The Journal of Physical Chemistry, Voi. 83, No. 22, 1979 2869
Spectral Study of Irradiated Naphthalene Crystal
only, as the computer simulation of the spectra show that all peaks are Lorentzian in nature with a very limited real half-width.
(11) A. V. Kiselev, V. A. Loknzsievskii, and V. I.Lygin, Zh. Fis. Chim., 49, 1796 (1975). (12) A. J. Tyler, F. M. Hambleton, and J. A. Hockey, J . Catal., 13, 35 (1969). (13) C. Morterra and M. J. D. Low, J. Phys. Chem., 73, 321 (1969). (14) C. Morterra, G. Ghiotti, E. Garrone, and F. Boccuzzi, J . Chem. Soc., Faraday Trans. 7 , 72, 2722 (1976). (15) M. L. Hair, "Infrared Spectroscopy in Surface Chemistry", Marcel Dekker, New York, 1967. (16) C. G. Armistead, A. J. Tyler, F. M. Hambleton, S.A. Mitchell, and J. A. Hockey, J . Phys. Chem., 73, 3347 (1969). (17) P. G. Rouxhet and R. E. Sempels, Trans. FaraAy SOC.,70, 70 (1974). (18) R. S. MacDonald, J . fhys. Chem., 62, 1168 (1958). (19) P. R. Ryason and B. G. Russel, J . Phys. Chem., 79, 1276 (1975). (20) J. B. Peri and A. L. Hensley, Jr., J . Phys. Chem., 70, 3168 (1966). (21) R. L. Mozzi and B. E. Warren, J. Appl. Crystallogr., 2, 164 (1969). (22) G. E. Ewing, J. fhys. Chem., 37, 2250 (1962); G. E. Ewing and G. C. Pimentel, ibid., 35, 925 (1961). (23) J. B. Davies and H. E. Hallam, Trans. Faraday Soc., 87, 3176 (1971); J. Chem. Soc., Faraday Trans. 2 , 68, 509 (1972). (24) G. B. Leroi, G. E. Ewing, and G. C. Pimentel, J . fhys. Chem., 40, 2298 (1964). (25) S.W. Charles and K. 0. Lee, Trans. Faraday Soc., 61, 614 (1965). (26) . . G. C. Pimentel and A. L. McLellan, "The Hydrogen . - Bond", W. H. Freeman, San Francisco, 1960. (27) N. S.Hush and M. L. Williams, J . Mol. Spectrosc., 50, 349 (1974). (28) J. A. Cusumano and M. J. D. Low, J . Catal., 23, 214 (1971). (29) H. Knozinger, Surf. Sci., 41, 339 (1974). (30) A. Ueno and C. 0. Bennett, J . Catal., 54, 31 (1978).
Acknowledgment. This work has been carried out with the partial financial support of the C.N.R. We gratefully thank Professor A. Zecchina for helpful discussions. References and Notes (1) L. H. Little, "Infrared Spectra of Adsorbed Species", Academic Press, New York, 1966, p 306. (2) A. V. Kiselev and V. I.Lygin, "Infrared Spectra of Surface Compounds", Wiley, Toronto, 1975, p 363. (3) N. Sheppard and D. J. C. Yates, f r o c . R . SOC.London, Ser. A , 238, 69 (1956). (4) A. Zecchina, G. Ghiotti, L. Cerruti, and C. Morterra, J. Chim. fhys., 68, 1479 (1971) (5) P. J. Fenelon and H. E. Rubalcava, J. fhys. Chem., 51, 961 (1968). (6) B. A. Morrow and I.A. Cody, J. fhys. Chem., 79, 761 (1975); 80, 1995, 1998 (1976); L. S. M. Lee, ibid., 80, 2761 (1976). (7) F. Boccuzzi, S.Coluccia, G. Ghlotti, C. Morterra, and A. Zecchina, J. fhys. Chem., 82, 1298 (1978). (8) E. Borello, A. Zecchina, C. Morterra, and G. Ghiotti, J. fhys. Chem., 71, 2945 (1967), and references therein. (9) A. M. Bradshaw and J. Pritchard, Surf. Sci., 17, 372 (1969). (10) V. M. Bermudez, J. fhys. Chem., 75, 3243 (1971).
