3232
J. Phys. Chem. 1992, 96, 3232-3236
Dynamics of the NH(X3E-) 4- NO(X2n) Reaction: Internal State Distribution of the OH( X2n)Product Dipti Patel-Misrat and Paul J. Dagdigian* Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21 21 8 (Received: November 7, 1991; In Final Form: December 30, 1991)
The dynamics of the NH(X3Z-) + NO(X211) .+ OH(X211) + N2(X’Zgt)reaction has been investigated by measurement of the nascent OH product internal state distribution in a crossed-beam experiment. The OH v = 1 to u = 0 vibrational population ratio was determined to be 0.30 f 0.06, and the average rotational energy was found to be 25 f 1 kJ/mol. A selectivity for formation of the lower ’IT3/2 spin-orbit levels over 211112was also found. In addition, the II(A’) A-doublet levels were preferentially formed over II(A”). The implications of the results of this experiment, as well as earlier studies of the related H + N 2 0 reaction, on the dynamics of these reactions are discussed.
1. Introduction The reaction of imidogen (NH) with nitric oxide is of importance in a variety of combustion processes, including the oxidation of ammonia, hydrazine, etc., and in schemes for the control of NO, emissions from fuel nitrogen.] The rate constant of the reaction of ground-state NH(X3Z-) near room temperature has been measured by a number of groups,2” and there is reasonable agreement that the rate constant is large, ca. 5 X lo-” cm3 molecule-’ S-I. A recent shock tube measurement of the rate of this reaction at high temperatures has been r e p ~ r t e d . ~This work also reviews earlier determinations of the high-temperature rate constant. Among reactions involving NH(X3Z-), the reaction with N O is interesting in that its rate is essentially the same as that for low-lying singlet NH(alA).6*8p9 This contrasts sharply with most other N H reactions, in which the rate constant for the ground electronic state is usually substantially smaller than for the singlet excited ~ t a t e . ~ J + I ~ There are two possible exothermic pathways for the reaction of NH(X3Z-) with NO:
NH
+ NO
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-
N 2 + OH, AHo” = -408 f 2 kJ/mol
N20
+ H,
AH,” = -147
f 2 kJ/mol
(la) (lb)
In calculating the exothermicities above, heats of formation were taken from a recent compilation,Is with the exception of NH, whose heat of formation was that determined by Anderson.16 There have been a number of experimental studies which have addressed the question of the product branching ratios for the reaction of both NH(X3Z-) and NH(aIA) with NO. For the reaction involving NH(X3Z-), Harrison et aL5considered channel l a to be the predominant pathway, although they were not able to detect the OH product by laser fluorescence excitation. Recently, Yamasaki et ale6detected both the N H reagent and OH product and concluded that NH(X3Z-) + N O produces N2 OH exclusively. In their recent high-temperature shock tube study, Mertens et ala7reported a branching ratio of 19 f 10% for pathway l a on the basis of fits to the time-dependent N H and OH concentration profiles. For the NH(alA) + N O reaction, Hack and Rathmann9 concluded that 40% of the collisions resulted in physical quenching. They also observed O H as a chemical reaction product. The N 2 0 yield from this reaction was measured by Fueno and co-workers17 in a static cell. Using mass spectrometric analyses they found that the pathway l b accounted for 70% of the total reaction. They did not take into account the electronic quenching of NH(alA) by NO. Yamasaki and co-workers6 report that this reaction is over 5 times less effective in producing O H than is NH(X3Z-) + NO. They also observed H atoms by vacuum-UV laser fluorescence excitation and concluded that pathway l b occurs with
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‘Present address: Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973.
