Dynamics following electron photodetachment in a complex of a chloride anion with ammonia is explored by a combination of electronic structure and quantum dynamical methods. This system serves as a prototype for investigating a hithertho unexplored class of chemical reactions - non-adiabatic proton transfer triggered by a detachment of an electron. All the reactive and non-reactive channels of this process are characterized and the respective quantum yields are presented

Jungwirth, P.

Nachtigallova, D.

Ronen, S.

Schmidt, B.

English

Repository: Freie Universität Berlin (FU), Math Department (fu_mi_publications)

P H Y S I C A L R E V I E W L E T T E R S week ending23 JULY 2004VOLUME 93, NUMBER 4Nonadiabatic Chemical Reaction Triggered by Electron Photodetachment:An Ab Initio Quantum Dynamical StudyShai Ronen,1,2,* Dana Nachtigallova´,1 Burkhard Schmidt,3,† and Pavel Jungwirth1,‡1Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republicand Center for Complex Molecular Systems and Biomolecules, Flemingovo namesti 2, 16610 Prague 6, Czech Republic2School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel3Freie Universita¨t Berlin, Institute of Mathematics II, Arnimallee 2-6, D-14195 Berlin, Germany(Received 6 February 2004; published 20 July 2004)048301-1Dynamics following electron photodetachment in a complex of a chloride anion with ammonia isexplored by a combination of electronic structure and quantum dynamical methods. This system servesas a prototype for investigating a hitherto unexplored class of chemical reactions-nonadiabatic protontransfer triggered by a detachment of an electron. All the reactive and nonreactive channels of thisprocess are characterized and the respective quantum yields are presented.DOI: 10.1103/PhysRevLett.93.048301 PACS numbers: 82.30.Fi, 31.15.Ar, 31.15.Qg, 31.50.GhChemical reactions triggered by electronic excitationof light absorbing molecules play an important role innature, prominent examples being the primary processesof photosynthesis or vision. The term photochemistry isgenerally used to encompass natural or technologicalreactions initiated by an optical excitation of a chromo-phore from the ground state to a discrete excited elec-tronic state. However, using radiation with a sufficientlyshort wavelength (typically in the UV region) valenceelectrons in molecular systems can also be excited to thedissociative continuum. While in most cases electronphotodetachment is a nonreactive process, for properlyselected systems chemistry can take place via the so-called transition state spectroscopy [1,2].It has been shown by photoelectron spectroscopy mea-surements as well as by ab initio quantum chemistry andquantum molecular dynamics calculations that photodis-sociation, which is the simplest photochemical reaction,follows photodetachment of the most weakly bound elec-tron in anionic complexes, such as XHY, where X andY represent halogen atoms [3–7]. Moreover, calculationsindicate that also more complex processes, such as ex-change or proton transfer reactions, can follow the prepa-ration of a ground state neutral complex from an anionicprecursor by electron photodetachment [8,9]. All thesechemical processes occur on a single electronic surface(i.e., the neutral ground state) and are, therefore, elec-tronically adiabatic.What happens if the molecular system is prepared in anexcited rather than ground state by electron photodetach-ment? Such an excited electronic state has been preparedand its vertical detachment energy characterized, e.g., byphotodetachment from a Cl   NH3 complex [10–12].While the neutral ground state lies 4.0 eVabove the anion,a threshold photon energy of 6.45 eV is necessary to createan excited charge-transfer state Cl   NH3 (experi-mental values for vertical transitions from Ref. [10]).What is the chemical dynamics following the preparation0031-9007=04=93(4)=048301(4)$22.50 of this charge-transfer state by vertical electron photo-detachment from the anionic precursor and which elec-tronic states get involved?In order to provide an answer to the above question, wefirst construct the ground and excited charge-transferpotential energy surfaces of the neutral Cl   NH3 clusterusing state of the art ab initio quantum chemistry meth-ods. We also evaluate surfaces of nonadiabatic couplingsbetween these two electronic states. To model the dynam-ics following electron photodetachment, we verticallypromote the vibrational wave packet corresponding tothe anionic precursor to either neutral surfaces. Subse-quently, the vibrational wave function is propagated bysolving numerically the time-dependent Schro¨dingerequation on the two coupled electronic