Spectroscopic Discrimination of Diastereomeric Complexes Involving an Axially Chiral Receptor

The infrared multiphoton dissociation (IRMPD) spectra of electrospray ionization (ESI)-formed proton-bound complexes between the axially chiral multifunctional macrocycle M^aR and d- and l-phenylalanine (P^D and P^L ) or d- and l-3,4-dihydroxyphenylalanine (D^D and D^L ) are recorded in the ν˜ =2800-3700 cm^-1 region. Whereas the diastereomeric [M^aR ⋅H⋅P^D ]⁺ and [M^aR ⋅H⋅P^L ]⁺ complexes do not show any significant spectral differences, the spectrum of [M^aR ⋅H⋅D^D ]⁺ clearly diverges from that of its [M^aR ⋅H⋅D^L ]⁺ diastereomer. A comparison of the experimental spectra with the B3LYP/6-31G**-calculated harmonic vibrational frequencies of a number of stable structures indicates the formation, in the ESI source, of very similar spectroscopically active structures, with the protonated amino acid placed outside the flat cavity of the macrocycle and hydrogen-bonded at its O₂ center. Different spectral signatures of the [M^aR ⋅H⋅D^D ]⁺ and [M^aR ⋅H⋅D^L ]⁺ complexes are attributed to the coexistence of several stable rotamers in the ESI source.