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Chemiluminescence in the gas phase reaction between tetrakis (dimethylamino)ethylene and oxygen
L Phorochem. Photobiok A:
Chem.,
67 (1992)
1-12
1
Chemiluminescence in the gas phase reaction between tetrakis(dimethylamino)ethylene and oxygen Sidney Toby,
Paul A.
Astheimer
and Frina S. Toby
Department of Chemistry, Rutgers University, PO Box 939, Piscataway, NJ 088.55 (USA)
(Received July 16, 1991; accepted February 20, 1992)
Abstract Chemiluminescence from the reaction between tetrakis(dimethylamino)e~hylene (TIMAE) and oxygen in the gas phase is reported. It was found that the spectrum of the emitted light showed a broad band with a maximum at about 500 nm, which is similar to what has been reported from liquid phase work and similar to the fluorescence spectrum of pure TMAE. The liquid phase chemiluminescence is well known and has been postulated to occur via a proton donor leading to an ionic complex. If the same emitter is formed in the gas phase, the reaction probably occurs via free radicals. The emission in the range lOA “C was studied as a function of pressures of oxygen and TMAE. The effect of added hydroxylic compounds in the gas phase was to increase the emitted intensity but their presence was not necessary for chemiluminescence, contrary to what has been claimed for liquid phase work. The emission was very long lived for a gas phase reaction in a closed system and was measurable after about 30 min under some conditions. Both peak and integrated light intensities decreased markedly as the temperature was raised at the same reactant concentrations and were very weak above 45 “C. A mechanism is postulated which accounts for the behavior of the light emission and which was satisfactorily computer simulated.
1. Inkduction It is more than 40 years since the bright chemiluminescence associated with the reaction of tetrakis(dimethylamino)ethylene (TIMAE ), [(CH~)Z~,C=C[N(CH,)ZI,, and molecular oxygen was discovered by Pruett et al. [l] and a large number of papers dealing with this compound have appeared (see listing in ref. 2). TMAE is useful as an oxygen probe [2J and its ease of ionization has led to its extensive use as a photocathode [3, 43. The chemiluminescent spectrum from the liquid phase reaction showed a structureless band with a maximum at 515 nm which appeared essentially equivalent to the TMAE fluorescence spectrum [5, 61. Since the known oxidation products did not fluoresce under the conditions used, the emitting species appeared to be electronically excited TMAE (TMAE*). Except for a study of the thermal decomposition of TMAE5 by Waring and Berard [7j, all published .work on the chemiluminescence and oxidation mechanism has been based on the reaction of gas phase oxygen with a liquid containing TMAE, an alcohol and usually a solvent. It is thus not clear what is the phase of the reaction zone. The form of the rate law for light emission is also in doubt. Paris [S] postulated that the chemiluminescent intensity was proportional to [TMAE]*[O2] [Cl&OH]‘. Fletcher and Heller [6] derived a mechanism to account for a rate law which is also second order
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in [TMAE) and first order in [O,] but is first order in alcohol. The role of the alcohol has been assumed to be that of a proton donor subsequently producing ionic intermediates, essential to light emission. We find that the reaction can be studied in the gas phase, which should eliminate the uncertainties associated with the phase of the reaction zone, and that added alcohol is not necessary for light emission.
2. Experimental
details
Commercial TMAE was slowly vacuum distilled several times with liberal rejection of head and tail fractions and then treated with sodium-potassium alloy before use [4]. Samples used after a single distillation and with no Na-K treatment gave erratic results. It has been claimed [9] that filtering through a bed of alumina gave TMAE that was gas chromatographically pure, but we could not confirm this and found that reproducible light emission with oxygen was probably a better test of purity than was gas chromatographic analysis. Other materials used were vacuum distilled at least once. The pale yellow liquid had a vapor pressure of about 0.5 Torr at room temperature. This low vapor pressure severely limited the range of TMAE pressures that could be studied and also set a limit of about 10 “C as the lowest usable temperature with the apparatus employed. The upper temperature limit was set by the unexpected finding that the emitted light intensity decreased as the temperature increased at constant reactant concentrations. Chemiluminescence was therefore investigated over the range 10-E “C. Two vacuum systems were used. Light emission was measured with the reactants in a cylindrical quartz cell 21.9 cm long and 5.6 cm in diameter using an RCA lP28 photomultiplier. A second lP28 was mounted at right angles to the cell axis for measurement of scattered light. Absorption measurements were done in a borosilicate tube 73.0 cm long and 3.2 cm in diameter fitted with quartz windows. The light source was a xenon high pressure arc and a Hamamatsu R636 photomultiplier was used. The photomultipliers were operated at room temperature and the output was measured with standard picoammeters. Both vessels were thermostatted and connected to vacuum systems which were evacuated to less than 10m5 Torr between experiments. Pressures were measured with MKS Baratron capacitance manometers.
