J. Phys. Chem. 1992, 96, 6668-6674
6668
of the potential energy surface in the prooxidant reaction of TocH. Acknowledgment. We thank the Computer Center of the Institute for Molecular Science (IMS) for the use of the HITAC M-680H and S-820/80 computer and the Library Program G A U S S I A N ~ ~We . ~ ~express our sincere thanks to Dr. Shiro Urano of Tokyo Metropolitan Institute of Gerontology for his help concerning the synthesis of 5,7-diethyl-8-methyltocopheroland 5,7-diisopropyl-8-methyltocopheroland to Mr. Yasuhiro Kohno of Ehime University for his help in the early stage of this work. RWtry NO, 1, 129666-54-2; 1-OH,142129-71-3; 2, 142129-70-2; 2-OH, 142129-72-4; 3, 129666-55-3; SOH, 142129-73-5; 4, 12966656-4; &OH, 124041-50-5; 5, 129666-52-0; SOH, 142129-74-6; 6, 129666-53-1; &OH, 142129-75-7; 7, 6257-34-7; 8, 4813-50-7; 9, 13020-06-9; 10, 75-91-2; 11, 6068-96-8; 12, 3031-75-2; 02, 7782-39-0.
Supplementary Material Available: Photoelectron spectra in the ionization energy range 8-21 eV of 8-12 (Figures 11-15) and optimized geometries of 8-12 with the ab initio method (Figures 16-20) (1 1 pages). Ordering information is given on any current masthead page.
References and Notes (1) Proton-Transfer Reactions; Caldin, E., Gold, V., Eds.; Chapman and Hall: London, 1975. (2) Spectroscopy and Dynamics of Elementary Proton Transfers in Polyatomic Systems; (special issue of Chem. Phys.); Barbara, P. F., Trommsdorff, H. P., Us.North-Holland: ; Amsterdam, 1989; Vol. 136, pp 153-360. (3) Nagaoka, S.;Nagashima, U. Chem. Phys. 1989, 136, 153. (4) Nagaoka, S.Kagaku To Kogyo 1991,44, 182. (5) Nagaoka, S.;Kuranaka, A.; Tsuboi, E.; Nagashima, U.; Mukai, K. J. Phys. Chem. 1992, 96, 2754. (6) Kuranaka, A.; Sawada, K.; Nagashima, U.; Nagaoka, S.; Mukai, K. Vitamins 1991, 65, 453. (7) Reference cited in ref 5. (8) Loury, M.; Bloch, C.; Francois, R. Rev. Fr. Corps Gras 1966,13,747. (9) Terao, J.; Matsushita, S. Lipids 1986, 21, 255. (10) References cited in ref 9. (1 1) Nagaoka, S.; Okauchi, Y.; Urano, S.;Nagashima, U.; Mukai, K. J. Am. Chem. Soc. 1990,112,8921. (12) Mukai, K.;Kohno, Y.; Ishizu, K. Biochem. Biophys. Res. Commun. 1988, 155, 1046.
(13) Mukai, K.; Kageyama. Y.; Ishida, T.; Fukuda, K. J. Org. Chem. 1989, 54, 552. (14) Nilsson, J. L. G.; Sievertsson, H.; Selander, H. Acta Chem. Scand. 1968, 22, 3160. (15) Mukai, K.; Kikuchi, S.; Urano, S.Biochim. Biophys. Acra 1990, 1035, 77. (16) Williams, H. R.; Mosher, H. S.J. Am. Chem. Soc. 1954, 76,2984. (17) Williams, H. R.; Mosher, H. S. J. Am. Chem. SOC.1954, 76,2987. (18) Mukai, K.; Watanabe, Y.; Ishizu, K. Bull. Chem. Soc. Jpn. 1986,59, 2899. (19) Mahoney, L. R.; Darooge, M. A. J. Am. Chem. Soc. 1970,92,4063. (20) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press/Halsted Press: Tokyo/New York, 1981. (21) Shindo, Y. Research Institute of Applied Electricity Technical Report (in Japanese) 1984, No. 3. (22) Frisch, M. J.; Binkley, J. S.;Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; DeFrees, D. J.; Seeger, R.; Whiteside, R. A.; Fox, D. J.; Fluder, E. M.; Topiol, S.; Pople, J. A. GAUSSIAN86; Camegie-Mellon Quantum Chemistry Publishing Unit, Camegie-Melon University: Pittsburgh, registered at IMS Program Library by N. Koga, S. Yabushita, K. Sawabe, and K. Morokuma (IMS). (23) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (24) Koller, J.; Hod&k, M.; Plesnihr, B. J. Am. Chem. Soc. 1990,112, 2124. (25) Boyd, S. L.; Boyd, R. J.; Barclay, L. R. C. J. Am. Chem. Soc. 1990, 112, 5724. (26) Although ROOH geometries may show tendencies toward intramolecular hydrogen bonding between OH and C (0)atoms (bridging by hydrogen), it does not have a significant influence on IK ( methylacetylene > ethylacetylene > phenylacetylene > benzene. A triplet-state mechanism involving long-lived nonradiative excited NO2was invoked to explain the production mechanism of this chemiluminescence.
