Origin of the diffuse vibrational signature of a cyclic intramolecular proton bond: Anharmonic analysis of protonated 1,8-disubstituted naphthalene ions

Abstract

We analyze the structures and spectral signatures of the cyclic intramolecular proton bond, N-H(+)-A, A = O and F, formed when an excess proton is added to derivatives of the 1,8-disubstituted naphthalene scaffold. These compounds provide a quasi-rigid framework with which to study the spectral complexity often associated with the N-H(+)-A entity. Vibrational spectra were obtained by monitoring photodissociation of weakly bound H2 adducts of the mass-selected ions cooled close to 10 K. Several bands across the 900-3500 cm(-1) spectral range were traced to involvement of the bridging proton by their telltale shifts upon selective H∕D isotopic substitution at that position. We account for the complex patterns that occur near the expected locations of the NH stretching fundamentals in the context of background levels mixing with a "bright" zero-order state through cubic terms in the potential energy expansion. Thus, this system provides a detailed picture of one of the mechanisms behind the line broadening often displayed by embedded excess protons. It does so in a sufficiently sparse density of states regime that many discrete transitions are observed in the vicinity of the harmonic stretching transition involving displacement of the trapped proton.

Investigation of the position of the radical in z3-ions resulting from electron transfer dissociation using infrared ion spectroscopy

“…The spectra recorded in H-stretching range generally show strong and sharp OH-stretch features and NH-stretch bands that are oen weaker than predicted; this is likely due to the fact that OH moieties are free in the computed structures, whereas NH moieties, especially on the Lys side chain, engage in strong hydrogen bonds. [83][84][85][86][87][88] In any case, most pertinently to the present discussion, the spectra in this range provide little or no diagnostic information that would allow us to distinguish between isomers.…”

On the character of the cyclic ionic H-bond in cryogenically cooled deprotonated cysteine

“…3b). We caution, however, that in related systems with such a cyclic intramolecular hydrogen bond (GlyGlyH + and protonated disubstituted naphthalene derivatives) 19,20 their characteristic stretching transitions (n CIDB and n CIHB ) occur hundreds of wavenumbers below harmonic predictions. 21 It is also important to note that the primary difference between the two isomers in traces (a) and (c) is the location of the proton (closer to the O and S atoms, respectively), and the stretching displacement of the shared proton would therefore largely contribute to isomer interconversion.…”

“…3a and c, which are variations of the -S À Á Á ÁH + Á Á Á À O 2 C-linkage. Bands due to excitation of the bridging proton in such cyclic linkages have proven difficult to identify, 19,20,22 and it is thus useful to compare the behavior of the (Cys-H + ) À system to that of a model compound containing a -CO 2 H group that is engaged in a cyclic intramolecular H-bond. This motif has been reported previously by Wang et al 23 in their study of deprotonated dodecanedioic acid, HCO 2 (CH 2 ) 10 CO 2 À , again using cryogenic photoelectron spectroscopy.…”

“…Although the extreme broadening of the shared proton stretch implied by this assignment is unprecedented, several recent reports have also found this motif to be unusually susceptible to strong anharmonic coupling. 19,20,22 Unfortunately, the expected anharmonic red shift would place the centroid of the shared deuteron stretch in the cluttered fingerprint region, thus frustrating the confirmation of this assignment using isotopic shifts. Nonetheless, we conclude that the structures with a strong intramolecular H-bond between -S À and -CO 2 H, best accommodates the spectroscopic data, and points to the need for further study to understand the strong anharmonicities that are associated with this motif.…”

Complexes of Crown Ether Macrocycles with Methyl Guanidinium: Insights into the Capture of Charge in Peptides

“…In the 18‐crown‐6 complex, two partly merged moderately broad bands are observed at 3150–3450 cm −1 , and a narrower and weaker band component is found at 3530 cm −1 . It should be noted that broadened bands in proton bonded systems often arise from proton delocalization effects . While such scenario would be appealing, the a priori expectation for the present systems is that the highly basic guanidine moiety retains the proton with a limited sharing with the crown ether.…”