim]Cl and [Amim]Cl.2,31 Sun et al. reported the solubility of chitosan in the temperature range from 40 C to 140 C and in 1-ethyl-3-methylimidazolium acetate ([Emim]OAc), [Bmim] OAc, 1-hexyl-3-methylimidazolium acetate ([Hmim]OAc), 1-octyl3-methylimidazolium acetate ([Omim]OAc) and 1-butyl-2,3dimethylimidazolium acetate ([Bmmim]OAc).32 The effects of cation type, alkyl chain structure, temperature and water content on the dissolution were investigated. In particular, according to the experimental data, [Bmim]OAc is very efficient for chitosan dissolution. Although these literatures revealed some significant aspects of the dissolution of chitosan in ILs, few theoretical studies on the mechanism of the interaction between chitosan and ILs were reported, and even few efforts have been made to explore the structure transformation in the dissolution chitosan in ILs as well. In this work, ILs [Emim]OAc, [Bmim]OAc, [Hmim]OAc, [Omim] OAc and [Bmmim]OAc are used to study the effect of cation structure of ILs on solubility of chitosan. Since experimental results show that [Bmim]OAc is the best solvent of chitosan among the five ILs, it is used as a represent IL to investigate the interactions between chitosan and IL. As the chitosan is too large for density functional theory (DFT) computation, chitobiose is considered to symbolize chitosan.33 On the basis of these investigations, a possible mechanism for the dissolution of chitosan in IL and the possible disruption of inherent inter- and intra-molecular hydrogen bonds of chitosan are proposed. 2. Computational methods DFT considers electron correlation in the self-consistent KohneSham procedure through the functions of the electron density. And it gives a good description for the systems, which require sophisticated treatments of the electron correlation in the
Fig. 2. The optimized structures and geometrical parameters in chitobiose optimized at B3LYP/6-31G(d,p) level (Distance in Å). The H bonds are indicated by dashed lines.
conventional ab initio approach. Therefore, it is a cost-effective and reliable method for calculation chemistry. Fig. 1 shows the structure of a bundle of several chitosan units. It usually comprises 30e40 chitobiose and is packed by intermolecular and intramolecular hydrogen bonds, which is a very large system for DFT calculation. As shown in Fig. 1, the fragment of chitosan in the equatorial position is depicted by the main hydrogen-bond patterns: OeH/O and OeH/N. The chitobiose (Fig. 2) contains O16eH17/N44 and O22eH23/O34 H-bonds, thus, it is chosen as a model compound for the assessment of the hydrogen bond and its chemical environment of chitosan. Geometries of [Bmim]OAc, [Emim]OAc, [Hmim]OAc, [Omim]OAc, and [Bmmim]OAc, and the complexes of [Bmim]OAc and chitobiose are fully optimized at B3LYP/6-31G(d,p) level.34 The interaction energy (DE) is defined as the energy difference between the ion pairs and the corresponding isolated ions. DE considering the basis set superposition errors (BSSE) correction using the counterpoise (CP) method is estimated. The vibrational frequencies of [Bmim]OAc, chitobiose and the complexes are calculated at B3LYP/6-31G(d,p) level too. Natural bond orbital (NBO) and atom in molecule (AIM) analyses are performed at same level to give second order perturbation stabilization energies E(2) and to confirm the existence of the hydrogen bond, respectively. In the NBO analysis,35 E(2) can be used to estimate the orbital
Fig. 1. The fragment of chitosan depicting by the main hydrogen-bond patterns: H/O, OeH/N.
Q. Tian et al. / Carbohydrate Research 408 (2015) 107e113
interactions between the proton donor and proton acceptor well, that is
Eð2Þ ¼ DEij ¼ qi
Fði; jÞ2 εj εi
where εi and εj are diagonal elements, qi is the donor orbital occupancy, and F(i,j) is the off-diagonal NBO Fock matrix element. The AIM analysis, which is known as a powerful tool, has been widely applied to study the hydrogen-bonded interaction.36e38 According to AIM theory, the electron density r(r), and the Laplacian of the electron density V2r(r), are used to describe the chemical bond. rBCP in Bond Critical Points (BCP) indicates the strength of hydrogen bonds and V2r(r) shows the nature of the bond. The hydrogen bonds and ionic bonds are characterized by V2r(r)>0 in the closedshell interactions, while covalent bonds are characterized by V2r(r) <0. li is an eigenvalue of the Hessian matrix of r, and there exists V2r(r)¼l1þl2þl3. When two of these three eigenvalues are negative and the other one is positive, the critical point is called a BCP, which indicates the existence of chemical bond between two atoms. When there are two positive and one negative eigenvalues, the critical point is called a Ring Critical Point (RCP), which indicates the existence of a ring structure. All the DFT calculations are computed in gas phase and carried out with Gaussian 03 package.39 3. Results and discussion 3.1. The solubility of chitosan in the imidazolium-based ILs As previous reported, the solubility data of chitosan in the ILs under study at different temperatures are summarized in Fig. S1 (Supplementary data).32 It can be seen that the solubility of chitosan at a given temperature generally decreases in the order [Bmim]OAc>[Emim]OAc>[Hmim]OAc>[Omim]OAc>[Bmmim]OAc. Among the investigated ILs, [Bmim]OAc is the most efficient one for the dissolution of chitosan. In order to study the possible dissolution mechanism of chitosan in ILs, The complexes formed by [Bmim]OAc, [Emim]OAc, [Hmim]OAc, [Omim]OAc and [Bmmim] OAc with chitobiose are optimized at the B3LYP/6-31G(d,p) level. The optimized structures of these complexes are shown in Fig. S2. For them, the interaction energies (DE) of chitobiose with [Emim] OAc, [Bmim]OAc, [Hmim]OAc, [Omim]OAc and [Bmmim]OAc are 2055.74, 2039.53, 2039.14, 2036.75 and 2021.31 kJ/mol, respectively. It indicates that DE of the complexes decrease in the order: [Emim]OAc>[Bmim]OAc>[Hmim]OAc>[Omim]OAc> [Bmmim]OAc. This result is broadly consistent with above experimental data. DE decreases with increasing alkyl length of imidazolium-based ILs when the anions are identical. In addition, the complexes formed by [Emim]OAc, [Bmim]OAc and chitobiose have lower DE than that by other ILs. However, the complex formed by [Emim]OAc and chitobiose has lowest interaction energy that is not in agreement with the experimental data. Since the alkyl chain length of [Emim]þ is shorter than that of [Bmim]þ, the steric hindrance in cation is relatively harder to attack oxygen atom in chitobiose. In a word, the dissolution mechanism of chitobiose in acetic-based ILs is complicated, and it is the outcome of many factors. The structures of the complexes formed by H2O, methanol, benzene and chitobiose optimized at B3LYP/6-31G(d,p) level are shown in Fig. S2. Interaction energy DE of the complexes is 147.56, 34.40 and 6.72 kJ/mol corresponding to the H2Oechitoiose complex, methanolechitobiose complex, and benzeneechitobiose complex, which is less than 1/10 time of that in [Bmim]OAc. It can be demonstrated that dissolution of chitosan in H2O, methanol and benzene is much weaker than of that in [Bmim]
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OAc. This result is consistent with the fact that chitosan have very low solubility in water and conventional organic solvents. 3.2. Geometries of [Bmim]OAc and chitobiose In order to give a visual understanding of the interaction between chitobiose and [Bmim]OAc, the electrostatic potential surfaces for the most stable geometries of [Bmim]OAc, [Emim]OAc, [Hmim]OAc, [Omim]OAc, [Bmmim]OAc and chitobiose are given in Fig. S3. As is shown, the highly negative region of OAc is on the electronegative oxygen atoms and the molecular electrostatic potential value is 0.212 a.u. While the highly positive areas in the [Bmim]þ cation are around the hydrogen atoms on C2 and C1 and the molecular electrostatic potential value of the hydrogen atoms on C2 and C1 are 0.171 a.u and 0.148 a.u, respectively. Fig. 3 shows the most stable geometries of [Bmim]OAc optimized at the B3LYP/ 6-31G(d,p) level. For the ionic pair, we have considered various possible orientations between anion and cation corresponding to the molecular electrostatic potential values. So, [Bmim]OAc is considered to have two possible conformers A (Fig. 3) and B (Fig. S4). As can be seen from the results, the energies of the structure A and B are 651.389902 a.u and 651.389645 a.u, respectively. Because the structure A has a lower energy, it is considered when the OAc anion occurs in the vicinity of the C2eH bond and C1eH of the cation. This can be attributed to the larger positive charge on the C2eH unit (0.26800e) than on other CeH units and the lager negative charge on C]O unit (0.79985e). These results are consistent with the above analysis on the electrostatic potential surfaces. In [Emim]þ, [Hmim]þ, [Omim]þ and [Bmmim]þ, the natural charge of C2eH unit on imidazole ring is 0.26698e, 0.26647e, 0.26644e, and 0.26440e, respectively, which shows the possible regions to form the hydrogen bonds with chitobiose. In addition, the possible structures of chitobiose are also optimized at B3LYP/6-31G(d,p) level and the most optimal structure is showed in Fig. 2. Comparing the energies of the structure I and structure II (Fig. S5), the structure I (Fig. 2) is the most stable conformer. From Fig. 2, the structure I contains two types of hydrogen bonds, including O16eH17/N44 and O22eH23/O34. In chitobiose, the highly negative and positive regions are around the oxygen atom on O16eH17 (0.0633 a.u) and the hydrogen atom on
Fig. 3. The optimized structures and geometrical parameters for various species in [Bmim]OAc optimized at B3LYP/6-31G(d,p) level (Distance in Å). The H bonds are indicated by dashed lines.
