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7942 J. Chem. Phys., Vol. 115, No. 17, 1 November 2001 Gutsev et al.TABLE III. Experimental and theoretical adiabatic electron affinities eV for CrO n (n1–5).CrO CrO 2 CrO 2 ) CrO 3 CrO 4 CrO 5Expt. 1.220.01 a 2.430.02 b 1.500.06 3.660.02 4.980.09 4.40.1Theor. 1.17 2.22 1.37 3.38 4.61 4.41a 1.2210.006 eV from Ref. 15.b 2.4130.008 eV from Ref. 16.quired to pin down the excited states of the CrO 3 neutralspecies. The seeming simplicity of the CrO 3 PES spectramay in fact contain complicated and overlapping detachmenttransitions.D. CrO 4 and CrO 4ÀThe ground state of CrO 4 is 1 A (C 2 symmetry and itselectronic configuration is 15a 2 13b 2 . But there are two lowlyingtriplet states, as shown in Fig. 7. The triplet peroxostate is only 0.12 eV above the ground state CrO 4 . The anionground state has an electronic configuration of13a 1 2 2a 2 2 7b 2 2 7b 1 1 , corresponding to a 2 B 1 (C 2v ) state, whichis degenerate in total energy with a 2 B 2 state. Detachmentfrom the 7b 1 MO results in the neutral 1 A 1 ground state withE tot (A, 1 A 1 )E tot (A, 2 B 1 6.13 eV, whereas detachmentfrom 7b 2 leads to a 3 A 2 state with E tot (A, 3 A 2 )E tot (A, 2 B 1 )4.95 eV, which is slightly above the peroxo3 A 2 state, the lowest triplet state of the neutral Fig. 7. Thesetwo detachment channels are in excellent agreement with thetwo main PES features at 5.07 eV (X) and 6.04 eV (A).Here, we met the case where the first vertical detachment ofan extra electron from the singly occupied MO results information of a neutral at C 2v symmetry, which would relaxto C 2 symmetry. This is a really unusual situation: this firsttransition to the singlet 1 A state has an energy higher by 1.2eV than the transition to the triplet 3 A 2 state, which is higherin total energy but has a similar geometry as the anion. Thissuggests that the A band corresponding to removal of the 7b 1electron should have a long low-energy tail extending to theleft of the X band, not inconsistent with our PES spectrumFig. 5a. Using Eq. 2 with two reference energies, weestimated energies of several other vertical transitions. Theestimated singlet states are: 5.29 eV ( 1 A 2 ), 5.60 eV ( 1 B 2 ),6.53 eV ( 1 A 2 ), and 6.71 eV ( 1 B 1 ); and the triplet states are:5.03 eV ( 3 B 2 ), 6.06 eV ( 3 B 1 ), and 6.09 eV ( 3 A 2 ). All thesetransitions except the 6.53 and 6.71 eV singlet states may becontained in the PES spectrum of CrO 4 Fig. 5a.E. CrO 5 and CrO 5ÀWe found CrO 5 to possess only one thermodynamicallystable state of a superoxo type (C s ) with a triplet state 3 A(8a) 2 (22a) 2 (9a) 2 (23a) 1 (10a) 1 . The ground state ofthe CrO 5 anion is 2 A ...(23a) 2 (10a) 1 and has a similarC s geometrical shape as its neutral parent, as shown inFig. 6. According to Eq. 1, detachment of an electron fromthe 23a orbital results in the 3 A neutral ground state with aVDE of 4.69 eV, whereas detachment from the 10a orbitalreaches a dissociative 1 A neutral state with a VDE of 5.17eV. These two computed detachment channels are in goodagreement with the broad band X, which should be due totwo overlapping features. The broad nature of this band isconsistent with the geometry changes between the anion andneutral ground state and the dissociative nature of the 1 Astate.Using Eq. 2 with the two reference energiesE tot (A, 3 A)E tot (A, 2 A)4.69 eV and E tot (A, 1 A)E tot (A, 2 A)5.17 eV, we estimated VDE’s from othervalence MO’s: 23a ( 1 A) – 5.60 eV, 9a ( 3 A) – 5.45 eV,9a ( 1 A) – 5.66 eV, 22a ( 3 A) – 6.21 eV, 22a ( 1 A) – 6.37eV, 8a ( 3 A) – 6.21 eV, 8a ( 1 A) – 6.45 eV. The 5.45 eVdetachment energy from the 9a orbital is in excellent agreementwith the A band at 5.33 eV. The 22a and 8a orbitalsare degenerate and the 6.21 eV detachment energies fromthese two MO’s are in good agreement with the B band at6.08 eV. The large number of transitions closely spaced inenergy is consistent with the complicated PES spectrum observedfor CrO 5 .F. Adiabatic electron affinitiesThe experimental and theoretical adiabatic electron affinitiesof the chromium oxides are summarized in Table III.The theoretical EA’s computed according to Eq. 3 are: 1.17eV CrO, 29 2.22 eV (CrO 2 ), 30 3.38 eV (CrO 3 ), 4.61 eV(CrO 4 ), and 4.41 eV (CrO 5 ). These computational data arein good agreement with the experimental values. The EA’sincrease with the number of O atoms and saturate at CrO 4 .This trend is consistent with our previous observations ofother oxide species and indicates a sequential oxidation ofthe Cr atom. Both CrO 4 and CrO 5 possess rather large EA’sexceeding that of atomic Cl 3.62 eV 48 and formally belongto the class of superhalogen compounds. 