High-Resolution Absorption and Fluorescence of the I-Hydrobinaphthyl Radical in the Irradiated Naphthalene Crystal T. Nakayama and S. J. Sheng" Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received January 15, 1979) fubllcation costs assisted by the US. Depatfment of Energy
The strong absorption band at 710 nm in an irradiated naphthalene crystal was studied at 4.2 K. Evidence from the deuterium shift, the absorption spectrum in a 5050 CloH8-Cl$, crystal, and the fluorescence spectrum indicates that the radical responsible for 710-nm absorption is the 1-hydrobinaphthyl radical. This radical is formed through the bimolecular reaction between a naphthyl radical and a naphthalene molecule and is stable at room temperature in the crystalline lattice.
Introduction When aromatic crystals are subjected to ionizing irradiation, some hydrogen atoms will be produced through the breakage of carbon-hydrogen bonds. The hydrogen atoms can add to aromatic rings, taking away one of the T electrons in forming cyclohexadienyl type radicals' which are rather stable. The phenyl type radical, on the other hand, will react with another aromatic ring to form phenylcyclohexadienyl type radicals. The absorption of phenylcyclohexadienyl radicals has not yet been observed. In the case of naphthalene, Chong and Itoh2p3and Piccini and Whitten4 have observed a strong absorption around 710 nm and attributed it to the dimer radical (I). The evidence for their assignment came from the observation that the absorption band at 710 nm gradually increases for a naphthalene crystal irradiated at 77 K and annealed at room temperature. Recently we have used a 50:50 isotopic mixed crystal to demonstrate the nature of the hydrogen atom addition in naphthalene crystals by studying the absorption bands at 'The research described herein was supported by the office of Basic Energy Sciences o f the Department of Energy. This i s Document No. NDRL-1958 from the Notre Dame Radiation Laboratory.
.
0022-3654/79/2083-2869$0 1.OO/O
I
539 and 635 nm. These two bands correspond to the 1-hydronaphthyl radical5 and 2-hydronaphthyl radical,6 respectively. The same type of experiment should provide additional information on the connection between the 710-nm absorption and the postulated reaction between naphthyl radicals and naphthalene molecules. From an analysis of the splitting in the 710-nm band of the spectrum of the 5050 mixed crystal and from the magnitude of the deuterium shift of the electronic origin, it will be shown that the 710-nm transition is indeed due to the dimer radical (I). 0 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 22, 7979
T. Nakayama and S. J. Sheng
TABLE I : Absorption of Dimer Radicals C,P, DlY Dp D, Ds
A, a
-v , cm-' ~
7050.0 7065.2 7072.3 7079.6
14 180 14 150 14 136 14 121
50:50 C,,D,-C,,H, A, a cm'l Da sa RlY Ha Ds
SY
RY
S6
Hp
Rs a
7050.7 7061.8 7066.1 7077.1 7081.4 7082.7 7089.1 7090.8 7092.2 7097.1 7099.2 7107.1
14 179 14 157 14 148 14 126 14 118 14 115 14 102 14 099 14 096 1 4 086 14 082 1 4 067
CI OH, A, a Ha Hp
cm-'
7077.9 7093.1 7100.9 7108.7
14 125 14 094 14 079 14 063
Corrected to wavenumber in vacuo.
Some additional support for this assignment comes from analyzing the vibrational structure of the fluorescence of these radicals. Cyclohexadienyl type radicals fluoresce weakly. This has been shown for the cyclohexadienyl r a d i ~ a l ,the ~ l-hydronaphthyl r a d i ~ a l ,and ~ ~ ~the dibenzocyclohexadienyl r a d i ~ a l . ~The laser-induced fluorescence from a pulsed tunable dye laser is extremely useful for the study of radicals because of the low concentration and multiple chemical species in the irradiated sample. The fluorescence of the hydrobinaphthyl radical was obtained in the present work.
