J. Phys. Chem. 1992, 96, 89-94 0.54 eV, respectively, and keep at these levels thereafter. This experimental phenomena can be explained by using the Hjalmarson theory of deep impurity levels. As increasing pressure, the “deep levels” bands do not shift like the ZnS energy gap with pressure. Beyond a certain pressure the deep levels T energy band falls below the ZnS conduction band. The 325-nm (3.82-eV) laser energy is not sufficient to excite the electrons to the conduction band but can excite the electrons to the deep levels T energy band. The deep levels T energy band plays a role similar to the ZnS conduction band; thus, the trap depths obtained at higher pressure are attributed to the electrons thermally released from the trap to the deep levels T energy band. As discussed above, there is a range of energies for the deep levels depending on the amount of C1 present. The trapping ‘level” also consists of a distribution or band of states which may be several tenths of an electronvolt in width. The irradiation times were intentionally kept constant at 2 min at all pressures. At low pressure we could saturate the traps in a few seconds as demonstrated by auxiliary experiments using short irradiation times. At high pressures only the lowest trap levels were filled due to the relative inefficiency of the ‘deep levels” as feeders. We demonstrated this by several partial heating experiments which showed a significantly narrower range of filled traps (smaller half-width of the thermoluminescence curves) at high pressure. For example, for the low-concentration (blue) sample at 0 kbar the fwhm is -60 K while at 43 kbar the fwhm is -45 K. The situation is represented in Figure 9 where the dashed cross hatching represents unfilled states in the trapping level at high pressure. ‘Deep levels” have been extensively studied in GaAs and A1,Gal-,As. In GaAs the deep levels are resonant with the conduction band at ambient pressure. The energy gap increases with pressure (by 11 meV/kbar) so that above -25 kbar the deep levels are exposed in the gap.26,27 Adding A1 to GaAs
-
89
increases the gap so that at x = 0.22 the deep levels appear in the gap at ambient pressure. The properties of deep levels in 111-V semiconductors have been reviewed by Mooney.28 The kas of thermoluminescence intensity with prtssure involves several factors:20.21the loss in luminescence intensity discussed above due to thermal quenching as T, increases and relative inefficiency of the localized deep levels as intermediate states.
Summary High-pressure luminescence and thermoluminescence studies have been made on well-characterized ZnS samples doped with Cu and Cl. By using a constant excitation energy of 3.82 eV, at low pressures electrons were excited to the conduction band, and at high pressures, to the “deep levels” initially buried in the conduction band. The differences caused by the different types of excitation were especially revealed in the trap depths and efficiency of thermoluminescence. The changes in luminescence energy and efficiency for the two emission peaks could be associated with the degree to which donor and acceptor states were pinned to the conduction and valence bands. Acknowledgment. We thank T. Brumleve for the samples and K. Hess and G. A. Samara for helpful and illuminating discussions of ‘deep levels” in semiconductors. It is a pleasure to acknowledge the continuing support of the Materials Science Division of the Department of Energy under Contract DEFG02-9 1ER 45439. Registry No. ZnS, 1314-98-3; Cu, 17493-86-6; C1, 16887-00-6. (26) Wolford, D. J.; Bradlex, J. A.; Fry, K.; Thompson, J.; King, H. E. Institute of Physics Conference Series; Institute of Physics: London, 1982; Vol. 65, p 477. (27) Mizuta, M.; Yachikawa, M.; Kukimoto, H.; Minomura, S. J . Appl. Phys. 1985, 24, L143. (28) Mooney, P. M. J . Appl. Phys. 1990, 67, R1.
High-Resolution Spectroscopy of Jet-Cooled Substituted Cyclopentadienyi Radicals David W. Cullin: Lian Yu,*James M. Williamson, and Terry A. Miller* Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, I20 West 18th Avenue, Columbus, Ohio 4321 0- I I73 (Received: July 12, 1991)
Rotationally resolved electronic spectra have been observed for a number of substituted cyclopentadienyl radicals, C5H4X, with X = F, C1, CN, and CH3. This paper reports the spectral analysis for the radicals with X = F and C1; results for X = CN and CH, have been reported previously. The geometric structural parameters for all the radicals are consistent with a distortion of the five-membered ring to a diene-like structure in the ground state, with C-C bond length alternations in excess of 0.1 A in some cases. The analysis also quite clearly shows a strengthening and shortening of the C-X bond in the radical compared to that of the similarly substituted benzene derivatives. The observed results are rationalized in terms of a simple molecular orbital picture involving conjugation and hyperconjugation.
