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Photodetachment of electrons from trifluoromethyl and trifluorosilyl ions; the electron affinities of CF3− and SiF3−
Volume
30,number
CHEMICAL
1
PHYSICS
1Jnnual-y
LETTERS
1975
.’
‘I
PHoT~DETAcHIMENT
~FELEC~ONS
..
FR~MTRIF~uQROM~HYLANDTRIFLU~~~~ILYL
IONS;
THE ELE~TRONAFF~NITIESDF-CF~~DS~;,I Jeffery
H. RICHARDSON*,
Depmtnhzt Received
I,.M. STEPHENS&+
of Cl~qnisfry, Stanford
Utriversiry, Sranford,
and John 1. BRAUMAN Californti
94305.
l$SA
6 September’1974
The relative in the wavelength
trifluoromcthyl
crass sections for the gas.phase photodetachment ofelectrpns have been determined for CF; and SiF; region 300-420 nm (4.13-2.995 cV). Thermochemical studies show thcclrctron affinity of the radical to be ca. 2.01 cV. The.large difference between the pholodctschmcnt threshold and adiabatic
electron aftinity is attributed to the different~geometries of the ion and neutral.
The variety of ions which dan be studied detachment experiments
by photo-
A- +hJ+A.+e-
(1)
has been greatly augmented by the use of ion cyclotron resonance techniques [ 1,2.] to generate and trap negative ions. For atomic systems, photodetachment experiments provide a precise and direct means of determining electron affinities [3]. It has become apparent, however, that molecular anions may have unusual photodetachment cross sections. There are many possible complications which might arise in photodetachmcnt experiments of molecular anions [4] ; one df the most obvious results from a change in the relative structure of the ion and neutral. Photodetachment.is a vertical process; poor Franck-Condon factors between the ion and neutral would result in a vertical detachment energy, determined by the photodetachment threshold, which exceeds the adiabatic electron affinity. Transitions to successively higher vibrational states of the neutral would result in a photodetachment curve which would.gradually tise over a rejatively wide energy interval [S-S] . Such behavior is in contrast with that frequently observed for atoms, where removal of an electro? from a p orbital results in a photodetachment cross section which * National Sdience Foundation Predoctoral Fe,Uow: ** Camille and Henry,Dreyfg Foundation Teacher.. ” Schok ,.
_.
rises sharply but eventually turns over eI;d flattens out as the pl;oton.energy is increased. A thermochemical cycle derived from proton transfer reactions in the gas phase provides an independent determination of the eiectron affinity: MtB-+&+A-.
(2)
Neglecting the change in standard entropy, the determination of the prkferred direction of reaction (2) identifies the sign of A@. Knowir.g DH” for AH and BH and EA for B allows a limit to be pIaced on EA(A). A substantial difference between the photodetachment threshold and the thermochemiw1 electron affmity’ may imply a significant change in the relative geometry of the ion and neutral (photodetachment of ap electron.from a more stable orbital, resulting in an excited state of the neutral, would also result in a large differknce between-the photodetachment threshold gnd the thermochemical electron afftity). In this paper; we report the measurement of the! relatice photode’tichment cross sections of trifluoromethyl anion, CFi, and trifIuorosilyI anion, SiFi. Comparison of the photodetachment threshold ?o appeararice potential and therrnochemical.determination of EA(CFj) suggests.there is’s si@ficant change -,in geometry between the ion tid neutral. This conelusion is further substantiated by semi-empirical cal-culations. Tetrafluoromethaqe
Vollime
30, number 1
,’
,:Cl-lE’+i
,. .,.:’ I Jar&y
PHYSICS LE’l-I’ERS
‘.
