On the Photoionization Spectrum of Propyne: A Fully ab Initio Simulation of the Low-Energy Spectrum Including the Jahn–Teller Effect and the Spin–Orbit Interaction

Abstract

The low energy photoionization spectrum of propyne (CH3-CCH), which reveals the vibronic structure of the propyne cation, is simulated using vibronic coupling theory. The spin-orbit interaction is included using an intensity borrowing approach, enabling determination of the (X̃(2)E1/2,3/2, v = 0) splitting and the relative photoionization intensity of these closely spaced levels. The results are compared with recent experimental studies and misstatements are corrected.

Photoionization spectroscopy of CH3C3N in the vacuum-ultraviolet range

“…The optimized equilibrium geometry of the cation ground state was found to be of C s symmetry due to distortion resulting from a Jahn-Teller interaction. Note that a similar situation is encountered in the propyne cation ground state (CH 3 C 2 H + ) [34]. The calculated equilibrium geometries of neutral and cationic CH 3 C 3 N are detailed in Table 1 with the labels defined in Fig.…”

“…In CH 3 C 3 N + , we can suspect a spin-orbit splitting of the same order of magnitude, slightly decreased by the off-axis H atoms [43,44]. Thus, higher-resolved spectra of theX þ2 E X1 A 1 transition should exhibit a complex rovibronic structure due to the combination of the Jahn-Teller effect and spin-orbit coupling like in propyne [34,36] and the 2-butyne [45] cations. Pulsedfield-ionization zero-kinetic-energy photoelectron spectroscopy would be perfectly suited for this purpose.…”

Internal rotation, spin–orbit coupling, and low-frequency vibrations in the ground state of CH3–CC–CH+3and CD3–CC–CD+3

“…[26], but in agreement with what is observed in the ground state of the acetylene radical cation (spin-orbit splitting of −30.86(44) cm −1 [27]). The calculations also indicated that the (E⊗e) Jahn-Teller effect in the ground state of the propyne radical cation is primarily linear, with a weak quadratic coupling and leads to a significant reduction of the spin-orbit coupling [23]. Given the structural similarities between the propyne and the 2-butyne radical cations, one may expect that the behaviour of the 2-butyne radical cation qualitatively resembles that observed for the propyne radical cation.…”

“…No information has been reported on the magnitude of the spin-orbit interaction in the ground electronic state CH 3 -CC-CH + 3 so far, but it is well known that the spinorbit interaction and the Jahn-Teller effect need to be considered jointly in similar molecular systems [22][23][24][25]. A recent investigation of theX + 2 E ground state of the propyne radical cation by PFI-ZEKE photoelectron spectroscopy has revealed a spin-orbit splitting of about 13.2 cm −1 [26].…”

“…A recent investigation of theX + 2 E ground state of the propyne radical cation by PFI-ZEKE photoelectron spectroscopy has revealed a spin-orbit splitting of about 13.2 cm −1 [26]. The splitting of the ground vibronic state was then determined to be −18.7 cm −1 [23] in an ab initio treatment of the Jahn-Teller effect in the propyne radical cation, which implies that the E 3/2 spin-orbit component is located below the E 1/2 component, opposite to the experimental assignment proposed in Ref. [26], but in agreement with what is observed in the ground state of the acetylene radical cation (spin-orbit splitting of −30.86(44) cm −1 [27]).…”