- Email: [email protected]
A study of the phenyl radical by vacuum ultraviolet photoelectron spectroscopy
261
Chemical Physics 115 (1987) 261-267 North-Holland, Amsterdam
A STUDY OF THE PHENYL RADICAL BY VACUUM ULTRAVIOLET PHOTOELECI’RON SPECTROSCOPY V. BUTCHER Department
M.L. COSTA ‘, J.M. DYKE *, A.R. ELLIS 3 and A. MORRIS
of Chemistry,
The Universi&
Southampton
SO9 5NH, UK
Received 2 February 1987
Part of a photoelectron band assigned to the phenyl radical, produced from the F + C,H, reaction, has been recorded. The adiabatic ionization energy was measured as 8.32 k 0.04 eV. Two vibrational components separated by 2790 + 100 cm-’ were observed in the C,H, band corresponding to excitation of a C-H stretching mode in .the ion. For C,D, this separation decreases to 237OkllO cm-t. Ab initio CI calculations on the low-lying CsHl states, 3B, and ‘At, show that the ground state of C,H: is ‘A, and the first adiabatic ionization energy is estimated as 8.OkO.l eV. Evidence is presented to show that the observed photoelectron band should be assigned to the ionization of CeH: (a 3B,) + CsH,(X ‘AI). In the F+ C,D, reaction, a photoelectron band assigned to C,DsF is observed at 7.8OkO.02 eV which is seen only very weakly in the F+ C,H, reaction.
1. Introduction The phenyl radical, C,H,, is the simplest aromatic radical. Theoretical and experimental evidence [l-6] shows conclusively that its ground electronic state is a 2A1 state, with the unpaired electron residing in an a,, u-type molecular orbital. In contrast, some uncertainty exists concerning the ground state of the phenyl cation. Published ab initio molecular orbital calculations performed for C&H: indicate that at the SCF level, the ground state is ‘B, with the ‘A, state approximately 0.3 eV higher in energy, where both ions are at their computed minimum energy geometries [7]. However, approximate allowance for the electron correlation energy difference between these two states by use of the known 3B,-1A, separation in CH, and comparison with the results of equivalent SCF molecular orbital calculations on CH,
’ Present address: Centre of Mass Spectrometry, University of Lisbon, INIC, Lisbon, Portugal. 2 To whom correspondence should be addressed. 3 Present address: Central Electricity Research Laboratories, CEGB, Leatherhead, Surrey, UK.
3B, and ‘A, leads to a corrected C,Hf 1A,-3B, minimum energy separation of 0.87 eV with the ‘A, state lying lower [7]. Semi-empirical calculations also indicate that the ground state of C,Hl is ‘A, [S]. Hence although the available evidence suggests that C,HT has a ‘A, ground state this result cannot be viewed as firmly established. The photoelectron spectrum of the phenyl radical was initially recorded with the aim of characterizing the ground state of C,Hf although it became clear at an early stage that ab initio calculations which include the effects of electron correlation would be necessary to compute vertical ionization energies of the phenyl radical and to establish the ground state of C,Hl. A number of measurements of the first adiabatic ionization energy of GH, have been made previously. Values of 8.1 f 0.1 eV and 8.8 + 0.3 eV have been derived by photoionization mass spectrometry [9] and electron impact mass spectrometry [lo], whereas use of available heats of formation of AHz2,,,of C,H, [11,12] and C,Hc [13-151 leads to a value of 8.33 + 0.15 eV. The present work uses the reaction of fluorine atoms with benzene to generate phenyl radicals in the gas phase. Previous kinetic studies [16-211
0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
262
V. Butcher et al. /A
demonstrate that this reaction can proceed via two distinct routes, proton abstraction to yield HF and C,H, and fluorine atom addition to yield C,H,F, which subsequently decomposes to give GH, and HF. Infrared chemiltinescence studies [16-191 of the vibrationally excited HF fragment indicate that, at room temperature, both the F + C,H, addition and abstraction reactions are rapid, with the initial addition reaction being approximately three times faster than the abstraction reaction.
2. Experimental In this work the phenyl radical was generated by the rapid reaction of atomic fluorine with benzene (Analar grade, BDH). Fluorine atoms were produced by a microwave discharge (2.45 GHz) of 5% molecular fluorine in helium. Photoelectron spectra were recorded using a multi-detector photoelectron spectrometer which has been described elsewhere [22] and the most intense spectra of the phenyl radical were recorded at a reagent mixing distance of 1.0-2.0 cm above the photon beam. Typical resolution under operating conditions as measured for argon (fwhm) using HeIol (21.22 eV) radiation was 25-30 meV. Similar experiments were performed with deuteriated benzene (C,D, Spectroscopic Grade BDH). A new band attributed to a short-lived molecule (C,D,F) was observed in this case and the most intense spectra of this feature were observed at a reagent mixing distance of 0.0-0.5 cm above the photon beam. All spectra were calibrated using the HeIa photoelectron spectrum of methyl iodide.
3. Computational
details
The ground state configuration of C,H,(X *At) is known from previous calculations [1,5] to be . . . (10a,)2(7b2)2(la2)2(2br)2(lla1)1, where the unpaired electron resides in the lla, level, a u-type molecular orbital. The molecular orbitals with a, and b, symmetry are all in the
study of
thephenyl
radical
plane of the phenyl ring whereas the b, and a, orbitals are perpendicular to the plane and thus form the T-system of the radical. In this work, vertical ionization energies have been calculated by performing restricted ab initio SCF molecular orbital calculations [23] for C,H,(X 2A1) and the C,Hc states ‘Al, lp3B,, ‘,3A2 (2b,)-‘, and rp3B2 arising from the (lla)-‘, ionizations respectively. (la,))’ and (7b,)-’ Gaussian basis sets (C[4s, 2p] and H[3s]) taken from ref. [24] augmented by polarization functions on each centre 1251 #’ were used in these calculations, which were carried out using the minimum energy geometry of C,H,(X 2A1) obtained in ref. [l] via a series of ab initio calculations with an STO-3G basis set. In this present work, these SCF calculations were supplemented by configuration interaction calculations on C,H,(X 2A1) and the above C,Hz states using the ATMOL direct CI method [26]. In all calculations, the lowest six molecular orbitals which correspond to C 1s orbitals were held frozen and all orbitals with energies greater than 1.7 au in C,H,(X 2A1), were also frozen. All single and double excitations were then considered within this active space in the CI expansions. Quadruple excitations were also allowed for by applying Davidson’s correction [27]. The corrected ASCF vertical ionization energies obtained from these calculations are given in table 1. Also with this method adiabatic ionization energies to the lowest C,Hf ‘A, and 3Bl states were calculated at the SCF CI level using the minimum energy geometries calculated with an STO-3G basis set [7]. The values obtained are also included in table 1. In order to assist in the assignment of the observed vibrational structure in the first photoelectron band of the phenyl radical, force field calculations were performed for C,H, and C,D, with the computed phenyl geometry of, ref. [l]. A Urey-Bradley force field was used in these calculations with force constants taken from those used to reproduce the experimental vibrational frequencies of pyridine and deuteriated pyridine [28]. For C&H,, five vibrational frequencies were computed
#’ The exponents used were C d 0.63, H p 0.88.
