J . Phys. Chem. 1986, 90,6184-6189
6184
that the reaction mechanisms can be made more transparent. Namely, the power of the collinear hypershperical coordinates can be. utilized straightforwardly in 3-D reactions. Some important qualitative features of the dynamics at each yi can be. understood, for instance, by considering the potential energy diagrams, E&) vs. p , and the relative position of the yi-dependent potential ridge. Thus, the effects of the potential energy surface topography and the masses of the constituent atoms on the reaction dynamics can be conveniently investigated by the theory presented in this paper.
In order to obtain values for differential and integral cross sections, one has to solve the Schriidinger equation. However, as a consequence of employing the hyperspherical coordinates, the numerical effort is reduced significantlyjust like in collinear reactions. Acknowledgment. This work was supported in part by a Grant in Aid from the Ministry of Education, Science, and Culture of Japan. Numerical calculations were carried out at the computer center of the Institute for Molecular Science.
Photodissociation Studies of HNCO: Heat of Formation and Product Branching Ratios Thomas A. Spiglanin,+Robert A. Perry, and David W. Chandler* Combustion Research Facility, Sandia National Laboratories, Livermore, California 94550 (Received: June 2, 1986; In Final Form: July 29, 1986)
The heat of formation (AHd298 K)) of HNCO is determined to be -249.;: kcal/mol (based on AHdNH) = 85.2 kcal/mol). This value is obtained by measuring the threshold for the production of NH(alA) and by determining the energy contents of the N H fragment and the CO cofragment produced by photolysis of HNCO it wavelengths near the threshold. Saturated laser-induced fluorescence is used to determine the internal state distribution of NH(a' A), and multiphoton ionization is used to measure the internal state distribution of CO. An upper limit for the branching ratio of NCO/NH production from photodissociation of HNCO at 193 nm is determined from an analysis of kinetic experiments to be 0.10. To clarify the mechanism of photodissociation, HNCO fluorescence-excitation and NH(alA) action spectra are also measured. They imply that two excited states of HNCO are present where only one had previously been considered.
I. Introduction Nitrogen chemistry in flames has been studied in great detail over the past 25 years, but the chemistry of radicals such as N H and NCO in combustion is still a major area of investigation. The photolysis of isocyanic acid (HNCO) is a potential source of both N H and N C O radicals and has received a great deal of attention. Two important dissociation channels are1-3 HNCO
+ hu
-
NH(alA)
-NCO+H
+ CO
(1) (2)
A study of the dissociation channels and the energetics of H N C O is of interest as isocyanic acid has been suggested as a potential intermediate in the conversion of fuel-bound nitrogen to oxides of nitrogen."8 Progress has been made toward a complete understanding of the mechanism of the photodissociation of HNCO. Dixon and Kirby measured and analyzed the absorption spectrum of H N C O from 2000 to 2820 A. The low-energy portion of this spectrum revealed that the lowest excited singlet state of H N C O has an equilibrium geometry with an N-C-0 bond angle of approximately 120°,9 in contrast with the nearly linear N - C - O bond of the ground state.1° Dixon and Kirby were not able to determine the absolute geometry of this excited state (cis or trans). Okabe investigated the excited states of H N C O with vacuum-UV light and found that excitation results in the production of several electronically excited species that subsequently fluoresce.' Several recent dynamics studies have investigated reaction 1. This reaction leaves NH in an electronically excited state and is the lowest energy, spin-conserving process that directly produces N H + CO. Fujimoto, Umstead, and Lin measured the vibrational state distribution of the CO fragment produced by HNCO photolysis at 6.4 eV (193 I I ~ ) The . ~ vibrational distribution roughly fits a statistical model that assumes all of the C O correlates with the production of N H in the a'A state. The populations of the quantum states of NH(alA) that result from 6.4-eV photolysis 'Bandia Postdoctoral Research Associate.
