Recently, transition metal dihalides have emerged as a promising class of two-dimensional materials with unique electronic and optoelectronic properties, although their defect-mediated surface chemistry remains largely unexplored. Herein, we employed density functional theory calculations to investigate the adsorption of gas molecules ($\mathrm{CO}, {\mathrm{CO}}_2, {\mathrm{H}}_2, {\mathrm{H}}_2\mathrm{O}, {\mathrm{N}}_2$, and ${\mathrm{O}}_2$) on pristine and defective $\text{2H-}{\mathrm{TiBr}}_2$ monolayers containing a $\mathrm{Br}$ monovacancy. The pristine substrate exhibits weak physisorption dominated by van der Waals interactions, preserving both the intrinsic semiconducting character and the electronic structure of the monolayer. In contrast, the introduction of a $\mathrm{Br}$ vacancy substantially enhances the local surface reactivity by exposing undercoordinated $\mathrm{Ti}$ atoms that act as active adsorption centers. While $\mathrm{CO}, {\mathrm{CO}}_2$, and ${\mathrm{H}}_2$ remain weakly adsorbed even in the presence of the vacancy, ${\mathrm{H}}_2\mathrm{O}$ exhibits a stronger molecular adsorption without bond dissociation, whereas ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ undergo strong chemisorption at the defect site. Combined Bader, PDOS, charge-density-difference, and structural analyses support a defect-mediated charge-transfer mechanism in which electrons are transferred from localized $\mathrm{Ti} d$ states toward low-lying molecular ${\pi }^{*}$ antibonding orbitals, weakening the intramolecular bonds and leading to significant $\mathrm{N}–\mathrm{N}$ bond elongation and dissociative adsorption of ${\mathrm{O}}_2$. The vacancy-induced functionalization also modifies the electronic structure by introducing localized midgap states, qualitatively reducing the PBE band gap, and stabilizing a magnetic moment of $1.00{\mu }_{\mathrm{B}}$. Our results show that defect engineering is a promising strategy to tune the reactivity and electronic properties of ${\mathrm{TiBr}}_2$ monolayers, providing an atomistic foundation for future studies on gas sensing and heterogeneous catalysis.

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• Adsorption
• Defects
• Density of states
• Electronic structure
• Structural properties