1975
_
Ferfluordpropane(Air Products) were used witk LJLI~ : $,. 5 .. ; *’;-’ -. 4 .’ ’ i’ ‘f&.&r pis&~ation. :,. .. ‘, .. j. $” 9 A Varian V-5900 ICR spedrometer @ai. used for . :. 5 .,:$.Iphotodetachment experiments and bracketing rew P actions. Detailed explhnations of the data &llection ” 0 and analysis‘have beeri previouslv reported [2,3] _ 3 .I Typical‘pre&ures* were 5 X IO-7 to 1.5 X iOM6 torr. 8 o.5 _ : ’ .’ .Conventiopal doub&. resonance [l] , ion ejection [9], 9 e . opticaldouble resonance, and experiments in a pulsed F d ,4 . ICR poj- wereused to.determine the direction of 0 Z ” :’vaiious reactions and eliminate any possibility of im-. . 0.0 -purities (vide inf&) influending the photodetachment i 300 400 450 350 kxpkiments.. .’ WiVELENGTH (nm) A 1000-W xenon br xenon--mercury lamp in conjunction with a pating monochrom~tqr was employei F@. 1. Rc,Iative cross sectipns fcr the photodetachment of CFi/CF4(e), bandwidth 14.1 nm. lle en&y range is 4.13as the light source. A grating blazed at 30d nm was 2.95 .eV. usGd:in ihe fEst order with matched~3.0,4.?, dr 7.7 .
mni slit&resulting in a bandwidth of 9.6, 14.1, or 24.6 -nm res@&tjiely (full vtidth at half height). Ldng waSelength pass filters were used to block light of higher orders.
Th$ the only process for ion ioss was phdtodetachnient was confrrnied by the observation that no additional ions were detected when irradiating with light. Thktias expected; reasonable assumptions about the electronic structure [l 1 -I 31 and the bond ditiociatidn energies [14,1.5] indicate that the photon.energy was insufficient for either' phctodissociation, photo. ,induced ‘chermcal reactions, or photod&chmei$ to an excited.electronic state of the neutral. Fig. 1 presents the results for the photodetachment of CF3 generated from tetrifluoromethane using the xenon lamp. Five runs were averageh togeth‘er, with an aver&e maximum fractiorial signal decrease of
0.03 1. Typical standard deviations are f ,370near the maxhum relative cross section, gradually increasing to + 15% near threshold. A linear 1elr.t squares fit ne’ar ,t?deshold (380-440 nm) yields an intercept of 438.6 f 13.nm(2.82~0.01 ev). Data were also obtained for the photcdetachrnent 6f CF; generated from hexafluoroethane. A pcQrer intrinsic negative ion signal resulted in Iess defiiiive .results; a linear least squares fit near’tllreshbld (360, .42O,ti) yields an intercept of 418.6 + 2;9 nm (a.96 + 0.02 bV)_. : .’ % F’resuke &a; q~cskred at thd Vat-Ion pump; rhe error for e?ima,tir$Ithe pressure adully in the ccl! nay bq ns Large
~asafactorof2.‘
..:.
Previqus estimates of the electron tifinity of trifluoromzthyl radical are approximatkly 2 eV. Page and Gocde [16] have estimated values ranging from 1.74 to 2.21 eV using the magnetron and a variety of substrates. Appearance potential measurements from several Ferfluorohydrocarbons yield values of 2;l eV (CzFJ [17], T.0 eV (C3Fs) [18], < 2.6 eV (C2Fs) 1201 and 0.2 eV (CF4) [21]. However, electron affhiiiies from i$pearance potential measurements necessarily assume there is no excess energy associated with th& products. For example, the anomalously’low value of EA (CF$ from CF, is rationalized by ascribing ca 1.6 eV excess energy to the products; in particular, LO internal excitation-of the CFT ion [21]. To determine EA(CFj) thermochemically, the following reactions were investigated: CFT +‘(CHs)3 COH 4 CF3H + (CH3j3 CO-,
(4)
CFT ,+ C&OH
+ CF,H + C,H,O-.,
(5)
CFY t CH;OH
-CF$+
(6)
CH,O-.
Both.CF4 and C2F, were used to generate CF;; identical r&Jlts w&e obtained in each case. Reactiqn (4) was facile, reaction (5) proce’eded to a significant extent, bu1 rehction (6) did not proceed at all. Equilibrlum measurements (W.N. Olinstead, Stanford, show that t-d.,&OH and CFsH haye almost identical acidi-’ ties. Thus, EA(CF;) g 2.01 eV (46 kcal/mole). The f?llowirg them-pdynamic data were used; DHO-EA a= 59:l kcal/mole for r-C,$OH [22] ; DHO(CF,-H) ‘.