3. Results The chemiluminescing gas phase TJMAE/02 system has not been previously reported and it was important to establish that the reacting system was homogeneous. Scattered light was therefore measured at right angles to the reaction cell axis. The ratio of scattered light to transmitted light at room temperature was 5.0~ 1W4 with a Th&KE pressure of at least 0.4 Torr. When 5 Torr O2 was added, the ratio did not measurably change. If products less volatile than TMAF, had been formed, aerosols would have increased the amount of scattered light. There was no evidence for a wall component of the chemiluminescence but we could not rule out this possibility, though we think it unimportant. Experiments were done which confirmed previous liquid phase findings [5, 61 that TMAE/02 emission gave a structureless band peaking at about 500 nm and that the
3
band was the same as that from fluorescence using 366 excitation. When oxygen was added to TMAE vapor ,at room temperature, light was emitted almost immediately, then decayed slowly and could still be measured after 30 min under optimum conditions. The peak intensity varied with the pressures of TMAJS, 0, and added gas and was also very dependent on temperature. For some runs the integrated light output was also measured; The various dependences were found to be very similar to those using peak intensities, and since the latter were much simpler to measure, integrated light intensities were not extensively recorded. Typical peak photocurrents were in the range l-100 nA. Data from experiments in which different hydroxylic compounds were added to the TM&Z/O2 system are given in Table 1, while Tables 2 and 3 show the effect of varying TMAE and O2 pressures respectively at different temperatures. When the light output was measured over a range of temperature, it was found that the intensity decreased considerably with temperature increase. This can be seen in Fig. 1, which is a plot of emission vs. ThIAJZ pressure with the O2 pressure kept approximately constant. Although it was not possible to work with TMAE pressures above 0.1 Torr
TABLE
1
Effect of added gases on emitted light
(Tom) 0.302 0.300 0.298 0.305 0.294 0.302 0.302 0.299 0.345 0.347 0.344 0.346 0.345 0,301 0.200 0.200 o-200 0.200 0.200 0.200 0,360 0.345 0.346 0.301 0.200 0.346 0.350
2.690
2.820 3.100 3.130 2.490 2.610 2.470 2.470 2.400 2.380 2.420 2.260 2.440 2.000 4.700 4.650 4.560 4.600 4.740 4.030 2.180 2.060 1.910 2.000 2.380 2.280 2.210
30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 22.0 22.0 22.0 22.0 22.0 22.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0
Added gas Crorr)
G? (nA)
0.672 0.470 0.259 0.174 0.019 0.176 0.003 0.008 0.176 0.256 0.088 0.829 0.018 0.000 0.139 0.275 0.965 2.300 3.230 0.000 0.000 1.870 3.480 0.000 0.194 0.558 I.270
44.16 28.53 34.73 28.14 22.03 24.32 22.46 25.60 49.67 47.60 45.14 59.79 47.42 21.89 133.1 135.3 149.4 187.5 218.0 123.2 26.91 51.83 79.91 21.90 19.11 60.14 83.66
H20 Hz0 Hz0 HZQ J320 J320 H20 H20
BuOH BuQH BuQH BuOH
BuOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH
TABLE
2
Effect of variation in temperature
10.0
10.0 10.0 10.0 10.0 10.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0
and TMAE
pressure on emitted light
P(TMAE) {Torr)
Jwz) (Torr)
0.065 0.020 0.040 0.020 0.058 0.050 0.250 0.200 0300 0.200 0.200 0.150 0.300 0.202 0.200 0.250 0.200 0.204 0.210 0.202 0.202 0.202 0.345 0.205 0.343 0.200 0.343 0.300 0.201 0.200 0.203 0.400 0.200
7.760 7.140 7.220 7.390 7.430 7.480 4.280 5.730 4.000 4.030 4.030 4.050 3.990 4.020 3.530 4.030 6.200 2.180 2.250 2.140 2.130 2.260 2.370 2.150 2.280 2.160 2.140 11.04 12.48 11.72 11.42 12.02 12.05 9.220 11.14 11.15
0.250 0.350 0.390
at 10 “C, it is clear that for a given ThJAE
IE
@A) 35.60 4.900 6.500 5.900 7.900 21.40 260.0 167.0 227.0 101.0 107.0 57.00 230.0 110.0 93.40 168.0 210.0 20.00 30.00 17.00 19.00 20.00 38.00 30.00 80.00 20.00 34.00 2.100 1.100 0.900 0.900 3.500 1.070 1.300 2.600 3.720
pressure the light output has a negative temperature coefficient. The effect of added methanol, butanol and water on the light emission is shown in Figs. 2 and 3. In spite of some scatter, it is evident that these additives increase the light emission but that the emission does not tend to zero at zero additive concentration, contrary to what has been found in liquid phase work. The absorption spectrum of TMAE in the near UV is shown as the upper curve in Fig. 4. When excess O2 was added, the absorbance was reduced but was still appreciable. This is seen in the lower curve in Fig. 4, where an E&fold excess of O2
5 TABLE
3
Effect of variation
in temperature
W-MAE) (Torr)
0.200 0.200 0.200 0.200 O-200 0.202 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.202 0.204 O-200 0.202 0.202 ‘0.204 0.204 0.208 0.200 0.201 0.199 0.200 0.202
22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 Vontained
up to 5 Torr
and O2 pressureon emittedlight
P(OzIJ
IE
(Torr)
@A)
4.030 2.160 6.200 11.25 9.380 4.020 11.09 3.530 1.360 9.920 5.730 11.08 4.030 11.60 4.910’ 2.140 2.180 2.160 2.130 4.270 8.180 6.170 14.30 8.030 6.010 9.7xP 9.72(r 2.260
107.0 30.00 210.0 215.0 277.0 110.0 227.0 93.40 14.30 240.0 167.0 298.0 101.0 280.0 45.00 17.00 20.00 20.00 19.00 64.00 189.0 115.0 220.0 190.0 110.0 168.0 130.0 20.00
COO.
reduced but did not eliminate the TMAE curve and no new absorption bands were seen except possibly above 400 nm. The absorption spectrum given in Fig. 4 differs from the near-U57 spectrum reported by Nakato et al_ [lo] and it may be pointed out that their far-W spectra have been disputed by Holroyd et al. [Ill. The mechanism was computer simulated with appropriate software [12]. None of the rate constants needed is known and the values chosen for best fit with the experimental data at 22 and 30 “C are given in Table 4.