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O38eH39 (0.689 a.u). The corresponding natural charge of O16 and H38 are 0.79257e and 0.49334e, respectively. The more positively charged regions of [Bmim]þ cation, the more negatively charged regions of OAc anion are the possible hydrogen bonding sites of the interaction with chitobiose. In addition, the highly negative and positive regions are around the oxygen atom and the hydrogen atom in H2O. The natural charge of O1 and H1 is 0.95357e and 0.47678e. As expected, the formation of the complexes between ILs, H2O and chitobisoes should occur in those regions possessing more positive charges and more negative charges. A series of possible initial geometries for the complexes are designed according to the above data. 3.3. Geometry of the complex Fig. 3 shows that the calculated H1/O1 and H2/O2 distances are 1.98 Å and 1.56 Å, respectively, which are both remarkably shorter than the summation (2.72 Å) of the van der Waals radii of O and H atoms. And C2eH/O2 angle and C1eH1/O1 angle are 174.41 and 155.99 , respectively. These data indicate the formation of C2eH2/O2 and C1eH1/O1 H-bonds between the anion and cation. The interaction energy of the ionic pair is calculated to be 1472.62 kJ/mol. The effective hydrogen bond and the strong Coulomb interaction between the cation and anion are in agreement with the experimental results.40 Fig. 2 shows two important features of the geometry: (i) the chitobiose is structurally suitable for mimicking a real chitosan chain and the saccharide rings keeping a chair conformation, and (ii) the ring oxygen (Oring) in a monosaccharide establishes an important hydrogen bridge with the hydroxyl group in the next monosaccharide. As shown in Fig. 2, the hydrogen bond distance is 2.66 Å and the O22eH23/O34 angle is 133.17. In addition, the distance between N44 and H17 in O16eH17 is 1.98 Å and the O16eH17/N44 angle is 172.54 , which implies the existence of OeH/N hydrogen bond. The parameters of these hydrogen bonds show obviously the existence of strong intramolecular hydrogen bonding in a chitosan chain. These intramolecular hydrogen bonds play an important role in the low solubility of chitosan in most solvents. Three complexes formed by chitobiose and [Bmim]OAc are optimized at the B3LYP/6-31G(d,p) level and shown in Fig. S6. The interaction energies (DE) of three complex conformations (a), (b) and (c) are 2039.53, 2019.68 and 2031.06 kJ/mol, respectively. The calculated data show that the most stable conformation is the complex (a). The optimal geometry of the complex (a) is shown in Fig. 4. As seen from Fig. 4, the main hydrogen bonds between [Bmim]OAc and chitobiose are obtained, the bond lengths are 2.34,
Fig. 4. The optimized structures and geometrical parameters in the complex formed by chitobiose and [Bmim]OAc optimized at the B3LYP/6-31G(d,p) level (Distance in Å). The H bonds are indicated by dashed lines.
1.89, 1.63 and 1.49 Å corresponding to H1/O16, H2/O16, H39/O1 and H41/O2, respectively; and angles are 148.85, 144.35, 171.65 and 174.48 corresponding to :C1eH1/O16, :C2eH2/O16, :O38eH39/O1 and :O40eH41/O2, respectively. It indicates that there exists strong intermolecular hydrogen bonding interaction between chitobiose and [Bmim]OAc. The N44/H45 distance is extended from 1.98 Å in the pure chitobiose to 2.02 Å in the complex. In addition, O34/H23 distance also varies from 2.66 Å to 2.69 Å. The corresponding angles of O16eH17/N44 and O22eH23/O34 vary ranging from 172.54 to 172.48 and from 133.17 to 136.22 , respectively. It means that the strength of these hydrogen bonds decreases in the complex comparing with that in chitobiose. These intermolecular hydrogen bonds are expected to be a key factor resulting in the high solubility of chitosan in [Bmim] OAc. The complexes formed by other ILs and chitobiose also have intermolecular hydrogen bonds. However, comparing with the hydrogen bonding interaction between chitobiose and [Bmim]OAc, they are weaker. For example, in chitobiosee[Emim]OAc complex, the interaction distances are 1.90 and 1.64 Å corresponding to H1/O16 and H39/O1, respectively. It shown that the hydrogen bonds C2eH2/O16 and O38eH39/O1 are weaker than the corresponding hydrogen bonds in chitobiosee[Bmim]OAc complex. In the complexes formed by [Hmim]OAc, [Omim]OAc, [Bmmim]OAc and chitobiose, the bond lengths of H2/O16 and H39/O1 are also larger at some extent. It is shown that the interaction of [Bmim]OAc with chitobiose is much stronger than other ILs. These results are consistent with the conclusion of the solubility of chitosan in ILs. 3.4. Vibration mode analysis Main variations of the vibration spectra in [Bmim]OAc, [Emim] OAc, [Hmim]OAc, [Omim]OAc, [Bmmim]OAc and chitobiose are calculated in GaussView41 and listed in Table S1, the corresponding calculated IR spectrum of the complexes are given in Fig. S7, and the data directly related to vibration mode analyses are given in Table 1. It can be seen that nas(C]O) is red-shift from 1648 cm1 in OAc to 1588 cm1 in [Bmim]OAc, and ns(C]O) is blue-shift from 1345 cm1 to 1381 cm1 in [Bmim]OAc. The reason for this is that O2eC3]O1 in OAc anion are sp2 hybridization, because of the effect of pep conjugation, the negative charge can be distributed to two O atoms averagely. When OAc anion approaches [Bmim]þ, it should break the pep conjugation, thus ns(C2eH) is red-shift from 3026 cm1 to 2308 cm1 and ns(C1eH1) is also red-shift from 3036 cm1 to 3001 cm1. Therefore, the C2eH2/O2, and C1eH1/O1 hydrogen bonds are formed because the interactions between the OAc and [Bmim]þ. This is the reason why [Bmim]OAc has unique properties as the efficient solvent for the polymer. Table 1 shows that ns(O38eH39) and ns(O40eH41) in chitobiose are red-shift from 3790 cm1 to 3171 cm1 and from 3835 cm1 to 2594 cm1, respectively. It manifests the existence O40eH41/O2 and O38eH39/O1 hydrogen bonds between chitobiose and OAc. At the same time, ns(C1eH1) and ns(C2eH2) are 3036 cm1 and 3026 cm1 in [Bmim]þ, which have a blue-shift to 3159 cm1 and 3130 cm1 in the complex, respectively. It can be explained that C1eH1/O16 and C2eH2/O16 are formed between chitobiose and [Bmim]þ. In addition, ns(N44eH45) and ns(O22eH23) in chitobiose are 3529 cm1 and 3774 cm1, respectively. In the complex, both ns(N44eH45) and ns(O22eH23) have a red-shift to 3450 cm1 and 3752 cm1, respectively. These hydrogen bonds in complex are better explanations why chitobiose dissolves easily in [Bmim]OAc. From Table 1, in the complexes formed by [Emim]OAc, [Hmim] OAc, [Omim]OAc, [Bmmim]OAc and chitobiose, ns(O38eH39) is red-shift to 3178, 3189, 3295 and 3166 cm1, respectively. It can be shown that O38eH39/O1 in these complexes is weaker than that
Q. Tian et al. / Carbohydrate Research 408 (2015) 107e113 Table 1 Main variation of the vibration spectrum in [Bmim]OAc, chitobiose and the complexes formed by ILs and chitobiose
OAc [Bmim]þ [Bmim]OAc
Chitobiose
Chitobioseþ[Bmim]OAc
Chitobioseþ[Emim]OAc Chitobioseþ[Hmim]OAc Chitobioseþ[Omim]OAc Chitobioseþ[Bmmim]OAc
Frequencies (cm1)
IR intensities (km mol1)
Vibrational mode assignment
