The Journal of
Physical Chemistry VOLUME 99, NUMBER 44, NOVEMBER 2,1995
0 Copyright 1995 by the American Chemical Society
LETTERS Generation of the Thermodynamically Stable Dications A1F2+ and SiF2+via Charge-Stripping Mass Spectrometry? Christoph Heinemann, Detlef Schroder, and Helmut Schwarz" Institut f i r Organische Chemie der Technischen Universitat Berlin, Strasse des 17. Juni 135, 0-10623 Berlin Received: July 21, 1995; In Final Form: September 18, 1 9 9 P
The theoretically predicted (Kolbuszewski; M.; Wright, J. S . J. Phys. Chem. 1995, 99, 3455) existence of the thermodynamically stable dications A1F2+ and SiF2+is demonstrated by charge-stripping mass spectrometry. M P e, M = Al, Si) of these species have been determined The vertical second ionization energies (MI? as 19.7 f 1.5 eV (AlF') and 21.7 f 0.6 eV (SiF+) and agree favorably with theoretically calculated values using both single-reference and multireference approaches. Evidence for the formation of the first excited triplet state of SiF+ (311) upon electron ionization of silicon tetrafluoride is presented.
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Introduction Diatomic dications have stimulated an increasing number of both experimental and theoretical investigations and comprehensive reviews covering both diatomic and polyatomic dications are available in the literature.' As early as in 1930, Pauling provided the basis for the conceptual understanding of chemical bonding between two atoms in the presence of two positive charges,2 and his ideas have propelled numerous studies of the celebrated doubly charged helium dimer.3 More recent studies have focused on the binding mechanisms and thermodynamics of these species: mean lifetime measurements of metastable states: and chemical reactivities.'d,6 Thermodynamically stable diatomic dications are characterized by a ground-state potential energy curve which has its minimum at an energy lower than the lowest atomic dissociation limit. As a first example of this class of molecules, the mixed rare-gas diatomic molecule NeXe2+ was observed in drift-tube experiments.' The thermodynamically by far most stable dications are simple metal compounds,8with triatomic ScOH2+ as an extreme example which is 4.9 eV more stable as compared
' Dedicated to Prof. Dr. Heinrich Kriegsmann on the occasion of his 70th birthday. Abstract published in Advance ACS Abstracts, October 15, 1995. @
0022-365419512099-16195$09.0010
+
to the isolated monocations Sc+ and OH+ and a bond dissociation energy (BDE) of 4.7 eV for the Sc2+-OH bond.9 In this letter, we report the existence of long-lived A1F2+and SiF2+ dications, which have recently been predicted as thermodynamically stable dications using theoretical arguments.lo For this purpose, AlF+ and SiF+ were subjected to chargestripping experiments,'," in which dications are produced by a kiloelectronvolt collision of the corresponding monocations with a neutral target, typically 0 2 .
Experimental and Computational Procedures The experiments were performed with a modified VG ZABI HFIAMD 604 four-sector mass spectrometer of BEBE configuration (B stands for magnetic and E for electric sectors), which has been described elsewhere.I2 AlF+ cations were generated by fast atom bombardment (FAB) of aluminum(II1) fluoride. SiF+ cations were generated by either electron ionization (EI, 70 eV) or chemical ionization (CI) of silicon tetrafluoride; note, that E1 of SiF4 also gives rise to formation of SiF2+ dications directly in the ion source. The ions of interest, having 8 keV 0 1995 American Chemical Society