4. Discussion
4.1. Mechanism and rate law We postulate the following emission
as reactant
mechanism to aazount for the behavior concentrations are varied.
of the light
00 0 10
0 00
Fig.
1. Emitted
Oi~~~l
0.20
0.30
0.40
0.50
P(TMAE)Prr
light intensity
1, VS. TMAE
pressure
at temperatures
shown.
IIII,l’ll”,,‘,““““‘,I”“,,‘,,
0
h/iethon&
TOT:
4
Fig. 2. Effect of added methanol on emitted light at 22 “C (filled circles) and 30 “C (unfilled circles). TMAE and O2 pressures kept approximately constant. Lines are from simulated mechanism.
TMAE+o,e
(k, k-d
TO2
TO2 -
2TMU
&a)
TO1-
ThJO+TMH
Wzb)
TQ-
R+
&A
TOz
+ RIOH
2R-TMAE*
RO, +
R + RIOOH
+ TMlJ
(W w4>
0
:7,,,,,.,,,,,,,.,,,,,,,,,,,...,,,,,,,.,,,,,,,,,,,,, 0
idde;i4
1
gas’:
Toris
Fig. 3. Effect of added water (unfilled circles) and added butanol (Wed circles) on emitted light at 30 “C. TMAE and O2 pressures kept approximately constant. Lines are from simulated mechanism_ 2500
7
1500
zz . isi
1000
ix a w
500
0 150
200
250
300
WAVELENGTH,
350
nm
400
450
Fig. 4. Absorbance of T’MAE (upper curve) and ThJAE P(TMAE) = 0.075 Torr, P( 0,) = 6.4 Torr.
R+TMAJ3+ R+OzTMAJz* ThaAE*+ TMAE*+Q-
products TMSJ
(ks) (W
TMAE+hv TMAE
‘I’MAE+Q
e7) W @9)
with added Oz (lower curve) at 23 “C.
TABLE
4
Rate constants chosen for simulation kinetics 22 “C 2.0x
kt
30 “C
3.0x lo-‘5
lo--=
7.5 x ld
k k,l
1.5x104 3.0x10' 1.0x10' 1.0x10'
1.5 x 10’ 1.0 x 10’ 1.0 x 10’
kzb
kzc
4.8x10-l7 1.2x lo-‘” 1.5x10-‘6 1.0x10-*5 5.0x10-‘” 1.0x lo-‘8 1.0x 108 1.0x 108 1.0x 10-14 l.ox10-‘6
4.8 x 1O-‘7
k3(MeOH) k,(BuOHj W=W) k4 kr, k, k7
1.0 x lo-‘5 5.0 x 10-16 1.0 x IO_‘8 1.0X108 1.0x 108 1.0x 10-‘4 1.0 x 1o-‘6
&TMAE) WOz)
First- and second-order rate constants have units of s-* and cm3 molecuIe-’
s-l
respectively.
The postulated intermediate TO* splits into two molecules of the product tetravia a symmetric split in step 2a. An unsymmetric methylurea (TMW), [(CH&N],C=O, split in step 2b leads to the products tetramethyloxamide (TMO), [(CH,),NC=0]2, and tetramethylhydrazine (TMH), [(CH&$?12. I n step 2c another unsymmetric split gives two free radicals, R and RO *, one of which dime&es to form electronically excited TI%%E* which we believe is the precursor of the chemiluminescence. A minor product, bis(dimethylamino)methane, [(CH3)2N12CHZ, is presumably formed by subsequent free-radical rearrangements. R,OH represents an added hydroxylic substance. Assuming a steady state in intermediates and postulating that 2k4[R12 we obtain the rate law for the emitted light intensity 1,: =k,[R][=l +k[RlP,l,
k&7
IE=
k,+ks+ks[Q]
k2 + kSIRIOH] kJIMAEl+k,[Oz]
’
kl t-1
Kbl
>( k_,+k,+k,[R,OH]
* >
(1)
where kz= kh + kzb+ k&. As seen in Figs. 2 and 3, light was emitted in the absence of added hydroxylics and this has also been observed in vapor phase work by Herek er al. [13]. The postulated mechanism therefore involves pathways to light formation corresponding to the absence (step 2c) and presence (step 3) of added RIOH. If the quenching step 9 is unimportant, eqn. (1) may be simplified in the absence of added RIOH to
where k
i
=
b+ks+ks[Ql
k&7
1Rk--l+k2 klkzc
9
Equation (2) was tested by plotting IB-lR VS. I/[TMAE] at constant [Q] (data from Table 2) in Fig. 5 and plorting &-ln us. I/[@] at constant [TMAJZJ (data from Table 3) in Fig. 6. Both graphs are reasonably linear. The rate law has not been agreed upon in the liquid phase work. Paris 181 reported thhatthe light emission was second order in TMAE and in hy&oxyIic activator and gave a mechanism which was in accord with these findings aad with first-order 0, dependence. Fletcher and Healer [4] gave a con@ex mechanism which gave a rate Iaw which reduced to first order each in TMAE, alcohol and Oz. They suggest that
o-0 7
2
3
4
I/P(TMAEJ
5
6
7
Tort-l
5. I,-ln as a function of 3/P(TMAE ) at 10 “C (circles), 22 “C (squares), 30 “C (triangles) and 45 % (diamonds) at constant P(0,). For clarity, values at 10 ‘C have been divided by 10. Lties at 22” aJld 30” are from sknulated mechanism. Fig.