Experimental Section Naphthalene single crystals were grown from melt in a Bridgman oven by using zone-refined CloHs and 98% isotopically pure C10D8purchased from Aldrich Chemical Co. Rectangular pieces of crystal were cut and polished to give typically a 5-mm optical path length. The axes were identified under a polarizing microscope. Naphthalene samples were irradiated at about 273 K by keeping the crystal, wrapped with aluminum foil, in contact with a metal can filled with ice water, which served as a heat sink to prevent the crystal from melting. The exact temperature of the crystal during irradiation was not known. Naphthalene single crystals were irradiated by a beam of 9-MeV electrons from a linear electron accelerator. Five minutes of irradiation with 50-11s pulses at a repetition rate of 120 pulses/s gave a total dose of about lo7 rd. The yield of deuterio radical was lower than its hydro counterpart. The irradiation time used for deuterio and mixed crystals was twice as long as for the hydro crystal. The light source of the direct absorption study was a 150-W Xe lamp. The output of the lamp was filtered with a Corning 3-72 glass filter, polarized by a prism polarizer and passed through the sample immersed in the liquid helium. The light beam after absorption was scrambled by a quarter wavelength plate and focussed on the entrance slit of a SPEX 1402 0.85-m double monochromator. The photoelectric current from a RCA 31034 PM tube, placed in a thermal electrically cooled housing purchased from Products for Research, was amplified by a Victoreen VTE-1 current amplifier. The spectrum was recorded on a Honeywell Electronik 194 strip chart recorder. The wavelength of the absorption bands was calibrated with iron standard lines of a hollow cathode tube and should be accurate within 1 %, of the absolute value. The experimental setup for the laser-induced fluorescence was described in a previous p~blication.~ Results and Discussion Optical Absorption around 710 nm. The 710-nm absorption in an irradiated naphthalene crystal was studied
7040
7080 Wavelength
7060
7100
(8)
7120
Figure 1. The c' polarized absorption spectra of irradiated C&, (A), CjoH, (c),and 5050 CI0H8-Cl0D8 (B)crystals. H, D, R, and S represent the radicals CloHTCloH8., CloD-rCIoD8., C10DTC10H8.1and CI~-I,-CI&,., respectively.
by Itoh et al.1° at liquid helium temperature. Vibrational structure associated with this absorption was observed. In the present work only the origin part of absorption spectrum was reproduced. Four lines at 7078,7093,7101, and 7109 A were observed in C10H8crystal. They are labeled Ha,H,, H,, and Ha,respectively, as shown in Figure 1C and Table I. These absorption origins are assigned as multiple-site splittings. Multiple-site splitting is a common feature in the spectra of radicals produced by ionizing radiation5or photolysisll in the solid phase at low temperature. The absorption origin of the deuterio radical in the C&8 crystal was also studied. The yield of the deuterio radical is a factor of 2 smaller than that of the hydro radical. The multiple origins, labeled D,, D,, D,, and Dg,were found at 7050, 7065,7072, and 7080 A (see Figure 1A and Table I.). The deuterium shifts of a,p, y,and 6 sites are 55, 56, 57, and 58 cm-l, respectively. The deuterium shift is a function of the molecular weight for a class of similar compounds. Benzene has a deuterium shift of 193 cm-l for the lowest electronic transition.12 This value drops to
The Journal of Physical Chemistty, Vol. 83, No. 22, 1979 2871
Spectral Study of Irradiated Naphthalene Crystal Center
Do
Ra
Sa
,+zzcm-lqa
~zzcrn-1'
53 cmCIOD?
CIOH?
CIOD?
:Id7
CIODB*
Ciob*
CiOne*
Cldl8.
I
I
I
Figure 2. The energy spacings of four isotopic l-hydrobinaphthylradicals in a 50:50 C1,H8-CloD8 single crystal.