VO~U’ITIC 30, number 1
CHESIICAL PI-JYSICSLETTERS
Table 1 Molecule
CFi CF;
‘.
CNDO/Z
Experimental
r(A)
0
r(A)
0
1.32
108” b)
1.32
105”
1.36
116O
CF4
1.34
log”28’b)
1.86
SiF;
1.90
109”B 104” 115O
1.32 b)
sir;;, SiF4
I .86
109” 28;
1SG C)
109”28’
1.19 I.23
lllD 121”
1.20 d) 1.24 c)
113” d) 122” c)
NO2
NOi
I
I 300
350
‘KY3
WAVELENGTH
a) 0 is the obtuse angle bctwecn a bond and the symmetry axis .CFj, CF;, SiF;, and SiF; assumed C-3,. b) Quoted in reF. [27]. C) Ref. [32] ; d) ref. [ 331; e) ref. [ 341.
= 106 rC_1 kcsl/mole [23,24] . It is of interest that only weak and non-reproducible evidence could be obtained for charge transfer between NO: and CFT (EA(NOz) ~2.38 5 0.06 eV> [S,25] ; this rcnction should be esothcrmic,
but it may be kinetically
slow
due to the
pronounced geometry changes required. It is apparent that there .is a significant difference jca. 0.8 ev) between the photodetachment threshold of CF; and the adiabatic electron affinity of CF;. For comparison, the difference between the vertical and adiabatic ionization potential of CF! is 1.6 eV. f26]. This difference is the result of a pronounced. geometry change in going from the neutral (pyramidal) to the cation (planar). A similai explanation is probably responsible for the difference in vertical detachment energy of CF, and the adiabatic electron affinity of CF;. Table 1 presents the results of CNDO!2 calculated [27] bqnd distances and bond angles; for comparison CF,, NO,, and NO; are included; The significant calculated difference in geometry is consistent with the experimental results. The monotonicalljr increasing photodetachment cross section is also consistent with poqr FranckCondon factcrs between the ion and neutral. The bandwidth was insufficient to resolve vibrational fme structure [28] _ It is possible to atiribute the difference & photodetachment &resholds between CF,/C,F, and.CF;/ CF, to some residual internal excitation of the latter 121.1. fiowevfk, CT; generated from CF4.0r C2F,5 was
Fig.
2.
Relative
cross sections
450
(nm)
for the photodctachmcnt
SiF; generated from SiF4 (a), and F-(a), The energy range is 4.13 -2.95 eV.
bandwidth
of
9.6 nm.
chemically equivalent; the difference in thresllalds may be due to the poorer ion signal and phatodetachIn any event, the difference, ment signal of CF;/C,F,. 0.14 eV, is considerably less than the differences in electron affinities based on appearance potential measurements_ This result su&ests that, if the CFTICF, ion is formed in an excited state; it is rapidly thermalized. Ions in our experiments can be expected to undergo approGmately 50 collisions, based on ion-molecule rate constants, ion lifetimes+, and phase coherence pulsed ICR experiments r29 j . Fig. 2 presents the results for the photodetachment of SiFS (generated from silicon tetraffuoride) using the xenon-mercury lamp. The best eight runs were averaged together with typical standard deviations of 5 2% near the maximum relative cress section, graduallyincreasing to 5 6% near threshold. The average maxitnum fractional sigr& decrease was 0.19. A Iinear least squares fit near threshold (360-420 nm) yields nn.intercept.of 421.8 f I,.3 nm (2.94 * 0.0 I eV). SiFT could also be generated
19
Volume 30, number 1 :
CHEMICAL PHYSICS LETTER!;
1 Jnnualy 1975 -
.’
; ‘.
‘.