Fig. 6. I,- LB as a function of I,!.~~~ at 22 ‘C (squares) and 30 “C (triangles) at constant P(TbbUT). Lines are from simulated mechanism,
3.0
3.1
3.2
3.5
3.3
1000/T,
3.6
I?”
Fig. 7. Arrhenius plot of slopes of Fig. 5. the light emission should show little or no temperature dependence but this was not verified experimentally. Our complex rate law (eqns. (1) and (2)) d oes not reduce to an emission which has a simple order in reactants. The slopes of the plots in Fig. 5 were converted into concentration units and given as an Arrhenius plot in Fig. 7. This yields an activation energy of 1651111.2 kcal mole-l (estimated error limits), corresponding to the temperature coefficient of the composite rate constant kaka. The temperature range in Fig. 6 is too small to warrant speculation on temperature effects. Computer simulation of the light emitted was carried out using the rate constants shown in Table 4 and applying the normalizing factor photocurrent
(LA) = #I v
where @ allows for the geometry of the system and quantum yield losses. The same value of @= 1.27~ 10s5 was used for all series except for the runs with added water where a value of @=&OX 10e6 was used. Some difficulties were encountered in the experimental work with added water and these data may not be reliable. Overall, however, the simulation was in accord with the postulate that hydroxylic compounds catalyze the light emission but are not essential for it. The simulation also lends weight to the conclusion that the reason for the light output decreasing with increasing temperature is because as the temperature rises, kl and k_l increase but k,/k_, decreases. 4.2. Reactive channels The predominant channels in the reaction of m with O2 do not produce light. The quantum yield in the liquid phase has been estimated at about 10m3 under typical conditions [6, 141. Urry and Sheet0 [15] made an extensive study of products from the liquid phase reaction and their findings may be summarized as TMAE+o*-
2TMs-J
‘z?MAE+o,-
TMo+TMH -
TMO+DAM
64%
36% >
where DAM signifies bis(dimethylamino)methane. If we assume similar product channels in the gas phase, we can represent the pathways as follows: me2
Me,N \
C
/
Me,N
C
/
+
\
02
NMe,
N
’
I
Me2 , 1 gives two molecules of TMU, while dimethylamino radicals which form TMH directly because of the cage effect) or DAM indirectly. The two radicals, one of which could dime&e to form very noticeable chemiluminescence. Cleavage
'\N
Me2 cleavage 2 gives TM0 and two (more likely in the liquid phase less likely skewed cleavage 3 gives TMAE*, giving the inefficient but
Acknowledgments We thank Dr. Ralph E. Weston Jr. of Brookhaven National Laboratory for kindly making available some unpublished work on Th&%E. We are grateful to the Rutgers University Research Council for partial support of this work.
References 1 R. L. Pruett, J. T. Barr, K. E. Rapp, C. T. Bahner, J. D. Gibson and R. W. Lafferty, J; Am. Chem. Sot., 72 (1950) 3646. 2 T. M. Freeman and W. R S&z, Anal. Chem, 53 (1981) 98. 3 K. Kuwata and D. H. Geske, J; Am. CItem Sot., 86 (1964) 2101. 4 R. A. Holroyd, S. Ehrenson and J. M. Preses, ,I. Phys. CIiem., 89 (1985) 4244. 5 H. E. Winberg, J. R. Downing and D. D. coffinan, 1. Am. Chem Sot., 87 (1965) 2054. 6 A. N. Fletcher and C. A. Heller, J. Phys. Chem, 71 (1967) 1507. 7 C. E. Waring and R. A. Berard, 3. Phys. Chem., 80 (1976) 1025. 8 J. P. Paris, Pbtochem. PhotobioL, 4 (1965) 1059. 9 C. A. Heller and A. N. Fletcher. J: Phys. Chem, 69 (1965) 3313. 10 Y. Nakato, M. Ozaki and H. Tsubomura, J. Phys- Chea, 76 (1972) 2105. 11 R. A. Holroyd, J. M. Preses, C. L. Woody and R. A. Johnson, Nuci. Imwrum. Methods Phys_ Res. A, 261 (1987) 440.
12 12 W. Braun, J. T. Herron and D. Kahaner, ACUCHEMfACUPLOT, National Institute of Standards and Technology, Gaithersburg, MD, 1987. 13 J. L. Herek, R. E. Weston Jr. and J. M. Preses, unpublished work, 1989. 14 F. K. Baibulatov, V. F. Minin and Z. A. Tsaryuk, I&J. Sib. O&d. Akori. Nuuic SSSR, Set. k7rim. Nuuk, 5 (1974) 119; Chem. Abs., (1975) 42492k. 15 W. H. Urry and J. Sheeto, Photochem. Photobioi., 4 (1965) 1067.
