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CHEMICAL PHYSICS Encoding of vinylidene isomerization · PDF fileCHEMICAL PHYSICS Encoding of...

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  • CHEMICAL PHYSICS

    Encoding of vinylidene isomerizationin its anion photoelectron spectrumJessalyn A. DeVine,1* Marissa L. Weichman,1* Benjamin Laws,2 Jing Chang,3

    Mark C. Babin,1 Garikoitz Balerdi,4 Changjian Xie,5 Christopher L. Malbon,6

    W. Carl Lineberger,7 David R. Yarkony,6 Robert W. Field,8 Stephen T. Gibson,2

    Jianyi Ma,3 Hua Guo,5 Daniel M. Neumark1,9

    Vinylidene-acetylene isomerization is the prototypical example of a 1,2-hydrogen shift,one of the most important classes of isomerization reactions in organic chemistry. Thisreaction was investigated with quantum state specificity by high-resolution photoelectronspectroscopy of the vinylidene anions H2CC and D2CC and quantum dynamics calculations.Peaks in the photoelectron spectra are considerably narrower than in previous work and revealsubtleties in the isomerization dynamics of neutral vinylidene, as well as vibronic couplingwith an excited state of vinylidene. Comparison with theory permits assignment of mostspectral features to eigenstates dominated by vinylidene character. However, excitation of then6 in-plane rocking mode in H2CC results in appreciable tunneling-facilitated mixing withhighly vibrationally excited states of acetylene, leading to broadening and/or spectral finestructure that is largely suppressed for analogous vibrational levels of D2CC.

    The 1,2-hydrogen shift is the simplest bond-breaking isomerization reaction in organicchemistry (1), and the prototypical exampleof this process is the isomerization of vi-nylidene (H2CC) to acetylene (HCCH). Vi-

    nylidene, the smallest unsaturated carbene (2),has been implicated as a transient intermediatein many chemical processes (36) but is of par-ticular interest as a high-energy form of acetylene(7). From the perspective of chemical physics, theH2CCHCCH isomerization (Fig. 1) is a benchmarkunimolecular reaction; the small number of atomsallows application of sophisticated theoreticalmethods to describe the isomerization dynamics(813), and the interplay between theory and ex-periment has provided a great deal of insightinto this reaction (14, 15). The lowbarrier (~0.1 eV)(Fig. 1) for vinylidene isomerization (8, 10) canlead to extensive tunneling interactions with acet-ylene states, and over the past several decades con-siderable effort has been invested in probing thisisomerization from both sides of the barrier. Onthe acetylene side, Field and co-workers (15, 16)have searched for spectroscopic signatures of vi-nylidene in highly vibrationally excited levels of

    HCCH, where the minimum-energy isomeriza-tion path lies along the local-bending vibrationalcoordinates. Alternatively, the vinylidene well canbe accessed directly by photodetachment of thevinylidene anion (H2CC ), and several researchgroups have used this approach to probe the spec-troscopy and dynamics of neutral H2CC (1721).Previous photodetachment-based experiments

    have led to differing views regarding the timescale on which vinylidene isomerizes to acetylene.In an anion photoelectron spectroscopy study,Ervin et al. (18) observed that transitions to the~X 1A1 state of H2CC were considerably broaderthan those arising from detachment to the higher-lying ~a3B2 state, for which the barrier to isomer-ization is considerably larger. The extra broadeningof ground-state band features was attributed toisomerization on a subpicosecond time scale. Incontrast, later Coulomb explosion imaging (CEI)experiments byVager and colleagues (19) indicatedthat neutral H2CC formed by anion photodetach-

    ment is stable on at least a microsecond timescale. It should be noted that lifetime is an ill-defined concept in such a system, because bothacetylene and vinylidene are bound species whoseeigenstates cannot form a true continuum. How-ever, individual eigenstates may have varying de-grees ofmixing between zeroth-order states of thetwo isomers, especially near and above the isom-erization barrier. This mixing has been exploredin quantum dynamical simulations of the anionphotoelectron spectrum starting with work byBowman and colleagues (10), who found the sim-ulated spectrum to be dominated by sharp peaksassociated with isolated vinylidene eigenstates.The aim of the current work was to experimen-

    tally characterize individual vibrational eigenstatesof vinylidene and to understand the vibrationalmode dependence of mixing with acetylene. Tothis end, we measured photodetachment spectraof H2CC and D2CC anions at higher resolutionthan previous work (18), using two complemen-tary experimentalmethods, high-resolution photo-electron imaging (HR-PEI) (22), and slow electronvelocity-map imaging of cryogenically cooled anions(cryo-SEVI) (23). The experiments are supple-mented by full-dimensional quantum dynamicscalculations on a highly accurate ab initiobasedpotential energy surface, carried out previouslyfor theH2CC-HCCH system (12, 24) and expandedhere by covering larger sections of configurationspace in both isomeric regions.The combination of experiment and theory

    shows that photodetachment directly accesseseigenstates that are mostly localized in the vi-nylidenewell. TheH2CC andD2CC isotopologuesboth undergo vibronic coupling to a high-lyingvinylidene electronic state, which results in theappearance of nominally Franck-Condon (FC) for-bidden transitions toneutral vibrational levels,withexcitation of nontotally symmetric (b2) modes.Most notable is the vibronic couplinginducedobservation of features involving odd quanta ofexcitation in the in-plane rocking (n6)mode,which,for the H2CC isotopologue, mixes strongly with thelocal-bending modes in the acetylene well. Isom-erization is largely encoded in the spectra of vibra-tional states that involve excitation of this mode.

