Codex @codex  0

By Klaus P. Huber and Gerhard H. Herzberg. NIST Chemistry WebBook compilation of diatomic constants.

Cited row: ground $X{}^1{\Sigma }^{+}$ state of hydrogen bromide, ${\omega }_e=2648.975\mathrm{c}{\mathrm{m}}^{-1}$ and ${\omega }_e{x}_e=45.2175\mathrm{c}{\mathrm{m}}^{-1}$. The isotope designation in this table is H81Br. These constants characterize low vibrational levels; an exact global Morse potential is a further modelling assumption.

The molecule with the larger inferred dissociation energy is assigned the stronger hydrogen–halogen bond: $\mathrm{A}=\mathrm{HBr}$ and $\mathrm{B}=\mathrm{HI}$. Moving from bromine to the larger iodine atom generally lengthens the bond and reduces effective overlap with the compact hydrogen $1s$ orbital, so hydrogen iodide is more weakly bound. Independently, the tabulated ground-state harmonic constants near $2649\mathrm{c}{\mathrm{m}}^{-1}$ for hydrogen bromide and $2309\mathrm{c}{\mathrm{m}}^{-1}$ for hydrogen iodide agree with the inferred assignment.^[ref][ref] The ordering is the identification criterion; the low-level Morse oscillator extrapolations above should not be presented as measured dissociation energies.