Efficient Access to Elusive 1D 13 C NMR Spectra through Highly Resolved 1 H, 13 C-Long-Range Correlation Spectroscopy.

A method for obtaining 1D 13 C NMR spectra from natural products or metabolites using proton detection is described. The approach delivers singlets for every 13 C signal without conducting any broadband 1 H decoupling (CPD) and is based on calculating 13 C projections from constant-time HMBC and conventional HSQC experiments, recorded at high digital resolution and processed to pure phases. Paramount to the proposed method is the implication of nonuniform sampling and echo processing. The echo processing produces phase-sensitive 2D CT-HMBC spectra with narrow 13 C signal line shapes. Two simple HMBC pulse sequences are utilized with the suppression of homo- and heteronuclear couplings. Due to the removal of the 1 H multiplet structure in F 1 (no tilt at higher digital resolution), 13 C singlets arise. An overall increase in 13 C signal-to-noise (SINO) for all types of carbon multiplicities is observed, making the proposed technique superior compared to direct 13 C excitation. For otherwise difficult-to-measure quaternary carbon atoms, a SINO enhancement of up to 6 and 12 depending on F 1 resolution (3 and 6 Hz/point) is reported. Echo/anti-Echo signal detection cleans up the spectrum. Nonuniform sampling (NUS) lays the groundwork to significantly reduce the total acquisition time. Final 1D 13 C projections are obtained by combining the 13 C projection from CT HMBC and conventional HSQC. This orthogonal concept of combining the 13 C projections from different spectra inherently minimizes the risk of missing 13 C cross-peaks by inappropriate setting of long-range n J HC coupling delays and the shortcoming of T 2 relaxations. The advantages and some limitations of the concept are discussed.