Theoretical Aspects of Isotope Effects on Nuclear Shielding

Probing Isotope Shifts in 103Rh and 195Pt NMR Spectra with Density Functional Theory

“…4,5,6,7 These effects are very large compared to those for other nuclei, when comparing the ranges of chemical shifts encountered. 8 McFarlane et al 4 2 ] complexes under carefully controlled spectroscopic conditions are resolved not only according to the statistically expected isotopologues, but also in some cases due to the isotopomers possible for these species, depending on whether a 35 Cl or 37 Cl ion is trans to the coordinated water molecule for each isotopologue in a given complex. Isotopomers or isotopic isomers are isomers with isotopic atoms, having the same number of each isotope of each element but differing in their positions.…”

The influence of charge on nuclear magnetic resonance isotope effects

“…[2] is a good approximation (3,4,8), whereo ( h r A H ) is the mean bond displacement, 15-20 X lop3 A (9). As mentioned in the introduction, u A (~-* H ) > u A (~-' H ) ; hence 'AA(*/'H) and (duA/dArA,,) are invariably negative.…”

“…A more complete analysis of isotope effects on u A involves contributions from mean square bond displacements and mean bond angle displacements. Thus terms such as (d2aA/dArAH2),((ArAH)*) and ( d~~/ d A c u ) , [ ( h c u )~~~h , -( A c~)~~~~~] must be considered; however, it is generally thought that these contributions to 'AA(*/'H) are relatively small (3,4,8,9,16).…”

“…They define the primary and secondary isotope effect on spin-spin coupling constants using eqs. [3] and [4], respectively. where B is either 'H or 'H.…”

“…This general periodic variation of */'H isotope effects in simple hydridei is supported by recent theoretical calculations of Chesnut and Foley (5,6), which show that the shielding derivative with respect to bond extension, (duA/dAr),, also exhibits a systematic variation as one moves across either the first or second row of the periodic table. The shielding derivative is intimately related to the isotope effect on u A because the main influence of deuterium substitution is to decrease the average mean bond displacement, ArA-211~ (3,(7)(8)(9).…”