J . Phys. Chem. 1990, 94, 1309-1313 as the conformer with the s-cis configuration, while the shortwavelength absorbing species was assigned to the s-trans conformer. The large difference in the vibronic structure between the A and B rotamers points to a difference in the nature of their lowest electronic state. The lowest excited state of the A rotamer shows a marked stilbenic character. The lowest excited state of the B rotamer is, however, characterized by an excitation localized on the naphthalenic group. Similar conclusions were drawn from a MO calculation (CNDO/S) by Bartocci et aI.l9 which assumed a planar geometry for the two conformers. The results we obtained with the A conformer of 2-styryl-3-methylnaphthaleneindicate that the difference in the nature of the lowest excited state of the two rotamers is related to differences in planarity. Obviously the release of the steric constraint in the A rotamer will involve both a slight deviation from planarity and a deformation of the P h - C a = C d angle. Ground-state calculations using the C-INDO method35indicate that this angle may be as large
1309
as 128O due to repulsive interaction between the ethylenic carbon atom C, and the aromatic carbon atom C,. Further support indicating that this deformation plays some role is the strong activity of the in-plane bending mode Ph-Ca==Cd in the spectra of the A rotamers. This result points out to the change, upon excitation, of the C,-C,=C, angle. It follows that the twist angle between the naphthalenic ring and the ethylenic plane in the A rotamer could be smaller than predicted if only the outof-plane deformation induced by the repulsion between the Hat and HI atoms were considered. Acknowledgment. We thank Professors U. Mazzucato, E. Fischer, and J. Joussot-Dubien for helpful comments. We also greatly acknowledge the technical assistance of J. C. Soulignac and C. Amine (Bordeaux) and the expert synthetic work of A. Jakob (Rehovot). Registry No. 2-StN, 2840-89-3; 2-StMN, 124200-35-7; 7-StQ, 1216 11-56-1 ; 2-NPy, 57356-01-1; 2-methylnaphthalene,91-57-6.
Proton Hyperfine Structure in the ESR AMs = f 2 Transitions of the Triplet States of 1-Naphthol and 1-Naphtholate in Glassy Matrices. A Parallel-Polarized ESR Study Mikio Yagi,* Akira Muramoto, and Jiro Higuchi* Department of Physical Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai. Hodogaya-ku, Yokohama 240, Japan (Received: April 25, 1989; In Final Form: August 1, 1989)
The effects of deprotonation on the spin distribution of 1-naphthol (1-ROH) in the lowest excited triplet (T,) state have been studied by the parallel-polarized ESR technique. The proton hyperfine structures in AMs = *2 transitions have been observed for the TIstates of 1-ROH and I-naphtholate (1-RO-) in random rigid solutions of ethanol at 77 K. The a-proton hyperfine splitting of I-RO- is smaller than that of I-ROH. With the aid of the spin-density calculation within McLachlan’s approximation, the effect of deprotonation on the spin densities at the a-carbon atoms of I-ROH in the TI state is discussed. This is the first report of the proton hyperfine structure of a TIorganic molecule in an ionic form.
1. Introduction
The zero-field splittings (zfs) of the lowest excited triplet (TI) states of organic molecules in ionic forms have been studied through conventional steady-state ESR,I4 time-resolved ESR,Ibl2 and optically detected magnetic experiments in rigid solutions at low temperatures. However, there are no literature data concerning the hyperfine structure of the TI states of organic molecules in ionic forms. The isotropic and anisotropic hyperfine structure of the TI states ( I ) Smaller, B.; Avery, E. C.; Remko, J. R. J . Chem. Phys. 1%7,46,3976. (2) Zuclich, J. J . Chem. Phys. 1970, 52, 3586. (3) Bulska, H.; Chodkowska, A.; Grabowska, A. Chem. Phys. Lett. 1972, 12, 508. (4) Chodkowska, A.; Grabowski, Z. R. Chem. Phys. Lett. 1974, 24, 11. (5) Bulska, H.; Chodkowska, A.; Grabowska, A.; Pakula, B.; Slanina, Z. J . Lumin. 1975, IO, 39. (6) Chodkowska, A.; Grabowska, A,; Herbich, J. Chem. Phys. Lett. 1977, 51, 365. (7) Yagi, M.; Higuchi, J. Chem. Phys. Lett. 1980, 72, 135. (8) Yagi, M.; Matsunaga, M.; Higuchi, J. Chem. Phys. Lett. 1982,86,219. (9) Komura, A.; Uchida, K.; Yagi, M.; Higuchi, J. J. Phofochem. Photobiol. A 1988, 42, 293. (10) Yagi, M.;Saitoh, A,; Takano, K.; Suzuki, K.; Higuchi, J. Chem. Phys.
