C O M M U N I C A T I O N S
dichloromethane and deuterochlorophorm solutions, as well as in
solid state; in particular, a Nujol mull was prepared with a
monocrystal of compound 1. No noticeable difference in its main
features were detected upon viewing the IR (Figure 2 left), except
for the splitting of the 1252 cm-1 band observed in the solid state
IR (1255 and 1245 cm-1). The few differences that are present are
due to the fact that only weak interactions occur in the supramo-
lecular arrangement in the solid state. Also, there is good agreement
between solution and solid state experimental and gas phase
calculated spectra, as shown in Figure 2 left. The two highest
frequencies in the experimental and calculated IR spectrum of 1,
assigned to the stretching of the ketal group (1097 cm-1) and to
asymmetric stretching of the aromatic ether group (1252 cm-1),
show a good fit. In the less intense bands slight displacements are
observed.
Figure 3. Experimental VCD absorption spectra of (R)-1 (blue) and (S)-1
(red) in Nujol mull.
excess in all the samples, but the preferred enantiomer seems
random (we obtained a 3:2 ratio of the R and S configuration).
In conclusion, we describe a method for determining the absolute
structure of flexible molecules composed of light atoms, based on
the combination of single crystal X-ray diffraction, solid state VCD,
and DFT calculations. This method has been applied to nonchiral
4-methoxy-4-(p-methoxyphenyl)-cyclohexanone ethylene ketal (1)
which describes the synthesis, crystal structure, and spontaneous
resolution. The procedure is simple and does not require a lengthy
calculation time, due to no conformational distribution analysis
being required before VCD calculation. Subsequently, we are
currently working on the implementation of this method to other
chiral materials.
Acknowledgment. The authors are grateful to the CICYT-
FEDER Spainish project (MAT96-1073-C02, CTQ2009-13129-
C02-02, and MAT2008-06522-C02) and the Government of Arago´n
and Madrid (Project MADRISOLAR, ref S-0505/PPQ/0225).
Thanks are given to the CTI (CSIC) for allocation of computer
time.
Figure 2. Left: IR spectra of 1 in CH2Cl2 solution (a), CDCl3 solution (b),
Nujol mull (c), and calculated IR spectrum of 1. Right: ((R)-1) calculated
VCD for (R)-1, (exp 1) experimental VCD absorption spectra of 1 in Nujol
mull, and ((S)-1 calculated VCD for (S)-1).
Supporting Information Available: Experimental details for syn-
thesis, characterization and X-ray structure determination (in CIF
format) for 1. This material is available free of charge via the Internet
The good agreement observed between the experimental and
calculated conformation as well as between the solid-sate experi-
mental and gas-phase calculated IR spectra allows us to have
confidence in the computational level employed to calculate VCD
spectrum. Thus, the experimental VCD spectrum of the a monoc-
rystal (Figure 2 right, middle) was compared with theoretical DFT-
calculated results for the optimized conformation of enantiomers
R and S (top and bottom). The overall profile of the experimental
VCD spectrum nicely coincides with that of the enantiomer (R)-1
calculated VCD, although the calculated VCD does not simulate
accurately the intensity of some peaks in the experimental spectrum
due to the interactions that are established in the solid. Actually,
apart from the overall coincidence of the spectrum, the most
important data for the assignment of the R or S absolute crystal
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