Introduction The visible photochemistry of NO2has been extensively studied due to the absorption in the entire visible spectral region, the relative ease of sample preparation, and the important role it plays in atmospheric chemistry. Recently much attention has focused on the photochemistry of NO2 following multiphoton excitation. Matsumi et a1.l determined that highly vibrationally excited 02(u’up to 25) was produced following the multiphoton excitation of NO2 with visible light. This study in addition to similar experiments by Nagai et al.2 and Jusinski et a1.j led to the determination of reactions involving O(IS) as follows: 0022-3654/92/2096-6668$03.00/0
-
-
+ nhu O(%) + N O N O + OZ(v?, AH = -597 kJ/mol
NO2
+ NO2 (1) A reaction scheme involving O( ‘D) production following multiphoton excitation was reported by Fujimura et al.:4 NO2 + h~ N O Z ( ~ B ~ ) NO2 + nhu O(ID) + N O
O(’S)
--
O(’D) + N02(2Bz) NO(A22+)+ 02 AH = -106 kJ/mol (A = 475 nm) 0 1992 American Chemical Society
(2)
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6669
NO A2Z+-X211 Chemiluminescence V"=O
2
1
I
I
r
3
I
I
4
5
!
I
I
I
I
6 I I
I
81
V'= 1 V'=O
20
p
n
h
0
10
20
30
40
SO0
h
J2 20
v v
/v
25
I
W
30'
L
I
220
240
260
280
300
Wavelength Inm Figure 1. Dispersed fluorescence spectra of NO A2F(u')-X21'I(u'') measured for the visible multiphoton absorption of a mkture of 150 Torr of N2, 10 Torr of NO2, and (a) 10 Torr of acetylene, (b) 10 Torr of methylacetylene, or (c) 10 Torr of benzene. The resolution was 5 nm.
-
The strong NO A22+-X211chemiluminescencewas observed with excitation at wavelengths shorter than 488 nm, coincident with the threshold of NO2 2hv O(lD) NO. In this experiment an NO,/hydrocarbon (HC) mixture was irradiated with laser light in the region of 500-570 nm, where the NO A2Z+-X211chemiluminescence is rather weak in the pure NO2 system. An enhancement in the NO chemiluminescencewas observed, when acetylene, methylacetylene, ethylacetylene, benzene, or phenylacetylene was added to pure N02. We propose an NO chemiluminescence production mechanism different from reaction 2, in which the triplet state of hydrocarbons plays an important role.
+
+
Experimental Section For NO chemiluminescence a XeCl excimer laser (Lambda Physik EMG 52 MSC or EMG 103) was used to pump a dye laser (Lumonics Hyperdye 300 or Lambda Physik FL-2002 or FL3001) operating on coumarin 500 or coumarin 54OA. Sometimes a nitrogen laser (Molectron UV 22 or 24) pumped dye laser (Molectron DL-11) was used as the excitation light source. A lens (f = 70 mm) focused this visible light (1 mJ/pulse) into a 5-cm long static cell. The fluorescence was detected at right angles to the excitation with a Corning 7-54 filter/solar blind photomultiplier (Hamamatsu R166) combination for pressure-dependent time profile measurements, and a monochromator (Nikon P250, 300 nm)/PMT combination was used for dispersed fluorescence. For the pressure-dependent total intensity and dispersed fluorescence, the PMT signal was amplified via a current amplifier (Keithley 427) and the output fed to a boxcar integrator (SRS SR-245). For the fluorescence time profile measurement, the PMT output was collected and integrated by a digital Wwpe (Gould DSO 4072). The pressures were measured with a capacitance manometer (MKS Baratron) and a Bourdon gauge. The quenching experiments on NO(AZZ+)were carried out in the same manner as the chemiluminescence time profile measurements with a few modifications. The dye was changed to coumarin 450 and the output doubled via a BBO crystal provided 226-nm radiation for exciting NO to the A22+state. Pure-grade acetylene, methylacetylene, and ethylacetylene and research-grade N2 and NO (99.9%) from Takachiho were used without further purification. Phenylacetylene (Tokyo Kasei 97%) was distilled prior to use. Reagent grade benzene (Kanto, 99.5%) was used after several freeze-pumpthaw cycles. The NO2 sample was pnparedby reacting NO with excess O2(Takachiho, 99.95%). R-tdts DispeReaunraw-
f~vspiMeLight
10 ' 20 a 30 Pressure (Torr) Figure 2. NO A22+-X21'Ichemiluminescence intensity and NO A2E+ yield as a function of aliphatic hydrocarbon pressure: (a) acetylene, (b) methylacetylene, and (c) ethylacetylene. The NO2 pressures were fiied at 10 Torr. The intensities are normalized such that the chemiluminescence intensity a t 0-Torr hydrocarbon pressure equals unity.
0
'
Irrediptionof N&/Hydrocarbon/N2,
Figure 1 shows the dispersed UV emission following 570-nm irradiation of N02/hydrocarbon/N2. In each case the band structure is indicative of the NO A2Z+-X211transition (yband).' The excitation wavelength of 570 nm was chosen such that a two-photon process would be incapable of direct excitation to the singlet state of the selected hydrocarbons. The SIstates for acetylene, methylacetylene, and ethylacetylene lie 6.5 eV above the ground state: The SIorigins for benzene and phenylacetylene are 4.74 and 4.45 eV, respectively.'.* A decrease in the v' = 1 population with increasing molecular weight of hydrocarbon may be due to more efficient vibrational relaxation of larger molecules. Note that the spectra are not corrected for system response and photodecomposition. The purpose of collecting the dispersed emission was to confirm that the chemiluminescence is from NO and not any other species; the relative intensities of the NO y-band transitions are well-
NO EohPncement by H y d " NOz (10 Torr) was placed in the cell, and the NO y-band intensity was measured following laser irradiation at 570 nm. Next, several pressures of the selected hydrocarbon were added to the cell, and the NO y-band intensity was measured after each addition. Irradiation time for a single run was 1-2 min, because some photochemical reactions took place during irradiation and the yband intensity decreased typically by 50% after 25-min irradiation. Thus, the intensity attenuation after each run was less than 5%. Figures 2 and 3 illustrate the chemiluminescence enhancement upon hydrocarbon addition. The yield curves, which we will discuss later, are also plotted. Each plot is a collection of three runs. The weak yband emission measured at 0 Torr of hydrocarbon (pure NO2) is due to reaction 2, which is a fourphoton process for X = 570 nm. When a hydrocarbon is added to the system, the yield of reaction 2 decreases since the hydrocarbon quenches O('D) and N02(2B2). Thus the true intensity ratio of hydrocarbon-dependentchemiluminescence to reaction 2 chemiluminescence is greater than that indicated in Figures 2 and 3. After achieving a maximum value, the y-band intensity decreases as the hydrocarbon pressure increases. This is a manifestation of the hydrocarbon quenching effect prevailing over
Sisk et al.