I. Introduction The implementation of supersonic free jet expansions and high-resolution optical laser induced fluorescence (LIF) techniques have advanced the understanding of the spectroscopy of organic free radicals in the last few years. A great deal of electronic and structural information has been obtained for radicals such as the fluorobenzene ions (c6F6+,C6F3H3+),’cyclopentadienyl (C5H5),233 methylnitrene (CH3N),4 diacetylene cation (C4H2+),5benzyl (C6H5CH2),6methoxy (CH30)? and others because of the ability to rotationally resolve the electronic spectra of these large organic species. We recently reported the spectroscopy of the cyclopentadienyl radical (Cp). We obtained accurate rotational constants along Present Address: Naval Surface Warfare Center, Dahlgren, VA 22448. *Present Address: Lilly Research Laboratories,Lilly Corporate Center, Indianapolis, IN 46285.
with other parameters including those describing the Jahn-Teller effect caused by three electrons occupying the doubly degenerate lel” orbital of Cp. It was found that as a consequence of the Jahn-Teller effect, the five-membered ring in Cp is significantly distorted from a regular pentagon. ( 1 ) Yu. L.; Foster, S.C.; Williamson, J. M.; Miller, T. A. J . Chem. Phys. 199b; 94, 5794. (2) Yu. L.: Foster. S. C.; Williamson, J. M.; Heaven, M. C.; Miller, T. A. J . Phys. Chem. 1988, 92,4263. (3) Yu, L.; Williamson,J. M.; Miller, T. A. Chem. Phys. Let?.1988, 162,
4263. (4) Carrick, P. G.; Brazier, C. R.; Bernath, P. F.; Engleking, P. C. J. Am. Chem. SOC.1987, 109, 5100. (5) Lecoultre, J.; Maier, J. P.; Rijsselein, M. J . Chem. Phys. 1988, 89, 608 1. (6) Cossart-Magos, C.; Goetz, W. J . Mol. Spectrosc. 1986, 115, 366. (7) Liu, X.; Damo, C. P.; Lin, T.-Y.; Foster, S . C.; Misra, P.; Yu, L.; Miller, T. A. J . Phys. Chem. 1989, 93, 2266.
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The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
More recently, we studied8 the effect of breaking of this vibronic symmetry by deuterium substitution, w_hich reduces the DSh symmetry of Cp to C2,. The degenerate XEIffstate is thus split into a B2 state and an A2 state. It was found that such a substitution stabilized the ground state into statically distorted structures rather than into the pseudorotating one of Cp, yet the magnitude of the distortion was precisely the same as in Cp itself. In this paper, we will report the effects of “chemical” substitution onto the Cp ring of the halogens, fluorine and chlorine. We will also extend our recent a n a l y s i ~ of ~ ~the ’ ~ spectrum of the pseudohalogen-substituted cyanocyclopentadienylradical (CNCp) . Additionally, we include in our discussion the results for the methylcyclopentadienyl radical.” The chloro- (ClCp) and fluorocyclopentadienyl (FCp) radicals were first observed by Porter and Ward in their classic flash photolysis experiments.12 Our lower resolution spectra of FCp and ClCp, taken with comparable resolution, are identical to those observed by Porter and Ward except that the radicals are colder due to the free jet expansion. The goal of this work is to rotationally resolve and analyze the band origin transitions of these radicals and thereby characterize their electronic and geometric structure. 11. Experimental Section The fluoro- and chlorocyclopentadienyl radicals were produced by the ArF excimer laser photolysis (193 nm) of suitable precursor molecules near the throat of a supersonic free jet expansion. The stable precursor molecules were entrained in the expansion by passing high-pressure helium over the precursors and then expanding the gas through a pinhole nozzle. The starting materials used were ortho- and para-substituted chloro- and fluoroanisoles as well as the corresponding para-substituted phenol. As we did in the cyanocyclopentadienyl work and as Porter and Ward did, we have exploited the photochemical ring contraction for generation of the five-membered ring radicals. In all cases, the precursors had to be heated to increase their vapor pressures. The probe laser crossed (after sufficient cooling) the jet flow downstream from the photolysis region, where the LIF signal was detected with a photomultipler tube (PMT). An optical slit was affixed to the front of the PMT so that only fluorescence from the center of the free jet expansion was viewed. This was done to eliminate detection of fluorescence from molecules with off-axis velocity components and thus reduce the doppler width of the spectral lines. The probe laser system is a pulse amplified CW ring dye laser. The dye laser (Coherent 699-29) was pumped with an argon ion laser (Coherent Innova 100) and operated with DCM laser dye for both halogen radicals. The pulse amplifier (Lambda Physik FL 2003) was also operated with DCM dye and was pumped by the XeCl(308 nm) output of a Lambda Physik EMG 103 MSG excimer laser. The resulting pulse-amplified light (line width approximately 100 MHz) was then frequency doubled in the C crystal of an Inrad Autotracker to generate the necessary UV radiation. Relative frequency calibration was accomplished by simultaneously recording the fringes of an external etalon (FSR approximately 475 MHz) with the spectral trace while absolute frequency calibration was accomplished by simultaneously recording an iodine spectrum.I3 111. Analysis
A. Electronic Structure. In order to understand the electronic transitions observed in the substituted cyclopentadienyl radicals, (8) Yu,L.; Cullin, D. W.; Williamson, J. M.; Miller, T. A. J. Chem. Phys., submitted for oublication. (9) Cullin, D. W.; Yu,L.; Platz, M. S.; Williamson, J. M.; Miller, T. A. J. Phys. Chem. 1990, 94, 3387. (IO) Cullin, D.W.; Soundararajan, N.; Platz, M. S.; Miller, T. A. J. Phys. Chem. 1990, 94, 8890. (11) Yu, L.; Cullin, D. W.; Williamson, J. M.; Miller, T.A. J. Chem. Phys. 1991, 95, 15. (12) Porter, G.; Ward, B. Proc. R. Sor. London A 1968,303, 139. (13) Gerstenkorn, S.;Luc, P.; Atlas du Spectre 6 absorption de la molecule d’ lode, Centre National de la Recherche Scientifique: Paris, 1978.
