4016
J . Phys. Chem. 1985,89, 4016-4020
Appendix. Computational Details and Summary of Convergence Tests The program used to solve the differential equation is the R X N l D code21with appropriate modifications to the way the effective potential (eq 8) is formed and the S matrix (eq 15) is calculated. The cross sections must be converged with respect to several parameters. Table VI shows the dependence of the collinear probabilities, as defined by eq 17, on the location of the turning center TC which defines the (u,p) coordinate system given in Figure 1. VLEm(RTc) = 0.83, 1.36, and 7.81 eV for the T C located at a distance of respectively 10, 1 1, and 12 a, from the origin. Closely related to the choice of RTCis the amount by which we shift the equilibrium position of the harmonic oscillator functions & , ( p l i ) away from the potential minimum to larger separations. This is regulated by a shift parameter. For a shift parameter of 0 the harmonic oscillator functions are centered at the potential minimum and for a value of 1 the location is at the intersection of the potential with V(RTc). For RTC = 10 a, in combination with shift = 0.7 the probabilities are most stable and these values are used in all other calculations. Next we have to choose the number (NVBASE) of harmonic oscillator functions & in which the locally adiabatic vibrational functions xu are expanded (eq 5 and 6). The number of functions xu is given by the parameter NCHP. Table VI1 shows that the collinear probabilities at the highest total energy considered are converged within 0.4% for the NVBASE = N C H P = 16 basis (except for probabilities less than 0.01, see table). This basis is used throughout in all the calculations. The sector widths in the u coordinate are chosen so that the (1,2) off-diagonal element of the overlap matrix between 2 basis sets ( x , ( u l i ) (x2(uli+l)) is also smaller than 0.07.
In the propagation procedure the maximum value for R , (H
+ C12 channel) is 9 a, and for R , (HCI + C1 channel) is 22 a,,.
This results in 75 propagation steps in the cy and 264 propagation steps in the y arrangement channel, and is sufficient to ensure convergence as is shown in Table VIII. In each section i the effective potential, as defined by eq 8, is taken to be independent of u and calculated at the midpoint of each sector. The potential is then expanded as a power series in ( p - pk), and pk is the position of the minimum. This power series consists of 11 terms and 20 points are used in the least-squares fit. The convergence test for this expansion is presented in Table IX. In order to calculate the bending correction we need a bending potential as given in eq 19. This potential is approximated by harmonic and quartic terms in (y - T). A 13-point least-squares fit to the LEPS potential is used to find the parameters k and q. The bending wave function +nb(y)(eq 20) is expanded in nine harmonic oscillator functions in y. Table X shows the convergence test for this fitting procedure in y. Finally the S matrix must be converged with respect to the summation over 1. Figures 13 and 14 show that we need to sum up to 1 = 90 at a total energy of 0.268 eV. At the lowest energy considered (0.101 eV) 50 1 values are enough. The following information is saved in each sector in the calculation at a total energy of 0.268 eV: the potential fits in u and y;the overlap matrices between the adiabatic function xu(pIi) and xu(&+ 1); and the eigenvalues from a diagonalization of the matrix defined by eq 11. This information is used in all the calculations at lower energies and this saves an enormous amount of computer time. The complete calculation for each 1 value takes about 58 s at the first energy whereas for subsequent energies it takes only 10 s. The CPU times quoted are those for an IBM 3081 computer. Registry No. H, 12385-13-6; CI,, 7782-50-5.
Vibronic Absorption Spectra of Condensed Ring Aromatic Cation Systems in Solid Argon Lester Andrews,* Ronald S. Friedman, and Benuel J. Kelsall Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: April 2, 1985)
The vapor from five condensed ring aromatic compounds was codeposited with excess argon at 20 K under concurrent vacuum-ultraviolet radiation. New product absorptions observed for the radical cations in solid argon are in agreement with glassy matrix spectra and predictions from photoelectron spectra and HMO calculations. Vibrational intervals correlate with strong Raman bands of the precursor. Full high-pressure mercury arc irradiation of samples containing precursor and CH,CI, electron trap produced radical cations for phenanthrene, 1,2-benzanthracene, and chrysene, but not anthracene and tetracene indicating that resonance two-photon ionization is more efficient for the less symmetric condensed ring systems.
