The involvement of skeletal deformations in the ultrafast excited-state proton transfer of 2-(2'-hydroxyphenyl)-benzothiazole (HBT) and the identification of the vibrational modes active in the process are reported. A multidimensional ab initio calculation of ground and excited states at the HF/DFT and CIS/TDDFT level renders the relevant portions of the potential energy surfaces around the minimum-energy path connecting the enol and keto configuration. The frequencies and potential energy distributions of the normal modes and the corresponding deformations of the molecule are calculated for all minimum-energy geometries. Along the minimum-energy path, the nuclear deformation is projected onto the relevant normal modes. This normal-mode analysis shows that mainly five low-frequency in-plane vibrations are associated with the electronic rearrangement and the transfer of the proton. The theoretical findings are in quantitative agreement with the experimental study presented in the accompanying paper.