Cullin et al.
G ,i
h
F i p e 1. Schematic diagram comparing and indicating the conventions of the axes for the substituted cyclopentadienyl radicals and the cyclopentadienyl radical itself. At the bottom of the diagram, the symmetry labels for the states involved in the observed electronic transitions are indicated on the left-hand side for cyclopentadienyl and on the right-hand side for the substituted species.
C H F 5
4
% Z
%.Z
X B,tB
8,
H 20 GHz
Figure 2. High-resolution LIF excitation spectrum of the fluorocyclopentadienyl band origin transition. The band origin is located at 30758.170 (3) cm-I. it is convenient to start with the corresponding electronic excitation in the cyclopentadienyl radical itself. As shown in_ Figure-1, the corresponding transition in the C5H5 radical is A2A2/’-X2Elff. Upon substitution onto the ring, the D5h symmetry is lowered to C2,and the doubly degenerate ground state of cyclopentadienyl splits into a 2A2and a *B2state. The 2A2/’excited state of C5H5 correlates to a 2B2 state in the substituted radicals. The inertial coordinate system for the cyclopentadienyl radical is also shown in Figure 1. When a substituent, X, is placed onto the ring, the radical changes from an oblate symmetric top to a near-prolate asymmetric top. As depicted in Figure 1, the 4 inertial axis is along the C2, figure axis (the C-X bond) and b axis is perpendicular to that B axis but in the plane of the ring, while the c^ a5is remajns perpendicular to the plane of the ring. The A2Azff-X2Elffelectronic transition of Cp has its electric dipole transition moment in the plane of the ring. As a result of the degeneracy splitting of the ground state, there can be two distinct types of electronic transitions. If the ground state is the 2B2 state, then the observed transition will have the transition moment along the ii axis leading to a type-a transition. If the lowest energy_state is the 2Azstate, the transition moment must be along the b inertial axis leading to a type-b transition. The type-a band structure of a near-prolate asymmetric top is characterized by distinct P-, Q-, and R-branch (AN = 0, f l ) structure. On the other hand, the type-b band structure lacks a
The Journal of Physical Chemistry, Vol. 96,No. 1 , 1992 91
Spectroscopy of Substituted Cyclopentadienyl Radicals C H CI 5
4
%2
X B2+B
1.2
TABLE I: Rotatio~lConstants of the Substituted Cyclopentadienyl
B,
Radicals (CHz) CcHdF A
B C
8.387 (67) 3.5745 (21) 2.5102 (14)
CqHdCN Ground State 8.07 (22) 1.9779 (12) 1.6050 (10)
CcHICl 8.47 (52) 2.1027 (25) 1.6967 (24)
Excited State A
B C 8
N
Figure 3. High-resolution LIF excitation spectrum of the chlorocyclopentadienyl band origin transition. The band origin is located at 29 192.726 (3) cm-’. 2 2
C H CN 5
4
X B2+B
-2
B,
H 20 GHz
Figure 4. High-resolution LIF excitation spectrum of the cyanocyclopentadienyl band origin transition. The band origin is located at 27 139.453 (21) cm-’. central Q branch and has a gap in the center of the band. It is obvious from Figures 2-4 that for all of the radicals, the type-a structure is observed indicating that the observed transitions are all 2B2-2B2. Because of the low temperatures in the free jet, it is very reasonable to assume that all-the transitions observed originate in the ground electronic state, X2B2. At presept, we have no information concerning the precise position of the A2A2state. B. Rotational Analysis. Figures 2-4 show the rotationally resolved spectra of the fluoro-,chloro- and cyanocyclopentadienyl radicals, respectively. All of these spectra were analyzed as type-a transitions. The rotational Hamiltonian used for both electronic states was that of a simple rigid rotor. Sa= BN;
+ CN; + AN;
(1)
