Wang Yu, Fang Zhijie £¬Jao Yan ,Wang Yuanxing
(Chemical Engineering School, Nanjing University of Science & Technology, Nanjing,210094)
Supported by the National Natural Science Foundation of China (No.29972020)
Abstract Ab intito Hartree-Fock(HF) and MØller-Plesset second order (MP2) perturbation theories for the correlation energy were applied to the investigation of the thermodynamic properties of 1,4-O-Bis(2,3,4- tri-O-acetyl -b-D-glucopyranosyloxy)benzene(I)and terephthalyl chloride(II) with 3-21G* basis sets. Cyclic structures were identified as the most stable conformation of the acetylized glycophane(III) , the energy change (DE) in the process of 2 I + 2 II III + 4HCl (Scheme 1)is about -27.37 kJmol-1 at the MP2/3-21G* level with dichloromethane as solvent. The Gibbs free energy change (DG) in the process was predicted to be -70.01 kJmol-1 at 273.15K. Acetylized glycophane(III) can be most easily spontaneously produced in the mixture of compound (I) and terephthalyl chloride(II) in dichloromethane solution at 273.15K, which is in agreement with the experimental fact.
Glycophane is a new type of neutral, chiral and water-soluble synthetic receptor which have hydrophobic cavities and can be concerned with the carbohydrate recognition processes. The recognition processes can be widely applied not only for the complexation between oligosaccharides and agglutinins or antibodies, but also for the fields of biology, medicine and catalysis etc. Since the synthesis of glycophane is very difficult, with complicated steps, only a few glycophanes have been synthesized in the last 20 years, and have not been in extensive use[1-4], our research group has primarily made lots of the research recently.
A novel synthesis of acetylized glycophane route has been designed, that is the annelation between 1,4-O-Bis (2,3,4-tri-O-acetyl-b-D-glucopyranosyloxy) benzene(I) terephthalyl chloride(II) to synthesize acetylized glycophane(III) as shown in Scheme
Studies on the atomic and molecular parameters of the compound can be expected to predict whether or how the reaction will happen, which leads to experiment for reference [6-8]. To illuminate the structure or synthesis of these glycophanes, the authors performed quantum chemistry calculations on a model for acetylized glycophane (III). As a result, acetylized glycophane(III) can be spontaneously synthesized in the mixture of compound (I) and terephthalyl chloride(II), which is in agreement with the experimental fact.
- CALCULATION MODEL AND METHODS
To study the changes of both the total energy and Gibbs free energy concerning the synthesis of such glycophane, either ab initio or semiempirical methods can be employed. Geometric model was primely optimized with PM3 of semi-empirical calculation to give initial conformation for HF/3-21G* calculations.
The energies in different solutions and the changes in thermodynamic properties at different temperatures in the process of synthesizing acetylized glycophane(Άσ) were investigated to predict optimal reaction condition[9-12].
All quantum chemical calculations were performed with the Gaussian 98 suite of programs.
- RESULATS AND DISCUSSION
3.1 Total Energies
Table 1 shows the total energies of three compounds in different solutions at the MP2/3-21G* levels, which indicating that the HF-optimized structure can be used to estimate the binding energies.
Table 1 The total energies of three compounds in different solutions at the MP2/STO-3G levels
The energies for the compound I, II and III in dichloromethane solution were estimated to be -55.47, -47.26 and -231.77 kJmol-1 at the MP2/3-21G* level respectively. The changes of the binding energies in the process of 2 I + 2 II III + 4HCl, that is the values of the energy change: DE=EIII-2EII-2EI– 4EHCl, are all negative at different computational levels, The binding energy of compound III is large enough to compensate for the energies needed for the dissociation of compound I and II. From the view point of energy, the formation of acetylized glycophane is feasible when the compound I and II are mixed. From the values of the binding energies, it can be predicted that the compound III might be the easiest synthesized in the mixture of compound Iand II with dichloromethane or chloroform as solvent, so we can select dichloromethane or chloroform as solvent to synthesize the acetylized glycophane.
3.2 Thermodynamic Properties
On the basis of vibrational analysis and statistical thermodynamic study, the standard thermodynamic functions, entropies(Smo) and enthalpies (Hmo), were obtained and listed in Table 2.
In the process of 2 I + 2 II III + 4HCl ,values of DST and DHT are positive and negative respectively, and both the values fluctuates as the temperature increases from 258.15 to 298.15K. The process is therefore an exothermic process accompanied by an increase in entropy. From the equation DG=DH-TDS, the change in Gibbs free energy (DG) in the process was predicted to be -70.01kJmol-1 at 273.15K, the acetylized glycophane(III) can be spontaneously produced, which is also in agreement with the experimental fact that the acetylized glycophane can relatively easily be synthesized at 273.15K than the case at other temperature.
Table 2 Thermodynamic properties of the three compounds at the MP2/3-21G* level.
|I||II||III||2 I + 2 II III + 4HCl|
|T(K)|| Smo Hmo
| Smo Hmo
| Smo Hmo
|΅χST ΅χHT ΅χGT
(Jmol-1K-1) (kJmol-1) (kJmol-1)
|258.15||584.73 47.77||513.25 40.28||2271.55 153.03||43.27 -39.52 -50.69|
|273.15||612.55 67.31||529.09 43.69||2289.57 170.27||68.70 -51.24 -70.01|
|288.15||640.47 80.05||545. 61 57.80||2318.75 216.68||23.33 -55.42 -62.14|
|298.15||653.76 95.22||559.20 70.56||2388.70 257.77||30.12 -50.22 -59.32|
|DST= (Smo)III+4(Smo)HCl –2(Smo)I–2(Smo)II; DHT= (Hmo)III+4(Hmo)HCl –2(Hmo)Άρ–2(Hmo)II; DGT=DHT-TDST|
The ab initio quantum chemistry calculations on 1,4-O-Bis (2,3,4- tri-O-acetyl-b-D-glucopyranosyloxy) benzene (I), terephthalyl chloride(II) and acetylized glycophane (III) shows, that the total energy change and the change in Gibbs free energy provide theoretical judgements and dichloromethane prediction that the formation of the compound (III) with the compound (II) and (I) as intermediates in dichloromethane or chloroform solvent at 273.15K might be the easiest, which is consonant with the experiments.