MOLECULAR-STRUCTURES AND FORCE-FIELDS OF MONOMERIC AND DIMERIC MAGNESIUM DICHLORIDE FROM ELECTRON-DIFFRACTION AND QUANTUM-CHEMICAL CALCULATIONS

Monomeric and dimeric MgCl2 have been investigated by high-temperature electron diffraction and ab initio calculation at the SCF and MP2 lev els of theory. The effect of the size of the basis set, and particular ly the number of polarization functions and their exponents, has been carefully investigated. The basis set size was increased until a conve rgence in the monomer bond length was reached at the TZ5P2f(+)/MP2 lev el. The highest level of calculation for the dimer was of DZP(+)/MP2 q uality. Harmonic vibrational frequencies were calculated and normal-co ordinate analyses were performed for both monomeric and dimeric molecu les. For the electron diffraction determination of the dimer geometry, constraints based on the quantum chemical calculation, such as the di fferences of the Mg-Cl bond lengths, were incorporated into the analys is, as the dimer was present only as a minor component of the vapor (1 2.8 +/- 1.3 mol %). Monomeric MgCl2 is linear with a bond length of r( g)(Mg-Cl) = 2.179 +/- 0.005 Angstrom. The converged calculated bond le ngth, r(e)(Mg-Cl) = 2.169 Angstrom, is consistent with the value estim ated from electron diffraction applying vibrational corrections, r(e)( M)(Mg-Cl) = 2.163 +/- 0.011 Angstrom. This agreement, however, can onl y be obtained with large basis sets; standard bases of DZP quality and standard exponents give much larger bond lengths than the experimenta l value. The calculated equilibrium structure of the dimer has a four- membered planar ring with two bridging chlorines (D-2h symmetry). At t he high temperatures used, the dimer appears strongly puckered if a st andard electron diffraction analysis is used, but the best fit to the experimental data was obtained by adopting a dynamic model, in which t he calculated rather anharmonic ring-puckering potential, together wit h the associated changes in the other geometrical parameters, was used as an additional constraint. Only a very minor increase in bond lengt h with bending was found for the monomer, whose bending potential is c lose to harmonic.