J. Phys. Chem. 1992, 96, 7876-7881
7876
delocalization in the small clusters with n = 3-5 is still in question. Spectroscopic study of (C6H6),,+with n = 4-8 is in progress and will provide much detailed information about the cluster structure.
(5) Schriver, K. E.; Paguia, A. J.; Hahn, M. Y.;Honea, E. C.; Camarena, A. M.; Whetten. R. L. J . Chem. Phys. 1987, 91, 31 31. (6) Krause, H.; Ernstberger, B.; Neusser, H. J. Chem. Phys. Lert. 1991,
References and Notes
1985, 82. 5288.
184, 41 1,
(7) Alexander, M. L.; Johnson, M. A.; Lineberger, W. C. J . Chem. Phys.
(1) Snodgrass, J. T.; Dunbar, R. C.; Bowers, M. T.J. Phys. Chem. 1990, 94, 3648.
(2) Ohashi, K.; Nishi, N. J . Chem. Phys. 1991, 95, 4002. (3) Ohashi, K.;Nishi, N. J . Phys. Chem. 1992, 96, 2931. (4) Beck, S. M.; Hecht, J. H. J. Chem. Phys. 1992, 96, 1975.
(8) Alexander, M. L; Johnson, M. A.; Levinger. N. E.; Lineberger, W. C. Phys. Rev. Lerr. 1986, 57, 976. (9) Hiraoka, K.; Fujimaki, S.; Aruga, K.; Yamabe, S.J. Chem. Phys. 1 9 9 1 , 95, 8413. (10) Ohashi, K.; Nishi, N. Unpublished results.
Infrared Spectrum of Matrlx- Isolated Naphthalene Radical Cation Jan Szczepanski, Dennis Roser, William Personette, Marc Eyring, Robert Pellow, and Martin Vala* Department of Chemistry and Center for Chemical Physics, University of Florida, Gainesville, Florida 3261 1-2046 (Received: March 24, 1992; In Final Form: June 8, 1992)
Radical cations of naphthalene (N) have been formed by electron bombardment of a vapor-phase mixture of naphthalene, carbon tetrachloride, and argon and trapped in a matrix (Ar) at 12 K. Infrared spectral scans of this matrix compared to ones containing neutral N alone or electron-bombarded CC14reveal new vibrational bands at 1525, 1519, 1401, 1218, 1215, 1023, and 1016 cm-I. These bands are attributable to the N cation by their positive correlation with the known N cation 2B,,(D,) X2A, (Do) band system at 675 nm. The observed IR band frequencies and relative intensities compare well with a recent ab initio calculation by Pauzat, Talbi, Miller, DeFrees, and Ellinger. The addition of CC14 to the mixture subjected to electron bombardment ionzation is considered and the probable role of CCll (and its products) as ionization enhancers and matrix charge stabilizers is discussed.
-
I. Introduction The field of interstellar chemistry is relatively young, tracing its beginnings to the detection in 1968 of the first polyatomic molecule, ammonia, in interstellar space.’ Four diatomics, CN, CH, CH+, and OH, had been observed earlier, the first in 1937. Since the discovery of ammonia, many molecules have been discovered in space, most by radioastronomical observations. In much of this work there has been a close interplay between laboratory measurements, theoretical calculations, and interstellar observations. One of the most perplexing problem^^,^ in astrochemistry to the present has been the origin of the so-called “unidentified infrared (UIR) emission features”, a series of bands observed at 3.3,6.2,7.8,8.7, and 11.3 pm (3030, 1613, 1282, 1149, and 885 cm-I). In 1984, Leger and Puget proposed4 that the UIR bands arise from the polycyclic aromatic hydrocarbons (PAH’s) which are excited by absorption of single ultraviolet photons which thermally heat the molecule and thereby provide excitation of the IR vibrational modes. These authors demonstrated the striking correspondence between the observed interstellar IR emission bands and the main IR absorption bands in a moderate-sized PAH (coronene): 3.3, 6.2, 7.8, 8.7, and 11.3 p m in space vs 3.3, 6.2, 7.6,8.8, and 11.9 pm in the molecule. Allamandola, Tielens, and Barker then proposed5that ionized, partially hydrogenated PAH’s were responsible for the IR emission features. These authors suggested that the PAH’s probably exist as monocations since their first ionization potentials are considerably below the energy obtainable from starlight. Other arguments regarding the probable stability and abundance of the PAH’s in the harsh interstellar environment, their absorption in the visible range, their possible hydrogen content, and state of ionization, though not as compelling as the IR spectral comparison, are sufficiently appealing to warrant further study. It is worth pointing out, as Allamandola has done: that if the hypothesis of PAH’s in interstellar space is shown to be valid (1) these molecules will be the first interstellar organic ring molecules
