J . Phys. Chem. 1989,93, 549-552 TABLE VI: Contributions of the Single Molecular Orbitals to the Efg of LiOH and LiOLi (in au)
molecule LiOH
orbital twe
electronic
nuclear
total
A
-0.150 0.001 -0.122 -0.189 -0.095 -0.095
0.150 0.000 0.084 0.084 0.150 0.150
0.000 0.001 -0.038 -0.105 0.055 0.055
total
-0.651
0.617
-0.034
1SO 1SLi
1F
-0.152 -0.019 0.002 -0.146 -0.205 -0.085 -0.085
0.152 0.019 0.000 0.081 0.081 0.152 0.152
0.000 0.000 0.002 -0.065 -0.124 0.067 0.067
total
-0.690
0.636
-0.054
1SO 1SLi U U
A
LiOLi
1SLi U U
A
atom and is included in the parameters of our model. To conclude, we may state that we "understand" the efg at the deuterium as being a neighbor effect. The situation is quite different for 14N. Whereas the efg's for ,H are typically in a range between 0.1 and 0.5 au, the values for I4N have a size of about 1 au and a negative sign. Therefore the main mechanism must be a different one. Previously, we gave a detailed breakdown into parts due to single molecular orbitals for several molecules.2 The single parts were rationalized by a qualitative model. If one further breaks down these molecular orbital shares into parts due to the atomic orbitals one finds a confirmation of what is well-known: the main part stems from the electrons in the occupied p-orbitals. Again a different situation is found for the efg at Li. There are no occupied porbitals on Li. Therefore we could assume that the situation is the same as a t H. But due to the inner shell of Li the bond distances between Li and its neighbor are much larger than for H. The neighbor model yields therefore only efg's be-
Photochemistry of O,/HN,
549
tween 0.005 au for Liz and 0.008 au for LiF. These values are not only very small but again show the wrong sign to explain the efg's at Li, which range from -0.004 au for Liz to -0.049 au for LiF. If we exclude Liz we find that all the values are between -0.034 au for LiOH and -0.054 au for LiOLi. In these two molecules Li has not only the same neighbor but also the Li-0 bond lengths are very similar (see Tables I11 and IV). It is hard to see the reason why these two molecules are the extrema of the Li efg scale. To provide some insight, we list the parts due to the single molecular orbitals for these two molecules in Table VI. The first column of Table VI gives an approximate characterization of the orbital type. The second column lists the electronic part of the efg due to that molecular orbital. The third column shows a nuclear part which we attributed to that orbital by the following rules. 1. For the s- and r-type orbitals it is the classical efg due to two charges at the position of the nucleus to which the orbital belongs. 2. The remaining three charges (two at the 0 and one at H or Li) were divided in two equal parts and attributed to the a-type orbitals. Finally, the last column shows the sum of the electronic and nuclear parts within the above arbitrary scheme. For both molecules we find positive contributions from the r-orbitals and larger negative contributions from the a-orbitals. The different parts mostly balance each other and the small overall difference between the two molecules is due to an extremely small change in this balance. Therefore, we conclude that the efg's at 7Li can neither be rationalized by an effect due to the neighbor atom as in ,H, nor by some large electronic effects as in 14N, which could be approximated by some crude electronic model. The small efg at 7Li is due to a balance of very fine electronic effects which may be changed even by substituents far away. Registry No'. Li2, 14452-59-6;LiCH3,917-54-4; LiCH2F,59189-61-6; LiCH20H, 59189-60-5; LiCH2NH2,59189-59-2; LiC3H5,3002-94-6; LiHCCH,, 917-57-7; LiCCH, 11 11-64-4; LiNCH2,99749-05-0; LiON, 36526-56-4; Li20, 12057-24-8;Li202,12031-80-0; 'Li, 13982-05-3.
Mixtures
A. P. Ongstad, R. D. Coombe,* Department of Chemistry, University of Denver, Denver, Colorado 80208
D. K. Neumann; and D. J. Stecht Department of Physics and Frank J. Seiler Research Laboratory, United States Air Force Academy, Colorado Springs, Colorado 80840 (Received: March 12, 1988; In Final Form: June 7. 1988)
Gaseous mixtures of HN, and O3were photolyzed with the 249-nm output of a pulsed KrF laser. The photolysis produces O(lD) atoms, which react with HN3to produce N, radicals and with the N3 radicals to produce electronically excited NO(AzZ+). Rate constants for these reactions were determined from comparison of the experimental NO A X emission time profiles with NO(A2Z+) densities calculated from a kinetic model for the system. The rate constants for O(lD) + HN3 and O('D) + N, were determined to be (3.2 f 1.0) X lo-''' cm3 s-l and (1.0 f 0.3) X cm3 s-I, respectively.
