J. Phys. Chem. 1993,97, 1256-1257
1256
Bound Excited States of Chloroethylene Anions Studied by Electron Capture Negative Ion Mass Spectrometry J. R. Wiley Division of Science and Engineering, University of Texas, Permian Basin, Odessa, Texas 79762
E. C. M. Chen School of Natural and Applied Sciences, University of Houston Clear Lake, Houston, Texas 77058
W. E. Wentworth’ Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: June 17, 1992; In Final Form: December 23, 1992
Electron detachment from the ground state of negative ions of chloroethylenes and dissociation from a bound excited state have been simultaneously observed by measuring the temperature dependence of [Cl-]/ [M-] in a thermal electron attachment chemical ionization mass spectrometer source. The data show two slopes in a typical reciprocal temperature plot. The lower slope is the difference in activation energies for attachment to the two negative ion states, and the higher slope gives the detachment energy from the ground state. The detachment energy for C2C14- is 0.5 eV while that for C2HCl3- is 0.3 eV. The differences in the activation energies for thermal electron attachment are 0.1 eV for both molecules.
Although it is generally believed that bound excited states of anions of large molecules should exist, very little experimental data supporting their existence have been reported. Recently, bound excited states of the anion of 7,7,8,8-tetracyanoquinodimethane (TCNQ-) have been observed using photodetachment spectroscopy in an ICR spectrometer.’ Bound excited states of SF6-have been observed using atmospheric pressure ionization mass spectrometry.2 Other bound excited states of anions of atoms, clusters, small molecular fragments, and radicals with high dipole moments are known to exist.’ We report the observation of bound excited electronic states of tetrachloroethylene and trichloroethylene by measuring the temperature dependence of the ratio, R = [Cl-]/[M-] obtained in a thermal electron attachment mass spectrometer source. The temperature dependence at low temperatures is dominated by dissociation from the excited state of the negative ion, AB-* while the temperature dependence at high temperatures is also influenced by electron detachment from the ground state negative ion, AB-. This leads to two activation energies in a plot of In R vs T’. The lower temperature slope is the difference in activation energies for attachment to the two negative ion states while the higher temperature slope minus the lower slope is the detachment energy from the ground state. The detachment energy for C2C14- is 0.5 eV, and that for C2HC13- is 0.3 eV in agreement with reported values of the electron affinitiesof these molecules. The differences in the activation energies for thermal electron attachment are 0.1 eV for both molecules in agreement with estimates obtained from pseudo-two-dimensional potential energy curves. The mass spectra were taken with a Hewlett-Packard 5988A GC-MS equipped for negative chemical ionization. The source gas was COz, dried and maintained at a pressure of 0.85 f 0.05 Torr in the ion source. The instrument was tuned using m / z values of 35, 137, and 452 and their isotopic masses. The tuning was optimized for the low-mass range and was unchanged throughout the temperature variations. The gas chromatograph was operated in the split mode with a fused silica capillary column which provided in line purification of the samples. Injections containing about 0.9 pmol of C2HCl3 and 0.5 pmol of C2C14 were made from a single mixture containing these compounds and SF6 as a reference diluted in hexane. Three 0022-3654/93/2097- 1256$04.00/0
well-resolved peaks at about 0.7,2, and 4 min were observed for the three compounds of interest with no extraneous peaks or interferences. The source temperature was set by computer. The lowest temperature was obtained by allowing the system to equilibrate overnight. The temperatures were then changed, and the ion source was allowed to equilibrate for 45-60 min. The mass spectra were obtained via the computer workstation, and the ratios between the parent ion and the fragment ion abundances were calculated using sums of all isotopic masses of the ions. The only ions observed are the parent negative ions and the dissociated anion, C1- or SF5- and their isotopic clusters. The experimental results are shown in Figure 1 where the In R is plotted vs lOOO/T. The values of R are [Cl-]/[M-] and [SFs-]/[SF6-]. The two regions are most obvious for czc14 but are clearly present for the other two compounds. The increase in the ratio at high temperatures is primarily due to the decrease in the intensity of the parent negative ion and to a lesser extent to the increase of the intensity of the dissociated species. The lines that are drawn through the data have been obtained by a least squares fit of the data to the equation obtained from the steady state for the kinetic model given below. ki
AB
AB
+ e- k-i
+ e-
Q
-
k2
AB- * A
k’i
+ B-
-
A
-
AB-
(2)
-
(3)
k2*
AB-*
k-1’
(1)
+ B-
kx
AB-, AB-*, or B-
+ P+
kk
neutrals
The pseudo-first-order rate constant for recombination with positive ions, P+,is k~ = k”[P+]. In addition to therecombination losses, the negative ions can be lost by diffusion. If the steadystate treatment is applied to the negative ion concentrations in this model, the ratio of [B-I/( [AB-] + [AB-*]) can be expressed in terms of the kinetic rate constants. On the basis of electron 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1257
Letters
7.0
I\ \
2 -
1 -
3.0
-
0 -
eV -1
-
-2
-
-
-3
-
-
-4
-5
-1.0
E.l = 0.5 eV
U
c c
\I \ I
I
I
C,CI,
t
CI
r
cp-
t
CI
-
C,CI,
+CI
I
l
I
1
Figure 1. In R vs 1000/T where R is the ion ratio [CI-]/[M-] or [SFs-]/[SF6-]. The curves for the chloroethylenes are least squares fits using eq 5. The SF6 curve is calculated using parameters given in ref 2.