The format& of.SiFg & F- was z facile rea?tion, _ .but it is unlikely that the degree of_exothermicity ., ‘&the partitioning of the energy among the prod-, ucts would be &r+r to. thzt found for SIFT gener-’ ated by dissociative electron capture. Fig. 2 also iresents the relative photodetachment cross section for $Fy generated via F-. The xenon-mercury l&p was used with a bandwidth of 9.6 nm. Three runs w&e averaged, with typical standard .deviations of + 10% tludughout the wavelength range studied. The average maximum fractional sipal decrease was 0.17. A linear least squares fit near threshnld (360-420 nm) yields a~ intercept’of 418.7 + 3.1 p-m (2.46 + 0.02 W The cross sections for photodetaclurient of SiF; generated under the two widely different methods tie identical within experimental error. This also suggests that under the condifions of our experiment iok are rapidly thermalized. A geometry cknge similar to that in the &bon analog in going from SiF,. to SiFj is predicted by @iDi)/ calculations (table 1). However, SiFT appears to be eithei a very stable or a very slowly reacting ion; we were.unable to obtain evidence of charge
transfer, proton transfer, or fluorine transfer with a Aumber of substrates. In view of the uncertainty about DHO(SiF,-H), we an draw no definite conclusions. The photodetachment experiment does, however, establish an upper limit to the electron affinity of the triflu&osilyl radical:
‘.. EA (SiF$
k 2.95 .+ 0.10 eV (68 f 2 kcal/mole),
where the &or reflects both the bandwidth and satting calibration. Previous +imates of EA(SiF$ were 3.4 eV [30] and.2.04
eV [31].
,.
We thank I%. CA. Liederand Mr. SJ. Reed for helpful dis&sions &d Mr. J.V. Garcia for techriical .assisiance. This tic& was supported by the National Science Foundation (GP-37044-X) and the Center for Materialb’Research, Stanford University.
Re&+es .y.._ ,’
.’
[li J.L Bkuchamp, Ann. Rev: Phys. Chem. 22 (1971) 527. ,. [?I KC. Smyth and ., 1132.:: “.,-
[ 2;.--. : ; _,_
: :
J.i. B;auman;.J. Chem.:Phys. 56 (1972)
.:
‘.
[3] H. Hotgp and Wk. Linebergcr. J. Chem. Phys. 58 (1973) 2379, :md ieferenscs therein. [4] J.H. Richard&, L.hi. Stephenson and J.I. Brauman, J. Chern. Phys. 59 (1973) 5068. [5] S. Golub and B. Steiner, J. Chem: Phys. 49 (1968) 5191. (61 J.H. Richar+on, L.M. Stephenson and J.I. Brauman, Chem. Phys. Letters 25 (1974) 318. [7] J.H. Richardson, L_M. &tephetion and J.I. Brauman, Chem. Phys. Letgrs 25 (1974) 321. [S] E. Herbst, T.A. Patterson and W.C. Lineberger. J. Chem. Phys., submitted for publiution, pxivaie communication. [9] J.L. Beauchamp and J.T. Armstrong, Rev. Sci. Instr. 40 (1969) 123. [lo] F..T. hlcIver Jr., Rev. Sci. Ins&, 41 (1970) 555. [ll] J.E. Hesser and K. Dressier, J. Chem. Phys. 47 (1967) 3443. [12] G.R. tiok and B.K. Ching, J. Chem. Phys. 43 (1965) 1794: [13] DE. Milligan, M.E. Jacox and W.A. Guillory, J. Chem. Phys. 49 (1968) 5330. [14] J.W. Cclomber and EWhittle, Trans. Faraday Sot. 63 (1967) 1394. [15] J.D. h4cDonald. C.H. Williams, J.C. Thompson and J.L. Margraq Advac. Chcm. Ser. 72 (1967) 261. [16] F-M. Page and G.C. Goode, Negative ions and the magnetron (Wiley, New York, 1969), [17] J.CJ. Thynne and K.A.G. MacNeil, Intern. J. hfms Spectram. Ian Phys. 5 (1970) 329. [18] C. Lifsliitzand R. Grajower, Intern. J. Mass Spectrom. Icr. Phlrs. 