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

Abstract

The threshold photoelectron spectrum (TPES) of halocyclohexanes C 6 H 11 X (X = Cl, Br, and I) was recorded at the Swiss Light Source and assigned with the help of density functional theory and equation-of-motion ionization potential coupled cluster calculations. Dyson orbitals show that the first two electronic states of the cation arise by symmetry breaking of the doubly degenerate eg orbitals in cyclohexane as perturbed by the halogen or by perturbation of the halogen lone pair by the cyclohexane ring scaffold in the case of light and heavy halogen substituents, respectively. When the resulting two states (A ′′ and A ′ ) are coupled via a conical intersection in C S symmetry, they are smoothly connected by molecular orbital rotation when the nuclear symmetry is relaxed. Even then, barriers at avoided crossings lead to distinct A ′ and pseudo-A ′′ minima, which contribute to the TPES separately. As axial and equatorial conformers are present in commensurate abundance at room temperature, four transitions are conceivable for each substituent in the low-energy range. Three of these could be identified, and their energy could be determined for each sample. Transitions to A ′ states are associated with a smaller geometry change and exhibit stronger origin transitions. Yet, most notably in X = Br, they do not correspond to the adiabatic ionization energy, which is indicated by a weak and broad band to the pseudo-A ′′ state with a lower onset energy. Franck-Condon vibrational analysis of the TPES coupled with quantum chemical calculations can provide insights into the behavior of conformers as well as strongly coupled electronic states.

Dissociative Photoionization of Chloro-, Bromo-, and Iodocyclohexane: Thermochemistry and the Weak C–Br Bond in the Cation

“…Only this rate information and the threshold ionization cross section (proportional to the TPES) are lost when the threshold ionization mass spectra are reduced by plotting the fractional ion abundances as a function of photon energy in the breakdown diagram (Figure 2). The TPES is plotted in the background in Figure 2 17 because it can reveal if the dissociation mechanism is nonstatistical. The statistical model assumes that the ergodic hypothesis holds and the system can explore the whole phase space prior to dissociation.…”

“…We also located the conical intersections coupling the low-lying electronic states and identified the conformer-specific spectral fingerprints of the heavier monohalogenated cyclohexanes by equation-of-motion-coupled cluster ionization energy calculations and Franck−Condon simulations of the TPES. 17 Thus, PEPICO was confirmed to be a conformer-selective detection tool. Using monochromatic vacuum ultraviolet ionizing radiation and discriminating for threshold, that is, near-zero kinetic energy electrons, we control the photoionization energy balance and can tune the internal energy deposited in the parent ion and available for fragmentation.…”

Ro-vibrational Distribution of NO+ Dissociated from NO2+ Ions in the a3B2 and b3A2 States: A Slow “Impulsive” Dissociation Example Revealed from Threshold Photoelectron–Photoion Coincidence Imaging

“…Additionally, using the optimized geometries, harmonic frequencies, and normal-mode vectors for each electronic state, the Franck–Condon factor (FCF) calculations were conducted as the overlaps between initial and target vibrational states in the harmonic approximation using the ezSpectrum program . Then, the TPE spectrum of NO 2 in a specific electronic state was simulated with a certain full width at half-maximum (fwhm), according to experimental spectra, to assign vibrational progressions of each specific electronic transition from neutral to cation as we did in other systems. − …”

From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF3SF5) by Double Velocity Map Imaging

“…Using pulsed-field ionization photoelectron photoion coincidence (PFI-PEPICO) techniques, − Cheuk-Yiu Ng and co-workers measured numerous dissociative photoionization onsets with unequaled accuracy and precision at the Advanced Light Source. − Continuous extraction PEPICO with electron velocity map (VM) imaging offers slightly inferior resolution but superior signal-to-noise ratio and has flourished at synchrotron light sources . Franck–Condon simulations of halocyclohexane TPE spectra revealed conformer-specific vibrational and electronic fingerprints and unveiled the coupling dynamics between the low-lying cation states. , Then again and along with recent analytical applications, PEPICO truly shines in dissociative ionization studies because the energy deposited in the parent ion is known and can be tuned by scanning the photon energy. Fractional parent and fragment ion abundances are plotted in the breakdown diagram (BD).…”