In this Communication, the plus and minus signs of the circular dichroism (CD) spectra in Figures 2b and 4b were incorrect. The corrected Figures are shown below. Additionally,after further theoretical investigation, it was found that the signs of the rotatory strengths (R)for pseudoephedrine (pED) vary depending on the type of the density functionals used in the time-dependent density functional theory (TDDFT) calculations. This variation is due to small CD effects of the S 0-S 1 transition of pED and large error bars of the CD values predicted by theory.T able S1 in the Supporting Information lists the R values estimated using various density functionals. Figure 2. a) R2PI spectrum of S-pED near the origin band of the S 0-S 1 transition. The inset shows the structure of S-pED. The number of ions produced by as ingle-laser pulse at the origin bands was roughly estimated as about 900. b) R2PI CD spectra of S-(blue line) and R-pED (red line). The g values at the bands of AG(a), AG(b), and GG(a) of S-pED were measured as + + 0.026 AE 0.005, À0.025 AE 0.006, and + + 0.024 AE 0.005, respectively.c)Theoretical CD spectra of S-(blue line) and R-pED (red line) obtained with the rotatory strength, R,c alculated using TDDFT at the M06-2X/6-311 + ++ + G(d,p) level. Figure 4. a) R2PI spectrum of RED near the origin band of the S 0-S 1 transition. The inset shows the structure of RED. The discontinuous region between 37825 and 37855 cm À1 is where the grating order of the dye laser changes. b) R2PI CD spectrum of RED. The g values of 1-3 bands were measured as + + 0.030 AE 0.011, À0.091 AE 0.007,a nd + + 0.041 AE 0.013, respectively.

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 resonant two-photon ionization circular dichroism (R2PICD) spectrum represents the cumulative circular dichroism (CD) of one-photon excitation and the subsequent one-photon ionization, whereas the fluorescence-detected circular dichroism (FDCD) spectra exhibit only the CD of one-photon excitation, similar to conventional CD spectra. We obtained the FDCD spectra of jet-cooled ephedrine (EPD) near the origin band of the S0-S1 transition to measure the CD of one-photon absorption and thus the CD of the ionization process in R2PI in comparison with the R2PICD spectra. The CD effects of the ionization following excitation of the A (0-0) and C (930 cm(-1)) bands in the spectrum are small, whereas those of the B band (530 cm(-1)) are anomalously large, leading to opposite CD signs for the FDCD and R2PICD spectra. Based on the intermediate state-selective fragmentation patterns in the R2PI spectra, this large CD effect is attributed to the state-selective isomerization that occurs after excitation of the B band. By comparing the experimental and theoretical spectra, we determined that the B band corresponds to an asymmetric ring distortion mode that involves torsional motions of the side chain, which may facilitate the isomerization process. This study demonstrates that FDCD spectroscopy combined with R2PICD spectroscopy provides a powerful tool to measure the CD effects of the excitation and ionization processes separately in R2PI and thus probe the structural changes that occur during the ionization process following excitation to an intermediate state.

We obtained resonant two-photon ionization circular dichroism (R2PICD) spectra of jet-cooled phenylalanine (Phe) and its hydrated clusters (Phe(HO), n = 1-2) near the origin band of the S-S transition. The R2PICD spectra of Phe exhibit well-resolved CD bands of six different conformers present in the jet, which vary in sign and magnitude depending on their conformations. We revised the previous structural assignments of the Phe conformers based on the comparison between the experimental and theoretical CD signs, infrared spectra, and rotational band contours. The R2PICD spectra of Phe(HO) reveal that hydration with one or two water molecule(s) does not affect the CD signs of Phe conformers but significantly increases their CD magnitudes. Furthermore, conformational selection by solvation alters relative populations of Phe conformers, leading to a sign inversion in the CD spectra of Phe(HO) compared with that of Phe monomer.

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.