117 cm-' for na~htha1ene.l~The deuterium shift for anthracene is reduced further to 65 cm-l.14 The same trend is observed for cyclohexadienyl type radicals. The deuterium shifts for the H adducts of benzene? naphthalene?p6 biphenyl: and anthraceneg are 139,96,85, and 71 cm-l, respectively. The assignment of the 710-nm absorption as a dimer radical is certainly consistent with its deuterium shift. Dimer Radical Confirmation. Upon ionizing irradiation of isotopic mixed naphthalene crystal, both C10H7. and C10D7. can be added to C10H8 and C10D8 to form hydrobinaphthyl radicals. Each absorption line of the radical formed in C l a 8 splits into four lines. The fully protonated and deuterated radicals (Cl0H7-Cl0H8.and C1,,D&ODg) are called H and D radicals, respectively. The other two hybrid radicals (C&,-C1oH8* and C1d-17-C&8.) are called R and S radicals, respectively. Figure 1B shows the c' polarized absorption origins for an irradiated 5050 CioH8-CiJI8 crystal. Eight prominent bands plus four shoulders can be identified. The 7077- and 7107-A bands are clearly derived from the H, and Hs lines of the radical from the C10H8 crystal, whereas the 7051and 7081-A bands are from the D, and Ds lines of the radical from the Cl,,D8crystal. The major new lines that appeared in the 50:50 crystal include those at 7062,7066, 7089, and 7097 A. The former two are assigned as the absorption origins of S and R radicals occupying a site; whereas the latter two are assigned as the absorption origins of the R radical occupying y and 6 sites. Once the intense lines of R and S radicals are identified, we can go back and look for the rest of the weak lines. It is noted that the spacing between P and y sites, equal to the spacing between y and 6 sites, is about 15 cm-l, whereas the spacing between a and 3( sites is about 30 cm-l. This is the same in both C10H8 and C10D8 crystals. Four shoulders (see Figure 1B) can satisfactorily be accounted for by S , S8, H and H,. The missing lines include D,, D,, S,, ana R,. T k s is not surprising at all since their intensities are expected to be weak. The yield of the D or S radical is less than half that of the H or R radical. Also the concentrations of radicals occupying the 0site are low. Thus despite the fact that site splitting complicates the analysis, it is concluded that four radicals (H, R, S, and D radicals) are created in 50:50 isotopic mixed crystals. The above result is similar to the addition of hydrogen or deuterium atoms to C l a Sor Cl,,Dsmolecules in forming the hydronaphthyl radicals. The important difference, however, exists in the pattern of energy spacings. In the case of the hydrobinaphthyl radical the absorptions of hybrid radicals are near the center of the gap between the absorption of the fully deuterated and protonated radicals. Whereas for 1- and 2-hydronaphthyl radical^,^^^ the absorption lines of hybrid radicals are near the two ends of the gap. This point is illustrated by using the radicals occupying a site as an example. The energy spacings of H, R, S, and D radicals are shown in Figure 2. The zero
7200
7400
7600
7800
8WO
Wavelength I%,
Flgure 3. The fluorescence spectrum of the l-hydroblnaphthyl radical in the Irradiated naphthalene crystal at 4.2 K. The laser wavelength is tuned to 6925 A, where it absorbs one of the vlbronic levels of l-hydrobinaphthyl radical.
