Circular dichroism studies of crystalline chiral and achiral α-nitronyl nitroxide † radicals in a KBr matrix

Abstract

The X-ray crystal structures of a series of four chiral phenyl α-nitronyl nitroxides substituted in the aromatic ring with an (R)-lactate moiety are compared, and reveal the presence of all four gross diastereomeric conformations resulting from the twisted conformations of the phenyl and α-nitronyl nitroxide moieties. The use of KBr discs as solid solutions for the study of the circular dichroism (CD) spectra of these radical crystals provides useful information concerning the conformational stereoisomerism displayed by the compounds, both intrinsically chiral and other achiral radicals which crystallise in chiral space groups. The optimum conditions for obtaining the solid state CD spectra have been investigated, and require a short ball-milling of the mixture followed by careful recording of the spectrum. The Cotton effects observed in the CD spectra have been correlated with the observed enantiomeric forms of the pseudo-anti and pseudo-eclipsed conformers in the crystals of chiral lactic acid-derived phenyl α-nitronyl nitroxides and an achiral phenyl α-nitronyl nitroxide that crystallises in a chiral space group. † The IUPAC name for α-nitronyl nitroxide is 4,5-dihydro-4,4,5,5tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl.

Synthesis, Structure and Magnetic properties of Chiral and Nonchiral Transition‐Metal Malates

“…To evaluate directly the chirality of the structures of the compounds 1 – 10 , measurements of the UV circular dichroism (CD) in the solid state were carried out by using a JASCO 810 spectrometer on powder samples embedded in KBr pellets 45. The results obtained for the Co II derivatives 3 and 7 , which exhibit the most effective absorption in the UV‐Vis spectral range, are shown in Figure 4.…”

The use of X‐ray crystallography to determine absolute configuration

“…For compounds that racemize rapidly in solution, solid-state CD in a KBr disk may be applied to the bulk compound and perhaps even to a powdered single crystal. [23][24][25] One may expect vibrational CD, either IR or Raman, to be used increasingly in the future. Enantioselective chromatography (EC): This sensitive technique is applicable to microgram quantities and provides estimates of the enantiomeric excess.…”

Spontaneous Resolution through Helical Assembly of a Conformationally Chiral Molecule with an Unusual Zwitterionic Structure

“…Since large crystals could not be grown, we have attempted to measure the CD of small single crystals by grinding with KBr, and forming solid pellets. As reported earlier, [27] considerable optimization of conditions was required. After several attempts with many crystals, we were able to obtain spectra showing a negative CD.…”

“…Figure 6 shows the CD spectrum along with the absorption profile; the molar ellipticity is calculated as reported in Ref. [27]. In spite of extensive efforts, the spectrum of the enantiomorphous crystal, which shows positive CD has proved elusive so far.…”