Infrared and Raman spectra and structures of matrix isolated magnesium dihalides: MgF2, MgCl2, MgBr2, and MgI2

Infrared and Raman spectra are presented for the magnesium dihalides isolated in matrices. Of particular interest is the Ramen observation of the ν1 symmetric stretch in argon matrices at 550.0, 326.5, 197.9, and 147.6 cm−1 for MgF2, MgCl2, MgBr2, and MgI2, respectively. For MgF2, ν1 had previously been assigned to a strong infrared feature near 480 cm−1, and this activity (along with isotopic frequency shifts) has been cited as evidence for a nonlinear structure for MgF2. The present results undermine this argument and a linear structure is favored, a result consistent with the failure to detect a dipole moment in gas phase electric deflection measurements. Force constants calculated for a linear structure are compared with values obtained from several theoretical models and the results favor an ion–ion interaction model in which the bonding is essentially ionic. Finally, features due to MgX2 dimers are assigned on the basis of a bridged D2h model, and some features attributed to weak complexes between MgX2 and impurities in the matrix are discussed.