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The induced circular dichroism (ICD) of phenol complexed with ( R )-(-)-2-butanol [PhOH-(-)BOH] in a supersonic jet is investigated using resonant two-photon ionization circular dichroism (R2PICD) spectroscopy. The R2PICD spectrum of PhOH-(-)BOH exhibits nonzero ICD bands near the absorption region of bare PhOH, where (-)BOH is transparent. Two different conformers containing a single hydrogen bond between PhOH and (-)BOH are identified using ultraviolet-ultraviolet hole-burning and infrared ion-dip spectroscopy combined with quantum theoretical calculations. The ICD values of the two conformers are similar to each other. To understand these similar ICD effects of the conformers, the geometrical asymmetry of the PhOH moiety bound to (-)BOH and the coupling strength of the electric transition dipole moments between PhOH and (-)BOH are estimated. Comparing the ICD values of PhOH-(-)BOH with those of PhOH-(-)-l-methyl lactate in the previous report [ Hong , A. ; J. Phys. Chem. Lett. 2018 , 9 , 476 -480 ], we investigate the physical properties that may govern the differences of the ICD values between the two complexes.

Induced circular dichroism (ICD) is the CD observed in the absorption of an achiral molecule bound to a transparent chiral molecule through noncovalent interactions. ICD spectroscopy has been used to probe the binding between molecules, such as protein-ligand interactions. However, most ICD spectra have been measured in solution, which only exhibit the averaged CD values of all conformational isomers in solution. Here, we obtained the first isomer-selective ICD spectra by applying resonant two-photon ionization CD spectroscopy to jet-cooled phenol complexes with (-)-methyl l-lactate (PhOH-(-)ML). The well-resolved CD bands in the spectra were assigned to two conformers, which contained different types of hydrogen-bonding interactions between PhOH and (-)ML. The ICD values of the two conformers have different signs and magnitudes, which were explained by differences both in the geometrical asymmetries of PhOH bound to (-)ML and in the electronic coupling strengths between PhOH and (-)ML.

The CD spectroscopy of a chiral compound in solution yields an average CD value derived from all of the conformations of a chiral molecule. By contrast, CD spectroscopy of cold chiral molecules in the gas phase distinguishes specific conformers of a chiral molecule, but the weak CD effect has limited the practical application of this technique. Reported herein is the first resonant two-photon ionization CD spectra of ephedrines in a supersonic jet using circularly polarized laser pulses, which were generated by synchronizing the oscillation of the photoelastic modulator with the laser firing. The spectra exhibited well-resolved CD bands which were specific for the conformations and vibrational modes of each enantiomer. The CD signs and magnitudes of the jet-cooled chiral molecules were very sensitive to their conformations and thus offered crucial information for determining the three-dimensional structures of chiral species, as conducted in combination with quantum chemical calculations., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)