point energy difference between H and D radicals is 53 cm-l. The absorption of R and S radicals are symmetric to the center of gravity of the gap. Presumably, the hydrobinaphthyl radicals can also be produced by irradiating the binaphthyl crystal. We have investigated the absorption spectrum of the irradiated 1,l'-binaphthyl crystal at 4.2 K. Surprisingly, no absorption around 710 nm can be found. Rather there are two intense absorption appearing at 544 and 597 nm. This negative result suggests that the addition reaction of hydrogen atoms to the 1position of binaphthyl molecules is unfavorable. The observed absorptions are likely related to the hydrogen addition to 2,4, or 5 positions which have higher free valence electron densities.15 Fluorescence. The fluorescence quantum yield of the 710-nm transition is very small. Emission can be observed, however, with single vibronic level excitation by using a tunable dye laser pumped by a high-peak-powered nitrogen laser. The fluorescence spectrum around 710 nm in the irradiated CloH8 crystal is shown in Figure 3. The origin as well as the vibrational lines appear to be doublets. The separation between them is 13 cm-l. This is attributed to site splitting similar to that in absorption. The origins are located at 7109 and 7116 A (labeled 6 and E , respectively). The 6 line is an origin in exact coincidence with the 6 site in direct absorption. There is no absorption line corresponding to the E line but a weak site absorption is possible. The Frank-Condon factor for the vibrational modes appearing in the fluorescence is very small. Nevertheless, more than 30 lines are measured and tabulated in Table 11. The number of vibrational modes appearing in the fluorescence is expected to be large because of the molecular size and the absence of any symmetry. The major vibrational structures include 161-, 237-, 378-, 476-, 524-, 617-, 651-, 1150-,and 1385-cm-' vibrations. The vibrational frequencies of the radicals are expected to be close to those of naphthalene and binaphthyl molecules. The 378-, 476-, 524-, 1150-, and 1385-cm-l vibrations are correlated with the Raman modes (399, 473, 515, 1152, 1387 cm-l) of naphtha1ene.l6 The 617-cm-' vibration, corresponding to the most intense vibrational line (601 cm-l) in the fluorescence of l-hydronaphthyl radical^,^ was correlated with the 620-cm-l IR active mode of naphthalene. The two low-frequency vibrations, 161 and 237 cm-l, only appear in the Raman spectrum of 1,l'-binaphthyl (see Table 11). This can serve as additional evidence that the 710-nm transition is indeed due to the hydrobinaphthyl radical. Geometry of the Dimer Radical. The reaction between the naphthyl radical and the naphthalene molecule takes place in the solid lattice. The crystal structure is expected to impose some reaction anisotropy. Frequently, solid state chemical reactions take place in defect sites. This has been shown for the photodimerizationof anthracene.l' Whether or not the present radical reaction also takes place in defect
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The Journal of Physical Chemistty, Vol. 83, No. 22, 1979
TABLE I1 : Fluorescence of t h e 1 - H y d r o b i n a p h t h y l R a d i c a l a n d R a m a n D a t a of 1 , l ‘ - B i n a p h t h y l fluorescence A, A
a
a,;
cm-’
A;,
cm-’
7109.2 7116.0 7122.9 7129.8 7153.2 7163.3 7183.7 7191.6 7198.3 7220.8 7231.2 7238.4 7287.5 7305.9 7313.2 7332.5 7358.5 7365.9 7378.7 7384.7 7391.8 7399.2 7409.1 7429.1 7435.7 7444.1 7454.5 7463.9 7515.2 7524.1 7597.5 7611.3 7652.5 7660.3
14 062 14 049 14 035 14 022 1 3 976 1 3 956 1 3 917 1 3 901 1 3 888 1 3 845 1 3 825 1 3 811 1 3 718 1 3 684 1 3 670 1 3 634 1 3 586 1 3 572 1 3 549 1 3 538 1 3 525 1 3 511 1 3 493 1 3 457 1 3 445 1 3 430 1 3 411 1 3 394 1 3 302 1 3 287 1 3 159 1 3 135 1 3 064 1 3 051
0 13 27 40 86 106 145 161 161 t 217 237 237 t 344 378 378 t 428 47 6 476 + 513 524 524 t 551 569 605 617 617 t 651 651 + 760 775 903 927 998 1011
7742.9 7748.7 7765.9 7789.3 7836.3 7854.1 7874.1 7885.9 7893.5 7957.5 7964.1 8001.3 8016.2
1 2 912 1 2 902 1 2 873 1 2 835 1 2 758 1 2 729 1 2 696 1 2 677 1 2 665 1 2 563 1 2 552 1 2 495 1 2 471
1150 1150 t 1 0 1189 1227 1304 1333 1366 1385 1385 t 1 2 1499 1510 1567 1591
13 14 14 14 13
15 17
int
23 100 12 6 4 7 2 5 2 1 6 1 2 4 0.5 1 10 4 1 16 5 6 1 1 9 2 5 2 1 2 1 1 0.5 0.5 2 1 1 1 1 1 2 2 1 1 1 0.5 1
T, c m - ’ (Raman)
180
207 259 398 472 518 535
687 859’
T. Nakayama and
S.J.
