1950
J. Phys. Chem. 1984,88, 1950-1954
the bleaching of bromide anion electron traps formed by photodissociation and electron capture. The observation that the bromine-substituted benzene cation absorptions were decreased a greater extent by the full mercury arc is expected because bromide electron traps detach electrons more easily than chloride electron traps. The stability of CB+ in the presence of visible light with more stable C1- electron traps shows that visible photodissociation of CB' does not occur in solid argon. The unusual photosensitivity of the 2,5-dichlorotoluene cation (the only cation studied which was completely destroyed by photolysis) is probably due to the reactivity of the methyl substituent which can readily participate in 1,3-hydrogen rearrangement processes like those found for toluene' and p-xylene8 cations. The failure to observe a large yield of MDCB' or ODCB' when these precursors were used, even though considerable PDCB' absorption was produced with both precursors, may be due to rearrangement of the MDCB+ and ODCB+ cations to PDCB+ during the condensation process. Since the argon resonance lamp (1 1.6 eV) can directly form the lone pair hole excited state,13 an excited bridged chloronium ion species could be formed as an intermediate for rearrangements among excited-state dichlorobenzene cation isomers.
Conclusions This study shows that stable chlorine- and bromine-substituted
benzene cations can be produced by matrix photoionization techniques and trapped in solid argon for spectroscopic investigation. Spectra for the cations exhibit absorptions in the visible and near-UV regions, which are in good agreement with predictions made from photoelectron spectra and with gas-phase emission and photodissociation spectra. The lower-energy transitions are due to promotion of an electron from a filled molecular orbital with lone pair character to the half-filled molecular orbital of the ground-state cation, while the higher-energy transitions are dominated by a* a excitations. Vibrational analysis reveals a number of fundamentals for the cations in solid argon that are slightly lower than neutral molecule values; this indicates a decrease in the net bonding in the cation ring consistent with the removal of an electron with bonding character. These experiments clearly demonstrate the important role of added electron trapping molecules in matrix photoionization studies of absorption spectra and photochemistry of molecular cations.
-
Acknowledgment. We gratefully acknowledge support for this research from NSF Grant CHE 82-17749 and assistance with the C,D5Br experiment by S. R. Davis. Registry No. PDCB*, 68170-44-5; MDCB', 68128-49-4; CB', 55450-32-3; PDBB', 74365-38-1; BBt, 55450-33-4; PBCB', 89104-50-7; 2,5-dichlorotoluene cation, 76563-46-7.
The First Bands in the Photoelectron Spectra of the CH,Br, CD,Br, CHBr,, and CH,I Free Radicals Lester Andrews,* John M. Dyke, Neville Jonathan, Noureddine Keddar, and Alan Morris Department of Chemistry, The University, Southampton, SO9 5NH U.K. (Received: November 23, 1983)
Reactions of F atoms with CH3X and CHzXzprecursors (X = Br, I) have been studied by photoelectron spectroscopy. A new structured band at 8.61 & 0.01 eV adiabatic and 8.72 & 0.01 eV vertical ionization energies is assigned to the CH2Br free radical. The 860 h 30 cm-I v ' = 0-1 separation assigned to the C-Br stretching fundamental in ground-state CHzBrf is higher than the 693-cm-I argon matrix value for CH2Br. The first band for CD2Br exhibits 780 f 30 and 1130 f 30 cm-' progressions in symmetric C-Br stretching and CD, scissor bending modes. Unstructured bands were observed at 8.41 f 0.03 and 8.52 h 0.03 eV for the first vertical ionization energies of the CHBr, and CHzI free radicals, respectively.
