On the chiroptical properties of racemic crystals.

In a recent letter on the "Optical activity from racemates", discussed at the 16th International Conference on Chiroptical Spectroscopy in Rennes (June 2017), scientists claimed to have measured the circular dichroism of colored, racemic crystals of two transition metal complexes, observations that "serve to correct a clear deficit in our understanding of the optical properties of racemates." In fact, there is no deficit in our understanding of the optical properties of racemates. Moreover, the measurements presented are unconvincing as evidence of circular dichroism measured in anisotropic crystals and therefore do not inform our understanding of the specific crystals investigated, or racemates generally speaking.

Revisiting polarimetry near the isotropic point of an optically active, non-enantiomorphous, molecular crystal.

Accurate polarimetric measurements of the optical activity of crystals along low symmetry directions are facilitated by isotropic points, frequencies where dispersion curves of eigenrays cross and the linear birefringence disappears. We report here the optical properties and structure of achiral, uniaxial (point group D2d ) potassium trihydrogen di-(cis-4-cyclohexene-1,2-dicarboxylate) dihydrate, whose isotropic point was previously detected (S. A. Kim, C. Grieswatch, H. Küppers, Zeit. Krist. 1993; 208:219-222) and exploited for a singular measurement of optical activity normal to the optic axis. The crystal structure associated with the aforementioned study was never published. We report it here, confirming the space group assignment I 4¯c2, along with the frequency dependence of the fundamental optical properties and the constitutive tensors by fitting optical dispersion relations to measured Mueller matrix spectra. k-Space maps of circular birefringence and of the Mueller matrix near the isotropic wavelength are measured and simulated. The signs of optical rotation are correlated with the absolute crystallographic directions.

Relationship between oxidation level and optical properties of secondary organic aerosol.

Brown carbon (BrC), which may include secondary organic aerosol (SOA), can be a significant climate-forcing agent via its optical absorption properties. However, the overall contribution of SOA to BrC remains poorly understood. Here, correlations between oxidation level and optical properties of SOA are examined. SOA was generated in a flow reactor in the absence of NOx by OH oxidation of gas-phase precursors used as surrogates for anthropogenic (naphthalene, tricyclo[5.2.1.0(2,6)]decane), biomass burning (guaiacol), and biogenic (α-pinene) emissions. SOA chemical composition was characterized with a time-of-flight aerosol mass spectrometer. SOA mass-specific absorption cross sections (MAC) and refractive indices were calculated from real-time cavity ring-down photoacoustic spectrometry measurements at 405 and 532 nm and from UV-vis spectrometry measurements of methanol extracts of filter-collected particles (300 to 600 nm). At 405 nm, SOA MAC values and imaginary refractive indices increased with increasing oxidation level and decreased with increasing wavelength, leading to negligible absorption at 532 nm. Real refractive indices of SOA decreased with increasing oxidation level. Comparison with literature studies suggests that under typical polluted conditions the effect of NOx on SOA absorption is small. SOA may contribute significantly to atmospheric BrC, with the magnitude dependent on both precursor type and oxidation level.

Transitions from functionalization to fragmentation reactions of laboratory secondary organic aerosol (SOA) generated from the OH oxidation of alkane precursors.

Functionalization (oxygen addition) and fragmentation (carbon loss) reactions governing secondary organic aerosol (SOA) formation from the OH oxidation of alkane precursors were studied in a flow reactor in the absence of NO(x). SOA precursors were n-decane (n-C10), n-pentadecane (n-C15), n-heptadecane (n-C17), tricyclo[5.2.1.0(2,6)]decane (JP-10), and vapors of diesel fuel and Southern Louisiana crude oil. Aerosol mass spectra were measured with a high-resolution time-of-flight aerosol mass spectrometer, from which normalized SOA yields, hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, and C(x)H(y)+, C(x)H(y)O+, and C(x)H(y)O(2)+ ion abundances were extracted as a function of OH exposure. Normalized SOA yield curves exhibited an increase followed by a decrease as a function of OH exposure, with maximum yields at O/C ratios ranging from 0.29 to 0.74. The decrease in SOA yield correlates with an increase in oxygen content and decrease in carbon content, consistent with transitions from functionalization to fragmentation. For a subset of alkane precursors (n-C10, n-C15, and JP-10), maximum SOA yields were estimated to be 0.39, 0.69, and 1.1. In addition, maximum SOA yields correspond with a maximum in the C(x)H(y)O+ relative abundance. Measured correlations between OH exposure, O/C ratio, and H/C ratio may enable identification of alkane precursor contributions to ambient SOA.