0022-3654/86/2090-6184$01.50/0
of H N C O have been measured by Drozdoski, Baronavski, and McDonald.2 They found the N H to be formed vibrationally cold (only Y = 0 seen) and the rotational state distribution in v = 0 describable by a temperature of 1100 K. These investigators were not successful in an attempt to observe N C O by laser-induced fluorescence and concluded that the initial production of N C O by the 193-nm photolysis of H N C O is negligible. Nonetheless, excimer laser photolysis of H N C O is used to generate N C O radicals for kinetics experiments.* Although N C O can be produced in these experiments by a secondary reaction of NH(alA) with H N C 0 , 2 the issue of the photodissociation braqching ratio [NCO]/[NH] is far from settled. Although H N C O and N C O are thought to be important in combustion systems, there still exists disagreement concerning the thermochemistry of these compounds. As part of his photodissociation study of HNCO, Okabe measured the appearance threshold for NH(c'II) and (based on a heat of formation of N H of 81 kcal/mol, a heat of formation of C O of -26.5 kcal/mol, and an electronic energy of NH(c'II) of 43 645 cm-' (124.8 kcal/mol)) calculated the heat of formation of H N C O to be -23 kcal/mol.lI Recent measurements of the heat of formation of N H range from 84 to more than 90 kcal/m01.'~-'~ Using AHf(NH) = 85.2 (1) Okabe,H. Photochemistry of Small Mo/ecu/es; Wiley-Interscience: New York, 1978. (2) Drozdoski, W. S.; Baronavski, A. P.; McDonald, J. R. Chem. Phys. Lert. 1979, 64, 421. (3) Fujimoto, G . T.; Umstead, M. E.; Lin, M. C. Chem. Phys. 1982, 65, 197. (4) Hayes, B. S.;Iverach, D.; Kirov, N. Y. Symp. (Int.) Combust. [Proc.], lSth, 1974 1975, 1103. ( 5 ) Fenimore, C. P. Combust. Flame 1976, 26, 249. (6) Morley, C. Combust. Flame 1976, 27, 189. (7) Haynes, B. S. Combust. Flame 1977, 28, 113. (8) Perry, R. A. J. Chem. Phys. 1985,82, 5485. (9) Dixon, R. N.; Kirby, G. H. Trans. Faraday SOC.1968, 64, 2002. (10) Yamada, K.J. Mol. Spectrosc. 1980, 79, 323. (11) Okabe, H. J . Chem. Phys. 1970, 53, 3507. (12) Melius, C. F.; Binkley, J. S. Presented at the Meeting of the Western States Section of the Combustion Institute, University of California, Los Angeles, CA Oct. 17-18, 1983; paper WSS/CI 83-61. (13) Melius, C. F.; Binkley, J. S.Presented at the 20th Symposium (International) on Combustion, The Combustion Institute, University of Michigan, Ann Arbor, MI, 1984.
0 1986 American Chemical Society
Photodissociation of HNCO PROBE LASER
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6185 multiphoton ionization (MPI). Laser-induced fluorescence is used to detect both electronically excited NH and ground electronic state NCO. These experiments are described separately. A. Detection of CO. A schematic of the apparatus used is shown in Figure 1. HNCO is injected into the source region of a differentially pumped time-of-flight (TOF) mass spectrometer via a thin glass tube. The TOF spectrometer is capable of mass resolution of 1 in 80. The pressure of H N C O in the tube is 1 Torr, as determined by the cold bath used to hold the sample. The base pressure at the ionization gauge in the source chamber is 1X Torr, although the pressure in the source region is higher due to the proximity of the injection tube to the photolysis region. The pressure in the detection chamber is 3 X IO-' Torr. An unfocused photolysis laser beam, either from an excimer laser operating at 193 nm (Lamdba Physik 200 EL) or from a tunable ultraviolet laser operating between 228 and 245 nm, is used to dissociate HNCO. The tunable ultraviolet light is generated by Raman shifting the output of a frequency-doubled dye laser (Quanta Ray) in a high-pressure cell containing H2. A focused (4.5-cm MgF2 lens) probe laser beam intersects the fragments in the source region of the TOF mass spectrometer after a variable time delay (10-500 ns). The wavelength of the probe X laser, near 230 nm, is two photon resonant with the B transition in CO. Following a state-selective, two-photon preparation of the B state of CO, the absorption of a single 230-nm photon ionizes the CO. This process is referred to as 2+1 multiphoton ionization (MPI). As the frequency of the probe laser is scanned, the ion signal is detected by a two-plate multichannel ion detector (Gallileo Model FTD 2003), amplified, and sent into a Stanford Research Systems boxcar averager. The gate of the boxcar is set to detect only the ion signal that arrives at the detector at a time characteristic of mass 28. The analog output of the boxcar is digitized by a Kinetic System C3553 12-bit analogto-digital converter and stored on hard disk with an LSI-11 laboratory computer. B. Detection ofNH(a'A). NH(a'A) (hereafter NH*) produced by the photolysis of H N C O is detected via laser-induced fluorescence. The apparatus used is essentially the same as shown in Figure 1 except that N H fluorescence is detected rather than CO' produced by MPI. HNCO vapor is allowed to effuse into a stainless steel cell that is being slowly evacuated through a conduction-limiting restriction until a steady-state pressure of 200 mTorr is obtained. In these experiments the probe beam is an unfocused, frequency-doubled dye laser beam with output wavelength tunable in the region of 320 nm. This wavelength is alA transition in NH. Drozdoski, resonant with the c'II Baronavski, and McDonald used this same transition in their study of H N C O photodissociation at 193 nm.2 In our experiments we detect NH*(v=O) after photolysis of HNCO near threshold. The photolysis beam is produced by Raman shifting (second anti-Stokes order) a frequency-doubled dye-laser beam in a high-pressure cell containing H2. This produces tunable light near the HNCO N H * + CO appearance threshold (227-242 nm) with 0.3 mJ of energy in an 8-ns pulse with a bandwidth of 0.4 cm-'. Laser-induced fluorescence is detected through a Corning 4-69 band-pass filter by a photomultiplier tube (Hammamatsu R955) withf/2 collection optics. Current at the photomultiplier's anode is amplified and processed by the same collection system used for ion detection. Fluorescence is observed coincident with the photolysis pulse when dissociation is induced in HNCO by using 230-nm photons. A time delay between the photolysis pulse and the probe pulse of 80 ns is used. Fluorescence that results from the photolysis pulse is sampled by a boxcar averager prior to the arrival of the probe pulse. Laser-induced fluorescence due to NH* is sampled by a boxcar averager after photolysis-induced fluorescence has decayed to a low level. Near the photodissociation threshold, very little emission results from the photolysis pulse and a time delay between photolysis and probe pulses of 10 ns is sufficient to observe only N H * emission. N H * fluorescence-excitation spectra are measured following photodissociation by fixing the wavelength of the photolysis laser and scanning the wavelength of the probe laser while monitoring
-
I
PHOTOLYSIS LASER
F I 1. Schematicof the apparatus used to measure the internal states of various dissociation products of HNCO. The photolysis pulse precedes the probe laser pulse by 5-100 ns. Product quantum state distributions are measured for CO by 2+1 resonant multiphoton ionization spectroscopy and for NH* by laser-induced fluorescence.
