Knowledge Bank

We previously reported that the infrared spectrum of monomeric $AlCl3$ implied that this molecule has a pyramidal structure. Since this is a somewhat surprising result we have studied the infrared spectra of $AlF3$ ($200cm-1$ - $4000cm-1$) and $AlBr3$ ($125cm-1$ - $800cm-1$) in solid argon. For $AlBr3$ the vibrational assignment is revised from previous assignments and it seems consistent with a planar structure. For $AlF3$, the vibrational assignments are also revised, but the infrared spectrum alone is insufficient as yet to fix a structure. The thermodynamic properties of these compounds have been calculated and the bonding will be discussed.