3 (1969) 211. [19] K.A.G. MacNeiland J.CJ. Thynne, Intern. J. ?,¶ass Spectrom. Ion Phys. 2 (1967) 1. IZO] M.M. Bibby and G. Carter, Trans. Faraday Sot. 59 (1963) 2455. [21] K.A.G. MacNei! and J.C.J. Thynnc, Intern. 5. Mass Spectrom. Ion Phys. 3 (1970) 455. [22] R.T. McIver Jr. and J.S. hii+, J. Am. Chom. Sot. 96 (1974) 4323. [23] J.A. Kerr, Chem. Rev. 66 (1966) 465. [24] J. Heicklen, Advan. Photochem. 7 (1969) 57. (251 D.B. Dunkin, F.C. Fehscnfeld and E.E. Ferguson, Chem. Phyr. Letters 15 (1972) 257. [26] C. Lifshitr and W.A. Chupka, J. Chem. Phys. 47 (1967) 3439.. [27] J.A. PO&Z and.D.L. Beveridge, Approximate molecular orbital theory (hicCraw-IIil, New York, 1970). 1281 D.E. hiilligan, ME. Jacox and JJ. Comeford, J. Chem. Phys..4l (1966) 4058. [29]P.A. Liede-r, R.W..Wien and R.T. McIver Jr., J. ohem. Phys. 515 (1972) 5184. [30] G.C. Good, private communication cited in ref. [21] _ [ 311 J.L. Wang, J.L. Margrave and J.L. Fran!Un,,J. Chem. Phys. 5:l (1973) 4417. [ 321 ,B:Beagley, l?.P. Brown and j.hf: Freeman, J. Mol. Struct. ‘. .16(1973) 337. [33j G.R. Bi:d, J;Chem. Phys. 25 (1956) 1040.. [34! $.R. Truter; Acta Cryst. 7 (1954) 73:
I’,
:.__
; ‘y
.,
.. _ .’ ._’ ]
30,number
CHEMICAL
1
PHYSICS
1Jnnual-y
LETTERS
1975
.’
‘I
PHoT~DETAcHIMENT
~FELEC~ONS
..
FR~MTRIF~uQROM~HYLANDTRIFLU~~~~ILYL
IONS;
THE ELE~TRONAFF~NITIESDF-CF~~DS~;,I Jeffery
H. RICHARDSON*,
Depmtnhzt Received
I,.M. STEPHENS&+
of Cl~qnisfry, Stanford
Utriversiry, Sranford,
and John 1. BRAUMAN Californti
94305.
l$SA
6 September’1974
The relative in the wavelength
trifluoromcthyl
crass sections for the gas.phase photodetachment ofelectrpns have been determined for CF; and SiF; region 300-420 nm (4.13-2.995 cV). Thermochemical studies show thcclrctron affinity of the radical to be ca. 2.01 cV. The.large difference between the pholodctschmcnt threshold and adiabatic
electron aftinity is attributed to the different~geometries of the ion and neutral.
The variety of ions which dan be studied detachment experiments
by photo-
A- +hJ+A.+e-
(1)
has been greatly augmented by the use of ion cyclotron resonance techniques [ 1,2.] to generate and trap negative ions. For atomic systems, photodetachment experiments provide a precise and direct means of determining electron affinities [3]. It has become apparent, however, that molecular anions may have unusual photodetachment cross sections. There are many possible complications which might arise in photodetachmcnt experiments of molecular anions [4] ; one df the most obvious results from a change in the relative structure of the ion and neutral. Photodetachment.is a vertical process; poor Franck-Condon factors between the ion and neutral would result in a vertical detachment energy, determined by the photodetachment threshold, which exceeds the adiabatic electron affinity. Transitions to successively higher vibrational states of the neutral would result in a photodetachment curve which would.gradually tise over a rejatively wide energy interval [S-S] . Such behavior is in contrast with that frequently observed for atoms, where removal of an electro? from a p orbital results in a photodetachment cross section which * National Sdience Foundation Predoctoral Fe,Uow: ** Camille and Henry,Dreyfg Foundation Teacher.. ” Schok ,.
