Medel, R.; Stelbrink, C.; Suhm, M. A.,

Contributed by Steven Bachrach

Reposted from Computational Organic Chemistry with permission

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InChIKey=GRWFGVWFFZKLTI-IUCAKERBSA-N

InChIKey=GRWFGVWFFZKLTI-RKDXNWHRSA-N

InChiKey=DTGKSKDOIYIVQL-QXFUBDJGSA-N

InChIKey=IAIHUHQCLTYTSF-OYNCUSHFSA-N

InChIKey=REPVLJRCJUVQFA-BZNPZCIMSA-N

InChIKey=WAPNOHKVXSQRPX-ZETCQYMHSA-N

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This work is licensed under a Creative Commons Attribution-NoDerivs 3.0 Unported License.

*Angew. Chem. Int. Ed*. 2019, 58, 8177Contributed by Steven Bachrach

Reposted from Computational Organic Chemistry with permission

Can vibrational spectroscopy be used to identify stereoisomers? Medel, Stelbrink, and Suhm have examined the vibrational spectra of (+)- and (-)-α-pinene, (±)-

**1**, in the presence of four different chiral terpenes**2-5**.^{1}They recorded gas phase spectra by thermal expansion of a chiral α-pinene with each chiral terpene.
For the complex of

**4**with (+)-**1**or (-)-**1**and**5**with (+)-**1**or (-)-**1**, the OH vibrational frequency is identical for the two different stereoisomers. However, the OH vibrational frequencies differ by 2 cm^{-1}with**3**, and the complex of**3/(+)-1**displays two different OH stretches that differ by 11 cm^{-1}. And in the case of the complex of α-pinene with**2**, the OH vibrational frequencies of the two different stereoisomers differ by 11 cm^{-1}!
The B3LYP-D3(BJ)/def2-TZVP optimized geometry of the

**2/(+)-1**and**2/(-)-1**complexes are shown in Figure 2, and some subtle differences in sterics and dispersion give rise to the different vibrational frequencies.2/(+)-1 | 2/(-)-1 |

**Figure 2**. B3LYP-D3(BJ)/def2-TZVP optimized geometry of the

**2/(+)-1**and

**2/(-)-1**

Of interest to readers of this blog will be the DFT study of these complexes. The authors used three different well-known methods – B3LYP-D3(BJ)/def2-TZVP, M06-2x/def2-TZVP, and ωB97X-D/def2-TZVP – to compute structures and (most importantly) predict the vibrational frequencies. Interestingly, M06-2x/def2-TZVP and ωB97X-D/ def2-TZVP both failed to predict the vibrational frequency difference between the complexes with the two stereoisomers of α-pinene. However, B3LYP-D3(BJ)/def2-TZVP performed extremely well, with a mean average error (MAE) of only 1.9 cm

^{-1}for the four different terpenes. Using this functional and the larger may-cc-pvtz basis set reduced the MAE to 1.5 cm^{-1}with the largest error of only 2.5 cm^{-1}.
As the authors note, these complexes provide some fertile ground for further experimental and computational study and benchmarking.

### Reference

1. Medel, R.; Stelbrink, C.; Suhm, M. A., “Vibrational Signatures of Chirality Recognition Between α-Pinene and Alcohols for Theory Benchmarking.”

*Angew. Chem. Int. Ed.***2019**,*58*, 8177-8181, DOI: 10.1002/anie.201901687.### InChIs

**(-)-1**, (-)-α-pinene: InChI=1S/C10H16/c1-7-4-5-8-6-9(7)10(8,2)3/h4,8-9H,5-6H2,1-3H3/t8-,9-/m0/s1

InChIKey=GRWFGVWFFZKLTI-IUCAKERBSA-N

**(+)-1**, (-)-α-pinene: InChI=1S/C10H16/c1-7-4-5-8-6-9(7)10(8,2)3/h4,8-9H,5-6H2,1-3H3/t8-,9-/m1/s1

InChIKey=GRWFGVWFFZKLTI-RKDXNWHRSA-N

**2**, (-)borneol: InChI=1S/C10H18O/c1-9(2)7-4-5-10(9,3)8(11)6-7/h7-8,11H,4-6H2,1-3H3/t7-,8+,10+/m0/s1

InChiKey=DTGKSKDOIYIVQL-QXFUBDJGSA-N

**3**, (+)-fenchol: InChI=1S/C10H18O/c1-9(2)7-4-5-10(3,6-7)8(9)11/h7-8,11H,4-6H2,1-3H3/t7-,8-,10+/m0/s1

InChIKey=IAIHUHQCLTYTSF-OYNCUSHFSA-N

**4**, (-1)-isopinocampheol: InChI=1S/C10H18O/c1-6-8-4-7(5-9(6)11)10(8,2)3/h6-9,11H,4-5H2,1-3H3/t6-,7+,8-,9-/m1/s1

InChIKey=REPVLJRCJUVQFA-BZNPZCIMSA-N

**5**, (1S)-1-phenylethanol: InChI=1S/C8H10O/c1-7(9)8-5-3-2-4-6-8/h2-7,9H,1H3/t7-/m0/s1

InChIKey=WAPNOHKVXSQRPX-ZETCQYMHSA-N

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This work is licensed under a Creative Commons Attribution-NoDerivs 3.0 Unported License.