Introduction In recent years, photoelectron spectroscopy has been used to study free radicals and high-temperature molecules.' The methyl radical, one of the most important transient species, has been produced by pyrolysis of a ~ o m e t h a n e ~and - ~ by hydrogen abstraction from methane6 for photoelectron spectroscopic study of the ground-state CH3+cation. Very recent investigations have employed the latter method and a new minicomputer-controlled multidetector spectrometer to observe photoelectron spectra (PES) of the CH2F, CH,Cl, CHCl,, and CHFCl free radicals and to provide symmetric carbon-halogen stretching fundamentals for the ground-state cations?-1° This paper describes a similar study of the CHzBr, CHBr,, and CHJ free radicals. Bromomethyl and iodomethyl radical transients have received little attention in the literature. In an earlier matrix investigation, infrared spectra of CHzBr and CD2Br provided evidence for planar radicals with some C-Br a bonding;" later ESR studies have supported the structure and bonding conclusions derived from the matrix infrared study.', The CH2Br+cation has been observed in photoelectron-photoion coincidence investigations on CH2Br,. The matrix infrared absorption spectrum of CHzI is also consistent
with a planar radical,I4 and gas-phase infrared emission has been observed from CHJ produced by ph~tofragmentation.'~The CHJ' cation has also been formed from CHzIzin a coincidence (1) Dyke, J. M.; Jonathan, N.; Morris, A. Znt. Rev. Phys. Chem. 1982,2,
3.
(2) Golob, L.; Jonathan, N.; Morris, A,; Okuda, M.; Ross, K. J. J . Electron Spectrosc. 197211973, 1 , 506. ( 3 ) Koenig, T.; Balle, T.; Snell, W. J . Am. Chem. SOC.1975, 97, 662. (4) Dyke, J.; Jonathan, N.; Lee, E.; Morris, A. J . Chem. SOC.,Faraday Trans. 2 1976, 72, 1385. (5) Houle, F. A.; Beauchamp, J. L. J . Am. Chem. SOC.1979, 101,4067. (6) Andrews, L.; Dyke, J. M.; Ellis, A. R.; Morris, A. unpublished work. (7) Andrews, L.; Dyke, J. M.; Jonathan, N.; Keddar, N.; Morris, A.; Ridha, A. Chem. Phys. Lett. 1983, 97, 89. (8) Andrews, L.; Dyke, J. M.; Jonathan, N.; Keddar, N.; Morris, A. J . Chem. Phys. 1983, 79, 4650. (9) Andrews, L.; Dyke, J. M.; Jonathan, N.; Keddar, N.; Morris, A. J. Am. Chem. Sac. 1984, 106, 299. (10) Andrews, L.; Dyke, J. M.; Jonathan, N.; Keddar, N.; Morris, A,; Ridha, A. J . Phys. Chem., in press. (11) Smith, D. W.; Andrews, L. J . Chem. Phys. 1971, 55, 5295. (12) Mishra, S . P.: Nielson, G.W.: Symons, M. C. R. J. Chem. Sac., Faraday Trans. 2 1974, 70, 1165. (13) Tsai, B. P.; Baer, T.; Werner, A. S.; Lin, S.F. J . Phys. Chem. 1975,
_.-.
7,, 0 5717
*Address correspondence to this author at the Chemistry Department, University of Virginia, Charlottesville, VA 22901,
(14) Smith, D. W.; Andrews, L. J . Chem. Phys. 1973, 58, 5222. (15) Baughcum, S . L.; Leone, S. R. J. C h e m Phys. 1980, 72, 6531.
0022-3654/84/2088-1950$01.50/00 1984 American Chemical Society
PES Study of CHzBr, CDzBr, CHBrz, and CHzI Radicals
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 1951 BOO0 :H3 I
v)
I-
Z 3 0
u 0‘ 13.0
11.0
I ESeVI
9.0
Figure 1. N e I photoelectron spectrum in the 8-14-eV ionization energy region of the F CH,Br reaction products a t a 2.5-cm mixing distance obtained by using relatively low F atom and relatively high CH3Br partial pressures. The N e Ia (16.67 eV) line in the ionization source gives PES bands at 0.18-eV higher apparent ionization energy with 15% of the intensity of the N e Ia (16.85 eV) line.
J
+
study.13 The following investigation reports first PES bands with resolved vibrational fundamentals for the CHzBr+and CDzBr+ cations and vertical ionization energies for the related CHBr, and CHzI free radicals.