1344 1648 3035 3026 1381 1588 2308 3000 3528 3774 3835 3789 3478 3752 3535 3450 3173 3170 3130 3070 2999 2594 1676 1439 3178 2595 3189 2594 3295 2600 3166 2616
337.6 674.5 57.4 41.0 364.7 283.9 1913.2 256.6 622.2 63.6 31.5 55.3 8.2 102.3 6.9 1.9 28.7 1953.4 221.9 38.2 2085.3 3018.5 497.7 439.2 1412.3 3028.8 1907.8 3040.2 1436.7 3002.9 1859.2 2888.3
ns(C]O) nas(C]O) ns(C1eH) ns(C2eH) ns(C]O) nas(C]O) ns(C2eH2) ns(C1eH1) ns(O16eH17) ns(O22eH23) ns(O40eH41) ns(O38eH39) ns(N44eH45) ns(O22eH23) nas(N44eH45) ns(N44eH45) nas(C1eH1) ns(O38eH39) ns(C2eH2) ns(C1eH1) ns(O16eH17) ns(O40eH41) nas(C]O) ns(C]O) ns(O38eH39) ns(O40eH41) ns(O38eH39) ns(O40eH41) ns(O38eH39) ns(O40eH41) ns(O38eH39) ns(O40eH41)
in the [Bmmim]OAcechitobiose complex. In addition, ns(O40eH41) is also red-shift to 2595, 2594, 2600 and 2616 cm1, respectively. It manifests that the formation of O40eH41/O2 in these complexes and O40eH41/O2 hydrogen bond between [Bmmim]OAc and chitobiose is stronger than that in other complexes. These results are consistent with the changes of the structures. 3.5. NBO analysis The NBO method characterizes hydrogen bonds in terms of hyper conjugative donoreacceptor interactions and has been used to study the bonding properties in the complexes. Table 2 shows that the main donoreaccepter interactions of chitobiose and the complexes formed between chitobiose with ILs and H2O, as well as their second order perturbation stabilization energies, E(2). The values of E(2) denote the intensity of the donoreaccepter interaction. The larger E(2) is, the stronger the interaction will be. For chitobiose, which contains two types of the intramolecular hydrogen bonds: O16eH17/N44 and O22eH23/O34, from Table 2 it is found that the value of E(2) (LP1(N44)/s*(O16eH17) is 70.25 kJ/mol. It indicates the existence of a stronger orbital interactions between electron donor of the long pair LP1(N44) and the sigma anti-bond orbital of the electron acceptor s*(O16eH17). Fig. S8a shows that it should be the best overlap, symmetrical matching orbital and the smaller energy gap (0.83 au). Interestingly, O22eH23/O34 intramolecular hydrogen bonds has not been discovered in NBO analyses, it may be due to interaction orbital, which exists asymmetrical matching and the bond length (2.66 Å) of O22eH23/O34 is longer than that (1.98 Å) of O16eH17/N44, which is in agreement with the above analysis. The complexes formed between chitobiose and H2O are considered
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Table 2 The main donoreaccepter interactions in the process and their second order perturbation stabilization energies E(2) at B3LYP/6-31G(d,p) level
Chitobiose ChitobioseþH2O (I) ChitobioseþH2O (II)
Chitobioseþ[Bmim]OAc
Chitobioseþ[Emim]OAc Chitobioseþ[Hmim]OAc Chitobioseþ[Omim]OAc Chitobioseþ[Bmmim]OAc
Donor (i)
Acceptor (j)
E(2) kJ/mol
LP1(N44) LP1(O1) LP2(O5) LP1(O22) LP2(O22) LP1(O1) LP2(O1) LP2(O1) LP1(O1) LP2(O1) LP1(O2) LP2(O2) LP2(O16) LP2(O16) LP1(O16) LP1(O16) LP2(O1) LP2(O2) LP2(O1) LP2(O2) LP2(O1) LP2(O2) LP2(O1) LP2(O2)
s*(O16eH17) s*(O38eH39) s*(O38eH39) s*(O1eH1) s*(O1eH1) s*(N1eH2) s*(O1eH1) s*(N1eH2) s*(O38eH39) s*(O38eH39) s*(O40eH41) s*(O40eH41) s*(C2eH2) s*(C2eN2) s*(C2eH2) s*(C2eN2) s*(O38eH39) s*(O40eH41) s*(O38eH39) s*(O40eH41) s*(O38eH39) s*(O40eH41) s*(O38eH39) s*(O40eH41)
70.25 1.72 88.53 2.97 74.43 0.84 0.21 24.64 115.65 3.93 217.07 3.14 38.95 0.67 19.04 1.13 116.16 216.95 42.82 216.58 87.65 214.32 116.91 212.68
and are shown in Figs. S8b and S8c for comparison. On the contrary, it can be found that interaction energy (146.07 kJ/mol) of the complex formed between O22eH23/O34 bond with H2O is larger than that (130.77 kJ/mol) between O16eH17/N44 bond with H2O. From Table 2, one can see that for former complex E(2)s of LP2(O1)/s*(O38eH39) and LP2(O2)/s*(O40eH41) are 90.67 and 74.43 kJ/mol, respectively. It can be seen that the interaction orbitals corresponding to Figs. S8b1 and S8b2 are symmetrical matching with the largest overlap. In addition, the interaction orbitals corresponding to LP1(O1)/s*(O38eH39) and LP1(O22)/ s*(O1eH1) are asymmetrical matching and the values of E(2) are 1.13 and 2.97 kJ/mol, respectively, which are much less than those above two interaction orbital. For the latter complex it can be seen that the interaction orbitals are LP1(O1)/s*(N44eH45), LP2(O1)/s*(O16eH17) and LP2(O1)/s*(N44eH45) from Figs. S8c1, S8c2 and S8c3, respectively. There is only an orbital of LP2(O1)/s*(N44eH45), which possess the symmetrical matching with the largest overlap. Its value (24.64 kJ/mol) is much less than those of above two orbital, so the interaction energy between O16eH17/N44 bond with H2O is less than that between O22eH23/O34 bond with H2O. Fig. 4 shows that for the complex formed between chitobiose and [Bmim]OAc, the optimized structure is [Bmim]þ combined with O16eH17/N44 in chitobiose and OAc connected with O22eH23/O34 in chitobiose. As shown in Table 2, there are six symmetrical matching orbitals: LP1(O1)/s*(O38eH39), LP2(O1)/s*(O38eH39), LP1(O2)/s*(O40eH41) and LP2(O2)/ s*(O40eH41) for OAc corresponding to the values of E2 are 43.22, 115.65, 43.47 and 217.07 kJ/mol, respectively. As well as LP1(O16) /s*(C2eH2) and LP2(O16) /s*(C2eH2) for [Bmim]þ corresponding to the values of E2 are 9.31 and 4.55 kJ/mol, respectively. The largest E2 (217.07 kJ/mol) is LP2(O2)/s*(O40eH41), which possesses the maximum overlap and minimum energy gap (0.78 a.u.) (Fig. S8d4). From above E2 values it can be found that the summation value (419.40 kJ/mol) of E2 for OAc is much larger than that (57.99 kJ/mol) of E2 for [Bmim]þ. In other words, OAc plays an important role in chitobiose dissolution by [Bmim]OAc. What is more, the summation value (165.10 kJ/mol) of E2 in chitobiosee [Bmim]OAc complex is much larger than that (114.10 kJ/mol) of
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chitobioseeH2O complex, even chitobiosee2H2O complex, where O16eH17/N44 and O22eH23/O34 hydrogen bonds in chitobiose are broken by two water molecules, and the summation value of E2 is only 189.74 kJ/mol. These results are consistent with the conclusion of the hydrogen bond distances discussed above. As shown in Table 2, in the complexes formed by [Emim]OAc, [Hmim]OAc, [Omim]OAc, [Bmmim]OAc and chitobiose, the largest E2 (LP2(O2)/s*(O40eH41)) is 216.95, 216.58, 214.32 and 212.68 kJ/mol, respectively. It can be found that the value of E2(LP2(O2)/s*(O40eH41)) decreases in the order: [Bmim]OAc> [Emim]OAc>[Hmim]OAc>[Omim]OAc>[Bmmim]OAc. These conclusions are in agreement with the experimental results.
consistent with the conclusion of the hydrogen bond distances discussed above. From Table 3, it can also be found that the largest r(r) (O40eH41/O2) is 0.077395 a.u, 0.077438 a.u, 0.076622 a.u and 0.076290 a.u, respectively, corresponding to [Emim]OAcechitobiose complex, [Hmim]OAcechitobiose complex, [Omim]OAcechitobiose complex and [Bmmim]OAcechitobiose complex. It can be found that the r(r) values of O40eH41/O2 in the [Bmim] OAcechitobiose complex is larger than other complexes. It indicates clearly that the hydrogen bonds in the chitobiosee[Bmim] OAc complex is much stronger than that in other complexes. These results are also consistent with the conclusion of the hydrogen bonds discussed above.
