Natural Abundance Carbon-13 Partially Relaxed Fourier Transform Nuclear Magnetic Resonance Spectra of Complex Molecules

Abstract

Proton‐decoupled partially relaxed Fourier transform (PRFT) NMR of carbon‐13 in natural abundance was used to determine spin–lattice relaxation times ${\mathrm{(T}}_1)$ of individual carbons in solutions of cholesteryl chloride, sucrose, and adenosine 5′‐monophosphate (AMP) at 15.08 MHz and 42°C. With the exception of a few side‐chain groups, all protonated carbons have $T_1$ values of less than 1 sec. Some side‐chain carbons on cholesteryl chloride show evidence of internal reorientation and have relaxation times of up to 2 sec. Nonprotonated carbons have $T_1$ values in the range 2–8 sec. These relaxation times are sufficiently short to make ordinary Fourier transform NMR a very sensitive technique in the study of complex molecules without the need for spin‐echo refocusing schemes. Integrated intensities and nuclear Overhauser enhancements prove that, except for two of the three nonprotonated carbons in AMP, all ¹³C nuclei in these compounds relax mainly through ¹³C–¹H dipolar interactions. Measured $T_1$ values of protonated carbons were used to determine rotational correlation times. The ring backbone of $\text{1M}$ cholesteryl chloride in CCl₄ reorients isotropically with a correlation time of 9 × 10⁻¹¹ sec. Methyl groups directly attached to the backbone show evidence of internal motion with a correlation time ≲ 5 × 10⁻¹² sec. A comparison of $T_1$ values on the side group attached at C‐17 indicates that the effect of internal motion is greatest for carbons near the free end. The two rings in sucrose behave as a single rigid entity reorienting isotropically with correlation times of 7 × 10⁻¹¹ and 3 × 10⁻¹⁰ sec for $\text{0.5M}$ and $\text{2M}$ aqueous solutions, respectively. There is evidence for internal reorientation of the CH₂OH side chains, with a correlation time much longer than that of methyl groups in cholesteryl chloride. The adenine group of $\text{1M}$ aqueous AMP is more restricted in its rotational motion than the sugar moiety. It appears that PRFT NMR spectra will be a useful addition to the arsenal of spectrochemical techniques. Relaxation times measured from PRFT spectra can be used in assignments. Furthermore, resonances that are unresolved in the normal spectrum, can be separated in PRFT spectra if the overlapping peaks arise from carbons with appreciably different $T_1$ values.