known, (2) they will be as abundant as the m a t abundant simpler interstellar polyatomic molecules known, (3) because of their complexity and abundance they may provide a link between interstellar gas and grains, and (4) their stability will imply lifetimes of the order of the age of the clouds in which they are found. Questions on the origin of interstellar and stellar matter, i.e., their astrochemistry, may then be probed on a more stable foundation. One of the major impediments to the conclusive identification of the PAH cations as the carrier(s) of the UIR bands is the lack of vibrational data on these species in low-temperature, isolated environments. In this paper we report on the infrared spectrum of the cation of the smallest PAH, naphthalene, isolated in an argon matrix at 12 K. The cationic species were produced by electron bombardment. By correlation of the new IR bands with the known naphthalene cation visible absorption band system at 675 nm, the IR bands could be assigned with certainty. The IR bands attributable to the naphthalene cation match reasonably well with the bands predicted by Pauzat et al.’ In the following paper, these authors report a b initio theoretical calculations of the infrared spectra of the cationic and neutral forms of naphthalene. They find the unexpected result that ionization effects the intensities of most vibrations very strongly. The intensities of the CC and in-plane C H vibrations increase while the CH stretching vibrations decrease. Experimentally, the question of the effect of ionization on the vibrational mode intensities is problematic, however, and is discussed in detail below. 11. Experimental Section
In the present study a specially-constructed electron bombardment source, sketched in Figure 1, was employed to ionize the naphthalene prior to deposition. The tungsten filament (0.1 mm diameter), was heated by a current of 1.20-1.45 A (at 6-8 V) with resultant electron emission. The anode was held a t a potential (U,)of +20 to +50 V, while the cathode potential (LIB) was maintained at -50 to -200 V. The electron beam intersected the vapor-phase mixture of argon, CCl,, and naphthalene just in
0022-365419212096-7876%03.00/0 0 1992 American Chemical Society
IR Spectrum of Naphthalene Radical Cation
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 I811
uv or
HSllKE BEAMS
Figure 1. General view of experimental setup and electron bombardment source.
front of the matrix deposition window. The naphthalene was sublimed into the flowing argon upstream from its outlet orifice. Without any gaseous mixture flowing, a copper ring situated above and adjacent to the matrix window registered 2 pA background current,with a potential (Vo)of +50 V. With the mixture flowing the current increased to 20-30 PA. Mixtures of CCl, and argon were premixed in a 5-Lglass bulb at ratios of 100/1 to 500/1 (Ar/CC14). The CC14 (Kodak, Spectrograde) and naphthalene (Baker) were used as received except for several cycles of freeze (77 K)-pumpthaw before introduction into the system. The electron-beam irradiated mixture was trapped on a BaFz window held on the 12 K cold finger of a closed-cycle helium cryatat (Displex 202,APD). BaFz was used since it is transparent over the 210 nm to 14 rcm range and is thus usable in both the IR and UV-vis regions. The infrared spectra were run on a Nicolet 7199 Fourier transform infrared spectrometer(typically with 300 scans, 1 cm-l resolution), while the UV-vis scans were done on a Cary 17 spectrophotometer (0.2-0.6 nm resolution over the 240-700 nm range). The sample cryostat was positioned in a specially-constructed sample housing for the Cary 17, which allowed the IR beam from the FTIR to pass perpendicularly to the visibltUV beam, through a set of KBr windows in the cryostat's outer shroud, impingc on a set of steering mirrors, and thence onto the detector (TGS OT MCT)located beneath the cryostat. By use of connecting tubes between the FTIR and UV-vis spectrometers, the entire IR beam path could be purged with dry N2.This cross beam arrangement permitted sample sicam on the same sample/matrix in the UV-vis and IR by a simple 90° rotation of the sample cryostat. Photolytic irradiation of the sample/matrix was performed for varying periods up to 15 min with a medium-pressure Hg lamp.