surfaces. Theprincipal result is that for the initial wave packet preparedon the excited charge-transfer potential the main channelis nonadiabatic and reactive, leading to the formation ofan HCl molecule and a NH2 radical, both in their groundelectronic state. To the best of our knowledge, this is thefirst described case of a nonadiabatic chemical reactiontriggered by electron photodetachment, and the presentcomplex thus serves as a benchmark system for investi-gating this new class of processes.The excited Cl   NH3 complex, which has aCl   H-NH2 hydrogen bonded structure (with dashand dots representing covalent bond and noncovalentinteractions, respectively), possesses nine vibrational de-grees of freedom. However, the investigated nonadia-batic reaction involving a proton transfer from theNH3 to the Cl unit is essentially a one-dimensionalprocess: a collinear H motion between the Cl and Natoms. To a lesser extent, the relative motion of the twoheavy atoms gets involved too. We assume that the re-maining modes are practically decoupled from the re-active mode and act merely as spectators. Therefore, weconstruct the potentials and nonadiabatic couplings, aswell as solve the quantum dynamical problem in a2004 The American Physical Society 048301-1P H Y S I C A L R E V I E W L E T T E R S week ending23 JULY 2004VOLUME 93, NUMBER 4reduced two-dimensional representation, involving theCl-H and H-N collinear coordinates. For the purpose ofsimplifying the ab initio calculations, we further restrictour geometry to be planar with a C2v symmetry. This isjustified by the fact that the charge transfer equilibriumgeometry is planar, and the reaction product Cl-H NH2is also well described by this symmetry. Moreover, theinitial (anionic) geometry is not very far from planarity[10,12]. On the other hand, the ground state equilibrium isfar from being planar. However, the exact dynamics inthis region are not the focus of our study and should beonly of minor effect.The quantum-chemical electronic structure calcula-tions were carried out using a multireference completeactive space self-consistent field (CASSCF) approachwith an augmented correlation consistent polarizationvalence double zeta (aug-cc-pvdz) basis set [13] usingthe MOLPRO program package [14]. For the constructionof the multireference wave function, all valence electronswere included in the active space. This wave function wasalso used for the evaluation of the nonadiabatic couplingsbetween the ground and excited charge-transfer poten-tials of the neutral complex by the method of finitedifferences. Important parts of the potential surfaceswere also recalculated using the multireference configu-ration interaction (MRCI) approach with single anddouble excitations, which accounts for both nondynamicand dynamic electron correlation effects. The result is thatthe inclusion of dynamic correlation practically does notchange the shape of the surfaces but merely causes con-stant shifts in energy. Therefore, the simpler CASSCFapproach is sufficient for the purpose of the present dy-namical study. It also allows for the evaluation of the first-order nonadiabatic couplings. The active space includedthe full valence space plus one additional orbital in the B2representation of the C2v point group.12312302468RH−N / 10−10 mRCl− H / 10− 10 mE(R) / eVCl−H+NH2 Cl+H− NH2  Cl − +H− NH2+ FIG. 1 (color online). Adiabatic potential energy surfaces forthe ground (neutral) and the lowest charge transfer state of theCl   NH3 complex. The contours represent the energy gap inthe avoided crossing region. The vertical bar indicates thecenter of the Franck-Condon region for vertical transitionfrom the anionic precursor.048301-2Figure 1 shows two-dimensional potential energy sur-faces for the two lowest doublet B2 states, i. e., the groundstate and the lowest charge transfer state. Note that twoother states, which are nearly degenerate with the elec-tronic ground state, have been omitted. While the uppersurface exhibits only one deep minimum correspondingto the charge transfer Cl   H-NH2 arrangement, thesituation on the lower surface is very different. Namely,two valleys exist there, reflecting the Cl   H-NH2 andCl-H   NH2 channels. Note that the two surfaces comeclose to each other exhibiting an avoided crossing along aline where the two coordinates are approximately equal(RCl-H  RH-N  60 pm). This is also the region wherethe first-order nonadiabatic coupling becomes significant.As is demonstrated in Fig. 2, the two components of thederivative coupling along the Cl-H and H-N coordinateshave, except for a sign change, a very similar shape withgrowing magnitude upon simultaneously increasing