Sheng
that the majority of reactions take place between naphthyl radicals and nearest-neighbor naphthalene molecules. In naphthalene crystals18 there are two nearest neighbors along each of three crystallographic axes and four nearest neighbors on the interchange equivalent positions. Since the reaction involves a u radical and a 7~ molecule, it is conceivable that the reaction between the entities occupying interchange equivalent positions will be favored. This is because the u orbital of the naphthyl radical is pointing toward the ?r orbitals of naphthalene. Presumably both 1-naphthyl and 2-naphthyl radicals are available for attacking the naphthalene 1and 2 positions. Since the 1 position has a higher free valence electron density,16”the resulting hydrobinaphthyl radical is expected to be either a 1,l’ or 1,2’ radical but not 2,l’ or 2,2’ radical. Furthermore, it is reasonable to assume the attack will take place between the closest carbon pairs of the nearest-neighbor interchange equivalent molecules. We have evaluated the interatomic distance between all carbon pairs and found the closest 1,l’ carbon is 3.8 A apart whereas the closest 1,2’ carbon is 4.0 A apart. Thus the trans-1,l’ radical (I) is expected to be slightly more favorable than the cis-l,2’ radical. This speculation needs further study. Acknowledgment. The authors thank Professor N. Itoh for providing us with a copy of their preprint cited as ref 10. We also thank Drs. G. Hug and G. N. R. Tripathi for their comments on the manuscript and Mr. Michael Kurtis for his assistance in irradiating the crystals. References and Notes R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 38,773 (1983); S. Ohnishi, T. Tanei, and I.Nitta, bid., 37, 2402 (1982).
1045 1084 1148 1255 1331 1378 1439 1587 1595
Corrected to wavenumber in vacuo
sites is not known. It is certainly possible, especially since many defects are created during the radiolysis. The fact that site splittings are observed in the irradiated sample and that the polarization of a and 6 sites are not the same could indicate that the hydrobinaphthyl radicals are formed in defects. On the other hand, it is also possible
T. Chong and N. Itoh, Mol. Cryst. Ll9. Cryst., 11, 315 (1970). T. Chong and N. Itoh, J . Phys. SOC. Jpn., 35, 518 (1973). A. Piccini and W. B. Whitten, Mol. Cryst. Liq. Cryst., 18,333 (1972). C. W. Jacobsen, H.-K. Hong, and S. J. Sheng, J. Phys. Chem., 82, 1537 (1978). S. J. Sheng, K. Nakagawa, T. Nakayama, Y. Kumazawa, and N. Itoh, Radiat. Phys. Chem., in press. S. J. Sheng, J . Phys. Chem., 82 442 (1978). T. Chong, Y. Shibata, and N. Itoh, Phys. Stat. Sol., 27a, 599 (1975). T. Nakayama and S. J. Sheng, unpublished results. K. Nakagawa and N. Itoh, Chem. Phys., 40, 89 (1979). Y. Udagawa and D. M. Hanson, J . Chem. Phys., 64, 3753 (1978). C. S. Parmenter and M. W. Schuyler, J. Chem. Phys., 52, 5388 (1970). D. S. McCiure, J . Chem. Phys., 24, 1 (1958). 0. J. Small, J . Chem. Phys., 52, 656 (1970). (a) K. Higasi, H. Baba, and A. Rembaum, “Quantum Organlc Chemistry”, Interscience, New York, 1965. (b) The free valence electron density of blnaphthyl was not calculated before. The free valence density of an analogous compound, 1-vinylnaphthalene, can be found inC. A. Coulson and A. Streitwieser, Jr., “Dictionary of ?r Electron Calculations”,?ergamon Press, New York, 1965. D. M. Hanson and R. Gee, J . Chem. Phys., 51, 5052 (1989); F. Stenman, ibM., 54, 4217 (1971). J. 0. Williams and J. M. Thomas, Mol. Cryst. Li9. Cfyst., 16, 371 (1972). S. 0. Abrahams, J. M. Robertson, and J. G. White, Acta Crystallogr., 2, 238 {1949).