Experimental Section Photoelectron spectra (PES) were recorded by using He Ia (21.22 eV) and N e Ia (16.85 eV) ionization and a new minicomputer-controlled transient spectrometer equipped with a channel plate/phosphor/SIT camera detector for rapid data collection;8s16the dc lamp employed here also emits about 2% He IP (23.09 eV) and 15% Ne Ia (16.67 eV) radiation.17 Free radicals were prepared by reaction of fluorine atoms, produced by microwave discharge of a 5% Fz-He mixture (B.O.C. Special Gases), with CH3Br (BDH Chemicals, Ltd.), CD3Br (Merck and Co., 99.5% D), CHzBr2,CH31, and CHJ2 (May and Baker, Ltd.), mixed in a flow reactor from 0.3 to 9.0 cm above the ionizing radiation. Spectra were calibrated against CH31 added to the sample chamber;” ionization energies quoted refer to vertical values and are accurate to fO.O1 eV unless otherwise stated. Results Photoelectron spectra for the products of fluorine atom reactions with five compounds will be presented in turn. CH,Br. The He I a PES of CH3Br is in accord with literature spectra;’* the first two sharp peaks were measured at 10.54 and 10.87 eV, respectively. The F CH3Br reaction was examined in 108 spectra under a variety of reaction conditions. At short mixing distances (1-2 cm), the introduction of F atoms reduced the CH3Br signal intensity, produced HF19 and Br atomZopeaks, and produced a new product signal around H e Ip ionization of the first CH3Br band at 8.67-eV apparent ionization energy. At long mixing distances (6-8 cm), HF and Br signals increased markedly at the expense of CH3Br and the new product signals, and the structured C F radical first band appeared with components at 9.11, 9.33,9.55, and 9.76 f 0.01 eV?’ Increasing the F atom partial pressure continued this trend; He I@ ionization of Br produced signals at 9.94 and 10.33 eV which were as strong as He Ia spectra of C F and CH3Br.
0
10.0
9.5
I.E.(eV)
9*0
8.5
Figure 2. Ne I photoelectron spectrum in the 8.5-10.5-eV ionization energy region of the structured product of the F CH3Br reaction at a 0.6-cm mixing distance. The presented spectrum is the sum of four sets of 1500 scans each recorded in 2 min. Methyl iodide was added to the ionization chamber for calibration.
+
CD2Br
a-I
goo(
v)
t-
z
3 0
u
+
(16) Dyke, J. M.; Francis, P. D.; Jonathan, N.; Keddar, N.; Mills, J. D.; Morris, A. Rev. Sci. Instrum. 1984, 55, 172. ( 1 7) Eland, J. H. D. “Photoelectron Spectroscopy”;Butterworths: London, 1974. (18) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. “Molecular Photoelectron Spectroscopy”; Wiley: New York, 1970. (19) Walker, T. E. H.; Dehmer, P. M.; Berkowitz, J. J . Chem. Phys. 1973, 59, 4292. Dyke, J. M.; Morris, A,; Winter, M. J., unpublished work. (20) Huffman, R. E.; Larrabee, J. C.; Tanaka, Y. J . Chern. Phys. 1967, 47, 856.
(21) Dyke, J. M.; Lewis, A. E.; Morris, A. J . Chem. Phys. 1984, 80, 1382.
0 9.0
I. E.( e V 1
8.5
Figure 3. N e I photoelectron spectrum in the 8.5-9.0-eV ionization energy region of the structured product of the F CD,Br reaction at a 0.6-cm mixing distance. The illustrated spectrum is the sum of seven sets of 2500 scans each recorded in 3.3 min with methyl iodide added for calibration (not shown).
+
In order to explore the new product near 8.7 eV, Ne Ior spectra were recorded; a typical survey spectrum, illustrated in Figure 1, was obtained by using a 2.5-cm mixing distance and low fluorine atom concentration. This spectrum reveals a structured product band at 8.72 eV, weak CF signals, strong Br atom bands, and weak HBr signals.22 Increasing the mixing distance increased Br and C F signals at the expense of the 8.72-eV product. The 8.5-10.5-eV region was investigated by using low F atom/high CH3Br partial pressures and a 0.6-cm mixing distance; Figure 2 shows four added sets of 1500 scans recorded in 120 s (22) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S . “Handbook of He I Photoelectron Spectra”; Halstead Press: New York, 1981.