3.6. AIM analysis
4. Conclusions
The molecular graphic corresponding to the complex of chitobiose with [Bmim]OAc is shown in Fig. S9. Table 3 shows r(r), V2r(r) and the matrix eigenvalues (l1, l2 and l3). The two topological properties of all hydrogen bonds in the complexes formed by H2O, five ILs and chitobiose, are in the range of the criteria (i.e., 0.002e0.035 a.u. for r(r), 0.014e0.139 a.u. for V2r(r)) proposed by Popelier42,43 for the existence of a hydrogen bond. V2r(r) values of the hydrogen bonds in chitobioseþH2O system are 0.097 a.u (O1eH1/O22) and 0.10 a.u (O38eH39/O1), respectively. In contrast, V2r(r) values of the hydrogen bonds in the complex are 0.15 a.u (O38eH39/O1), 0.16 a.u (O40eH41/O2), 0.039 a.u (C1eH1/O16) and 0.090 a.u (C2eH2/O16), respectively. It can be observed that V2r(r) value of the chemical bonds in the complexes in the range of the criteria, which indicates that the main chemical bonds of the complexes are hydrogen bonds. In addition, r(r) values of O40eH41/O2 and O38eH39/O1 hydrogen bonds are 0.035776 a.u and 0.36129 a.u, respectively. In the same way, r(r) values of O38eH39/O1, O40eH41/O2, C1eH1/O16 and C2eH2/O1 are 0.052740 a.u, 0.077440 a.u, 0.013249 a.u and 0.031455 a.u, respectively. It is shown that r(r) values of O40eH41/O2 in the complex are much higher than that of O38eH39/O1 in chitobioseþH2O system. As r(r) values are related to the bond strength, it indicates clearly that the hydrogen bond in the complex is much stronger than that in chitobioseþH2O system. These results are also
[Emim]OAc, [Bmim]OAc, [Hmim]OAc, [Omim]OAc and [Bmmim] OAc are used to investigate the dissolution mechanism of the chitosan in ILs. For simplify the DFT calculations, the chitobiose is chosen as the model of chitosan. The interaction energies (DE) of five ILs with chitobiose are calculated at B3LYP/6-31G(d,p) level. The comparison of DE indicates the complexes formed by [Emim] OAc and [Bmim]OAc with chitobiose are larger than that by other complexes. The complexes formed by H2O, benzene, methanol and chitobiose are optimized at B3LYP/6-31G(d,p) level. DE of the complexes is much less than 1/10 time of that in [Bmim]OAc. These results show that [Bmim]OAc is the best solvent to dissolve the chitosan. In addition, the electrostatic potential surfaces are taken into consideration for the initial geometry design for the complexes formed by chitobiose and ILs. The smaller intermolecular H-bond distance and larger H-bond angle suggest that the hydrogen bonds formed between [Bmim]OAc and chitobiose are stronger than those of other complexes. The vibration mode analyses indicates that the formation of complexes between chitobiose and [Bmim]OAc is due to hydrogen bond interactions. These hydrogen bonds have been characterized by the NBO and AIM analysis to the unique properties. In chitobiosee[Bmim]OAc complex, the largest E2 (217.07 kJ/ mol) is LP2(O2)/s*(O40eH41), which is larger than that in other complexes. The r(r) and V2r(r) of all the hydrogen bond of the
Table 3 The electron density (rBCP, a.u.), Laplacian of the electron density (V2rBCP, a.u.) and matrix eigenvalues (l1, l2, and l3) at the B3LYP/6-31G(d,p) level
[Bmim]OAc ChitosanþH2O Chitosanþ[Bmim]OAc
Chitosanþ[Emim]OAc
Chitosanþ[Hmim]OAc
Chitosanþ[Omim]OAc
Chitosanþ[Bmmim]OAc
AeB bond
r
V2r
ε
l1
l2
l3
C2eH2/O2 C1eH1/O1 O1eH1/O22 O38eH39/O1 O38eH39/O1 O40eH41/O2 C1eH1/O16 C2eH2/O16 O38eH39/O1 O40eH41/O2 C1eH1/O16 C2eH2/O16 O38eH39/O1 O40eH41/O2 C1eH1/O16 C2eH2/O16 O38eH39/O1 O40eH41/O2 C1eH1/O16 C2eH2/O16 O38eH39/O1 O40eH41/O2 C1eH1/O16
0.068580 0.027075 0.035776 0.036129 0.052740 0.077440 0.013249 0.031455 0.052710 0.077395 0.014842 0.031144 0.052632 0.077438 0.016777 0.029261 0.047501 0.076622 0.016356 0.030648 0.052899 0.076290 0.021392
0.1522 0.07245 0.09780 0.1015 0.1469 0.1553 0.03926 0.08996 0.1469 0.1551 0.04188 0.08898 0.1466 0.1553 0.04579 0.08205 0.1665 0.1554 0.05180 0.09997 0.1472 0.01563 0.05696
0.02980 0.01487 0.02063 0.05432 0.002596 0.008127 0.1340 0.1037 0.002678 0.008193 0.1188 0.1087 0.002869 0.007952 0.1100 0.1062 0.02310 0.02056 0.1138 0.1061 0.002212 0.007976 0.08557
0.1344 0.03347 0.05268 0.5415 0.09228 0.1664 0.01465 0.04352 0.09242 0.1672 0.01681 0.04290 0.09220 0.1664 0.01949 0.03944 0.08135 0.1685 0.01929 0.04348 0.09288 0.01626 0.02611
0.1305 0.03298 0.05161 0.05136 0.09204 0.1651 0.01292 0.03943 0.09217 0.1659 0.01503 0.03869 0.09194 0.1651 0.01755 0.03565 0.07951 0.1651 0.01732 0.03931 0.09268 0.1613 0.02405
0.4172 0.1389 0.2021 0.2070 0.3312 0.4868 0.06682 0.1729 0.3315 0.4882 0.07372 0.1706 0.3307 0.4868 0.08283 0.1571 0.3274 0.5461 0.08841 0.1828 0.3328 0.4803 0.1071
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complexes formed by ILs and chitobiose are in the range of the criteria proposed by Popelier. These results show that the electrostatic interaction between chitobiose and [Bmim]OAc is much stronger than that between H2O, other ILs and chitobiose. The reason for this is that chitobiose has high solution ability in [Bmim] OAc, which is in good agreement with experimental results. Acknowledgments This work was funded by the National Natural Science Foundation of China (21473252). Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carres.2015.02.015. References 1. Dias AM, Cortez AR, Barsan M, Santos J, Brett CM, de Sousa HC. ACS Sustain Chem Eng 2013;1:1480e92. 2. Chen QT, Xu AR, Li ZY, Wang JJ, Zhang SJ. Green Chem 2011;13:3446e52. 3. Geng X, Kwon O-H, Jang J. Biomaterials 2005;26:5427e32. 4. Ravi Kumar M. N React Funct Polym 2000;46:1e27. 5. Sashiwa H, Kawasaki N, Nakayama A, Muraki E, Aiba S-i. Chitin Chitosan Res 2002;8:249e51. 6. Nah JW, Jang MK. J Polym Sci Part A Polym Chem 2002;40:3796e803. 7. Hallett JP, Welton T. Chem Rev 2011;111:3508e76. 8. Petkovic M, Seddon KR, Rebelo LPN, Pereira CS. Chem Soc Rev 2011;40: 1383e403. 9. Wasserscheid P, Keim W. Chem Int Ed 2000;39:3772e89. 10. Bara JE, Camper DE, Gin DL, Noble RD. Acc Chem Res 2010;43:152e9. 11. Armand M, Endres F, MacFarlane DR, Ohno H, Scrosati B. Nat Mater 2009;8: 621e9. 12. Hu S, Zhang Z, Zhou Y, Song J, Fan H, Han B. Green Chem 2009;11:873e7.
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13. Vijayaraghavan R, Izgorodin A, Ganesh V, Surianarayanan M, MacFarlane DR. Angew Chem Int Ed 2010;49:1631e3. 14. Zhang YQ, Gao HX, Joo YH, Shreeve JM. Angew Chem Int Ed 2011;50:9554e62. 15. Zhao C, Burrell G, Torriero AAJ, Separovic F, Dunlop NF, MacFarlane DR, et al. J Phys Chem B 2008;112:6923e36. 16. Zhao C, Bond AM, Lu XY. Anal Chem 2012;84:2784e91. 17. Olivier J-H, Camerel F, Selb J, Retailleau P, Ziessel R. Chem Commun 2009: 1133e5. 18. Bates ED, Mayton RD, Ntai I, Davis JH. J Am Chem Soc 2002;124:926e7. 19. Gutowski KE, Maginn EJ. J Am Chem Soc 2008;130:14690e704. 20. Swatloski RP, Spear SK, Holbrey JD, Rogers RD. J Am Chem Soc 2002;124: 4974e5. 21. Zhang H, Wu J, Zhang J, He JS. Macromolecules 2005;38:8272e7. 22. Xu A, Wang J, Wang H. Green Chem 2010;12:268e75. 23. Zhu SD, Wu YX, Chen QM, Yu ZN, Wang CW, Jin SW, et al. Green Chem 2006;8: 325e7. 24. Vitz J, Erdmenger T, Haensch C, Schubert US. Green Chem 2009;11:417e24. 25. Andanson J-M, Bordes E, Devemy J, Leroux F, Padua AAH, Gomes MFC. Green Chem 2014;16:2528e38. 26. Brandt A, Grasvik J, Hallett JP, Welton T. Green Chem 2013;15:550e83. 27. King AWT, Asikkala J, Mutikainen I, Jarvi P, Kilpelainen I. Angew Chem Int Ed 2011;50:6301e5. 28. Wang H, Gurau G, Rogers RD. Chem Soc Rev 2012;41:1519e37. 29. Xie H, Zhang S, Li S. Green Chem 2006;8:630e3. 30. Wu Y, Sasaki T, Irie S, Sakurai K. Polymer 2008;49:2321e7. 31. Sun X, Xue Z, Mu T. Green Chem 2014;16:2102e6. 32. Sun X, Tian Q, Xue Z, Zhang Y, Mu T. RSC Adv 2014;4:30282e91. 33. Loerbroks C, Rinaldi R, Thiel W. Chem Eur J 2013;19:16282e94. 34. Yui T, Kobayashi H, Kitamura S, Imada K. Biopolymers 1994;34:203e8. 35. Reed AE, Curtiss LA, Weinhold F. Chem Rev 1988;88:899e926. 36. Shukla M, Srivastava N, Saha S. J Mol Struct 2010;975:349e56. 37. Dunitz JD, Gavezzotti A. Angew Chem Int Ed 2005;44:1766e87. 38. Arnold WD, Oldfield E. Am Chem Soc 2000;122:12835e41. 39. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian 03, revision C. 02. Wallingford, CT: Gaussian, Inc.; 2004. p. 26. 40. Hunt PA, Kirchner B, Welton T. Chem Eur J 2006;12:6762e75. 41. Frisch A, Nielsen AB, Holder AJ. Gaussview users manual. Pittsburgh, PA: Gaussian Inc.; 2000. 42. Bader RF: Oxford University Press. Oxford: 1990. 43. Popelier PLA. J Phys Chem A 1998;102:1873e8.
