J . Phys. Chem. 1986, 90, 1990-1992
1990
TABLE 11: Experimental Measurements of the Reaction Rate Constant for Chlorine Atoms with H2S
temp, K
species monitored
methodo
press., Torr
10"kl, cm3 molecule-I s-I
298 296 296 211-353 232-359
HCI C1 HZS CI CH2=CH3*CI
LP-CL DF-RF DF-MS FP-RF RGC
10 1-1.7 1 25-100 4000
7.3 f 0.9' 4.00 f 0.08 5.1 f 0.7 6.29 f 0.46 10.5 f 0.4
ref 8 9 10 11 this
work LP, laser photolysis: CL, chemiluminescence: DF, discharge flow; RF, resonance fluorescence: MS, mass spectrometry; FP, flash photolysis; RGC, radiochemical gas chromatography. Rate Constant for formation of HCI(v=l). I?
intense laser action possible with this reaction, and the ratio (v=O)/(u=l) has been estimated as 0.6 from the work of Dill and Heydtmann.I5 If the latter estimate were correct, then the overall rate for kl from the data of Nesbitt and Leone would be (1.6) X (7.3 f 0.9) X lo-]', or (11.7 f 1.4) X lo-" cm3 molecule-l s d . The latter value is in agreement within the respective error limits of our measurement in Table I. The differences among the various values for k l in Table I1 appear to be beyond the respective error limits of the experiments, which in turn represent widely different experimental conditions and techniques. These radiochemical experiments are in a pressure ~~
~~
(15) Dill, B.: Heydtmann, H. Chem. Phys. 1978, 35, 161
regime at least 40 times higher than any of the other experiments, and our higher values may be indicative of a pressure dependence for k , . The only previous attempt to detect any change in k , with pressure used argon as the bulk gas and a pressure range of 25-100 Torr, well below that of our measurements." A possible mechanistic source of such variations can be that the reaction of C1 with H2S is not a direct abstraction of H but instead involves a more complex attack on the sulfur atom with the formation of an H2SCl intermediate, as suggested by Leone and colleagues.7*8 The absence of any observed activation energy in our experiments or in those of Nava et a1.I' does not provide much additional information about the mechanism of the reaction because both direct abstraction and complex formation can be made to occur with little temperature dependence, e.g., the abstraction of H from C,H, shown in Table I. An inconsistency also appears to exist among the various rate constant measurements at pressures of 100 Torr and less. In any case, the possibilities of a pressure dependence in the measured value for k , or of a complex intermediate H2SC1 surviving long enough to make collisions at 1-atm pressure introduce substantial uncertainty as to the appropriate value for k , to be used in atmospheric calculations or modeling. Our own experiments can presumably be modified to determine the value for k , with a mixture of N 2 / 0 2at 1-atm pressure as the moderator gas although some molecule such as CC1F3 must still be present as the source molecule for the 37Cl(n,y)38C1nuclear reaction.
Acknowledgment. This research was supported by the Office of Chemical Sciences, Division of Basic Sciences, U. S. Department of Energy, Contract No. AT-DE-ER76-70126.
A Novel Investigation of Vapor-Phase Charge-Transfer Complexes of Halogens with n Donors by Electron Energy Loss Spectroscopyt P. Vishnu Kamath, M. S. Hegde, and C. N. R. Rao* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560 012, India (Received: December 3, 1985; In Final Form: February 19, 1986)
Complexes of I2 with diethyl ether and triethylamine and of Br, with diethyl ether have been investigated in the vapor phase for the first time by employing electron energy loss spectroscopy. Besides the CT bands, blue-shifted vacuum-UV bands of the halogens have been assigned; the amine-I, system appears to exhibit two CT bands, associated with two different excited states of the complex.
Introduction Interactions between electron donors and acceptors are generally characterized by the occurrence of a charge-transfer (CT) tran~ition.'-~Besides the C T bands, in the case of halogen acceptors such as 12, a blue shift of the visible absorption band (520 nm) is ~ b s e r v e d . ~ The - ~ charge transfer and the blue-shifted visible I, bands have been investigated widely in many complexes of 1, in the solution p h a ~ e ,and ~ . ~the spectra of a few of the complexes have been characterized in the vapor We considered it most valuable to investigate the electronic spectra of some of the halogen complexes in the vapor phase by employing electron energy loss spectroscopys-I0 (EELS) since this technique would permit a study of the entire region from vacuum ultraviolet to visible. This would enable us to examine whether the vacuum UV transition of I2 (1 80 nm) also undergoes a blue shift on complexation. The interest in obtaining information on the vacuum UV transition of complexed I2 is due to the close rela'Contribution No. 342 from the Solid State and Structural Chemistry Unit.