1952 The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
each with CH31calibrant in the sample chamber during the run. The structured product band at 8.61 i 0.01 eV adiabatic and 8.72 i 0.01 eV vertical ionization energies exhibits a single progression with a u’ = 0-1 separation of 860 i 30 cm-’ (measured relative to the CH31 band separation) and a weak band 700 i 50 cm-’ below the adiabatic ionization peak. CD3Br. Similar spectra were recorded the next day for CD3Br under the same reaction conditions. Figure 3 shows only the new product band taken from seven added sets of 2500 scans recorded in 200 s each; again CH31was used as the calibrant. The product band exhibits two isotopic effects: the adiabatic and vertical ionization energies show small shifts to 8.60 f 0.01 and 8.70 f 0.01 eV and two vibrational progressions were observed with u’ = 0-1 separations of 780 i 30 and 1130 i 30 cm-I. In addition a weak band was observed 650 50 cm-’ below the adiabatic ionization. He I a PES were also recorded for the CD3Br reaction at a 5-cm mixing distance with relatively high F atom partial pressure; the structured 8.70-eV band and most of the CD3Br were consumed, and DF was observed at 16.07, 16.33, and 16.59 eV with unreacted F atoms at 17.42 eV. Strong Br atom and weak C F radical products were also ob~erved.l~-~’ The C F first band was carefully calibrated against CH31 and the first two vibronic components were measured at 9.1 1 f 0.01 and 9.33 f 0.01 eV, the same found in the F + CH3Br experiments. CH2Br2. The F CH2Br2reaction was examined in 37 spectra under a variety of reaction conditions. Reaction of low F/high CH2Br2relative partial pressures at a 0.6-cm mixing distance produced a weak new unstructured 8.41-eV band, strong Br atom bands, and a moderately intense H F first band. Increasing the mixing distance decreased 8.41-eV and CH2Br2bands and increased Br atom and H F signals. The new product band was calibrated relative to the first three bands of CH2Br2recorded with He I@radiation at 9.38,8.96, and 8.76 eV apparent ionization energies; these measurements yielded 8.30 0.03 eV adiabatic and 8.41 i 0.03 eV vertical ionization energies. CH3Z. Methyl iodide was also allowed to react with fluorine atoms at several fluorine partial pressures and mixing distances. At a 0.6-cm reagent mixing distance and relatively low F atom partial pressures, the reaction consumed about 40% of the incident CH31 and produced a weak, structureless band at 8.52 f 0.03 eV, a strong, sharp band at 9.84 eV due to the CH3 r a d i ~ a lstrong ,~ sharp I atom signals at 10.45 and 12.15 eV,20strong structured band systems at 10.62 and 11.32 eV due to IF,23 a strong structured band at 13.00 eV in agreement with CH3F,’*and the H F first band. The 11.32-eV IF band was double the intensity of the 16.06-eV H F band. Increasing the mixing distance to 1.5 cm then 3.0 cm decreased and then destroyed the 8.52-eV band, decreased the CH3 radical and CH31 bands, and increased the strong CH3F band. CH2Z2. A similar reaction with CH212at low F atom/high CH212partial pressures and mixing the reagents 0.3 cm above the photon beam consumed about 50% of the incident CH21Z,and a typical product spectrum is illustrated in Figure 4. A new structureless band was observed at 8.52 f 0.03 eV vertical ionization energy with an 8.40 i 0.03 eV onset; weak He I@bands of CH212at 8.42 and 8.76 eV apparent ionization energy also contributed to this band. In addition to CH212bands at 9.52, 9.83, and 10.29 eV, sharp, weak new bands appeared at 10.07 and 11.07 0.03 eV, and product bands due to IF and I atoms were observed at 10.62 and 11.32 and at 10.45 and 12.15 eV, respectively; a weak H F band was detected at 16.06 eV (not shown). The 8.52-eV product band was calibrated by using the sharp CH212and I atom bands at 9.83 and 12.15 eV, respectively.’8,20 Intensity measurements show that the 11.32-eV IF band was seven-fold stronger than the 16.06-eV H F band. Finally, increasing the mixing distance decreased CH212 bands, destroyed the new 8.52-eV product band, decreased the I, IF, and 10.07-eV bands, and increased the sharp 11.07-eV band due to HI.22
Andrews et al. IF 2500
I1/1
z 3
u 0
*
+
*
*
(23) Colburn, E. A.; Dyke, J. M.; Fayad, N. K.; Morris, A. J . Electron Spectrosc. Rel. Phenom. 1978, 14, 443.
