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Reports: DNI653476-DNI6: Spectroscopic Investigation of Vibronic Interactions in Molecules with Low Symmetry

Jinjun Liu, PhD, University of Louisville

The research supported by this DNI grant from the ACS PRF investigates the vibronic coupling in molecular species, especially free radicals. During this second year of funding, several important reaction intermediates were studied by laser-induced fluorescence/dispersed fluorescence (LIF/DF) spectroscopy. They are either Jahn-Teller (JT) or pseudo-Jahn-Teller (pJT)-active, depending on whether they have degenerate or semi-degenerate electronic states.

1.     T-butoxy

Alkoxy radicals are among the most important reaction intermediates in atmospheric chemistry and “low-temperature” (T<1000 K) combustion. Among alkoxy radicals, t-butoxy is especially interesting in dynamic studies because of its β C-C bond scission. Moreover, T-butoxy is the second smallest alkoxy radical subject to the JT effect (JTE), after methoxy (CH3O). In both radicals, the JTE is coupled with spin-orbit (SO) interaction. The comparison between t-butoxy and methoxy can separate the mass effect from the effect of the force field. Furthermore, it is interesting to examine how methyl torsion influences the vibronic interaction and the SO effect. We have obtained jet-cooled DF and LIF spectra of t-butoxy that reveal vibrational structures of its X̃2E (ground) and Ã2A1 (first-excited) electronic states, respectively. The observed transitions were assigned based on calculated vibrational frequencies of both states and Franck-Condon factors (FCFs) for transitions between them. The SO splitting was measured to be 36(5) cm-1 for the lowest vibrational level of the X̃ state, which is significantly smaller than that of methoxy (61 cm-1), and increases with increasing vibrational quantum number of the CO stretch mode. Vibronic analysis of the DF spectra suggests that JT-active modes of the ground-state t-butoxy radical are similar to those of methoxy. The rotationally and fine structure resolved LIF transition to the first CO stretch overtone level of the Ã2A1 state has also been recorded and simulated (Fig. 1), yielding an accurate determination of the rotational constants of both à and X̃ states.


Fig. 1. Rotationally resolved LIF spectrum of the 720 band of t-butoxy.

 

2.     Methylcyclohexoxy

In the first year of funding, we carried out the spectroscopic analysis of the cyclohexoxy radical, a pJT molecule with Cs symmetry. It is of both theoretical and practical interests to extend the spectroscopic investigation of cyclohexoxy to the methylcyclohexoxy (MCHO) radicals. Especially important is the effect of the methyl group position at the six-membered ring. We have obtained the jet-cooled DF spectra of the four positional isomers of MCHO, which reveal the vibrational structures of their nearly degenerate X̃ and à states (Fig. 2). All four isomers of MCHO have close-lying X̃ and à states although their energy separations are affected by the position of the methyl group. Intensities of vibronic transitions in the DF spectra are dependent on the pumped LIF bands, which can be explained by the difference in geometry and symmetry between the lower X̃/à states and the highly excited B̃ state. Both FCFs and Duschinsky mixing between CO-stretch and other vibrational modes have to be taken into account in the simulation of the LIF/DF spectra. Excited-state vibrational population relaxation from the CO stretch level to the vibrational ground level and from combination levels of the CO stretch mode and other vibrational modes to the non-CO stretch modes was observed.


Fig. 2. DF spectra of 2, 3-, and 4-MCHOs by pumping the LIF origin bands compared with calculated transition intensities and simulated spectra.

 

3.     Calcium ethoxide

Metal-containing free radicals are important intermediates in metal-surface reactions and in the interaction between metals and organic molecules. Laser spectroscopy is a power tool for the investigation of metal-ligand binding and the precise measurement of ionization energies. Previously, monomethyl radicals such as MgCH3, CaCH3 and SrCH3, as well as their corresponding monomethoxide radicals (MgOCH3, CaOCH3 and SrOCH3), have been studied in LIF experiment. These free radicals are subject to JTE. Asymmetric methyl substitution lowers the molecular symmetry from C3v to Cs and reduces the JTE to the pJT effect (pJTE). Most recently, we have combined the laser ablation technique with supersonic jet expansion and obtained the jet-cooled LIF/DF spectra of calcium ethoxide (CaOCH2CH3). In the LIF spectrum (Fig. 3a), which illustrates the rotational contours of the Ã2A’-X̃ 2A’ and B̃2A”-X̃ 2A’ origin bands, the energy separation between the nearly degenerate à and B̃ states has been determined. Vibronic analysis of the LIF/DF spectra (Fig. 3) will unravel the interplay between pJT and SO interactions. Such analysis will be done by a new version of the SOCJT program first developed in the research group of Prof. Terry A. Miller at the Ohio State University. The original program can simulate and fit vibronic spectra of C3V molecules subject to E×e JTE coupled with SO interaction. We will upgrade the program so that it is capable of analyzing the pJTE and SO interactions in Cs molecules. The updated SOCJT program will be used to simulate and fit LIF spectra of calcium ethoxide as well as other monovalent derivatives of alkaline earth metals including  ethyl calcium (CaCH2CH3), isopropyl calcium (CaCH(CH3)2), and calcium isopropoxide (CaOCH(CH3)2).


Fig. 3. (a) LIF and (b) DF spectra of CaOCH2CH3.

 

Impact of PRF DNI Award: The PRF DNI award has helped Dr. Liu launch his career as an independent principle investigator. This award provided financial support to a postdoctoral research fellow and will be used to support a graduate student. Dr. Liu’s group now consists of eight graduate students, four of which work in a gas-phase laser spectroscopy lab that is partially supported by this ACS PRF award. In addition to the three published papers, results summarized in the present and last year’s annual reports have been reported in several manuscripts that are either under review or in preparation. These results served as preliminary data in Dr. Liu’s NSF CAREER proposal in 2015, which has been granted funding. In June 2015, two graduate students of Dr. Liu won the Student Travel Awards of the University of Louisville to present their works in the 70th International Symposium on Molecular Spectroscopy at UIUC.