III. Results
A. Speetnl Correlation ia UV-vir and IR. The crossed IR/ UV-vir boam confuuration is a powerful method for the deter-
x4
300
400
500
t
600
700
WAVELENGTH [ n m l
Figure 2. VisibltUV spectra of neutral and cationic naphthalene in Ar at 12 K in the 290-7Wnm spectral region.
mination of which new IR bands belong to the parent naphthalene radical cation. This approach has been used previously for the investigation of the IR spectra of p-dichlorobenzene*and p-dimetho~ybenzene~ radical cations and in the study of the visible-UV spectra of certain carbon clusters.lOJ1The method is dependent on an a priori knowledge of either the visible-UV spectrum or IR spectrum of the sought-after species. By variation of the conditions under which the cation is produced, a variety of concentrations may be obtained. The IR and UV-vis spectra, run on the same matrix/sample, are then used to search for a correlation between the band intensities in the two spectral regions. If a good correlation is found, it is strong evidence that the species giving rise to the one spectrum is also responsible for the other. In the present case, the visible spectrum of the naphthalene cation is well-known, having been studied by Andrews and coworkersI2 in Ar matrices, Shida and co-workersI4 in sec-butyl chloride and Freon matrices, and very recently by Salama and AllamandolaI5 in argon and neon matrices. It is characterized
1878 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
Szczepanski et al.
.4 eY
3
0s
2
I7
Q)
i
B
e
r(
I
z
0
.2 -
I
I
1400
1200
1
1000
Wavenumbers (cm'l ) Figure 3. Infrared spectral of neutral (bottom) and neutral solid circles.
+ cationic naphthalene (top) in Ar at 12 K.
by a well-developed series of vibronic bands commencing at 675 nm (in Ar). The visible spectrum obtained in the present work, shown in Figure 2, matches closely that obtained by the above authors. The identification of the observed bands to naphthalene cation transitions and their comparison to photoelectron (PE) spectral3is discussed by Andrews and co-workers.I2 Only in the first PE transition is vibrational fine structure observed." It is a progression in the totally symmetric carbon-carbon stretching mode of frequency -1370 cm-'. We have found that the addition of CCl, to the Ar isolant gas greatly enhances (by a factor of 5 to 8) the intensity of the visible bands due to the naphthalene cation. A maximum absorbance of 1.4 was obtained for the 675-nm band after 6 h of deposition. (Recall that the ionization process (via electron bombardment) occus in the vapor phase prior to deposition and is not a solid-state irradiation as in the previous studies.) The possible reasons for this enhancement are discussed later. The IR spectrum of the matrix/sample producing the 675-nm visible band system is shown in Figure 3 (top). It coIlsists of bands belonging to neutral naphthalene plus its radical cation. Beneath it is shown the spectrum of neutral naphthalene in Ar (for which the 675-nm band system is completely absent). A comparison of the two " Ishows that new bands appear at 1537,1525,1519, 1401, 1218, 1215, 1037, 1023, 1019, 1016,927, and 898 cm-I. Previous studies have identified three of these as originating from CC,: 1037 em-' (CC13+),16927 em-I (CCl4+),I6and 898 cm-I (CC13).17 However, a recent studyla reassigns the 927-cm-' and 1019-cm-1bands to the unusual complex, CC13.Cl, in which a C1 is weakly bound to one of the chlorine in CC13. The 1525, 1215, 1016, and 1023-cm-I N+ bands overlap with the neutral N and complex CC13-Clbands (cf. Figure 3). The most intense of those remaining is the 1218/1215 cm-l doublet band. It was subjected to a correlation analysis, the results of which are sketched in Figure 4. The experimental points represent ten different experiments in which the absorbance of the 675-nm band varied (via length of deposit, intensity and ionizing electron beam, and concentration of CCl, and naphthalene) from a low of 0.16 to a high of 1.4. It can be seen that the correlation between the visible band and the 1218/1215 cm-' doublet is excellent; the correlation coefficient is 0.997, and the
New peaks due to cation are marked with
0.0
0.1 4 ABSCREANCE
-R
0.2 8
(1218/1215 &')
Figure 4. Correlation of 675-nm visible band absorbance of naphthalene cation with 1218/1215-cm-' doublet IR band absorption. The datum from an experiment with CCI, absent is marked wth an empty square.