0 12
11
10
9
1.E.leV)
Figure 4. He I photoelectron spectrum in the 8-13-eV region of the F + CH212reaction products at a 0.3-cm mixing distance obtained by using relatively low F atom and relatively high CH212partial pressures. The dashed spectrum is the He I@spectrum recorded for CH,I, with normalized intensity relative to He Ia bands for CH21zin the reaction
mixture.
Discussion The new product bands observed in the present fluorine atom reactions will be assigned to halomethyl radicals. Structure and bonding in these radicals and reaction mechanisms will also be considered. CH3Br and CD3Br. The new structured 8.72-eV band is assigned to the CH2Br product of reaction 1. This reaction is slower F + CH3Br H F + CH2Br (1)
-
+
than the analogous F CH, reaction ( k , = 0.2kcH, at 25 0C)24 which provided CH3for PES observation.6 Changing the reaction conditions to longer mixing distances and higher F atom partial pressures reduced the 8.72-eV band intensity and increased HF, Br, and HBr signals, which is consistent with the 8.72-eV band being due to a primary reaction product transient species. The latter species are products of secondary reactions suggested in (2). F
+ CH2Br
--
-
[CH2FBrIt
+
CHzF
+ Br
(2a)
C H F HBr (2b) The formation of a new C-F bond in the [CH2FBr] intermediate provides more than enough activation energy for elimination of Br or HBr. Similar “atom-switching”reactions have been proposed in the secondary reactions of F atoms with chloromethyl radic a l ~ . ~The * ~relative * ~ ~ strength of the H-Br bond is manifested here in that (2a) is clearly favored over (2b), but, in the analogous F + CH2C1reacti~n,’?~ the HC1 product is favored over chlorine atoms. Unfortunately, CHzF and CHF were not detected in these experiments; these species undergo further reactions with F atoms to give the more chemically stable C F radical, which was produced here as described in the PES study of CH,F.’O The adiabatic ionization energy of CH2Br has not been previously determined directly, but the present 8.61 f 0.01 eV PES value is in good agreement with an indirect determination of 8.65 i 0.12 eV based on the 11.35 f 0.02 eV appearance potentialI3 of CH2Br+ from CH2Br2 and the pyrolysis measurement25 of D(CH2Br-Br) = 2.7 f 0.1 eV. In fact a more accurate D(CH2Br-Br) = 2.74 i 0.03 eV value can be determined from appearance potential and 8.61 i 0.01 eV PES observations. These determinations supercede an earlier 8.34 f 0.1 1 eV electron-impact ionization energy measurement for CH2Br.26 The present observation of CH2Br is supported by the recent assignment of the first photoelectron band of CH2C1 at 8.87 eV with a similar vibronic band which also implies that autoionization does (24) Jones, W. E.; Skolnick, E. G. Chem. Rev. 1976, 76, 563. Bozzelli, J. W.; Kaufman, M. J . Phys. Chem. 1973, 77, 1748. ( 2 5 ) Szwarc, M.; Sehon, A. H. J. Chem. Phys. 1951, 19, 656. (26) Reed, R. I.; Snedden, W. Trans. Faraday Soc. 1959, 55, 876.
