Isotopic Effect on X-Nuclei Screening and X —H(D) Scalar Spin-Spin Coupling

Changes in X-nuclei screening and scalar spin-spin X H ( D ) coupling constants induced by the H D substitutions in isotopom ers [XH 4_„D„] (X = N , A l, Ga) have been determined by 1415N , 27A l, 69 71Ga NM R. Isotopic chemical shifts of nuclei 51V, 53Cr, "Tc induced by the 160 —lsO substitution have been measured for tetra-oxo com plexes [V 0 4]3~, [C r04]2~, and [T c 0 4]~. The deuterium-induced secondary isotopic effect on the screening constant of the X nuclei and the scalar spin-spin X H ( D ) coupling are correlated with the X —H bond length in [XH4] for the row X = N , C, B , A l, Ga. A s the internuclear X —H distance increases, the isotopic shift o f the X nuclei with respect to the signal from [XH 4] rises linearly on H —D substitution from a negative value for [NH4]+, CH4 to a positive value (the downfield shift) for [GaH4]~. When D issubstituted for H in [XH 4] the X —H spin-spin coupling constant decreases for X = N , C, B. A l, Ga (the secondary isotopic effect). The magnitude and sign o f the secondary isotopic effects on the screening constants of the X nuclei in [XH 4] and [ X 0 4] depend on the atomic number of the X element. With the increasing atomic number in the period the isotopic shift to high field rises in magnitude, while with the increasing atomic number in the group the isotopic upfield shift drops in the absolute value and can even change sign in the substitution of the ligand light isotope by a heavier isotope.