49 The reason ofsuch high EA’s for the 3d TM oxides was first proposed onthe basis of calculations by an X-method. 50 Our recentcalculations 26,27 of FeO 4 and FeO 4 by the BPW91 methodconfirmed that the reason is in the structure of the topmostMO, which accepts the extra electron. This MO is almostentirely composed of oxygen AO’s and is bonding with respectto oxygens. Note that the classical superhalogen MnO 4has the A ad of 4.96 eV computed at the BPW91/6-311G*level of theory, in good agreement with the experimentalvalue of 4.8 eV obtained from the PES spectra of MnO 4 . 25The topmost MO of MnO 4 is a pure oxygen one as is requiredby the superhalogen theory. 49G. Fragmentation patternsTable IV reports the fragmentation energies of CrO n andCrO n computed according to Eq. 4. Experimental data areavailable for CrO and CrO 2 , 51,52 and they are in fair agree-Downloaded 29 Mar 2007 to 130.20.226.164. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
J. Chem. Phys., Vol. 115, No. 17, 1 November 2001 Electronic structure of chromium oxides7943TABLE IV. Fragmentation energies (D e in eV of CrO n and CrO n , computedaccording to Eq. 4.ment with our computational results. Since Cr has a 4s 1 3d 5electronic configuration, it has a formal valence of six. However,the Cr–O bond is stronger in CrO 2 than that in CrO 3 incontrast to the anticipation based on the notion of maximumsaturation of formal vacancies. CrO 4 is stable by about 55kcal/mol while CrO 3 is stable by only 10 kcal/mol towardevolution of molecular oxygen.The low-energy decay channels of CrO n anion correspondto detachment of an extra electron for n1–3. CrO 4is almost equally stable toward detachment of an extra electronor dissociation of atomic and molecular oxygen. WhileCrO 3 is rather stable towards detachment of an extra electronand dissociation of an atomic oxygen, it is much lessstable toward evolution of molecular oxygen, as would beexpected due to the superoxo O 2 bonding in CrO 5 .V. SUMMARYCrO nWe report a comprehensive theoretical and experimentalinvestigation of CrO n (n1 – 5) and their correspondingneutral species. Photodetachment photoelectron spectra wereobtained for the anions produced from a laser vaporizationcluster source at various photon energies. Calculated adiabaticand vertical binding energies of CrO n are in goodagreement with the experimental values. We show that theoreticalcalculations are very helpful in interpretations of thephotoelectron spectra of the anions, in understanding theelectronic and geometrical structures of their ground and excitedstates, and chemical bonding for both the anions andthe neutral species.ACKNOWLEDGMENTSCrO nChannel De Channel DeCrO→CrO 4.94 a CrO →CrOe 1.17→CrO 1 4.78CrO 2 →CrO 2 5.11 CrO 2 →CrO 2 e 2.22→CrOO 5.98 b →CrO O 6.11→CrO 2 6.22CrO 3 →CrOO 2 5.48 CrO 3 →CrO 3 e 3.35→CrO 2 O 5.31 →CrO 2 O 6.44→CrO O 2 7.66CrO 4 →CrO 2 O 2 2.42 CrO 4 →CrO 4 e 4.56→CrO 3 O 2.94 →CrO 3 O 4.15→CrO 2 O 2 4.76CrO 5 →CrO 3 O 2 0.49 CrO 5 →CrO 5 e 4.41→CrO 4 O 3.37 →CrO 3 O 2 1.55→CrO 4 O 3.22a Experimental D 0 value is 4.780.09, see Ref. 51.b Experimental D 0 value is 5.47 0.3 0.65 , see Ref. 52.This work was supported in part by a grant to VirginiaCommonwealth University by the Department of EnergyGrant No. DE-FG02-96ER45579. The experimental workwas supported by NSF L.S.W. under grant CHE-9817811and performed at the W. R. Wiley Environmental MolecularSciences Laboratory, a national scientific user facility sponsoredby Department of Energy’s Office of Biological andEnvironmental Research and located at Pacific NorthwestNational Laboratory, which is operated for the U.S. Departmentof Energy by Battelle.1 H. H. Kung, Transition Metal Oxides: Surface Chemistry and CatalysisElsevier, New York, 1989.2 P. C. Thune, R. Linke, W. J. H. van Gennip, A. M. de Jong, and J. W.Niemantsverdriet, J. Phys. Chem. B 105, 3073 2001.3 H. Brandle, D. Weller, S. S. Parkin et al., Phys. Rev. B 46, 13889 1992.4 M. A. Korotin, V. I. Anisimov, D. I. Khomskii, and G. A. Sawatzky, Phys.Rev. Lett. 80, 4305 1998.5 C. W. Walter, C. F. Hertzeler, P. Devynck, G. P. Smith, and J. R. Peterson,J. Chem. Phys. 95, 824 1991.6 E. B. Rudnyi, O. M. Vovk, and E. A. Kaibicheva, J. Chem. Thermodyn.21, 2471989.7 H. Wu, S. R. Desai, and L. S. Wang, J. 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Rosh, Topics in Organome-Downloaded 29 Mar 2007 to 130.20.226.164. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
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