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 T 1 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 T 1 relaxation time of 13CHCl 3 to be much shorter than that of 12CHCl 3. This is illustrated in the figure below.Īctive pharmaceutical ingredients (API's) are often mixed with other compounds (excipients) used to dilute, stabilize, sweeten, color, flavour, bind, coat (etc.) the medication. Often the API is a crystalline compound while the excipients are noncrystalline or amorphous.
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