bothcoordinates.The dynamics following electron photodetachmentfrom the Cl   NH3 cluster is modeled by a verticalpromotion of the vibrational wave packet of the anioniccomplex to one of the two potential surfaces of the neutralsystem, depending on the photon energy. The vertical barin Fig. 1 denotes the center of this Franck-Condon region.The quantum dynamical simulation starts with the prepa-ration of the initial wave function, which we took as theharmonic ground state vibrational function of the anionicCl   NH3, projected on the Cl-H and H-N coordinates.This wave function is then placed on either of the twoneutral surfaces. The subsequent time evolution of the twocoupled states is governed by the following Hamiltonian:H  E h22rTM1r1 2CTM1r T; (1)0.511.5211.52−10−50510RH− N / 10− 10 mRCl H / 10− 10 mC(R) / 1010  m − 1−FIG. 2 (color online). First-order nonadiabatic couplingCCl-HR (positive surface) and CH-NR (negative surfaceand contours) between ground and lowest charge transfer stateof the Cl   NH3 complex. Note that the orientation differsfrom that of Fig. 1.048301-2P H Y S I C A L R E V I E W L E T T E R S week ending23 JULY 2004VOLUME 93, NUMBER 4where M1 is the metric tensor for the collinear Cl-H andH-N coordinates. The first two terms on the right-handside represent adiabatic quantum dynamics along therespective potential energy surfaces EiR. The remainingtwo terms result from the action of the nuclear kineticoperator on the adiabatic electronic statesCkij R iR@@RkjR; k  Cl-H;H-N;TijR  hiRjrTRM1rRjjRi;(2)where jiRi stands for the adiabatic electronic statevector which parametrically depends on the nuclear co-ordinates R. In most studies of nonadiabatic effects onmolecular quantum dynamics the second order term isneglected, thus rendering the truncated Hamiltonian tobe non-Hermitian. In the simulations described below,this leads to an unphysical decrease of population bymore than 30% during the time scale under consideration(14 fs). This problem can be circumvented by virtue of thefollowing identity:Tij  rTM1Cij  hrTijM1jrji; (3)which allows one to decompose the second order non-adiabaticity into an anti-Hermitian and a Hermitian con-tribution [15]. Including the former and neglecting onlythe latter term yields a first-order approximation to thefull Hamiltonian operator of Eq. (1), which is Hermitianagain and, hence, leads to unitary time evolution. Thecoupled time-dependent Schro¨dinger equations aresolved numerically using a finite differencing schemefor the temporal problem. Wave functions are representedon an equidistant grid consisting of 256 256 pointsallowing for the use of fast Fourier transforms [16]. The1.522.5RCl−H / 10−10 mt = 0 t1 1.5 21.522.51RH− NFIG. 3 (color online). Wave packet dynamics upon vertical excitastate of the neutral complex (upper panel). Upon reaching the avoidground state occurs (lower panel). Solid contours: density. Dotted048301-3outgoing parts of the wave functions are smoothly ab-sorbed near the edges of the grid.Quantum dynamical simulations were carried out fortwo different photodetachment scenarios. First, the vibra-tional ground state wave function of the anionic complexis vertically promoted to the lower surface of the neutralsystem. The dominant part of the wave packet remainsconfined to the region of the bound Cl   H-NH2 com-plex without overcoming the barrier to the reactiveCl-H   NH2 channel. Also, practically no nonadiabatictransfer of population to the upper surface occurs. In asecond set of simulations, the initial vibrational wavefunction is vertically placed on the excited potentialsurface of the neutral complex; see Fig. 3. The wavepacket oscillates essentially along a line (RCl-H  RH-N const) thus passing the avoided crossing region. As can beseen in Fig. 4, only 25% of the population remain in theupper state after 14 fs. The emerging lower state popula-tion can be divided into the nonreactive and reactivechannels. Only a small portion (21%) is found in thenonreactive Cl   H-NH2 region, while the remainingpopulation (54%) is found in the reactive domain. Thispopulation partly remains bound in the Cl-H   NH2potential well (11%), while the remainder dissociatesinto the Cl-H NH2 channel with a total quantum yieldof 43%. The outgoing portion of the wave packet isabsorbed near the edges of the grid, which causes thedecrease of total population down to 57% during the first14 fs; see Fig. 4. On a longer time scale the populationdynamics remains qualitatively unchanged. After 50 fssuccessive nonadiabatic transitions deplete the upper statepopulation down to 10%, while the population of thebound regions of the lower state decays down to 15%.In conclusion, we have