Application of 1D BIRD or X-filtered DEPT long-range C-C relay for detection of proton and carbon via four bonds and measuring long-range 13C-13C coupling constants

Abstract

We propose the 13C-detecting 1D DEPT long-range C-C relay to detect super long-range H-C connectivity via four bonds (1H-13C-X-X-13C, X represents 12C or heteronuclear). It is derived from the DEPT C-C relay which detects the H-C correlations via two bonds (1H-13C-13C) by setting the delays for J(CC) in the C-C relay sequence to the (LR)J(CC). This sequence gives correlation signals split by small (LR)J(CC), which seriously suffers from residual center signal. The unwanted signal is due to long-range C-H couplings ((LR)J(CH)). The expected relayed magnetization transfer 1J(CH) --> (LR)J(CC) occurs in the 1H-13C-X-(X)-13C isotopomer, whereas the unwanted signal of (LR)J(CH) comes from 1H-12C-(X)-13C isotopomers, whose population is 100 times larger than that of the 1H-13C-X-(X)-13C isotopomer. The large dispersive line of this unwanted center signal would be a fatal problem in the case of detecting small (LR)J(CC) couplings. This central signal could be removed by an insertion of BIRD pulse or X-filter. DEPT spectrum editing solved a signal overlapping problem and enabled accurate determination of particular (LR)J(CC) values. We demonstrate here the examples of structure determination using connectivity between 1H and 13C via four bonds, and the application of long-range C-C coupling constants to discrimination of stereochemical assignments.