kcal/mol (we will discuss our choice of this value later in this paper), AHdCO) = -27.2 kcal/mol,14 and the electronic energy of NH(c'II) corrected for zero-point energies (123.4 kcal/mol),20 we calculate an updated value of the heat of formation of HNCO from Okabe's dissociation threshold of -20.7 kcal/mol. Using a larger value for AHdNH) would lead to a larger value of AHdHNC0,298 K). Sullivan, Smith, and Crosley recently determined a lower limit for the heat of formation of NCO of 48 kcal/mol.2' Using this value and the energy threshold for the appearance of NCO(aZZ)measured by Okabe, they placed a lower limit on the heat of formation of HNCO of 3-14 kcal/mol, higher than that calculated from Okabe's NH(clII) threshold unless AHANH) is taken as 291.9 kcal/mol. Melius and Binkley recently calculated the heat of formation of H N C O to be -25.7 kcal/mol.'2 As they accurately calculated A& values for a number of other compounds, the difference between this and the two experimental values is disturbing. In this paper we report (1) a determination of the heat of formation of HNCO, (2) measurements of internal energy distributions in the dissociation products of near-threshold photolysis of HNCO that bracket AHdHNC0,298 K), and (3) an upper limit to the photodissociation branching ratio [NCO] / [NH] for 193-nm photolysis. We also present spectra that evidence the presence of two electronically excited states, only one of which leads to the formation of NH and CO. These data help to clarify the mechanism of photodissociation of HNCO. 11. Experimental Section H N C O is prepared by heating cyanuric acid (HNCO polymer) to 650 K. H N C O vapor is condensed in a liquid nitrogen trap, purified by vacuum distillation, and stored in a dry ice/acetone, bath at 200 K. At this temperature the vapor pressure of HNCO above the liquid is 1 Torr. Purity of the sample is checked by measuring the FTIR spectrum of the vapor and is better than 99%; the largest impurity is C 0 2 . Three dissociation products are observed by using three different experimental arrangements. CO is detected by resonant 2+1 (14) Stull, D. R., Ed. JANAF Thermochemical Tables; Dow Chemical: 1974; addenda. (15) Piper, L. J. Chem. Phys. 1979, 70, 3417. (16) Dean, A. M.;Chou, M.-S.; Stern, D. In The Chemistry ofcombusrion Processes; Sloane, T . M.;Ed.; American Chemical Society: Washington, DC, 1984; ACS Symp. Ser. No. 249. (17) Chase, Jr., M. W.; Curnutt, J. L.; Downey, Jr., J. R.; McDonald, R. A.; Syverud, A. N.; Valenzuela, E. A. J . Phys. Chem. ReJ Data 1982, 1 1 , 695. (lS).Hofzumanhaus, A,; Stuhl, F. XI1 International Conference on Photochemistry, 1985, abstract 2A03. (19) Gibson, S. T.;Green, J. P.; Brekowitz, J. J. Chem. Phys. 1985.83, 4319. (20) Huber, K.P.; Herzberg, G. Constants ofDiotomic Molecules; Van Nostrand: Princeton, NJ, 1979. (21) Sullivan, B. J.; Smith, G. P.; Crosley, D. R. Chem. Phys. Lett. 1983, 96, 307.
Midland, MI,
-
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6186 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
Spiglanin et al.
