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1 Chemicl Physics Letters 496 (2010) Contents lists vilble t ScienceDirect Chemicl Physics Letters journl homepge: Mtrix isoltion nd computtionl studies of the CF 2 I rdicl Ptrick Z. El-Khoury, Lis George b, Aimble Klume b, Igor Schpiro, Mssimo Olivucci,c, Alexnder N. Trnovsky, Scott A. Reid b, * Deprtment of Chemistry nd Center for Photochemicl Sciences, Bowling Green Stte University, Bowling Green, OH 43403, USA b Deprtment of Chemistry, Mrquette University, P.O. Box 1881, Milwukee, WI 53201, USA c Universit di Sien, Diprtimento di Chimic vi Aldo Moro I Sien, Itly rticle info bstrct Article history: Received 6 July 2010 In finl form 9 July 2010 Avilble online 14 July 2010 We report experimentl nd computtionl studies of the spectroscopy nd photochemistry of the CF 2 I rdicl, which ws generted using pulsed dischrge or photolytic techniques nd trpped in Ar or Ne mtrices held t 5 K. Ground stte clcultions were performed using UM062X nd UB3LYP density functionls nd the UMP2, UCCSD, nd CASPT2 b initio methods, ll with high-qulity bsis sets. The photochemicl rection pth from the lowest-lying electroniclly excited stte ws mpped using the CASPT2//CASSCF methodology. A conicl intersection ws found to drive the excited stte species to the photoproduct/rectnt wells on the ground stte potentil energy surfce. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Hlocrbons (hlons) re clss of compounds with wide-rnging importnce in tmospheric nd environmentl chemistry, nd the photochemistry of these compounds in vrious medi hs received extensive scrutiny [1 9]. As primry products of the photochemicl nd therml decomposition of hlolknes, hlolkyl rdicls re importnt trgets for experimentl nd computtionl investigtion. However, s compred with their prent molecules, much less is known regrding the structure, properties, rectivity, nd photochemistry of these species. Among the hlomethnes, difluorodiiodomethne (CF 2 I 2 ) hs proven model system for exploring stte selective photochemistry. In the gs-phse, excittion of CF 2 I 2 into the first excited singlet S 1 stte locted t bout 350 nm leds to direct C I bond clevge, CF 2 I 2? CF 2 I + I. The bond-breking process from the first excited stte hs been shown to occur on timescle much fster thn rottionl period of prent molecule [10], nd the quntum yields for the I( 2 P 3/2 ) nd I( 2 P 1/2 ) toms re 0.98 nd 0.02, respectively [11]. Photochemicl bond breking yields highly vibrtionlly excited CF 2 I rdicls, with the mjority of vibrtionl excittion loclized in the C I stretching mode. At shorter excittion wvelengths, the two-body decy chnnel of CF 2 I 2 is followed by C I bond clevge in vibrtionlly hot rdicls [10,12], due to smll bond dissocition energy of 12 kcl mol 1 [10], in the rdicl. The popultion of higher lying electronic stte (k nm) leds to concerted three-body decy process: CF 2 I 2? CF 2 +I+I[11]. The formtion of * Corresponding uthor. Fx: E-mil ddress: scott.reid@mu.edu (S.A. Reid). moleculr iodine hs been observed only upon excittion of CF 2 I 2 t 193-nm (6.5 ev) [13 16]. In contrst to the gs-phse photochemistry, 350 nm excittion of CF 2 I 2 in solution (n-hexne) leds to n ultrfst formtion of moleculr iodine in 32% quntum yield [17]. The severl picosecond build up of I 2 ws observed in wide rnge of solvents (methnol, cetonitrile, liner lknes, nd chlorinted lknes) [17], lthough resonnce Rmn spectr of CF 2 I 2 re strongly solventdependent [18,19]. Computtionl