6670 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
-~(a,c6H5cc",*
l.O/
0.6 0
1
2
,
3
4
1
11.0
5
b) Ph/NO,
e
,I.
2
I
.."
k
I c) AclNO,
l
.
o
~
C . .
~
~
.
o
0.6
0 1.o
(b) CeH, 0.2
5
0
10
15
20
25
Pressure (Torr) Flgure 3. NO A'Z+-X*Il chemiluminescenceintensity and NO A2Z+
yield as a function of aromatic hydrocarbon pressure: (a) phenylacetylene and (b) benzene. The NO2 prcssures were fixed at 10 Torr. The inkmitics are normalized such that the Chemiluminescenceintensity at &Torr hydrocarbon pressure equals unity. A 7-54 filter was placed in front of the PMT to block most of the radiation below 242 nm (a. 5% transmission), since phenylacetylene radiatively trap emission below 242 nm.
hvdrocarbon HCCH CH$CH C2H5CCH a
molecule-l s-') 3.2 k 0.3 5.1 1.1 5.5 1.0
*
hydrocarbon C6H6 CsHsCCH
hydrocarbon HCCH
CZH&CH molecule-' 7.0 10.9
C6H6 5-l)
C~HSCCH
0.2 4.7
the ability of hydrocarbon to promote NO A22+-XZIIchemiluminescence. The effectiveness of enhancing the chemiluminescence in order of decreasing efficiency is acetylene > methylacetylene > ethylacetylene > phenylacetylene > benzene. Within the aliphatic acetylene series (acetylene, methylacetylene, ethylacetylene) it is noted that the pressure for maximum enhancement in intensity decreases with increasing molecular size. The enhancement data contain two contributions to the fluorescence: the efficiency of the hydrocarbon to enhance the chemiluminescence and the effectiveness at quenching the N O y-band fluorescence. To obtain a measure of the relative yield of NO(AZZ+)production, it became necessary to measure the quenching rate constants of NO(A*Z+) by hydrocarbons. of NO(AW) by Hydmwbcwu The Qpondbhae Rate fluorescence decay of NO(A22+,u'=0) in pure NO2 was well explained by the previously determined radiative and selfquenching rate constants.I0-" The quenching experiments were performed by filling the cell with 0.20 Torr of N O while varying the pressure of the quenching gas between 0 and 0.65 Torr. The decay rate (kf)for a fmed hydrocarbon pressure was calculated over an observation time of two lifetimes or greater. The first 10% decay was excluded in the fitting, because this region is convoluted with the laser profile (10-ns fwhm). At longer times there was a characteristic positive deviation from the least-squares line attributed to a low signal-to-noiseratio. For each quenching gas, the decay rates measured were plotted as a function of pressure with the quenching rate constant by hydrocarbon (kHc) equal to the slope according to the Stern-Volmer relation:
+ k ~ o [ N o +] kHc[hydrocarbon]
500
600
700
: :T"( [
[Hydrocarbon]" (Torr) 6.9 10.0 3.0 10.3 3.8 10.2 1.6 10.0 0.9 3.1
(X106 kl s-1)
10.0 9.4 10.0 10.0 10.1 10.1 10.1 10.0 10.1 9.9
8.0 8.2 7.0 9.3 8.0 10.4 17.1 9.4 9.5 11.1
k26
(X 108 s-I)
2.0 2.2 1.8 3.0 2.0 3.1 1.6
3.5 1.6 2.3
"Two pressure regimes were utilized for each hydrocarbon gas: (1) 10 Torr of NO2 and 10 Torr of hydrocarbon; (2) 10 Torr of NO2 and
Error bars are 20..
kf = kp
300 400 Time (ns)
200
NO A2Z+-X211chemiluminescencetemwral Drofiles nenera t 2 from 570-nm multiphoton absorption of (a) 1O'Torr i f NO2,lb) 10 Torr of NO2 and 10 Torr of benzene, and (c) 10 Torr of NO2 and 10 Torr of acetylene.
CHBCCH TABLE I: OUesehian Rate Constmts for NO(A*??)"