Lett. 1985, 118, 275. ( 1 1 ) Yagi, M.; Deguchi, Y.; Shioya, Y.; Higuchi, J. Chem. Phys. Lett. 1988, 144, 412. (12) Yagi, M.; Komura, A.; Higuchi, J. Chem. Phys. Lett. 1988, 148, 37. (13) Co, T.-T.; Hoover, R. J.; Maki, A. H. Chem. Phys. Lett. 1974, 27, 5.
(14) Rousslang, K. W.; Kwiram, A. L. Chem. Phys. Lett. 1976,39,226. (15) Svejda, P.;Anderson, R. R.; Maki, A. H. J . Am. Chem. Soc. 1978, 100, 7131. (16) Herbich, J.; van Noort, H. M.; van der Poel, W. A. J. A,; van der Waals, J. H. Chem. Phys. Lett. 1979, 65, 266. (17) Motten, A. G.; Kwiram, A. L. J . Chem. Phys. 1981, 75, 2608.
0022-3654/90/2094- 1309$02.50/0
of organic molecules provides valuable information concerning their spin distribution since Hutchison and Mangum first observed the hyperfine structure of the TI states of naphthalene oriented in a single crystal of durene.l* They detected the proton hyperfine structure in AMs = f l transitions when the microwave magnetic field Bfi was applied perpendicular to the external magnetic field B. For the acid-base equilibria of organic molecules, the experiments should be carried out in water or in alcoholic-aqueous solutions by changing the pH of the medium. Because of anisotropic broadening, however, the measurements in such random rigid solutions do not permit one to observe the proton hyperfine structure when Brf is perpendicular to B. The use of random rigid solutions was first proposed by van der Waals and de Gr00t.I~ They observed the proton hyperfine structure of naphthalene in AM8 = f 2 transitions when Brf is parallel to B. This fact suggests that hyperfine structure may be observable even in AMs = f 2 transitions if the electron spinspin tensor and the electron-nuclear spin-spin tensor have common principal axes and B is along one of these principal axes.20,21 When B is parallel to the principal axis, however, the ESR AM, = f2 transition probability drops to zero with B r f I B (perpendicular polarization), while it is largest with BrfI/B(parallel polarization) .21 Recently, we have succeeded in observing proton hyperfine structures in AM, = f 2 transitions for the TI states of naphthalene and azanaphthalenes in glassy matrices and in stretched poly(viny1 (18) Hutchison Jr., C. A,; Mangum, B. W. J . Chem. Phys. 1961,34,908. (19) van der Waals, J. H.; de Groot, M. S. Mol. Phys. 1959, 2, 3 3 3 .
(20) de Groot, M. S.; van der Waals, J. H. Mol. Phys. 1960, 3, 190. (21) Kottis, Ph.; Lefebvre, R. J . Chem. Phys. 1963, 39, 393.
0 1990 American Chemical Society
1310 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 TABLE I: Zfs Parameters (in cm-I), Lifetimes (in s), 1-Naphthdate in Ethanol a t 77 K
molecule I-naphthol 1 -naphtholate OD = (-3/2)X.
T,
and a-Proton Hyperfine Splittings (in mT) of tbe TIStates of 1-Napbtbol and
Y 0.0449 0.0389
X -0.0622 -0.0563
Yagi et al.
