ZEKE Photoelectron Spectroscopy of the cis and trans Isomers of Formanilide

Abstract

Spectroscopic characterization of cationic amides: ZEKE photoelectron spectroscopy has been successfully applied to isomers of the model peptide formanilide. The spectra indicate that significant charge delocalization occurs from the aromatic ring to the side chain in the ion. The picture shows the spectrum of the trans isomer together with the side‐chain in‐plane bending and amide stretching modes that are associated with the B′ and Σ′ bands. IE=ionization energy.

Stepwise Microhydration of Aromatic Amide Cations: Formation of Water Solvation Network Revealed by Infrared Spectra of Formanilide+–(H2O)n Clusters (n ≤ 5)

“…68 Zero-kinetic-energy (ZEKE) photoelectron spectra of t-/c-FA + yield information on a few low-frequency modes of their amide group as well as on the amide torsion. 68 The important amide I−II and hydride stretch frequencies (ν CH/NH/OH ), which are sensitive to protein folding, are available from recent IRPD spectra of cold FA + −L n clusters Table 2. Relative energies (E 0 ), binding energies (D 0 ), and bond lengths are given in cm −1 and Å, respectively.…”

“…72 Ionization of t-/c-FA into the D 0 state induces remarkable structural changes, which have been discussed in detail previously. 68,72 The adiabatic ionization energies of t-/c-FA calculated as IE = 66114/66514 cm −1 agree well with the measured values of IE = 67408 ± 5/(67710 ± 5) cm −1 . 68 The N−H bond of c-FA + is weaker, longer, and more acidic than that of t-FA + (r NH = 1.015 versus 1.012 Å), leading to a much lower ν NH frequency (3348 versus 3381 cm −1 ).…”

“…68,72 The adiabatic ionization energies of t-/c-FA calculated as IE = 66114/66514 cm −1 agree well with the measured values of IE = 67408 ± 5/(67710 ± 5) cm −1 . 68 The N−H bond of c-FA + is weaker, longer, and more acidic than that of t-FA + (r NH = 1.015 versus 1.012 Å), leading to a much lower ν NH frequency (3348 versus 3381 cm −1 ). Thus, the NH group in c-FA + is a better proton donor and forms stronger H-bonds to solvent molecules.…”

“…The ionization energy of t-FA + (8.358 eV) 68 is much lower than that of (H 2 O) n clusters (>10 eV), 96,97 and the positive charge in [t-FA−(H 2 O) n ] + clusters is therefore largely localized on the t-FA moiety, justifying the employed notation of t-FA + − (H 2 O) n . Nonetheless, as the ionization energy of (H 2 O) n clusters decreases as a function of n, there is a monotonic increase in the charge transfer from t-FA + to the solvent cluster, which reaches around 80 me at n = 4.…”

Resonance Raman spectroscopy and theoretical study on the photodissociation dynamics of formanilide in S3 state

“…Substitution of phenyl group with the N or C terminal ofCONHhas considerable influence on the nature of a peptide bond because of the addition of noticeable conjugation interaction. In formanilide (FA), the simplest N‐terminal‐substituted aromatic amide, its excited state charge transfer dynamics was noted to involve the whole molecule after ionization 8, 9. The S 2 excited‐state photochemistry of formanilide and benzamide (BA, a typical C‐terminal aromatic amide) was systematically explored by Fang et al using the CASSCF computational method 10.…”