Observation of Optical Chemical Shift by Precision Nuclear Spin Optical Rotation Measurements and Calculations

Abstract

*Nuclear spin optical rotation (NSOR) is a recently developed technique for detection of nuclear magnetic resonance via rotation of light polarization, instead of the usual long-range magnetic fields. NSOR signals depend on hyperfine interactions with virtual optical excitations, giving new information about the nuclear chemical environment. We use a multi-pass optical cell to perform first precision measurements of NSOR signals for a range of organic liquids and find clear distinction between proton signals for different compounds, in agreement with our earlier predictions. Detailed first principles quantum-mechanical NSOR calculations are found to be in good agreement with the measurements. The effect of light on nuclear magnetic resonance (NMR) has been the subject of considerable interest, as it can be used to further increase the power of NMR spectroscopy [1,2]. While NMR frequency shifts due to laser light turned out to be too small to be easily measured [3], the inverse effect of optical rotation (OR) caused by nuclear spin polarization was observed in water and liquid 129 Xe [4]. The rotation of light polarization is similar to the Faraday effect caused by nuclear magnetization, but is enhanced by the hyperfine interaction between nuclear spins and virtual electronic excitations. Recent firstprinciples calculations of the nuclear spin OR (NSOR) predict different signals from chemically non-equivalent nuclei, opening the possibility of a new chemical analysis technique combining optical and NMR spectroscopy [5]. However, the signal-to-noise ratio (SNR) of NSOR detection has been poor in the first experiments utilizing low-field CW NMR [4] and in high-field pulsed experiments [6,7]. NSOR differences between the same nuclei in different molecular positions have not been clearly observed [7].Here we use a multi-pass optical cell [8,9] to measure proton NSOR with an SNR greater than 15 after 1000 seconds of integration and perform systematic NSOR measurements on organic liquids with absolute uncertainty of 5%. Contrary to recent report [7], we find that the NSOR signals do not scale with the Verdet constant V of the liquid since the hyperfine interaction between electrons and nuclei is influenced by the chemical environment. The ratio of NSOR to Faraday rotation and to inductive 1 H signals ranges by more than a factor of 2 for the simple chemicals studied. We also apply the recent first-principles theoretical method [5] for calculation of NSOR and obtain results in good qualitative agreement with our measurements for both 1 H and 19 F spins.

Nuclear spin circular dichroism

“…6,10,15,16 Calculated NSOR results at off-resonant wavelengths for ethene, listed in Table S6 of the supplementary material, 63 confirm that far from the transition region the damped response theory results indeed are in a very good agreement with the results of the standard approach. Figure 3 and Table S7 of the supplementary material 63 contain the data for the basis-set convergence of the calculated NSCD ellipticity and NSOR angle for both the 13 C and 1 H nuclei in ethene, obtained at the BHandHLYP level.…”

“…Various means of creating and controlling the nuclear magnetization in NMOS measurements have been discussed. [9][10][11][12] These include transferring the magnetized sample from a separate polarization vessel to the optical apparatus, 9, 10 a method relying on relatively slow nuclear spin relaxation processes. Alternatively, the optical measurement can be placed transversally to the bore of a nuclear magnetic resonance spectrometer, enabling the creation and manipulation of the magnetization in the same volume.…”

“…Later work has demonstrated, using a multi-pass cavity apparatus, the existence of an optical chemical shift, the differing NSOR angles caused by nuclear spin polarization in different molecular liquids. 10 This phenomenon was predicted using firsta) juha.vaara@iki.fi b) coriani@units.it principles calculations according to the response theory formulation of the underlying antisymmetric polarizability. 6 Further experimental [11][12][13] and theoretical [14][15][16] work has been reported for NSOR.…”

“…The antisymmetric polarizability underlying NSOR increases rapidly as the incident light frequency approaches optical excitations, [23][24][25] which has been proposed to facilitate a means of gaining chemical resolution of the nuclear magneto-optic rotation signals arising from the different chromophores of the molecule. 6 The first-principles NSOR calculations carried out so far 6,10,15,16 have all been performed with conventional quadratic response theory, 26 disregarding the fact that the perturbational approach is not valid in the vicinity of the excitation energies. Hence, it remains a question whether the conclusions drawn in the earlier work remain valid in a more realistic treatment of the near-resonant spectral regions.…”

Nuclear‐Spin‐Induced Circular Dichroism in the Infrared Region for Liquids

“…[6,7] Ref. [4] made efficient first-principles calculations of the NSOR of 11-cis-retinal protonated Schiff base (PSB11) and predicted that very sharp increases are expected near resonance by 10 1 -10 2 ;t his result is typical for NSOR.…”