A constant-time (13)C- (1)H HSQC with uniform excitation over the complete (13)C chemical shift range.

An improved version of the constant-time HSQC experiment is presented that gives uniform sensitivity over the complete (13)C bandwidth in (13)C-(1)H correlation experiments without creating artifacts in the methyl and aromatic regions of the spectra. The improvement is achieved by replacing the refocussing (13)C 180° pulse in the evolution time by a combination of a full-power (22 kHz) hyperbolic secant 180° pulse that inverts and refocusses the entire (13)C window, immediately followed by a selective 180° pulse on the CO region. Further improvement in signal-to-noise in the aromatic and methyl regions, although less spectacular, is obtained by replacing the other two 180° (13)C pulses in the INEPT parts of the pulse sequence by full-power hyperbolic secant pulses. Results of simulations and experimental data are presented that demonstrate the excellent performance of the hyperbolic secant pulse for broadband inversion and show that refocussing of transverse magnetization occurs over the same bandwidth, albeit with a (13)C signal phase that depends quadratically on offset. A further modification, in which one of the selective pulses on the CO region is omitted, is also presented. Implications for other 2D and 3D experiments performed at high fields, where uniform (13)C inversion and refocussing is desirable, are discussed.