12C/13C Isotope Effects on 1H T1 Relaxation Times
What is the 1H
T1 relaxation time of chloroform? It seems like a simple enough question, but the answer is not so simple. The relaxation rate for any proton is the sum of relaxation rates resulting from several different mechanisms (eg. homonuclear dipolar coupling, heteronuclear dipolar coupling, chemical shielding anisotropy, spin rotation etc...). Each of these mechanisms of relaxation depends on dynamic effects and the extent of those processes occurring at the Larmor frequency. Often, in proton-rich organic compounds, 1H T1 relaxation is dominated by the homonuclear dipolar coupling interaction. For chloroform, with only a single proton, there can be no intra-molecular homonuclear 1H dipolar interaction and the 1H relaxation rate must depend on other mechanisms. One of these mechanisms is the result of the heteronuclear dipolar coupling interaction. For the 13C isotopomer of chloroform, one would expect a significant heteronuclear dipolar interaction between the directly bound 1H and 13C. This interaction is absent in the 12C isotopomer and one would therefore expect the T1 relaxation time of 13CHCl3 to be much shorter than that of 12CHCl3. This is illustrated in the figure below.
The 1H T1 relaxation times for both 12CHCl3 and 13CHCl3 were measured with the
inversion recovery method for a
degassed, dilute (1%) sample of chloroform in acetone-d6. The inversion recovery delay was varied from from 1 sec. to 300 sec. The recycle delay was 300 sec. Relaxation is much more efficient for 13CHCl3 compared to 12CHCl3. The T1 for 13CHCl3 is only 46% that of 12CHCl3, indicating the significance of the heteronuclear 1H - 13C dipolar coupling interaction as a relaxation mechanism.
Source:
University of Ottawa NMR Facility Blog