J . Phys. Chem. 1984,88, 6087-6089 species such as 12(A') (and likely I,) can thermally dissociate in -1 ps.
Acknowledgment. These experiments were performed while
J.T. was on leave at the University of Canterbury. The work was
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supported by the N S F (US.-NZ. Cooperative Research Grant INT-8319286) and the Air Force (Grant AFOSR-83-0110). Registry No. 12, 7553-56-2; N2, 1727-37-9; I,, 12596-31-5; Ar, 7440-37-1; I, 14362-44-8.
Resonance-Enhanced Multiphoton Ionization Spectroscopy of ABCO and ABCU: Core Splitting of the 3p Rydberg Orbitals Ann M. Weber, Ananth Acharya, and David H. Parker* Chemistry Board, University of California, Santa Cruz, Santa Cruz, California 95064 (Received: August 17, 1984)
Two-photon resonance-enhanced multiphoton ionization (REMPI) spectra of I-azabicyclo[2.2.2]octane (ABCO) and 1azabicyclo[3.3.3]undecane (ABCU) are reported for the lowest excited electronic states. The differences in the REMPI and one-photon absorption spectra are explained as differing sensitivity of the two techniques to the 3p and 3d Rydberg core components of these molecules.
Introduction The electronic structure of the tertiary amines is simple and well studied, experimentally and theoretically.' All excited states observed thus far can be characterized as Rydberg in nature and are usually diffuse to the point that vibrational structure is unobservable. l-Azabicyclo[2.2.2]octane,ABCO, is an exception,
showing rich vibronic spectra due to its rigid cagelike structure and C3, symmetry. In this paper two-photon spectroscopy via resonance-enhanced multiphoton ionization (REMPI) and laser polarization analysis* is used to examine in finer detail the lowest excited states of ABCO. l-Azabicyclo[3.3.3]undecane,ABCU, has a less rigid structure and lower symmetry (e3), but its lower ionization potential allows more of its transitions to be experimentally observed, and the analysis of trends in its spectra allows for confirmation of the effects observed in ABCO. Symmetry selection rules for C3,and C3molecules suggest that all transitions from the A, ground state to the possible A, and E excited states of ABCO and ABCU are. both one-photon and two-photon allowed. The lowest excited transitions are from the nonbonding lone-pair orbital n to the 3s(a), 3p,(e), and 3p,(a) Rydberg orbitals, respectively. In ABCO, two low-lying Rydberg states, SI and S2, have previously been observed in both onephoton, and two-photon4 studies. The first, a 3s(a)-n transition, is nearly identical in both situations. Assignment of the second, a 3p-n transition, is complicated by very distinct intensity differences of several bands between the one- and two-photon spectra. We propose in this paper that these intensity differences are due to the resolution of the two core components, 3px, and 3p,, by the ( 1 ) M. B. Robin, "Higher Excited States of Polyatomic Molecules", Vol. 1, Academic Press, New York, 1974. (2) D. H. Parker, "Ultrasensitive Laser Spectroscopy", D. Kliger, Ed., Academic Press, New York, 1983. (3) A. M. Halpern, J. L. Roebber, and K. Weiss, J . Chem. Phys., 49, 1349 (1968). (4) D. H. Parker and P. Avouris, Chem. Phys. Lett., 53, 515 (1978).
two techniques. Such resolution of core splitting is rare for first-row molecules with more than 10 atoms.'
