ChemInform Abstract: NMR SPECTROSCOPIC STUDIES ON CHALCOGEN COMPOUNDS. 4. CARBON‐13 ISOTOPE EFFECT T ON SELENIUM‐77 AND TELLURIUM‐125 NUCLEAR SHIELDING AND ITS CORRELATION WITH CARBON‐SELENIUM BOND DISTANCES. TELLURIUM‐123 ISOTOPE EFFECT ON TELLURIUM‐125 NUCLEAR SHIELDING