NH(alA), but not with NH(X3Z-), as reagent. Several theoretical studies of the reaction of N O with the imidogen radical have been reported. Melius and Binkley,18and more recently Marshall et al.,I9 using fourth-order Moller-Plesset perturbation theory with bond additivity corrections calculated the heats of formation and the free energies of formation of the various reactants, products, intermediates, and the activated complexes along the reaction pathways. For the NH(X3Z-) reaction, they concluded that pathway l a was the favored product channel, although both channels can be reached without an activation barrier through an initial H N N O adduct. Fueno and co-workers17 have also carried out a b initio calculations on this reaction system. They find that a HNNO(*A”) intermediate can be formed from the addition of NH(X3Z-) to NO, while the lower-lying HNNO(ZA’) is accessed from NH(a’A) + NO. The products of both pathways l a and l b can be accessed from HNNO(ZA’) with similar activation barriers. The calculations of Melius and BinkleyI8 and Fueno and co-workers17 are not in conflict since the HNNO(2A’) minimum can be reached from NH(X3Z-) N O if nonplanar geometries are allowed. Using approximate semiclassical trajectory calculations, Phillips” calculated the rate constant for this reaction and obtained a close
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( I ) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, I S , 287. (2) Gordon, S.; Mulac, W.; Nangia, P. J . Phys. Chem. 1971, 75, 2087. (3) Hansen, I.; Hoinghaus, K.; Zetesch, C.; Stuhl, F. Chem. Phys. Lett. 1976, 42, 370. (4) Cox, J . W.; Nelson, H . H.; McDonald, J. R . Chem. Phys. 1985, 96, 175. ( 5 ) Harrison, J. A.; Whyte, A. R.; Phillips, L. F. Chem. Phys. Lett. 1986, 129, 346. (6) Yamasaki, K.; Okada, S.;Koshi, M.; Matsui, H. J . Chem. Phys. 1991, 95. 5087. (7) Mertens, J. D.; Chang, A . Y.; Hanson, R. K.; Bowman, C. T. Int. J . Chem. Kinet. 1991, 23, 173. (8) Hack, W.; Wilms, A. J . Phys. Chem. 1989, 93, 3540. ( 9 ) Hack, W.; Rathmann, K. J . Phvs. Chem. 1990. 94. 4155. (IO) Hoinghaus, K.; Biermann, H . W.; Zetzsch, C.; Stuhl, F. Z . Natur’forsch.. A 1976. 31. 239. ( I I ) Drozdoski, W.S.: Baronvaski, A. P.; McDonald, J. R . Chen,. Phys. Lett. 1979. 6 4 . 421. (12) Bower; R. D.; Jacoby, M. T.; Blauer, J. A. J . Chem. Phys. 1987,86, 1954. (13) Freitag, F.; Rohrer, F.; Stuhl, F. J . Phys. Chem. 1989, 93, 3170. (14) Nelson, H . H.: McDonald, J. R.; Alexander, M. H . J . Phys. Chem. 1990, 94, 329 1 . (IS) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R . A.; Syverud, A. N . J . Phys. Chem. ReJ Data 1985, 14 (Suppl. 1).
(16) Anderson, W. R . J. Phys. Chem. 1989, 93, 530. ( I 7 ) Fueno, T.; Fukuda, M.; Yokoyama, K. Chem. Phys. 1988, 124, 265. ( I 8 ) Melius, C . F.; Binkley, J. S. In Proceedings ofthe 20th Symposium (International) on Combusrion; The Combustion Institute: Pittsburgh, PA, 1984; p 575. (19) Marshall, P.: Fontijn, A.; Melius, C . F. J . Chem. Phys. 1987, 86, 5540. (20) Phillips, L. F. J. Chem. Soc., Faraday Trans. 2 1987, 83>857
0022-365419212096-3232%03.00/0 0 1992 American Chemical Society
Dynamics of the NH(X3Z-) + NO(X211) Reaction agreement with the previously determined experimental rate constant from his laboratory5 and concluded that the height of the barrier to H atom migration is much less than the HNNO well depth as viewed from the entrance channel. More recently, Walch21 has been performing new a b initio electronic structure calculations on this reaction system. It is also relevant to our understanding of this reaction to note that there have been several studies of a reverse reaction, namely H N20. Marshall et al.19 have recently reported the bimolecular rate constant for this reaction over a wide range of temperature. They observed