Millimeter Wave Rotational Spectra of AlCl, AlBr, and AlI

Abstract

Rotational transitions of Al³⁵Cl, Al³⁷Cl, Al⁷⁹Br, Al⁸¹Br, and AlI have been measured in the millimeter wave region. To our knowledge, this is the initial microwave investigation of AlBr and AlI, and for AlI is the first report of any rotational constants. Transitions in excited vibrational states have been measured (up to $\text{v=14}$ for AlI), allowing accurate experimental determination of rotation—vibration interaction terms Y₁₁, Y₂₁, Y₃₁, and Y₁₂ of the Dunham theory. Data analysis yields: for ${\mathrm{Al}}^{35}{\mathrm{Cl,\; B}}_e\text{=7313.206(4)\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\; D}}_e\text{=7.503(4)\hspace{0.17em}}\mathrm{kHz}{\mathrm{,\; r}}_e\text{=2.13011\hspace{0.17em}Å;}{\mathrm{for\; Al}}^{37}\mathrm{Cl}{\mathrm{,\; B}}_e\text{=7141.131 (4)\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\; D}}_e\text{=7.154(4)\hspace{0.17em}}\mathrm{kHz}{\mathrm{,\; r}}_e\text{=2.13011 Å}$ ; for ${\mathrm{Al}}^{79}\mathrm{Br}{\mathrm{,\; B}}_e\text{=4772.825(2)\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\; D}}_e\text{=3.3832(11)\hspace{0.17em}}\mathrm{kHz}{\mathrm{,\; r}}_e\text{=2.29480\hspace{0.17em}Å}$ ; for ${\mathrm{Al}}^{81}\mathrm{Br}{\mathrm{,\; B}}_e\text{=4742.799(2)\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\; D}}_e\text{=3.3408(11)\hspace{0.17em}}\mathrm{kHz}{\mathrm{,\; r}}_e\text{=2.29480\hspace{0.17em}Å}$ ; and for AlI, $B_e\text{=3528.5533(8)\hspace{0.17em}}\mathrm{MHz}{\mathrm{,\; D}}_e\text{=1.9550(3)\hspace{0.17em}}\mathrm{kHz}{\mathrm{,\; r}}_e\text{=2.53709\hspace{0.17em}Å}$ . From nuclear quadrupole couplings the ionic character of the AlI bond is found to be 85%; that of AlBr is found to be ≥87%. From the ionic character of the aluminum monohalides, the electronegativity value of monovalent aluminum is found to be 1.3.