Chem.,
67 (1992)
1-12
1
Chemiluminescence in the gas phase reaction between tetrakis(dimethylamino)ethylene and oxygen Sidney Toby,
Paul A.
Astheimer
and Frina S. Toby
Department of Chemistry, Rutgers University, PO Box 939, Piscataway, NJ 088.55 (USA)
(Received July 16, 1991; accepted February 20, 1992)
Abstract Chemiluminescence from the reaction between tetrakis(dimethylamino)e~hylene (TIMAE) and oxygen in the gas phase is reported. It was found that the spectrum of the emitted light showed a broad band with a maximum at about 500 nm, which is similar to what has been reported from liquid phase work and similar to the fluorescence spectrum of pure TMAE. The liquid phase chemiluminescence is well known and has been postulated to occur via a proton donor leading to an ionic complex. If the same emitter is formed in the gas phase, the reaction probably occurs via free radicals. The emission in the range lOA “C was studied as a function of pressures of oxygen and TMAE. The effect of added hydroxylic compounds in the gas phase was to increase the emitted intensity but their presence was not necessary for chemiluminescence, contrary to what has been claimed for liquid phase work. The emission was very long lived for a gas phase reaction in a closed system and was measurable after about 30 min under some conditions. Both peak and integrated light intensities decreased markedly as the temperature was raised at the same reactant concentrations and were very weak above 45 “C. A mechanism is postulated which accounts for the behavior of the light emission and which was satisfactorily computer simulated.
1. Inkduction It is more than 40 years since the bright chemiluminescence associated with the reaction of tetrakis(dimethylamino)ethylene (TIMAE ), [(CH~)Z~,C=C[N(CH,)ZI,, and molecular oxygen was discovered by Pruett et al. [l] and a large number of papers dealing with this compound have appeared (see listing in ref. 2). TMAE is useful as an oxygen probe [2J and its ease of ionization has led to its extensive use as a photocathode [3, 43. The chemiluminescent spectrum from the liquid phase reaction showed a structureless band with a maximum at 515 nm which appeared essentially equivalent to the TMAE fluorescence spectrum [5, 61. Since the known oxidation products did not fluoresce under the conditions used, the emitting species appeared to be electronically excited TMAE (TMAE*). Except for a study of the thermal decomposition of TMAE5 by Waring and Berard [7j, all published .work on the chemiluminescence and oxidation mechanism has been based on the reaction of gas phase oxygen with a liquid containing TMAE, an alcohol and usually a solvent. It is thus not clear what is the phase of the reaction zone. The form of the rate law for light emission is also in doubt. Paris [S] postulated that the chemiluminescent intensity was proportional to [TMAE]*[O2] [Cl&OH]‘. Fletcher and Heller [6] derived a mechanism to account for a rate law which is also second order
lOlq-6030/92/$5.00
(8 1992 - Elsevier
Sequoia.
All
rights reserved
2
in [TMAE) and first order in [O,] but is first order in alcohol. The role of the alcohol has been assumed to be that of a proton donor subsequently producing ionic intermediates, essential to light emission. We find that the reaction can be studied in the gas phase, which should eliminate the uncertainties associated with the phase of the reaction zone, and that added alcohol is not necessary for light emission.
2. Experimental
details
Commercial TMAE was slowly vacuum distilled several times with liberal rejection of head and tail fractions and then treated with sodium-potassium alloy before use [4]. Samples used after a single distillation and with no Na-K treatment gave erratic results. It has been claimed [9] that filtering through a bed of alumina gave TMAE that was gas chromatographically pure, but we could not confirm this and found that reproducible light emission with oxygen was probably a better test of purity than was gas chromatographic analysis. Other materials used were vacuum distilled at least once. The pale yellow liquid had a vapor pressure of about 0.5 Torr at room temperature. This low vapor pressure severely limited the range of TMAE pressures that could be studied and also set a limit of about 10 “C as the lowest usable temperature with the apparatus employed. The upper temperature limit was set by the unexpected finding that the emitted light intensity decreased as the temperature increased at constant reactant concentrations. Chemiluminescence was therefore investigated over the range 10-E “C. Two vacuum systems were used. Light emission was measured with the reactants in a cylindrical quartz cell 21.9 cm long and 5.6 cm in diameter using an RCA lP28 photomultiplier. A second lP28 was mounted at right angles to the cell axis for measurement of scattered light. Absorption measurements were done in a borosilicate tube 73.0 cm long and 3.2 cm in diameter fitted with quartz windows. The light source was a xenon high pressure arc and a Hamamatsu R636 photomultiplier was used. The photomultipliers were operated at room temperature and the output was measured with standard picoammeters. Both vessels were thermostatted and connected to vacuum systems which were evacuated to less than 10m5 Torr between experiments. Pressures were measured with MKS Baratron capacitance manometers.