demonstrated two differentscenarios for photodetachment of the Cl   H-NH2 = 3 fs t = 6 fs1.5 2 / 10− 10 m1 1.5 2tion of the Cl   NH3 complex to the lowest charge transfered crossing region (dashed line), nonadiabatic transition to thecontours: adiabatic potential energy surfaces (spacing 0.5 eV).048301-32 6 10 1400.20.40.60.81time / fspopulationstotalupper st.lower st.Cl+HNH2Cl−H+NH2FIG. 4 (color online). Population dynamics upon verticalexcitation of the Cl   NH3 complex to the excited state ofthe neutral complex. The lower state population emerging uponnonadiabatic population transfer is divided into nonreactive(Cl   H-NH2) and reactive (Cl-H   NH2) regions. Note thatthe effect of absorbing boundary conditions acting on thereactive Cl-H NH2 channel leads to a 43% loss of totalpopulation during the time interval considered (14 fs).P H Y S I C A L R E V I E W L E T T E R S week ending23 JULY 2004VOLUME 93, NUMBER 4complex. Upon vertical excitation to the lower surface ofthe ground state, the dynamics can be viewed asnonreactive and electronically adiabatic. In contrast, theprocess initiated on the upper charge-transfer surface isdominated by an electronically nonadiabatic transitionleading to the reactive Cl-H NH2 channel. Thus, thepresent system serves as a prototype of a new class ofchemical reactions: nonadiabatic proton transfer trig-gered by electron photodetachment. Our prediction willhopefully motivate future ultrafast spectroscopic experi-ments. Currently, the only data available for the presentsystem come from time-independent photoelectron spec-troscopy [10,11]. Detachment of an electron from ammo-nia in the Cl   NH3 complex results in a detectablevibrational progression assigned to umbrella motion ofNH3 which is not included in our model. However,from the width of the experimental vibrational features,we can conclude that at least some of the populationsurvives in the charge transfer state for tens of femto-seconds, which is consistent with our theoretical results.It is also possible that the inclusion of the umbrella modeinto the model would slow down the decay of the chargetransfer state population, since the reactive hydrogenwould explore regions of the potential with a weakernonadiabatic coupling.048301-4We are grateful to Ori Cheshnovsky for valuable dis-cussions. Support of the Czech Ministry of Educationto the Center for Complex Molecular Systems andBiomolecules (Grant No. LN00A032) is acknowledged.S. R. is grateful for support by the James Franck German-Israeli Program, the U.S.-Israel Binational Foundation,and the Israel Science Foundation.*Electronic address: sronen@post.tau.ac.il†Electronic address: burkhard@math.fu-berlin.de‡Electronic address: pavel.jungwirth@uochb.cas.cz[1] D. N. Neumark, Science 272, 1446 (1996).[2] D. N. Neumark, Acc. Chem. Res. 26, 33 (1993).[3] R. B. Metz, T. Kitsopoulos, A. Weaver, and D. M.Neumark, J. Chem. Phys. 88, 1463 (1988).[4] A. Weaver, R. B. Metz, S. E. Bradforth, and D. M.Neumark, J. Phys. Chem. 92, 5558 (1988).[5] K. Yamashita and K. Morokuma, J. Chem. Phys. 93, 3716(1990).[6] R. B. Metz and D. M. Neumark, J. Chem. Phys. 97, 962(1992).[7] A. B. McCoy, R. B. Gerber, and M. A. Ratner, J. Chem.Phys. 101, 1975 (1994).[8] D.Y. Wang, J. Z. H. Zhang, and C. H. Yu, Chem. Phys. Lett.273, 171 (1997).[9] R. L. Schwartz, G. E. Davico, J. B. Kim, and W. C.Lineberger, J. Chem. Phys. 112, 4966 (2000).[10] G. Markovich, O. Cheshnovsky, and U. Kaldor, J. Chem.Phys. 99, 6201 (1993).[11] G. Markovich and O. Cheshnovsky, J. Phys. Chem. 98,3550 (1994).[12] U. Kaldor, Z. Phys. D 31, 279 (1994).[13] T. H. Dunning, Jr., J. Chem. Phys. 90, 1007 (1989); R. A.Kendall, T. H. Dunning, Jr., and R. J. Harrison, J. Chem.Phys. 96, 6796 (1992); D. E. Woon and T. H. Dunning, Jr.,J. Chem. Phys. 98, 1358 (1993).[14] MOLPRO is a package of ab initio programs written byH.-J. Werner, P. J. Knowles, R. Lindh, M. Schu¨tz,P. Celani, T. Korona, F. R. Manby, G. Rauhut, R. D.Amos, A. Bernhardsson, A. Berning, D. L. Cooper,M. J. O. Deegan, A. J. Dobbyn, F. Eckert, C. Hampel,G. Hetzer, A.W. Lloyd, S. J. McNicholas, W. Meyer,M. E. Mura, A. Nicklass, P. Palmieri, R. Pitzer,U. Schumann, H. Stoll, A. J. Stone, R. Tarroni, andT. Thorsteinsson.[15] A. Hofmann and R. de Vivie-Riedle, Chem. Phys. Lett.346, 299 (2001).[16] R. Kosloff, Annu. Rev. Phys. Chem. 45, 145 (1994).048301-4

Non-Adiabatic Chemical Reaction Triggered by Electron Photodetachment: An ab initio Quantum Dynamical Study

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Non-Adiabatic Chemical Reaction Triggered by Electron Photodetachment: An ab initio Quantum Dynamical Study

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