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Theoretical studies on the dissolution of chitosan in 1-butyl-3methylimidazolium acetate ionic liquid Qingqing Tian a, Shuangyue Liu a, Xiaofu Sun b, Haitao Sun a, *, Zhimin Xue c, Tiancheng Mu b, * a b c
School of Chemistry, Qufu Normal University, Qufu 273165, China Department of Chemistry, Renmin University of China, Beijing 100872, China College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 December 2014 Received in revised form 13 February 2015 Accepted 20 February 2015 Available online 12 March 2015
In this work, the dissolution mechanism of chitosan in imidazolium acetic-based ionic liquid (IL) 1-butyl3-methylimidazolium acetate was investigated by density functional theory (DFT). Chitobiose is considered to symbolize chitosan during the DFT calculations. [Bmim]OAc is supposed to be the best suitable IL among the investigated ILs for the dissolution of chitosan since the complex formed between [Bmim]OAc and chitobiose has the lowest energy. The hydrogen bonds formed by IL and chitobiose were studied by discussing the geometric parameter variations and the vibration mode analyses. Four strong hydrogen-bond patterns C1eH1/O16, C2eH2/O16, O38eH39/O1 and O40eH41/O2 were found, which means the existence of strong interaction between chitosan and [Bmim]OAc. In addition, natural bond orbital (NBO) analysis was used to study the second order perturbation stabilization energies (E(2)) that denotes the intensity of the interactions between chitobiose with H2O and ILs. The E(2) of chitobiose with [Bmim]OAc is larger than that of chitobiose with other ILs and solvents studied, which proves that chitobiose can be dissolved in [Bmim]OAc but cannot in water and other solvents. Atom in molecules (AIM) theory shows that hydrogen bonds between chitobiose and [Bmim]OAc are stronger than that between chitobiose and other solvents. It means that the interactions between [Bmim]OAc and chitobiose interrupt the initial hydrogen bonds in the chitobiose due to the formation of new hydrogen bonds in the complexes. The calculation data provide the interaction mechanism of the dissolution of chitosan in [Bmim]OAc. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chitobiose Acetic-based ionic liquid Vibration mode analysis Natural bond orbital analysis Atom in molecules analysis
1. Introduction Chitosan is one of the most abundant biorenewable and biocompatible materials with a long and well-established technological base.1 It is a highly crystalline polymer with D-glucosamine units joined together in long chains by b-(1,4) glucosidic bonds.2 Chitosan and its derivatives are widely used in industry as drug carriers, water treatment additives, wound-healing agents and so on because they have antimicrobial activity.1,3,4 For these applications, chemical modifications of these biomacromolecules are sometimes needed. Thus, it is essential to form a stable
* Corresponding authors. Zhongguancun St. No 59, Haidian, Beijing 100872, China. Tel.: þ86 10 62514925; fax: þ86 10 62516444. E-mail addresses: [email protected] (Q. Tian), liushuangyue0919@163. com (S. Liu), [email protected] (X. Sun), [email protected] (H. Sun), [email protected] (Z. Xue), [email protected] (T. Mu). http://dx.doi.org/10.1016/j.carres.2015.02.015 0008-6215/© 2015 Elsevier Ltd. All rights reserved.
homogeneous solution to improve the efficiency of modification. However, owing to their stiff molecules and close chain packing via numerous intramolecular and intermolecular hydrogen bonds, they have very low solubility in water and conventional organic solvents, which limits their application in practical processes. Nevertheless, trifluoroacetic acid and various aqueous solutions of mineral and organic acids and hexafluoro-2-propanol have been found as solvents for chitosan.3e5 However, these solvent systems have disadvantages such as volatile, toxic or corrosive. In addition, the applications of chitosan as drug carriers limited for bioactive agents may be influenced by these solvents.6 Therefore, to propose new solvents for chitosan dissolution is important. Ionic liquids (ILs) have unique properties, such as nondetectable vapor pressure, low melting point, high chemical and thermal stability, non-flammability, and chemical tunabilities.7,8 They have attracted much attention as greener replacements for traditional volatile organic solvents in various fields such as
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catalytic reactions, organic synthesis, functional materials, life science and electrochemical reactions.9e16 The properties of ILs can be changed by altering the cation or anion, which dramatically increases their potential fields of application.17e19 ILs were first applied as solvents for cellulose.20 It led to continuing and increasing explosion of interest in the use of ILs for biomass treatment, mainly on cellulose.21e28 Xie et al. reported that chitosan could be efficiently dissolved in 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) with a solubility of about 10% at 110 C.29 Subsequently, 1-allyl-3-methylimidazolium chloride ([Amim]Cl) was studied as good solvent for chitosan.30 Then, 1-butyl-3methylimidazolium acetate ([Bmim]OAc) was viewed as a better solvent than [Bmim]Cl and [Amim]Cl.2,31 Sun et al. reported the solubility of chitosan in the temperature range from 40 C to 140 C and in 1-ethyl-3-methylimidazolium acetate ([Emim]OAc), [Bmim] OAc, 1-hexyl-3-methylimidazolium acetate ([Hmim]OAc), 1-octyl3-methylimidazolium acetate ([Omim]OAc) and 1-butyl-2,3dimethylimidazolium acetate ([Bmmim]OAc).32 The effects of cation type, alkyl chain structure, temperature and water content on the dissolution were investigated. In particular, according to the experimental data, [Bmim]OAc is very efficient for chitosan dissolution. Although these literatures revealed some significant aspects of the dissolution of chitosan in ILs, few theoretical studies on the mechanism of the interaction between chitosan and ILs were reported, and even few efforts have been made to explore the structure transformation in the dissolution chitosan in ILs as well. In this work, ILs [Emim]OAc, [Bmim]OAc, [Hmim]OAc, [Omim] OAc and [Bmmim]OAc are used to study the effect of cation structure of ILs on solubility of chitosan. Since experimental results show that [Bmim]OAc is the best solvent of chitosan among the five ILs, it is used as a represent IL to investigate the interactions between chitosan and IL. As the chitosan is too large for density functional theory (DFT) computation, chitobiose is considered to symbolize chitosan.33 On the basis of these investigations, a possible mechanism for the dissolution of chitosan in IL and the possible disruption of inherent inter- and intra-molecular hydrogen bonds of chitosan are proposed. 2. Computational methods DFT considers electron correlation in the self-consistent KohneSham procedure through the functions of the electron density. And it gives a good description for the systems, which require sophisticated treatments of the electron correlation in the
Fig. 2. The optimized structures and geometrical parameters in chitobiose optimized at B3LYP/6-31G(d,p) level (Distance in Å). The H bonds are indicated by dashed lines.
conventional ab initio approach. Therefore, it is a cost-effective and reliable method for calculation chemistry. Fig. 1 shows the structure of a bundle of several chitosan units. It usually comprises 30e40 chitobiose and is packed by intermolecular and intramolecular hydrogen bonds, which is a very large system for DFT calculation. As shown in Fig. 1, the fragment of chitosan in the equatorial position is depicted by the main hydrogen-bond patterns: OeH/O and OeH/N. The chitobiose (Fig. 2) contains O16eH17/N44 and O22eH23/O34 H-bonds, thus, it is chosen as a model compound for the assessment of the hydrogen bond and its chemical environment of chitosan. Geometries of [Bmim]OAc, [Emim]OAc, [Hmim]OAc, [Omim]OAc, and [Bmmim]OAc, and the complexes of [Bmim]OAc and chitobiose are fully optimized at B3LYP/6-31G(d,p) level.34 The interaction energy (DE) is defined as the energy difference between the ion pairs and the corresponding isolated ions. DE considering the basis set superposition errors (BSSE) correction using the counterpoise (CP) method is estimated. The vibrational frequencies of [Bmim]OAc, chitobiose and the complexes are calculated at B3LYP/6-31G(d,p) level too. Natural bond orbital (NBO) and atom in molecule (AIM) analyses are performed at same level to give second order perturbation stabilization energies E(2) and to confirm the existence of the hydrogen bond, respectively. In the NBO analysis,35 E(2) can be used to estimate the orbital
Fig. 1. The fragment of chitosan depicting by the main hydrogen-bond patterns: H/O, OeH/N.
