J. Phys. Chem. 1985, 89, 4550-4553
4550
Vibronic Absorption Spectra of Dimethylnaphthalene Cations in Solid Argon Lester Andrews,* Ronald S. Friedman, and Benuel J. Kelsall Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: January 28, 1985; I n Final Form: June 21, 1985)
Argon matrix samples containing dimethylnaphthalenes and methylene chloride were subjected to full high-pressure mercury arc photolysis. This treatment produced five new product absorption band systems for each of the dimethylnaphthalene cations in solid argon. Substituent effects were noted on the band origins and the vibronic structure.
Introduction The methyl group is one of the most important substituents in organic chemistry. Methyl substitution of an unsaturated or aromatic hydrocarbon lowers the energy of the doublet ground or excited states of the radical cation thus leading to a red shift in the corresponding vertical ionization energies.' However, methyl substitution can affect the relative ordering of transition energies depending on the ?r-electron population at the center of s u b s t i t ~ t i o n . ~Reasonably ,~ good agreement has been obtained for changes based on a simple Huckel model and the 2A *X and 2B *X transitions for naphthalene cations, and no significant improvement was found in more sophisticated configuration interaction calculations.' Since matrix two-photon ionization methods have given large yields of naphthalene cations for observation of five electronic transitions$ it was decided to explore the experimental effects of dimethyl substitution and to use simple Hiickel calculations to predict the dimethylnaphthalene cation transition energies.
-
-
Experimental Section The cryogenic apparatus has been described previously.5,6 Due to the relatively low volatilities of dimethylnaphthalenes (Aldrich), the precursors were evaporated directly through a needle valve into the sample formed by condensing an Ar/CH2C12 = 300/ 1 mixture at 20 f 2 K for 2-3-h period^.^ On the basis of comparison with naphthalene precursor absorption in similar ~ t u d i e s , ~ the Ar/dimethylnaphthalene ratios are greater than 1000/ 1. Matrix samples were irradiated for 20-40 min periods with a high-pressure mercury arc (Illumination Industries, Inc., BH-6- 1, loo0 W) 20 cm from the sample focussed by a quartz lens through a water filter. The photon flux was measured with a thermopile power meter using an identical illumination geometry, as described previously; the 220-1000-nm light intensity at the sample was 8 X 10'' photons/(cm2 s ) . ~
Results Matrix photoionization experiments with dimethylnaphthalenes will be described in turn. 2,3-Dimethylnaphthalene.Figure l a displays the spectrum of an argon/methylene chloride/2,3-dimethylnaphthalenemixture; note the sharp weak 316.8-nm and strong broader 286-nm vibronic band systems of the precursor. Irradiation of the sample first with the 290-1 000-nm output from the high-pressure mercury arc gave no new absorptions, but a similar 220-1000-nm irradiation produced the spectrum illustrated in Figure lb. Five new band origins were observed at 677.4 nm ( A = absorbance = 0.51), 487.1 nm (1) Heilbronner. E.; Hoshi, T.; von Rosenberg, J. L.; Hafner, K. Nouu. J . Chim. 1977, 1, 105. (2) Longuet-Higgins, H. C.; Sowden, R. G. J. Chem. SOC.1952, 1404. (3) Coulson. C. A. ProE. R. Sor. London. Ser. A 1952, 65, 933. (4) Andrews, L.; Kelsafi, B. J.; Blankenship, T. A. J. Phys. Chem. 1982,
86, 2916. (5) Andrews, L. J . Chem. Phys. 1975, 63, 4465. (6) Kelsall, B. J.; Andrews, L. J . Chem. Phys. 1982, 76, 5005.
TABLE I: Vibronic Absorption and Vibrational Spacings for 2,3- and 2.6-Dimethvlna~hthaleneCations in Solid Argon 2,3 A. nm
u.
cm-'
~~
2,6
Av. cm-' site origin site
682.4 677.4 662.9 657.8 639.8 622.5 617.1 606.0 601.1 586.0 571.5 567.4 553.8 540.8
14650 14760" 15090 15200 15630 16070 16200 16500 16640 17060 17500 17620 18060 18490
487.1 472.3 453.6 431.7
20530 21 170 22050 23160
origin
386.5 379.6 373.3 370.1 360.1
25870 26340 26790 27020 27770
origin
316.9 312.5 309.7 305.5 303.0 296.5
31560b 32000 32290 32730 33000 33730
origin
283.7
35250
origin
440 430
site 1440
site 440 430 440 1420 440 430 640 1520 2630 470 470 1150 7 50
440 730 1170 710 730
A. nm
u.