_.
rises sharply but eventually turns over eI;d flattens out as the pl;oton.energy is increased. A thermochemical cycle derived from proton transfer reactions in the gas phase provides an independent determination of the eiectron affinity: MtB-+&+A-.
(2)
Neglecting the change in standard entropy, the determination of the prkferred direction of reaction (2) identifies the sign of A@. Knowir.g DH” for AH and BH and EA for B allows a limit to be pIaced on EA(A). A substantial difference between the photodetachment threshold and the thermochemiw1 electron affmity’ may imply a significant change in the relative geometry of the ion and neutral (photodetachment of ap electron.from a more stable orbital, resulting in an excited state of the neutral, would also result in a large differknce between-the photodetachment threshold gnd the thermochemical electron afftity). In this paper; we report the measurement of the! relatice photode’tichment cross sections of trifluoromethyl anion, CFi, and trifIuorosilyI anion, SiFi. Comparison of the photodetachment threshold ?o appeararice potential and therrnochemical.determination of EA(CFj) suggests.there is’s si@ficant change -,in geometry between the ion tid neutral. This conelusion is further substantiated by semi-empirical cal-culations. Tetrafluoromethaqe
Vollime
30, number 1
,’
,:Cl-lE’+i
,. .,.:’ I Jar&y
PHYSICS LE’l-I’ERS
‘.
1975
_
Ferfluordpropane(Air Products) were used witk LJLI~ : $,. 5 .. ; *’;-’ -. 4 .’ ’ i’ ‘f&.&r pis&~ation. :,. .. ‘, .. j. $” 9 A Varian V-5900 ICR spedrometer @ai. used for . :. 5 .,:$.Iphotodetachment experiments and bracketing rew P actions. Detailed explhnations of the data &llection ” 0 and analysis‘have beeri previouslv reported [2,3] _ 3 .I Typical‘pre&ures* were 5 X IO-7 to 1.5 X iOM6 torr. 8 o.5 _ : ’ .’ .Conventiopal doub&. resonance [l] , ion ejection [9], 9 e . opticaldouble resonance, and experiments in a pulsed F d ,4 . ICR poj- wereused to.determine the direction of 0 Z ” :’vaiious reactions and eliminate any possibility of im-. . 0.0 -purities (vide inf&) influending the photodetachment i 300 400 450 350 kxpkiments.. .’ WiVELENGTH (nm) A 1000-W xenon br xenon--mercury lamp in conjunction with a pating monochrom~tqr was employei F@. 1. Rc,Iative cross sectipns fcr the photodetachment of CFi/CF4(e), bandwidth 14.1 nm. lle en&y range is 4.13as the light source. A grating blazed at 30d nm was 2.95 .eV. usGd:in ihe fEst order with matched~3.0,4.?, dr 7.7 .
mni slit&resulting in a bandwidth of 9.6, 14.1, or 24.6 -nm res@&tjiely (full vtidth at half height). Ldng waSelength pass filters were used to block light of higher orders.
Th$ the only process for ion ioss was phdtodetachnient was confrrnied by the observation that no additional ions were detected when irradiating with light. Thktias expected; reasonable assumptions about the electronic structure [l 1 -I 31 and the bond ditiociatidn energies [14,1.5] indicate that the photon.energy was insufficient for either' phctodissociation, photo. ,induced ‘chermcal reactions, or photod&chmei$ to an excited.electronic state of the neutral. Fig. 1 presents the results for the photodetachment of CF3 generated from tetrifluoromethane using the xenon lamp. Five runs were averageh togeth‘er, with an aver&e maximum fractiorial signal decrease of
0.03 1. Typical standard deviations are f ,370near the maxhum relative cross section, gradually increasing to + 15% near threshold. A linear 1elr.t squares fit ne’ar ,t?deshold (380-440 nm) yields an intercept of 438.6 f 13.nm(2.82~0.01 ev). Data were also obtained for the photcdetachrnent 6f CF; generated from hexafluoroethane. A pcQrer intrinsic negative ion signal resulted in Iess defiiiive .results; a linear least squares fit near’tllreshbld (360, .42O,ti) yields an intercept of 418.6 + 2;9 nm (a.96 + 0.02 bV)_. : .’ % F’resuke &a; q~cskred at thd Vat-Ion pump; rhe error for e?ima,tir$Ithe pressure adully in the ccl! nay bq ns Large
~asafactorof2.‘
..:.
