Intermolecular bonding and vibrations of phenol⋅oxirane

Induced photoelectron circular dichroism onto an achiral chromophore

An achiral chromophore can acquire a chiral spectroscopic signature when interacting with a chiral environment. This so-called induced chirality is documented in electronic or vibrational circular dichroism, which arises from the coupling between electric and magnetic transition dipoles. Here, we demonstrate that a chiroptical response is also induced within the electric dipole approximation by observing the asymmetric scattering of a photoelectron ejected from an achiral chromophore in interaction with a chiral host. In a phenol-methyloxirane complex, removing an electron from an achiral aromatic π orbital localised on the phenol moiety results in an intense and opposite photoelectron circular dichroism (PECD) for the two enantiomeric complexes with (R) and (S) methyloxirane, evidencing the long-range effect (~5 Å) of the scattering chiral potential. This induced chirality has important structural and analytical implications, discussed here in the context of growing interest in laser-based PECD, for in situ, real time enantiomer determination.

CHIRAL DISCRIMINATION IN HYDROGEN-BONDED COMPLEXES OF 2-METHYLOL OXIRANE WITH ETHANOL

We present an ab initio investigation on the chiral discrimination of 2-methylol oxirane (M-olOx)· · · ethanol (EtOH) complexes, for the sake of comparison with previous report on propylene oxide (PO)· · · EtOH complexes. Second-order Møller–Plesset perturbation theory (MP2) with the 6-311++G(d,p) basis set was used to elucidate the diastereomeric interactions between ethanol ( EtOH ), a transient chiral alcohol, and the chiral molecule 2-methylol oxirane (R). Six complexes of M-olOx· · · EtOH have been identified and their structures as well as their calculated stability ordering have been determined. The six complexes were defined in a similar way as for PO· · · EtOH . The primary O–H· · · O hydrogen bonds are predicted to be important contributions to chiral discrimination in M-olOx· · · EtOH . The three syn structures, with ethanol and the methylol group on the same side of the oxirane ring, are energetically favored over the three anti structures. The larger chirodiastaltic energy between synG- and synG+ is 0.52 kJ mol-1. The largest diastereofacial energy between synG- and antiG- is 13.90 kJ mol-1. The obtained results are compared with previously reported results on the PO· · · EtOH complexes and the mechanisms of chiral discrimination in PO· · · EtOH and M-olOx· · · EtOH are discussed. The harmonic frequencies, IR intensities, rotational constants, and dipole moments for the M-olOx· · · EtOH complexes are also presented. Such a theoretical study should be valuable to further spectroscopic investigations on M-olOx· · · EtOH complexes.

Internal Rotation of the Hydroxyl Group in Isopropanol and the Chirality of the Gauche Form: Fourier Transform Microwave Spectroscopy of (CH(3))(2)CHOD

The rotational spectrum of the deuterated isopropanol (CH(3))(2)CHOD has been observed by Fourier transform microwave spectroscopy and analyzed to yield tunneling splitting of 4431.4613 (17) MHz, between the antisymmetric and the symmetric gauche forms, which is much larger than the 2400 MHz estimated from the internal-rotation potential function reported in the literature. The potential function for the OH internal rotation has been examined in view of the discrepancy between the observed and estimated tunneling splitting, and it was accounted for by taking into account isotope effects on the potential constants. The deuterium quadrupole coupling effect has been included together with the Coriolis terms in the off-diagonal block of the Hamiltonian matrix for the gauche form. The deuterium quadrupole coupling constants obtained for the trans form were employed to calculate the components of the coupling constants as functions of the internal-rotation angle, and the components at around 120 degrees were compared with the values observed for the gauche form, thereby leading to unambiguous determination of the signs of the constants in the off-diagonal block; the signs are not obtainable from an ordinary analysis of the rotational spectra. The chirality of the gauche form was discussed by placing special emphasis on the effect of intermolecular interactions between two chiral molecules. Copyright 2001 Academic Press.