263
V. Butcher et al. / A stu& of the phenyl radical Table 1 Computed vertical ionization energies (ev) of C,H,(X
*At) a)
Orbital ionized b,
Ionic state
ASCF ‘) value
A SCF + CI ‘) value
Experimental
2b,
3%
lla, 2b,
lAt lB, 3A2
la2
‘A2
8.62 (8.53) 8.79 (7.92) 9.22 8.87 8.87
8.67+0.02 -
la2
8.25 (8.24) 9.11 (8.19) 8.73 8.54 8.62
000
V,F
C&H,
600
_
a) See text for details of these calculations. In this work, the total energy computed for C,H: (3B1) with the neutral C,H,(X ‘Ai) STG-3G geometry at the SCF level is - 229.780392 au; at the CI level this drops to -230.243629 au. The SCF energy computed with a 4-31G basis in ref. [7] with an optimized C,H: (3Bt) geometry determined with an STO-3G basis is -229.43357 au (the corresponding SCF energy computed in this work is - 229.780619 au). b, The electronic configuration of CeH,(X ‘A,) is (lOa,)‘ (7b2)2(la2)2(2b,)2(11a,)‘. Ionization from the 7b, molecular orbital gives rise to ionic states above 11.0 eV ionization energy. ‘) Values in parentheses are calculated adiabatic values.
above 3000 cm- ’ and approximate to C-H stretching modes. They are 3044 (a,), 3047 (a,), 3051 (a,), 3045 (b2) and 3049 cm-’ (b,). In C,D,, the corresponding values are 2235, 2243 and 2252 cm-’ (ai) and 2235 and 2240 cm-’ (b2).
4. Results and discussion The He1 photoelectron spectrum recorded for the reaction of fluorine atoms with benzene over the ionization energy range 4.5-11.0 eV is shown in fig. 1. This spectrum represents 500 scans accumulated in a time of 4 min. The sharp feature at an apparent ionization energy of 4.99 eV has been identified as helium, from the F,/He mixture used, ionized by He11 radiation. The bands in the 9.0-10.0 eV ionization energy region have been assigned to unreacted benzene [29,30] and monofluorobenzene [31,32]. Also present are weak features in the 8.0-9.0 eV region which show a maximum in intensity at a reagent mixing distance of 1.0-2.0 cm above the photon beam. An expanded signal averaged spectrum of the 7.5-9.2 eV ionization energy region collected over 2500 scans is shown in fig. 2, recorded at a reagent mixing
HakieJl)
h
200 C,H, A___-
400
9
10
11
0
7 IONIZATION
6
5 ENERGY
(cd;
Fig. 1. The He1 photoelectron spectrum recorded for the F+C,H, reaction over the ionization energy range 4.5-11.0 eV. The reagent mixing distance above the photon beam was 1.5 cm. Ordinate: counts, abscissae: ionization energy (ev).
distance of 1.5 cm above the photon beam. The vertical ionization energy of the band shown in this figure has been measured as 8.67 f 0.02 eV and the adiabatic ionization energy was measured as 8.32 + 0.04 eV. The separation of the two components was measured as 2790 f 100 cm-‘. Having inspected many spectra of the type shown in fig. 2, it is clear that the two components observed in this spectrum are part of a vibrational series which continues under the bands assigned to benzene and monofluorobenzene. The adiabatic ionization energy of 8.32 k 0.04 eV compares
600
094
I
99
86
82
l0NZLL
EN&
WI
Fig. 2. The He1 photoelectron spectrum recorded for F + GH, over the ionization energy range 7.5-9.2 eV. The reagent mixing distance above the photon beam was 1.5 cm. Ordinate: counts, abscissae: Ionization energy (ev).
264
V. Butcher et al. / A study of the phenyl radical
favourably with values of 8.1 + 0.1 eV and 8.33 + 0.15 eV derived from photoionization mass spectrometry [9] and heats of formation AHf(298) of C,H, and C,Hl [ll-151 respectively. This evidence, combined with the transient nature of the observed components, has been used to assign the band to ionization of the phenyl radical. Unfortunately, none of the expected higher bands of this radical (see table 1) were observed experimentally because they are thought to overlap with the much more intense bands due to benzene and monofluorobenzene. Also, at reagent mixing distances greater than 2.0 cm above the photon beam, interpretation of the experimental spectra was further complicated by contributions from higher fluorinated benzenes, which all have bands at greater than 9.0 eV ionization energy [32,33]. The spectra were also checked for bands arising from biphenyl [33,34] and benzyne [35], two possible secondary reaction products, but no evidence of contributions from these molecules was found. This was probably because the phenyl radical was produced using relatively high atomic fluorine: benzene partial pressure ratios and the main fate of the phenyl radical appeared to be reaction with fluorine atoms to give mono-fluorobenzene. Similar spectra to that shown in fig. 2 for F + C,H, were also obtained for the F + C,D, reaction. Fig. 3 shows a spectrum obtained for the F + C,D, reaction, recorded over the ionization energy range 6.5-9.2 eV at a reagent mixing distance of 0.7 cm above the photon beam. It consists of a total of 1250 scans accumulated in a time of 2.5 min. As in the F + C,H, reaction, two components were observed in the 8.0-9.0 eV ionization energy region which were assigned to C,D,. The adiabatic component was measured as 8.37 * 0.04 eV, the vertical component measured as 8.67 f 0.03 eV and the vibrational separation measured as 2370 + 110 cm-‘. Fig. 3 shows, however, an extra band in the 7.0-8.0 eV region which was observed much more weakly in the F + C,H, reaction. This feature was only observed at reagent mixing distances of less than 1 cm above the photon beam and clearly exhibited a different mixing distance profile from the band assigned to the phenyl radical showing maximum intensity at shorter mixing distances (i.e. reaction times) than
* eool ,
$
600.
I O94
!)