studies of the photochemicl rection pthwy in S 1 CF 2 I 2 showed tht n S 1 /S 0 conicl intersection drives the photochemistry of this molecule from the excited stte potentil energy surfce to wekly bound ground-stte ICF 2 I vn der Wls complex, which ultimtely collpses to form iso-f 2 C I I, n isomer of CF 2 I 2 [20]. We recently chrcterized this isomer in both the frequency domin (using mtrix isoltion) nd ultrfst time domin (in solution, using ultrfst infrred (IR) nd UV/Visible spectroscopy), ll supported by b initio clcultions [21]. Despite the fct tht CF 2 I is primry photoproduct of CF 2 I 2 photolysis t 350 nm, there hve been few studies of this rdicl [10,11,17 26]. Prochsk nd Andrews observed the vibrtionl spectrum of the rdicl following open dischrge photoioniztion of CF 3 I/Ar smple nd reported two bsorptions t 1126 nd 627 cm 1 tht were ssigned to m 1 (symmetric C F stretch) nd m 2 (C I stretch) [22]. There is no informtion vilble on the electronic spectroscopy or photochemistry of the rdicl. In this work, the CF 2 I rdicl ws trpped in Ar nd Ne mtrices t 5 K, nd the infrred nd electronic bsorption spectr were mesured. Two different pproches were used to trp the rdicl: () 266 nm photolysis of mtrix isolted CF 2 I 2, nd (b) pulsed jet dischrge mtrix isoltion. The photochemistry of the rdicl ws investigted computtionlly using b initio multiconfigurtionl quntum chemicl clcultions /$ - see front mtter Ó 2010 Elsevier B.V. All rights reserved. doi: /j.cplett

2 P.Z. El-Khoury et l. / Chemicl Physics Letters 496 (2010) The photochemicl rection pth following promotion of CF 2 I into its lowest lying excited singlet electronic stte (S 1 ) is mpped using the complete ctive spce self-consistent field (CASSCF) method. The decy of the electroniclly excited CF 2 I * molecules from the Frnk Condon region leds through conicl intersection either to CF 2 + I, or bck to the rectnt. 2. Experimentl nd computtionl methods 2.1. Experimentl methods The mtrix isoltion pprtus used in this work hs been described in detil in recent studies [21,27,28], nd therefore only detils pertinent to this work will be given. The CF 2 I 2 :M (M = Ar, N 2, Ne) smples were generted by pssing the high purity mtrix gs over the liquid held in stinless steel bubbler tht ws plced in refrigerted, temperture controlled bth. The bth temperture ws typiclly held t 258 K. Before ech experiment the smple line ws pumped under vcuum to remove ny voltile impurities. Typicl deposition conditions were: 1 ms gs pulse durtion, 5 Hz repetition rte, 1 2 h deposition time. Two different types of experiments were conducted to produce the CF 2 I rdicl: () photolysis of mtrix isolted CF 2 I 2, nd (b) pulsed jet dischrge smpling [28]. For the photolysis experiments, the s-deposited mtrix ws irrdited using 2 mj pulses of 266 nm light generted from frequency qudrupled Nd:YAG lser (Continuum Minilite). The lser bem ws expnded using home-built 4:1 bem expnder to fill the mtrix window nd void dmge to the opticl (KBr) windows. Typicl irrdition times were min with the lser operting t repetition rte of 5 Hz. For the pulsed jet dischrge mtrix isoltion experiments, the dischrge ws initited by pplying negtive DC high voltge (typiclly 800 V) through 15 kx current-limiting bllst resistor to n luminum ring electrode, which ws seprted from the wll of the ssembly by 3 mm Delrin spcer. The body of the ssembly, connected to the power supply ground, served s the second electrode. The dischrge current ws monitored