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ABSTRACT Circular birefringence (CB) is defined as the difference in refractive index for opposite circular polarization states and has played a crucial role in the development of stereochemistry and the concept of chirality. It manifests experimentally as optical rotatory dispersion (ORD), that is, the wavelength‐dependent optical rotation of the plane of light polarization. However, most methods for probing ORD rely on analyzing transmitted light asymmetry at single wavelengths (usually the sodium D‐line at 589 nm) with linear polarizers, which cannot discern between unpolarized and circularly polarized light, limiting the apparatus to analyze a single phenomenon. Here we showcase the use of Stokes spectropolarimetry (SSP), a versatile and cost‐effective technique, to probe ORD of circularly birefringent materials. This technique allows complete analysis of the dispersive changes in polarization caused by anisotropic media, portraying a versatile experimental framework to study different types of optical anisotropies with a single spectropolarimeter. Here, aqueous solutions of chiral sucrose, fructose, and their mixtures are investigated. The ORD acquired verify that the optical rotation is proportional to the concentration of the chiral species and follows an inverse proportion with wavelength. As a case study, we show via SSP that ORD at 589 nm (D‐line of sodium) is in good agreement with literature (+63.5° ± 1.4° mL g−1 dm−1for sucrose and −83.7° ± 2.0° mL g−1 dm−1for fructose).
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Isolated and solvated chiroptical behavior in conformationally flexible butanamines
Abstract The nonresonant optical activity of two highly flexible aliphatic amines, (2R)‐3‐methyl‐2‐butanamine (R‐MBA) and (2R)‐(3,3)‐dimethyl‐2‐butanamine (R‐DMBA), has been probed under isolated and solvated conditions to examine the roles of conformational isomerism and to explore the influence of extrinsic perturbations. The optical rotatory dispersion (ORD) measured in six solvents presented uniformly negative rotatory powers over the 320–590 nm region, with the long‐wavelength magnitude of chiroptical response growing nearly monotonically as the dielectric constant of the surroundings diminished. The intrinsic specific optical rotation, (in deg dm−1[g/mL]−1), extracted for ambient vapor‐phase samples ofR‐MBA [−11.031(98) and −2.29 (11)] andR‐DMBA [−9.434 (72) and −1.350 (48)] at 355 and 633 nm were best reproduced by counterintuitive solvents of high polarity (yet low polarizability) like acetonitrile and methanol. Attempts to interpret observed spectral signatures quantitatively relied on the linear‐response frameworks of density‐functional theory (B3LYP, cam‐B3LYP, and dispersion‐corrected analogs) and coupled‐cluster theory (CCSD), with variants of the polarizable continuum model (PCM) deployed to account for the effects of implicit solvation. Building on the identification of several low‐lying equilibrium geometries (nine forR‐MBA and three forR‐DMBA), ensemble‐averaged ORD profiles were calculated atT = 300 K by means of the independent‐conformer ansatz, which enabled response properties predicted for the optimized structure of each isomer to be combined through Boltzmann‐weighted population fractions derived from corresponding relative internal‐energy or free‐energy values, the latter of which stemmed from composite CBS‐APNO and G4 analyses. Although reasonable accord between theory and experiment was realized for the isolated (vapor‐phase) species, the solution‐phase results were less satisfactory and tended to degrade progressively as the solvent polarity increased. These trends were attributed to solvent‐mediated changes in structural parameters and energy metrics for the transition states that separate and putatively isolate the equilibrium conformations supported by the ground electronic potential‐energy surface, with the resulting displacement of barrier locations and/or decrease of barrier heights compromising the underlying premise of the independent‐conformer ansatz.
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- Award ID(s):
- 2154840
- PAR ID:
- 10422197
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chirality
- Volume:
- 35
- Issue:
- 9
- ISSN:
- 0899-0042
- Page Range / eLocation ID:
- p. 586-618
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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