Chiral Self-Recognition:  Direct Spectroscopic Detection of the Homochiral and Heterochiral Dimers of Propylene Oxide in the Gas Phase

Abstract

In this report, we describe rotational spectroscopic and high-level ab initio studies of the 1:1 chiral molecular adduct of propylene oxide dimer. The complexes are bound by weak secondary hydrogen bonds, that is, the O(epoxy)...H-C noncovalent interactions. Six homochiral and six heterochiral conformers were predicted to be the most stable configurations where each monomer acts as a proton acceptor and a donor simultaneously, forming two six- or five-membered intermolecular hydrogen-bonded rings. Rotational spectra of six, that is, three homochiral and heterochiral conformer pairs, out of the eight conformers that were predicted to have sufficiently large permanent electric dipole moments were measured and analyzed. The relative conformational stability order and the signs of the chiral recognition energies of the six conformers were determined experimentally and were compared to the ab initio computational results. The experimental observations and the ab initio calculations suggest that the concerted effort of these weak secondary hydrogen bonds can successfully lock the subunits in a particular orientation and that the overall binding strength is comparable to a classic hydrogen bond.

Chirality Recognition between Neutral Molecules in the Gas Phase

“…[28] Indeed, calculations at the MP2/6-311 + + G(d,p) or MP2/aug-cc-pVTZ level fail to reproduce the experimental sign of the chirodiastaltic energy in methyl oxirane dimers. [92] The second aspect of these calculations is to determine the global structure and hydrogen-bond topology of the complexes by direct comparison with the experimental spectra. It is crucial that the method used allows a complete exploration of complex potential energy surfaces (PES) of molecules with high conformational flexibility.…”

“…[112] Its dimer has been studied by highresolution rotational spectroscopy combined with high-level correlated ab initio calculations. [92] Evidence was found for three homochiral and three heterochiral dimers. In all of them, the two methyl oxirane subunits are tethered through four nonconventional hydrogen bonds: the oxygen atom of each molecule acts as an acceptor for two aliphatic CH units from the other molecule.…”

Molecular and supramolecular chirality: R2PI spectroscopy as a tool for the gas‐phase recognition of chiral systems of biological interest

“…More recently, the rotational spectra of gas phase homo-and heterochiral dimers of propylene oxide have been reported by Xu and coworkers. 44 Several conformers, bound by O HÀ ÀC interactions, were observed and assigned either for the homoand for the heterochiral dimers.…”

Hydration of a Chiral Molecule: The Propylene Oxide⋅⋅⋅(Water)2 Cluster in the Gas Phase

“…[6] Very recently, the rotational spectra of a number of PO-containing chiral molecular complexes have been studied, and detailed information about the structures of conformers and their relative stability ordering was reported. [7][8][9] One of the systems studied is the PO···H 2 O binary complex, where both syn and anti PO···H 2 O were detected experimentally. [7] While this experimental result represents a significant step towards understanding the intermolecular interactions taking place in an aqueous environment, there is still a huge gap to bridge between the isolated binary PO···H 2 O complex and PO in water.…”

“…[15] Only the basis set 6-311 + + G(d,p) [16] was employed in this study since it has been tested and proven to give the best agreement with experiments according to our previous studies of related systems. [7,9,17] The three most stable conformers of the PO··· (H 2 O) 2 complex, namely, syn PO···H 2 O) 2 , anti PO···(H 2 O) 2 , and bi PO···(H 2 O) 2 , were located (see Figure 1). (Detailed structural coordinates from the ab initio calculations are listed in Table S1 in the Supporting Information.)…”