100
1
(3)
kp and kNO are the zero-pressure NO fluorescence decay and selfquenching rate constants, respectively. Under the present experimental arrangement we could neglect diffusion out of the field of view and wall collisions. The standard Stem-Volmer plots
hydrocarbon at the pressure that led to maximum enhancement as obtained from Figures 2 and 3. bCalculated by eq 3 using &? = 4.61 X IO6 s-' (ref IO),, ,& = 3.75 X cm3 molecule-' s-I (ref 12) and kCH values obtained from Table I. for NO(A2Z+)quenching give the quenching rate constants, which are tabulated in Table I. The quenching rate constant increased with the molecular size of the quencher. ' h e profik 0fNO AW-XSII Inasimilar experiment we measured the NO chemiluminescence time profile generated by focused 570-nm irradiation of the N02/hydrocarbon (HC)mixture. Figure 4a illustrates the time profile of chemiluminescence produced from the NOZ/0('D) mechanism described earlier (eq 2). The observed profile (fwhm ca. 20 ns) involves the interplay of the bimolecular reaction rate, the system response time, and the quenching rate of NO(A2Z+)by NO2.l2 Upon addition of 10 Torr of hydrocarbon, the time profile develops a longer fluorescence lifetime as illustrated in Figure 4b,c for benzene and acetylene, respectively. This indicates the occurrence of a mechanism different from that observed in the pure NOz irradiation. If NO(A22+) is formed through some intermediate, then the temporal profile of NO(A2Z+)concentration is expressed as [NO(A2Z+)] = (const)kl{exp(-k2t) - exp(-klf))/(kl - kz) (4) Here, kl is the formation rate of NO(A22+)and kz is the depletion rate, being equal to k? + kN01[N02]+ k ~ ~ [ h y d m ~ a r b o nIf] . k2 >> k,,then [NO(A*Z+)] is proportional to exp(-klt): the apparent decay rate equals the formation rate. For the NO2/ hydrocarbon mixture the long decay times could be evidence of the two-step pmcess described above, such that the apparent decay rate equals the formation rate. The formation rate k, (apparent decay rate of chemiluminescence) and the decay rate k2 (=kp + ~ N O ~ [ N+Ok~c[hydrocarbon]) ~] are tabulated for each gas in Table 11.
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6671
NO A2Z+-X211Chemiluminescence
J
e
L
0
I
I
200
400
I
600
800
I
l
1000
Time (ns)
Figure 5. Time decay profile following 532-nm excitation of 10 Torr of acetylene/lO Torr of NO2 mixture monitoring N0z(2Bz) visible fluorcscence, and NO(A22*) ultraviolet chemiluminescence. The inset shows the same data on a logarithmic vertical scale.
In the case of benzene, a lower pressure of benzene leads to a faster formation rate. This is attributed to the low chemiluminescence yield in the N02/benzene system. Low benzene pressure leads to time profiles dominated by the fast NOz(2B2)/O(lD)mechanism; high pressures of benzene tend to quench the N02(2B2)/O(1D)mechanism, leading to low yields, but produces NO(A2Z+) in a much slower fashion compared to that produced via N02(2Bz)/O('D),leading to lower kl values. Since enhancement of chemiluminescence is small for ethylacetylene, phenylacetylene, and benzene, the kl values for these cannot be considered with the same degree of confidence as the kl values for acetylene and ethylacetylene. In an attempt to observe the fluorescence rise and decay as well as discern the NO A-X chemiluminescence decay (observed rise) from the response time of the system (204s laser pulse width, 10-ns PMT response time), a series of time profiles were recorded for low pressures of acetylene and NO2. When the pressure was lowered, the N02(2B2)/O(1D) mechanism described by eq 2 becomes the domnant mechanism, thus obscuring the chemiluminescence of the NOz/acetylene system. Even when Nz was added as a selective quencher of O(lD), there was no difference in the observed rise time. It may seem puzzling that the rate constants change slightly with pressure. This is primarily due to two factors. First the rate constants were obtained by fitting the decay profiles to a single exponential rather than a double exponential, due to our failure to qualitatively resolve the first 10% decay. Finally as mentioned earlier, NO(A2Z+) from N02(2B2)/0(1D)perturbs the hydrocarbondependent chemiluminescence, with the perturbation becoming larger at lower pressures. When N02(2B2)and NO(A2Z+) were monitored for the N02/acetylene system under the same conditions, the N02(2Bz) fluorescence decayed faster than the observed NO(A2Z+) fluorescence (Figure 5). This might lead one to believe that N02(2B2)is not necessary for the observed NO A-X chemiluminescence; however, this may not be the case as we will discuss in the next section. Relative NO(AfZ+) Yields. From the enhancement and quenching experiments, we may determine the relative yield of NO(A2Z+)produced. The measured total fluorescence (IF)of the enhancement experiment is proportional to the timeinkgrated [NO(AzZ+)] population, which is obtained by integrating eq 4 over all time: Xm[NO(A22+)]dt = (const)[NO(A2Z+)lo/k2
(5)
where [NO(A2Z+)IOrepresents the net yield of NO(A2Z+) generated by the chemiluminescence reactions of interest. Therefore, the relative yield of NO(A2Z+) can be estimated by combining an NO y-band emission intensity (IF)and the quenching rate (k2): [N0(A2Z+)]o= (oonst')k21p The correction factor is related to
0-l
530
540
550
Wavelength Inm
Figure 6. Excitation spectra. (a) Monitoring NO A2Z+-XZIIultraviolet chemiluminescence in a mixture of 11.0 Torr of NOz and 10.6 Torr of acetylene. (b) Monitoring N0#BZ) visible fluorescence for 6.1 Torr of NOz.
the fluorescence quantum yield of the system. The [NO(AzZ+)]o values are plotted as NO(A2Z+)yields in Figures 2 and 3. The results show that the NO(A) production in order of decreasing efficiency is: acetylene(HCCH) > methylacetylene(CH,CCH) > ethylacetylene(C2HSCCH)> phenylacetylene(C6H5CCH)> benzene. To examine other aliphatic hydrocarbons, a mixture of 1-10 Torr of HC' (HC' = methane, ethane, propane, cyclopropane, ethylene, or propylene) and 10 Torr of NO2 was irradiated with 540-nm light. The results showed only increased fluorescence quenching of NO(AZZ+) from N02(2Bz)/O(rD)for higher pressures rather than fluorescence enhancement. Excitation Spectrum of NO 7 - B d Chemiluminescence h NOz/Aoetykae. Figure 6a shows the excitation spectrum obtained by monitoring the NO A-X chemiluminescence in the NO2/ acetylene mixture. The fluorescence excitation spectrum shown in Figure 6b was recorded by monitoring the visible 2Bz %Al fluorescence of NOz with the pump laser intensity attenuated by a factor of about 10. Upon comparison, these two excitation spectra reveal similar structure with essentially the same intensity distribution. This result indicates that the present photochemical reaction is initiated by the absorption of visible light by NO2, and NO2 absorption alone is responsible for NO(A2Z+) formation. Thus it is safe to say that NO (produced by the multiphoton dissociation of NOz) and acetylene cannot be excited directly in the present system to generate NO(A2Z+).