16196 J. Phys. Chem., Vol. 99, No. 44,1995 kinetic energy, were mass-selected using B( ])E( 1) at a resolution of m/Am = 4000. Charge stripping was performed by colliding the ions with oxygen (50-80% transmission, T). The energy necessary to remove an electron from a fast moving monocation is taken from its kinetic energy and leads to a shift of the dication signal in the kinetic energy scale from the expected E/2 value to a slightly lower one; this energy difference is referred to as em,,value.'" These Qmln values were determined by using two different scan modes: (i) an energy scan was done with E(l) for B(l) mass-selected species and (ii) with E(2) using B(1)/ E( 1)/B(2) for mass selection; the reported values are averages of at least five independent measurements, and within the experimental errors both methods gave identical results. As references for the calibration of the energy scale,'" we used the Mg+ cation (Qmin= 15.1 eV)I3 in the FAB measurements and the toluene cation radical (Qmln = 15.7 eV)' for E1 and CI experiments; the multiplicative correction method was used. I The energy resolutions (EIAE) of the electrostatic analyzers varied between 1500 and 13000. As the data system of the first two sectors is not functioning at the moment, the E ( ] ) spectra were acquired as single scans with an x/y recorder. For E(2) scans the spectra were accumulated and on-line processed with the AMDLntectra data system; 5- 15 spectra were averaged to improve the signal-to-noise ratio. Quantum chemical calculations were carried out using singlereference and multireference approaches. Since close to their equilibrium internuclear distances, the ground states of AIFff and SiF" ( n = 1, 2) are reasonably well described by single electronic configurations (Le., AlF?', 5a26022;t'7d: AlF+ and SiF?+, 5a26a'2?t'7a'; SiF+. 5026022vd7a2),the respective geometries were optimized using second-order Mdler-Plesset perturbation theory (MP2) based on (unrestricted) Hartree-Fock wave functions expanded in the 6-311+G(3df) basis of the GAUSSIAN94 program package.I4 The obtained equilibrium internuclear distances (i.e., AlF+ (2Z+),1.612 A; AIF2+ (IX-), 1.581 A; SiF+ (IZ+), 1.544 A: SiF- ("), 1.559 A: SiF2+ (?E+), 1.509 A) and harmonic frequencies (Le., AlF+ (?E-). 930 cm-I; AlF?' ('P 986 )cm-I: , SiF+ (Il?), 1025 cm-': SiF' (313), 988 cm-'; SiF'- (?X+),1117 cm-I) compare well with earlier treatments based on multireference configuration-interaction techniques. 10.15 Vertical ionization energies of the monocations (IE,) were calculated as total energy differences between the mono- and the dications at the equilibrium distances of the singly charged species. For this purpose, electron correlation was treated using the standard (unrestricted) single-reference methodologies of GAUSSIAN94I4 (UMPn perturbation theory, quadratic configuration interaction and coupled-clusters including all single and double as well as a perturbational estimate of triple excitations, UQCISD(T) and UCCSD(T)) with standard Pople-type basis sets. Vertical ionization energies were also calculated using internally contracted multireference configuration interaction' (IC-MRCI), average coupled-pair functional" (IC-ACPF) methods, and the spin-restricted RCCSD(T) codeIx of MOLPR094.I9 Natural orbitals for multireference treatments were generated using full-valence complete active-space selfconsistent field (CAS-SCF) calculations, and all wave functions were expanded in an atomic natural orbital (ANO) basis?" of the size (17~12p5d4f)/[7~6p4d3fl for A1 as well as Si and ( 1 4 ~ 9 ~ 4 d 3 Q / [ 6 ~ 5 ~ 3for d 2 fF.l Core electrons (1s-2p for A1 and Si: I s for F) were generally not correlated, and this constitutes probably the largest source of error for the computed second ionization energies.
Results and Discussion Upon collision with oxygen, AlF+ as well as SiF+ undergo not only dissociation to the corresponding atomic fragments but
Letters AI'
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i
1
AIF~+
I si+ TIZ
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Figure 1. BQj-CS mass spectra of ( a ) AIF' and (b) SiF+ monocations mass-selected by means of B( 1 ) and E( 1 I: oxygen ( 7 0 9 T) was used as a collision gas.
