Conformation of 1,2-Dimethoxyethane in Water

Vol. 17No. 32001Chemical Research in Chinese U niversities315~317Conformation of 1 , 2-Dimethoxyethane in Water *LIU Yue'3°*，YANG Xiao zhen2 , DAI Bai-qing.1 , SU Zhong min'* and WANG Qi'(1. Institute of Opto-electronics , Harbin Insitute of Technology, Harbin 150001, P. R. China;2. Polymer Physics Laboratory, Institute of Chemistry, Academia Sinica, Beijng 100080, P. R. China;3. Department of Chemistry, Harbin Normal University, Harbin 150080, P. R. China;4. Department of Chemistry, Northeast Normal Universiy, Changchun 130024, P. R. China)(Received July 10, 2000)To understand the conformation of 1 ,2-dimethoxyethane (DME) in water, a system of twokinds of molecules, DME and H2O, was focused. The interaction of various conformers of DMEwith water was studied by means of ab initio molecular orbital calculation with 6-31G (d) basisset. It is shown that there are two forms of interactions between the two molecules in the sys-tem, the close touched (H2O attaches to the two oxygen atoms of DME) and the open touched(H2O attaches to one oxygen atom of DME) structures. The conformation of DME is remark-ably influenced by the interactions. Instead the ttt conformer is preferred in the gas state, with aclose touched H2O the tgt conformer becomes the most stable one. The obtained hydration ener-gies show that the stabilized order of DME conformers by water is tgt > >tgg' ; > ttt.Keywords 1, 2-Dimethoxyethane , Conformation, Ab initio, Molecular orbital calculationArticle ID 1005- 9040(2001 )-03- 315-03IntroductionThere are trans (t) and gauche (g or g' ) conformations for each of the three backbonebonds of 1,2 dimethoxyethane (DME). Since the experiments[1.2] show that the most stableconformer of DME is g for the middle bond and t for the others, the tgt conformation haslong been accepted with the lowest conformational energy. However, quantum mechanicsanalysis results in a different conclusion, that is, the ttt conformation is the most stable. Ithas puzzled scientists for years until Astrup' s discovery[3] that the conformational distributionsof DME in the gas state are different from those in the liquid state. Yoshida[4] supported thispoint again by IR spectra that the most stable conformation for an isolated DME is tt. Manyliteratures indicated that the conformational distribution of DME in the gas state differs fromthat in the liquid state or in an aqueous solution[5-8. .Why does DME in liquid or in water show the stable conformer tgt? This has receivedmuch attention in literatures. Andersson et al. [] found that nure nantim chemical calcula-tion, or quantum chemical calculation together with r中国煤化工annot be usedto predict experimental observations since the reactidCNMHGr the solvent# Supported by Polymer Physics Laboratory, Chinese Academy of Sciences, Beijing.# # To whom correspondence should be addressed.316Chemical Research in Chinese UniversitiesVol. 17molecule is much smaller than the solute molecule. Acoording to pure quantum chemical cal-culation together with a statistical mechanical simulation of the dielectric medium, the tgt con-former is more stable than the tt conformer by 4. 18- - 6.27 kJ/ mol even through this resultappears to be lack of the structural details of the interaction of DME and water.The present paper is aimed at the interaction details of various conformers of DME withwater to build up the interaction models. Since the conformational energies of DME can notbe properly calculated by using reaction field techniques[9] and Amsterdam Density Functionalmethod[10], the ab initio molecular orbital calculation together with a model of a DME and areal size water molecule should be a suitable method in realizing our mission here.MethodThe study was carried out by Gaussian 94 (Revision D.1)m1. The interactions of a wa-ter molecule with three lowest conformers ofDME, i.e. ，tt, tgt, and tgg' were investi-gated by RHF at 6-31G(d) basis set level.9Taking one molecules of water and one omolecule of DME as a model system， the(A(Bmolecules of water and DME can form twoFig. 