I 140 7)
120
I
4
MPI o f
I
1
CO
I
s .-
Y
0
*O1
40
20
i1
0 1 0
200
I
800
400
Following HNCO
Photolysis a t 230 nm
T
800
-
Fit
491K
Time (msec)
L P
J
4 eeooo
I
I
2000
4000
6000
8000
10000
[H21/[H NCOI, Figure 2. NCO laser-induced fluorescence intensities plotted as functions of H2concentrations that have been normalized to the original concentration of HNCO. The circles and triangles represent the results of two different sets of experiments. The difference between the two sets results from the 193-nm photolysis pulse's decreasing of the actual concentration of HNCO in the reactor. The inset shows a typical NCO decay curve. The fluorescence intensities at time 0, determined by fitting an exponentially decay function to the data (see text), are used for the NCO concentrations.
the fluorescence intensity. By tuning the frequency of the probe laser to the Q2 transition of NH* and scanning the frequency of the photolysis laser, we obtained an action spectrum. This spectrum represents the absorption spectrum of the state of HNCO that produces NH*(v=O,J=2), multiplied by a dissociation efficiency. By monitoring fluorescence coincident with the photolysis pulse, a fluorescence-excitation spectrum of HNCO is measured simultaneously with the NH* action spectrum, allowing the two spectra to be directly compared. C. Detection of NCO. The apparatus used to determine the branching ratio of NH/NCO for the photolysis of HNCO is identical with that described in ref 22. Only basic features of the apparatus will be outlined here. HNCO vapor at 1 Torr is mixed with 760 Torr of Ar gas and flowed slowly through a quartz reactor at room temperature. The pressure inside this reactor is typically 10 Torr. The sample is excited by the unfocused output of an A r F excimer laser. The NCO that is produced is detected by laser-induced fluorescence using a CW Ar+-pumped dye laser transition in NCO resonant with the AZZ+l,o,o X2110,0,0(P2) (416.7 nm).u Fluorescence is detected by a photomultiplier whose output is sampled by a multichannel analyzer.
-
111. Results We divide the results of our experiments into three sections: results that determine the branching ratio for the products of photolysis, those that directly measure the threshold for dissociation of HNCO to NH* CO, and spectroscopy of HNCO that probes the dissociating state. A . Branching Ratio. NCO concentration in the flow reactor decays exponentially as a function of time following the photolysis of HNCO in the gas phase, as shown in Figure 2. In the absence of H2, the decay results from diffusion of NCO out of the region of intersection between the probe laser and the detection optics. NCO decay curves are measured as functions of H2 pressure and are fitted by a function of the form Z = Ae-kr. The magnitude of the time-zero intercept of the fit ( A ) decreases as a function of the Hz pressure, as shown in Figure 2. The decrease in intensity of the NCO fluorescence on addition of Hz is primarily due to reactive collisions of Hz with NH(alA), which would otherwise have reacted with HNCO to give NCO NH2.
+
+
(22) Perry, R. A.; Melius, C. F. Presented at the 20th Symposium (International) on Combustion, The Combustion Institute, University of Michigan, Ann Arbor, MI, 1984. (23) Dixon, R. N. Philos. Tram. R. SOC.London, A 1960, 252, 165.
~eeio
1 88b20
88930
86940
86960
Frequency (cm") Figure 3. Multiphoton ionization spectrum of CO that is produced by photodissociation of HNCO near 230 nm, plotted against twice the frequency of the laser. This measurement is made using only one laser since the probe laser also serves to photolyze the sample. (The absorption spectrum of HNCO is virtually flat over the range over which the probe laser wavelength is tuned.) The dashed curve represents the spectrum calculated by using a statisticalmodel that weights line strengths by their Boltzmann factors. The temperature defined by fitting this model to the data is 491 f 10 K.
HNCO can dissociate following 193-nm photolysis to give either NH* or NCO (reactions 1 and 2). Drozdoski et al. concluded that following dissociation NH(a'A) also reacts with HNCO:2 HNCO NH(alA) NCO + NH2 (3)
-
+
-
It is also known that in the presence of H224
NH(a'A)
+ Hz
NH2
+H
(4)
Since the predominant chemical pathway to NCO is represented by reaction 3: in the absence of H2the amount of NCO produced is the sum of that produced via channels 2 and 3. Since H2 effectively removes NH(a'A) (eq 4), the amount of NCO produced is expected to decrease with increasing H2 pressure until a limiting value is reached. That is, the NCO detected in the presence of high H2 pressures must be produced exclusively by direct dissociation (eq 2). In these experiments quenching of NCO(A2Z+) due to H2 is of minor importance. At the highest [H2]/[HNCO] ratios, changes in the absolute concentration of [H2]by a factor of 3 make no difference in relative signal intensity. The branching ratio for process 1 to process 2 is therefore given by [NCO]/[NH] = [NCO1exH2/([NCOlnoH2 - [NCO1excessH2) As shown in Figure 2, the intensity of the laser-induced fluorescence from NCO drops dramatically with increasing H2 pressure. If one uses the NCO signal intensity at the highest [H,]/[HNCO] ratio as an upper limit to the signal due to NCO formed by direct dissociation, an upper limit to the [NCO]/[NH] branching ratio of 0.10 results. In a separate experiment, we measure a small yield of H atoms using MPI following HNCO photodissociation at 243 nm, consistent with a small yield of NCO. B. Threshold for the Formation of NH* CO. The MPI spectrum of CO produced by photolysis near 230 nm is shown in Figure 3. This spectrum, which encompasses the 0-0 band for the B X transition of CO, is fit by a statistical model that weights the rotational lines (only a Q branch is observed) by the degeneracy of the rotational level of the origin state and by the state's Boltzmann factor. The rotational lines are convolved with Gaussian functions, and the model is fitted to the measured spectrum. The model defines a temperature of the CO fragment produced via eq 1 of 490 f 25 K. This temperature corresponds to an average rotational energy content of 341 cm-'. The region of the 1-1 band of the same electronic transition is also scanned by no v = 1 CO is detected, consistent with photolysis energy being near the photodissociation threshold.
