Improved ¹³C¹H shift correlation spectra for indirectly bonded carbons and hydrogens: The FLOCK sequence

Abstract

Abstract Several modifications of the basic 13 C 1 H shift correlation sequence as optimized for polarization transfer via long‐range 13 C 1 H coupling constants are described and evaluated. Modifications involve incorporation of BIRD pulse(s) at the mid‐point of the evolution period and/or at the mid‐point of the fixed delay prior to polarization transfer, in addition to the previously discussed incorporation of a BIRD pulse during the final fixed delay. These modifications give significantly improved sensitivity for many cross‐peaks, excellent suppression of one‐bond peaks and, in the case of the sequence incorporating two additional BIRD pulses, some ability to distinguish two‐bond and three‐bond connectivity peaks. The latter sequence is, overall, comparable in sensitivity to the fixed time sequences XCORFE and COLOC. It is also easier to parameterize and gives better suppression of one‐bond peaks and other artifacts.