a non-Arrhenius behavior of the rate constant and suggested two mechanisms. In a direct pathway, the H atom attaches to the 0 end of N 2 0forming an unstable NNOH species, which immediately dissociates to the products N2 + OH. The second mechanism involves addition of the H atom to the nitrogen end of N 2 0 leading to the HNNO intermediate. As in the NH + NO reaction, this intermediate is thought to undergo a 1,3hydrogen shift to form NNOH, which then falls apart. They derived the branching ratio for the NH NO pathway compared to the total reaction to be approximately 1 5 X at 873 K. The dynamics of this reaction has been studied by Hollingsworth et through laser fluorescence detection of the OH product from the reaction of photolytically produced hot H atoms with N20. They found that the OH product had considerable rotational excitation, with a preference for the II(A') A-doublet levels.23 The latter propensity suggests that the reaction proceeds through a planar intermediate. The reaction of hot H atoms with N20 has also been studied by Hoffmann et both in bimolecular collisions and in photoexcited N20-HBr complexes. In the former situation, O'H A2Z+ X211 chemiluminescence was observed, while only ground-state OH(X211) products were detected for reaction within the complex. In both situations, NH(X3Z-) products were observed, with a yield for reaction within complexes similar to that for OH(X211), despite a large energy difference favoring the N 2 + OH channel. In the present paper, we report a study of the dynamics of the NH(X3Z-) NO(X211) N2 OH(X211) reaction pathway, by measurement of the internal state distribution of the OH product though laser fluorescence detection in a crossed-beam experiment. It is anticipated that such an experiment will shed some light on the complicated dynamics of this reaction.
The Journal of Physical Chemistry, Vol. 96, No. 8,I992 3233
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2. Experimental Section These experiments were carried out in a crossed-beam apparatus A brief sumwhich has been previously described in mary of the experimental setup is presented here, with emphasis on recent modifications. The imidogen free radical was prepared in a separate, differentially pumped source chamber (evacuated with an unbaffled 10-in. diffusion pump) by 193-nm excimer laser (Lambda Physik EMG101MSC) photolysis of a mixture of ammonia diluted in a seed gas at the tip of a quartz tube (0.1 cm i.d., 0.3 cm o.d., 1 cm long) mounted on a pulsed solenoid valve (General Valve), which was operated at IO-Hz repetition rate. The typical excimer laser pulse energy at the beam source was 8 mJ; the laser beam was focused with a 30 cm focal length lens to yield a -0.3-cmdiameter spot at the quartz tube. For most experiments, ammonia (21) Walch, S. P. Private communication. (22) Hollingsworth, W. E.; Subbiah, J.; Flynn, G. W.; Weston, R. E., Jr. J . Chem. Phys. 1985, 82, 2295. (23) Alexander, M. H.; Andresen, P.; Bacis, R.; Bersohn, R.; Comes, F. J.; Dagdigian, P. J.; Dixon, R. N.; Field, R. W.; Flynn, G. W.; Gencke, K.-H.; Howard, B. J.; Huber, J . R.; King, D. S.; Kinsey, J. L.; Kleinermanns, K.; Luntz, A. C.; MacCaffery, A. J.; Pouilly, B.; Reisler, H.; Rosenwaks, S.; Rothe, E.; Shapiro, M.; Simons, J. P.; Vasudev, R.; Wiesenfeld, J. R.; Wittig, C.; Zare, R. N. J . Chem. Phys. 1988, 89, 1749. (24) Hoffmann, G.; Oh, D.; lams, H.; Wittig, C. Chem. Phys. Lerr. 1989, 155, 356. Hoffmann, G.;Oh, D.; Wittig, C. J . Chem. Soc., Faraday Trans. 2 1989,85, 1141. (25) Dagdigian, P. J . J . Chem. Phys. 1989, 90, 2617. (26) Dagdigian, P. J. J . Chem. Phys. 1989, 90, 61 10. (27) Sauder. D. G.; Dagdigian, P. J . J . Chem. Phys. 1990. 92, 2389. (28) Patel-Misra, D.; Sauder, D. G.; Dagdigian, P. J . J . Chem. Phys. 1991, 95, 955.
