Disentangling the Contribution of Multiple Isomers to the Infrared Spectrum of the Protonated Water Heptamer

We use infrared/infrared double-resonance population labeling ((IRMS2)-M-2) spectroscopy in the spectral region of the free and hydrogen-bonded OH stretching fundamentals (2880-3850 cm(-1)) to identify the number and to isolate the vibrational signatures of individual isomers contributing to the gas-phase IR spectra of the cryogenically cooled protonated water clusters H+(H2O)(n)center dot H-2/D-2 with n = 7-10. For n = 7, four isomers are identified and assigned. Surprisingly, the (IRMS2)-M-2 spectra of the protonated water octa-, nona-, and decamer show no evidence for multiple isomers. The present spectra support the prediction that the quasi-2D to 3D structural transition occurs in between n = 8 and 9 in the cold cluster regime. However, the same models have difficulty explaining the remarkable size dependence of the isomer population reported here.