Using laser-induced Coulomb explosion of aligned chiral molecules to determine their absolute configuration

The absolute configuration of chiral molecules in the gas phase is determined by femtosecond laser-induced Coulomb explosion and recording of the emission direction of critical ionic fragments. The spatial orientation of the $3,5-\mathrm{dibromo}-3^{\prime },5^{\prime }-\mathrm{diflouro}-4^{\prime }-\mathrm{cyanobiphenyl}$ molecules studied is determined by a combination of laser-induced alignment and time-of-flight measurement of ${\text{F}}^{+}$ ion recoils. Hereby, the enatiomeric state of the molecules could be uniquely identified through analysis of the correlation of the angular distribution of ${\text{F}}^{+}$ and ${\text{Br}}^{+}$ ions. The method will apply to a range of molecules and allow imaging of chiral changes on ultrafast time scales.

Figure 1

Effect of orientation and chirality on the sign of the dihedral angle as seen from the detector. (a) Orientation that leads to an early ${\text{F}}^{+}$ ion signal. The sign of the dihedral angle seen by the detector is shown for the (b) $M$ and (c) $P$ enantiomers. (d) Orientation that leads to a late ${\text{F}}^{+}$ ion signal. The sign of the dihedral angle seen by the detector is shown for the (e) $M$ and (f) $P$ enantiomers. The circular axes in (b), (c), (e), and (f) indicate ${\varphi }_{\text{d}}$. The atoms in the molecule are labeled by color.

Figure 2

Time-of-flight mass spectrum recorded for 1D aligned BFCbP molecules ionized by the probe pulse. The inset shows a magnification of the region around the arrival time of the ${\text{F}}^{+}$ ions with the experimental data marked by light red circles. The $E$ and $L$ subscripts label the early and late peaks used to identify the orientation of the molecules (see the text for details).

Figure 3

Time-of-flight covariance map of 1D aligned BFCbP. (b)–(e) show magnifications of selected regions (marked by white squares) of the covariance map. In each panel the range of the color scale is adjusted to emphasize the important features.

Figure 4

(a) Covariance map of the angular distribution from the ${\text{F}}^{+}$ ions in the early peak and the angular distribution from all the ${\mathrm{Br}}^{+}$ ions. Each broad line of high intensity parallel to the diagonal arises from a single enantiomer. (b) Integration of the covariance maps along lines of constant ${\theta }_{{\text{F}}^{+}}-{\theta }_{{\text{Br}}^{+}}$ with the light red (dark blue) points showing the result for the early (late) ${\text{F}}^{+}$ ion peak. Fits of four Gaussian functions to the experimental data are shown as solid lines with corresponding colors. For both the early peak data and the late peak data each enantiomer gives rise to two peaks in (b). The enantiomers are labeled $P$ or $M$ below the peaks in the same color as the experimental data.