Geminal Isotope Effects on the Proton Magnetic Resonance of Some First-Row Molecules

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Abstract
Geminal substitution of H by D in first-row molecules generally causes upfield shifts in the proton magnetic resonance. These isotope shifts are sometimes explained in terms of intramolecular electrostatic effects. This work shows that they can also be considered as being due to the changes in bond rehybridization accompanying small changes in molecular geometry upon isotopic substitution. The diamagnetic part of the magnetic shielding at the protons in NH3, CH4, and CH3D was evaluated as a function of HNH or HCH bond angle using localized equivalent orbitals constructed from Slater atomic orbitals. The treatment for CH3D differed from that for CH4 in that the shorter experimental CH bond distance was used. The results of the calculation showed that the magnetic shielding at the proton in a NH or CH bond increased with decreasing bond angle, and with a decrease in bond distance. For small changes in geometry, similar to what is experimentally found upon isotopic substitution, the calculated change in shielding was the same order of magnitude as that experimentally observed. Consideration of isotope shift data on first-row molecules reveals that the magnitude of the shift is also a function of the initial molecular geometry, being larger for molecules with smaller HXH bond angles (X=C, O, N) and H–H internuclear distances. Isotope shifts were measured for CH2Cl2, CH2Br2, and CH2I2. Upon monodeuteration the proton magnetic resonance shifted upfield by 0.0125±0.0003, 0.0137±0.0002, and 0.0135±0.0002 ppm, respectively. The measured JHD spin—spin coupling constants were also measured for these molecules and found to be in agreement with previous work.