Introduction Molecular cations are high-energy species whose characterization requires special experimental methods. Aromatic hydrocarbons occupy a special place in chemistry, and information on their electronic structure can be obtained from spectroscopic studies of condensed ring cation systems. The naphthalene cation has been studied extensively by phot~electronl-~ and absorption spectroscopy4-* and by calculation^.^^^^^-^^ Matrix isolation ( I ) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. 'Molecular Photoelectron Spectroscopy"; Wiley-Interscience: New York, 1970. (2) Eland, J. H. D.; Danby, C. J . Z . Naturforsch. A 1968, 23, 355. (3) Clark, P. A.; Brogli, F.; Heilbronner, E. Helu. Chem. Acra 1972, 55, 1415. (4) Badger, B.; Brockelhurst, B. Trans. Faraday SOC.1969, 65, 2588. (5) Shida, T.; Iwata, S. J . Am. Chem. SOC.1973, 95, 3473. (6) Haselbach, E.; Bally, T.; Gschwind, R.; Klemm, U.; Lanyiova, 2. Chimica 1979, 33, 405. (7) Andrews, L.; Blankenship, T. A. J. Am. Chem. Sac. 1981,103, 5977. ( 8 ) Andrews, L.: Kelsall, B. J.; Blankenship, T. A. J. Phys. Chem. 1982, 86, 2916.
techniques provided vibronic absorption spectra for the naphthalene cation,'%*and resonance two-photon ionization of dilute precursor samples allowed the observation of five electronic transitions for the naphthalene cation.I2 Condensed aromatic systems of three and four rings were examined to test the matrix two-photon ionization method and to observe absorption spectra of their radical cations in solid argon. Absorption spectra for several of these cations have been observed in y-irradiated glasses at 77 K,S in UV-irradiated boric acid glasses,13in sulfuric acid,14 and in solution following pulse r a d i o l y s i ~ . ~ ~ ~ ~ ~ (9) Coulson, C. A.; Streitweiser, A., Jr. "Dictionary of Electron Calculations"; Freeman: San Francisco, 1965. (IO) Zahradnik, R.; Carsky, P. J. Phy. Chem. 1970, 74, 1240. (11) Helmstreit, W.; Hanschmann, G. J. Prakt. Chem. 1980, 322, 981. (12) Kelsall, B. J.; Andrews, L. 1.Chem. Phys. 1982, 76, 5005. (13) Kahn, 2. H.; Khanna, B. N. Can. J . Chem. 1974, 52, 827. (14) Kimura, K.; Yamazaki, T.; Katsumata, S. J. Phys. Chem. 1971, 75, 1768. (15) Kira, A,; Arai, S.; Imamura, M. J . Phys. Chem. 1972, 76, 1119.
0022-365418512089-4016$0 1.5010 0 1985 American Chemical Society
Condensed Ring Aromatic Cations in Solid Ar
9 00
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4017
600
500
700
WAVELENGTH I n t nI
Figure 1. Absorption spectrum of sample prepared by depositing anthracene vapor into excess argon at 20 K with concurrent argon resonance irradiation for 6 h. The vertical lines denote band positions given in Table I. TABLE I: Vibronic Absorptions Assigned to Anthracene Cation in Solid Argon at 20 K A, nm v, cm-' Au, cm-' assignment 0 origin 722.0 13 850
a
704.0 686.8 660.0 644.5 630.5 618.0 606.0
14210 14560 15 150 I5 510 15 560 16 180 16500
360 350 1300 360 350 1030 1350
(394)' (394) (1403) (394) (394) ( 1006) (1403)
564.6 521.1
17 710 19 190
0 1480
origin (1 505)
428.0 404.0
23 360 24 750
0 1390
origin (1403)
Fundamentals from Raman spectrum of polycrystalline anthracene,
ref 19.
Experimental Section The cryogenic and vacuum apparatus, and vacuum ultraviolet and two-photon ionization have been described previously. The precursor samples were heated to 50-130 OC in an external apparatus controlled by a needle valve or in a small internal Knudsen cell and evaporated or sublimed directly into the argon matrix gas often containing 0.2% CH2C12added to serve as an electron trap. Samples were deposited for 3-6-h periods and spectra were recorded on a Cary 17 spectrometer; the matrix was irradiated by the high-pressure arc two-photon ionization sourceI2and more spectra were recorded. In other experiments samples were deposited 3-6 h with concurrent irradiation from a 3-mm-orifice windowless argon resonance lamp.18 Samples (Aldrich) were heated under vacuum to deposition temperature before use. Precursor concentrations in argon were on the order of 1:400 in these experiments based on comparison with nearultraviolet precursor absorption in similar naphthalene experiments. Results Observations for the five precursors will be presented in turn. Anthracene. Anthracene (1) was examined in several exper-
iments. Exposure of a sample with CH2Cll to the full highpressure mercury arc gave no evidence for product formation. However, concurrent sample deposition with argon resonance radiation gave the sample whose absorption spectrum is illustrated (16) Brede, 0.; Helmstreit, W.; Mehnert, R. Chem. Phys. Lett. 1974, 28, 43. (17) Andrews, L. J . Chem. Phys. 1975, 63, 4465. (18) Andrews, L.; Tevault, D. E.; Smardzewski, R. R. Appl. Spectrmc. 1978, 32, 157.