C, D,
,1,,
90
80
82
70’74
70
66
IONIZATION ENERGY b/v)
Fig. 3. The He1 photoelectron spectrum recorded for F + C,D, over the ionization energy range 6.5-9.2 eV. The reagent mixing distance above the photon beam was 0.7 cm. Ordinate: counts, abscissae: ionization energy (ev).
the phenyl radical components. The band maximum of this feature was measured as 7.80 f 0.02 eV. In accordance with previous spectroscopic [36,37] and kinetic studies [16-211 of the F + C,H, and F + C,D, reactions, this band was assigned to the first photoelectron band of the l-fluorocyclohexadienyl radical, C, Ds F. The reason for the observation of C,D,F in the F + C,D, reaction with more intensity than C,H,F in the F + C,H, reaction is not immediately apparent. However, the reaction of fluorine atoms with aromatic molecules and olefins has been shown to follow two distinct reaction paths [16-211; the addition reaction which produces a fluorinated intermediate, and the direct hydrogen abstraction reaction which produces a polyatomic radical and HF. Simple collision theory shows that neither the initial addition step nor the direct proton abstraction reaction are likely to exhibit any appreciable isotopic differences in rate constant between F + C,H, and F + C,D,. In contrast, once the addition products are formed the decomposition rate of C$H,F* is expected to be much greater than that of C,D,F*. This conclusion is reached from the results of kinetic studies on the F + C,H, and F + C,D, reactions [38-401 in which C,H,F* was found to decompose much faster than C2D4F*, and from relative rate constants for C,H,F* and &D,F* decomposition expected from RRKM calculations [40,41]. This
V. Butcher et al. / A sttiy
would then provide support for the observation of C,D,F in the F + C,D, photoelectron spectrum with more intensity than C,H,F from the F + C,H, spectrum. Assignment of the vibrational separation observed in the first photoelectron band of the phenyl radical is relatively straightforward because of its high frequency and experimental deuterium shift. It is attributed to excitation of a C-H stretching mode of a, symmetry in the ion, assuming that both the neutral molecule and cation in their ground electronic states have planar C,, equilibrium geometries. Although the frequencies of the C-H stretching modes in C,H,(X 2A1) and C,D,(X 2A1) have not been measured experimentally, some support for the above assignment is provided by the force field calculations performed in this work which gave three a, modes at 3044, 3047 and 3051 cm-’ for C,H, which decrease to 2235, 2243 and 2252 cm-’ on deuteration. The corresponding values for the symmetric C-H stretching mode in C,H, and C,D,, as obtained from liquid phase Raman measurements [42], are 3062 and 2293 cm-’ respectively. Returning to the issue of the ground state of the phenyl cation and the assigmnent of the observed phenyl photoelectron band, table 1 shows that at the SCF level the vertical ionization energy to the 3B, ionic state is lower than that to the ‘Ai ionic state. This is also true after configuration interaction (CI) has been performed for C,H, X 2A1 and the 3B, and ‘A, ionic states. However, previously it has been estimated [7] that the ‘A, state would be the ground state of C,Hf . A ‘A, ground state was also expected from INDO semiempirical molecular orbital calculations on C,Hc [S] and some qualitative evidence from solution experiments [43]. With this in mind, adiabatic ionization energies to the C,H,’ ‘Ai and 3B, states were computed at the SCF CI level using their respective minimum energy geometries calculated with an STO-3G basis set [7]. All other computational details were as adopted previously. The results of these calculations are also shown in table 1. These calculations clearly show that the ground state of C,H: is the *A, state with the ‘B, state = 0.61 eV higher in energy, where both states are at their STO-3G minimum energy geom-
of the phenyl radical
265
etries. This separation compares with a value of 0.87 eV estimated previously from 4-31G SCF energies calculated at the STO-3G optimized cation geometries corrected empirically by the known relative correlation energies in CH, 3B, and ‘A,
171.
The calculated ionization energies presented in table 1 have important implications for the assignment of the phenyl photoelectron band observed in this work and the following pieces of evidence should be considered: (1) For the ionization C,Hl(3B,) + C,H, (X 2A1), the difference between the adiabatic and vertical ionization energy is expected to be small as the STO-3G minimum energy geometries for both states are very similar [1,7]. The configuration interaction calculations give 0.09 eV for this separation. However, this value neglects zero-point vibrational contributions and any residual correlation energy contributions not accounted for by the CI calculations. The calculated value is also dependent on using STO-3G geometries and will obviously change when more reliable equilibrium geometries become available. Nevertheless, for C, H:(3B1) 6 C,H,(X 2A1), these calculations clearly indicate that the separation of the adiabatic and vertical ionization energies is small for this ionization. (2) In contrast, for the process C,Hl(‘A,) t C,H,(X’A,), the CI calculations yield a much larger adiabatic-vertical ionization energy difference of 0.87 eV. This arises because the STO-3G minimum energy geometries of C,Hz(lA,) and C,H,(X 2A1) are very different. The largest change occurs in the C-C-C angle at the radical centre which increases from 123.7O to 144.9O on ionization [1,7] and as a result the corresponding broad photoelectron band is expected to show extensive vibrational structure in the ring bending modes of a, symmetry. The 3A2 and ‘A2 states arising from the (la,))’ ionization are also expected to make contributions above = 8.7 eV ionization energy (see table 1). (3) Experimentally, the observed phenyl envelope shows only two components with the second component more intense than the first. Although a third component is not observed as it is almost certainly masked by the first band of unre-
266
V. Butcher et al. / A study of the phenyl radical
acted benzene, it is clear from figs. 2 and 3 that it must be weaker than the second component. The peak maximum of this second component, 8.67 + 0.02 eV, is also in reasonably good agreement with the computed vertical ionization energy for the process C,Hc(3B,) + C,H,(X 2A1) of 8.62 eV. On the basis of this evidence, the observed phenyl components are assigned as vibrational components of the ionization C,Hc(a 3B,) +- C,H, (X 2At). This assignment means that the first band of the phenyl radical, corresponding to the process C,Hl(X ‘A,) +- C,H,(X 2A1), is not observed experimentally. From table 1, this ionization is expected to give a broad band with an adiabatic ionization energy of 7.92 eV and a vertical ionization energy of 8.79 eV. The reason why this is not seen in fig. 2 is presumably because the Franck-Condon factors in the 7.9-8.6 eV ionization energy region are too low. Some support for this suggestion is provided by the fact that the first adiabatic ionization energy measured by photoionization mass spectrometry, 8.1 k 0.1 eV [9], is lower than the value measured here of 8.32 f 0.04 eV for the C,Hc(a 3B,) +- C,H,(X 2A1) process. Using the heat of formation (AH:2,,,) of the phenyl radical, C,H,(X 2A1), of 3.37 + 0.08 eV [11,12], the adiabatic ionization energy of the second band of C,H, measured in the work allows the heat of formation of C,Hc(a 3B,) (AHg298j) to be determined at (11.69 f 0.12) eV. If the first adiabatic ionization energy of C,H, is taken as 8.0 f 0.1 eV, a value consistent with the calculations performed in this work and the analysis of the photoelectron spectrum, AHi2,,, of C,H: (X ‘A,) can be calculated as 11.37 f 0.18 eV. In conclusion, part of the first photoelectron band of the phenyl radical has been recorded and the spectrum obtained has been interpreted in terms of the ionization C,H:(a 3B,) t C,H, (X 2A1). On the basis of ab initio configuration interaction calculations, the first adiabatic ionization energy, corresponding to the process C,H:(X ‘Ai) + C,H,(X 2A1), is deduced to be 8.0 f 0.1 eV. Clearly, however, higher resolution spectroscopic studies will be required to characterize the X’A, and a3B, states of the phenyl cation and determine the separation of their zeroth
vibrational levels more accurately. It is hoped that this present work will stimulate interest in this area.