Z 0.0173 0.0173
Do 0.0934 0.0845
Eb -0.0138 -0.0108
f
IAWI
1.22 0.57
0.77 0.73
b E = (1/2)(Z - Y). cObtained from the decay of the AMs = f l transition signal. 2‘
T X‘
5
L-
Y’
4
Figure 1. Molecular structure and coordinate systems chosen for 1naphthol.
alcohol) (PVA) films with parallel polarization.22 In the present work, we have studied the proton hyperfine structures of 1naphthol (1-ROH) and 1-naphtholate (1-RO-) in glassy matrices at 77 K using the same method. The effects of deprotonation on the proton hyperfine structure are discussed. The preliminary results have been given in a previous paper.23
2. Experimental Section 1 -ROH (Tokyo Kasei, G.R. grade) was purified by sublimation in the dark. Ethanol (Wako S.S. grade) was used without further purification. All the sample solutions of randomly oriented molecules were prepared at the concentration of 1 X mol dm-3. The deprotonated species of 1-ROH was observed in NaOHethanol mixtures. A PVA film was obtained by the same method as described p r e v i o ~ s l y . ~The ~ film was stretched about 180% at room temperature with a Shibayama SS-60 film stretcher. The ESR spectra were measured at 77 K by a JEOL-FElXG spectrometer equipped with a homemade 100-kHz power amplifier and a Varian E-236 bimodal cavity (TEIo2rectangular, 9.2 GHz with perpendicular polarization and 9.3 GHz with parallel polarization, 100-kHz magnetic field modulation). The static magnetic field was calibrated with an Echo Electronics EFM-2000 proton NMR gauss meter. Measurements of the microwave frequency were made with a Takeda Riken TR-5212 electronic counter. To improve the signal-to-noise ratio, the ESR signals were accumulated by using an Electronica ELK-5125-1 waveform storage and an NEC PC-9801 microcomputer system. The excitations were carried out using a Hanovia I-kW X e H g arc lamp through 5 cm of distilled water, a Toshiba UV-D33S glass filter, and a Copal DC-495 electromechanical shutter.
3. Results The principal axes of the zfs tensor D(x,y,z) and the hyperfine interaction tensor Ak(x’,y’,z’)for the kth nucleus and the molecular axes (L,M,N) were taken to be in accordance with the case of naphthalene as shown in Figure 1. The y axis deviates an angle of 0 from the L and y’axes. For the hyperfine interaction tensors of protons, the z’axis is parallel to the C-H bond and they’axis is perpendicular to the z’axis in the molecular plane, while the x’ axis is perpendicular to the molecular plane. The ESR spectra of the TI state of 1-ROH in ethanol solutions with various concentrations of NaOH were measured at 77 K with B r f l B as shown in Figure 2. As is clearly seen in this figure, the ESR spectrum depends on the amount of NaOH. In ethanol (22) Yagi, M.; Uchida, K. S.; Higuchi, J. J . Magn. Reson. 1987, 71, 303. (23) Muramoto, A.; Uchida, K.; Yagi, M.; Higuchi, J. Chem. Phys. Lett. 1988, 150, 325. (24) Higuchi, J.; Ito, T.; Yagi, M.; Minagawa, M.; Bunden, M.; Hoshi, T.Chem. Phys. Lett. 1977, 46, 477.
260
220
300
0 /mT
Figure 2. ESR spectra of the low-field AM, = f l transitions for the T, states of I-naphthol and I-naphtholate (a) in ethanol, (b) in NaOHethanol (0.03 wt % NaOH), and (c) in NaOH-ethanol (0.5 wt % NaOH) with B r f I B at 77 K. Y
I
2
I f+
v
I
n
2-’
2 20
260
300
B /mT
Figure 3. ESR spectra of the low-field AMs = f l transitions for the TI states of 1-naphthol (a, b) and I-naphtholate (c, d) with B r f I B in stretched PVA films at 77 K.
solution (Figure 2a), the observed spectrum is ascribed to 1-ROH because the intensity of the peaks decreases with an increase in the amount of NaOH. On the other hand, in basic solution (Figure Zc), the observed spectrum is interpreted as due to 1-RObecause the peak intensity increases with an increase in the amount of NaOH. The zfs parameters obtained are listed in Table I. The g value was assumed to be isotropic and equal to the free-electron value.
The Journal of Physical Chemistry, Vol. 94, No. 4 , 1990 1311
Hfs in Triplet 1-Naphthol and 1-Naphtholate
150
I
1
159
162 158
160 B/mT
150
160 0 /mT
Figure 4. (a, c) ESR spectra of the AM, = 1 2 transitions for the TI states of 1-naphthol with B,,#B in ethanol glasses at 77 K. (b,d) Computer-simulated ESR spectra.