slope is 5.90 f 0.20. This provides strong evidence that the 1218/ 1215 m-l doublet band is due to naphthalene radical cation vibration(s). Because of their weakness, it was not possible to perform such an analysis on the remaining six bands (1537,1525, 1519,1401,1023,and 1016 cm-').However, it was apparent that all these bands, except the 1537 cm-' one, tracked the intensity of the 1218/1215 cm-l doublet in a parallel manner. Thus, we believe these bands also belong to the naphthalene radical cation and that 1537 cm-' is due to a different species. B. Pbotolys& Photolysis of the deposited sample with a medium-pmsure Hg lamp has a sirylifcant effect on various band intensities. The spectra obtained in the UV-vis and IR ranges before photolysis are shown in Figures 5 (top) and 6 (top), respectively. After irradiation for 5 min, the spectra shown in F w 5 (bottom) and Figure 6 (bottom) were observed. Notice that the broad unstructured visible band centered at 420 nm almost disappears upon photolysis. (Although previously assigned to CC14+,it is here reassigned to CC13*C1on the basis of evidence presented below.) Further, the 927-cm-1 band (with a 929-m-' shoulder) is very photosensitive, almost completely disappearing
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7879
IR Spectrum of Naphthalene Radical Cation
the results of this calculation (only those vibrations displaying nonzero intensities are included); both scaled and unscaled values are presented in the table. A matchup of the experimental and calculated frequencies is also given in the table. Several interesting observations can be made from this comparison. (1) The band calculated to be most intense, the 1211 (1 lOl)-cm-' band (first value is from unscaled 3-21G calculation and value in parentheses is from scaled 6-31G* calculation'), is very close to the most intense observed cation peak at 1218 cm-I. There is a resolved shoulder on the 1218-cm-' band at 1215 cm-I. This shoulder may be due to a separate vibrational mode, correlated, for example, with the calculated 1207 (1097)-cm-' band, or may arise because of a Fermi resonance with a combination mode of several lower energy modes. We argue against the former suggestion on the basis of intensities. The ratio of calculated intensities for the 1211/1207 (1101/1097) cm-I pair is -51:l (60:1), whereas the experimental ratio of the 1218/1215 cm-I set is -3:l. Thus, it seems unlikely that the 1215-cm-l band correlates with the 1207 (1097)-cm-I band. Rather, we believe it arises from a Fermi resonance effect, and assign the 1218/1215 cm-' doublet to the bZusymmetry CH in-plane bending vibration of N+, calculated at 1211 (1101) cm-I. (2) After the 1211 (llOl)-cm-l band, the three next-most intense bands are calculated at 1058 (1004), 1659 (1490), and 1658 (1519) cm-I. These correspond reasonably well with the observed bands at 1016, 1525, and 1519 cm-I, respectively. (3) A comparison of the theoretical relative intensities (taking the band at 1211 cm-' as unity) to the experimental relative intensities (with the 1215- and 1218-cm-' pair as unity) shows a quite reasonable match (cf. Table I). (4) Two other bands are predicted to have intensities comparable to the above ones; however, the 898 (777)-cm-' band may be overlapped by the CC14 or CC13bands while the 635 (484)-cm-I band is out-of-range due to the use of the BaF2 window. In conclusion, the calculated and experimental frequencies match reasonably well. B. M e d " of Enhanced Ionization and Matrix Stabilization, The fact that the carbon tetrachloride fragments, CC13.Cl, CC13+, and CC13 (plus C1 and/or C1-, by deduction) are observed in the
4ZOnm:CCljQ
675Onm:N'
I
0.0 , 300
I
I
400
1
I
1
I
600
500
I
I
700
WAVELENGTH/nm
Figure 5. Visible-UV spectrum of neutral ( N ) and cationic (N+) naphthalene in Ar at 12 K prior to photolysis (top); spectrum after photolysis for 5 min with medium-pressure Hg lamp (bottom). Note that the intensity of the band system in the 300-500-nm range has been compressed 2-fold for visual clarity.