where the rotational constants A, B, and C have their usual significance in terms of the axes defined in Figure 1. This Hamiltonian was sufficient to adequately fit the high-resolution spectra within experimental error. As in the previously published cases2,338 of cyclopentadienyl and the deuterated cyclopentadienyl radicals, it was not necessary to include spin-related terms in the Hamiltonian since no spin splitting was observed. It is assumed that any such splitting is within the experimental line width. Similarly, because of the low temperature of the observed spectra, centrifugal distortion terms were not included. Table I shows the rotational constants resulting from the least-squares analysis using the rigid rotor model described above. As is evident, the B and C constants in both the ground and excited states are quite well determined. However, since this is a type-a transition, only the difference between A constants ( A ” - A’) is precisely determined. The correlation between the A values is broken by the asymmetric rotor terms which couple K, and K, f 2 states, which results in a less precise determination of the individual A constants. As one might expect, the residuals in the fluorocyclopentadienyl radical are the smallest because the resolution and signal-to-noise
8.405 (66) 3.4005 (18) 2.4313 (12) 45 53 lines
8.04 (21) 1.9557 (12) 1.5883 (11) 58 93 linesb
8.43 (52) 2.0412 (23) 1.6616 (22)
71
92 linesC
Overall standard deviation in MHz (values represent 1 u confidence limits). Represents simultaneous fit of three independently recorded spectra. cRepresents simultaneous fit of two independently recorded spectra. ratio of this spectrum are superior to those of the other two. Conversely, the overall fit of the chlorocyclopentadienyl radical spectrum is the poorest, as both the resolution and signal-to-noise ratio are degraded somewhat. We think this arises because of the larger mass of the molecule and the presence of the two isotopomers involving 37Cland 35Cl;the 37Clisotope being roughly one-third as abundant as 35Cl. The spectra of the isotopomers will be slightly shifted, because of both the slight difference in their rotational constants and the shift in their zero-point energies. Apparently, the respective rotational lines observed from the two species are unresolved, leading to lower resolution and also a lower signal-tenoise ratio due to dilution. Regardless, we have analyzed the chlorocyclopentadienyl spectrum in the same manner as that of the other two radicals, simply using the atomic mass of chlorine rather than that of the individual isotopes to obtain structural information. C. Geometric Analysis. For the ground and excited states of each of the radicals, rotational constants are determined. The B and C constants are well determined, but as explained previously the A constants are not so well determined. This is not enough information to uniquely determine all of the structural parameters. However, with some reasonable assumptions, the structural details can be determined to a degree. The undistorted ring carboncarbon and the carbon-hydrogen bond lengths are knowns from previous cyclopentadienyl work, and based upon that work, it is also reasonable to assume ring planarity. We, of course, must determine the carbon substituent bond length, Rc-x. In addition in going from Cp to a substituted Cp, the symmetry is lowered from D,, to C, (assuming the C-C=N bond is linear and treating CH3 as structureless). The e,’ and e,’ asymmetric distortions of Cp itself now each resolve into one component which is totally symmetric in the lowered C, symmetry of substituted Cp. There are of course multiple normal modes of el’and q’symmetry. In our previous analysis of Cp, we argued that modes involving primarily H motions could be neglected because (i) H bond lengths and angles are relatively insensitive to fine details of the ?r structure and (ii) the observed rotational constants are relatively insensitive to H positions in any case. Similarly, we earlier found that changes in rotational constants could generally be simulated by changes in either the ring bond lengths or angles alone or obviously by a linear combination of such changes. Following theoretical arguments that the bond length changes were most important, we previously neglected ring bond angle changes. We follow the same logic here. Thus we can logically expect to describe ring distortion in substituted Cp’s by a nominal C-C bond length (taken to be that of Cp itself) and the two bond length alternation parameters allowed by C,, symmetry. Clearly if our data can be described by a single alternation parameter, that is preferable. As with Cp itself, we define the ring C-C bond alternation parameter by R&(ring) = Rc-&ing) + ARC- cos (47rk/5) (2) where k = 0, fl, f 2 correspond to the bonds opposite the carbon atoms labeled in Figure 5 . Here the &