Q. Tian et al. / Carbohydrate Research 408 (2015) 107e113
interactions between the proton donor and proton acceptor well, that is
Eð2Þ ¼ DEij ¼ qi
Fði; jÞ2 εj εi
where εi and εj are diagonal elements, qi is the donor orbital occupancy, and F(i,j) is the off-diagonal NBO Fock matrix element. The AIM analysis, which is known as a powerful tool, has been widely applied to study the hydrogen-bonded interaction.36e38 According to AIM theory, the electron density r(r), and the Laplacian of the electron density V2r(r), are used to describe the chemical bond. rBCP in Bond Critical Points (BCP) indicates the strength of hydrogen bonds and V2r(r) shows the nature of the bond. The hydrogen bonds and ionic bonds are characterized by V2r(r)>0 in the closedshell interactions, while covalent bonds are characterized by V2r(r) <0. li is an eigenvalue of the Hessian matrix of r, and there exists V2r(r)¼l1þl2þl3. When two of these three eigenvalues are negative and the other one is positive, the critical point is called a BCP, which indicates the existence of chemical bond between two atoms. When there are two positive and one negative eigenvalues, the critical point is called a Ring Critical Point (RCP), which indicates the existence of a ring structure. All the DFT calculations are computed in gas phase and carried out with Gaussian 03 package.39 3. Results and discussion 3.1. The solubility of chitosan in the imidazolium-based ILs As previous reported, the solubility data of chitosan in the ILs under study at different temperatures are summarized in Fig. S1 (Supplementary data).32 It can be seen that the solubility of chitosan at a given temperature generally decreases in the order [Bmim]OAc>[Emim]OAc>[Hmim]OAc>[Omim]OAc>[Bmmim]OAc. Among the investigated ILs, [Bmim]OAc is the most efficient one for the dissolution of chitosan. In order to study the possible dissolution mechanism of chitosan in ILs, The complexes formed by [Bmim]OAc, [Emim]OAc, [Hmim]OAc, [Omim]OAc and [Bmmim] OAc with chitobiose are optimized at the B3LYP/6-31G(d,p) level. The optimized structures of these complexes are shown in Fig. S2. For them, the interaction energies (DE) of chitobiose with [Emim] OAc, [Bmim]OAc, [Hmim]OAc, [Omim]OAc and [Bmmim]OAc are 2055.74, 2039.53, 2039.14, 2036.75 and 2021.31 kJ/mol, respectively. It indicates that DE of the complexes decrease in the order: [Emim]OAc>[Bmim]OAc>[Hmim]OAc>[Omim]OAc> [Bmmim]OAc. This result is broadly consistent with above experimental data. DE decreases with increasing alkyl length of imidazolium-based ILs when the anions are identical. In addition, the complexes formed by [Emim]OAc, [Bmim]OAc and chitobiose have lower DE than that by other ILs. However, the complex formed by [Emim]OAc and chitobiose has lowest interaction energy that is not in agreement with the experimental data. Since the alkyl chain length of [Emim]þ is shorter than that of [Bmim]þ, the steric hindrance in cation is relatively harder to attack oxygen atom in chitobiose. In a word, the dissolution mechanism of chitobiose in acetic-based ILs is complicated, and it is the outcome of many factors. The structures of the complexes formed by H2O, methanol, benzene and chitobiose optimized at B3LYP/6-31G(d,p) level are shown in Fig. S2. Interaction energy DE of the complexes is 147.56, 34.40 and 6.72 kJ/mol corresponding to the H2Oechitoiose complex, methanolechitobiose complex, and benzeneechitobiose complex, which is less than 1/10 time of that in [Bmim]OAc. It can be demonstrated that dissolution of chitosan in H2O, methanol and benzene is much weaker than of that in [Bmim]
109
OAc. This result is consistent with the fact that chitosan have very low solubility in water and conventional organic solvents. 3.2. Geometries of [Bmim]OAc and chitobiose In order to give a visual understanding of the interaction between chitobiose and [Bmim]OAc, the electrostatic potential surfaces for the most stable geometries of [Bmim]OAc, [Emim]OAc, [Hmim]OAc, [Omim]OAc, [Bmmim]OAc and chitobiose are given in Fig. S3. As is shown, the highly negative region of OAc is on the electronegative oxygen atoms and the molecular electrostatic potential value is 0.212 a.u. While the highly positive areas in the [Bmim]þ cation are around the hydrogen atoms on C2 and C1 and the molecular electrostatic potential value of the hydrogen atoms on C2 and C1 are 0.171 a.u and 0.148 a.u, respectively. Fig. 3 shows the most stable geometries of [Bmim]OAc optimized at the B3LYP/ 6-31G(d,p) level. For the ionic pair, we have considered various possible orientations between anion and cation corresponding to the molecular electrostatic potential values. So, [Bmim]OAc is considered to have two possible conformers A (Fig. 3) and B (Fig. S4). As can be seen from the results, the energies of the structure A and B are 651.389902 a.u and 651.389645 a.u, respectively. Because the structure A has a lower energy, it is considered when the OAc anion occurs in the vicinity of the C2eH bond and C1eH of the cation. This can be attributed to the larger positive charge on the C2eH unit (0.26800e) than on other CeH units and the lager negative charge on C]O unit (0.79985e). These results are consistent with the above analysis on the electrostatic potential surfaces. In [Emim]þ, [Hmim]þ, [Omim]þ and [Bmmim]þ, the natural charge of C2eH unit on imidazole ring is 0.26698e, 0.26647e, 0.26644e, and 0.26440e, respectively, which shows the possible regions to form the hydrogen bonds with chitobiose. In addition, the possible structures of chitobiose are also optimized at B3LYP/6-31G(d,p) level and the most optimal structure is showed in Fig. 2. Comparing the energies of the structure I and structure II (Fig. S5), the structure I (Fig. 2) is the most stable conformer. From Fig. 2, the structure I contains two types of hydrogen bonds, including O16eH17/N44 and O22eH23/O34. In chitobiose, the highly negative and positive regions are around the oxygen atom on O16eH17 (0.0633 a.u) and the hydrogen atom on
Fig. 3. The optimized structures and geometrical parameters for various species in [Bmim]OAc optimized at B3LYP/6-31G(d,p) level (Distance in Å). The H bonds are indicated by dashed lines.
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O38eH39 (0.689 a.u). The corresponding natural charge of O16 and H38 are 0.79257e and 0.49334e, respectively. The more positively charged regions of [Bmim]þ cation, the more negatively charged regions of OAc anion are the possible hydrogen bonding sites of the interaction with chitobiose. In addition, the highly negative and positive regions are around the oxygen atom and the hydrogen atom in H2O. The natural charge of O1 and H1 is 0.95357e and 0.47678e. As expected, the formation of the complexes between ILs, H2O and chitobisoes should occur in those regions possessing more positive charges and more negative charges. A series of possible initial geometries for the complexes are designed according to the above data. 3.3. Geometry of the complex Fig. 3 shows that the calculated H1/O1 and H2/O2 distances are 1.98 Å and 1.56 Å, respectively, which are both remarkably shorter than the summation (2.72 Å) of the van der Waals radii of O and H atoms. And C2eH/O2 angle and C1eH1/O1 angle are 174.41 and 155.99 , respectively. These data indicate the formation of C2eH2/O2 and C1eH1/O1 H-bonds between the anion and cation. The interaction energy of the ionic pair is calculated to be 1472.62 kJ/mol. The effective hydrogen bond and the strong Coulomb interaction between the cation and anion are in agreement with the experimental results.40 Fig. 2 shows two important features of the geometry: (i) the chitobiose is structurally suitable for mimicking a real chitosan chain and the saccharide rings keeping a chair conformation, and (ii) the ring oxygen (Oring) in a monosaccharide establishes an important hydrogen bridge with the hydroxyl group in the next monosaccharide. As shown in Fig. 2, the hydrogen bond distance is 2.66 Å and the O22eH23/O34 angle is 133.17. In addition, the distance between N44 and H17 in O16eH17 is 1.98 Å and the O16eH17/N44 angle is 172.54 , which implies the existence of OeH/N hydrogen bond. The parameters of these hydrogen bonds show obviously the existence of strong intramolecular hydrogen bonding in a chitosan chain. These intramolecular hydrogen bonds play an important role in the low solubility of chitosan in most solvents. Three complexes formed by chitobiose and [Bmim]OAc are optimized at the B3LYP/6-31G(d,p) level and shown in Fig. S6. The interaction energies (DE) of three complex conformations (a), (b) and (c) are 2039.53, 2019.68 and 2031.06 kJ/mol, respectively. The calculated data show that the most stable conformation is the complex (a). The optimal geometry of the complex (a) is shown in Fig. 4. As seen from Fig. 4, the main hydrogen bonds between [Bmim]OAc and chitobiose are obtained, the bond lengths are 2.34,
Fig. 4. The optimized structures and geometrical parameters in the complex formed by chitobiose and [Bmim]OAc optimized at the B3LYP/6-31G(d,p) level (Distance in Å). The H bonds are indicated by dashed lines.