Experimental Section A Nd:YAG laser pumped tunable dye laser was used as the excitation source, and the spectral regions of interest were covered by overlapping a number of different dyes. The dye laser was scanned with a stepping motor controlled by a SlOO-Z80 based microcomputer, with wavelength steps ranging from 0.002 to 0.08 nm. Uncertainty in peak positions are estimated as f5 cm-', due mainly to the widths of the transitions. The dye laser output was focused into an evacuated glass parallel plate ionization cell containing the sample. After passing through the cell, the laser pulse energy was measured by a pyroelectric detector and fed into the computer. The ions were collected on a 250-V biased plate, and the amplified ion current signal was fed into a separate channel of the computer which then averaged and normalized the spectra. A Fresnel rhomb assembly was placed in front of the ionization cell to obtain polarization data. ABCO (quinclidine) was obtained commercially, and a sample of ABCU ( m a n ~ a n e )was ~ generously provided by Professor Nelson J. Leonard. Results and Discussion ABCO. As in ammonia, the excited states of ABCO have a more planar geometry about the nitrogen atom than the ground state. This causes long vibrational progressions in the a, cagesquashing mode, v12/1 (604 ~ r n - ' ) . ~ In the S,-So transition, the first quanta of the alvll' (766 cm-I) mode and a 914-cm-I mode have progressions of v12/ (618 crn-') built upon them., A hot band corresponding to the 914-cm-' mode has been found at 969 cm-' by Vaida and co-workers6 for this transition. This was assigned in earlier vibrational studies' as v3311 (965 cm-I), an e-type vibration, while another active mode at 990 cm-I was assigned as vgII, of a, symmetry. Polarization analysis of the REMPI signal for this transition reveals that all bands strongly favor linear polarization; Le., the REMPI signal polarization ratio for circular to linear polarized photons is < < 3 / 2 , (5) J. C. Coll, D. R. Crist, M. G. Barrio, and N . J. Leonard, J . Am. Chem. SOC.,94, 7092 (1972). ( 6 ) A. M. Halpern, D. P. Gerrity, L. J. Rothberg, and V. Vaida, J . Chem. Phys., 76, 102 (1982). (7) P. Bruesch, Spectrochim. Acta, 22, 867 (1966).
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The Journal of Physical Chemistry, Vol. 88, No. 25, 1984
Letters TABLE I: Band Origins and Vibrational Energies for the Lowest Electronic Transitions in ABCO and ABCU
state
assignment" ABCO
energy, cm-I
0-0
39 076 618 166 914 43 750 625 950 44 390 620
3 Sb
v12/ Vll'
vgl 0-0
3P,'
Vlzl
Ygl
3P,d
0-0 Vlzl
u11'
775
vgl
925
ABCU~ 3s
3P, 3d,,X, 3dxz,p 3d,,, 4s 4Pxy "Vibrational mode noitation 3. dThis work.
(0)
J 44
45
46
47
WAVE NUMBER (sm-l)X IO-'
Figure 1. Comparison of (a) one-photon absorption3and (b) two-photon REMPI spectrum for the S2-So transition of ABCO.
indicating that the observed vibrational modes are all totally symmetric. This suggests that the 965-cm-' vibration should be assigned as vgll (al), which has an energy of 914 cm-' in the S1 state. This reassignment is also supported by fluorescence studies of ABCO by Halpern.' The one-photon absorption and two-photon REMPI spectrum for the Sz region are compared in Figure 1. The S2-So transition in ABCO has previously been assigned as a 3p,-n transition by Halpern et aL3and as a 3pxy-n transition by Parker and A v o ~ r i s . ~ Ab initio calculations by Avouris and Rossi: utilizing a Gaussian 70 STO-3G minimal basis set augmented by special diffuse functions for the s and p Rydberg orbitals, predict that the 3p core splitting of ABCO, is -0.1 eV, with 3p, component appearing at lower energy. The two-photon resonant MPI spectrum using linear polarization, Figure lb, shows some very interesting differences from the corresponding one-photon spectrum, Figure l a , which may be interpreted as follows. First, the 3p, transition almost dis(8) A. M. Halpern, J . Am. Chem. SOC.,96, 7655 (1974). (9) P. Avouris and A. R. Rossi, J . Phys. Chem., 85, 2340 (1981).