I
32300
I
I
32400
I
I
32500
l l
32600
laser wavenumber (cm-'1
Figure 1. Laser fluorescence excitation spectrum of the A2Z+-X211 (0,O) band of the OH product from the NH(X3Z') + NO reaction.
(Linde, stated purity 99.99%) was mixed in nitrogen (typically 180 Torr in 10-1 2 atm); a few runs employed helium as seed gas to probe the effect of higher reagent collision energies. The pulsed NH beam passed into the scattering chamber through a skimmer (orifice diameter 0.13 cm) and intersected at 90° with a beam of N O (Matheson) generated from a pulsed solenoid valve (General Valve). To avoid formation of NO dimers in this beam, the nozzle (0.1-cmdiameter orifice) backing pressure (neat NO) was kept at 150-200 Torr. This pressure is considerably below that typically employed for the generation of beams of the dimer.2g The N O beam source was mounted in the scattering chamber and was not differentially pumped. The scattering chamber was pumped with a water-baffled 10-in. diffusion pump. The average pressure in this chamber with both beams on was 0. The relative populations of the various fine-structure levels, in particular the A-doublet levels, can provide further insight into the dynami c ~ In . ~both ~ the present study of the NH(3Z-) NO reaction and a previous investigation22of the H + N,O reaction, a preference for the II(A’) A-doublet levels was observed. Such a A-doublet propensity implies that these reactions proceed through
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(38) Huber, K . P.; Herzberg, G. Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (39) Wardlaw, D. M.; Marcus, R. A. Ado. Chem. Phys. 1988, 70, 231. (40) Pechukas, P.; Light, J . C.; Rankin, C. J . Chem. Phys. 1966,44, 794. (41) Wittig, C.; Hoffmann, G.;Chen, Y.; lams, H.; Oh, D. J . Chem. Soc., Faraday Tram. 2 1989,85, 1292. Bohmer, E.; Shin, S.K.; Chen, Y.; Wittig, C . To be published.
J . Phys. Chem. 1992, 96, 3236-3239
3236
a planar intermediate; indeed, the transition state leading to N, + O H products is ~alculatedl’*’~ to have a planar structure. The observed preference for II(A’) over II(A”) levels is consistent with the expected bonding in this complex. As in the HN, molecule,42one expects that the N-0 bond in NNOH will be formed by breaking a N N A bond in the N, moiety and by combining 2 p r orbitals on the adjacent N and 0 atoms to form the N-0 bond with a strongly bent N-O-H angle. In such a structure, the out-of-plane 0 2p7r orbital will be doubly occupied. If planarity is maintained in the formation of the free N2 + OH products, the in-plane n component of the O H molecule will be singly occupied, while the out-of-plane A component will be doubly
occupied, thus leading to preferential production of II(A’) levels, as observed. In the N H + N O reaction, the lower 2113,2spin-orbit levels of the OH product are preferentially formed over the 2111,2levels. This contrasts with the observation by Hollingsworth et a1.22of no spin-orbit selectivity in the OH product from the H N,O reaction. The relative product spin-orbit populations will be governed by the evolution of the system from the NN-OH transition state, in which the spin-orbit splittings are quenched, to the asymptotic OH spin-orbit levels. The difference in the OH spin-orbit populations for the N H + N O and H + N 2 0reactions is surprising but may reflect the differing rotational distributions.
(42) Walch, S. P.; Duchovic, R.J.; Rohlfing, C . M. J . Chem. Phys. 1989, 90,3230.
Acknowledgment. This research was supported by the National Science Foundation under Grant CHE-9020727 and by the US. Army Research Office under Grant DAAL03-9 1-G-0129.