(-99.596) upon photolysis. There are other interesting changes: the 898-cm-l (CCI3) band increases by 27% the 1037-cm-l (CC13+)band decreases almost 40% while all the neutral naphthalene peaks decrease about 9%. Simultaneously,the 1218-cm-' (N+)line increases by 1096, as does the 675-nm (N+) visible band. The probable causes of these changes are discussed in section IVC.
-
Iv. Discwpion A. Comparison to Theoretical Frequencies. Using a SCF HartretFock approach with 3-21G and 6-31G* basis sets, DeFrees, Ellinger, and co-workers' calculated the frequencies and intensities of neutral and monocationic naphthalene. Table I gives
N/N+in
Ar, 12K
I
e
I
.2
iz,
4
1400
1200 Wavenumbers (cm-l )
1000
Figure 6. Infrared spectra of neutral and cationic naphthalene in Ar, 12 K, prior to photolysis (top) and after photolysis (bottom); same sample as in Figure 5 ([CCl,]/[Ar] = 3/1000).
7880 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
Szczepanski et al.
TABLE I: Theoretical" and Experimentalb IR Bands for Naphthrleae Cation ROHF/3-21G
symmetry and description b2, r(CH) bi, r(CH) b,, r(CH) b2u r(CH) b2, R(CC) + P(CH) + a(CCC) b,, R(CC) + @(CH) a(CCC) bi, @(CHI buR(CC) + B(CH) b,, R(CC) @(CH)+ a(CCC) b2, W C C ) + B(CW b2, N C C ) + @ ( C W b,, R(CC) + j3(CH) + a ( C C C ) b3, 4 C H ) b2, R(CC) + @(CH) a(CCC) bU4 C H ) b2, a W C ) b,, 7(CCC) b3, ~ ( C c c )
+
+
+
experiment v/cm-l IRE2
ROHF/6-3 1G*
?/cm-l
P/km/mol
-4/cm-'
3406 3394 3376 3375 1659 1658 1583 1466 1431 1299 121 1 1207 1165 1058 898 635 483 175
1 1 2 11 180 (0.16) 147 (0.13) 19 (0.02) 3 5 31 1123 (1.00) 22 (0.02) 1 662 (0.60) 162 324 48 5
3114 3104 3087 3090 1490 1519 1387 1289 1280 1140 1101 1097 1002 1004 777 484 426 156
Ic/(km/mol) 7 0.3 0 13 323 (0.28) 133 (0.12) 33 (0.03) 10 9 14 1136 (1.00) 19 (0.02)
1525 1519 1401
0.16 0.08 0.04
1218/1215 1023
0.06
1016
0.20
.oo
1
1
835 (0.70) 1 I6 563 27 3
"Theoretical symmetries, band descriptions, frequencies,and intensities from ref 7. Present work. Unscaled values, ref 7. dScaled values, cf. ref 7 for scaling factors. eIntensitiesin parentheses relative to 121 I-cm-' band (3-21G) and 1lOl-cm-' band (6-31G*). /Intensities relative to combined 1215- and 1218-cm-' bands. matrix points to the following processes occurring under electron bombardment:
+ + N
CC14
+ e-
+ + 2eCC13 + C1 + e-
e-
CC14
N+ + 2e-
CC13+ C1
e-
Because of the high electron affinity of chlorine, carbon tetrachloride is often added to matrices as an electron trap to provide charge balance for the cations under study. In the present work, however, CCl, and argon gas appear to behave additionally as ionization enhancers. We believe this may occur via the charge transfer process
+
CC13+ N
-
CC13
- + - + + - + + + - +
+ eAr + eAr*
Ar+
N
Ar*
e-
Ar+
2e-
N+
N
Ar
N+
e-
Ar
The presence of the ring electrode (with a +SO V potential) close to the deposition window serve9 as a negative ion and electron trap. The electron current monitored at this electrode was observed to increase 10-15-fold when the Ar/CCl,/naphthalene gas mixture was admitted to the deposition chamber, indicating the presence of significant numbers of charged species in the gaseous mixture just prior to deposition. In addition to the above vapor phase processes, the addition of CCll to the gas stream plays an important role in the matrix. Electron impact with Ar atoms produces low energy electrons; these slow electrons can be captured by C1 atoms (from C C 4 dissociation) to produce C1 anions. These anions (plus perhaps other impurity ions such as OH-) act to balance the excess charge created by the deposited naphthalene cations. To emphasize this point, it is instructive to point out that the potential 1 cm in front of the matrix window, created by a sample/matrix containing only positive ions and the rare gas (with a cationic absorbance of 1.O), is approximately a megavolt! Clearly, such a matrix would explode Coulombically; thus the presence of negative balancing charges (preferably infrared-invisible) is mandated for the successful