The zfs parameters of I-ROH and of I-RO- shown in Table I are similar to those observed in methanol-water mixtures at 77 K.] However, in general, random rigid solutions do not permit one to determine the principal axes of D relative to the molecular axes even if certain anisotropic components of ESR signals are detected. To determine the direction of the magnetic principal axes, we measured the ESR spectra of 1-ROH in stretched PVA films, changing the pH of the medium. The observed ESR spectra of the low-field AMs = f l transitions are shown in Figure 3. According to the general relations concerning the orientation of guest molecules,25 the assignment of the resonance fields is straightforward. In the cases of aromatic molecules with planar TI states, the intensity of Y peaks is very strong when B is parallel to the stretched direction of a film s, as can be seen in Figure 3a,c. On the other hand, the intensity of X peaks is relatively enhanced when B is parallel to the normal of the film plane n, as shown in Figure 3b,d. As a result, we can reasonably assign all of the observed peaks as in Table I. The ESR spectra of the T, states of 1-ROH and I-RO- were measured at 77 K with B,dlB and are shown in Figures 4 and 5, respectively. Peaks with hyperfine structures and so-called Bmin signals were observed. Both of the observed peaks with hyperfine structures have already been assigned to the Y peaks by using the stretched PVA film method. Both of the Y peaks of I-ROH and I-RO- split into four components with relative intensity approximately 1 :3:3: I . The splitting between adjacent peaks is approximately uniform. The average value of the hyperfine splittings is estimated to be 0.77 0.02 mT in I-ROH and 0.73 f 0.02 mT in 1-RO-. We assigned the observed multiplets as being due to hyperfine interactions with the three a-protons. The fact that in the T, state these approximately equivalent hyperfine splittings are obtained for three inequivalent a-protons is quite similar to phenomena with those obtained and discussed for aprotons of aza- and diazanaphthalenes (with different symmetries
*
( 2 5 ) Higuchi, J.; Yagi, M.; Iwaki, T.; Bunden, M.; Tanigaki, K.; Ito, T. Bull. Chem. SOC.Jpn. 1980, 53,890.
163
B / mT
B/mT
Figure 5. (a, c) ESR spectra of the AM, = f2 transitions for the T, states of 1-naphtholate with B,dlB in ethanol glasses at 77 K. (b, d) Computer-simulated ESR spectra.
0.L
0.3
0.2
0.1
I
Figure 6. Plots of lu/lmin versus the angles between B and s.
from that of naphthalene).22 The most notable observation is that the average value of the a-proton hyperfine splitting of I-ROis smaller than that of 1-ROH. Assuming a planar structure of the molecule in the TI state and an isotropic g tensor, the direction of the x'axis of Ak coincides with that of the x axis of D as shown in Figure 1. Thus, if 0 of the neutral molecule is nearly equal to that of the ionic molecule, these results suggest that, in the absolute value, the principal values of Ak of the a-protons in the T, state decrease on deprotonation of I-ROH as discussed in section 4.2. To examine this point more precisely, we have estimated the 0's of 1-ROH and 1-RO- using the stretched PVA film method. The relative intensities of Y peaks for the AMs = f l transitions (Zy)to that of Bminpeaks (Imin) were observed by changing the angles between B and s in the film plane with B , , I B , F. The results are shown in Figure 6 . As is clearly seen in this figure, the Iy/Zhn value gives a maximum at $. = 0' for both 1-ROH and 1-RO-. This apparently implies the fact that the 0 value of I-ROH is nearly equal t o that of 1-RO- within the resolution of
1312 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Yagi et al.
TABLE 11: Calculated %-ElectronSpin Densities for the TIStates of 1-Naphthol and 1-Naphtholate atom position molecule I-naphthol I-naphtholate I-naphtholate 0kc4
h0"
I
2.0 1.5 1.0
0.185 0.162 0.099
4 0.218 0.214 0.195
8 0.193 0.178 0.152
5 0.202 0.194 0.180
2 0.086 0.099 0.114
3 0.036 0.025 0.006
6 0.050 0.046 0.037
7 0.054 0.053 0.049
11 (0) 0.041 0.082 0.200
= 1.0
our stretched PVA film experiments, about IOo.
