Low-Energy Photoelectron Spectrum and Dissociative Photoionization of the Smallest Amides: Formamide and Acetamide

Abstract

The threshold photoelectron spectrum and low-energy dissociative photoionization processes of formamide and acetamide were studied using photoelectron photoion coincidence spectroscopy and vacuum ultraviolet synchrotron radiation. Ab initio calculations and Franck− Condon simulations helped us assign the main vibrational progressions in the spectra and enabled the first conclusive assignment of the first electronically excited states. The adiabatic ionization energies to the X ̃+ and A ̃+ states of formamide (10.236 ± 0.004 and 10.643 ± 0.015 eV) and acetamide (9.734 ± 0.008 and 10.282 ± 0.020 eV) have been re-evaluated and spectroscopic transitions were assigned using a Franck−Condon approach. The cationic potential energy surface was explored to rationalize the observed fragmentation patterns and to construct a statistical model, which was fitted to the experimental breakdown diagram. Thermochemical thresholds were measured and calculated for H, CO, and NH 2 loss from HCONH 2 + as well as for CH 3 , NH 2 , CO, HCCO, and NH 3 loss from CH 3 CONH 2 + . We present the first comprehensive, experimental and theoretical treatise of these fragmentation processes. The statistical model confirms fast internal conversion between the X ̃+ and A ̃+ states in formamide, as H transfer in CO loss is shown to take place on the excited state surface. It also explains the five almost simultaneously opening dissociation channels in the acetamide cation quantitatively. The derived 0 K appearance energies have been confirmed by ab initio calculations and by comparison with state-of-the-art thermochemical data, and some of the previous results have been revised by more than 10 times their stated uncertainty.

The Threshold Photoelectron Spectrum of Fulvenone: A Reactive Ketene Derivative in Lignin Valorization

“…[24] In fact this result shows the limit of the widely applied FC modeling approach to fit experimental ms-TPE spectra for the isomer-selective assignment of reactive intermediates in complex environments. However, we have found that, as was also observed in formamide and acetamide, [25] the lowest energy state at the optimized geometry of theà þ state is indeed theà þ state. This applied to wave function theory methods, which allowed us to carry out, among others, ground-state CBS-APNO and G3 composite method calculations to obtain the adiabatic ionization energies to the excitedà þ state, as well (Table 1).…”

UPS, XPS, NEXAFS and Computational Investigation of Acrylamide Monomer

“…1395 cm −1 ). A similar trend was also observed in the spectrum of formamide [35], and it was tentatively assigned to the asymmetric stretching of the N-C=O frame, an interpretation supported by recently reported vibronic structure simulations [36]. According to this interpretation, we observe that the shape of peak "D" is akin to that of "A".…”

“…S=0.156 S=0.134 S=0.410 The positioning of the D 1 ← S 0 transition is more challenging. In Figure 5, we propose a tentative assignment with an adiabatic ionization energy ∆E 0,0 = 10.31 eV, corresponding to peak H. This value is 0.033 eV smaller than that of formamide (10.64(2) eV [36]) and very close to that of acetamide (10.28(2) [36]). According to the Huang-Rhys factors (Figure 7), the most active normal modes are δ CCC = 38 meV (306 cm −1 ), ν C−N = 172 meV (1388 cm −1 ), δ CCO = 80 meV (648 cm −1 ), and ν C=C = 161 meV (1297 cm −1 ).…”

“…By aligning the calculated and experimental spectra, we determine the adiabatic ionization energies to the D 0 state as ∆E 0,0 = 9.755 eV. This value is 0.475 eV smaller than that of formamide (10.23(6) eV [36]) and 0.021 eV greater than that of acetamide (9.734(8) [36]). According to the Huang-Rhys factors (Figure 7), the main vibrational structure is related to the skeletal stretching motions associated with the largest geometry changes upon ionization, specifically ν C−C = 179 meV (1444 cm −1 ), ν C−N = 196 meV (1584 cm −1 ) and ν C=O = 132 meV (1061 cm −1 ).…”

Conformers, electronic states, and diabolical conical intersections in the valence photoelectron spectroscopy of halocyclohexanes

“…Photoelectron spectroscopy combined with Franck-Condon simulations can be used to characterize cation ground and excited states. 28 In previous experiments on monohalogenated silacyclohexanes, 29 trends could be established in the electronic and bonding structure as a function of the halogen size. This motivated us to study the photoelectron spectra of heavier halocyclohexanes (C 6 H 11 X, X = Cl, Br, and I) to gain insights into their electronic structure and conformational behavior as a function of the halogen.…”