1.89, 1.63 and 1.49 Å corresponding to H1/O16, H2/O16, H39/O1 and H41/O2, respectively; and angles are 148.85, 144.35, 171.65 and 174.48 corresponding to :C1eH1/O16, :C2eH2/O16, :O38eH39/O1 and :O40eH41/O2, respectively. It indicates that there exists strong intermolecular hydrogen bonding interaction between chitobiose and [Bmim]OAc. The N44/H45 distance is extended from 1.98 Å in the pure chitobiose to 2.02 Å in the complex. In addition, O34/H23 distance also varies from 2.66 Å to 2.69 Å. The corresponding angles of O16eH17/N44 and O22eH23/O34 vary ranging from 172.54 to 172.48 and from 133.17 to 136.22 , respectively. It means that the strength of these hydrogen bonds decreases in the complex comparing with that in chitobiose. These intermolecular hydrogen bonds are expected to be a key factor resulting in the high solubility of chitosan in [Bmim] OAc. The complexes formed by other ILs and chitobiose also have intermolecular hydrogen bonds. However, comparing with the hydrogen bonding interaction between chitobiose and [Bmim]OAc, they are weaker. For example, in chitobiosee[Emim]OAc complex, the interaction distances are 1.90 and 1.64 Å corresponding to H1/O16 and H39/O1, respectively. It shown that the hydrogen bonds C2eH2/O16 and O38eH39/O1 are weaker than the corresponding hydrogen bonds in chitobiosee[Bmim]OAc complex. In the complexes formed by [Hmim]OAc, [Omim]OAc, [Bmmim]OAc and chitobiose, the bond lengths of H2/O16 and H39/O1 are also larger at some extent. It is shown that the interaction of [Bmim]OAc with chitobiose is much stronger than other ILs. These results are consistent with the conclusion of the solubility of chitosan in ILs. 3.4. Vibration mode analysis Main variations of the vibration spectra in [Bmim]OAc, [Emim] OAc, [Hmim]OAc, [Omim]OAc, [Bmmim]OAc and chitobiose are calculated in GaussView41 and listed in Table S1, the corresponding calculated IR spectrum of the complexes are given in Fig. S7, and the data directly related to vibration mode analyses are given in Table 1. It can be seen that nas(C]O) is red-shift from 1648 cm1 in OAc to 1588 cm1 in [Bmim]OAc, and ns(C]O) is blue-shift from 1345 cm1 to 1381 cm1 in [Bmim]OAc. The reason for this is that O2eC3]O1 in OAc anion are sp2 hybridization, because of the effect of pep conjugation, the negative charge can be distributed to two O atoms averagely. When OAc anion approaches [Bmim]þ, it should break the pep conjugation, thus ns(C2eH) is red-shift from 3026 cm1 to 2308 cm1 and ns(C1eH1) is also red-shift from 3036 cm1 to 3001 cm1. Therefore, the C2eH2/O2, and C1eH1/O1 hydrogen bonds are formed because the interactions between the OAc and [Bmim]þ. This is the reason why [Bmim]OAc has unique properties as the efficient solvent for the polymer. Table 1 shows that ns(O38eH39) and ns(O40eH41) in chitobiose are red-shift from 3790 cm1 to 3171 cm1 and from 3835 cm1 to 2594 cm1, respectively. It manifests the existence O40eH41/O2 and O38eH39/O1 hydrogen bonds between chitobiose and OAc. At the same time, ns(C1eH1) and ns(C2eH2) are 3036 cm1 and 3026 cm1 in [Bmim]þ, which have a blue-shift to 3159 cm1 and 3130 cm1 in the complex, respectively. It can be explained that C1eH1/O16 and C2eH2/O16 are formed between chitobiose and [Bmim]þ. In addition, ns(N44eH45) and ns(O22eH23) in chitobiose are 3529 cm1 and 3774 cm1, respectively. In the complex, both ns(N44eH45) and ns(O22eH23) have a red-shift to 3450 cm1 and 3752 cm1, respectively. These hydrogen bonds in complex are better explanations why chitobiose dissolves easily in [Bmim]OAc. From Table 1, in the complexes formed by [Emim]OAc, [Hmim] OAc, [Omim]OAc, [Bmmim]OAc and chitobiose, ns(O38eH39) is red-shift to 3178, 3189, 3295 and 3166 cm1, respectively. It can be shown that O38eH39/O1 in these complexes is weaker than that
Q. Tian et al. / Carbohydrate Research 408 (2015) 107e113 Table 1 Main variation of the vibration spectrum in [Bmim]OAc, chitobiose and the complexes formed by ILs and chitobiose
OAc [Bmim]þ [Bmim]OAc
Chitobiose
Chitobioseþ[Bmim]OAc
Chitobioseþ[Emim]OAc Chitobioseþ[Hmim]OAc Chitobioseþ[Omim]OAc Chitobioseþ[Bmmim]OAc
Frequencies (cm1)
IR intensities (km mol1)
Vibrational mode assignment
1344 1648 3035 3026 1381 1588 2308 3000 3528 3774 3835 3789 3478 3752 3535 3450 3173 3170 3130 3070 2999 2594 1676 1439 3178 2595 3189 2594 3295 2600 3166 2616
337.6 674.5 57.4 41.0 364.7 283.9 1913.2 256.6 622.2 63.6 31.5 55.3 8.2 102.3 6.9 1.9 28.7 1953.4 221.9 38.2 2085.3 3018.5 497.7 439.2 1412.3 3028.8 1907.8 3040.2 1436.7 3002.9 1859.2 2888.3
ns(C]O) nas(C]O) ns(C1eH) ns(C2eH) ns(C]O) nas(C]O) ns(C2eH2) ns(C1eH1) ns(O16eH17) ns(O22eH23) ns(O40eH41) ns(O38eH39) ns(N44eH45) ns(O22eH23) nas(N44eH45) ns(N44eH45) nas(C1eH1) ns(O38eH39) ns(C2eH2) ns(C1eH1) ns(O16eH17) ns(O40eH41) nas(C]O) ns(C]O) ns(O38eH39) ns(O40eH41) ns(O38eH39) ns(O40eH41) ns(O38eH39) ns(O40eH41) ns(O38eH39) ns(O40eH41)
in the [Bmmim]OAcechitobiose complex. In addition, ns(O40eH41) is also red-shift to 2595, 2594, 2600 and 2616 cm1, respectively. It manifests that the formation of O40eH41/O2 in these complexes and O40eH41/O2 hydrogen bond between [Bmmim]OAc and chitobiose is stronger than that in other complexes. These results are consistent with the changes of the structures. 3.5. NBO analysis The NBO method characterizes hydrogen bonds in terms of hyper conjugative donoreacceptor interactions and has been used to study the bonding properties in the complexes. Table 2 shows that the main donoreaccepter interactions of chitobiose and the complexes formed between chitobiose with ILs and H2O, as well as their second order perturbation stabilization energies, E(2). The values of E(2) denote the intensity of the donoreaccepter interaction. The larger E(2) is, the stronger the interaction will be. For chitobiose, which contains two types of the intramolecular hydrogen bonds: O16eH17/N44 and O22eH23/O34, from Table 2 it is found that the value of E(2) (LP1(N44)/s*(O16eH17) is 70.25 kJ/mol. It indicates the existence of a stronger orbital interactions between electron donor of the long pair LP1(N44) and the sigma anti-bond orbital of the electron acceptor s*(O16eH17). Fig. S8a shows that it should be the best overlap, symmetrical matching orbital and the smaller energy gap (0.83 au). Interestingly, O22eH23/O34 intramolecular hydrogen bonds has not been discovered in NBO analyses, it may be due to interaction orbital, which exists asymmetrical matching and the bond length (2.66 Å) of O22eH23/O34 is longer than that (1.98 Å) of O16eH17/N44, which is in agreement with the above analysis. The complexes formed between chitobiose and H2O are considered
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Table 2 The main donoreaccepter interactions in the process and their second order perturbation stabilization energies E(2) at B3LYP/6-31G(d,p) level
Chitobiose ChitobioseþH2O (I) ChitobioseþH2O (II)
Chitobioseþ[Bmim]OAc
Chitobioseþ[Emim]OAc Chitobioseþ[Hmim]OAc Chitobioseþ[Omim]OAc Chitobioseþ[Bmmim]OAc
Donor (i)
Acceptor (j)
E(2) kJ/mol
LP1(N44) LP1(O1) LP2(O5) LP1(O22) LP2(O22) LP1(O1) LP2(O1) LP2(O1) LP1(O1) LP2(O1) LP1(O2) LP2(O2) LP2(O16) LP2(O16) LP1(O16) LP1(O16) LP2(O1) LP2(O2) LP2(O1) LP2(O2) LP2(O1) LP2(O2) LP2(O1) LP2(O2)
s*(O16eH17) s*(O38eH39) s*(O38eH39) s*(O1eH1) s*(O1eH1) s*(N1eH2) s*(O1eH1) s*(N1eH2) s*(O38eH39) s*(O38eH39) s*(O40eH41) s*(O40eH41) s*(C2eH2) s*(C2eN2) s*(C2eH2) s*(C2eN2) s*(O38eH39) s*(O40eH41) s*(O38eH39) s*(O40eH41) s*(O38eH39) s*(O40eH41) s*(O38eH39) s*(O40eH41)
70.25 1.72 88.53 2.97 74.43 0.84 0.21 24.64 115.65 3.93 217.07 3.14 38.95 0.67 19.04 1.13 116.16 216.95 42.82 216.58 87.65 214.32 116.91 212.68
and are shown in Figs. S8b and S8c for comparison. On the contrary, it can be found that interaction energy (146.07 kJ/mol) of the complex formed between O22eH23/O34 bond with H2O is larger than that (130.77 kJ/mol) between O16eH17/N44 bond with H2O. From Table 2, one can see that for former complex E(2)s of LP2(O1)/s*(O38eH39) and LP2(O2)/s*(O40eH41) are 90.67 and 74.43 kJ/mol, respectively. It can be seen that the interaction orbitals corresponding to Figs. S8b1 and S8b2 are symmetrical matching with the largest overlap. In addition, the interaction orbitals corresponding to LP1(O1)/s*(O38eH39) and LP1(O22)/ s*(O1eH1) are asymmetrical matching and the values of E(2) are 1.13 and 2.97 kJ/mol, respectively, which are much less than those above two interaction orbital. For the latter complex it can be seen that the interaction orbitals are LP1(O1)/s*(N44eH45), LP2(O1)/s*(O16eH17) and LP2(O1)/s*(N44eH45) from Figs. S8c1, S8c2 and S8c3, respectively. There is only an orbital of LP2(O1)/s*(N44eH45), which possess the symmetrical matching with the largest overlap. Its value (24.64 kJ/mol) is much less than those of above two orbital, so the interaction energy between O16eH17/N44 bond with H2O is less than that between O22eH23/O34 bond with H2O. Fig. 4 shows that for the complex formed between chitobiose and [Bmim]OAc, the optimized structure is [Bmim]þ combined with O16eH17/N44 in chitobiose and OAc connected with O22eH23/O34 in chitobiose. As shown in Table 2, there are six symmetrical matching orbitals: LP1(O1)/s*(O38eH39), LP2(O1)/s*(O38eH39), LP1(O2)/s*(O40eH41) and LP2(O2)/ s*(O40eH41) for OAc corresponding to the values of E2 are 43.22, 115.65, 43.47 and 217.07 kJ/mol, respectively. As well as LP1(O16) /s*(C2eH2) and LP2(O16) /s*(C2eH2) for [Bmim]þ corresponding to the values of E2 are 9.31 and 4.55 kJ/mol, respectively. The largest E2 (217.07 kJ/mol) is LP2(O2)/s*(O40eH41), which possesses the maximum overlap and minimum energy gap (0.78 a.u.) (Fig. S8d4). From above E2 values it can be found that the summation value (419.40 kJ/mol) of E2 for OAc is much larger than that (57.99 kJ/mol) of E2 for [Bmim]þ. In other words, OAc plays an important role in chitobiose dissolution by [Bmim]OAc. What is more, the summation value (165.10 kJ/mol) of E2 in chitobiosee [Bmim]OAc complex is much larger than that (114.10 kJ/mol) of
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chitobioseeH2O complex, even chitobiosee2H2O complex, where O16eH17/N44 and O22eH23/O34 hydrogen bonds in chitobiose are broken by two water molecules, and the summation value of E2 is only 189.74 kJ/mol. These results are consistent with the conclusion of the hydrogen bond distances discussed above. As shown in Table 2, in the complexes formed by [Emim]OAc, [Hmim]OAc, [Omim]OAc, [Bmmim]OAc and chitobiose, the largest E2 (LP2(O2)/s*(O40eH41)) is 216.95, 216.58, 214.32 and 212.68 kJ/mol, respectively. It can be found that the value of E2(LP2(O2)/s*(O40eH41)) decreases in the order: [Bmim]OAc> [Emim]OAc>[Hmim]OAc>[Omim]OAc>[Bmmim]OAc. These conclusions are in agreement with the experimental results.
