Journal zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA M olecular Structure (Theochem), 170 (1988) 69-74 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 69 zyxwvutsrq THE CONFORMATIONAL STRUCTURE OF l,S-DIMETHOXY- ETHANE IN THE GAS PHASE MARIO BARZAGHI, ALDO GAMBA and GABRIELE MOROSI C.N.R. Center for the Study of Structure/Reactivity Relations and Department of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, I-20133 Milan (Italy) (Received 12 August 1987) ABSTRACT The conformational structure of 1,2-dimethoxyethane in the gas phase has been studied by Hartree-Fock and Msller-Plesset perturbation theory. Full geometry optimizations within Cz symmetry constraints for the aga structure and within CPh symmetry constraints for the uoa struc- ture were performed using both 3-21G and 6-31G* basis sets. Our best energy estimate favors the aaa structure by 0.613 kcal mol-‘, which reduces to 0.518 kcal mol-’ when the (unscaled) 3-21G zero-point energy contribution is included. The transition states for the conversion of the ooa structure into the oga structure and for the degenerate rearrangement of the og zyxwvutsrqponmlkjihg +a structure into the ag -a structure have been determined. INTRODUCTION In the last decade the dynamics of ion pairs in ethereal solutions has been the topic of many experimental investigations in ourlaboratory [ 11. In reac- tions involving ions or ion pairs, the solvent plays a fundamental role: even a small modification of its structure may cause a large change in the rate and in the pattern of a process. A thorough characterization of solvents is required before quantitative understanding of their role can be contemplated. There- fore, a program of theoretical investigation of ethereal solvents was started, which will ultimately lead to a Monte Carlo statistical analysis of ion pairing phenomena involving anion radicals of aromatic compounds. In this commu- nication a quantum mechanical ab initio investigation of 1,2dimethoxyethane (DME) of increasing level of sophistication is reported in order to establish the minimum level needed to describe properly the available experimental data. Such information will decide the choice of the quantum mechanical model by which the potential energy surface, required for statistical calculations, must be evaluated. Since the ab initio evaluation of the full potential hypersurface is very expensive, the capability of the MNDO method [ 21 to reproduce the ab initio results for a few stationary points of the surface has also been tested.