cm-I
Au. cm-I
667.4 663.8 657.6 646.9 640.2 630.0 624.3 606.9 601.7 592.3 587.6 578.5 558.5 546.1
14980 15O6Oc 15210 15460 15620 15870 16020 16480 16620 16880 17020 17290 17910 18310
site origin site
481.7 460.0 448.0 430.0
20760 21740 22320 23260
origin
388.8 383.7 369.8 359.8
25720 26060 27040 27790
origin
319.5 315.3 311.4 308.1 304.6 301.2
31300 31 720' 32110 32460 32830 33200
site
283.1
35320
origin
400
site 410
site 1420
site 400
site 410 1430 400 980 1560 940 340 1320 750
origin 390 740 370 740
"Accuracy f 1 0 cm-I, full width at half-maximum = 100 cm-l. bAccuracy f 1 0 cm-', fwhm = 150 cm-I. cAccuracy f 1 0 cm-I, fwhm = 130 cm-'. ( A = 0.07), 386.5 nm ( A = 0.21), 316.9 nm ( A = 1.5), and 283.7 nm ( A = 0.3). Vibronic structure is given in Table I. In another 2,3-dimethylnaphthalene experiment, the sample was exposed to argon resonance vacuum-UV radiation by using methods described previ~usly,~ and the same product band systems were observed with 0.2 of the absorbance reported above. However, exposure of the sample to 220-1000-nm mercury arc radiation increased the band systems 5-fold. 2,6-Dimethylnaphthalene,Several experiments were performed with 2,6-dimethylnaphthalene. Figure 2a shows the spectrum of the deposited sample in a typical experiment; three sharp welldefined precursor origins were observed at 321.2, 289.8, and 247.7 nm. Irradiation for 30 min using the full arc reduced the precursor absorptions about one-third and produced five new product band origins at 663.8 nm ( A = 0.22), 481.7 nm ( A = 0.06), 388.8 nm ( A = 0.16), 315.3 nm ( A = 0.75), and 283.1 nm ( A = 0.24), as shown in Figure 2b. The 663.8-nm origin band exhibited a sharp
0022-3654/85/2089-4550$01.50/00 1985 American Chemical Society
Spectra of Dimethylnaphthalene Cations
i
The Journal of Physical Chemistry, Vol. 89, No.21, 1985 4551
I
A.0.32
A.0.15
w
u
z U
m
a 0
m m U
250
350
'i 50
550
650
350
50
'i 50
550
650
WAVELENGTH [nm 1
WFIVELENGTH [nm1
Figure 1. Absorption spectra of argon/methylene chloride/2,3-dimethylnaphthalene sample: (a) after 2-h deposition at 20 K; (b) after 220-1000-nm irradiation for 20 min.
Figure 3. Absorption spectra of argon/methylene chloride/l,3-dimethylnaphthalene sample: (a) after 3.5-h deposition at 20 K; (b) after 220-1000-nm irradiation for 12 min.
TABLE I 1 Vibronic Absorption and Vibrational Spacings for 1,3and 1,4-DimethylnaphtbaleneCations in Solid Argon"
695.6 675.6 658.2 632.8 616.4 580.0
1,3 cm-' 14380 14800 15 190 15800 16220 17 240
47 1.3 461.3 454.0 450.6 439.4
21 220 21 680 22 030 22 190 22 760
origin 460 810 970 1540
448.1
22320
origin
389.8 374.8 363.5
25 650 26 680 27 510
origin 1030 1860
387.2
25830
origin
313.6 308.9 307.2 304.7 298.6
31 890 32 370 32 550 32 820 33 490
origin 480 660 930 1600
311.8
32070
origin
282.7 279.0
35 370 35 840
origin 470
278.2 273.2 269.7
35950 36660 37080
origin 650 1130
A, nm
250
350
9 50
550
650
WAVELENGTH lnml
Figure 2. Absorption spectra of argon/methylene chloride/2,6-dimethylnaphthalene sample: (a) after deposition at 20 K for 2.5 h; (b) after 220-1000-nm photolysis for 30 min.
shoulder at 657.6 nm; a similar contour was observed for the more intense members of the vibrational progression. 1,3-DimethyInaphthalene. Two studies were done with 1,3dimethylnaphthalene using different photolysis periods, which gave similar spectra. The spectrum of the deposited sample is illustrated in Figure 3a; a sharp weak precursor band was observed at 319.9 nm and a stronger, broad band peaked at 278.5 nm. Irradiation with the full mercury arc for 2 min produced five new band systems and halved the precursor absorption; irradiation for 10 more min increased the bands 20% producing origins at 695.6 nm ( A = 0.13), 471.3 nm ( A = O.ll), 389.8 nm ( A = 0.21), 313.6
Y,
1,4
AY,cm-I
A, nm
v, cm-l
AY,cm-l
origin 426 810 1420 420 1440
728.2 701.2 659.4 637.4
13730 14260 15170 15690
origin 530 1440 520
"Accuracy f 2 0 cm-I.