-
Introduction
Photolysis of HN3 in the near-UV region has been shown to produce electronically excited "("A) with unit quantum efficiency.1 The excited NH radicals then react with the parent azide to generate excited NH, and azide radicals: "(ala) f
+ HN3
Department of Physics. Frank J. Seiler Research
-
",(*A1)
+ N3
(1)
Laboratory. 0022-3654/89/2093-0549$01.50/0
The rate constant for this processZhas been determined to be 9.3 X lo-" cm3 SKI. The fate of the azide radical (N3) in this system is not well-known. The chemistry of N3 radicals has been Of considerable recent interest, since their reactions with atoms (R N, NR + N2) are often constrained3s4 to produce elec-
+
-
( I ) McDonald, J. R.; Miller, R. G.; Baronavski, A. V. Chem. Phys. Lett. 1977, 51, 57. (2) McDonald, J. R.; Miller, R. G.; Baronavski, A. P. Chem. Phys. 1978, 30, 133.
0 1989 American Chemical Society
550
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
tronically excited diatomics NR. In this paper we describe our investigation of a system which is chemically similar to reaction 1 but also is sensitive to the generation of N3. The experiments involved pulsed photolysis of mixtures of HN, and 0, with the 249-nm output from a KrF laser. Photolysis of O3 at this wavelength dissociates the molecule5 to O(lD) 02(a1A,). Since O(lD) is isoelectronic to NH(alA), our hypothesis was that a reaction analogous to process 1 above would occur in the presence of HN,:
+
O('D)
+ HN3
+
OH
+ N,
1500
-0 c
-
(2)
The corresponding reaction of O(,P) with HN, is known6 to cm3 s-l). Both Clark and Clyne' and be quite slow ( k < Piper and co-workerss have shown that the reaction of O(3P) atoms with N 3 produces excited NO(A2Z+),with subsequent emission in the A2Z+ X2nsystem ( y bands). The rate constant of this reactions is approximately 1 X IO-" cm3 s-l. Hence, production of electronically excited N O is expected subsequent to the generation of N3 radicals in reaction 2, by the reaction of these radicals with either O(ID) or any O(3P) present. The issues addressed by the present experiments were thus the occurrence of reaction 2 and its rate constant, and the production of excited N O by the subsequent O(lD) N 3 reaction. Since O(lD) N 3 is substantially more exothermic than O(,P) + N3,and is subject to different angular momentum correlations, the rate constant of this reaction and the electronic states of the product N O are also of interest.
-
+
Ongstad et al.
+
210
215
220
-
I
I
225
230
235
240
Wavelength (nm)
Figure 1. A portion of the NO A X emission spectrum resulting from 249-nm photolysis of O3 (1.2 X l O I 5 cm-)) and HN, (3.3 X l O I 5 cm-').
0
400
-
0,
5 a!
._ t
u
[li
Experimental Section The apparatus used in the pulsed photolysis experiments consisted of four components, which were an excimer laser photolysis source, the reaction chamber, an optical multichannel analyzer detection system, and a detection system for collection of the time histories of individual emitters. The photolysis source was a Lambda Physik EMG 202 excimer laser operated on the KrF transition at 249 nm. The nominal output was 0.7 J in a 3.5 cm X 1.O cm beam, with a full width at half-maximum pulse duration of 30 ns. The laser illuminated the photolysis chamber without focusing. The reaction chamber was constructed from a 10-cm cubic block of aluminum. Four small 2.5-cm windows were used for in situ species density measurements as well as observation of spontaneous emission from the electronically excited species produced in the reaction medium. A 5.0 cm diameter window was used for admission of the laser beam to the cell. A 60 L m-l mechanical pump was used to establish flow through the chamber. Pressures in the chamber were monitored with a capacitance manometer. Emission from excited species generated in the reaction medium was detected and spectrally resolved with an optical multichannel analyzer (PAR) consisting of a 0.25-m polychromator with a 1024-element silicon detector array. The dispersion of the polychromator was 0.55 nm per channel. A lens system cf/25) was used to focus light emitted in the reaction medium onto the polychromator slits. Time histories of particular emitters were collected with a 0.3-m McPherson monochromator coupled to a cooled RCA C31034 photomultiplier tube. Signals detected by the PMT were amplified and stored with a transient digitizer (Biomation) which was coupled to a Vax 730 computer for final data reduction. The time constant of this system was approximately 100 ns. Hydrazoic acid was generated by the reaction of stearic acid with sodium azide at 115 O C . The gas was collected in a 5-L Pyrex (3) Pritt, A. T., Jr.; Patel, D.; Coombe, R. D. Inr. J . Chem. Kinet. 1984, 16, 971. (4) David, S.J.; Coombe, R. D. J . Phys. Chem. 1985, 89, 5206. ( 5 ) Brock, J. C.; Watson, R . T . Chem. Phys. Lett. 1980, 71, 371. (6) MacDonald, M . A.; Coombe, R. D., unpublished results. (7) Clark, T. C.; Clyne, M . A. A. Trans. Faraday SOC.1970, 66, 877. (8) Piper, L. G.; Krech, R. H.; Taylor, R. L. J . Chem. Phys. 1979, 71,
2099.