4. Discussion 4 . I. Spin Hamiltonian. Assuming the isotropic g value, the spin Hamiltonian describing the TI states of 1-ROH and I-ROin an external magnetic field B is taken to be H = gbBB*S S*D*S- CgNkbNB'Ik + CS'Ak'Ik k
k
= gpBB*S- XS,' - YS; - 2s: - ZgNkbNB'Ik -k Ls'Ak'Ik k
k
(1) Here, -X,-Y, and -Z are the principal values of D. The other symbols have their usual meaning. From ESR experiments on it follows that for the /3-protons (2, 3,6, and 7 ring positions) of naphthalene the magnitude of all the hyperfine tensor components is less than 0.23 mT, and these may be safely neglected. For the a-protons (1, 4, 5, and 8 ring positions) of naphthalene the hyperfine interactions are &I, i= 0.773 mT, IAJ = 0.561 mT, and lAiil zz 0.248 mT.26 Hence, Af,, and of a-protons give the important contribution in the hyperfine terms. 4.2. Computer-Simulated ESR Spectra with BAIB. To make sure of the assignment of the hyperfine structure mentioned above, the ESR spectrum of the randomly oriented TI state with B,AIB was simulated in the same manner as presented by Kottis and Lefebvre21 with some modifications concerning the hyperfine interactions. The present simulation is based on the following assumptions. I . The hyperfine interactions of the unpaired electron spins with the three a-protons are dominant, and those with the four /?-protons can be safely neglected. 2. The contribution of the a-electron spin density on the ith carbon atom, pi, to the principal values of Ak for the proton adjacent to the ith carbon atom is proportional to pi. The proportionality constant is taken to be in accordance with the cases of the T I states of naphthalene,26 fl~oronaphthalene,~' anthracene,'* and a~ridine:'~ AXjd= - 2 . 4 6 ~mT ~ A,,,y, = -3.70p, mT A,,
(2)
= - 1 . 2 0 ~mT ~
3. The three a-protons are equivalent in the TI states of 1-ROH and 1-RO- according to the present observation. 4. The 19 value of 1-ROH is equal to that of 1-RO-, 0'. In the present simulation, the Gaussian line width of 0.55 mT and the zfs parameters listed in Table I were used. Figure 4b,d shows the simulated ESR spectra of 1-ROH with B,I(IB. The principal values of Ak for the three a-protons used in the simulation were A,, = -0.510 mT, A,,,,, = -0.770 mT, and AZ,, = -0.250 mT with pa = 0.208. Figure 5b,d shows the simulated ESR spectra of 1-RO- with B,AIB. The principal values of Ak for the three a-protons used = -0.490 mT, A,,,, = -0.730 mT, and in the simulation were Aii = -0.240 mT with pa = 0.197. These results may show that within the assumption of use of the same proportionality constants as in eq 2, the a-electron spin densities at the a-carbon atoms of 1-ROH in the TI state decrease on deprotonation. Actually, these proportionality constants are slightly changeable according to the (26) Hirota, N.; Hutchison Jr., C. A.; Palmer, P. J . Chem. Phys. 1964,
40, 3717.
(27) Mispelter, J.; Grivet, J.-Ph.; Lhoste, J.-M. Mol. Phys. 1971, 21, 999. (28) Grivet, J.-Ph. Chem. Phys. Lett. 1969, 4, 104. (29) Grivet, J.-Ph. Chem. Phys. Lett. 1971, I I , 267.