using the voltge drop cross smll (100 X) resistor plced in the dischrge circuit. The cryostt ws mounted on homebuilt ril system tht llowed quick interchnge between two spectrometers. Infrred spectr were obtined with n FTIR spectrometer (Mttson, Glxy series), which ws purged t flow rte of 20 L/min using purge gs genertor (Prker-Blston 75-52A). IR spectr were recorded t 2cm 1 resolution nd typiclly verged over 128 scns. UV/Visible spectr were recorded using n Agilent 8453 diode rry system. The IR nd UV reference spectr were recorded for the cold smple holder immeditely prior to mtrix deposition. All spectr were trnsferred to spredsheet nd nlysis progrm (ORIGIN 8.0) for subsequent workup. were used for CASPT2 geometry optimiztion. The corresponding vibrtionl frequencies were computed to ensure tht ll eigenvlues were positive t these criticl points. Verticl trnsition energies were clculted using TD DFT nd CASPT2 methods; the fully optimized minim were used for these clcultions. A logicl sequence of ANO-RCC contrctions ws employed in CASPT2 clcultions to test the convergence of the clculted verticl trnsition energies with respect to further increse in bsis description. In CASPT2 clcultions, the qudruple-f nd ANO-RCC single point verticl trnsition energy clcultions were performed on the fully optimized triple-f structure. To probe the photochemistry of the rdicl long minimum energy pths, we mpped the photochemicl rection t the CASS- CF(9,8)/Sdlej-pVTZ level of theory. The ctive spce moleculr orbitls re shown in Fig. 1S of the Supporting Informtion section. Unconstrined geometry optimiztion ws used to locte the CF 2 I minimum on the S 0 potentil energy surfce, nd the first singlet excited stte Frnk Condon (FC) geometry ws used s the strting point for the conicl intersection (CI) serch nd optimiztion between S 1 nd S 0. The ctive spce moleculr orbitls of the CASSCF conicl intersection re shown in Fig. 2S of the Supporting Informtion section. A minimum energy pth which connects the FC point to the CI ws computed in terms of the Intrinsic Rection Coordinte (IRC) using two-root stte-verge CASSCF grdients. Strting from the optimized CI point, two dditionl IRC pths were clcu Computtionl methods The reported minimum energy structures were fully optimized using two different density functionls nd b initio methods, including the UB3LYP [29], UM06-2X [30], UMP2 [31,32], nd UCCSD [33,34] methods in combintion with two triple-f qulity bsis sets: (i) smll-core reltivistic correltion consistent triple zet vlence bsis for C nd F toms (ug-cc-pvtz) in combintion with pseudo potentil to describe the core of iodine toms (ugcc-pvtz-pp) [35], nd (ii) the Sdlej-pVTZ [36] full tomic bsis set with polriztion on ll toms. Geometry optimiztion t the CAS- PT2 level of theory ws performed using numericl grdient procedure [37]. The reltivistic bsis sets of tomic nturl orbitls (ANO-RCC) [38] contrcted to the double-f or triple-f schemes Fig. 1. () Difference spectrum obtined following 266 nm irrdition of n sdeposited CF 2 I 2 :Ar (1:1000) smple held t 5 K. Assignments re noted. (b) Spectrum obtined following pulsed jet dischrge smpling of CF 2 I 2 :Ar (1:1000) mixture. The brs indicte theoreticl predictions (UB3LYP/Sdlej-pVTZ) of the positions nd reltive intensities of the CF 2 I rdicl infrred bsorptions.

