Chiral hide-and-seek: Retention of enantiomorphism in laser-induced nucleation of molten sodium chlorate

Abstract

We report the observation of non-photochemical laser-induced nucleation (NPLIN) of sodium chlorate from its melt using nanosecond pulses of light at 1064 nm. The fraction of samples that nucleate is shown to depend linearly on the peak power density of the laser pulses. Remarkably, we observe that most samples are nucleated by the laser back into the enantiomorph (dextrorotatory or levorotatory) of the solid prior to melting. We do not observe a significant dependence on polarization of the light, and we put forward symmetry arguments that rule out an optical Kerr effect mechanism. Our observations of retention of chirality can be explained by decomposition of small amounts of the sodium chlorate to form sodium chloride, which provide cavities for retention of clusters of sodium chlorate even 18 °C above the melting point. These clusters remain sub-critical on cooling, but can be activated by NPLIN via an isotropic polarizability mechanism. We have developed a heterogeneous model of NPLIN in cavities, which reproduces the experimental data using simple physical data available for sodium chlorate.

Role of Impurity Nanoparticles in Laser-Induced Nucleation of Ammonium Chloride

“…[1][2] Non-photochemical LIN (NPLIN) generally requires relatively lower intensities of light at wavelengths where there are no strong absorption bands in the system; short, unfocussed pulses (ps to ns) of laser light are typically used. Systems that have been studied using NPLIN include small molecules such as glycine and carbamazepine, [3][4][5][6][7][8][9] simple salts such as KCl, [10][11][12] singlecomponent systems such as acetic acid and sodium chlorate, [13][14] and large molecules such as proteins. [15][16] The main advantage of NPLIN is that it appears to be similar to homogeneous 2 nucleation: it requires only solute and solvent, occurs in the bulk of the solution, and there is no chemical damage to molecules.…”

“…For nucleation of simple salts from solution, the interaction of the transient E-field with a sub-critical cluster acts to lower its free energy, causing a change in its structure. 10,12 In the case of small molecules, the difference in polarizability along different directions of the molecule means that molecules can become aligned with respect to the direction of the E-field, analogous to the optical Kerr effect (OKE). 5,19 The resulting increase in structural order due to the interaction between the E-field and solute cluster gives rise to nucleation.…”

“…For example, it was observed that the nucleation probability is linearly proportional to E 2 , as expected for the OKE. 12,20 Needle-like crystals of urea that were nucleated by NPLIN were observed to be aligned with the electric field vector of linearly polarized light. 3 In the cases of glycine and of L-histidine it was possible to control the product polymorph by use of circular versus linear polarized light.…”

“…3,7,19 Aging was not required for halide salts, which may indicate that these clusters form more rapidly. 10,12 There are several unresolved issues with the polarizability mechanism, as summarized recently by Agarwal and Peters. 21 The observation of a minimum power required for nucleation at low laser powers is not readily explained.…”

Nonphotochemical Laser-Induced Nucleation of Potassium Halides: Effects of Wavelength and Temperature

“…29 Theoretical simulations by Knott et al, however, suggest that the free energy gain from the orientational bias at the electric field strengths involved are not sufficient to explain NPLIN by the OKE mechanism. 30 Alexander and co-workers have studied NPLIN of a number of metastable systems, 31,32 in particular the benchmark system of aqueous potassium chloride (KCl). 24,33,34 It was demonstrated that samples of KCl with supersaturations (S = C/C sat ) in the range 1.05-1.10 can be nucleated by a single pulse from a nanosecond laser.…”