1
T
'100
600 WRVELENGTH ( h m )
BOO
Figure 2. Absorption spectrum of sample prepared by codepositing phenanthrene vapor into excess argon with CH2CI2at 20 K under argon resonance irradiation for 3 h.
in Figure 1; the major product band appeared at 722.5 nm ( A = absorbance = 0.08, fwhm = 400 cm-I); product band positions are listed in Table I. This sample contained no CH2C12,and exposure to visible and near-ultraviolet irradiation for 15-min periods successively diminished the product absorptions to 20% of their original absorbances. Similar experiments with CH2C1, added gave increased product absorbances. Phenanthrene. New absorptions were produced from phenanthrene (2) by both photoionization sources. Figure 2 shows the
0 2
spectrum of a sample formed by concurrent argon resonance irradiation of a matrix containing phenanthrene and CH2CI2for 3 h; the major product absorption at 899.8 nm was A = 0.34 with fwhm = 140 cm-l; irradiation by the full mercury arc increased this band and the other product absorptions by 30%. Absorption band positions are given in Table 11. A similar sample was deposited without irradiation and then exposed to the full mercury arc for 15 min; a spectrum essentially identical with Figure 2 was produced. Another sample deposited with argon resonance radiation for 2 h produced the same product bands (899.8 nm, A = 0.22), and a 15-min full mercury arc irradiation doubled the product absorbances. Tetracene. Tetracene (3, also 2,3-benzanthracene or naph-
3
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The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
TABLE 11: Vibronic Absorptions Assigned to Phenanthrene Cation in Solid Argon at 20 K A, nm Y, cm-' Au, cm-' assignment 899.8 11 115 0 origin 857.8 a1(548)" 545 11 660 837.8 820 11 935 a1(829) 820.0 1080 2 X 540 12 195 802 sh 1355 a,(1350) 12470 795.0 1465 a , (1 442) 12580 789.2 1555 12670 a,(1527) 781.6 1680 12795 a, ( 1600) 762.5 2000 13115 1460 + 540 756.8 2100 13215 1560 + 540 750.0 2220 13 335 1680 + 540 715 sh 2875 2 X 1438 13 990 705 3065 14 180 2 X 1533 694 14410 3295 2 X 1648
Andrews et al. TABLE 111: Vibronic Absorptions Assigned to Tetracene Cation in Solid Argon at 20 K A, nm
u, cm-'
Au, cm-'
assignment
875.2 867.8 840.0 801.5 773.8 765 sh
1 1 430 11 520 11 900 12480 12920 13 070
0 380 960 1400 155Q
site origin (445)" (999) (1 385) (1 544)
745.4
13 420
0
origin
392.6 388.4 387.7
25 470 25 750 26 200
0 280 730
origin (3 16) (754)
347.9 339.6 336.4 33 1.5
28 740 29 450 29 730 30 170
0 710 990 1430
origin (754) (999)
._
64 1 624
15 600 16020
0 420
origin a,(406)
473.7 461.5
21 110 21 670
0 560
origin al(548)
22.
430 421 412
23 250 23 750 24 250
0 500 1000
origin a1(548) 2 X 500
398
25 130
0
bands were observed with threefold more absorbance. After full mercury arc irradiation the product band absorbances increased 25%. Chrysene. Two similar studies were done with matrices containing chrysene (5) and CH2C12. The sample was deposited for
Fundamentals from Raman spectrum of crystalline tetracene, ref
origin
"Symmetric fundamentals from Raman spectrum of phenanthrene, ref 21.
5
2 h, and full mercury arc irradiation for 30 min produced a sharp 1166-nm band ( A = 0.05) with 1102- and 998-nm satellites, a weak 675-nm band ( A = 0.01), a broad 473-nm band ( A = 0.03) with a 456-nm satellite, and a broad 407-nm band ( A = 0.03). The sample was again deposited for 2 h, this time with concurrent argon resonance radiation, and the same product absorptions were observed with half-again greater absorbance. A final 20-min irradiation with the mercury arc increased these bands by 20%.