3. Results The chemiluminescing gas phase TJMAE/02 system has not been previously reported and it was important to establish that the reacting system was homogeneous. Scattered light was therefore measured at right angles to the reaction cell axis. The ratio of scattered light to transmitted light at room temperature was 5.0~ 1W4 with a Th&KE pressure of at least 0.4 Torr. When 5 Torr O2 was added, the ratio did not measurably change. If products less volatile than TMAF, had been formed, aerosols would have increased the amount of scattered light. There was no evidence for a wall component of the chemiluminescence but we could not rule out this possibility, though we think it unimportant. Experiments were done which confirmed previous liquid phase findings [5, 61 that TMAE/02 emission gave a structureless band peaking at about 500 nm and that the
3
band was the same as that from fluorescence using 366 excitation. When oxygen was added to TMAE vapor ,at room temperature, light was emitted almost immediately, then decayed slowly and could still be measured after 30 min under optimum conditions. The peak intensity varied with the pressures of TMAJS, 0, and added gas and was also very dependent on temperature. For some runs the integrated light output was also measured; The various dependences were found to be very similar to those using peak intensities, and since the latter were much simpler to measure, integrated light intensities were not extensively recorded. Typical peak photocurrents were in the range l-100 nA. Data from experiments in which different hydroxylic compounds were added to the TM&Z/O2 system are given in Table 1, while Tables 2 and 3 show the effect of varying TMAE and O2 pressures respectively at different temperatures. When the light output was measured over a range of temperature, it was found that the intensity decreased considerably with temperature increase. This can be seen in Fig. 1, which is a plot of emission vs. ThIAJZ pressure with the O2 pressure kept approximately constant. Although it was not possible to work with TMAE pressures above 0.1 Torr
TABLE
1
Effect of added gases on emitted light
(Tom) 0.302 0.300 0.298 0.305 0.294 0.302 0.302 0.299 0.345 0.347 0.344 0.346 0.345 0,301 0.200 0.200 o-200 0.200 0.200 0.200 0,360 0.345 0.346 0.301 0.200 0.346 0.350
2.690
2.820 3.100 3.130 2.490 2.610 2.470 2.470 2.400 2.380 2.420 2.260 2.440 2.000 4.700 4.650 4.560 4.600 4.740 4.030 2.180 2.060 1.910 2.000 2.380 2.280 2.210
30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 22.0 22.0 22.0 22.0 22.0 22.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0
Added gas Crorr)
G? (nA)
0.672 0.470 0.259 0.174 0.019 0.176 0.003 0.008 0.176 0.256 0.088 0.829 0.018 0.000 0.139 0.275 0.965 2.300 3.230 0.000 0.000 1.870 3.480 0.000 0.194 0.558 I.270
44.16 28.53 34.73 28.14 22.03 24.32 22.46 25.60 49.67 47.60 45.14 59.79 47.42 21.89 133.1 135.3 149.4 187.5 218.0 123.2 26.91 51.83 79.91 21.90 19.11 60.14 83.66
H20 Hz0 Hz0 HZQ J320 J320 H20 H20
BuOH BuQH BuQH BuOH
BuOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH
TABLE
2
Effect of variation in temperature
10.0
10.0 10.0 10.0 10.0 10.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0 45.0
and TMAE
pressure on emitted light
P(TMAE) {Torr)
Jwz) (Torr)
0.065 0.020 0.040 0.020 0.058 0.050 0.250 0.200 0300 0.200 0.200 0.150 0.300 0.202 0.200 0.250 0.200 0.204 0.210 0.202 0.202 0.202 0.345 0.205 0.343 0.200 0.343 0.300 0.201 0.200 0.203 0.400 0.200
7.760 7.140 7.220 7.390 7.430 7.480 4.280 5.730 4.000 4.030 4.030 4.050 3.990 4.020 3.530 4.030 6.200 2.180 2.250 2.140 2.130 2.260 2.370 2.150 2.280 2.160 2.140 11.04 12.48 11.72 11.42 12.02 12.05 9.220 11.14 11.15
0.250 0.350 0.390
at 10 “C, it is clear that for a given ThJAE
IE
@A) 35.60 4.900 6.500 5.900 7.900 21.40 260.0 167.0 227.0 101.0 107.0 57.00 230.0 110.0 93.40 168.0 210.0 20.00 30.00 17.00 19.00 20.00 38.00 30.00 80.00 20.00 34.00 2.100 1.100 0.900 0.900 3.500 1.070 1.300 2.600 3.720
pressure the light output has a negative temperature coefficient. The effect of added methanol, butanol and water on the light emission is shown in Figs. 2 and 3. In spite of some scatter, it is evident that these additives increase the light emission but that the emission does not tend to zero at zero additive concentration, contrary to what has been found in liquid phase work. The absorption spectrum of TMAE in the near UV is shown as the upper curve in Fig. 4. When excess O2 was added, the absorbance was reduced but was still appreciable. This is seen in the lower curve in Fig. 4, where an E&fold excess of O2
5 TABLE
3
Effect of variation
in temperature
W-MAE) (Torr)
0.200 0.200 0.200 0.200 O-200 0.202 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.202 0.204 O-200 0.202 0.202 ‘0.204 0.204 0.208 0.200 0.201 0.199 0.200 0.202
22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 Vontained
up to 5 Torr
and O2 pressureon emittedlight
P(OzIJ
IE
(Torr)
@A)
4.030 2.160 6.200 11.25 9.380 4.020 11.09 3.530 1.360 9.920 5.730 11.08 4.030 11.60 4.910’ 2.140 2.180 2.160 2.130 4.270 8.180 6.170 14.30 8.030 6.010 9.7xP 9.72(r 2.260
107.0 30.00 210.0 215.0 277.0 110.0 227.0 93.40 14.30 240.0 167.0 298.0 101.0 280.0 45.00 17.00 20.00 20.00 19.00 64.00 189.0 115.0 220.0 190.0 110.0 168.0 130.0 20.00
COO.
reduced but did not eliminate the TMAE curve and no new absorption bands were seen except possibly above 400 nm. The absorption spectrum given in Fig. 4 differs from the near-U57 spectrum reported by Nakato et al_ [lo] and it may be pointed out that their far-W spectra have been disputed by Holroyd et al. [Ill. The mechanism was computer simulated with appropriate software [12]. None of the rate constants needed is known and the values chosen for best fit with the experimental data at 22 and 30 “C are given in Table 4.
