Nuclear Spin Optical Rotation in Organic Liquids

Abstract

Nuclear spin induced optical rotation (NSOR) is a novel technique for the detection of nuclear magnetic resonance (NMR) via optical rotation instead of conventional pick-up coil. Originating from hyperfine interactions between nuclei and orbital electrons, NSOR provides a new method to reveal nuclear chemical environments in different molecules. Previous experiments of NSOR detection have poor signal-to-noise ratio (SNR), which limits the application of NSOR in chemistry. In this work, based on a continuous-wave NMR scheme at a low magnetic field (5 G), we employ a multi-pass cavity and a 405 nm laser to improve the sensitivity of NSOR. By performing precision measurements of NSOR detection in a range of pure liquid organic chemicals, we demonstrate the capability of NSOR to distinguish 1H signals in different chemicals, in agreement with the first-principles quantum mechanical calculations. The NSOR of 19F is also measured at low fields with high SNR, showing that heavy nuclei have higher optical rotation signals than light nuclei.

In addition, in order to obtain NSOR at different chemical sites in the same molecule via chemical shift, we make efforts to develop a novel scheme based on liquid-core hollow fiber for the detection of NSOR under high magnetic fields. By coiling a long liquid-core fiber densely for many loops around a small rod combined with RF coils, it is possible to measure optical rotation signals inside a narrow-bore superconducting magnet. Manufactured by filling liquids into capillary tubings, those liquid-core fibers perform like multimode step-index fibers, and thereby exhibit linear birefringence and depolarization, significantly reducing the light polarization for the measurement of optical rotation. According to our attempts, it is possible to suppress the linear birefringence by filling chiral liquids in hollow fibers, and approach near single-mode operation by means of launching light beam into the fiber core under the mode match condition. Although some issues of hollow fibers obstruct the final measurement of high-frequency NSOR, our work on the liquid-core fiber provides the basis for future fiber-based NSOR experiments under high magnetic fields.