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 H5O2(+) 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.