+
-
(24) Cox, J. W.; Nelson, H. H.; McDonald, J. R.Presented at the 8th International Symposium on Gas Kinetics, 1984.
Photodissociation of H N C O
30400
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6187
30800
30800
Probe Laser Frequency (cm-') 1
> r v)
a=l -C
''
"HNCO (photolysis pulse)
! , l,\ N H
I
1
(probe pulse)
1'
~
t-'
\J\\
--o-
1
1
0
200
400
-
600
0
43300 800
Time (nsec) frequency (top) and time (bottom) following photodissociation of HNCO at 230 nm. Included in the top frame are the rotational state populations of the NH* determined from the spectrum. The first peak in the bottom frame is due to fluorescence from the HNCO sample that results during photolysis while the second peak (about 100 ns delayed in this case) is due to NH* fluorescence induced by the probe laser. 1 ,
I
41500
41600
41700
41800
41900
Photolysis Frequency (cm-') Figure 5. NH* laser-induced fluorescence intensity (produced by exciting Q2of the NH(c'lI) NH(aIA) 0-0 transition) as a function of the frequency of the laser used to dissociate HNCO. The dashed line indicates the extrapolation used to determine the threshold for the production of NH* (41 530 & 150 cm-I) (see text). The temperature of the NH* produced by photolysis in this region is approximately 162 K.
-
The rotational distribution of the N H * produced by photolysis' at 230 nm is determined by measuring the fluorescence excitation spectrum of the NH(alA), as shown in Figure 4. The intensity of the laser is sufficient to saturate the absorption transitions (saturated laser-induced fluorescence), and the rotational line intensities are proportional to the original population of the absorbing state multiplied by the fraction of the population moved prt), where pp and psj are the to the upper state ( p r / ( p r degeneracies of the upper and lower states, respectively, assuming that the excited-state lifetime is long compared to the duration of the laser pulse. The rotational-state populations are determined by using the P- and Q-branch lines (AJ = -1, 0, respectively), also shown in Figure 4. The intensity of the NH* laser-induced fluorescence is measured as a function of photolysis energy very near the threshold for photodissociation, as shown in Figure 5 . The N H * transition excited is Q2 so the intensity of the signal is proportional to the population of the lowest possible rotational level of NH(a'A). The intensity of fluorescence due to N H * decreases in a nearly linear fashion toward zero as the frequency of the photolysis laser is scanned to lower frequency. Extrapolation of this curve to zero signal leads to a threshold for the formation of NH(a'A) of
+
43450
43600
43750
43900
Photolysis Frequency (cm-'1 Figure 6. NH(a'A) action spectrum (upper curve, A) and fluorescence-excitation spectrum (lower curve, B) of HNCO vapor at 200 mTorr. The laser that induces NH* fluorescence intersects the sample 50-100 ns after the HNCO excitation pulse.
1000
Figure 4. Fluorescence intensity of NH* as a function of probe laser
41400
~~
I
approximately 41 530 cm-I. Fluorescence-excitation spectra of the NH* are measured following photolysis of H N C O at 50-cm-' intervals over the range of photon energies shown in Figure 5 . These spectra show virtually identical peak intensity distributions within the experimental uncertainty. These distributions reflect a rotational temperature of 162 f 5 K. At a photolysis energy of 41 475 cm-' there is very little intensity in the N H * excitation spectrum, evidencing only a very small yield of NH*. At a photon energy of 41 425 cm-I, the excitation spectrum contains no peaks due to NH*, only a weak background believed to be H N C O fluorescence. C. Spectra of HNCO. Two methods of detection are used to monitor the absorption of light by HNCO. One method involves only a single laser that induces fluorescence in the gaseous sample. The intensity of this fluorescence is measured as a function of the frequency of the light used for excitation. This spectrum is referred to as the fluorescence-excitation spectrum. The other method of detecting absorption in H N C O uses laser-induced fluorescence to monitor the formation of N H * from photodissociation of H N C O as a function of photolysis frequency. This spectrum is referred to as the N H * action spectrum. The fluorescences are detected by the same photomultiplier tube (RCA C 3 1034A) through the same filter (Corning 4-69) and are separated by the timing of the photolysis and probe lasers (shown in the bottom of Figure 4) by independent boxcar averagers. Portions of both the fluorescence-excitation spectrum and the NH* action spectrum of HNCO at 200 mTorr are shown in Figure 6. The two spectra are clearly quite different; the N H * action spectrum exhibits significantly less structure than does the fluorescence-excitation spectrum. The structured appearance of the fluorescence-excitation spectrum is real (in other words, it is not noise); there is no corresponding structure in the N H * action spectrum. IV. Discussion A . Branching Ratio NCOINH for 193-nm Photolysis of HNCO. The results of the experiment aimed at determining the branching ratio for the various products of photodissociation N C O dissociation directly confirm the low yield of the H channel in the low-energy photolysis of HNCO. This conclusion is in agreement with that of Drozdoski et al., who unsuccessfully tried to detect N C O by laser-induced fluorescence following 193-nm photolysis of H N C 0 . 2 Based on the heat of formation of NCO determined by Okabe,the dissociation limit for H + NCO is lower in energy than that for N H * + CO." It is tempting to use the preference for rupture at the N-C bond over the H-N bond as evidence for the involvement of excited-state dynamics on the dissociative process. The excited state thought to be involved in the photodissociation is the A" state extensively in-
