To understand how the D2d oxalate scaffold (C2O4)(2-) distorts upon capture of a proton, we report the vibrational spectra of the cryogenically cooled HO2CCO2(-) anion and its deuterated isotopologue DO2CCO2(-). The transitions associated with the skeletal vibrations and OH bending modes are sharp and are well described by inclusion of cubic terms in the normal mode expansion of the potential surface through an extended Fermi resonance analysis. The ground state structure features a five-membered ring with an asymmetric intramolecular proton bond. The spectral signatures of the hydrogen stretches, on the contrary, are surprisingly diffuse, and this behavior is not anticipated by the extended Fermi scheme. We trace the diffuse bands to very strong couplings between the high-frequency OH-stretch and the low-frequency COH bends as well as heavy particle skeletal deformations. A simple vibrationally adiabatic model recovers this breadth of oscillator strength as a 0 K analogue of the motional broadening commonly used to explain the diffuse spectra of H-bonded systems at elevated temperatures, but where these displacements arise from the configurations present at the vibrational zero-point level.
Survey vibrational predissociation spectra of several representative protonated peptides and model compounds reveal very diffuse absorptions near 2500 cm −1 that are traced to pentagonal cyclic ionic hydrogen bonds (C 5 interactions) involving the excess charge centers. This broadening occurs despite the fact that the ions are cooled close to their vibrational zero-point levels and their spectra are obtained by predissociation of weakly bound adducts (H 2 , N 2 , CO 2 ) prepared in a cryogenic ion trap. The C 5 band assignments are based on H/D isotopic substitution, chemical derivatization, solvation behavior, and calculated spectra. We evaluate the extent to which this broadening is caused by anharmonic coupling in the isolated molecules by including cubic coupling terms in the normal mode expansion of the potential energy surface. This analysis indicates that the harmonic H-bonded stretching vibration is mixed with dark background states over much of the energy range covered by the observed features. The difficulty with identifying these features in earlier studies of dipeptides is traced to both the breadth and the fact they are calculated to be intrinsically weaker than cases involving linear variations of the N•••H + •••O motif.
b S Supporting Information T he formate ion, HCO 2 h , is ubiquitous in aqueous chemistry and biology, and its structural properties have been extensively catalogued over many decades using a wide variety of spectroscopic techniques. 1À5 There is renewed interest, however, in this ion's intrinsic behavior due to its increasingly important role in the chain of events leading to activation of CO 2 to transportable fuels, for example, by reductive addition of hydride. 6,7 The electronic and vibrational level structures of the isolated ion were accurately calculated in 2007 by Botschwina and co-workers, 8 and two aspects of the potential curve describing CH dissociation are nonintuitive. First, the ∼18700 cm À1 bond dissociation energy 5,9,10 is much lower than is typical for CH bonds of closed-shell singlets (e.g., 43370 cm À1 for HCN) 11 and second, the shape of the CH potential does not conform to a Morse form, instead exhibiting a sharp change in slope around a bond length of 2 Å where it flattens toward the low dissociation asymptote. Both these features are consistent with the 2003 experimental observation of the CH stretching (ν CH ) fundamental of formate in a Ne matrix 12 at 2456 cm À1 , which is far below the typical 2950 cm À1 value found for aliphatic hydrocarbons. 13 More interestingly, this value is also much lower than that long known for HCO 2 h in aqueous media (2803 cm À1 ) 4 and in crystals (2830 cm À1 ). 14 In this paper, we explore the mechanism underlying the unusual shape of the CH potential, the low intrinsic ν CH frequency, and its anomalously large solvent dependence. This is accomplished by analyzing the vibrational
Ultraviolet and infrared-ultraviolet (IR-UV) double-resonance photofragment spectroscopy has been carried out in a tandem mass spectrometer to determine the three-dimensional structure of cryogenically cooled protonated C-terminally methyl esterified leucine enkephalin [YGGFL-OMe+H](+). By comparing the experimental IR spectrum of the dominant conformer with the predictions of DFT M05-2X/6-31+G(d) calculations, a backbone structure was assigned that is analogous to that previously assigned by our group for the unmodified peptide [ Burke, N.L.; et al. Int. J. Mass Spectrom. 2015 , 378 , 196 ], despite the loss of a C-terminal OH binding site that was thought to play an important role in its stabilization. Both structures are characterized by a type II' β-turn around Gly(3)-Phe(4) and a γ-turn around Gly(2), providing spectroscopic evidence for the formation of a β-hairpin hydrogen bonding pattern. Rather than disrupting the peptide backbone structure, the protonated N-terminus serves to stabilize the β-hairpin by positioning itself in a pocket above the turn where it can form H-bonds to the Gly(3) and C-terminus C═O groups. This β-hairpin type structure has been previously proposed as the biologically active conformation of leucine enkephalin and its methyl ester in the nonpolar cell membrane environment [ Naito, A.; Nishimura, K. Curr. Top. Med. Chem. 2004 , 4 , 135 - 143 ].
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