We report a density functional theory (DFT) investigation of the chemical reactivity of the S=C=S...Cl complex toward 2,3-dimethylbutane (DMB) and compare it to the related reaction of a Cl atom with DMB. The DFT calculations predict that the reaction of a Cl atom with DMB will have little tertiary selectivity toward chlorination consistent with experimental results for photochlorination of alkanes in inert solvents. However, the DFF calculations indicate that the reaction of the S=C=S...Cl complex with DMB is different with the primary H abstraction reaction being noticeably more difficult than the tertiary H abstraction reaction. This indicates there will be significant tertiary selectivity for the reaction of the S=C=S...Cl complex with DMB and this is consistent with experimental results for chlorination of DMB in CS2 solvent. We examine the reaction mechanism and properties of the S=C=S... Cl complex and explore potential implications for tertiary selectivity for other related photochlorination reactions.