nm ( A = 0.45), and 282.7 nm ( A = 0.78), which are shown in Figure 3b. Vibronic data are listed in Table 11. A 30-min 420-1000-nm irradiation reduced the product bands by 5%. In the second experiment, the product band absorbance after 5-min exposure to 220-1000-nm radiation was decreased by 5% after a further 20-min irradiation. 1,I-Dimethylnaphthalene.Two experiments were done with 1,4-dimethylnaphthaIene.A 2-min full arc irradiation produced five band systems which increased by 60% after 10-min additional
4552
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
TABLE 111: Vibronic Absorption and Vibrational Spacings for Acenaphthene Cation in Solid Areon A, nm Y , cm-' Au, cm-' 677.1 14770 site origin 655.5 15 260 380 639.4 15640 600 630.7 15860 970 16 230 6 16.0 1440 598.8 16 700 600 578.0 17300 970 566.0 17 670 1450 55 1.6 18 150 620 532.8 18 770 499.2 442.5 429.5 416.7
22 260 22 600 23 310 24 000
origin 340 1050 1740
384.5 375.5 369.5 362.3
26010 26 630 27 060 27 600
origin 620 1050 1590
316.0
31 650
origin
271.8 267.1
36 790 37 240
origin 550
Andrews et al. TABLE V Comparison of Major Repeated Vibronic Intervals (cm-') in Red Absorption System of Substituted Naphthalene Cations cation 1 2 Nt 1422 505 2-MeNt 1423 436 I-MeN' 1425 514 2,3-diMeNt 1440 440 2,6-diMeNt 1420 400 420 1,3-diMeNt 1420 530 1,4-diMeNt 1440 acenaphthene 1440 600
-
bands to the B 2B3, X2Au(long axis polarized) and C 2B2, X2A, (short axis polarized) transitions has been discussed in detail for naphthalene cation^.^ The three ultraviolet transitions are due to electron-promotion transitions with upper states not attained by PES. (3) In the one case where data are available, the first three band origins of acenaphthene cation 1 compare favorably with the +-
1
TABLE I V Origin - . i d s (cm-I) of I L ? Five Transitions Observed for Naphthalene and Substituted Naphthalene Cations in Solid Areon 1 4 cation 2 3 5 Nt 14810 21 697 26 240 32510 36417 2-MeNt 14903 21 137 25 994 32051 35 753 I-MeN' 14031 22095 26 076 32 300 36 193 2,3-diMeNt 14650 20 530 25 870 31 560 35 250 2,6-diMeNt 14980 20 760 25 720 31 720 35 320 1,3-diMeNt 14380 21 220 25 650 31 890 35 370 1,4-diMeNt 13 730 22 320 25 830 32 070 35 950 acenaphthene 14770 22 260 26010 31 650 36 760
irradiation giving band systems at 728.2 nm ( A = 0.09), 448.1 nm ( A = 0.03), 387.2 nm ( A = 0.17), 311.8 nm ( A = 0.14), and 278.2 nm ( A = 0.95); a further 290-1000-nm irradiation decreased the band absorbances by 10%. In the second experiment, a 5-min full arc irradiation produced the five band systems, which increased only 20% after 20 min more irradiation. It thus appears that the optimum irradiation time for production and isolation of the new product species is between 10 and 20 min. Acenaphthene. Several studies were conducted with acenaphthene. A 20-min full arc irradiation produced strong absorption origins at 655.5, 384.5, and 271.8 nm and weaker band origins at 449 and 316 nm. These bands and vibronic fine structure are listed in Table 111. Similar spectra were produced by argon resonance photoionization of precursor molecules during sample condensation.
