Isotopic Effect on X-Nuclei Screening and X–H(D) Scalar Spin-Spin Coupling in XH4 and XO4

Abstract

Changes in X-nuclei screening and scalar spin-spin X-H(D) coupling constants induced by the H-D substitutions in isotopomers [XH4-nDn] (X = N, Al, Ga) have been determined by 14,1l5N, 27 AI, 69,71Ga NMR. Isotopic chemical shifts of nuclei 51V, 53Cr, 99Tc induced by the 16O-18O substitution have been measured for tetra-oxo complexes [VO4]3-, [CrO4]2-, and [TcO4]-. 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 [XHJ for the row X = N, C, B, Al, Ga, As the internuclear X-H distance increases, the isotopic shift of the X nuclei with respect to the signal from [XHJ rises linearly on H-D substitution from a negative value for [NH4]+, CH4 to a positive value (the downfield shift) for [GaH4]-. When D is substituted for H in [XH4] the X-H spin-spin coupling constant decreases for X = N, C, B, Al, Ga (the secondary isotopic effect). The magnitude and sign of the secondary isotopic effects on the screening constants of the X nuclei in [XH4] and [XO4] 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.