Acknowledgement The authors gratefully acknowledge financial support for this research from the SERC. One of us (MLC) also acknowledges support from the Calouste Gulbenkian Foundation (Portugal).
References 111R.P. Johnson, J. Org. Chem. 49 (1984) 4857. 121G. Porter and B. Ward, Proc. Roy. Sot. A 287 (1965) 457. t31 G. Porter and B. Ward, Proc. Chem. Sot. (1964) 288. [41 N. Ikeda, N. Nakashima and K. Yoshihara, J. Am. Chem. Sot. 107 (1985) 3381. and D.W. Brown, J. Phys. Chem. 87 (1983) 1553. WI P.H. Kasai, E. Hedaya and E.B. Whipple, J. Am. Chem. Sot. 91 (1969) 4364. t71 J.D. Dill, P. von R. SchIeyer, J.S. BinkIey, R. Seeger, J.A. Pople and E. Haselbach, J. Am. Chem. Sot. 98 (1976) 5428. PI H.H. Jaffe and G.F. Koser, J. Org. Chem. 40 (1975) 3082; E.M. Evleth and P.M. Horowitz, J. Am. Chem. Sot. 93 (1971) 5636. t91 Y.L. Sergeev, M.E. Akopyan and F.I. Vilesov, Opt. Spectry. (USSR) 32 (1972) 230. WI R.A.W. Johnstone and F.A. Mellon, J. Chem. Sot. Faraday Trans. II 68 (1972) 1209. VI G.A. Chamberlain and E. Whittle, Trans. Faraday Sot. 67 (1971) 2077. WI A.S. Rodgers, D.M. Golden and S.W. Benson, J. Am. Chem. Sot. 89 (1967) 4578. [I31 Y. Mahnovich and C. Lifshitz, J. Phys. Chem. 90 (1986) 2200. 1141 R.C. Dunbar and J.P. Honovich, Intern. J. Mass Spectrom. Ion Phys. 58 (1984) 25. v51 J. Damracher, H.M. Rosenstock, R. Buff, A.C. Parr, R.L. Stockbauer, R. Bombach and J.P. Stadelmann, Chem. Phys. 75 (1983) 23. P61 D.J. Smith, D.W. Setzer, K.C. Kim and D.J. Bogan, J. Phys. Chem. 81 (1977) 898. P71 D.J. Bogan and D.W. Setzer, J. Chem. Phys. 64 (1976) 586. WI J.G. Moehlmann and J.D. McDonald, J. Chem. Phys. 62 (1975) 3061. [I91 J.G. MoehImann and J.D. McDonald, J. Chem. Phys. 62 (1975) 3052. [20] K. Shobatake, J.M. Parson, Y.T. Lee and S.A. Rice, J. Chem. Phys. 59 (1973) 1427.
[51 J. Pacansky
V. Butcher et al. / A study of the phenyl radical
[21] J.A. Cramer and F.S. Rowland, J. Am. Chem. Sot. 96 (1974) 6579. [22] A. Morris, N. Jonathan, J.M. Dyke, P.D. Francis, N. Keddar and J.D. Mills, Rev. Sci. Instrum. 55 (1984) 172. [23] V.R. Saunders and M.F. Guest, ATMOL Reference Manual, SERC, Didcot, UK (1976). [24] T.H. Dunning, J. Chem. Phys. 53 (1970) 2823. [25] B. Roos and P. Siegbahn, Theoret. Chim. Acta 17 (1970) 199. [26] V.R. Saunders and J.H. van Lenthe, Mol. Phys. 48 (1983) 923. [27] E.R. Davidson, The world of quantum chemistry, eds. R. Daudel and B. Pullmann (Reidel, Dordrecht, 1974); S.R. Langhoff and E.R. Davidson, Intern. J. Quantum Chem. 8 (1974) 61. [28] G. Zerbi, B. Crawford and J. Overend, J. Chem. Phys. 38 (1963) 127. [29] L. Karlsson, L. Mattson, R. Jadmy, T. Bergmark and K. Siegbahn, Physica Scripta 14 (1976) 230. [30] L. Asbrink, 0. Edqvist, E. Lindolm and L.E. Selin, Chem. Phys. Letters 5 (1970) 192. [31] P. Natalis, J.E. Collin and T. Momigny, Intern. J. Mass Spectrom. Ion Phys. 1 (1968) 327. [32] D.G. Streets and G.P. Ceasar, Mol. Phys. 26 (1973) 1037. [33] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, Handbook of He1 photoelectron spectra of funda-
[34] [35] [36] [37] [38] [39] (401
[41] [42]
[43]
26-l
mental organic molecules (Japan Scientific Societies Press, Tokyo, 1981). J.P. Maier and D.W. Turner, Faraday Discussions Chem. Sot. 54 (1972) 149. M.J.S. Dewar and T.P. Tien, J. Chem. Sot. Chem. Commun. (1985) 1243. M.E. Jacox, J. Phys. Chem. 86 (1982) 670. E.L. Co&ran, F.J. Adrian and V.A. Bowers, J. Phys. Chem. 74 (1970) 2083. R.L. Williams and F.S. Rowland, J. Am. Chem. So-c. 94 (1972) 1047. J.P. Frank and F.S. Rowland, J. Phys. Chem. 78 (1974) 850. F.S. Rowland, F. Rust and J.P. Frank, Proceedings of the 169th ACS Symposium, Pennsylvania, 1975, Am. Chem. Sot. Symp. Ser. 66 (1978) 26. P.J. Robinson and K.A. Holbrook, Unimolecular reactions (Wiley-Interscience, New York, 1972). T. Shimanouchi, Tables of molecular vibrational frequencies consolidated, Vol. 1 (1972) NSRDS NBS 39 (US Govt. Printing Office, Washington, 1972). C.G. Swain, J.E. Sheats and K.G. Harbison, J. Am. Chem. Sot. 97 (1975) 783; C.G. Swain, J.E. Sheats, D.G. Gorenstein and K.G. Harbison, J. Am. Chem. Sot. 97 (1975) 791.
