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 to which those processes occur 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 isotopologue of chloroform, one would expect a significant heteronuclear dipolar interaction between the directly bound ¹H and ¹³C.   This interaction is absent in the ¹²C isotopologue 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.