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, H-2, 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 H-2 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 H-2) 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. (C) 2014 AIP Publishing LLC.