    RESEARCH

    DeVine et al., Science 358, 336339 (2017) 20 October 2017 1 of 4

    1Department of Chemistry, University of California, Berkeley,CA 94720, USA. 2Research School of Physics and Engineering,Australian National University, Canberra, ACT 2601, Australia.3Institute of Atomic and Molecular Physics, Sichuan University,Chengdu, Sichuan 610067, China. 4Departmento de QumicaFsica, Facultad de Ciencias Qumicas, Universidad Complutensede Madrid (Unidad Asociada I+D+I CSIC), 28040 Madrid, Spain.5Department of Chemistry and Chemical Biology, University ofNew Mexico, Albuquerque, NM 87131, USA. 6Department ofChemistry, Johns Hopkins University, Baltimore, MD 21218, USA.7JILA and Department of Chemistry and Biochemistry, Universityof Colorado, Boulder, CO 80309, USA. 8Department ofChemistry, Massachusetts Institute of Technology, Cambridge,MA 02139, USA. 9Chemical Sciences Division, Lawrence BerkeleyNational Laboratory, Berkeley, CA 94720, USA.*These authors contributed equally to this work.Corresponding author. Email: [email protected] (J.M.);[email protected] (D.M.N.)

    Fig. 1. Energy diagram for theneutral vinylidene-acetyleneisomerization. Energies (in eV,relative to HCCH) and geometrieswere obtained from (21). Experi-mental energies for the anions ofboth isomers are shown in gray;the H2CC value was obtainedfrom the present work, whereasthe HCCH value was estimatedfrom electron-scatteringexperiments (28). The CHCHJacobi coordinate system used todescribe the isomerization isshown as an inset.

    0

    1

    2

    eV

    VINYLIDENEACETYLENE

    ~2.6HCCH-

    HCCH

    H2CC-

    H2CC1.925

    1.437

    2.054

    0.000

    r1

    r2

    1r0

    2 CM2

    CM1

    on October 19, 2017

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  • The experiments reported here used velocity-map imaging (VMI) detection schemes to mea-sure the electron kinetic energy (eKE) distributionand photoelectron angular distribution (PAD) thatresult from electron photodetachment of mass-selected anions. The VMI spectrometer used intheHR-PEImeasurements (fig. S1) was optimizedto provide 0.7 to 25 cm1 resolution over a widerange of eKE, so that measurements at a singlephoton energy (hn) could be used to obtain vib-rationally resolved spectra with reliable intensi-ties and PADs. The cryo-SEVI spectrometer (fig.S2) provided higher resolution (sub-meV) over anarrower range of eKEs, assisted by cooling theanions to ~10 K before detachment to reducespectral congestion arising from anion rotationaland vibrational excitation. Together, the HR-PEIand cryo-SEVI techniques yield a more completepicture of the photoelectron eKE spectrum andPADs than when used separately.The cryo-SEVI spectra of H2CC and D2CC

    (Fig. 2A) and the HR-PEI spectrum of H2CC (Fig. 2B) display photoelectron intensity versuselectron binding energy (eBE), where eBE hn eKE . All three spectra are dominated by thevibrational origin (A) and show transitions tovibrational levels up to ~4000 cm1 above thevinylidene vibrational ground state. PADs arereadily obtained fromphotoelectron images [sup-plementary materials (SM), section B], an exam-ple of which is shown in Fig. 2B. For each peak,the PADs yield the anisotropy parameter (b),which by definition falls between 1 and 2. Theselimits correspond to perpendicular and paralleldetachment, respectively (25). Figure 2C showsb for several peaks as a function of eKE, obtainedfrom HR-PEI measurements at several photonenergies. The PADs extracted from the cryo-SEVIspectra (fig. S3) are in agreement with the HR-PEI results; with the exception of features B, I,and K, all peaks in the cryo-SEVI spectra of bothisotopologues have b < 0 for eKEs below 1 eV,and peaks B, I, and K show distinctly positive bvalues at these kinetic energies.The enhanced resolution of cryo-SEVI is evi-

    dent in the considerably narrower linewidths inFig. 2A compared with previous photoelectronspectra (18), and a direct comparison is shown infig. S4. The linewidths of the vibrational originsand most of the other peaks are 10 cm1 and30 cm1 in the H2CC a

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