capture detector studies of the chloroethylenes, estimates of the preexponential and energetic terms for the rate constants for detachment and dissociation from the ground state have been made. For example, in the temperature range 100-300 'C, the rateconstant for dissociation,k2, is less than that for detachment, k-l .3 If it is assumed that k2* > (k*-I + kN + k,J and k2 < (k-1 + kN),the steady-state treatment of this model for the various anions leads to the expression
[B-I [AB-] + [AB-*]
-- k*,(kN + k-l) k,kN
(4)
By substituting the relationships kN = AN, k l = A I exp(EI/RT), kl* = A I * T 1 I 2exp(-E1*/RT), and k-l = A-,T exp(-E-I/RT), the equation becomes
[B-I [AB-] + [AB-*]
-- A,* expI-(E,*
- E,)/RT) AI A,*A-,Texp(-(E,* - E, + E-,)/RT) +
(5)
The first term predominates at low temperatures while the second term predominates in the high-temperature region, leading to two activation energies. The quantities AB- and AB-* cannot be distinguished in the mass spectrometer so that the measured ratio is that shown in eqs 4 and 5 . From these data, the difference in activation energies for attachment to the two negative ion states, El* - El, and the detachment energy from the ground state, E-],are obtained. The detachment energy for C2C14-is 0.5 eV while that for C2HCh is 0.3 eV, in agreement with literature estimates of the electron affinity of these molecule^.^*^ The differences in the activation energies for thermal electron attachment are 0.1 eV for both molecules, which is in agreement with pseudo-two-dimensional potential energy curves calculated from thermal electron attachment, electron beam, electron swarm, and electron transmission data in the l i t e r a t ~ r e . ~ The potential energy curves for tetrachloroethylene and its anions given in Figure 2 clearly show two bound excited states which give rise to the curvature in the data. Indeed the present ECNIMS results could have been predicted from the thermodynamic and kinetic data in the literature, which were used to construct the potential energy curves. Alternatively, the current results are additional experimental verification for the literature values. The adiabatic separation between the ground state in
,2.0
1.5
'
I
2
I
I
3
l
'
4
I
5
r Angstroms Figure 2. Morse potential energy curves for tetrachloroethylene and its negative ions calculated using parameters given in ref 3. The insert is an expansion of the region around 2 A.
which the added electron is in a u* orbital and the first excited state in which the added electron is in a r* orbital is 0.2 eV. The data for SFs were included to give a measure of the electron energy distribution. Based on the value of the ion ratio of 0.004 at 373 K, the electron distribution is essentially thermalss The curve that is shown in Figure 1 has been drawn using quantities obtained from API mass spectrometry2clearly showing that the kinetic model used previously is applicable to the current experiment. In conclusion the temperature dependence of the ion ratios of the dissociated species to the parent negative ion for C2C14and C2HC13show two activation energies in the temperature region of the current thermal electron attachment mass spectrometry experiments. This is interpreted as evidence for dissociation of an excited state of the anion and detachment from the groundstate anion. These molecules serve as a prototype of a general class of anions in which dissociative attachment and nondissociative electron attachment can be observed in a single experiment. Such anions must havea moderateelectron affmityanda relatively low threshold energy for dissociative electron attachment. Other molecules of this sort could be fluoro, chloro, bromo, iodo, or nitrocompounds. For these types of molecules, it may be possible to determine the detachment energy for the ground state negative ion and hence the electron affinity of the molecule. Finally, a procedure for obtaining fundamental thermodynamic and kinetic data for thermal electron attachment using a negative ion mass spectrometric source is presented.
Acknowledgment. We are grateful to the Robert A. Welch Foundation Grant E095 and Chemistry Department Grants to the University of Texas Permian Basin and Universityof Houston Clear Lake. We appreciate the helpful comments of the reviewers. References and Notes ( 1 ) Brinkman, E. A.; Gunther, E.; Brauman, J . I. J . Chem. Phys. 1991. 95, 6185 and refs 1-5 cited therein. (2) Chen, E. C. M.: Shuie, L. R.: D'sa, E. D.; Batten, C. F.; Wentworth, W. E. J . Chem. Phys. 1988, 88, 41 1 1 . (3) D'sa, E. D.: Wentworth, W. E.: Chen, E. C. M. J . Phys. Chem. 1988, 92, 285. (4) Wiley, J. W.: Chen, E. C. M.: Chen, E. S. D.; Richardson, P.; Reed, W. R.; Wentworth, W. E, J . Electroanal. Chem. 1991, 307, 1969. (5) Knighton, W. B.:Grimsrud,E. P. Inr.J.MassSpcctrom.lonProccsses
1991, 8383.
