A new technique is introduced for the acquisition of size-selected neutral cluster spectra on the basis of argon-mediated, population-modulated electron attachment. This method is demonstrated and used to obtain the vibrational spectrum of the neutral water hexamer precursor to the (H₂O)₆^- cluster ion. The mid-infrared spectrum of the neutral species is dominated by four intense features above 3400 cm^-1, clearly indicating that significant structural rearrangements occur upon slow electron attachment to form the “magic” hexamer cluster anion. Comparison with previous spectroscopic reports and theoretical predictions indicates that the low-energy “book” isomer is most consistent with the observed band pattern and is suggested to be the species that captures a low-energy electron to form the hexamer anion.