Previqus estimates of the electron tifinity of trifluoromzthyl radical are approximatkly 2 eV. Page and Gocde [16] have estimated values ranging from 1.74 to 2.21 eV using the magnetron and a variety of substrates. Appearance potential measurements from several Ferfluorohydrocarbons yield values of 2;l eV (CzFJ [17], T.0 eV (C3Fs) [18], < 2.6 eV (C2Fs) 1201 and 0.2 eV (CF4) [21]. However, electron affhiiiies from i$pearance potential measurements necessarily assume there is no excess energy associated with th& products. For example, the anomalously’low value of EA (CF$ from CF, is rationalized by ascribing ca 1.6 eV excess energy to the products; in particular, LO internal excitation-of the CFT ion [21]. To determine EA(CFj) thermochemically, the following reactions were investigated: CFT +‘(CHs)3 COH 4 CF3H + (CH3j3 CO-,
(4)
CFT ,+ C&OH
+ CF,H + C,H,O-.,
(5)
CFY t CH;OH
-CF$+
(6)
CH,O-.
Both.CF4 and C2F, were used to generate CF;; identical r&Jlts w&e obtained in each case. Reactiqn (4) was facile, reaction (5) proce’eded to a significant extent, bu1 rehction (6) did not proceed at all. Equilibrlum measurements (W.N. Olinstead, Stanford, show that t-d.,&OH and CFsH haye almost identical acidi-’ ties. Thus, EA(CF;) g 2.01 eV (46 kcal/mole). The f?llowirg them-pdynamic data were used; DHO-EA a= 59:l kcal/mole for r-C,$OH [22] ; DHO(CF,-H) ‘.
VO~U’ITIC 30, number 1
CHESIICAL PI-JYSICSLETTERS
Table 1 Molecule
CFi CF;
‘.
CNDO/Z
Experimental
r(A)
0
r(A)
0
1.32
108” b)
1.32
105”
1.36
116O
CF4
1.34
log”28’b)
1.86
SiF;
1.90
109”B 104” 115O
1.32 b)
sir;;, SiF4
I .86
109” 28;
1SG C)
109”28’
1.19 I.23
lllD 121”
1.20 d) 1.24 c)
113” d) 122” c)
NO2
NOi
I
I 300
350
‘KY3
WAVELENGTH
a) 0 is the obtuse angle bctwecn a bond and the symmetry axis .CFj, CF;, SiF;, and SiF; assumed C-3,. b) Quoted in reF. [27]. C) Ref. [32] ; d) ref. [ 331; e) ref. [ 341.
= 106 rC_1 kcsl/mole [23,24] . It is of interest that only weak and non-reproducible evidence could be obtained for charge transfer between NO: and CFT (EA(NOz) ~2.38 5 0.06 eV> [S,25] ; this rcnction should be esothcrmic,
but it may be kinetically
slow
due to the
pronounced geometry changes required. It is apparent that there .is a significant difference jca. 0.8 ev) between the photodetachment threshold of CF; and the adiabatic electron affinity of CF;. For comparison, the difference between the vertical and adiabatic ionization potential of CF! is 1.6 eV. f26]. This difference is the result of a pronounced. geometry change in going from the neutral (pyramidal) to the cation (planar). A similai explanation is probably responsible for the difference in vertical detachment energy of CF, and the adiabatic electron affinity of CF;. Table 1 presents the results of CNDO!2 calculated [27] bqnd distances and bond angles; for comparison CF,, NO,, and NO; are included; The significant calculated difference in geometry is consistent with the experimental results. The monotonicalljr increasing photodetachment cross section is also consistent with poqr FranckCondon factcrs between the ion and neutral. The bandwidth was insufficient to resolve vibrational fme structure [28] _ It is possible to atiribute the difference & photodetachment &resholds between CF,/C,F, and.CF;/ CF, to some residual internal excitation of the latter 121.1. fiowevfk, CT; generated from CF4.0r C2F,5 was
Fig.