0-0 0-0 0-0 0-0 0-0 0-0
35 930 38 700 43 200 44 700
46 660 47 870 from ref 7. bReference6. 'Reference
appears, with only the origin and the 950-cm-I band at 44 700 cm-' observable. A new transition, which we assign as the 3p,-n excitation, with an overall band-shape pattern identical with the SI and S3(both of a l symmetry) transitions of the one-photon spectra3 emerges with its origin nearly coincident with the first v I 2 quanta of the 3p, transition. Because the two 3p, peaks lie on a strong background, due to overlap with the 3s and 3p, transitions, their REMPI polarization ratios are not quantitative. These peaks grow using circular polarization relative to the 3p, (and 3s) bands, whose polarization ratios are all very much less than 3/2. Since the two peaks show the same polarization ratio, they should arise from the same (3pJ excited state, and they must be separated by an a, vibration. From this data and the separation of the peaks (950 cm-I), the peak at 44 690 cm-' can be assigned as an al vibration, most likely the vgl mode (vgll = 965 cm-'). The origin of the 3p, transition observed in the REMPI spectrum lies at 44 390 cm-I, with a polarization ratio of much less than 3/2 (-0.2), and the observed vibrational modes built off of it have essentially the same ratio. The excited-state energies for these totally symmetric modes are very similar to those of the 3s-n transition. As seen in Figure 1, almost all of the 3p, vibronic bands are nearly coincident with the 3p, bands seen in one-photon absorption, since the core splitting for the 3p orbitals (-640 cm-,) is very close to the main v I 2progression (-620 cm-') of the 3p, transition. Table I shows the origins and active vibrations of the lowest ABCO transitions. The possibility that the REMPI spectrum corresponds to vibronically coupled 3p, bands rather than a 3p, state can be examined. Besides appearing at a different energy (assuming calibration of both the one- and two-photon spectra is correct), the v I 2 mode (a, symmetry) is not active in intensity borrowing. No nontotally symmetric vibrations have been identified' at the energy (640 cm-I) of the 3p, state, and no evidence of the expected nontotally symmetric components appears with the 3p, origin, by using polarization analysis. Finally, the overall band shape of the 3p, transition is identical with the SI and S3states and is not typical of a vibronically induced transition. The preceding analysis of the S2-S0transition is in agreement with the calculations performed on ABC0,9 in which the 3p,-n transition was predicted to be higher in energy than the 3p,,,-n transition by -0.1 eV. The experimental value for this core splitting is 0.08 eV. ABCU. As in ABCO, the S1-So transition of ABCU is identical in the one-photon and two-photon spectra. It has been previously assigned as a 3s-n transition,' and this assignment has been confirmed by the polarization ratios of much less than 3/2 in the MPI study.
J . Phys. Chem. 1984, 88, 6089-6090 The ABCU S2-Sotransition differs substantially between the one-photon and two-photon spectra, probably because of the overlap of the 3p,(A)-n and 3p,(E)-n transitions. Spectral congestion in the two-photon case makes vibrational analysis difficult. As in ABCO, the same Sz-So transition which appears strongly in the one-photon spectra also appears weakly (compared to S,) in the two-photon spectra, overlapped by stronger transitions and with a polarization ratio approaching 3 / 2 (estimated by subtracting base lines). The 3p,-n transition could not be identified in the REMPI spectrum. The S3-So transition is identical in the one-photon and twophoton spectra. The previous assignment of the 0-0 band at 43 197 cm-I, based on quantum defect, is uncertain, with either a 4s-n or 3d-n transition as a possibility.* From the MPI polarization ratios, this transition can be assigned to an E state based on a pair of degenerate 3d Rydberg orbitals. There is also a possible band origin at 44 695 cm-I, which is present in the one-photon spectrum but not assigned. The MPI polarization ratios allow this transition to be assigned to the second E state based on the other pair of degenerate 3d Rydberg orbitals. The E state composed of the 3d, and 3d, orbitals is expected to lie at a lower than the E state made of the 3d, and 3dy, orbitals, since orbitals containing a z component are expected to be higher in energy. This prediction is based on the 3p splitting in ABCO, and calculations on ABCU must be done before the state assignments of the 3d E states can be more definitive. The area of the spectrum between the S3-So transition and the S,-So transition contains no peaks in the one-photon spectrum, but there is a band origin at 46665 cm-I in the two-photon spectrum. This peak has a hot band of 174 cm-I and some vibrational structure. MPI polarization studies show that it corresponds to excitation of an A state. Two A states are possible at this energy, the 4s orbital or the 3d, orbital, and it is not possible to distinguish between the two alternatives.
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The &-So transition has a 0-0 band at 47 874 cm-’, and the one-photon study has assigned it as a 4s-n transition. From the quantum defect of 0.47, it seems more likely that it is a 4p-n transition. The preliminary polarization evidence suggests that it corresponds to an E state formed by the 4p, orbitals. This is analogous to the splitting of the 3p orbitals.