Discussion The new product absorptions will be identified and substituent effects on the vibronic spectra will be examined. Identification. The five band systems observed here and listed in Table IV are assigned to substituted naphthalene radical cations for the following reasons: (1) The five band origins are near values for naphthalene cation and the two methylnaphthalene cation isomer^,^ also listed in the table. (2) The two visible transitions are in excellent agreement with energy differences between photoelectron bands where data are available.' For example, 1,3-dimethylnaphthaleneexhibits four sharp PES bands at 7.86, 8.61, 9.65, and 10.51 eV. The appropriate differences, 1.79 f 0.02 and 2.65 f 0.02 eV, predict transitions at 14440 f 160 and 21 370 160 cm-l, which are in excellent agreement with the 14380 f 10- and 21 220 f 20-cm-l origins in solid argon. Although the agreement is within the measurement error, a small red shift (on the order of 100 cm-') is expected in the solid argon matrix. Assignment of analogous
*
spectrum of the cation produced in a Freon mixture at 77 K by radiolysis.' The bands in solid Freon at 14990,21980, and 25 770 cm-I are at slightly lower energy (260 f 20 cm-') than the solid argon bands due to the larger cation-matrix interaction for the more polarizable Freon matrix. (4) The vibronic intervals are in agreement with spacings for the C(9)-C( 10) stretching and ring deformation modes of naphthalene and methylnaphthalene cations, which will be discussed below. The naphthalene cations were produced here by two-photon ionization as discussed previously,6 using intense 220-1000-nm radiation. Absortion of the first photon in the strong precursor 250-280-nm absorption, followed by relaxation to the lowest singlet state (near 310 nm) and absorption of a second photon into an autoionizing state, produced the precursor cations. It is important to note here that 290-1000-nm irradiation failed to produce cation absorptions. Methylene chloride served as a dissociative electron capture trap to retain electrons removed in photoionization so that cations can be preserved from neutralization. In these two-photon ionization experiments, a dynamic photochemical equilibrium is established between two-photon ionization of parent molecules and photoneutralization of parent cations by photodetachment from chloride anion electron traps. photoionization, 250 nm t 300 nm
Nt + CH2Cl + C1CH2C12* photoneutralization, 290 nm Finally, the 2,3-dimethylnaphthalene, methylnaphthalene, and naphthalene cations were also produced by using single vacuumUV (1 1 eV) photons from an argon resonance lamp,4 which is capable .of direct photoionization of precursor molecules, and the same spectra were observed with reduced intensity. This observation supports the above identification of dimethylnaphthalene cations from their absorption spectra. Previous photoionization experiments with CF3' in the presence of C1- showed a small decrease in CF3+ on prolonged full-arc irradiation, which was attributed to photoneutralization by photodetachment from C1- electron traps, since CF,+ itself is photochemically stable in this region.* In the previous methylnaphthalene cation studies4 and the present dimethylnaphthalene cation experiments, however, the cation itself is potentially photosensitive as determined from photodissociation ~pectroscopy.~ Visible irradiation of methylnaphthalene4 and 1,3-dimethylnaphthalene cation systems with added electron traps, which do +
(7) Shida, T.; Iwata, S . J . Am. Chem. Soe. 1973, 95, 3473. (8) Prochaska, F. T.; Andrews, L. J . Am. Chem. SOC.1978, 100, 2102. (9) Dunbar, R. C.; Klein, R. J . Am. Chem. SOC.1976, 98, 7994.
J . Phys. Chem. 1985,89,4553-4560
Although the shifts for 1,3-dimethyl substitution are not canceled as compared to naphthalene cation itself, they are intermediate between 1-methyl and 2-methyl values. These effects are summarized by H M O calculations of the transition origins, which are compared with the observed values in Table VI. The purpose here is not to present state-of-the-art calculations, but to show that H M O calculations done on a small laboratory computer support trends in experimental observations. It has been pointed out that H M O calculations work essentially as well as configuration interaction calculations for naphthalene cation.' The vibrational intervals reflect the mechanical position of the substituent. The C(9)-C( 10) stretching mode is independent of substituent within experimental error since the 9 and 10 carbons contain no substituent. The ring deformation motion active in the vibronic transition, however, is dependent on the substituent position. Although naphthalene exhibits only one strong Raman band in the 400-600-cm-' region,12 the reduced symmetry for 1-methyl- and 2-methylnaphthalene gives at least two symmetric modes in this r e g i ~ n , *and ~ J ~any of these could, in principle, be active in the vibronic progression. The 1-methyl- and 1,4-dimethylnaphthalene cations exhibit the higher vibronic deformation mode whereas the 2- and 3-substituted cations show activity in the lower deformation mode. The acenaphthene cation with the ethano bridge between 1 and 8 gives the highest deformation vibronic interval, as might be expected.