molecular species treated here,30,31although their optimum values are uncertain. In consideration of the general trend that, in the absolute value, the proportionality constants for neutral radicals are slightly larger compared with those of anion radicals, the relative magnitude on the a-electron spin densities at the a-carbon atoms of 1-ROH and 1-RO- cannot conclusively be estimated at this stage, because for determining it the difference between the corresponding proton hyperfine splittings is not sufficiently large. 4.3. Calculated Spin Densities. Using the simple LCAO-MOs, the a-electron spin densities in the T1states of 1-ROH and 1-ROwere calculated within McLachlan's appr~ximation,~' as given in Table 11. Parameters appearing in the Coulomb and bond integrals, a. = ac + h d o and Oca = k c d o , respectively, were chosen to be in agreement with those used for the photochemically generated neutral and ion radicals of the perinaphthenone system by Rabold et al.j3 That is, ho = 2.0 and kc4 = 1.0 for 1-ROH and ho = 1.5 and 1.0 and kca = 1.0 for 1-RO-. As is clearly seen in Table 11, pa's are fairly close to each other and apparently large compared with p i s . This is a similar situation to the cases of naphthalene and azanaphthalenes in their TI states.'* The a-electron spin densities at the a-carbon atoms decrease and become less equivalent with decreasing ho at constant pea. This is in accordance with the observed effect of deprotonation on the hyperfine interaction with the a-protons. As a result, the observed multiplets were assigned as being due to the nearly equivalent three a-protons of the TIstates of I-ROH and 1-RO- within the resolution of our parallel-polarization experiments. Therefore, it may be possible to infer the fact that, on deprotonation of 1-ROH, the a-electron spin densities at a-carbon atoms decrease and the variation of the proportionality constants appearing in eq 2 is not sufficiently large for changing the sequence in the a-electron spin densities against the proton hyperfine splittings. It may be noted here that, in the T1 states of ROH and RO-, the electron structures associated with a-orbitals are not significantly different from each other, because their singly occupied orbitals are the highest occupied and the lowest unoccupied orbitals of their ground states. On the other hand, in the anion and cation radicals of ROH and their deprotonated radical species (R0"- and RO'), the p:s are actually different from each other as can be deduced from the corresponding proton hyperfine constant and from the different nature of each singly occupied orbital (corresponding to the highest occupied orbital and the lowest unoccupied orbital of the neutral molecule ROH).
5. Conclusion The proton hyperfine structure has not yet been observed for AM, = f l transitions of triplet molecules oriented randomly in solution. On the other hand, it has been observed in the AM, = f 2 transitions. This is due to the fact that the anisotropic broadening of AMs = f 2 transitions is much smaller than that of AM, = fl transitions. Although the resolution of the present work is lower than that of the single-crystal work, the present method is useful for studying the anisotropic hyperfine structures of molecules under acid-base equilibria, where the single-crystal method is not applicable. We have clarified the effects of deprotonation on the proton hyperfine splitting of 1-ROH in the TI state by the parallel-polarized ESR technique. The a-proton hyperfine splitting of 1(30) Clarke, R. H.; Hutchison Jr., C. A. J . Chem. Phys. 1971,54,2962. ( 3 1 ) Yu, H.-L.; Lin, T.-S.; Chem. Phys. Lett. 1983, 102, 529. (32) McLachlan, A. D. Mol. Phys. 1960, 3, 233. (33) Rabold, G. P.; Bar-Eli, K. H.; Reid, E.; Weiss, K. J . Chem. Phys. 1965, 42, 2438.
J . Phys. Chem. 1990, 94, 1313-1316
ROH in the TI state decreases on deprotonation. This is the first report of the proton hyperfine structure of a TI molecule in an ionic form. Acknowledgment. The authors express their sincere thanks to Professor K. Obi of Tokyo Institute of Technology for his kindness
1313
in putting the Varian E-236 bimodal cavity at their disposal. The present work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science and Culture (No. 61430002 and No. 63540330). Registry NO. 1-ROH, 90-15-3; 1-RO-, 17545-30-1.
Laser Vaporization of Alloys: Fluorescence and Electronic Structure of AiCu M. F. Cai, S. J. Tsay, T. P. Dzugan, K. Pak, and V. E. Bondybey* Department of Chemistry, The Ohio State University, Columbus, Ohio, 43210-1 167 (Received: May 4, 1989)
Products of laser vaporization of the copper-aluminum alloys are studied by time-resolved, laser-induced fluorescence. The diatomic AlCu is identified among the products. Three different electronic transitions of the molecule are analyzed, and its electronic structure and bonding are discussed.