Conclusion The S2-S0 transition of ABCO has been analyzed by using two-photon MPI techniques. It was found that the difference between the one-photon and two-photon spectra is due to the presence of two states, the A, state from the 3p, orbital and the E state from the 3p, orbital. The two-photon spectrum is dominated by the 3p,-n transition, which is obscured by the strongly allowed 3p -n transition in the one-photon spectrum. As in ammonia’Ox&d other examples,2 multiphoton techniques with polarization analysis can provide useful information even for molecules lacking a center of symmetry. The first few Rydberg transitions of ABCU were assigned by using two-photon REMPI techniques. The previous state assignments were checked, and some modifications were made. The broader absorption bands of ABCU did not allow for as quantitative vibrational analysis as was done on ABCO, but the trends observed in ABCU showed that the two-photon enhancement of A-A transitions is a phenomenon that is not restricted to ABCO. Acknowledgment. We gratefully acknowledge the donors of Petroleum Research Fund, administered by the American Chemical Society, for financial support of this work. We also thank Professor N. Leonard for a sample of ABCU. (10) J. H. Glownia, S. J. Riley, S. D. Colson, and G. C. Nieman, J . Chem. Phys., 73, 4296 (1980).
Nature of the Lowest Emitting States of trans-(N,),W(dppe), J. G . Brummer and G . A. Crosby* Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99164-4630 (Received: April 30, 1984; In Final Form: October 8, 1984)
Thermal dependence of the spectra of the title compound implies an excited-state manifold with a 3CT term lying lowest followed by a 3LF term higher in energy. The former is responsible for the structured emission observed at 77 K, whereas the latter is assigned to the precursor state for photochemical dissociation. An electronic thermal barrier to photochemical dissociation is predicted.
Since the initial discovery that trans-(N2)zM(dppe)z is photoreactive’ (M = Mo(O), W(0); dppe = Ph2PCHzCH2PPhz),many studies have appeared in the l i t e r a t ~ r e . ~Both , ~ the substitution of I5N2for CO or l4N2and the reaction of organic halides with the W(0) complex proceed only in the presence of light. In contrast, these reactions take place in the dark for the analogous Mo(0) complex, although their rates are enhanced considerably by irradiation. Both modes of photoreactivity are given by
Recent studies have indicated that the initial process in these reactions is nitrogen dissociation2”qband that irradiation enhances this step. Dissociative photochemistry is believed to occur following the population of bond weakening ligand field (LF) states. Population of charge transfer (CT) excited states is expected to lead to electron transfer chemistry or ligand chemistry. Moreover, one might anticipate the lowest energy state of triplet spin parentage to be the dominant precursor state since it is populated
hu
t r a n ~ - ( ~ ~ N ~ ) ~ M ( d+p p2Y e), (cis~rans)-(Y)~M(dppe), + 2I5N2 +
+
t r a n ~ - ( N ~ ) ~ M ( d p p e ) ,R X
hu
trans-X(N,R)M(dppe),
+ N,
R = alkyl, acyl, aroyl; X = C1, Br, I (1) Darensbourg, D. J. Inorg. Nucl. Chem. Lett. 1972, 8, 529.
0022-3654/84/2088-6089$01.50/0
(2) (a) Caruana, A.; Hermann, H.; Kisch, H. J . Organomet. Chem. 1980, 287, 349. (b) Chatt, J.; Head, R. A.; Leigh, G . J.; Pickett, J. J . Chem. Soc., Dalton Trans. 1978, 1638. ( c ) George, T. A.; Iske, S. D. A. “Proceedings of the 1st International Symposium on Nitrogen Fixation, June 1974, Pullman, WA“; Newton, W. E., Nyman, N. J., Eds.; Washington State University: Pullman, WA, 1976; p 27. (d) Chatt, J.; Diamantis, A. A.; Heath, G. A.; Leigh, G. J.; Richards, R. L. “Proceedingsof the 1st International Symposium on Nitrogen Fixation, June 1974, Pullman, WA”; Newton, W. E., Nyman, N. J., Eds.; Washington State University: Pullman, WA, 1976; p 17. ( e ) Chatt, J.; Pearman, A. J.; Richards, R. L. J . Chem. SOC.,Dalton Trans. 1977, 1852.
0 1984 American Chemical Society