also charge stripping (CS) to the dications A1F2+ and SiF?+. respectively (Figure 1). From a purely energetical point of view, the somewhat larger ratio of the CS product AIF'- to the homolysis product Al+ as compared to that of SiF?+/Si+ can be regarded as a first hint to our finding that IE(A1F') < IE(SiF+) as outlined below. The stability of the so-formed dications can be inferred from the fact that within the sensitivity of the experiments neither F-atom losses nor Coulomb explosions,'."' Le.. A1F'Al' Ff and SiF2SiF+, were observed, when the dications generated by CS preceding E(1) were mass-selected using either E(l) or E(I)/ B(2) and their unimolecular fragmentations were monitored by scanning B(2) or E(2), respectively.?' Thus, we conclude that both dications are stable for at least 25 pus, which is the time the dications generated by CS in the collision cell preceding E( 1) need to reach the detector behind E(2). As this time is certainly sufficient for a complete energy randomization in the dications, it is reasonable to assume that both dications indeed represent stable species as predicted by Kolbuszewski and Wright;") recently, also the related MgF2+ dicatiodh has been observed in the gas phase.xh For the process AlF+ AlF", we determined Qmlnas 19.7 f 1.5 eV, which is slightly higher than the second IE of Al(18.8 eV)." The relatively large error in the determination of Qmlnis due to the fact that the ion signal for AlF+ from FAB is not very abundant. The intensity of SiF+ is much higher, and SiF'+ we determined Qmln = 17.1 f for the process SiF0.4 eV as compared to IE(Si+) = 16.3 eV.13 Note, however, that the lower Qmln for the process SiF SiF?- is contradictory to the intuitive statement that the IE(SiF+) should be higher than IE(AlF+), based on the AlF?+/AI' and SiF'+/Si+ ratios in the CS spectra.
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Letters
J. Phys. Chem., Vol. 99, No. 44, 1995 16197
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TABLE 1. Calculated Vertical Ioniz tion Energies (IE,)for AlF+ (2E+ lE+ e) at R = 1.612 (MP2/6-311+G(3df) Equilibrium Distance) Using Various Levels of Theoryb
1
+
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IE, Lev1 method 2Z+ ]Z+ e Single-Reference Methods UMP2/6-311G(d) 18.31 UMP4/6-3llG(d) 18.26 UQCISD(T)/6-311G(d) 18.36 UMP2/6-311+G(3df) 18.32 18.38 UCCSD(T)/6-31 liG(3df) RCCSD(T)/ANO 18.38 Multireference Methods IC-MRCVANO' 18.17 IC-MRCI+Q/ANO 18.21 IC-ACPF/ANO 18.21
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The equilibrium distance of A1F2+(1X+) amounts to 1.581 A; IE, = 0.02 eV. Only the valence electrons have been correlated. The leading references in the IC-MRCI wave functions have weights of 94.0% (AlF') and 93.2% (A1F2+),respectively. a
- IE,
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TABLE 2. Calculated Vertical Ionization Energies (E,) for SiF+ (lE+ 2E+ e) at R = 1.544 .& and SiF+ (311 *E+ e) at R = 1.559 .& (MP2/6-31l+G(3df) Equilibrium Distances") Using Various Levels of Theoryb IE, [eV] IX+ IE, [eV] 311 G [eVIcIZ+ method -*Z++e --Z++e -3n Single-Reference Methods UMP2/6-311G(d) 20.61 16.13 4.48 UMP4/6-311G(d) 20.80 16.07 4.73 UQCISD(T)/6-311G(d) 20.84 16.12 4.72 UMP2/6-31lfG(3df) 20.72 16.24 4.48 UCCSD(T)/6-311+G(3df) 20.97 16.31 4.66 RCCSD(T)/ANO 20.98 16.30 4.66 Multireference Methods IC-MRCI/ANOd 20.60 16.11 4.49 IC-MRCI+Q/ANO 20.75 16.24 4.5 1 IC-ACPF/ANO 20.73 16.23 4.50
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The equilibrium distance of SiF2+(*Z+) amounts to 1.509 A; IE,
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- IE, = 0.02 eV. Only the valence electrons have been correlated. G, (]E+ 311)- Go( I F 311)= 0.005 eV. The leading references
in the IC-MRCI wave functions have weights of 91.5% (SiF+, IZ+), 93.2% (SiF+, 311)and 92.4% (SF*+),respectively. Therefore, we performed ab initio MO calculations for the monocations and dications of AlF and SiF. In a CS experiment the interaction time of the fast moving monocation with the s), such that ionization is considered target is very short (< to occur as a vertical process.'." Table 1 shows the vertical ionization energies (IE,) of AlF+ to the A1F2+ dication, and if not stated otherwise, we will refer to the coupled-cluster (UCSSD(T) and RCCSD(T)) results in the following discussion. Irrespective of the computational method used, we arrive at an IE,(AlF+) of 18.3 f 0.1 eV (18.38 eV with UCCSD(T)/631 l+G(3df)), which implies that both species are reasonably well described at these levels of theory. In this respect, particularly the good agreement between single-reference and multireference methods is convincing. Not surprisingly, the difference between IE, and the adiabatic IE (Ea)of AlF+ is rather small (0.02 eV), since the equilibrium distances R(A1F) are very similar in the mono- and dication (1.6 12 and 1.581 A, respectively). The calculated vertical ionization energies (IE,) for SiF+ are shown in Table 2. Again the spread of calculated values is rather small in that all fall in the range between 20.6 and 21.0 eV (20.98 eV with UCCSD(T)/6-31lfG(3df)); further, also here single-reference and multireference approaches yield comparable results. On the basis of Koopmans' theorem, a very similar IE, for SiF+ (20.8 eV) has been deduced earlier.22 In addition,