-
+
+
The 927 (9)- and 1019-cm-I bands due to CC13Cl disappear upon photolysis (cf. Figure 6). The C1 atom and CC13+(as well as other cations) will act as traps for any slow electrons produced in the two-photon photoionization of naphthalene N + hvl N*
- + + + - + + -+
N* + hu2
+ N+
or via Penning ionization. Argon, as the dominant gas in the Ar/CCl,/naphthalene mixture, may be ionized and excited by electron impact. Since the energy of the excited metastable Ar is higher than the IP of naphthalene, Penning ionization of naphthalene is possible Ar
spectroscopic interrogation of cationic species. The results of photolyzing the matrix clarifies the role of CCl, addition. The 27% increase in CC13ooncentration probably results from the photolytic decomposition of CC13Cl whose binding energy is less than the incident photon energy CC13.Cl hv CC13 C1
C1 CCl,
e-
hv
N+
e-
C1-
CC13
CC13+ e-
C1
CC13
where N* represents naphthalene in its singlet excited state (with T(SJ = 280 ns) or triplet state (with T(TJ > 2.5 s). Thus,neutral naphthalene and CC13+ are expected to decrease when the naphthalene cation and the CC13 increase, as observed. For the following reasons, we believe the broad visible band at 420 nm should be assigned to the CC13.CI complex. Earlier, it had been assignedI6to CC14+and, more recently,Is to CC13+*Cl. (1) In our photolysis experiment the 927 (9)cm-I band and the 420-nm band both disappear almost completely and are the only two UV-vis-IR band pairs to do so. The 927 (9)-cm-l band has previously been assigned'* to CC13CI. (2) Maier and co-workers, using near 200-nm (6.2-eV) photons, observedI8 929 (7)- and 1019-cm-' lines but not the 1037-cm-l (CC13+)band upon photolysis of CCI, in an Ar matrix. (3) The dissociation energy of CC14 to yield CC13 via'* CCl, hv (23.06 eV) CC13 + C1
-
+
is 3.06 eV while the ionization/dissociation of CCl, to yield CC13+ via I CCl,
+ hu ( 2 11.5 eV)
-
-
CC13++ C1 + e-
requires l l .5 eV. The direct ionization of CCl, via CCl,
+ hv
CC14++ e'
is not a favorable pathway since CC14+is unstable according to mass spectrometric studies.19 (We have also found that with electron bombardment ionization using the same electron gun,
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7881
IR Spectrum of Naphthalene Radical Cation no parent CC14+peak appears in the mass spectrometry trace.) Thus, the 200-nm (6.2-eV) radiation used by Maier and coworkers and the medium-pressure Hg radiation used by us is energetic enough to lead to CCI3and C1 products but not to CC13+ and C1. (4) Ab initio calculationsI8 (6-31GS basis) of the IR frequencies of CC13.Cl agree with the observed experimental lines at 927 and 1019 cm-I, provided typical scaling factors are used. In summary,the parallel behavior of the 420-nm and 929 (7)-cm-', 1019-cm-l pair bands upon photolysis, the nonappearance of the 1037-cm-' (CC13+)band upon 200-nm photolysis, the favorable energetics of the dissociation of CCl, to CC13 and Cl, plus the results of the ab initio calculations all point to the conclusion that the 420-nm band belongs to the CC13Cl complex. C. Cation Yield and Absolute Infrared Band Intensities. The molar absorption coefficient of the naphthalene cation in the visible was determined by Kekll and AndrewsIzb(KA) as 8,(675 nm) = 24900 M-I cm-' (in Ar) and recently by Salama and Allamandola15(SA) as ~+,~,(675nm) = 1130 M-l cm-I (in Ar). To calculate the yield of cations-to-neutrals (c+/co) in the matrix, the following absorbance formulas are applicable: i
