Q.E.COSY: Sign and Size of Small Deuterium Residual Quadrupolar Couplings based on extended E.COSY principle

Residual quadrupolar couplings contain important structural information comparable to residual dipolar couplings. However, the measurement of sign and size of especially small residual quadrupolar couplings is difficult. Here, we present an extension of the E.COSY principle to spin systems consisting of a spin 1 coupled to a spin 1⁄2 nucleus, that allows the determination of sign of the quadrupolar coupling of the spin 1 nucleus relative to the heteronuclear coupling between the spins. The so-called Q.E.COSY approach [1] is demonstrated with its sign-sensitivity using variable angle NMR, stretched gels, and liquid crystalline phases applied to various CD and CD3 groups. The sign-sensitive measurement of residual quadrupolar couplings that remain unresolved in conventional deuterium 1D spectra is particularly beneficial with the approach (Figure 1).