Introduction Metals and alloys are among the key industrial materials, and understanding their properties and structure is of considerable importance. A significant effort went into theoretical modeling of metals and their surfaces.’ Dimers are the simplest building blocks of solid metals, and elucidating their bonding and structureZ is an important first step toward understanding the bulk solid. Particularly interesting and particularly poorly known are the properties of heteronuclear metal dimers and clusters. The knowledge of the bonding potentials between two unlike metal atoms is of course of importance in the study of alloys. Thorough understanding of the bonding might lead to the development of alloys with enhanced properties, such as greater strength or corrosion resistance. Metals, and in particular transition metals, are important catalyst^.^*^ It is well-known that the catalytic property of the metal surface can be greatly enhanced or “poisoned” by the presence of small amounts of surface impurities. Again, in order to understand how the surface properties and catalytic activity will be affected by the impurity atom, it is essential to know the appropriate pairwise interaction potentials. The laser-vaporization-laser-induced fluorescence technique developed in our laboratory5 is very easily applied to studies of heteronuclear clusters. When classical furnace techniques are used, such studies are quite difficult, in particular if the two components have disparate vapor pressures and boiling points; by laser-vaporization of an appropriate alloy or mixture,6 one can produce metal or semiconductor vapors of any desired composition. Some time ago, we examined using this technique the diatomic CuGa and Culn speciesa6 In the present paper, we extend this study to the lighter, but equally interesting, AlCu molecule. AlCu has been previously observed by mass spectroscopic techniques,’ which permitted an estimate of its binding energy, but there are to the best of our knowledge no previous data on its optical properties. ( I ) Langhoff, S.R.; Bauschlicher, C. W., Jr. Annu. Reu. Phys. Chem. 1988, 39, 181. (2) Morse, M. D. Chem. Rev. 1986,86, 1049. (3) Bimetallic Cafalysts: Discoveries, Concepts and Applications; Sinfelt, John H., Ed.; Wiley: New York, 1983. (4) Goodman, D. W. Annu. Rev. Phys. Chem. 1986, 37,425. (5) Bondybey, V. E. Science 1985, 227, 125. (6) Bondybey, V. E.; Schwarz, G. P.; English, J. H. J . Chem. Phys. 1983, 78, 11-15. (7) Uy, 0. M.; Drowart, J . Trans. Faraday SOC.1971, 67, 1293.
Very recently, we succeeded in obtaining the laser excitation spectra of AIz, and demonstrateds that its molecular properties are in very good agreement with recent state of the art ab initio quantum mechanical calculations.+12 Also, the heteronuclear AlCu was recently theoretically examined by Bauschlicher et aI.,I3 but experimental data about its molecular properties were not available for comparison. In the present paper, we report the first experimental observation of several electronic transitions of AICu. Experimental Section The experimental techniques used in this investigation were similar to those employed in our previous studies,6 and the procedure was identical with that recently used in a study of the aluminum dimer.8 Briefly, the metal sample was vaporized in a Teflon fixture by focusing the fundamental radiation of a pulsed Nd:YAG laser (Quanta Ray DCR-1) on the metal sample; the fmture is mounted on the end of a Lasertechnics piezoelectric valve. The opening of the valve, synchronized with the laser pulse, allows the carrier gas, He in this case, to mix with the vaporized metal and sweep it out of the fixture. Carrier gas stagnation pressures in the range of 2-6 atm were used in the present work. These expansion conditions typically yielded products with rotational temperatures of 10-30 K. The metal sample was prepared as a ‘/4-in. rod to permit its rotation in the vaporization fixture. This prevents deep drilling or pitting of the sample and aids in generating a more uniform metal “flame”. The copper-aluminum metal samples were prepared by melting pure copper and aluminum chips in a quartz tube under vacuum, typically in a 1:l weight ratio. The metals were obtained from Aldrich Chemical Co. The resultant molecular beam was probed with a nitrogenpumped dye laser (Molectron DL-14); the laser-induced fluorescence of the metal species was amplified, detected by a photomultiplier tube (RCA QB9659), digitized in a LeCroy waveform recorder, and finally averaged and recorded by an IBM PC-XT computer. Atomic aluminum lines, as well as lines of known atomic or molecular impurities, were used to calibrate the spectra. In (8) Cai, M. F.; Dzugan, T. P.; Bondybey, V. E. Chem. Phys. Lett., in press. (9) Sunil, K. K.; Jordan, K. D. J . Phys. Chem. 1988, 92, 2774. (10) Pacchioni, G. Theor. Chim. Acra 1983,62, 461. ( 1 1 ) Basch, H.;Stevens, W.; Krauss, M. Chem. Phys. Left. 1984,109,212. (12) Bauschlicher, C. W.; Partridge, H.; Langhoff, S.R.; Taylor, P. R.; Walch, S.P. J . Chem. Phys. 1987, 86, 7007. (13) Bauschlicher, C. W., Jr.; Langhoff, S. R.; Partridge, H.; Walch, S. P. J . Chem. Phys. 1987, 86(10), 5603.
0022-3654/90/2094-13 13%02.50/0 0 1990 American Chemical Society
