Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for interrogating systems containing transition metal ions, and can provide direct evidence for the presence of SACs since the spectroscopic fingerprint of isolated metal ions on the surface of TiO2 is significantly different from EPR spectra of metal oxide clusters or nanoparticles that are dominated by long-range dipolar interactions. The complex EPR spectrum a-TiO2-Cu SA (Figure ), consists of five components as determined by computer simulation. The main constituent (~85%), denoted Cu-1, exhibits magnetic parameters g||  2.353, A|| 113 G and g⊥  2.067 with not resolved perpendicular hyperfine structure (HFS) typical for surface-bound single Cu(II) ions electronic configuration), or Cu(II) substituting lattice Ti(IV), both expected to be spectroscopically the same. 23 G) can also be undoubtedly assigned to isolated copper centers in a slightly different environment. The last two components with very low intensity can be assigned to the electrons trapped at oxygen vacancies (R-1, g = 2.002) and to Ti(III) centers present in TiO2 matrix (Ti-1, g||  1.924, and g⊥  1.945). Similarly, an EPR analysis of the sample a-TiO2-Fe SA (Supporting Information, Figure ) reveals a main signal that has been typically attributed to isolated Fe(III) ions at the surface or near surface of TiO2, though a minor presence of small iron oxide-type clusters wrapped up by the anatase matrix is also possible (for details see Supporting Information, Note 1). At very low surface coverages on metal oxide supports, vanadate species are known to be mostly present as isolated VOx species in typically distorted structures in between tetrahedral (VO4) or octahedral (VO6) arrangements. In EPR analysis, vanadate species become active after photoreduction of V(V) to V(IV) (d 1 electronic configuration). The complex EPR spectrum of the sample a-TiO2(400)-V SA registered after 120 min of UV irradiation (Figure ) consists of four components as determined by computer simulation, suggesting the presence of at least four different vanadate species in distinct local environments. EPR signals show nearly axial symmetry with well-resolved hyperfine structure (HFS) due to the interaction between electron and nuclear ( 51 V, 99.75%, I = 7/2) spin magnetic moments. On the basis of the similarity of magnetic parameters, the components can be classified into two groups (VO-1A/B and VO-2A/B; see also whereby the parameters collected in Table suggest that the symmetry of VO-1 centers is more distorted. , where P, a parameter describing the relation between Bohr and nuclear magnetons, was set for 140 G as 128•10 ─4 cm ─1 (calculated for averaged giso=1.96). The lower (𝛽 2 * ) 2 value expresses delocalization of the electron onto the oxygen ligand and a higher degree of covalence of the V-O bond.