-
PresSlpeDq~a~keofNOy-BM F di ~i 7 displays the NOz prtssure dependence of NO ./-band emission
intensity. The data was fitted to IF = ( c o r ~ s t ) [ N O ~ ] ' . ~by ~*~.~~ a least-squares procedure, where IF is the NO y-band chemiluminescence intensity. This result indicates that more than one NO2 molecule participates in the formation of one NO(A2Z+) molecule. Since the NO2 dimer, N204.has very little oscillator strength in the visible spectral region, it cannot contribute to the reaction. Thus, we conclude that the NO(A2Z+) formation requires at least two NOz and/or some species produced by the visible absorption of NO2. The quenching of the chemiluminescence by helium and nitrogen was studied in an attempt to gain insight into the reaction mechanism. The chemiluminescence intensity varies with He and N2. The apparent halfquenching pressures of He and N2for the NO A-X chemiluminwnce were 200 and 400 Torr,respectively.
Sisk et al.
6612 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 5.0
-
4.0
-
1.0
1 0
/
I 1
I
I
I
I
I
2
3
4
5
6
7
NC$ Pressure (Torr) Figure 7. NO A22+-X211chemilumintscenceintensity as a function of NO2pressure: Ah, = 530 nm. Acetylene pressure = 7.0 Torr. The solid line is simulated by IF = ( c o n ~ t ) [ N O ~ ] ' . ~ ~ .
It is important to note that He and N2 are poor quenchers of NO(A2Z+).I3The half-quenching pressure of NO(AzZ+) by He and N2 is estimated to be more than 75 000 Torr with 8.5 Torr of NO2 and 10.2 Torr of acetylene. Thus, helium or nitrogen should quench not NO(AzZ+)but other intermediate species which contribute to the NO y-band chemiluminescence. Lurer Pow= Depenaenca The NO(A2Z+)chemiluminexence was measured as a function of laser power for 532-, 540-, and 570-nm light. The N02(2B2)fluorescence was also measured at each wavelength. Figure 8 shows one data set for 570-nm excitation. All the data sets show the power index (yN02) for the visible N02(2B2)fluorescence to be less than 1, and that (yNO) for the NO(A2Z+)chemiluminescence to be between 2 and 3. An NOz absorption should be a single-photon process at low laser power, and thus the results (for example, yNo2 = 0.71 at 570 nm) indicate saturation. As stated earlier, the visible absorption of NO2 initiates the formation of NO(A2Z+), and hence one must count this photon. The power index difference for NO(A2Z+) chemiluminescence and the visible N02(2B2)fluorescence, Ay = 1.54 for the 570-nm excitation, indicates that at least two additional photons are required. Possibly, saturation also occurs in NO(A2Z+) generation steps following the first NO2 visible absorption step. Thus, the precise number of photons could not be obtained, but we can conclude that at least three photons are necessary for NO(A2Z+)formation in the NOZ/acetylenesystem. The time profile of N02(2B2)and NO(A2Z+) chemiluminescence was also recorded for increasing laser power for 532-nm radiation. Each decay was fitted to a single exponential for the form 1, = 10 exp(-kt). The nascent decay rate,-[dlF/dt]o = kZ0, was measured as a function of laser power, and the corresponding log-log plot was constructed. A slope of 0.8 implies that the fluorescence decay of N02(2B2)varies linearly with laser power, whereas a slope of 1.9 indicates a quadratic dependence with laser power for the NO(A2Z+)decay rate.
Discussion Mecbrdsmg We will now discuss a few mechanisms as possible candidates for the observed chemiluminescence in the NO2/ hydrocarbon systems. For simplicity, we consider the NO2/ acetylene system alone. Molecular Mechanisms. The direct reaction of excited NO2 with acetylene may lead to NO(AZZ+) as illustrated by
NO2 + hv
+
N02(2B2)
N02(2B2) + CzH2 NO(A2Z+) + C2H20(possibly ketene and/or CH2+CO) This scheme may be discounted due to the fact that it is inconsistent with the observation of two or more NO2 moleculcs and three or more photons consumed. In addition to this, the reaction is also endothermic for X = 570 nm.
