Why is N···Be distance of NH3H(+)···DBeH shorter than that of NH3D(+)···HBeH? Paradoxical geometrical isotope effects for partially isotope-substituted dihydrogen-bonded isotopomers.

The partial isotope substitution for the change of geometrical parameters, interaction energies, and nuclear magnetic shielding tensors (σ) of dihydrogen-bonded NH3X(+)···YBeH (X, Y = H, D, and T) systems is analyzed. Based on the theoretical calculation, the distance between heavy atoms RN···Be of NH3H(+) ···DBeH is clearly found to be shorter than that in NH3D(+)···HBeH. Such apparently paradoxical geometrical isotope effect (GIE) on RN···Be is revealed by the cooperative effect of two kinds of (1) primary covalent-bonded GIE and (2) secondary dihydrogen-bonded one. We have demonstrated that (1) the covalent bond lengths become shorter by heavier isotope-substitution and (2) the dihydrogen-bonded distance RX···Y becomes shorter by heavier Y and lighter X isotope-substitution due to the difference of electronic structure reflected by the nuclear distribution. We have also found that interaction energy of NH3H(+)···DBeH is stronger than that of NH3D(+)···HBeH and isotopic deshielding effect of magnetic shielding becomes large in lighter isotope.