3 70 P.Z. El-Khoury et l. / Chemicl Physics Letters 496 (2010) Tble 1 Fully optimized geometricl prmeters of CF 2 I clculted using different wvefunctions nd density functionls. Bond lengths re in Angstroms nd ngles in degrees. Method/bsis set C F C I F C F F C I F C F I UB3LYP/ ug-cc-pvtz-pp Sdlej-pVTZ UM062x/ ug-cc-pvtz-pp Sdlej-pVTZ UMP2/ ug-cc-pvtz-pp Sdlej-pVTZ UCCSD/ ug-cc-pvtz-pp Sdlej-pVTZ CASPT2(9,8)/ ANO-RCC-VDZP ANO-RCC-VTZP Clculted using numericl grdients. Fig. 2. Infrred spectrum of the CF 2 I rdicl trpped in Ne mtrix t 5 K. The brs indicte theoreticl predictions (UB3LYP/Sdlej-pVTZ) of the positions nd reltive intensities of the CF 2 I rdicl infrred bsorptions. lted to follow the evolution of the system fter S 1 S 0 decy t the CI long the S 0 single-root CASSCF potentil energy surfce. Single point CASPT2/Sdlej-pVTZ clcultions were performed on severl geometries long the two CASSCF IRC pths to ccount for the effect of dynmic electron correltion on the energy profiles [39]. All CASPT2 clcultions were performed using developer s version of Molcs [37], nd the Cholesky decomposition [40,41]. All other DFT nd b initio clcultions were performed using the methodologies vilble in GAUSSIAN 09 [42]. 3. Results nd discussion 3.1. Mtrix isoltion experiments Figs. 1 nd 2 disply mtrix infrred spectr of the CF 2 I rdicl in Ar (Fig. 1) nd Ne (Fig. 2) mtrices t 5K. Fig. 1 shows the difference spectrum obtined following 266 nm irrdition of n s-deposited CF 2 I 2 :Ar (1:1000) smple, while Fig. 1b displys spectrum obtined following pulsed jet dischrge spectroscopy of CF 2 I 2 :Ar (1:1000) smple together with theoreticl predictions (UB3LYP/Sdlej-pVTZ) of the positions nd reltive intensities of the prominent infrred bsorptions. Prochsk nd Andrews observed the rdicl following open dischrge photoioniztion of CF 3 I/Ar smple nd reported two bsorptions t 1126 nd 627 cm 1, corresponding to m 1 (symmetric C F stretch) nd m 2 (C I stretch) [22]. In ddition to these modes, we observe m 3 (CF 2 scissors) t 534 cm 1 nd m 5 (symmetric C F stretch) t 1172 cm 1. In the Ne mtrix, these re found t 1128 (m 1 ), 627 (m 2 ), 537 (m 3 ), nd 1180 cm 1 (m 5 ). The fully optimized minimum energy structures using the UM06-2X nd UB3LYP density functionls nd UMP2 nd UCCSD methods with the ug-cc-pvtz-pp nd Sdlej-pVTZ bsis sets re shown in Tble 1. The corresponding vibrtionl frequencies (Tble 2) re in excellent greement with the mtrix results. By inspecting the geometricl prmeters (Tble 1), it is immeditely noticed tht the optimized geometricl prmeters t ll levels of theory considered in this work re identicl within tight error br. The structure is pyrmidl, in contrst to the plnr CH 2 I Tble 2 Vibrtionl frequencies in cm -1 (intensities in km/mol) of CF 2 I clculted t different levels of theory, compred with experimentl results from mtrix isoltion. Method/bsis sets m 1 m 2 m 3 m 4 m 5 m 6 (sym. C F stretch) (C I stretch) (CF 2 scissor) (CF 2 I umb.) (sym. C F stretch) (CF 2 rock) UB3LYP/ ug-cc-pvtz-pp 1135 (467) 624 (35) 544 (17) 262 (2) 1171 (229) 260 (0) Sdlej-pVTZ 1135 (453) 623 (34) 539 (17) 268 (1) 1176 (227) 264 (0) UM062x/ ug-cc-pvtz-pp 1186 (463) 645 (42) 556 (16) 275 (1) 1242 (229) 272 (0) Sdlej-pVTZ 1186 (448) 640 (40) 548 (17) 278 (1) 1247 (229) 275 (0) UMP2/ ug-cc-pvtz-pp 1166 (425) 651 (42) 571 (5) 290 (0) 1208 (223) 274 (0) Sdlej-pVTZ 1141 (433) 637 (47) 562 (7) 284 (0) 1186 (229) 271 (0) CASPT2(9,8)/ ANO-RCC-VDZP 1145 (364) 626 (43) 550 (14) 280 (0) 1277 (353) 258 (0) ANO-RCC-VTZP 1158 (347) 642 (40) 560 (15) 290 (0) 1293 (322) 266 (0) Experimentl/ Ref. [22] This work (Ar) This work (Ne) Clculted using numericl grdients.