5 10 Laser Power (Arb.) Figure 8. NO A22+-X211chemiluminescence intensity as a function of laser power in a mixture of 10.0 Torr of NO2and 11.O Torr of acetylene (Ak = 570 nm). (a) Monitoring NO A22+-X211ultraviolet chemiluminescence. The slope is 2.25. (b) Monitoring N0#B2) visible fluorescence. The slope is 0.7 1. l1
2
If NO2 excited by two or more photons reacts with acetylene, it may be energetically possible to produce NO(A22+) C 2 H z 0 NO2 NO2**
+ nhv(nl2)
+ CZHZ
+
-
+
NOz**
+
NO(A22+) C2H20
This mechanism consumes only one NO2 molecule, and furthermore the dissociation lifetime of NO2** will be in the picosecond range, much shorter than the 1-ns mean collision time expected for NO2** and hydrocarbon under the present experimental conditions. Thus this NOz** mechanism is deemed improbable for NO(A2Z+) formation. The next molecular mechanism to consider is that of forming the NO2.-C2Hz complex followed by direct excitation with multiphoton radiation:
+ C2H2 (+M) N02-C2H2 + nhv NO2
-
N 0 2 4 2 H 2 (+M) NO(A2Z+) + C2H20
This V i b i l i t y was investigated by carrying out a supersonic jet experiment which offered collision-free conditions. A mixture of 10 Torr of N02/20 Torr of C2Hz/760Torr of Ar was expanded into a vacuum through a 0.5" orifice and the molecular jet crossed with focused visible laser light at 505 nm. However, NO yband emission could not be detected. Moreover, this complex photodissociation mechanism does not explain some other experimental results observed. For example, only one NO2 molecule contributes to the formation of NO(A2Z+) in this model, which conflictswith the fact that two or more NO2 molecules have been observed to participate in NO(A2Z+) formation. A three-body collision, expressed as 2N02(2B2) + C2H2 .-* NO(A2Z+) + NO2 + C2H20 can be neglected due to the laser power dependence (n 1 3) of the chemiluminescence. When NO2 is dissociated by multiphoton absorption, NO is generated as the counter product of an 0 atom. We will consider whether this NO product can be excited into NO(A2Z+) in the present system. If some electronically excited species is formed, energy transfer to NO might be possible. But this mechanism utilizes only one NO2 molecule, whereas the data show two NO2 molecules to be necessary. A decrease in NO y-band chemilu-
The Journal of Physical Chemistry. Vol. 96, No. 16, 1992 6673
NO A22+-X211 Chemiluminescence minescence was observed by NO addition, contrary to predictions from this model. Atomic Oxygen Mechanism. It has been shown that the visible multiphoton dissociation of NOz generated oxygen atoms in both the ground (3P) and excited ('D/'S) states2+
+ nhu
NO2
+
NO
+ O(3P/1D/1S)
For O('D) atoms the following scheme must be considered for an NOZ/C2H2mixture:I4 O('D) CzH2 CHZ(Z'A1) + CO(X'Z+)
-
+
+ CHZ(1'AI)
NOZ('B2)
+
NO(A2Z+) + CHZO
This scheme appears promising in that the number of NO2 molecules and photons consumed are consistent with observed results. By making use of the O(lD) quenching rate constants," we would expect N2 to quench by a factor of 5 at 500 Torr of N2 (10 Torr of N02/10 Torr of C2H2),whereas He should do little quenching under the same conditions. As described earlier the quenching of the chemiluminescence seems to be slightly more effective for He than for N2;thus this mechanism may be rejected. We disregard any schema involving O(lS) on the k i s of O(%) quenching by NO. It is known that NO deactivates O('S) and NO(A22+) rather efficiently.'J5 The 7-band intensity is not weakened by NO addition as much as expected from the quenching rate constants. Thus O('S) does not contribute to NO(A22+) formation. Proposed Chemiluminescence Mechanism. The above arguments coupled with the experimental observations suggest that hydrocarbons cannot be excited directly by visible laser light for the following reasons. (1) The excitation spectrum monitoring NO 7-band chemiluminescence does not reflect SI SO absorption of acetylene (Figure 8). (2) No ultraviolet emission could be detected in the pure hydrocarbon system with visible laser irradiation. We propose the following mechanism in which an NO2.hydrocarbon complex (N02.-HC) is excited by borrowing oscillator strength from NOz:
-
NO2 + hu
-
N02('B2)
(6)
+ HC (+M)
e [NOz***HC]*(+M) NO2 + HC (+M) s NOz..*HC (+M)
N02('B2) N02..*HC
+2 h ~
+ - +
+
[N02*..HC]*
[NOpHC]**
+ hv
[N02...HC]**
N02(2B2) M HC*(TI)
+ NO,(*)
+
NO2 +
NO2
(8) (9)
+ HC*(TI) (10)
+M
(11)
(oxidized HC)
(12)
NO$*)
NO(A22+)
+ HC*(TI)
(7)
In this scheme NO,(*) represents NO2 in a long-lived dark state. Note that electronically-excited hydrocarbon (HC*) is assumed to be in the T I state, because the two-photon energy at 570 nm lies below the SI origin of acetylene, whereas three-photon absorption would lead to the dissociation. Complex formation (NOz-HC) is necessary for the absorption of visible photons, due to the @-forbidden nature of the HC*(TI) HC(&) transition. For all hydrocarbons studied two photons would suffice for the TI excitation. The long lifetime of the hydrocarbon triplet state (70 ccs for benzene and 2.5 ms for acetylene)16J7is also in accordance with the observed long formation time of the NO A22+-X211chemiluminmce. This mechanism is in accordance with the laser power dependence ( n 1 3) and the observation that two NO2 molecules are consumed. In the following sections we will elaborate on the credibility of this mechanism. Acetylene, benzene, and the derivatives, possessing low-lying triplet states, enhanced the NO chemiluminescence in the NOz/hydrocarbon mixtures; whereas methane, ethane, propane, ethylene, propylene, and cyclopropane did not. This fact illustrates the importance of low-lying triplet states for chemiluminescence. Energetic considerationsalso add credibility to the triplet state +