+
6188 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
vestigated by Dixon and Kirby.g These investigators found that this state has an equilibrium geometry quite different from that of the ground state. The principal change is in the NCO bond angle which goes from nearly 180" in the ground statelo to approximately 120° in the excited state.9 This presumably takes place via a rehybridization around the C atom and results in a reduction in the order and an increase in the length of the N - C bond. Exaggerated motion along this surface from the equilibrium geometry of the ground state logically leads to the formation of N H * and CO. As will be discussed in section C below, a clear picture of the dynamics involved in the photodissociation is not yet possible due to an incomplete knowledge of the excited state structure of HNCO. B. Heat of Formation of HNCO. The results of three experiments are used to determine the heat of formation of HNCO. These experiments are the NH* action spectrum for HNCO CO dissociation threshold, the photolysis near the N H * fluorescence excitation spectrum of NH* as a function of HNCO photolysis wavelength, and the MPI spectrum of CO produced by photodissociation of HNCO near 230 nm. The NH* action spectrum of HNCO near 239 nm (Figure 5) shows that the dissociation efficiency falls dramatically toward zero as the photolysis energy decreases toward 41 530 cm-I. We obtain an estimate for the threshold energy by using the frequency at which a linear extrapolation of the photolysis efficiency curve crosses zero. The value thus determined is 41 530 f 150 cm-I. The error range results from considering the inability to draw a unique line through the data of Figure 5. In Figure 5, a broad band appears near 41 450 cm-' that we believe is due to structure in the absorption spectrum of HNCO or to a hot band absorption. At 41 475 cm-I, N H * fluorescence-excitation spectra show that only small amounts of NH* are formed; at 41 425 cm-' virtually no NH* is produced. The A' ground state of HNCO dissociates to NH(a'A) CO." In the absence of a barrier to dissociation, a measurement of the dissociation threshold reflects the ground state dissociation energy for rupture of the N - C bond in HNCO, regardless of the details of the mechanism for dissociation. It is possible that there is a small barrier to dissociation (or a weak dynamical constraint). In this case the measured threshold energy for the production of N H * from HNCO would be higher than the true dissociation energy. Such a barrier would result in rotational, translational, and possibly vibrational excitation of the products, accounting for the difference in energy between the top of the barrier and the true dissociation energy. Fluorescence-excitation spectra of NH* produced near threshold evidence some rotational excitation of the NH* corresponding to an average energy content of 78 cm-' or a temperature of 162 K. Absorption by thermally excited molecules could account for the NH* product excitation. We are unable to measure CO rotational distributions following photolysis photon energies very near the dissociation threshold due to the intense background ion signal introduced by the probe laser. In addition to the spectra reported above, N H * fluorescence excitation spectra have been measured for photolysis energies to 52295 ~ m - ' . Except ~ ~ for the results from the low-energy photolysis experiments, the average NH* rotational energies increase linearly with increasing photon energy. Extrapolating this behavior to zero rotational energy (and ignoring the upward curvature at low photon energies believed due to absorption by thermally excited molecules), we obtained an upper limit to the dissociation energy of HNCO of 42 500 cm-I. Since the measured threshold energy for NH* formation is below that determined by extrapolating the NH* internal energy to zero energy, there appears to be no barrier to dissociation. The absence of a barrier means that measuring the threshold is equivalent to measuring the dissociation limit for HNCO. (Interestingly, the threshold energy (41 530 cm-I) and the N H * rotational energy extrapolation (42 500 cm-I) can be brought into agreement by assuming the threshold is due to absorption by rotationally and vibrationally excited molecules ( k T N 200 cm-I; v5(HNCO) = 577 cm-I)). Using our threshold energy of 41 530 cm-I (1 18.7 kcal/mol), the heat of formation of NH (85.2 k ~ a l / m o l ) , 'the ~ a'A-X32-
+
+