the difference between IE, and IE, is quite small (0.02 eV) due to similar equilibrium geometries of SiF+ and SiF2+(1 S44 and 1SO9 A, respectively). Note, that with respect to the equilibrium bond lengths (Re)and frequencies ( W e ) of the Alp' (Re = 1.581 A, We = 986 cm-I) and SiF2+(Re= 1SO9 A, oe= 1117 cm-') ground states, our computations agree very well with the previous predictions by Kolbuszewski and Wright (A1F2+: Re = 1.583 A, we = 984 cm-I; S i p + : Re = 1.513 A, W , = 1186 cm-').l0 A rough estimate for the accuracy of the calculated second IEs can be derived from the corresponding values for atomic A1 and Si, for which we computed second IEs of 18.61 and 16.23 eV using the UCCSD(T)/6-31lSG(3df) approach, as compared to the experimental datal3 of 18.82 and 16.34 eV, respectively. Thus, the level of theory applied appears to be quite reasonable, and the slight underestimation of the IEs can be attributed to the missing treatment of core correlation in the calculations. To a first approximation, we can expect similarly underestimated IEs for the diatoms AlF+ and SiF+, such that we arrive at corrected values of IE,(AlF+) = 18.6 eV and IE,(SiF+) = 21.2 eV using the UCCSD(T)/6-31l+G(3df) approach. In the comparison of and SiF2+a significant qualitative difference becomes apparent which can be interpreted in terms of the different nature of the bonding in the corresponding species together with thermochemical arguments: While IE,(AlF+) is close to that of atomic Al+ (AIE, = -0.23 eV), that of SiF+ is by 4.74 eV larger as compared to IE(Si+). Formally, upon CS of AlF+ an electron is removed from the 3s orbital of A1 leading to A1F2+ in its 'E+ground state, such that the BDE (A12+-F) of 3.43 eV (UCCSD(T)/6-311+G(3df), a result of 3.29 eV was reported in ref 10) is similar to BDE (AI+-F) in the monocation (3.22 eV).23 In contrast, for the IE+ground state of the SiF+ monocation BDE (Si+-F) is quite high (6.60 eV)22 and upon CS to the 2C+ ground state of the dication an electron is removed from the bond, such that BDE (Si2+-F) shrinks to only 1.93 eV (1.89 eV in ref 10). Consequently, while the IEs of Al+ and AlF+ should be quite similar, that of SiF+ should exceed IE(Si+) significantly. In view of the relatively large experimental error, the measured Qmln(19.7 f 1.5 eV) for the process AlF+ A1F2+ is in agreement with the theoretical prediction (18.6 eV). In the silicon case, however, the discrepancy between experimental and calculated figures (17.1 f 0.4 versus 2 1.2 eV) is simply too large to be accounted for by uncertainties. Rather, a previous study' of the second-row congener, the CF+/CF2+ couple, suggests that electronically excited SiF+(31-I)monocations are generated under E1 ionization and contribute to the CS is shifted to lower values. The singledtriplet signal in that gap G of SiF+('Z+/311)has previously been calculated as 4.78 eV using a MRD-CI approach,i5eand we computed G as 4.484.73 eV at various levels of theory (Table 2; 4.66 eV with UCCSD(T)/6-31lfG(3df)). If we assume that the measured value reflects the transition SiFf(311) SiF2+(2Z+) e, the IE of the 'Z+ ground state of SiF+ can be calculated according to eq 1:
m+
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'
em,,
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em,,
+
+
+
IE(SiF+('E+)) = Q,,,(SiF+) G = 17.1 f 0.4 eV 4.6 f 0.2 eV = 21.7 f 0.6 eV (1) To test this hypothesis, we ionized SiF4 in a chemical ionization source at a relatively high pressure, such that thermalizing collisions can allow for a decrease of the amount of electronically excited SiF+ in the precursor ion beam. Figure 2 shows the signal for the SiF2+ dication obtained under high energy resolution. In fact, the CS peak exhibits two features which are separated by ca. 2 eV. As we measure dications, this difference translates to an energy difference of ca. 4 eV, which
16198 J. Phys. Ckem., Vol. 99, No. 44,1995
1 7 . 1 eV
Letters
Q ,
'eV1
Figure 2. Energy resolved signal for the SiF?' dication in the E( 1 )CS mass spectrum of B(1)-selected SiF' cations generated by chemical ionization of SiF+ The width of the SiF' precursor beam was 0.6 eV at half-height (corresponding to an energy resolution of EIAE z 13000): oxygen (80% T! was used as a collision gas. In the figure. seven single scans were superimposed.
