Communication: He-tagged vibrational spectra of the SarGlyH⁺ and H⁺(H₂O)_2,3 ions: Quantifying tag effects in cryogenic ion vibrational predissociation (CIVP) spectroscopy

To assess the degree to which more perturbative, but widely used "tag" species (Ar, H₂, Ne) affect the intrinsic band patterns of the isolated ions, we describe the extension of mass-selective, cryogenic ion vibrational spectroscopy to the very weakly interacting helium complexes of three archetypal ions: the dipeptide SarGlyH⁺ and the small protonated water clusters: H⁺(H₂O)_2,3, including the H₅O₂⁺ "Zundel" ion. He adducts were generated in a 4.5 K octopole ion trap interfaced to a double-focusing, tandem time-of-flight photofragmentation mass spectrometer to record mass-selected vibrational predissociation spectra. The H₂ tag-induced shift (relative to that by He) on the tag-bound NH stretch of the SarGlyH⁺ spectrum is quite small (12 cm^-1), while the effect on the floppy H₅O₂⁺ ion is more dramatic (125 cm^-1) in going from Ar (or H₂) to Ne. The shifts from Ne to He, on the other hand, while quantitatively significant (maximum of 10 cm^-1), display the same basic H₅O₂ ⁺ band structure, indicating that the He-tagged H₅O ₂⁺ spectrum accurately represents the delocalized nature of the vibrational zero-point level. Interestingly, the He-tagged spectrum of H⁺(H₂O)₃ reveals the location of the non-bonded OH group on the central H₃O⁺ ion to fall between the collective non-bonded OH stretches on the flanking water molecules in a position typically associated with a neutral OH group.