Relation between molecular electronic structure and nuclear spin-induced circular dichroism

Abstract

The recently theoretically described nuclear spin-induced circular dichroism (NSCD) is a promising method for the optical detection of nuclear magnetization. NSCD involves both optical excitations of the molecule and hyperfine interactions and, thus, it offers a means to realize a spectroscopy with spatially localized, high-resolution information. To survey the factors relating the molecular and electronic structure to the NSCD signal, we theoretically investigate NSCD of twenty structures of the four most common nucleic acid bases (adenine, guanine, thymine, cytosine). The NSCD signal correlates with the spatial distribution of the excited states and couplings between them, reflecting changes in molecular structure and conformation. This constitutes a marked difference to the nuclear magnetic resonance (NMR) chemical shift, which only reflects the local molecular structure in the ground electronic state. The calculated NSCD spectra are rationalized by means of changes in the electronic density and by a sum-over-states approach, which allows to identify the contributions of the individual excited states. Two separate contributions to NSCD are identified and their physical origins and relative magnitudes are discussed. The results underline NSCD spectroscopy as a plausible tool with a power for the identification of not only different molecules, but their specific structures as well.

Spatial localization in nuclear spin-induced circular dichroism – a quadratic response function analysis

“…15,16 Most recently, nuclear spin-induced circular dichroism (NSCD) has been theoretically investigated. [17][18][19][20] NSCD is intimately connected to NSOR, stemming from the same physical origins. However, in contrast to NSOR, which is a dispersive effect, NSCD occurs at energies of electronic excitations, i.e., where the molecule has optical absorption bands.…”

“…9 However, available theoretical analyses have dealt with this topic only in a passing manner and the experimental measurements so far have not been performed near electronic resonances in order to confirm this prediction. Moreover, it has also been shown that the overall shape of the electron density and NSCD spectra seem to qualitatively correlate, 20 leading to the hypothesis that the analysis of the excited state densities may hold a key to new, nucleus-specific information hidden in NSCD.…”

“…To date, NSCD spectra have been obtained via the complex polarization propagator (CPP) approach 17,18,20 also known as damped response theory. [21][22][23] The CPP method has the advantage of taking into account contributions from all excited states at any calculated energy point, but it comes with the requirement of selecting beforehand a peak broadening parameter g, and the need to calculate many discrete points on a frequency grid to obtain a sufficiently smooth spectral curve.…”

“…Moreover, in order to extract the NSCD contribution from a particular excited state, one needs to resort to numerical fitting of the CPP curve. 20 The results of such fitting are prone to errors due to the fact that even low energy regions of the CPP curve contain contributions from ''tailing'' peaks of excited states lying at higher energies.…”

Low Concentration Measurements of Nuclear Spin-Induced Optical Rotation Using SABRE Hyperpolarization

“…Out of the five described NMOEs [4,5,6,2,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,1,3], so far only nuclear spin-induced optical rotation (NSOR) has been observed, for a range of molecules including water [4,6,13,12], organic liquids, such as alcohols [12], hydrocarbons [6,12], halogenated hydrocarbons [6,7,12], organic phosphines and thiols, [7] and for xenon [4]. In general, these experiments share two common features: a) in most cases neat substances (pure liquids) were used as samples and b) they were polarized thermally in a strong magnetic field.…”