PES Study of CH2Br, CD2Br, CHBr,, and CH21 Radicals TABLE I: Vertical Ionization Energies (eV) of Substituted
Methyl Radicals Determined by Photoelectron Spectroscopya X
radical
CH,
F
C1
Br
I
CH,X CHX,
8.51b 7.62b
9.22c
8.87d 8.54d
8.72e 8.4Ie
8.52e
a The ionization energy of CH, is 9.84 eV, ref 4. Dyke, J. M.; Ellis, A. R.; Keddar, N . ; Morris, A. J. Ph s. Chem., in press and unpublished results. Reference 10. $Reference 9. e This
work. TABLE 11: Vibrational Fundamentals (cm-')for
Monohalomethyl Radicals'and Cations X=FF
X=CLb
C-F
C-C1 mode
species
mode
CH,X CD,X CH,X+ CD,Xt
1163 1191
1450
826 186 1040
1530
Reference 10 and references therein. ences therein. This work and ref 1 1. a
X = BrC
C-BI
mode 693 656 860 780
CH, scis 1356 1016 1130
Reference 9 and refer-
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 1953 C-X stretching fundamental is expected on ionization from this orbital. These calculations also provide total electron density differences between radical and cation ground states and show that, although the free radical electron is located mainly on carbon, the electron density changes on C, H, and halogen centers are significant for the cation. This is due to electronic reorganization in the cation, which is responsible for a significant increase in the C-X bond strength in the cations. CH2Br2. The unstructured band at 8.41 f 0.03 eV in CH2Br2 experiments is assigned to the CHBr2 product of reaction 3. This F + CH2Br, H F + CHBr2 (3)
-
band behaved as a primary product transient species with appropriate changes in reaction conditions. Secondary F atom reactions with CHBr2 give Br atoms and other products, which unfortunately escaped detection. Structure was not resolved on the first PES band of CHBr,, presumably owing to the smaller symmetric C-Br, stretching mode for CHBr,'; this is reasonable since the symmetric C-C1 stretching mode for CHClzCwas not as well resolved as for CH2Cl+ in their respective first photoelectron bands.g The 8.30 f 0.03 eV measurement of the adiabatic ionization energy for CHBr2 determined directly from PES is in agreement with a value of 8.3 f 0.1 eV determined indirectly from the 10.70 f 0.02 eV appearance potentialI3 of CHBr2+ from CHBr, and D(CHBr2-Br) = 2.4 f 0.1 eV measured by pyrolysis.25 CH31and CH212. The common product band at 8.52 f 0.03 eV in CH31and CH212experiments is assigned to the CHJ free radical product of reactions 4 and 5. Although CH3 radical and F + CH31 H F CH21 (4)
not alter the CH2Br band profile significantly. Table I compares vertical ionization energies for substituted methyl radicals and shows a small halogen effect. The vibrational structure in the first PES band is appropriate for the ground state of CH2Br+. Since matrix infrared and ESR evidence indicates a planar CH,Br radical and the CH2Br+ground state is expected to be planar as well, ionization should produce IF CH2I F + CHZI, (5) a change in only the C-Br bond as the outermost half-filled orbital I F products were observed for the F + CHJ reaction, a comis (C2p-Br4p)antibonding 7~ in character. The observation of a parable H F signal was also observed and the CH21radical is an single progression in the C-Br stretching mode is in accord with expected product of the reaction. Reaction 5 gave an increased this expectation. The 860 f 30 cm-' interval in the first PES band yield of the CH21band as the increased yield of I F relative to H F of CH2Br+ is larger than the 693-cm-l C-Br stretching mode for attests. The observation of the strongest band of vinyl iodide, the radical," which is the relationship found for other halomethyl CHzCHI, at 10.07 f 0.03 eV29provides evidence for CH21radicals cations and Matrix infrared spectra of CHBr2 and through the recombination reaction 6. The sharp weak secondary CHBr,+ show an increase in the antisymmetric C-Br, stretching mode from 786 cm-' for the radical to 897 cm-I for the c a t i ~ n . ~ ' , ~ ~ CH21 + CH21 [CH21CHzI]t H I + CH2CHI (6) The first band in the PES of CD2Br provides isotopic data in F + CHJ [CH2FIIt CHzF + I support of the above assignment. Table I1 compares vibrational (7a) data for monohalomethyl radicals and cations. The 780 f 30 cm-I CHF HI (7b) first vibrational interval for CD,Br+ is in accord with the isotopic shift for the free radical;." The dppearance of a second vibrational reaction product band at 11.07 f 0.03 eV is due to from interval in the PES of CD2Br is due to mixing of the symmetric reactions 6 and 7b. It is clear from the spectrum that (7a) is the CD2 scissor bending and C-Br stretching modes, which is in part predominant secondary reaction. Finally, the observation of CH,F responsible for the deuterium shift in the C-Br stretching mode. in the F + CH31 system, and the increase in CH,F yield with The 1130 f 30 cm-I interval is assigned to the CD2scissor bending longer reaction time, may provide evidence for the Walden inmode for ground-state CD2Br+,which is higher than the 1016-cm-' version reaction in the gas phase, a process addressed in a recent value for CD,Br, in part due to interaction with the C-Br communi~ation.