Two-Dimensional Single- and Multiple-Quantum Correlation Spectroscopy in Zero-Field Nuclear Magnetic Resonance

We present single- and multiple-quantum correlation $J$-spectroscopy detected in zero ($<\!\!1$~$\mu$G) magnetic field using a \Rb vapor-cell magnetometer. At zero field the spectrum of ethanol appears as a mixture of \carbon isotopomers, and correlation spectroscopy is useful in separating the two composite spectra. We also identify and observe the zero-field equivalent of a double-quantum transition in ${}^{13}$C$_2$-acetic acid, and show that such transitions are of use in spectral assignment. Two-dimensional spectroscopy further improves the high resolution attained in zero-field NMR since selection rules on the coherence-transfer pathways allow for the separation of otherwise overlapping resonances into distinct cross-peaks.