A+(675 nm) = e+,,,(675 nm)c+l AO(309.2 nm) = ~ ~ ~ ~ ~ (nm)cOl 309.2
photolysis. It assumes that all neutral parent lost is converted to ionic product. If this is not the case, the value determined represents a lower limit. Thus, if a competitive process such as N+ + hul + hv2 N+2+ e- were occurring, the molar absorption coefficient for the IR band would be higher than found here. Finally, Pauzat et al. predict' that the intensities of the C H stretch modes should decrease (factor of about 50) upon ionization. It would have been very interesting to verify this fmding. However, due to spectral congestion from the C H stretches of the neutral parent, no new bands were located in this region. In any case, verification of this prediction may prove difficult because of the congestion from the neutral molecule, the low concentration of the cationic species present, and the predicted low molar absorption coefficients of these vibrations.
-
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. For partial support of this research, we also gratefully acknowledge the National Aeronautics and Space Administration, the US. Air Force, Environics Laboratory, and the National Science Foundation. We also express our gratitude to Drs. D. J. DeFrees and Y.Ellinger for a copy of ref 7 prior to publication and for fruitful discussions. Registry No. Ar, 7440-37-1;CC14, 56-23-5;naphthalene radical ion
The yield of cations is thus
(l+), 34512-27-1.
c+/co = [A+(675 nm)/A0(309.2 nm)] X [eomax(309.2 nm) /&,ax(675 nm)]
References and Notes
The absorbance ratio (first on the RHS) is 1.46 based on our spectra (cf. Figure 2), while the molar absorption coefficient ratio is 0.185, using the results of SA. From this, the percentage of cations (to neutrals) is determined to be 27%. On the other hand, using KA's results leads to a percentage yield of 1.2%. We believe the 27% figure is probably too high and by implication the SA molar absorption coefficient is too low. Allowed electronic transition in PAH's is usually in the 104 to lo5 range.20 The 1.2% yield deduced from KA's results apppears, however, reasonable. While the comparison between theoretical and experimental relative IR intensities for the naphthalene cation is quite good, it is of interest to look at absolute intensities also. If we accept the KA molar absorption coefficient of 24 900 M-' cm-' for the 675-nm peak and use our result that the 1218/1215-Cm1IR band is 5.9 times weaker than the 675-nm band (cf. slope of the correlation plot in Figure 4), this leads to a value for e+-( 1218 cm-') = 4220 M-'cm-I. To compare with theoretical values, the integrated band intensity is needed 2.303sc(v) du = 2.303~+,,,(1218 cm-I) A Y ~=/ ~ 195 km mol-' This value is considerably smaller (about 5.7 times) than the theoretical value of 1123 km/mol. The experimental value of 195 km/mol is of course dependent on the value used for the 675-nm band intensity. A higher value for e+,,,(675 nm) would lead to better agreement; a value equal to 1.4 X lo5 M-' cm-' would be necessary for a match. The KA method of determining c+,(675 nm) entails the measurement of the intensity decrease of the neutral parent and the increase of the cationic product upon
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