4 P.Z. El-Khoury et l. / Chemicl Physics Letters 496 (2010) Tble 3 Verticl trnsitions in nm (oscilltor strength) clculted using the TD DFT nd CASPT2 methods, compred with experimentl results from mtrix isoltion. The corresponding fully optimized structures were used unless otherwise indicted. Method/bsis set Tv S 0 -S 1 (f) Tv S 0 -S 2 (f) Tv S 0 -S 3 (f) TD UB3LYP/ ug-cc-pvtz-pp (0.0039) (0.0003) (0.0418) Sdlej-pVTZ (0.0056) (0.0001) (0.0385) TD UM062x/ ug-cc-pvtz-pp (0.0148) (0.0002) (0.0255) Sdlej-pVTZ (0.0140) (0.0001) (0.0248) CASPT2(9,8)/ ANO-RCC-VDZP (0.0096) (0.0390) ANO-RCC-VTZP (0.0191) (0.0437) ANO-RCC-VQZP (0.0198) (0.0441) ANO-RCC (0.0200) (0.0447) Experiment (0.08) b (0.16) b (this work) (0.12) c Fig. 3. UV/Visible spectrum obtined following 266 nm irrdition of n sdeposited CF 2 I 2 :Ar (1:1000) smple held t 5 K. The brs indicte theoreticl predictions (TD UM062x/Sdlej-pVTZ) of the positions nd reltive intensities of the CF 2 I rdicl electronic bsorptions. rdicl, which hs recently been observed in the gs-phse using microwve spectroscopy [43]. Fig. 3 shows the mesured UV/Visible spectrum following 266 nm photolysis of CF 2 I 2 :Ar mixture, which displys bnds ssigned to I 2,CF 2 I, nd CF 2. Tble 3 lists the clculted verticl trnsition energies nd oscilltor strengths of the prominent trnsitions. Under the ssumption tht the IR nd UV/Visible spectr smple the sme region of the mtrix, the integrted IR nd UV/Vis intensities were combined with clculted IR intensities to estimte the oscilltor strength of the UV/Vis trnsitions. Thus, the integrted IR bsorbnce of given feture ws divided by the clculted intensity (in km/mol) to derive column density in the mtrix, nd n verge vlue ws obtined over the observed IR bsorptions. b c The oscilltor strength of given electronic (UV/Visible) bnd ws then obtined ccording to the following formul [44]: f ¼ Clculted using the optimized ANO-RCC-VTZP structures. Argon mtrix. Neon mtrix. R AUV ð~tþd~t N IR ð1: mol=kmþ ð1þ where N IR is the column density derived from the IR mesurements. The oscilltor strengths of the CF 2 I bnds derived using this pproch re in good greement with the clculted vlues (Tble 3). It is instructive to compre the mesured electronic spectrum of CF 2 I in n Ar mtrix with the previously mesured gs-phse spectrum of the relted iodomethyl rdicl, CH 2 I [45]. As shown in Fig. 3S of the Supplementry informtion, there is good correspondence between these spectr. Reltive Energy (kcl/mol) CF 2 I * CF 2 I λ = nm CASPT2 // CASSCF S 1 CASPT2 // CASSCF S 0 CASSCF S 1 CASSCF S 0 CF 2 +I Intrinsic Rection Coordinte Fig. 4. Intrinsic Rection Coordinte (IRC) scns clculted t the CASSCF(9,8)/Sdlej-pVTZ level of theory. Single point CASPT2 clcultions were performed to include the effects of dynmic electron correltion. See text for more detils.