is in good agreement with the decrease of the measured Qlnin value by interference of electronically excited SiF+(?n)cations. Unfortunately, the poor resolution of CS mass spectrometry as well as the overlap of both peaks render it impossible to derive more precise threshold information for the excitation gap G of SiF+ cation. In conclusion, the theoretically predicted existence of AlF?+ and SiF2+as stable dications could be established experimentally. Nevertheless, the CS technique suffers from interferences by electronically excited ions in the precursor beam as well as relatively large experimental uncertainties, and altemative methods for the examination of the thermochemistry of dications, as universal as charge-stripping mass spectrometry, however, are desirable. Furthermore, the dependence of the CS efficiencies on the magnitude of remains to be examined. Finally, the similarity of the computed IEs of Alf and AlF+ contradict the simple picture that electron withdrawing ~~ ligands as for example fluorine should increase I E S ; ~a .finding which highlights the need for a deeper understanding of role of ligand effects on first and second ionization energies of metal compounds.
en,,,
Acknowledgment. We thank Marcin Kolbuszewski, NRC Ottawa, and Carlito B. Lebrilla, UC Davis, for preprints and helpful comments. Financial support by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the Volkswagen Stiftung is gratefully acknowledged. References and Notes (1) (a) 4st. T. Adz,. M c i s i Specfrorn. A 1980. 8. 5 5 5 . !bl Lanimertsma.
K.: Schleyer. P. v. R.: Schuarz, H. Arigew. Ciiem., lr7r. Ed. E q [ . 1989. 28. 1321. (c) Schuarz. H. Piire Appl. Chern. 1989. 61. 685. ( d j Roth. L. M.: Freiser. B. S. Moss S p e c r ~ n R. e x 1991. 10. 303. (e) Mathur. D.Ph? s. Rep. 1993. 225. 193. ( 2 ) Pauling. L. J. Ci7en7. Ph?.c. 1933. 1. 56 (3) (ai Guilhaus. M.: Brenton, A. G.: Beynon. J. H.: Rabrenolic. M.: Schleyer. P. \ . R. J. Phy.s. B. 1984. 17. L605. !b) Belkacem. A,: Kanrer. E. P.: Mitchell. R. E.: Vager. Z.: Zabransky. B. J. Phys. Rerm. L e n 1989. 63. 2555. (c) Zaifman. D.: Kanter. E. P.: Vager. Z.: Zaifman. J. Pit!\. Re!.. 1991. 43, 1608. id) Bacalis. N.C.: Komninos. Y.: Nicolaide\. C.4 . P i i , ~ . Rei? A 1992. 45, 2701. (4) ( a ) Bruna. P. J.: Wright. J. S. J. Chern. Phxr. 1990. 93. 2617. ( b J Koch. W.: Liu. B.: Frenking. G. J. Chern. Phyc. 1990. 92. 2464.i c ) Wong.