tionship between the vacuum UV and the visible absorption bands.j The 180-nm band of iodine in complexes has not been studied hitherto probably because of the difficulty in recording optical spectra in the vacuum UV region. Another interest in the vacuum UV region is the possibility of observing an additional C T band (1) Mulliken, R. S.J. Am. Chem. SOC.1952, 74, 8 11. (2) Mulliken, R. S. J . Chim. Phys. 1963, 20. (3) Mulliken, R. S.; Person, W. B. Molecular Complexes: Wiley-Interscience: New York, 1969; p 142. (4) Breigleb, G. Electron-Donator-Acceptor Komplexe: Springer-Verlag: Berlin, 1961. ( 5 ) Rao, C. N. R.: Bhat, S. N.: Dwivedi, P. C. Appl. Spectrosc. Rev. 1971, 5, 1. (6) Rao, C. N. R.; Chaturvedi, G. C.; Bhat, S. N. J . Mol. Spectrosc. 1970, 33, 554. (7) Tames, M. In Molecular Complexes, Vol. 1, Foster, R. Ed., Elek Science: London, 1973; p 49. (8) Kupperman, A,; Flicker, W. M.; Mosher, 0. Chem. Rev. 1979, 79,77. (9) Rao, C. N. R.; Srinivasan, A,; Jagannathan, K.; Hegde, M. S. J . Sei. Ind. Res. (India) 1980, 39, 212. (10) Celotta, R. J.; Huebner, R. H. Electron Spectroscopy, Theory, Techniques and Applications, Brundle, C. R., Baker, A. D., Ed.; Academic Press: New York, 1979; p 41.
0022-3654/86/2090- 1990$01.SO10 0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 1991
Letters
I .
I
I
I
1
12
0
L
0
A € (eV) I
I
12
8
0 A E (eV)
Figure 1. Electron energy loss spectra of diethyl ether, I*, and the complex. Positions of the CT band and blue-shifted I2 bands are shown.
in amine-I, complexes.2 We have carried out preliminary investigations of the halogen complexes of diethyl ether and triethylamine in the vapor phase by employing EELS with the hope of obtaining information on the C T bands as well as the blueshifted vacuum UV bands of halogens. The present study does indeed indicate that EELS can be effectively employed for the study of molecular complexes in the vapor phase.
Experimental Section The electron energy loss spectra were recorded with a homebuilt spectrometer.” In this spectrometer, a thoriated tungsten filament is heated and the thermionically emitted electrons are accelerated to a primary energy E, of 40 eV. The electron beam so obtained is monochromatized by using a spherical sector analyzer of 127 mm mean diameter (working in the velocity-selector mode). The monochromatized beam (fwhm = 0.32 eV) is then focussed into the sample chamber with suitable electron optics. In the sample chamber, the electron beam encounters a beam of differentially pumped sample molecules. The electrons, after scattering, enter another spherical sector analyzer (working in the analyzer mode) which measures the kinetic energy of the electrons. The inelastically scattered electrons emerge at energies E, which are different from the primary energy E,. The differences in the energies of the electrons before and after scattering, AE, = E, - E,, give the energies absorbed by the molecule. The scattered electron current was collected at the normal (0’) angle. The working resolution of the spectrometer was 0.8%. Freshly distilled diethyl ether (or triethylamine) was taken in a small glass bulb with an extended neck. A calculated quantity of freshly sublimed I2 (or liquid Br2) was added ensuring that the donor was in slight excess. The bulb and the long stem of the ( 1 1 ) Hegde, M. S.; Jayaram, V.; Kamath, P. V.; Rao, C. N. 1985, 24, 293.
R. Pramana
Figure 2. Electron energy loss spectra of bromine and the ether-bromine
complex. bulb were gently warmed. The vapors were first permitted to equilibrate in the long stem and then admitted into the spectrometer through a needle valve. While the presence of a considerable amount of unreacted donor (especially in the case of the diethyl ether-12 complex) is expected, we believe that our spectra are largely due to the complex alone, on account of the different pumping speeds required for the donor molecules compared to the molecules of the complex. The spectrum initially corresponded to that of the donor but soon changed to that of the complex (as seen by the appearance of the C T band). It is likely that, on account of the vacuum in the sample chamber and the high speed differential pumping of the sample, the molecules of the complex are not in equilibrium during the spectral measurement. Different spectral features were assigned to the complex only when they appeared in the same scan as the C T band at its maximum intensity; these features decreased in intensity or disappeared when the C T band was weak or absent. All the spectral measurements were made at a sample pressure of torr and an equilibration temperature of 350 K. The base vacuum in the spectrometer was torr.
