Solvation of Propylene Oxide in Water: Vibrational Circular Dichroism, Optical Rotation, and Computer Simulation Studies

Abstract

The solvation of propylene oxide (PO) in water has been studied using vibrational circular dichroism (VCD) spectroscopy, optical rotation dispersion (ORD) spectroscopy, molecular dynamics simulations, and ab initio calculations. VCD and ORD measurements were carried out for PO as neat liquid, in CCl4, and in water solutions. The classical molecular dynamics simulations were carried out for the PO + water binary mixtures at different concentrations, and the solvation information was derived from the radial distribution functions obtained in the simulations. The total number of water molecules within the closest vicinity of PO was predicted to be about 3. The geometry optimizations, vibrational frequencies, and VCD intensities were evaluated for the PO monomer and the PO-(H2O)n clusters with n = 1-3 , using density functional theory calculations at the B3LYP/aug-cc-pVTZ level of theory. The chirality transfer VCD feature, which is a direct result of the explicit H-bonding between water and the chiral PO solute, was detected experimentally at the water bending band region. This feature exhibits high sensitivity to the solvation structure around PO. Comparison of the calculated and experimental chirality transfer features leads to the conclusion that the PO-water binary complex is the dominating species in aqueous solution at room temperature and the anti conformation, where water is on the opposite side of the oxirane ring of the PO methyl group, is preferred over the syn one. This conclusion is also supported by the complementary ORD studies. Possible contributions from the ternary and quaternary PO-water complexes are also discussed.

Gas phase optical rotation calculated from coupled cluster theory with zero‐point vibrational corrections from density functional theory

“…A promising, if computationally expensive, alternative is to combine molecular dynamics (MD) simulation with quantum chemical sampling of the solute and explicit solvent molecules. [40][41][42] In an attempt to keep the computational cost of the MD-based approach at a minimum, Kundrat and Autschbach 43,44 have shown that a simple point charge representation of the solvent molecules (water) may be used to simulate explicit solvent effects in the quantum chemical sampling. As noted by these authors, the MD-based scheme may also be used as an alternative to Boltzmann averaging to account for conformational effects.…”

Stereochemical Communication within a Chiral Ion Pair Catalyst

“…[13] This unique sensitivity also led to the discovery of chirality-transfer phenomena. Fori nstance,c hiral oxiranes in water were found to induce aVCD signal for the H 2 Obending modes, [14] and more recently,a mmonia was shown to become optically active upon interacting with methyl lactate under matrixisolation conditions. [15] Furthermore,chiral induction in an ion pair has been reported by Bas et al [16] This encouraged us to apply VCD spectroscopy to study chirality-induction phenomena in ion-pair-based asymmetric catalysis with the focus on establishing structure-spectra relationships which can be used to predict catalytic selectivity.…”

Infrared diode laser spectroscopic investigation of four C–H stretching vibrational modes of propylene oxide

“…Second-order Møller-Plesset perturbation theory (MP2) [38] with the basis set 6-311++G(d,p) [39] was chosen because of its proven performance in predicting geometries of organic molecules [40] and their IR spectra [21]. For comparison with the experimental data, the harmonic frequencies have been uniformly scaled with a factor of 0.94.…”

“…It is both rigid and stable, and has one stereogenic center, namely the center carbon atom bonded with the methyl group. PO has been a valuable prototype molecule for theoretical and rotational spectroscopic studies of chiral recognition effects [16,17] and chiroptical activities [18][19][20][21]. A series of rotational spectroscopic studies of PO in its ground vibrational and excited internal rotational states were published [22][23][24][25].…”