M, W.: Radom, L. J. Phxs. Chern. 1990. 94. 638. (d) Miller. P. J.: Rogers, S. A,: Senekowitsch. J.: O'Neil, S. V.: Leone. S. R.; Werner. H.-J.: Knowles. P. J. h i t . J. A4u.s.s Spectrorn. lor7 P ~ o ~ ~ . s 1990. . w . s 100. 505. (e) Gutchina. N, A,: Nikuiin. V. K. Opr. Specrro\c,. iCSSR) 1991. 71. 432. (0Senekowitsch. J.: O'Neil. S. J. Chrrii. Phyx 1991. Y5. 1847. (g) Cachoncinlle. C.: Pouvesle. J. M.: Durand. G.: Spiegelmann. F. J. Cliem. PIiys. 1992. 96. 6085. (11) Kolbuszewski. M.: Wright. J . S. Car7. J. Chem. 1993. 71. 1562. ( i ) Kolbuszewski. M.:Wright. J. S. Chert?. Phys. Lett. 1993. 218, 338. (j) Kolbuszenski. M.: Wright. J. S.. Buenker. R. J.. J. Chern. Phyv. 1995. 102. 7519. ( k ) See also: Tallor. P. R. Moi. P/7\\. 1983. 49. 1297. ( 5 ) (a) Ben-lrrhak. I.: Gertner. I.: Heber. 0.; Rosner. B. Cliern. P h y ~ . Leu. 1993. 212. 467. I b JBen-Itzhak. I.: Chen, Z.: Esry, B. D.: Gertner. I.: Heber. 0.:Lin. C. D.: Rower. B.: P h ~ . sReL. . A 1994. 49. 1773. ( 6 ) ( a ) Price. S.D.: Manning. bl.: Leone. S. R. Cherti. Phv,s. Leri. 1993, 214. 553. ( b ) Price. S. D.: Manning. M.: Leone. S . R. J. Ari7. Chern S O , . 1994. 116. 8673. ( 7 ) ( a )Johnson. R.: Biondi. .LI.A. Ph\.s. Rei,.'41979.20. 87. (b) Smith. D.: Adams. N. G.: A g e . E.: Villinger. H.: Lindinger. W. J. Phys. B 1980. l.?. 2787. !c) See also: Helm. H.: Stephan. K.: Mark. T. D.: Huestii. D. L. J. C 1 7 e ~P/lJ.\. 1981. 71.3844. 18) ( a ) McCullough. S. M:Jones. D. A,: Lebrilla. C.B./ f i r . J. M L I . ~ . ~ \ 1991. 107. 545. (b) Dai. P. Q.: McCulloughCatalano. S.: Bolton. M.: Jane\. .A. D.: Lebrilla. C.B. / ? i f . J. r M ~ s Specfrom ~s loti P r o c 1995. 144. 67. ( 9 ) !a) McCullough-Catalano. S.: Lebrilla, C. B. J. h 7 . Ciiern. Soc. 1993. l l j . 1441. ( b J For NbCH22' as a similar example. see: Gord. J. R.: F r e i w . B. S.: Buckner. S . W . J. Chrrn. Phy.5. 1989. 91. 7530. i 10) M.Kolbu$zeuski. J . S. Wright J. P17ys. Chen7. 1995. 99. 3455. ( 1 1 ) ( a ) A t . T.: Proctor. C. J.: Poner. C. J.: Bejnon h r . J. !Mci,sr Specfrmr. Iori P/i?.c. 1981. 40.I 1 I . ( b ) Proctor. C. J.: Porter. C. J.: Asr. T.: Bebnon. J . H. / f i r . J. M(ic.s Spec.rroi?i. lor7 P/7ys. 1982. 41. 251. ic) Porter. C. J. : Proctor. C.J.: Ast. T.: Be) non. J. H. lr7r. J. Mirss S~~c(~rrorn. lori Piiy.~.1982. I/.265. ( d ) RabrenoLic. M.:A% T.: Beynon. J. H. h r . J . M t r \ s Sprc~rrorri.I o r i P r o ~ ~ e s . c1984. ~ ' \ 6 1 . 3 I . (el Dreuello. T. Ph.D. Thesis. TU Berlin D8i. 1989. ( 1 2 ) ( a ) SriniLac. R.: Sulrle. D.: m'eiske. T.: Schuarz. H. lrir. J. Mac5 Spwrrorti. l ~ t Pi ~ O W \ S L 1991. J Y 107. 