Chemical Physics 115 (1987) 261-267 North-Holland, Amsterdam
A STUDY OF THE PHENYL RADICAL BY VACUUM ULTRAVIOLET PHOTOELECI’RON SPECTROSCOPY V. BUTCHER Department
M.L. COSTA ‘, J.M. DYKE *, A.R. ELLIS 3 and A. MORRIS
of Chemistry,
The Universi&
Southampton
SO9 5NH, UK
Received 2 February 1987
Part of a photoelectron band assigned to the phenyl radical, produced from the F + C,H, reaction, has been recorded. The adiabatic ionization energy was measured as 8.32 k 0.04 eV. Two vibrational components separated by 2790 + 100 cm-’ were observed in the C,H, band corresponding to excitation of a C-H stretching mode in .the ion. For C,D, this separation decreases to 237OkllO cm-t. Ab initio CI calculations on the low-lying CsHl states, 3B, and ‘At, show that the ground state of C,H: is ‘A, and the first adiabatic ionization energy is estimated as 8.OkO.l eV. Evidence is presented to show that the observed photoelectron band should be assigned to the ionization of CeH: (a 3B,) + CsH,(X ‘AI). In the F+ C,D, reaction, a photoelectron band assigned to C,DsF is observed at 7.8OkO.02 eV which is seen only very weakly in the F+ C,H, reaction.
1. Introduction The phenyl radical, C,H,, is the simplest aromatic radical. Theoretical and experimental evidence [l-6] shows conclusively that its ground electronic state is a 2A1 state, with the unpaired electron residing in an a,, u-type molecular orbital. In contrast, some uncertainty exists concerning the ground state of the phenyl cation. Published ab initio molecular orbital calculations performed for C&H: indicate that at the SCF level, the ground state is ‘B, with the ‘A, state approximately 0.3 eV higher in energy, where both ions are at their computed minimum energy geometries [7]. However, approximate allowance for the electron correlation energy difference between these two states by use of the known 3B,-1A, separation in CH, and comparison with the results of equivalent SCF molecular orbital calculations on CH,
’ Present address: Centre of Mass Spectrometry, University of Lisbon, INIC, Lisbon, Portugal. 2 To whom correspondence should be addressed. 3 Present address: Central Electricity Research Laboratories, CEGB, Leatherhead, Surrey, UK.
3B, and ‘A, leads to a corrected C,Hf 1A,-3B, minimum energy separation of 0.87 eV with the ‘A, state lying lower [7]. Semi-empirical calculations also indicate that the ground state of C,Hl is ‘A, [S]. Hence although the available evidence suggests that C,HT has a ‘A, ground state this result cannot be viewed as firmly established. The photoelectron spectrum of the phenyl radical was initially recorded with the aim of characterizing the ground state of C,Hf although it became clear at an early stage that ab initio calculations which include the effects of electron correlation would be necessary to compute vertical ionization energies of the phenyl radical and to establish the ground state of C,Hl. A number of measurements of the first adiabatic ionization energy of GH, have been made previously. Values of 8.1 f 0.1 eV and 8.8 + 0.3 eV have been derived by photoionization mass spectrometry [9] and electron impact mass spectrometry [lo], whereas use of available heats of formation of AHz2,,,of C,H, [11,12] and C,Hc [13-151 leads to a value of 8.33 + 0.15 eV. The present work uses the reaction of fluorine atoms with benzene to generate phenyl radicals in the gas phase. Previous kinetic studies [16-211
0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
262
V. Butcher et al. /A
demonstrate that this reaction can proceed via two distinct routes, proton abstraction to yield HF and C,H, and fluorine atom addition to yield C,H,F, which subsequently decomposes to give GH, and HF. Infrared chemiltinescence studies [16-191 of the vibrationally excited HF fragment indicate that, at room temperature, both the F + C,H, addition and abstraction reactions are rapid, with the initial addition reaction being approximately three times faster than the abstraction reaction.
2. Experimental In this work the phenyl radical was generated by the rapid reaction of atomic fluorine with benzene (Analar grade, BDH). Fluorine atoms were produced by a microwave discharge (2.45 GHz) of 5% molecular fluorine in helium. Photoelectron spectra were recorded using a multi-detector photoelectron spectrometer which has been described elsewhere [22] and the most intense spectra of the phenyl radical were recorded at a reagent mixing distance of 1.0-2.0 cm above the photon beam. Typical resolution under operating conditions as measured for argon (fwhm) using HeIol (21.22 eV) radiation was 25-30 meV. Similar experiments were performed with deuteriated benzene (C,D, Spectroscopic Grade BDH). A new band attributed to a short-lived molecule (C,D,F) was observed in this case and the most intense spectra of this feature were observed at a reagent mixing distance of 0.0-0.5 cm above the photon beam. All spectra were calibrated using the HeIa photoelectron spectrum of methyl iodide.
3. Computational
details
The ground state configuration of C,H,(X *At) is known from previous calculations [1,5] to be . . . (10a,)2(7b2)2(la2)2(2br)2(lla1)1, where the unpaired electron resides in the lla, level, a u-type molecular orbital. The molecular orbitals with a, and b, symmetry are all in the
study of
thephenyl
radical
plane of the phenyl ring whereas the b, and a, orbitals are perpendicular to the plane and thus form the T-system of the radical. In this work, vertical ionization energies have been calculated by performing restricted ab initio SCF molecular orbital calculations [23] for C,H,(X 2A1) and the C,Hc states ‘Al, lp3B,, ‘,3A2 (2b,)-‘, and rp3B2 arising from the (lla)-‘, ionizations respectively. (la,))’ and (7b,)-’ Gaussian basis sets (C[4s, 2p] and H[3s]) taken from ref. [24] augmented by polarization functions on each centre 1251 #’ were used in these calculations, which were carried out using the minimum energy geometry of C,H,(X 2A1) obtained in ref. [l] via a series of ab initio calculations with an STO-3G basis set. In this present work, these SCF calculations were supplemented by configuration interaction calculations on C,H,(X 2A1) and the above C,Hz states using the ATMOL direct CI method [26]. In all calculations, the lowest six molecular orbitals which correspond to C 1s orbitals were held frozen and all orbitals with energies greater than 1.7 au in C,H,(X 2A1), were also frozen. All single and double excitations were then considered within this active space in the CI expansions. Quadruple excitations were also allowed for by applying Davidson’s correction [27]. The corrected ASCF vertical ionization energies obtained from these calculations are given in table 1. Also with this method adiabatic ionization energies to the lowest C,Hf ‘A, and 3Bl states were calculated at the SCF CI level using the minimum energy geometries calculated with an STO-3G basis set [7]. The values obtained are also included in table 1. In order to assist in the assignment of the observed vibrational structure in the first photoelectron band of the phenyl radical, force field calculations were performed for C,H, and C,D, with the computed phenyl geometry of, ref. [l]. A Urey-Bradley force field was used in these calculations with force constants taken from those used to reproduce the experimental vibrational frequencies of pyridine and deuteriated pyridine [28]. For C&H,, five vibrational frequencies were computed
#’ The exponents used were C d 0.63, H p 0.88.
