Communication: He-tagged vibrational spectra of the SarGlyH~+ and H~+(H_2O)_(2,3) ions: Quantifying tag effects in cryogenic ion vibrational predissociation (CIVP) spectroscopy

摘要

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~+(H_2O)_(2,3), including the H_5O_2~+ "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 H_5O_2~+ 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 H_5O_2~+ band structure, indicating that the He-tagged H_5O_2~+ spectrum accurately represents the delocalized nature of the vibrational zero-point level. Interestingly, the He-tagged spectrum of H~+(H_2O)_3 reveals the location of the non-bonded OH group on the central H_3O~+ 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.