Results and Discussion The electron energy loss spectrum of the diethyl ether-12 complex in the vapor phase is shown in Figure 1. The chargetransfer band in this complex is found at 4.91 eV (252 nm) which corresponds closely to the known optical spectrum of the comp l e ~ . ~The - ~ 2.38-eV (520-nm) visible band of I2 appears as a shoulder around 2.8 eV (450 nm) in the complex and the observed blue shift is in accordance with the value reported from optical spectra. What is more interesting is the observation of the blue shift of the vacuum UV band of I,. The 7.92-eV (1 56-nm) band in the spectrum is assigned to complexed iodine because of its high intensity (being a u u* band) and also based on a comparison with the spectrum of the Br2 complex with diethyl ether (Figure 2). This assignment implies that the 180-nm (6.88-eV) band of 1, has undergone a blue shift of 1 eV compared to a 0.5-eV blue shift of the 520-nm (2.38-eV) band.
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1992 The Journal of Physical Chemistry, Vol. 90, No. IO, 1986
Letters to that for the visible band. Based on the transition energies from EELS and the known orbital energy of the ug level of I2 (12.80 eV), we estimate the (T, level to be shifted from -5.9 eV in the free I, to -4.9 eV in the complex. The electron energy loss spectrum of the diethyl ether-Br, complex is shown in Figure 2. Bromine exhibits the IZg+ 311u and 'Zg+ I&+ transitions at 2.9 (423 nm) and 8.2 eV (150 nm) in the EELS. The charge-transfer transition in the complex appears at 6.2 eV (200 nm), the hvCT being higher than in the I, complex as expected. We could not obtain a distinct blue-shifted visible band of Br2 in the complex, but the blue-shifted vacuum UV band is seen at 9.0 eV (138 nm). The observed blue shift of the vacuum UV band of Br, is comparable to that of the I, band. The 4pu, level of Brz in the complex is estimated to be at -5.44 eV compared to -6.24 eV in free Br,. The observations made in the case of the ether-I, complex also hold for the triethylamine-I, complex (Figure 3) which shows the characteristic CT band at 4.82 eV (256 nm). The hvCT of this complex is slightly lower than that of the ether-Iz complex. The vacuum UV transition of molecular I2 at 6.88 eV shifts to 8.0 eV. The larger blue shift in the amine-I, complex when compared to the ether-I, complex is consistent with the higher enthalpy of formation of the former. Examination of the EEL spectra in Figures 1 and 3 clearly shows the presence of a band at 7.28 eV (170 nm) in the spectrum of the amine-I, complex. This peak cannot be assigned to either the transitions of the amine or I,. We suggest that this band may be due to the second CT transition of the amine-I, complex predicted by Mulliken., It is well-known that the triethylamine-I, complex undergoes a transformation to a charge-separated inner complex [Et3NI+]Iin a polar e n ~ i r o n m e n t . ~Whenever J~ we used unpurified Et3N (probably contaminated with water), we obtained a spectrum which was quite different from that of the Et3N-12 complex discussed earlier. The spectrum showed two distinct peaks attributable to iodine and also showed a shift of the Et3N band to higher energies (Figure 3). We attribute this spectrum to the inner complex, [Et3NI+]I-. This inner complex is a solid and it is normally difficult to record its spectrum in the vapor phase. However, due to the operating conditions, while recording the spectrum ( lo4 Torr vacuum at 350 K), it seems that we have been able to obtain the spectral characteristics of the inner complex. Further studies on electron donor-acceptor complexes in the vapor phase by EELS are now in progress in this laboratory.
-
12
8
4
0
A € (ev)
Figure 3. Electron energy loss spectra of triethylamine and its complex with I,; the spectrum of the inner complex [Et3NI+]I- is also shown.
The visible band of Iz at 2.38 eV corresponds to a transition from the '2: ground state having the configuration 5pui5p?ri5p?ri to the first excited state 311u with the configuration 5pui 5pr: 5pr: 5puA. The blue shift of this band on complexation arises from the Coloumb repulsion between the excited electron and the donor lone pair of electrons, as the lone-pair orbitals of the donor are expected to overlap extensively with the u,, orbital of I,, the latter having a large radial e x t e n ~ i o n . ~The , ~ vacuum UV band of I2 at 6.88 eV is due to a transition to the lZu+ state having the electron configuration ui 7r: ?ri ut. This transition is more like an intramolecular two-way charge-transfer (V N) transition, polarized along the internuclear axis. The explanation of the blue shift of the vacuum UV band of I, would be similar
-
-
Acknowledgment. The authors thank the Department of Science and Technology, Government of India, for support of this research. (12) Cotton, F. A,; Wilkinson, G . Advanced Inorganic Chemistry; Wiley Eastern: New Delhi, 1972.