263
V. Butcher et al. / A stu& of the phenyl radical Table 1 Computed vertical ionization energies (ev) of C,H,(X
*At) a)
Orbital ionized b,
Ionic state
ASCF ‘) value
A SCF + CI ‘) value
Experimental
2b,
3%
lla, 2b,
lAt lB, 3A2
la2
‘A2
8.62 (8.53) 8.79 (7.92) 9.22 8.87 8.87
8.67+0.02 -
la2
8.25 (8.24) 9.11 (8.19) 8.73 8.54 8.62
000
V,F
C&H,
600
_
a) See text for details of these calculations. In this work, the total energy computed for C,H: (3B1) with the neutral C,H,(X ‘Ai) STG-3G geometry at the SCF level is - 229.780392 au; at the CI level this drops to -230.243629 au. The SCF energy computed with a 4-31G basis in ref. [7] with an optimized C,H: (3Bt) geometry determined with an STO-3G basis is -229.43357 au (the corresponding SCF energy computed in this work is - 229.780619 au). b, The electronic configuration of CeH,(X ‘A,) is (lOa,)‘ (7b2)2(la2)2(2b,)2(11a,)‘. Ionization from the 7b, molecular orbital gives rise to ionic states above 11.0 eV ionization energy. ‘) Values in parentheses are calculated adiabatic values.
above 3000 cm- ’ and approximate to C-H stretching modes. They are 3044 (a,), 3047 (a,), 3051 (a,), 3045 (b2) and 3049 cm-’ (b,). In C,D,, the corresponding values are 2235, 2243 and 2252 cm-’ (ai) and 2235 and 2240 cm-’ (b2).
4. Results and discussion The He1 photoelectron spectrum recorded for the reaction of fluorine atoms with benzene over the ionization energy range 4.5-11.0 eV is shown in fig. 1. This spectrum represents 500 scans accumulated in a time of 4 min. The sharp feature at an apparent ionization energy of 4.99 eV has been identified as helium, from the F,/He mixture used, ionized by He11 radiation. The bands in the 9.0-10.0 eV ionization energy region have been assigned to unreacted benzene [29,30] and monofluorobenzene [31,32]. Also present are weak features in the 8.0-9.0 eV region which show a maximum in intensity at a reagent mixing distance of 1.0-2.0 cm above the photon beam. An expanded signal averaged spectrum of the 7.5-9.2 eV ionization energy region collected over 2500 scans is shown in fig. 2, recorded at a reagent mixing
HakieJl)
h
200 C,H, A___-
400
9
10
11
0
7 IONIZATION
6
5 ENERGY
(cd;
Fig. 1. The He1 photoelectron spectrum recorded for the F+C,H, reaction over the ionization energy range 4.5-11.0 eV. The reagent mixing distance above the photon beam was 1.5 cm. Ordinate: counts, abscissae: ionization energy (ev).
distance of 1.5 cm above the photon beam. The vertical ionization energy of the band shown in this figure has been measured as 8.67 f 0.02 eV and the adiabatic ionization energy was measured as 8.32 + 0.04 eV. The separation of the two components was measured as 2790 f 100 cm-‘. Having inspected many spectra of the type shown in fig. 2, it is clear that the two components observed in this spectrum are part of a vibrational series which continues under the bands assigned to benzene and monofluorobenzene. The adiabatic ionization energy of 8.32 k 0.04 eV compares
600
094
I
99
86
82
l0NZLL
EN&
WI
Fig. 2. The He1 photoelectron spectrum recorded for F + GH, over the ionization energy range 7.5-9.2 eV. The reagent mixing distance above the photon beam was 1.5 cm. Ordinate: counts, abscissae: Ionization energy (ev).
264
V. Butcher et al. / A study of the phenyl radical
favourably with values of 8.1 + 0.1 eV and 8.33 + 0.15 eV derived from photoionization mass spectrometry [9] and heats of formation AHf(298) of C,H, and C,Hl [ll-151 respectively. This evidence, combined with the transient nature of the observed components, has been used to assign the band to ionization of the phenyl radical. Unfortunately, none of the expected higher bands of this radical (see table 1) were observed experimentally because they are thought to overlap with the much more intense bands due to benzene and monofluorobenzene. Also, at reagent mixing distances greater than 2.0 cm above the photon beam, interpretation of the experimental spectra was further complicated by contributions from higher fluorinated benzenes, which all have bands at greater than 9.0 eV ionization energy [32,33]. The spectra were also checked for bands arising from biphenyl [33,34] and benzyne [35], two possible secondary reaction products, but no evidence of contributions from these molecules was found. This was probably because the phenyl radical was produced using relatively high atomic fluorine: benzene partial pressure ratios and the main fate of the phenyl radical appeared to be reaction with fluorine atoms to give mono-fluorobenzene. Similar spectra to that shown in fig. 2 for F + C,H, were also obtained for the F + C,D, reaction. Fig. 3 shows a spectrum obtained for the F + C,D, reaction, recorded over the ionization energy range 6.5-9.2 eV at a reagent mixing distance of 0.7 cm above the photon beam. It consists of a total of 1250 scans accumulated in a time of 2.5 min. As in the F + C,H, reaction, two components were observed in the 8.0-9.0 eV ionization energy region which were assigned to C,D,. The adiabatic component was measured as 8.37 * 0.04 eV, the vertical component measured as 8.67 f 0.03 eV and the vibrational separation measured as 2370 + 110 cm-‘. Fig. 3 shows, however, an extra band in the 7.0-8.0 eV region which was observed much more weakly in the F + C,H, reaction. This feature was only observed at reagent mixing distances of less than 1 cm above the photon beam and clearly exhibited a different mixing distance profile from the band assigned to the phenyl radical showing maximum intensity at shorter mixing distances (i.e. reaction times) than
* eool ,
$
600.
I O94
!)