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Phosphorescence from the Triplet Spin Sublevels of a Hexanuclear Molybdenum(I I ) Chloride Cluster Ion, [Mo,CI,,]*-. Relative Radiative Rate Constants for Emitting Sublevels Hisayuki Miki, Takeshi Ikeyama,+Yoichi Sasaki,l and Tohru Ammi* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: August 29, 1991; In Final Form: December 2, 1991)
Phosphorescence of [ Mo6C114]2-is investigated focusing attention on the temperature-dependent spectral shift. The relative radiative rate constants for the emitting sublevels T,,, E,, and TI,are 1:5.8:100. The large radiative rate constant for the TI, sublevel substantiates the previous assignment of the sublevels.
Introduction Hexanuclear molybdenum( 11) chloride cluster ion, [ Mo6ClI4]*-, exhibits red photoluminescence both in solutions and in the crystalline phase (Figure la).1-3 It is important to understand the origins and the mechanisms of the luminescence. However, probably because of the broad and structureless spectrum shown in Figure la, understanding of the excited-state properties is poor. In previous papers4v5we have shown that the luminescence of solid [(C2H5)4N]2[M~6C114] is composed of emissions from three Boltzmann populated sublevels associated with the lowest triplet state, 3Tlu.The energy spacings among the sublevels and the lifetimes of the individual sublevels have been determined by analyzing the temperature dependence of the lifetime. The results are shown in Figure 1b. Even though the emitting levels have thus been clarified, there are still several unsolved questions concerning the radiative mechanism. First of all, the decay rate constant obtained for individual sublevels is the sum of the radiative and nonradiative rate constants, and nothing is known about the radiative rate constant. Without a knowledge of the radiative rate constants the mechanism of the luminescence remains unsolved. Second, the observed luminescence as a whole exhibits a peculiar temperature dependence. As will be described in detail below, as the temperature increases from liquid helium temperature, the spectrum first shifts to the red and then, at temperatures higher than 140 K, shifts to the blue. Such a peculiar temperature dependence of the spectral distribution is in sharp contrast to the monotonic temperature dependence of the lifetime reported in previous paper^.^-^
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’ Miyagi University of Education, Sendai.
*Present address: Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo.
In this paper we, first of all, aim to determine the relative radiative rate constants for the sublevels and, further, to understand the origin of the peculiar temperature dependence. The relative radiative rate constants associated with the sublevels should, in principle, be determined from the simultaneous measurements of the relative quantum yield (or emission intensity) and lifetime. The exact measurements of the emission intensity at various temperatures, however, encounter some difficulty due to the above-mentioned spectral shift in the wavelength region where the sensitivity of the photomultiplier tube drastically depends on the wavelength. Efforts to obtain reproducible data for the intensity integrated over the whole energy scale have been unsuccessful. Therefore, as an alternative method, we focus attention on the spectral distribution which can be determined fairly reproducibly. Experimental Section All the experiments were carried out for crystalline samples of the tetraethylammonium salt, [(C2H5)4N]2[Mo6C114]. The cluster was synthesized as reported previ~usly.~The phosphorescence was observed at temperatures between 13 and 280 K by a Spex 1702 monochromator equipped with a red sensitive Hamamatsu R406 photomultiplier tube. The temperature was controlled with a closed cycle helium cryostat. Excitation source was a 500-W high-pressure mercury lamp. The signal from the photomultiplier tube was fed into a microcomputer for sensitivity ( I ) Marverick, A. W.; Gray, H. B. J. A m . Chem. SOC.1981, 103, 1298. (2) Marverick, A. W.; Najdzionek, J. S.; MacKenzie, D.; Nocera, D. G.; Gray, H. B. J. Am. Chem. Sor. 1983, 105. 1878. (3) Zietlow, T. C.; Hopkins, M. D.; Gray, H. B. J. Solid Store Chem.
1985. 57, 112. (4) Saito, Y . ;Tanaka, H. K.; Sasaki, Y.; Azumi, T. J. Phys. Chem. 1985, 89, 4413. ( 5 ) Azumi, T.; Saito. Y. J. Phys. Chem. 1988, 92, 1715.
0022-365419212096-3236%03.00/0 0 1992 American Chemical Society