~~ stretching mode. In the CH2Br+cation, the CH2 scissor mode The 8.40 f 0.03 eV adiabatic ionization energy for CH21 is expected slightly above the 1356-cm-*value for CH2Br in solid observed here is in agreement with an indirect determination of argon," and interaction with the 860 f 30 cm-I C-Br stretching 8.33 f 0.07 eV based on the appearance of CHJ+ from CH21z mode of CH2Br+is not sufficient to give observable intensity to at 10.55 rt 0.02 eVI3 and a recent 2.22 f 0.05 eV spectroscopic this progression in the first PES band of CH2Br. mea~urementl~ of D(CH21-I). The latter bond energy can also The weak bands below the adiabatic ionization energies for be determined from the CHJ+ appearance potential and the CH2Br and CD2Br are assigned to hot bands in the C-Br fun8.40-eV onset of the CH,I PES band; a D(CH,I-I) = 2.13 f 0.05 damental of the radicals. Their separations from the adiabatic eV value is obtained. The steady decrease of ionization energy component are in agreement with these fundamentals for the for the CHzX radicals with increasing size of X shown in Table radicals." I supports the CH21 assignment, and the small halogen effect The increased C-Br stretching fundamentals at 860 and 780 further suggests that the free radical electron is located primarily f 30 cm-' for CH2Br+and CD2Br+,respectively, as compared on carbon. to these modes for CH2Br and CDzBr at 693 and 656 cm-I in solid Conclusions argon," demonstrate a substantial increase in C-Br bond strength The fluorine atom-methyl bromide reaction produced CH2Br in the cations. Ab initio c a l ~ u l a t i o n performed s ~ ~ ~ ~ for CH2F and free radical for photoelectron spectroscopic observation-of 8.6 1 CH2Cl indicate that the half-filled orbital in the radicals is es& 0.01 eV adiabatic and 8.72 f 0.01 eV vertical first ionization sentially an antibonding (CzP-Xnp) K orbital, and an increase in
--
--
+
(27) Carver, T. G.; Andrews, L. J. Chem. Phys. 1969, 50, 4223. (28) Andrews, L.; Prochaska, F. T.; Auk, B. S. J. Am. Chem. SOC.1979, 101, 9.
+ +
+
--
(29) Boschi, R. A. A.; Salahub, D. R. Can. J. Chem. 1974, 52, 1217. (30) Venkitachalam, T. V.; Das, P.; Bersohn, R. J . Am. Chem. SOC.1983, 105, 7452.
J. Phys. Chem. 1984, 88, 1954-1959
1954
band at 8.52 f 0.03 eV for the first vertical ionization energy of CH21. These direct observations of ionization energies for CH2Br, CHBr2, and CH,I are in agreement with indirect determinations from appearance potential and bond dissociation energy measurements.
energies. The u' = 0-1 separation of 860 f 30 cm-' in the structured first PES band is due to the C-Br stretching fundamental of CH2Br+;this interval shifts to 780 30 cm-' for CD2Br+ and a weaker 1130 f 30 cm-' progression in the CD, scissor mode appears due to interaction between these symmetric modes for CD2Brf. Substantial increases in the C-Br stretching fundamentals in the cations as compared to the radicals is due to increased net C-Br bonding in the cation. The fluorine atommethylene bromide reaction produced a transient band at 8.41 f 0.03 eV, which is assigned to the first vertical ionization of CHBr,. Similar reactions with CH31and CHzIzgave a common
*
Acknowledgment. The authors gratefully acknowledgefinancial support for this research from the S.E.R.C. (U.K.). L.A. acknowledges a Sesquicentennial Associateship from the University of Virginia, a Senior Visiting Fellowship from the S.E.R.C., and a Fulbright Senior Research Fellowship.
Mechanism of Ammonia Decomposition on (I00) Oriented Polycrystalline Tungsten and Single-Crystal W (100) Allyson P. C. Reed and Richard M. Lambert* Department of Physical Chemistry, University of Cambridge, Cambridge CB2 I EP, England (Received: August I O , 1983)
The interaction of NH3 with single-crystal W(100) and (100) oriented polycrystalline W has been characterized by using LEED, Auger spectroscopy,and temperature-programmedreaction measurements. At 300 K chemisorption occurs rapidly (sticking probability N 0.4) with partial decomposition of the adsorbate; the overall decomposition which takes place upon subsequent heating exhibits two distinct stages. At 600 K decomposition (evolving only H2) and desorption of NH, become competing processes. Preadsorbed N inhibits the decompositionvery efficiently, but preadsorbed H has no such effect; D, and NH3 do not appear to compete for a common adsorption site. Rapid equilibration between H (from NH,) and coadsorbed D occurs in this regime, although exchange into the N-H bond itself is slow. The stoichiometry of this intermediate surface phase and the mechanism of its subsequent decomposition are investigated. Simultaneous evolution of N, and a small amount of Hz occurs above 750 K. The principal pathway for this stage is shown to involve a small amount of NH, species in the presence of an otherwise completely dehydrogenated layer. In contradistinction with earlier work, no evidence was found for the presence of stable uniform intermediate phases such as NH or NH2; these apparent discrepancies are accounted for. Auger spectroscopy reveals the onset of W nitridation under reaction conditions, and experiments with N2H, confirm certain aspects of the mechanism proposed here for NH, decomposition.