mechanism. This NO(AZZ+)production consumes three photons in which one photon is utilized for NOz(2Bz)production, leaving two photons available for HC*(TI) production. The energy content of two 570-nm photons (4.35 eV) absorbed by the N02-hydrocarbon complex is below the SIthresholds of the hydrocarbons in this investigation. Therefore only the triplet states of hydrocarbons arc accessible via [N02**-HC] [N02-HC*] energy transfer within the complexes. Laser Power Dependence and Tripkt Mechanism. As noted earlier, the NO A22+-XZIIchemiluminescence has a cubic dependence on laser power. This is consistent with the proposed mechanism in which three visible photons are absorbed. The rate of the NO(A22+) decay at t = 0 (the maximum of the decay profile), [ d I ~ / d t ] was ~ , observed to vary quadratically with the laser power. This may be reconciled with the last step of the mechanism in which the observed decay rate is proportional to the NO(AzZ+)production rate as mentioned earlier. The observed decay rate, dZF/dt, is proportional to the NO(A22+) production rate, kl[NOZ(*)][HC*(Tl)]. If [NO2(*)]>> [HC*(T,)], pseudo-fmtsrder kinetics will follow: dZF/dt = kl'[HC*(Tl)], where kl' = kl[NO,(*)]. For t = 0, [ d l ~ / d t = ] ~kl'[HC*(T,)lo. Since [HC*(Tl)lo is produced via two-photon absorption, it follows that [dlF/dt]o is proportional to (ZLaseJ2. A few words should be said about the assumption that [NO,(*)] >> [HC*(T,)], especially since it was demonstrated that the N0z(2B2)fluorescence decayed faster than the NO 7-band chemiluminescence (Figure 5). Since HC*(TI).is produced by two-photon absorption compared to the single-photon absorption of NOz(2Bz),one would expect the amount of NOz(2&) prduction to be larger due to efficient single-absorption processes as compared to inefficient multiphoton processes. Even after accounting for collisions, Dulcey et a1.18 obtained the fluorescence quantum yield of 0.16 for excitation in the 520-650-nm range, by comparing the NO2 fluorescence signal to the Nz Raman signal for which the absolute scattering yield is known. This result suggests that the remaining energy is emitted as infrared energy outside of the visible detection range or retained as internal energy in the molecule present after the visible fluorescence has ceased. In an infrared experiment McAndrew and ~o-workers'~ determined the quenching rate constant of N02(2Bz)to be 3.3 X cm3 molecule-' s-l from the rise time of N02%(0,0,1) %(O,O,O) infrared emission. For a 10 Torr of N02/10 Torr of acetylene mixture, if we assume equal quenching rate constants for the quenching of NO#Bz) by NOz and acetylene (3.3 X 10-l2 an3 molecule-' s-'), we obtain a quenching lifetime of 0.47 ps. This is much longer than the 130-ns lifetime observed for NO(AZ2+) in the present system. This indicates that long-lived excited NO,(*) molecules are involved in the chemiluminescence process. Candidates for the metastable states responsible for this long lifetime include the vibrationally excited levels of the ground state as well as the optically forbidden C2Azstate (To = 2.028 eV).20 Curve crossing from the optically excited state to the optically forbidden states may account for the metastability of excited NOz.'* The reaction of acetylene(TI)with NOZ(X2Al)to produce NO(A22+) ketene is exothermic by 3275 cm-la21Our data, the excitation spectra (Figure 8), and the threephoton dependence suggests that N0#B2) is necessary for producing the excited species NOz(*) which reacts with acetylene(T,) to produce NO(A2Z+). Since the optically excited ,B2 and ground X2Al states of NO2 are strongly coupled vibronically, the photoexcitation to ,B2 readily populates high vibrational levels of the ground state. Even after undergoing a number of collisions a residual amount of vibrational energy remains in NO,(*). One possibility for why NO,(*)is preferred over NO, in the vibrationless level for NO(A2Z+) production is the structure of the potential energy surface. It may be that the NOz(%ZAl)+ acetylene(T,) potential energy surface, although thermodynamically favorable, possesses a kinetic barrier (activation energy) along the path leading to NO(A2Z+) CH2C0 production due to molecular rearrangement. This experiment, however, does not suggests a value for the activation energy. A large fraction of NO,(*) in collision-resistant nonradiative states would persist beyond the cessation of visible
-
-
+
+
6674
J. Phys. Chem. 1992,96,6674-6679
fluorescence. This implies that [HC*(Tl)] should always be less than [NO+*)], as required to reconcile the proposed mechanism with the observed power dependence of the chemiluminescence rate. Enhancement Trends. Finally we address the question as to why the benzene/N02 system yields such a low NO(A2Z+) chemiluminescenceyield in relation to the acetylene compounds. Some evidence may come from the study of heterogeneous phenylacetylene clusters. Stanley and CastlemanZ2noted that NH3 is centered above the benzene ring in benzenwNH3 clusters, whereas NH3occupies a site between the ring and acetylenic group in phenylacetylene-NH3 clusters. They postulate that the aelectron density of the acetylenic group attracts NH, more strongly than benzene. The implications for this work is that the acetylene derivatives possibly may form complexes with NOz more readily than benzene, a key step in the proposed mechanism. In the above argument we have merely focused on one point (complex formation). However it should be noted that a number of other factors affect the efficiency of NO(AzZ+) production such as the ability of HC*(TI) to be deactivated by NOz and the ability of N0#B2) to be deactivated by hydrocarbons.
Conclusions In this experiment we determined the relative efficiency of NO(A2Z?) production from a mixture of NOz and hydrocarbons containing a low-lying triplet state. The trend of NO(A2Z+) production in order of decreasing efficiency is HCCH > CH3CCH > CzHsCCH > C&CCH > C&. On the basis Of the laser power dependence on NO(A2Z+)chemiluminescence, the pressure-dependent variation, and energetic considerations, a triplet mechanism for chemiluminescenceis suggested. To reconcile the present investigation with the triplet mechanism, we concluded that a significant fraction of excited NOz molecules must remain in a nonradiative collision-resistant excited state for a significant period of time.
Acknowledgment. This work was partly supported by the Grant-in-Aid for Scientific Research (No. 03453019) from the Ministry of Education, Science, and Culture. We are grateful to the Japan Society for the Promotion of Science (JSPS). The assistance and advice of Mr. Masayuki Arai, Dr. Yoshihisa Matsushita, and Dr.Masaru Fukushima were greatly appreciated.