Spiglanin et al. energy difference for N H (35.8 kcal/mol),20 and the heat of formation of CO (-27.2 kcal/mol),14 we calculate the heat of formation of HNCO to be -24.9 kcal/mol. Recent values for the heat of formation of N H range from 84.2 to 90.0 k ~ a l / m o l . l ~ - ' ~ We choose a value of 85.2 kcal/mol determined from photoionization threshold measurements (NH2 NH') because we believe it to be the most accurate experimental value currently available. Using the upper limit to the dissociation energy of 42 500 cm-I (121.5 kcal/mol), we obtain a lower limit to the heat of formation of HNCO of 3-27.7 kcal/mol. The rovibrational energy distribution of the CO produced by photodissociation of HNCO at 230 nm provides further support for the heat of formation determined above. A study of the CO rovibrational energy distribution following 193-nm photolysis of HNCO suggests that there are no dynamical constraints on the formation of vibrationally excited C0.3 This assumption, along with the absence of vibrationally excited CO following 230-nm photolysis, places an upper limit on the amount by which the photolysis energy exceeds the dissociation energy to one quantum of C O stretch (2200 cm-I). This leads to an upper limit to the heat of formation of HNCO of -24.2 kcal/mol. We therefore report AHf(HNC0,298 K) = -24.9 ?&' kcal/mol. This value reflects the errors associated with measuring the dissociation energy of HNCO and the electronic energy of the NH*. The uncertainties associated with the values of AHdNH) and W d C O ) are not considered. A brief comparison of this result with the previous determinations is necessary. Okabe determined the onset of NH(c'I1) formation from HNCO at a photon energy of 70671 cm-l.ll We calculate the same threshold at 72215 f 150 cm-'. Since our threshold is above Okabe's and since we have been very careful about ensuring our measurement's accuracy, the previously reported threshold must be in error (possibly biased by the filtering used to isolate NH(c'II) emission from NH(A311) emission and a low signal-to-noise ratio). Okabe's NCO(AZZ) threshold does not appear to be biased by structure in the absorption spectrum nor by signal-to-noise ratios. Therefore, the value of AH, (HNC0,298 K) calculated by Sullivan, Smith, and Crosley (-14.3 kcal/mol) should agree with ours. To reconcile the two values, we would need to use a value of AHdNH) = 95.8 kcal/mol, higher than that supported by even the largest literature values. Alternately, the threshold determined by Sullivan, Smith, and Crosley for NCO dissociation could reflect a threshold for some other nonradiative process. There are not sufficient data a t this time to discriminate between these two reconciliatory possibilities. C. Mechanism of Photodissociation. The NH* action spectrum and the fluorescence-excitation spectrum of HNCO, shown in Figure 6, are different from one another in both shape and structure. These differences could result from a fundamental difference in the fluorescence/dissociation branching ratio of a single state of HNCO. In order to rationalize the two spectra shown in Figure 6 as coming from a single state, the fluorescence/dissociation branching ratio for that state would have to vary as spectrum B. This scenario is unlikely since the sharp bands in the excitation spectrum can be assigned as members of a long progression of bands supposedly emanating from the lowest excited singlet state of HNCOS9 Another possibility is that the source of the prompt emission (spectrum B) is not HNCO. The spectrum of the fluorescencedetected in Figure 6 is extremely broad, peaking near 290 nm for excitation at 42 500 cm-' and extending for about 16000 ~ m - ' . ' ~This spectrum evidences a great deal of vibrational structure and can be analyzed as arising from several vibrational modes with one very active mode with a fundamental frequency of 575 cm-l. A literature search of spectra of polyatomic molecules failed to turn up a candidate for the fluorescent species other than HNCO (possible candidates included NCO, COz, NH3, and HCN). We therefore conclude that two distinct excited states are present in H N C O in the region probed in Figure 6, a conclusion supported by the closeness of the 575-cm-' mode of the
-
( 2 5 ) Spiglanin, T. A.; Perry, R. A.; Chandler, D. W., manuscript in preparation.
J. Phys. Chem. 1986, 90, 6189-6194 ground state fundamental of H N C O (577 cm-1).26 One of the two excited states has a much higher probability of dissociation to N H * CO than the other. A discussion of the dynamics of the photodissociation of HNCO will be presented in detail in a future p ~ b l i c a t i o n . ~ ~
+
V. Conclusions We have measured the branching ratio [NCO]/INH] for photodissociation of H N C O at 193 nm and have found that the formation of NH(a'A) dominates that of N C O by about an order of magnitude. The photolysis of H N C O is thus an efficient precursor for NH(a'A). We have also determined the heat of (26) Steiner, D. A.; Wishah, K. A,; Polo, S.R.;McCubbin, Jr., T. K. J . Mol. Specfrosc. 1979, 76, 341.
6189
formation of H N C O to be -24.9 kcal/mol by both direct (threshold for photoproduction of NH*) and indirect (absence of vibrationally excited CO product for a single photolysis wavelength) means. The internal states of the N H * and C O products were measured at several wavelengths including wavelengths very near the threshold for their production. Rotation of the, products accounts for only a small fraction of the energy available. Finally, H N C O fluorescenceexcitation and NH* action spectra were measured. The two spectra are quite dissimilar, evidencing the involvement of two excited states in the photodissociation of HNCO.
Acknowledgment. This research was supported by the US. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.