4. Discussion
4.1. Mechanism and rate law We postulate the following emission
as reactant
mechanism to aazount for the behavior concentrations are varied.
of the light
00 0 10
0 00
Fig.
1. Emitted
Oi~~~l
0.20
0.30
0.40
0.50
P(TMAE)Prr
light intensity
1, VS. TMAE
pressure
at temperatures
shown.
IIII,l’ll”,,‘,““““‘,I”“,,‘,,
0
h/iethon&
TOT:
4
Fig. 2. Effect of added methanol on emitted light at 22 “C (filled circles) and 30 “C (unfilled circles). TMAE and O2 pressures kept approximately constant. Lines are from simulated mechanism.
TMAE+o,e
(k, k-d
TO2
TO2 -
2TMU
&a)
TO1-
ThJO+TMH
Wzb)
TQ-
R+
&A
TOz
+ RIOH
2R-TMAE*
RO, +
R + RIOOH
+ TMlJ
(W w4>
0
:7,,,,,.,,,,,,,.,,,,,,,,,,,...,,,,,,,.,,,,,,,,,,,,, 0
idde;i4
1
gas’:
Toris
Fig. 3. Effect of added water (unfilled circles) and added butanol (Wed circles) on emitted light at 30 “C. TMAE and O2 pressures kept approximately constant. Lines are from simulated mechanism_ 2500
7
1500
zz . isi
1000
ix a w
500
0 150
200
250
300
WAVELENGTH,
350
nm
400
450
Fig. 4. Absorbance of T’MAE (upper curve) and ThJAE P(TMAE) = 0.075 Torr, P( 0,) = 6.4 Torr.
R+TMAJ3+ R+OzTMAJz* ThaAE*+ TMAE*+Q-
products TMSJ
(ks) (W
TMAE+hv TMAE
‘I’MAE+Q
e7) W @9)
with added Oz (lower curve) at 23 “C.
TABLE
4
Rate constants chosen for simulation kinetics 22 “C 2.0x
kt
30 “C
3.0x lo-‘5
lo--=
7.5 x ld
k k,l
1.5x104 3.0x10' 1.0x10' 1.0x10'
1.5 x 10’ 1.0 x 10’ 1.0 x 10’
kzb
kzc
4.8x10-l7 1.2x lo-‘” 1.5x10-‘6 1.0x10-*5 5.0x10-‘” 1.0x lo-‘8 1.0x 108 1.0x 108 1.0x 10-14 l.ox10-‘6
4.8 x 1O-‘7
k3(MeOH) k,(BuOHj W=W) k4 kr, k, k7
1.0 x lo-‘5 5.0 x 10-16 1.0 x IO_‘8 1.0X108 1.0x 108 1.0x 10-‘4 1.0 x 1o-‘6
&TMAE) WOz)
First- and second-order rate constants have units of s-* and cm3 molecuIe-’
s-l
respectively.
The postulated intermediate TO* splits into two molecules of the product tetravia a symmetric split in step 2a. An unsymmetric methylurea (TMW), [(CH&N],C=O, split in step 2b leads to the products tetramethyloxamide (TMO), [(CH,),NC=0]2, and tetramethylhydrazine (TMH), [(CH&$?12. I n step 2c another unsymmetric split gives two free radicals, R and RO *, one of which dime&es to form electronically excited TI%%E* which we believe is the precursor of the chemiluminescence. A minor product, bis(dimethylamino)methane, [(CH3)2N12CHZ, is presumably formed by subsequent free-radical rearrangements. R,OH represents an added hydroxylic substance. Assuming a steady state in intermediates and postulating that 2k4[R12 we obtain the rate law for the emitted light intensity 1,: =k,[R][=l +k[RlP,l,
k&7
IE=
k,+ks+ks[Q]
k2 + kSIRIOH] kJIMAEl+k,[Oz]
’
kl t-1
Kbl
>( k_,+k,+k,[R,OH]
* >
(1)
where kz= kh + kzb+ k&. As seen in Figs. 2 and 3, light was emitted in the absence of added hydroxylics and this has also been observed in vapor phase work by Herek er al. [13]. The postulated mechanism therefore involves pathways to light formation corresponding to the absence (step 2c) and presence (step 3) of added RIOH. If the quenching step 9 is unimportant, eqn. (1) may be simplified in the absence of added RIOH to
where k
i
=
b+ks+ks[Ql
k&7
1Rk--l+k2 klkzc
9
Equation (2) was tested by plotting IB-lR VS. I/[TMAE] at constant [Q] (data from Table 2) in Fig. 5 and plorting &-ln us. I/[@] at constant [TMAJZJ (data from Table 3) in Fig. 6. Both graphs are reasonably linear. The rate law has not been agreed upon in the liquid phase work. Paris 181 reported thhatthe light emission was second order in TMAE and in hy&oxyIic activator and gave a mechanism which was in accord with these findings aad with first-order 0, dependence. Fletcher and Healer [4] gave a con@ex mechanism which gave a rate Iaw which reduced to first order each in TMAE, alcohol and Oz. They suggest that
o-0 7
2
3
4
I/P(TMAEJ
5
6
7
Tort-l
5. I,-ln as a function of 3/P(TMAE ) at 10 “C (circles), 22 “C (squares), 30 “C (triangles) and 45 % (diamonds) at constant P(0,). For clarity, values at 10 ‘C have been divided by 10. Lties at 22” aJld 30” are from sknulated mechanism. Fig.