TABLE VI: Comparison of Observed and Calculated Transition Energies (1000 em-') for Substituted Naphthalene Cations
transition 1 obsd
3
2
N+
14.8
2-MeN+ l-MeN+ 2,3-diMeN+ 2,6-diMeN+ 1,3-diMeN+ 1,4-diMeN+
14.9 14.0 14.8 15.1 14.4
calcd 15.1 15.2 14.9 15.3 15.3 15.2
13.7
15.1
22.3
acenaphthene
14.8
14.7
22.3
cation
obsd 21.1 22.1 20.5
calcd 22.0 21.2 22.4 20.4
21.0
20.5
21.2
21.4 22.7 22.6
21.6
obsd 26.2 26.0 26.1 25.9 25.7 25.7
calcd 27.2 27.0 26.9
27.0 26.9
25.8
26.8 26.5
26.0
26.8
not photodetach with visible light, did reduce the product absorptions slightly (5-lo%), and this is attributed to photodissociation of the cation. The optimum irradiation time for production of maximum dimethylnaphthalene cation absorbance in solid argon by the above photochemical equilibrium is between 10 and 20 min. During this period where cation formation by photoionization dominates, photodissociation of the cation itself depletes a relatively small fraction of the cation population. Substituent Effects. Two substituent effects are worth considering: the positions of the band origins (Table IV) and the vibrational intervals (Table V). The ?r-bond orders from H M O calculationslO*"for naphthalene show that the positive hole is concentrated in the C ( 1)-C(2) region in the ground 2A, state, in the C(l)-C(9) region in the excited 2B3, state, and in the C(2)-C(3) region in the higher 2B2, states4 An electron donating substituent (-CH,) is expected to stabilize the electron deficient M O at the position of substitution, which is in agreement with the observed shifts in transition origins. The effects of the 2-methyl substituent are increased with 2,3- or 2,6-dimethyl substitution since the 3 and 6 positions are equivalent to 2. Likewise the effects of 1-methyl substitution are increased by 1,Cdimethyl substitution.
4553
Conclusions The matrix 2-photon ionization technique provides a sufficient concentration of dimethylnaphthalene cations for observation of five electronic transitions. These transition origins exhibit shifts due to stabilization of the M O a t the position of the substituent. Vibronic fine structure in these absorptions also shows a dependence on the position of the substituents. Acknowledgment. We gratefully acknowledge support from N.S.F. Grant CHE-82-17749 and copies of original PES from E. Heilbronner.
(10) Clark, P. A,; Brogli, F.; Heilbronner, E. Helu. Chim. Acta 1972, 55,
1415.
(12) Mitra, S. S.; Bernstein, H. J. Can. J . Chem. 1959, 37, 553. (13) Claverie, N.; Garrigou-Lagrange, C. J . Chim. Phys. 1964, 61,889. (14) Claverie, N. Ann. Chim. 1965, IO, 5.
(1 1) Cotton, F. A. "Chemical Applications of Group Theory", 2nd ed.; Wiley-Interscience: New York, 1971; Chapter 7.
Studies of the Interaction of Pd3+ and Pd+ with Organic Adsorbates, Water, and Molecular Oxygen in Pd-Ca-X Zeolite by Electron Spin Resonance and Electron Spin-Echo Modulation Spectroscopy J. Michalik,+M. Narayana, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: February 15. 1985)
Electron spin resonance and electron spin-echo modulation (ESEM) spectroscopic studies have been carried out for several paramagnetic palladium species in Ca-X zeolite. Oxidation at 773 K of the Ca-X zeolite exchanged with Pd(NH3)42+resulted in the formation of Pd3+which on prolonged evacuation at 773 K partly converts into Pd+. Subsequent hydrogen reduction at temperatures of 295 to 500 K substantially increases the concentration of Pd' at the cost of Pd3+. Adsorption of water or oxygen on activated Pd-Ca-X zeolite results in almost identical oxygen based paramagnetic radicals indicating decomposition of water molecules by Pd" cations. The interaction of Pd' with various organic adsorbates as studied by ESEM is discussed in terms of possible locations and coordination geometries of Pd+ in Ca-X zeolite.
Introduction The efficiency of palladium-loaded zeolites as catalysts in hydrocracking,I hydrogenation, and dimerization of small olefins2J On leave from the Institute of Nuclear Chemistry and Technology, Department of Radiation Chemistry and Technology, 03-195 Warsaw, Poland.
0022-365418512089-4553$0 1.5010
is well-known. Despite this catalytic importance, little work has been done on the nature of the active palladium site, its valence
0) Rabo, J. A. "Zeolite Chemistry and Catalysis"; American Chemical Society: Washington, DC, 1976; 1st ed, Chapters 3 and 13. (2) US.Patent 3 544 565, 1972. 0 1985 American Chemical Society
