Calculation and electronic analyses of 55Mn and 13C nuclear magnetic shielding constants for Mn(CO)5X (X=H, F, Cl, Br, I, and CH3) and M(CO)(NH3)3 (M=Cr2+, Fe2+, Cu+, and Zn2+)

We calculated 55Mn and 13C magnetic shielding constants for Mn(CO)5X (X=H, F, Cl, Br, I, and CH3) and M(CO)(NH3)3 (M = Cr2+, Fe2+, Cu+, and Zn2+), respectively. For the first molecular group, we compared the calculated 55Mn chemical shifts with the experimental values, and clarified effects of the basis sets. The calculated magnetic shielding constants using the second-order Douglas-Kroll-Hess (DKH2) method showed good agreement with the experimental results. According to the atomic orbital (AO) contribution analysis, the origin of the chemical shifts was attributed to the d-d transitions of Mn. In particular, the 3dπ orbital mainly contributed to the paramagnetic term of the Mn chemical shift. For the second molecular group, the 13C chemical shifts were dependent on the metal atoms. When the metal centers were Cr2+ or Fe2+, lower field shifts were seen. When the metal centers were Cu+ or Zn2+, upper field shifts were observed. These results were in good agreement with the experimental trends. The change of the paramagnetic term mainly depended on the d orbital configurations of the metal of centers, and the donation from the metal d orbital to the CO anti-bonding π* orbitals is expected to affect the chemical shift. (author)