C, D,
,1,,
90
80
82
70’74
70
66
IONIZATION ENERGY b/v)
Fig. 3. The He1 photoelectron spectrum recorded for F + C,D, over the ionization energy range 6.5-9.2 eV. The reagent mixing distance above the photon beam was 0.7 cm. Ordinate: counts, abscissae: ionization energy (ev).
the phenyl radical components. The band maximum of this feature was measured as 7.80 f 0.02 eV. In accordance with previous spectroscopic [36,37] and kinetic studies [16-211 of the F + C,H, and F + C,D, reactions, this band was assigned to the first photoelectron band of the l-fluorocyclohexadienyl radical, C, Ds F. The reason for the observation of C,D,F in the F + C,D, reaction with more intensity than C,H,F in the F + C,H, reaction is not immediately apparent. However, the reaction of fluorine atoms with aromatic molecules and olefins has been shown to follow two distinct reaction paths [16-211; the addition reaction which produces a fluorinated intermediate, and the direct hydrogen abstraction reaction which produces a polyatomic radical and HF. Simple collision theory shows that neither the initial addition step nor the direct proton abstraction reaction are likely to exhibit any appreciable isotopic differences in rate constant between F + C,H, and F + C,D,. In contrast, once the addition products are formed the decomposition rate of C$H,F* is expected to be much greater than that of C,D,F*. This conclusion is reached from the results of kinetic studies on the F + C,H, and F + C,D, reactions [38-401 in which C,H,F* was found to decompose much faster than C2D4F*, and from relative rate constants for C,H,F* and &D,F* decomposition expected from RRKM calculations [40,41]. This
V. Butcher et al. / A sttiy
would then provide support for the observation of C,D,F in the F + C,D, photoelectron spectrum with more intensity than C,H,F from the F + C,H, spectrum. Assignment of the vibrational separation observed in the first photoelectron band of the phenyl radical is relatively straightforward because of its high frequency and experimental deuterium shift. It is attributed to excitation of a C-H stretching mode of a, symmetry in the ion, assuming that both the neutral molecule and cation in their ground electronic states have planar C,, equilibrium geometries. Although the frequencies of the C-H stretching modes in C,H,(X 2A1) and C,D,(X 2A1) have not been measured experimentally, some support for the above assignment is provided by the force field calculations performed in this work which gave three a, modes at 3044, 3047 and 3051 cm-’ for C,H, which decrease to 2235, 2243 and 2252 cm-’ on deuteration. The corresponding values for the symmetric C-H stretching mode in C,H, and C,D,, as obtained from liquid phase Raman measurements [42], are 3062 and 2293 cm-’ respectively. Returning to the issue of the ground state of the phenyl cation and the assigmnent of the observed phenyl photoelectron band, table 1 shows that at the SCF level the vertical ionization energy to the 3B, ionic state is lower than that to the ‘Ai ionic state. This is also true after configuration interaction (CI) has been performed for C,H, X 2A1 and the 3B, and ‘A, ionic states. However, previously it has been estimated [7] that the ‘A, state would be the ground state of C,Hf . A ‘A, ground state was also expected from INDO semiempirical molecular orbital calculations on C,Hc [S] and some qualitative evidence from solution experiments [43]. With this in mind, adiabatic ionization energies to the C,H,’ ‘Ai and 3B, states were computed at the SCF CI level using their respective minimum energy geometries calculated with an STO-3G basis set [7]. All other computational details were as adopted previously. The results of these calculations are also shown in table 1. These calculations clearly show that the ground state of C,H: is the *A, state with the ‘B, state = 0.61 eV higher in energy, where both states are at their STO-3G minimum energy geom-
of the phenyl radical
265
etries. This separation compares with a value of 0.87 eV estimated previously from 4-31G SCF energies calculated at the STO-3G optimized cation geometries corrected empirically by the known relative correlation energies in CH, 3B, and ‘A,
171.
The calculated ionization energies presented in table 1 have important implications for the assignment of the phenyl photoelectron band observed in this work and the following pieces of evidence should be considered: (1) For the ionization C,Hl(3B,) + C,H, (X 2A1), the difference between the adiabatic and vertical ionization energy is expected to be small as the STO-3G minimum energy geometries for both states are very similar [1,7]. The configuration interaction calculations give 0.09 eV for this separation. However, this value neglects zero-point vibrational contributions and any residual correlation energy contributions not accounted for by the CI calculations. The calculated value is also dependent on using STO-3G geometries and will obviously change when more reliable equilibrium geometries become available. Nevertheless, for C, H:(3B1) 6 C,H,(X 2A1), these calculations clearly indicate that the separation of the adiabatic and vertical ionization energies is small for this ionization. (2) In contrast, for the process C,Hl(‘A,) t C,H,(X’A,), the CI calculations yield a much larger adiabatic-vertical ionization energy difference of 0.87 eV. This arises because the STO-3G minimum energy geometries of C,Hz(lA,) and C,H,(X 2A1) are very different. The largest change occurs in the C-C-C angle at the radical centre which increases from 123.7O to 144.9O on ionization [1,7] and as a result the corresponding broad photoelectron band is expected to show extensive vibrational structure in the ring bending modes of a, symmetry. The 3A2 and ‘A2 states arising from the (la,))’ ionization are also expected to make contributions above = 8.7 eV ionization energy (see table 1). (3) Experimentally, the observed phenyl envelope shows only two components with the second component more intense than the first. Although a third component is not observed as it is almost certainly masked by the first band of unre-
266
V. Butcher et al. / A study of the phenyl radical
acted benzene, it is clear from figs. 2 and 3 that it must be weaker than the second component. The peak maximum of this second component, 8.67 + 0.02 eV, is also in reasonably good agreement with the computed vertical ionization energy for the process C,Hc(3B,) + C,H,(X 2A1) of 8.62 eV. On the basis of this evidence, the observed phenyl components are assigned as vibrational components of the ionization C,Hc(a 3B,) +- C,H, (X 2At). This assignment means that the first band of the phenyl radical, corresponding to the process C,Hl(X ‘A,) +- C,H,(X 2A1), is not observed experimentally. From table 1, this ionization is expected to give a broad band with an adiabatic ionization energy of 7.92 eV and a vertical ionization energy of 8.79 eV. The reason why this is not seen in fig. 2 is presumably because the Franck-Condon factors in the 7.9-8.6 eV ionization energy region are too low. Some support for this suggestion is provided by the fact that the first adiabatic ionization energy measured by photoionization mass spectrometry, 8.1 k 0.1 eV [9], is lower than the value measured here of 8.32 f 0.04 eV for the C,Hc(a 3B,) +- C,H,(X 2A1) process. Using the heat of formation (AH:2,,,) of the phenyl radical, C,H,(X 2A1), of 3.37 + 0.08 eV [11,12], the adiabatic ionization energy of the second band of C,H, measured in the work allows the heat of formation of C,Hc(a 3B,) (AHg298j) to be determined at (11.69 f 0.12) eV. If the first adiabatic ionization energy of C,H, is taken as 8.0 f 0.1 eV, a value consistent with the calculations performed in this work and the analysis of the photoelectron spectrum, AHi2,,, of C,H: (X ‘A,) can be calculated as 11.37 f 0.18 eV. In conclusion, part of the first photoelectron band of the phenyl radical has been recorded and the spectrum obtained has been interpreted in terms of the ionization C,H:(a 3B,) t C,H, (X 2A1). On the basis of ab initio configuration interaction calculations, the first adiabatic ionization energy, corresponding to the process C,H:(X ‘Ai) + C,H,(X 2A1), is deduced to be 8.0 f 0.1 eV. Clearly, however, higher resolution spectroscopic studies will be required to characterize the X’A, and a3B, states of the phenyl cation and determine the separation of their zeroth
vibrational levels more accurately. It is hoped that this present work will stimulate interest in this area.