1 The tgt conformer of DME forming akinds of structures. One is the close touchedclose touched structure with a waterstructure, in which the two hydrogen atoms ofmolecule (A), and the ttt conformer ofa water molecule attach to the two oxygenDME forming an open touched structureatoms of DME by hydrogen bond. The otherwith a water molecule (B).is the open touched structure, in which only one hydrogen atom of a water molecule attachesto one of the oxygen atom of a DME molecule. The two structures are shown in Fig. 1.Results and DiscussionSince tt, tgt and tgg' are the main conformers for DME5- 8, the three conformers arebrought into focus in the following discussion. For an isolated DME molecule, we found thatthe tt conformer is the lowest in energy, while the tgt and tgg' conformers are 5. 85 and6.90 kJ/mol respectively higher than the ttt conformer in energy (Fig. 2 curve a). Our re-sults with 6-31G(d) basis set are comparable to the D95+ (2d,f,p) basis set resultsl12,i.e. , the tgt and tgg' conformers are 6. 35 and 7. 44 kJ/ mol respectively higher than the ttconformer in energy.Conformational results of a DME molecule interacting with a water molecule are quitedifferent from those of isolated DME molecules. The tgt conformer is the most stable confor-mation (Fig. 2 curve b). The geometry optimization中国煤化Inergies obtainedshow that the ttt conformer can be converted to thHCN MH G in water, andthat the open touched structure for the tgt cannot exist in water. In the conformation searchwithout constraint, a starting structure of an open touched structure for the tgt always result-ed in a close touched structure, while a close touched structure for ttt remained with higherNo.3LIU Yue, YANG Xiao-zhen, DAI Bai-qing et al.317energy，couldn't geometrically be furtheroptimized, and easily resulted in an opentouched structure. Among the examined inter-acting conformations, the close touched struc-bture of the ttt is the highest in energy. EvenA一25the dihedral angles of the three backbonecbonds in the close touched structure of the ttt一30become - 175.1°，- - 150.4°, and一175.0°,tgg'Conformerwhich means that the interaction of water with Fig. 2 The conformational energies relative toDME is the driving force to bend the t confor-the ttt conformer for isolated DME (a),mation of CH2- CH2 bond to the g conforma-relative to the tgt conformer for the as-tion. The close touched and the open touchedsociated structure of DME and water(b), and the hydration energies (c).structures of the ttt with a water molecule haveThe data for curve (b) or (c) was taken fromhigher energies of 13. 84 and 2. 01 kJ/mol, re-the most stable structure, the close touched orspectively, than that of the close touchedthe open touched structure of each conformer.structure of the tgt. The energies of the close touched and the open touched structures of thetgg' conformer are comparable, which are 5. 94 and 5. 81 kJ/mol, respectively, higher thanthe close touched structure of the tgt conformer. The hydration energy of a conformer ofDME is defined as the energies of the associated molecules minus the energies of the isolatedDME conformer and a water molecule. In Fig. 2, curve c shows the hydration energies of thethree conformers，which demonstrates the population of the tgt conformer increases with thedecrease of the ttt conformer when DME is going from the gas to the liquid state. This con-forms with the experimental results[5.6]. It is concluded that the model of a DME molecule in-teracting with a water molecule reveals the main features of the interactions of various DMEconformers with water.References[1 ] Iwamoto R. , Spectrochim. Acta A.，27(11), 2 385(1971)[2] Abe A., Tasaki K., Mark J. E., Polymer J., 17, 883(1985)[3] Astrup E. E., Acta Chem. Scand. , A33, 655(1979)[4] Yoshida H.，Kaneko I.，Matsura H. et al. , Chem. Phys. Lett. ，196(6), 601(1992)[5] SmithG. D., JaffeR. L., YoonD. Y., J. Am. Chem. Soc., 117(1), 530(1995)[6] YangX. 2., SuZ., Wu D. et al. , Macromolecules, 30(30), 3 796(1997)[7] WANG Yi-gui, SUN Chang gun, BIAN Wen sheng et al. , Chemical Rsearch in Chinese Universities, 16(2), 136(2000)[ 8 ] Yoshimi Sueishi, Yuko Nishihara, Chemical Research in Chinese Universities, 16(4): 313(2000)[9] Andersson M, Karlstrom G., J. Phys. Chem. , 89(23), 4 957(1985)[10] LIU Yue, DAI Baiqing, LIU Ying. Journal of Molecule Science, 16(2): 110(2000)[11] Frisch M. J，Trucks G. w., Schlegel H. B. et al. , Gaussian中国煤化工Inc., PitburghPA, 1995jY HCNMHG[12] JaffeR. L., SmithG. D., YoonD. Y, J. Phys. Chem. , 97(4y): 1Z 745019951