COMPARISON OF DEUTERIUM & TRITIUM ISOTOPE EFFECTS ON CHEMICAL SHIFTS

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NMR chemical shifts are very sensitive probes for changes in the local environment around the nuclei being detected. In particular there are substantial isotope effects on chemical shifts, both primary (being detected on the atom whose mass is changing) and secondary (being detected on other nuclei near that with altered mass). The isotopes of hydrogen provide large relative mass changes, and have been used fairly extensively to investigate hydrogen bonding. Both primary and secondary shifts can be quite substantial, propagating many bonds from the site of hydrogen bond formation. A majority of studies have been done comparing chemical shifts with compounds containing protons vs. deuterium. The deuterium lines are somewhat broader than proton due to quadrupole coupling, and the magnetic moment is 1/6th that of a proton, limiting the accuracy of the shift measurements. We have now undertaken a study to extend measurements to tritium. TITLE: Comparison of Deuterium and Tritium Isotope Effects on Chemical Shifts (Continued) The main motivation is to compare the isotope shifts of deuterium and tritium relative to proton and see whether the induced effect scales with mass difference in the same way that the proton - deuterium shift scales. Deviations from this behavior could reflect asymmetry in the hydrogen bond potential, and could help understanding of strong hydrogen bonds. A wide variety of organic compounds were studied dissolved in organic solvents which contained intramolecular hydrogen bonds. The donors and acceptors were varied over a wide range of chemical functionalities, including O, N and S on both donor and acceptor ends. Substitution of tritium for protons was accomplished through addition of small amounts of water. The spin 1/2 character of tritium made possible much more accurate determinations of shifts than with deuterium. Only very small deviations from the expected H-D / H-T ratios of 1.4 were observed; for most compounds the value was at the expected theoretical value.