Interfacial Electrical Potential In Micelles: I t s Influence on N ,N ,N',N'-Tetr amethylbenzidfne Cation Decay A. Bemas, D. Grand, S. Hautecloque, U A 75, CNRS- Universite' Paris-Sud, 91405 Orsay Cedex, France
and C . Giannotti Institut de Chimie des Substances Naturelles, CNRS, 91 190 Gif Sur Yvette, France (Received: November 21, 1985; In Final Form: April 23, 1986) The decay and lifetimes 71/2 of photoproduced N,N,N',N'-tetramethylbenzidine cation radicals (TMB') in cationic (Mi'), neutral (Mio), and anionic (Mi-) micelles, in the absence or presence of electrolytes to modify the electrical interface potential, have been determined. In Mi', first-order kinetics are obeyed, in Mio a second-order law is observed, whereas in Mi- the reaction order appears ill-defined. When the decay rate constant K1 is expressed in terms of the interfacial electrical potential A$, an exponential law is found which reflects the existence of an activation energy barrier. The role of the micellar environment thus appears twofold (i) it affects the electron-transfer reaction rates through A+, and (ii) it affects reaction rates by solubilizing the reactants in two distinct phases.
Introduction The influence of organized media-micelles, vesicles, membranes-on the course of a large variety of chemical and biochemical reactions is now well documented. Such an effect is particularly marked when charge-transfer reactions are involved. In the typical case of the photoionization of various hydrophobic micellized solutes, pulsed or continuous photolysis e~perimentsl-~ have repeatedly shown that hydrated electron yields are lower for cationic than for neutral and for neutral than for anionic micelles. The proposed interpretation was based on mere electrostatic considerations: the micellized cation is somewhat stabilized by the negatively charged interface* whereas the photoejected electron, whether in a dry or hydrated state, is repelled from the surface. In two previous s t ~ d i e s ,the ~ . ~effect of the interfacial electrical potential A$ on perylene photoionization yields (cp,-) was explicited. Values of A$ were deduced from various lipoid pH indicators,lOJ1 (1) Wallace, S.C.; Gratzel, M.; Thomas, J. K. Chem. Phys. Letf. 1973, 23, 359. (2) Gratzel, M.; Thomas, J. K. J . Phys. Chem. 1978, 78, 2248. (3) Alkaitis, S.A.; Beck, G.; Gratzel, M. J . Am. Chem. SOC.1975, 97, 5723. (4) Alkaitis, S. A.; Gratzel, M.; Henglein, A. Ber. Bunsen-ges Phys. Chem. 1975, 79, 54. ( 5 ) Alkaitis, S.A.; Gratzel, M. J . Am. Chem. SOC.1976, 98, 3549. (6) Bemas, A,; Grand, D.; Hauteclcque, S.;Chambaudet, A. J. Phys. Chem. 1981, 85, 3684. (7) Grand, D.; Hautecloque, S.;Bemas, A.; Petit, A. J. Phys. Chem. 1983, 87, 5236. (8) Beck, S. M.; Brus, L. E. J . Am. Chem. SOC.1983, 105, 1106. (9) Hautecloque, S.; Grand, D.; Bemas, A. J. Phys. Chem. 1985,89,2705. (10) James, A. D.; Robinson, B. H. J. Chem. Soc.,Faraday Trans. I 1978, 74, 10. (11) Fernandez, M. S.; Fromhertz, P. J. Phys. Chem. 1977, 81, 187.
and a linear relationship between 'pc- and A# was ~ b t a i n e d .The ~ surface electric potential thus proved to be a significant, if not determinant, parameter in charge separation efficiency, in accordance with other authors' conclusions.12 In cases where the net reaction is not a mere electron ejection and cation-electron recombination but of a more complex type, organized assemblies may also stabilize reaction intermediates and lead to greatly enhanced ion lifetimes compared with homogeneous solutions. One of the most striking example is that of the tetramethylbenzidine cation (TMB') whose lifetime increases by several orders of magnitude on going from a 50/50 water-ethanol solution (7112 = 1.5 ms13) to the sodium lauryl sulfate (NaLS) micelle where slj2is of the order of hours or days.5 Similarly, the half-life of the methylphenothiazine cation (MPTH+) was f o ~ n d ' ~to, ' ~decrease in the following order: anionic > neutral > cationic indicating that the interface potential might play a role in the cation decay kinetics. Most recent studies on micellized TMB have focused on chromophore localization,I6 on electronic energy relaxation competitive paths,I3 on the influence of micellar structural characteristics on photoionization yields,I7 and on radical cation stability. (12) Calvin, M.; Willner, I.; Laane, C.; Otvos, J. W. J. Photochem. 1981, I 7 , 195.
(13) Hashimoto, S.; Thomas, J. K. J . Phys. Chem. 1984, 88, 4044. (14) Mc Intire, G. L.; Blount, H. N. J. Am. Chem. SOC.1979, 101,7720. (15) Lardet, D.; Lauren, E.; Thomalla, M.; Genies, M. Nouu. J . Chim. 1982, 6, 349. (16) Narayana, P. A.; Li, A. S. W.; Kevan, L. J. Am. Chem. SOC.1981, 103, 3603. (b) J. Am. Chem. SOC.1982, 104, 6502. (17) Szajdinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R.M. J . Am. Chem. SOC.1984, 106, 4675.
0022-3654/86/2090-6189$OlSO/O 0 1986 American Chemical Society