consistent with the conclusion of the hydrogen bond distances discussed above. From Table 3, it can also be found that the largest r(r) (O40eH41/O2) is 0.077395 a.u, 0.077438 a.u, 0.076622 a.u and 0.076290 a.u, respectively, corresponding to [Emim]OAcechitobiose complex, [Hmim]OAcechitobiose complex, [Omim]OAcechitobiose complex and [Bmmim]OAcechitobiose complex. It can be found that the r(r) values of O40eH41/O2 in the [Bmim] OAcechitobiose complex is larger than other complexes. It indicates clearly that the hydrogen bonds in the chitobiosee[Bmim] OAc complex is much stronger than that in other complexes. These results are also consistent with the conclusion of the hydrogen bonds discussed above.
3.6. AIM analysis
4. Conclusions
The molecular graphic corresponding to the complex of chitobiose with [Bmim]OAc is shown in Fig. S9. Table 3 shows r(r), V2r(r) and the matrix eigenvalues (l1, l2 and l3). The two topological properties of all hydrogen bonds in the complexes formed by H2O, five ILs and chitobiose, are in the range of the criteria (i.e., 0.002e0.035 a.u. for r(r), 0.014e0.139 a.u. for V2r(r)) proposed by Popelier42,43 for the existence of a hydrogen bond. V2r(r) values of the hydrogen bonds in chitobioseþH2O system are 0.097 a.u (O1eH1/O22) and 0.10 a.u (O38eH39/O1), respectively. In contrast, V2r(r) values of the hydrogen bonds in the complex are 0.15 a.u (O38eH39/O1), 0.16 a.u (O40eH41/O2), 0.039 a.u (C1eH1/O16) and 0.090 a.u (C2eH2/O16), respectively. It can be observed that V2r(r) value of the chemical bonds in the complexes in the range of the criteria, which indicates that the main chemical bonds of the complexes are hydrogen bonds. In addition, r(r) values of O40eH41/O2 and O38eH39/O1 hydrogen bonds are 0.035776 a.u and 0.36129 a.u, respectively. In the same way, r(r) values of O38eH39/O1, O40eH41/O2, C1eH1/O16 and C2eH2/O1 are 0.052740 a.u, 0.077440 a.u, 0.013249 a.u and 0.031455 a.u, respectively. It is shown that r(r) values of O40eH41/O2 in the complex are much higher than that of O38eH39/O1 in chitobioseþH2O system. As r(r) values are related to the bond strength, it indicates clearly that the hydrogen bond in the complex is much stronger than that in chitobioseþH2O system. These results are also
[Emim]OAc, [Bmim]OAc, [Hmim]OAc, [Omim]OAc and [Bmmim] OAc are used to investigate the dissolution mechanism of the chitosan in ILs. For simplify the DFT calculations, the chitobiose is chosen as the model of chitosan. The interaction energies (DE) of five ILs with chitobiose are calculated at B3LYP/6-31G(d,p) level. The comparison of DE indicates the complexes formed by [Emim] OAc and [Bmim]OAc with chitobiose are larger than that by other complexes. The complexes formed by H2O, benzene, methanol and chitobiose are optimized at B3LYP/6-31G(d,p) level. DE of the complexes is much less than 1/10 time of that in [Bmim]OAc. These results show that [Bmim]OAc is the best solvent to dissolve the chitosan. In addition, the electrostatic potential surfaces are taken into consideration for the initial geometry design for the complexes formed by chitobiose and ILs. The smaller intermolecular H-bond distance and larger H-bond angle suggest that the hydrogen bonds formed between [Bmim]OAc and chitobiose are stronger than those of other complexes. The vibration mode analyses indicates that the formation of complexes between chitobiose and [Bmim]OAc is due to hydrogen bond interactions. These hydrogen bonds have been characterized by the NBO and AIM analysis to the unique properties. In chitobiosee[Bmim]OAc complex, the largest E2 (217.07 kJ/ mol) is LP2(O2)/s*(O40eH41), which is larger than that in other complexes. The r(r) and V2r(r) of all the hydrogen bond of the
Table 3 The electron density (rBCP, a.u.), Laplacian of the electron density (V2rBCP, a.u.) and matrix eigenvalues (l1, l2, and l3) at the B3LYP/6-31G(d,p) level
[Bmim]OAc ChitosanþH2O Chitosanþ[Bmim]OAc
Chitosanþ[Emim]OAc
Chitosanþ[Hmim]OAc
Chitosanþ[Omim]OAc
Chitosanþ[Bmmim]OAc
AeB bond
r
V2r
ε
l1
l2
l3
C2eH2/O2 C1eH1/O1 O1eH1/O22 O38eH39/O1 O38eH39/O1 O40eH41/O2 C1eH1/O16 C2eH2/O16 O38eH39/O1 O40eH41/O2 C1eH1/O16 C2eH2/O16 O38eH39/O1 O40eH41/O2 C1eH1/O16 C2eH2/O16 O38eH39/O1 O40eH41/O2 C1eH1/O16 C2eH2/O16 O38eH39/O1 O40eH41/O2 C1eH1/O16
0.068580 0.027075 0.035776 0.036129 0.052740 0.077440 0.013249 0.031455 0.052710 0.077395 0.014842 0.031144 0.052632 0.077438 0.016777 0.029261 0.047501 0.076622 0.016356 0.030648 0.052899 0.076290 0.021392
0.1522 0.07245 0.09780 0.1015 0.1469 0.1553 0.03926 0.08996 0.1469 0.1551 0.04188 0.08898 0.1466 0.1553 0.04579 0.08205 0.1665 0.1554 0.05180 0.09997 0.1472 0.01563 0.05696
0.02980 0.01487 0.02063 0.05432 0.002596 0.008127 0.1340 0.1037 0.002678 0.008193 0.1188 0.1087 0.002869 0.007952 0.1100 0.1062 0.02310 0.02056 0.1138 0.1061 0.002212 0.007976 0.08557
0.1344 0.03347 0.05268 0.5415 0.09228 0.1664 0.01465 0.04352 0.09242 0.1672 0.01681 0.04290 0.09220 0.1664 0.01949 0.03944 0.08135 0.1685 0.01929 0.04348 0.09288 0.01626 0.02611
0.1305 0.03298 0.05161 0.05136 0.09204 0.1651 0.01292 0.03943 0.09217 0.1659 0.01503 0.03869 0.09194 0.1651 0.01755 0.03565 0.07951 0.1651 0.01732 0.03931 0.09268 0.1613 0.02405
0.4172 0.1389 0.2021 0.2070 0.3312 0.4868 0.06682 0.1729 0.3315 0.4882 0.07372 0.1706 0.3307 0.4868 0.08283 0.1571 0.3274 0.5461 0.08841 0.1828 0.3328 0.4803 0.1071
Q. Tian et al. / Carbohydrate Research 408 (2015) 107e113
complexes formed by ILs and chitobiose are in the range of the criteria proposed by Popelier. These results show that the electrostatic interaction between chitobiose and [Bmim]OAc is much stronger than that between H2O, other ILs and chitobiose. The reason for this is that chitobiose has high solution ability in [Bmim] OAc, which is in good agreement with experimental results. Acknowledgments This work was funded by the National Natural Science Foundation of China (21473252). Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carres.2015.02.015. References 1. Dias AM, Cortez AR, Barsan M, Santos J, Brett CM, de Sousa HC. ACS Sustain Chem Eng 2013;1:1480e92. 2. Chen QT, Xu AR, Li ZY, Wang JJ, Zhang SJ. Green Chem 2011;13:3446e52. 3. Geng X, Kwon O-H, Jang J. Biomaterials 2005;26:5427e32. 4. Ravi Kumar M. N React Funct Polym 2000;46:1e27. 5. Sashiwa H, Kawasaki N, Nakayama A, Muraki E, Aiba S-i. Chitin Chitosan Res 2002;8:249e51. 6. Nah JW, Jang MK. J Polym Sci Part A Polym Chem 2002;40:3796e803. 7. Hallett JP, Welton T. Chem Rev 2011;111:3508e76. 8. Petkovic M, Seddon KR, Rebelo LPN, Pereira CS. Chem Soc Rev 2011;40: 1383e403. 9. Wasserscheid P, Keim W. Chem Int Ed 2000;39:3772e89. 10. Bara JE, Camper DE, Gin DL, Noble RD. Acc Chem Res 2010;43:152e9. 11. Armand M, Endres F, MacFarlane DR, Ohno H, Scrosati B. Nat Mater 2009;8: 621e9. 12. Hu S, Zhang Z, Zhou Y, Song J, Fan H, Han B. Green Chem 2009;11:873e7.
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