300
I
I
500
700
I
900
WAVELENGTH i n m )
Figure 3. Absorption spectrum of sample prepared by codepositing tetracene vapor into excess argon with CH2Cl2at 20 K for 2 h then for an additional 2 h with concurrent argon resonance irradiation. The dots indicate precursor absorptions.
thacene) was the subject of three experiments. After sample deposition with Ar/CH2CI2 for 2 h, full mercury arc exposure produced no new absorptions. However, continued sample deposition with concurrent argon resonance radiation produced the spectrum illustrated in Figure 3; 290-1000-nm photolysis resulted in a 10% decrease in product absorptions, which are listed in Table 111. 1,2-Benzanthrpcene, A pair of experiments was done with 1,2-benzanthraceqe (4). In the first experiment, the sample was
4
deposited for 3 h and then irradiated with the full mercury arc for 20 min; new product bands were observed at 894 ( A = 0.07), 844 ( A = 0.01), 650 ( A = 0.01), 473 ( A = 0.02), and 4 0 3 nm ( A = 0.03). In the second experiment, the sample was irradiated with argon resonance radiation during deposition and the same
Discussion The product bands reported here can be assigned to parent radical cations on the basis of comparison to photoelectron and glassy matrix absorption spectra and calculations of transition energies. Anthracene. The strong band origin at 722 nm in solid argon is in excellent agreement with the 725-nm origin produced by y-radiolysis of 1 in butyl chloride glass at 77 K and the 720-nm transient band observed after pulsed radiolysis of 1 in cyclohexane, ~ phowhich have been assigned to the 1 c a t i ~ n . ~In, ' addition, toelectron spectra predict a 2Au 2B2gtransition ( x , x s electron promotion) at 1.75 f 0.02 eV (708 nm),3 in excellent agreement with the observed band; the difference can be attributed to a small (265 cm-I) red matrix shift due to the solid argon host. Calculations also predict this transition at 775 (HMO)9 and at 738 nm (HMO + perturbation for changes in bond length on i~nization).~ The 565- and 428-nm band origins in solid argon agree with 575- and 437-nm glassy matrix meas~rements.~ The 575-nm band does not correlate with photoelectron spectra so it must involve antibonding molecular orbitals. The former is in excellent agreement with the transition ZB,,(s8*) 2Bz,(x7)predicted at 548 nm by H M O calculation^.^ The 428-nm transition is near the 2.8 1 f 0.02 eV (441 nm) interval predicted by photoelectron spectra for the ZBlu ZB (T, x4electron promotion) transition calculated at 454 ( H M 8 ) and 429 nm ( H M O + PERT).3*9 The vibronic fine structure included in Table I for the 1 cation is in accord with the Raman spectrum of polycrystalline 1 at room t e m p e r a t ~ r e . ' ~The major vibronic intervals are near the 1403-
-
- -
-
-
(19) Ohta, N.; Ito, M. Chem. Phys. 1977, 20, 71
Condensed Ring Aromatic Cations in Solid Ar
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4019
and 1556-cm-l values for the strong totally symmetric C-C stretching vibrations for the neutral molecule. Phenanthrene. The sharp origin band at 899.8 nm in solid argon is in excellent agreement with 920-nm glassy matrix and solution pulse radiolysis measurement^,^-'^ the 1.40 f 0.02 eV (886 nm) difference between photoelectron bands for the ZAz 2Bl (a7 aselectron promotion) transition,20and the 846-nm prediction from H M O calculations9for the 2 cation. The weak 641-nm origin is in agreement with the 2.00 f 0.02 eV (620 nm) and 648-nm predictions from photoelectron spectraZoand H M O calculationsg for the 2Bl *B1(a7 a4electron promotion) transition. The three stronger bands at 474,430, and 398 nm compare favorably with glassy matrix5 bands near 483,440, and 402 nm; the 474-nm band is in agreement with 2.68 f 0.02 eV (463 nm) and 498-nm predictions from photoelectron spectra and H M O calculations for the 2BI 2Bl (a7 a3electron promotion) transition. The next possibilities from H M O calculations for the 430- and 398-nm azelectron bands are 'A2(ae*) ZB,(r7)and 2A2 2B, ( A , promotion) transitions predicted at 375 and 337 nm, respectively; better agreement can be expected with more sophisticated calculations. Considerable vibrational fine structure was resolved in the argon matrix absorption spectrum of the 2 cation which exhibited better resolution than the glassy matrix absorption spectrum. The major vibronic intervals correspond closely with strong a, Raman bands for 2 in solution.21 The strong Raman signals at 1350, 1442, 1527, and 1600 cm-' corresponding to C-C stretching modes have nearby counterparts that are active in the vibronic spectrum. The symmetric deformation mode at 548 cm-' in the Raman spectrum is active in the vibronic spectrum as a 545-cm-l fundamental, overtone, and in combination with the C-C stretching modes. In contrast to the as upper state, the a4 upper state involves the 420-cm-' symmetric deformation mode at 406 cm-l in the Raman spectrum for phenanthrene. Tetracene. The four band origins listed in Table I11 for the 3 cation agree to f 5 nm with glassy matrixs
