Reaction field effects on nitrogen shielding

Solvation effects on nuclear magnetic shielding at nitrogen in the CH₃CN, CH₃NO₂, CH₃NCS, and CH₃ONO₂ molecules are studied using electronic structure calculations. Reaction field theory together with a dielectric continuum is invoked to describe solute-solvent electrostatic interactions, using a solute electronic isodensity contour to define the cavity surface. With the common approach that treats only surface polarization effects, it is found that the nitrogen shielding is very sensitive to the cavity size and usually provides results in reasonable agreement with experiment only for rather large cavity sizes corresponding to contour values of ∼0.000 25-0.0005 a.u. With a more complete treatment that also includes volume polarization effects arising from penetration of the solute charge density outside the cavity, the nitrogen shielding becomes much less sensitive to the cavity size and reasonable agreement with experiment can be obtained with contour values ∼0.001-0.003 a.u., which is more consistent with previous findings on the optimum cavity size for determination of free energies of solvation.