Chiroptical anisotropy of crystals and molecules.

Optical activity, a foundational part of chemistry, is not restricted to chiral molecules although generations have been instructed otherwise. A more inclusive view of optical activity is valuable because it clarifies structure-property relationships however, this view only comes into focus in measurements of oriented molecules, commonly found in crystals. Unfortunately, measurements of optical rotatory dispersion or circular dichroism in anisotropic single crystals have challenged scientists for more than two centuries. New polarimetric methods for unpacking the optical activity of crystals in general directions are still needed. Such methods are reviewed as well as some of the 'nourishment' they provide, thereby inviting to new researchers. Methods for fitting intensity measurements in terms of the constitutive tensor that manifests as the differential refraction and absorption of circularly polarized light, are described, and examples are illustrated. Single oriented molecules, as opposed to single oriented crystals, can be treated computationally. Structure-property correlations for such achiral molecules with comparatively simple electronic structures are considered as a heuristic foundation for the response of crystals that may be subject to measurement.

Tuning the optical isotropic point of mixed crystals of ethylenediammonium sulfate/selenate.

In anisotropic crystals, optical isotropic points are wavelengths where linear birefringence disappears because the refractive indices for both eigenmodes are accidentally equivalent. Here, the optical isotropic point of ethylene-diammonium selenate (EDSe) is tuned by generating a solid-solution series of EDSe doped with sulfate. Mueller matrix polarimetry and single-crystal X-ray diffraction are used to correlate changes in linear birefringence with the crystal composition of EDS x Se1-x . A scheme for using mixed crystals with isotropic points as tunable birefringent optical bandpass filters is proposed and their performance is modeled.

Polarized light through polycrystalline vaterite helicoids.

Vaterite helicoids [W. Jiang et al., Nat. Commun., 2017, 8, 15066] are chiral, polycrystalline suprastructures grown in the presence of the amino acids, aspartic (Asp) or glutamic (Glu) acid, that are abundant in proteins regulating biomineralization. These complex objects are composed of hexagonal vaterite nanocrystals assembled as curved-edge platelets that form chiral ensembles. The sense stacked platelets is correlated with the stereochemistry of the amino acid additive: l-Asp gives counterclockwise architectures while d-Asp gives the clockwise enantiomorphs. As new layers stack, platelets become progressively inclined with respect to the substrate suface. The growth and structure of vaterite helicoids was originally evidenced by electron microscopy and atomic force microscopy. Here, we develop an optical model for describing polarized light transmission through helicoids as measured by Mueller matrix polarimetry. The close agreement between experimental measurements and simulation confirms that the propellor-like organization of inclined platelets creates optically active structures determined by growth additive stereochemistry. The microscopy employed demonstrates the information that can be obtained by complete polarimetry using a camera as a light detector, a technique that could be applied profitably to all manner of complex structures organized from anisotropic particles.

Double cone of eigendirections in optically active ethylenediammonium selenate crystals.

Circular birefringence (CB) is generally responsible for only a small perturbation to the state of light polarization in crystals that also exhibit linear birefringence (LB). As such, the magnetoelectric tensor of gyration, which gives rise to CB and optical activity, is less well determined than the electric permittivity tensor in optical models of the Mueller matrix. To visualize the effect of the magnetoelectric tensor on polarimetric measurements, reported here are experimental mappings of the Mueller matrix and of the CB in a new chiral crystal with accidental null LB at an accessible optical frequency. Single crystals of ethylenediammonium selenate (EDSeO4) were synthesized and characterized by X-ray diffraction and Mueller matrix measurements in transmission and reflection. The crystals are isomorphous with the corresponding sulfate salt. They are tetragonal, space group P41(3)212. The constitutive relations of EDSeO4 were recovered using a partial wave summation of incoherent reflections. The extraordinary and ordinary refractive indices cross at 364 nm (3.41 eV), a scenario commonly called the 'isotropic point' or 'iso-index point'. At this wavelength, the magnetoelectric tensor fully describes the polarization transformation, giving rise to a double cone of eigendirections.

Oriented attachment by enantioselective facet recognition in millimeter-sized gypsum crystals.

Crystal growth by oriented attachment involves the spontaneous self-assembly of adjoining crystals with common crystallographic orientations. Herein, we report the oriented attachment of gypsum crystals on agitation to form stereoselective mesoscale aggregates.