368. ( b ) SriniLas. R.: Sulzle. D.: Koch. W.: DePu). C.H.: Schwarz. H. J . Am. Chern. Soc. 1991. 113, 5970. i 13) Moore. C.E. A ~ o ~Erier,q ~ I c Lrwls. National Standard Reference Data S e r i e ~National Bureau of Standards. NSRDS-NBS 35. Washington D.C.. 1971. (14) GAUSSlAN 94. Re\i\ion B3. Frisch. M J.: Trucks. G. W.: Schlegel. H. B.: Gill. P. hl. iV.: Johnson. B. G.: Robb. M. A,: Cheeseman. J. R.: Keith. T.: Petersson. G . A,: Montgomer!. J . A,: Raghavachari. K.: AI-Laham. M. A,: Zakrzeuski. V. G.: Ortiz. J . V.: Foresman. J . B.: Peng. C . Y.: A y a h P. Y.: Chen. LV.: Wong. M. W.: Anders. J. L.: Replogle. E. S.: Gomperrs. R.: Martin. R. L.: Fox. D. J.. Binkleq. J . S.: Defrees. D. J.: Baker. J.: Steu art. J . J . P.: Head-Gordon. M.: Gonzalez. C.: Pople. J . .A. GAUSSIAN Inc.. Pittsburgh PA. 1995. i 15) AIF-: (a1 Klein. R.: R o m u s . P. 777eor. Chrrn. Acru 1984. 66. 21. i h ) Knight. L. B. Jr.: Earl. E.: Ligon. A. R.: Cobranchi. D. P.: Wooduard. J. R.: Boctick. J. M.: Da\idson. E. R.: Feller. D. J. Ani. C/iern. SOC. 1986. 108. 5065. ( c ) Gleneninkel-Me!er. T.: Miiller. B.; Otringer. C.: Rosmus. P.; Knoules. P. J.: Werner. H.-J. J . C17rrn. P/7\s. 1991. Y5. 5133. SiF-: rd) Petrmichl. R. H.: Peter\on. K. A , : Woods. R. C. J. Cher17. Phy.5. 1988. 89. 5454. ( e )Peterson. ti. A: Woods. R. C.: Ram" P.: Werner. H.-J. J. Cl7em. Pi7~c.1990. Y.?. 1889. tfl Peterson. K . A : Woods. R. C.J. C/iern Pii!.s. 1990. Y2. 6061 i 16) i a J Werner. H.-J.: Knoules J . Ci7em. P17y.5. 1988. 89. 5803. ( b l Knoule$. P. J.: Werner. H.-J. C h e m P/7?.s. Lerr. 1988. 145. 514. (17) Gdanitz. R. J.: Ahlrichs. R. Chern. PhJc. Lerr. 1988, 143. 413. ( 1 8 ) Knoulei. P. J.: Hampel. C.: Werner H.-J. J. Chert7. P/i,v.s. 1993. YY. 5219. i 19) XIOLPRO is a package of ab initio programs Lvritten by Werner.
H.-J.. and Knoules. P. J.. a i t h contributions from: Almlof. J.: Amos. R. D.: Deegan. hl, J. 0.:Elbert. S. T.: Hampel. C.: Meyer. Vi,: Peterson. ti.: Pitrer. R.: Stone. A. J.: Tallor. P. R. ( 2 0 ) Widmark. P . - 0 . : Perswn. B. J.: Roos. B. 0. Ti7eor. Chirn. Acrci 1991. 7Y. 419. (21 J In addition. also SiF" dication mass-celected directly from the E1 source doe\ not undergo an) significant unimolecular decal. ( 2 2 ) O'Hare. P. A. G.: Wahi. A. C.J. Chern. Ph,u 1971. 5. 666. ( 2 3 1 Lias. S. G . : Banmes\. J . E.: Liebman. J. F.: Holmes. J . L.: Lexin. R . D.: Mallard. W. G . Gcir P17citr /o/i(ind Neitrrcil ~/ir~rnoc~/ter,ri\r~~\~, J. Ph?.\. C1ierr7. Re/. Dcirci, Sirppl. 11988. 17.
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