2.
Relative
cross sections
450
(nm)
for the photodctachmcnt
SiF; generated from SiF4 (a), and F-(a), The energy range is 4.13 -2.95 eV.
bandwidth
of
9.6 nm.
chemically equivalent; the difference in thresllalds may be due to the poorer ion signal and phatodetachIn any event, the difference, ment signal of CF;/C,F,. 0.14 eV, is considerably less than the differences in electron affinities based on appearance potential measurements_ This result su&ests that, if the CFTICF, ion is formed in an excited state; it is rapidly thermalized. Ions in our experiments can be expected to undergo approGmately 50 collisions, based on ion-molecule rate constants, ion lifetimes+, and phase coherence pulsed ICR experiments r29 j . Fig. 2 presents the results for the photodetachment of SiFS (generated from silicon tetraffuoride) using the xenon-mercury lamp. The best eight runs were averaged together with typical standard deviations of 5 2% near the maximum relative cress section, graduallyincreasing to 5 6% near threshold. The average maxitnum fractional sigr& decrease was 0.19. A Iinear least squares fit near threshold (360-420 nm) yields nn.intercept.of 421.8 f I,.3 nm (2.94 * 0.0 I eV). SiFT could also be generated
19
Volume 30, number 1 :
CHEMICAL PHYSICS LETTER!;
1 Jnnualy 1975 -
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The format& of.SiFg & F- was z facile rea?tion, _ .but it is unlikely that the degree of_exothermicity ., ‘&the partitioning of the energy among the prod-, ucts would be &r+r to. thzt found for SIFT gener-’ ated by dissociative electron capture. Fig. 2 also iresents the relative photodetachment cross section for $Fy generated via F-. The xenon-mercury l&p was used with a bandwidth of 9.6 nm. Three runs w&e averaged, with typical standard .deviations of + 10% tludughout the wavelength range studied. The average maximum fractional sipal decrease was 0.17. A linear least squares fit near threshnld (360-420 nm) yields a~ intercept’of 418.7 + 3.1 p-m (2.46 + 0.02 W The cross sections for photodetaclurient of SiF; generated under the two widely different methods tie identical within experimental error. This also suggests that under the condifions of our experiment iok are rapidly thermalized. A geometry cknge similar to that in the &bon analog in going from SiF,. to SiFj is predicted by @iDi)/ calculations (table 1). However, SiFT appears to be eithei a very stable or a very slowly reacting ion; we were.unable to obtain evidence of charge
transfer, proton transfer, or fluorine transfer with a Aumber of substrates. In view of the uncertainty about DHO(SiF,-H), we an draw no definite conclusions. The photodetachment experiment does, however, establish an upper limit to the electron affinity of the triflu&osilyl radical:
‘.. EA (SiF$
k 2.95 .+ 0.10 eV (68 f 2 kcal/mole),
where the &or reflects both the bandwidth and satting calibration. Previous +imates of EA(SiF$ were 3.4 eV [30] and.2.04
eV [31].
,.
We thank I%. CA. Liederand Mr. SJ. Reed for helpful dis&sions &d Mr. J.V. Garcia for techriical .assisiance. This tic& was supported by the National Science Foundation (GP-37044-X) and the Center for Materialb’Research, Stanford University.
Re&+es .y.._ ,’
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[li J.L Bkuchamp, Ann. Rev: Phys. Chem. 22 (1971) 527. ,. [?I KC. Smyth and ., 1132.:: “.,-
[ 2;.--. : ; _,_
: :
J.i. B;auman;.J. Chem.:Phys. 56 (1972)
.:
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