5 72 P.Z. El-Khoury et l. / Chemicl Physics Letters 496 (2010) b Ψ GS r Ψ ES Reltive Energy (kcl/mol) Angle (degrees) 1.0 Orbitl Occupncy Ψ GS Ψ ES MO6 MO Angle (degrees) Fig. 5. () A vector representtion of the grdient difference (solid) nd derivtive coupling (dshed) vectors. (b) Left Pnel: schemtic digrm of the photochemicl funnel. Right Pnel: reltive energies nd chnge in electronic wvefunction (the occupncies of MOs 3 nd 6) s function of ngulr displcement from the CI t trust rdius of 0.1 Å for severl geometries trcing the lower cone of the CI The photochemicl rection pth We strted out by mpping the excited stte portion of the photochemicl rection pth long the Intrinsic Rection Coordinte t the CASSCF(9,8)/Sdlej-pVTZ level of theory. An S 1 /S 0 conicl intersection is found to drive the electroniclly excited species either to the photoproduct (CF 2 + I) or bck to the rectnt (CF 2 I) on the ground stte potentil energy surfce, Fig. 4. The minimum energy conicl intersection ws optimized, nd the grdient difference nd derivtive coupling vectors were computed (Fig. 5). The degenercy-lifting vectors were used to determine the shpe of the lower CI cone. A circulr cross-section centered t the minimum energy CI nd spnning the brnching plne formed by the grdient difference nd derivtive coupling vectors ws generted using the Moplot softwre [46]. Single point CASSCF clcultions were performed on severl structures trcing the ground stte circulr cross-section to determine the ground stte energy profile nd trce the chnges in electronic wvefunction long this cross-section, Fig. 5b. The resulting energy profile exhibits the formtion of two chnnels denoted by W ES nd W GS t trust rdius of 0.1 Å, nd the computed ground-stte CASSCF IRC scns were directed long these two cross-section minim, Fig. 5b. The clculted IRC scns demonstrte tht the W ES chnnel leds to the photoproduct (CF 2 + I), while the W GS chnnel leds bck to the rectnt (CF 2 I), Fig. 4. Moreover, nd to trce the chnges in the electronic wvefunction long the ground stte circulr cross-section, the occupncies of moleculr orbitls (MOs) 3 nd 6 were extrcted from the digonl elements of the first order density mtrices, shown in the lower right pnel of Fig. 5b. At n ngle of bout 360 (corresponding to the W GS chnnel), the electronic configurtion of MOs 3 (singly occupied) nd 6 (empty) reflects the ground-stte electronic configurtion, providing n dditionl proof tht this chnnel leds to the rectnt structure (CF 2 I). On the other hnd, the promotion of n electron from MO 3 to MO 6 t n ngulr displcement of bout 180 (corresponding to W ES ) reflects the excited stte electronic configurtion, suggesting tht this rection chnnel points dibticlly towrds the CF 2 + I photoproducts. The smll moleculr system investigted in this work is idel for the ppliction of ccurte stte-of-the-rt but computtionlly demnding multi-reference b initio clcultions to probe photochemicl rections. Specificlly: (i) it consists of only four toms (six vibrtionl degrees of freedom in totl), (ii) it hs lbile

6 P.Z. El-Khoury et l. / Chemicl Physics Letters 496 (2010) crbon iodine bond which suggests model bond-breking/bondmking photochemistry, nd (iii) in this work, the excited stte species re found to be funneled to the product (or rectnt) wells ground stte potentil energy surfce through conicl intersection. The locus trced by the conicl intersection points in this system forms four-dimensionl intersection spce, rther thn highdimensionl sems often encountered in reltively lrge molecules. Thus, this model system is idel to explore the intersection spce using ccurte b initio clcultions. This will be reported in future publiction from our groups. 4. Conclusions The spectroscopy of the CF 2 I rdicl, primry photoproduct of CF 2 I 2 decomposition following excittion into the first bsorption bnd t 350 nm, hs been studied from the infrred to UV/Visible region. The rdicl ws generted using photolytic nd pulsed dischrge techniques nd trpped in Ar or Ne mtrices t 5 K. The observed infrred nd electronic spectr of the rdicl re in excellent greement with DFT nd high-level b initio clcultions. The photochemicl rection pth following the promotion of the CF 2 I rdicl into its lowest lying excited stte excittion ws mpped using the CASPT2//CASSCF methodology. A conicl intersection is found to funnel the excited stte species either to the CF 2 +I photoproducts or bck to the rectnt. Acknowledgments Support of this reserch by the Ntionl Science Foundtion (Grnt CHE ), the Donors of the Petroleum Reserch Fund of the Americn Chemicl Society (Grnt ND6), nd the Wehr Foundtion Trust (Mrquette University) is grtefully cknowledged. This work ws lso supported by n NSF CAREER wrd (Grnt CHE , ANT). An lloction of computer time from the Ohio Supercomputer Center is grtefully cknowledged. Appendix A. Supplementry dt Supplementry dt ssocited with this rticle cn be found, in the online version, t doi: /j.cplett References [1] L. Ji, Y. Tng, R.S. Zhu, Z.R. Wei, B. 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