The infrared spectra of monomeric and dimeric magnesium chloride and monomeric magnesium bromide trapped in solid argon at 4 to 10°K have been obtained. For the monomer l2, two band systems were observed at 603 and 585 cm−1. The two bands appear to result from different matrix sites. For the MgBr2 monomer an intense band system was observed in the region 490 to 530 cm−1 with three strong bands at 502, 509, and 520 cm−1 and shoulders at 495 and 526 cm−1. The origin of these observed multiplets is uncertain but may also result from different matrix sites. For the dimer Mg2Cl4, bands were observed at 504, 410, and 515 cm−1. These frequencies are shown to be consistent with an assumed chlorine bridged structure.
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It is interesting to note that when a sample of dry MgCl2 that had been stored in a small closed container with NH4Cl for several weeks was vaporized and trapped in an argon matrix, bands that could be attributed to NH4Cl were also observed in the infrared spectra. Apparently the solid MgCl2 sample was contaminated by vapor transport of the NH4Cl and the latter was subsequently co-trapped with the MgCl2. The MgCl2 absorption bands were all shifted to lower frequencies by 10 to 30 cm−1 from those observed when only MgCl2 was trapped. This could be due to either a strong interaction or weak bonding between MgCl2 and NH4Cl. Supporting this possibility is the observed decrease in the MgCl stretching frequency which might be expected when a strong interaction exists between MgCl2 and NH4Cl. The fact that both the monomer and polymer bands of MgCl2 were shifted indicated that NH4Cl was interfering with most of the trapped MgCl2 molecules. Further work is called for to understand better the effect of NH4Cl on the trapped magnesium chloride molecules.
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