Fig. 6. I,- LB as a function of I,!.~~~ at 22 ‘C (squares) and 30 “C (triangles) at constant P(TbbUT). Lines are from simulated mechanism,
3.0
3.1
3.2
3.5
3.3
1000/T,
3.6
I?”
Fig. 7. Arrhenius plot of slopes of Fig. 5. the light emission should show little or no temperature dependence but this was not verified experimentally. Our complex rate law (eqns. (1) and (2)) d oes not reduce to an emission which has a simple order in reactants. The slopes of the plots in Fig. 5 were converted into concentration units and given as an Arrhenius plot in Fig. 7. This yields an activation energy of 1651111.2 kcal mole-l (estimated error limits), corresponding to the temperature coefficient of the composite rate constant kaka. The temperature range in Fig. 6 is too small to warrant speculation on temperature effects. Computer simulation of the light emitted was carried out using the rate constants shown in Table 4 and applying the normalizing factor photocurrent
(LA) = #I v
where @ allows for the geometry of the system and quantum yield losses. The same value of @= 1.27~ 10s5 was used for all series except for the runs with added water where a value of @=&OX 10e6 was used. Some difficulties were encountered in the experimental work with added water and these data may not be reliable. Overall, however, the simulation was in accord with the postulate that hydroxylic compounds catalyze the light emission but are not essential for it. The simulation also lends weight to the conclusion that the reason for the light output decreasing with increasing temperature is because as the temperature rises, kl and k_l increase but k,/k_, decreases. 4.2. Reactive channels The predominant channels in the reaction of m with O2 do not produce light. The quantum yield in the liquid phase has been estimated at about 10m3 under typical conditions [6, 141. Urry and Sheet0 [15] made an extensive study of products from the liquid phase reaction and their findings may be summarized as TMAE+o*-
2TMs-J
‘z?MAE+o,-
TMo+TMH -
TMO+DAM
64%
36% >
where DAM signifies bis(dimethylamino)methane. If we assume similar product channels in the gas phase, we can represent the pathways as follows: me2
Me,N \
C
/
Me,N
C
/
+
\
02
NMe,
N
’
I
Me2 , 1 gives two molecules of TMU, while dimethylamino radicals which form TMH directly because of the cage effect) or DAM indirectly. The two radicals, one of which could dime&e to form very noticeable chemiluminescence. Cleavage
'\N
Me2 cleavage 2 gives TM0 and two (more likely in the liquid phase less likely skewed cleavage 3 gives TMAE*, giving the inefficient but
Acknowledgments We thank Dr. Ralph E. Weston Jr. of Brookhaven National Laboratory for kindly making available some unpublished work on Th&%E. We are grateful to the Rutgers University Research Council for partial support of this work.
References 1 R. L. Pruett, J. T. Barr, K. E. Rapp, C. T. Bahner, J. D. Gibson and R. W. Lafferty, J; Am. Chem. Sot., 72 (1950) 3646. 2 T. M. Freeman and W. R S&z, Anal. Chem, 53 (1981) 98. 3 K. Kuwata and D. H. Geske, J; Am. CItem Sot., 86 (1964) 2101. 4 R. A. Holroyd, S. Ehrenson and J. M. Preses, ,I. Phys. CIiem., 89 (1985) 4244. 5 H. E. Winberg, J. R. Downing and D. D. coffinan, 1. Am. Chem Sot., 87 (1965) 2054. 6 A. N. Fletcher and C. A. Heller, J. Phys. Chem, 71 (1967) 1507. 7 C. E. Waring and R. A. Berard, 3. Phys. Chem., 80 (1976) 1025. 8 J. P. Paris, Pbtochem. PhotobioL, 4 (1965) 1059. 9 C. A. Heller and A. N. Fletcher. J: Phys. Chem, 69 (1965) 3313. 10 Y. Nakato, M. Ozaki and H. Tsubomura, J. Phys- Chea, 76 (1972) 2105. 11 R. A. Holroyd, J. M. Preses, C. L. Woody and R. A. Johnson, Nuci. Imwrum. Methods Phys_ Res. A, 261 (1987) 440.
12 12 W. Braun, J. T. Herron and D. Kahaner, ACUCHEMfACUPLOT, National Institute of Standards and Technology, Gaithersburg, MD, 1987. 13 J. L. Herek, R. E. Weston Jr. and J. M. Preses, unpublished work, 1989. 14 F. K. Baibulatov, V. F. Minin and Z. A. Tsaryuk, I&J. Sib. O&d. Akori. Nuuic SSSR, Set. k7rim. Nuuk, 5 (1974) 119; Chem. Abs., (1975) 42492k. 15 W. H. Urry and J. Sheeto, Photochem. Photobioi., 4 (1965) 1067.