References and Notes (1) Matsumi, Y.; Murasawa, Y.; Obi, K.; Tanaka, I. Laser Chem. 1983, 1, 113. (2) Nagai, H.; Kusumoto, T.; Shibuya, K.; Tanaka, I. J . Chem. Phys. 1986, 85, 5061. (3) Jusinski, L. E.; Sharpless, R. L.; Slanger, T. G. J. Chem. Phys. 1987, 86, 5509. (4) Fujimura, Y.; Homma, K.; Kajimoto, 0. Chem. Phys. Lett. 1987,140,
320. ( 5 ) Engleman, R., Jr.; Rouse, P. E. J . Mol. Spectrosc. 1971, 37, 240. (6) Flicker, W. M.; Mosher, 0.;Kupperman, A. J . Chem. Phys. 1978,69,
--..
3 511. .~~~
(7) Duncan, D. A.; Dietz, T. G.; Liverman, M. G.; Smalley, R. E. J. Phys. Chem. 1981,85, 7. (8) Chia, L.; Goodman, L. J. Chem. Phys. 1982, 76, 4745. 19) Nicholls. R. W. J. Res. Notl. Bur. Stond. Sect. A 1964. 68. 535. (IO) McDerkid, S.; Laudenslager, J. B. J . Quont. Spectrosc. Rbdiat. Tromfer 1982, 27, 1. (11) Greenblatt, G.D.; Ravishankara, A. R. Chem. Phys. Lett. 1987,136,
501. (12) Asscher, M.; Haas, Y. J. Chem. Phys. 1982, 76, 2115. (13) Broida, H. P.; Carrington, T. J. Chem. Phys. 1963, 38, 136. (14) Shaub, W. M.; Burks, T. L.; Lin, M. C. J. Phys. Chem. 1982,86,757. (15) Schofield, K. J. Photochem. 1978, 9, 55. (16) Miller, R. G.;Lee,E. K. C. Chem. Phys. Lett. 1974, 27,475. (17) Musahid, A.; Callear, A. B. Chem. Phys. Lett. 1989, 156, 35. (18) Dulccy, C. S.;McGee, T. J.; McIlrath, T. J. Chem. Phys. Lett. 1980, 76, 80.
(19) McAndrew, J. J. F.; Preses, J. M.;Weston, R. E., Jr.; Flynn, G.W. J. Chem. Phys. 1989, 90,4772. (20) Weaver, A.; Metz, R. B.; Bradforth, S. E.; Neumark, D. M. J. Chem. Phys. 1989, 90, 2070. (21) Barin, I. Thermodynamical Data of Pure Substances; VCH: Weinheim, Germany, 1989. (22) Stanley, R. J.; Castleman, A. W., Jr. J . Chem. Phys. 1990,92, 5770.
Substltuent Effects on the Spectral, Acid-Base, and Redox Properties of Indolyl Radlcals: A Pulse Radiolysis Study Slobodan V. Jovanovic*lt and Steen Steenkent Laboratory 030, The Boris Kidric Institute, P.O. Box 522, 11001 Beograd, Yugoslavia, and Max-Planck-Institut ftir Strahlenchemie, Stifstrasse 34-36, 0-4330 Mtilheim a.d. Ruhr 1, Germany (Received: March 2, 1992)
Spectral and acid-base properties and reduction potentials of various substituted indolyl radicals were studied by pulse radiolysis in aqueous solutions at 20 O C . Except for the 5-methoxyindolyl and 5-carboxyindolyl radicals, the spectra of the substituted indolyl radicals resemble the previously published 320- and 520-nm spectra of the neutral and 330- and 580-nm spectra of the cation indolyl and tryptophan radicals. The substitutionof indolyl radical cation by electron-attractinggroups (positive a') results in a blue shift of the 580-nm band by -30 nm, whereas the spectra of methylindolyl (a' = -0.31) are similar to those of unsubstituted indolyl radicals. The 430- and 455-nm bands appearing in the spectra of the 5-carboxyindolyl and 5-methoxyindolyl radical cations, respectively, indicate even stronger interaction of the unpaired electron with the 5-substituent. The radical cations of various indole-3-acetic acids decarboxylate at pH values below their pK, to produce allyl radicals. The 5-bromoindolyl radical undergoes solvolysis to 5-hydroxyindolyl radical in acidic and alkaline media. The dissociation constants and reduction potentials of the substituted indolyl radicals correlate with the Brown substituent constants: pK, = 4.14 - 2.1321~+, correlation coefficient 0.987, and E0/0.059 = 22.29 + 3.52a', correlation coefficient 0.980. The p values from these correlations (-2.13 and 3.5) are similar to that of the Hammett correlatirm of the dissociation constants of the protonated indole nitrogen in various substituted indoles, p = -2.49, but smaller than the p value of the dependence on the substituent of the reduction potentials of phenoxy1 radicals, p = 5.4.
Introduction The free radical chemistry of methylindoles,l,z dimethoxy-, dihydroxy-, and (methoxyhydroxy)indoles,3 and hydroxyindoles,4" The Boris Kidric Institute.
* Max-Planck-Institut fiir Strahlenchemie.
as well as tryptophan and its derivatives,'3*610has been extensively studid However, the effect of S~bstit~tiOn On the Physicochemical characteristics of indoie radicals has remained unclear. For example, it was reported'J that methyl substitution at positions 2 and 3 of the indole ring influenced considerably the pK, values of indolyl radicals, whereas the effect on their reduction potentials
0022-365419212096-6674$03.00/0 0 1992 American Chemical Society