Acknowledgement The authors gratefully acknowledge financial support for this research from the SERC. One of us (MLC) also acknowledges support from the Calouste Gulbenkian Foundation (Portugal).
References 111R.P. Johnson, J. Org. Chem. 49 (1984) 4857. 121G. Porter and B. Ward, Proc. Roy. Sot. A 287 (1965) 457. t31 G. Porter and B. Ward, Proc. Chem. Sot. (1964) 288. [41 N. Ikeda, N. Nakashima and K. Yoshihara, J. Am. Chem. Sot. 107 (1985) 3381. and D.W. Brown, J. Phys. Chem. 87 (1983) 1553. WI P.H. Kasai, E. Hedaya and E.B. Whipple, J. Am. Chem. Sot. 91 (1969) 4364. t71 J.D. Dill, P. von R. SchIeyer, J.S. BinkIey, R. Seeger, J.A. Pople and E. Haselbach, J. Am. Chem. Sot. 98 (1976) 5428. PI H.H. Jaffe and G.F. Koser, J. Org. Chem. 40 (1975) 3082; E.M. Evleth and P.M. Horowitz, J. Am. Chem. Sot. 93 (1971) 5636. t91 Y.L. Sergeev, M.E. Akopyan and F.I. Vilesov, Opt. Spectry. (USSR) 32 (1972) 230. WI R.A.W. Johnstone and F.A. Mellon, J. Chem. Sot. Faraday Trans. II 68 (1972) 1209. VI G.A. Chamberlain and E. Whittle, Trans. Faraday Sot. 67 (1971) 2077. WI A.S. Rodgers, D.M. Golden and S.W. Benson, J. Am. Chem. Sot. 89 (1967) 4578. [I31 Y. Mahnovich and C. Lifshitz, J. Phys. Chem. 90 (1986) 2200. 1141 R.C. Dunbar and J.P. Honovich, Intern. J. Mass Spectrom. Ion Phys. 58 (1984) 25. v51 J. Damracher, H.M. Rosenstock, R. Buff, A.C. Parr, R.L. Stockbauer, R. Bombach and J.P. Stadelmann, Chem. Phys. 75 (1983) 23. P61 D.J. Smith, D.W. Setzer, K.C. Kim and D.J. Bogan, J. Phys. Chem. 81 (1977) 898. P71 D.J. Bogan and D.W. Setzer, J. Chem. Phys. 64 (1976) 586. WI J.G. Moehlmann and J.D. McDonald, J. Chem. Phys. 62 (1975) 3061. [I91 J.G. MoehImann and J.D. McDonald, J. Chem. Phys. 62 (1975) 3052. [20] K. Shobatake, J.M. Parson, Y.T. Lee and S.A. Rice, J. Chem. Phys. 59 (1973) 1427.
[51 J. Pacansky
V. Butcher et al. / A study of the phenyl radical
[21] J.A. Cramer and F.S. Rowland, J. Am. Chem. Sot. 96 (1974) 6579. [22] A. Morris, N. Jonathan, J.M. Dyke, P.D. Francis, N. Keddar and J.D. Mills, Rev. Sci. Instrum. 55 (1984) 172. [23] V.R. Saunders and M.F. Guest, ATMOL Reference Manual, SERC, Didcot, UK (1976). [24] T.H. Dunning, J. Chem. Phys. 53 (1970) 2823. [25] B. Roos and P. Siegbahn, Theoret. Chim. Acta 17 (1970) 199. [26] V.R. Saunders and J.H. van Lenthe, Mol. Phys. 48 (1983) 923. [27] E.R. Davidson, The world of quantum chemistry, eds. R. Daudel and B. Pullmann (Reidel, Dordrecht, 1974); S.R. Langhoff and E.R. Davidson, Intern. J. Quantum Chem. 8 (1974) 61. [28] G. Zerbi, B. Crawford and J. Overend, J. Chem. Phys. 38 (1963) 127. [29] L. Karlsson, L. Mattson, R. Jadmy, T. Bergmark and K. Siegbahn, Physica Scripta 14 (1976) 230. [30] L. Asbrink, 0. Edqvist, E. Lindolm and L.E. Selin, Chem. Phys. Letters 5 (1970) 192. [31] P. Natalis, J.E. Collin and T. Momigny, Intern. J. Mass Spectrom. Ion Phys. 1 (1968) 327. [32] D.G. Streets and G.P. Ceasar, Mol. Phys. 26 (1973) 1037. [33] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, Handbook of He1 photoelectron spectra of funda-
[34] [35] [36] [37] [38] [39] (401
[41] [42]
[43]
26-l
mental organic molecules (Japan Scientific Societies Press, Tokyo, 1981). J.P. Maier and D.W. Turner, Faraday Discussions Chem. Sot. 54 (1972) 149. M.J.S. Dewar and T.P. Tien, J. Chem. Sot. Chem. Commun. (1985) 1243. M.E. Jacox, J. Phys. Chem. 86 (1982) 670. E.L. Co&ran, F.J. Adrian and V.A. Bowers, J. Phys. Chem. 74 (1970) 2083. R.L. Williams and F.S. Rowland, J. Am. Chem. So-c. 94 (1972) 1047. J.P. Frank and F.S. Rowland, J. Phys. Chem. 78 (1974) 850. F.S. Rowland, F. Rust and J.P. Frank, Proceedings of the 169th ACS Symposium, Pennsylvania, 1975, Am. Chem. Sot. Symp. Ser. 66 (1978) 26. P.J. Robinson and K.A. Holbrook, Unimolecular reactions (Wiley-Interscience, New York, 1972). T. Shimanouchi, Tables of molecular vibrational frequencies consolidated, Vol. 1 (1972) NSRDS NBS 39 (US Govt. Printing Office, Washington, 1972). C.G. Swain, J.E. Sheats and K.G. Harbison, J. Am. Chem. Sot. 97 (1975) 783; C.G. Swain, J.E. Sheats, D.G. Gorenstein and K.G. Harbison, J. Am. Chem. Sot. 97 (1975) 791.