Introduction The chemisorption and decomposition of NH, on W is a classical problem in surface chemistry which has been extensively investigated from the earliest days of the subject. However, no coherent picture has emerged and the literature contains many disagreements on almost every point concerning the identity of the crucial surface species, the stoichiometry of various surface phases, the formation of surface compounds, and the mechanism of the reaction.' The state of understanding of the Fe-NH, interaction stands in marked contrast with this2,, thanks to the application of modern physical techniques which have been used to characterize directly the species present at the metal surface. Much of the W-NH3 work has relied exclusively on observations of the rate of production of N2 and H2; the resulting kinetic rate laws cannot be interpreted unambiguously in terms of a particular molecular pathway for the surface reaction. In not a few cases, a close reading of the published work suggests that experimental artefacts may have led to significant difficulties. Ammonia is notoriously difficult to handle in vacuum systems. Conventional dosing techniques can lead to extensive decomposition of the reactant before it encounters the specimen surface, and the resulting very strong chemisorption of nitrogen on tungsten could undoubtedly have a serious effect on the results. The problem is exacerbated by the lack of surface analytical measurements in these studies. (1) Lambert, R. M.; Bridge, M . E. "The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis"; Elsevier: New York, 1983; Vol. 3B. (2) Ertl, G. Catal. Reu. Sci. Eng. 1980, 21, 202. (3) Spencer, N. D.; Somorjai, G. A. J . Phys. Chem. 1982, 86, 3495.
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Opinion is divided as to whether or not chemisorption at 300 K occurs with dissociation of the adsorbate; a brief UPS investigation4 led to the conclusion that dissociative and nondissociative chemisorption occur simultaneously. Initial sticking probabilities ranging from >0.45 at 300 K5 to at 700 K6 have been reported. Most workers agree that some desorption occurs below 700 K leaving an intermediate surface phase which decomposes above 800 K. The nature of this intermediate has been the subject of much controversy, the results of an early single-crystal study on W(100) by Estrup and AndersonS proving particularly contentious. Their main conclusion was that a layer of bridge-bonded NH2 is formed, and that this decomposes leading to simultaneous evolution of N 2 and H2. The results of later work' on W(211) supported this view, but FEM and desorption measurements by Hansen and his c o - w o r k e r ~suggested ~~~ a stoichiometry of NHo for this phase. A recent molecular beam studylo identified the surface intermediate as N H and the decomposition reaction was ascribed to a most unusual reaction involving this species ("(a) + ",(a) N2(g) + 2H2(g)). In addition to NH, species (0 < x < 2) the decomposition of various tungsten nitride/hydride compounds has been proposed6,8J1J2at the rate-limiting step.
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(4) Egelhoff, W. F.; Linnett, J. W.; Perry, D. L. Faraday Discuss. Chem. SOC.1975, 60, 127.
(5) Estrup, P. J.;f Anderson, J. J. Chem. Phys. 1968, 49, 523. (6) Peng, Y. K.; Dawson, P. T. J . Chem. Phys. 1971, 54, 950. (7) May, J. W.; Szostak, R. J.f Germer, L. H. Surf. Sci. 1969, 15, 37. (8) Dawson, P. T.; Hansen, R. S. J . Chem. Phys. 1968, 48, 6 2 3 . (9) Matsushita, K.; Hansen, R. S. J. Chem. Phys. 1969, 51, 472. (10) Steinbach, F.; Schutte, J. Surf. Sci. 1979, 88,498.
0 1984 American Chemical Society