o f
I
290
300
-
310
1
I
320
330
1
340
350
-
Wavelength (nm)
Figure 2. JH A X and OH A X emissions resulting from 24 1-nm photolysis of 0,(1.2 X 10l5 ~ m - and ~ ) H N , (3.3 X 1015cm").
bulb and diluted with Ar (UHP) to produce a 20% mixture. Ozone was produced by passing a small flow of O2 through a commercial ozonizer (PCI Corp.), and collected on silica gel held at 197 K. After a sufficient quantity had been collected, the excess 0, was removed by pumping. The ozone was transferred to a 3-L Pyrex bulb by allowing the silica gel trap to warm. The purity of the ozone prepared in this manner was typically 65% as determined by absorbance measurements at 253.7 nm. The gas flows from the 0, and HN, storage bulbs were mixed with one another upstream of the photolysis chamber. Since the larger HN, flow tended to suppress the 0, flow, a 10-cm absorption cell was placed in the 0 3 / H N 3 line between the mixing zone and the photolysis chamber to monitor the 0, density in the flow. The 0, flow was adjusted to maintain a constant O3density as the HN, flow was changed. Measurements of O3densities in the presence of HN, under static (Le., nonflowing) conditions indicated that no spontaneous prereaction occurred between these species over a period of minutes.
Results Experiments were performed in which mixtures containing O3 at approximately 1 X lOI5 cm-, and H N 3 at 3 X 1015to 1 X 10l6 cm-, were irradiated at 249 nm. Since the absorption cross section of O3 at 249 nm is large (5 X and that for HN, is small (2 X cm2),10the O3was selectively dissociated even though the HN, was in excess. Intense N O A X emission was produced by the photolysis. Figure 1 shows the portion of the emission spectrum (recorded with the OMA as described above) at wavelengths shorter than the 249-nm laser pulse. Population of the u = 0, 1, and 2 vibrational levels of NO(A) is evident. N o evidence of the production of other excited states of N O was found. +
(9) Braun, W. et al., J. Phys. Chem. ReJ Data 1973, 2, 267. (IO) McDonald, J. R.; Rabalais, .I. W.; McGlynn, S. P. J . Chem. Phys. 1970, 52, 1332.
Photochemistry of O,/"3
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 551
Mixtures
a lo
r
3
3.2 2.0
I
2.6
2.4
i
:/I Slope
2.2 b
2 1 14.5
I
14.6
= 2.03 k .043
1
1
I
I
14.7
14.8
14.9
15
LOG[O('D) Number Density]
-
Figure 4. Dependence of the time integrated NO A X intensity on the initial O(ID) density. These data were taken with mixtures containing 0,at 1 . 1 X lOI5 cm-3 and HN3 at 2.6 X 10l5cm-'.
the following expressionI2for the density of ozone remaining after the laser pulse n(z) = no(l
-
Figure 3. Temporal profiles of NO A X emission: (a) O3 (1.6 X l O I 5 cm-') and HN3(1.0 X 1OI6 cm-'); (b) O3(1.0X lok5~ m - and ~ ) HN3 (4.2 X l O I 5 cm-'). The broken lines shown in the figures and the number densities indicated on the intensity axis are the results of calculations with the kinetic model discussed in the text.
In particular, NO B211
X211 bands above 300 nm were not present in the spectra. The spectra in this region indicated features attributable to N H A311 X32- and OH A2Z+ X211 transitions, however, as shown in Figure 2. Taking into account the spectral response of the OMA, the wavelength-integrated intensities of the N H and OH emissions were approximately 2 orders of magnitude smaller than the intensity of the N O A X bands noted above. N o emission was found in the near-UV or visible regions. In particular, no N2 first positive (B311, A,&+) bands were found. Time profiles of the N O A X emission were recorded for numerous different combinations of the O3 and H N 3 flow rates. In one series of experiments, the 0,density in the cell was held fixed near 1 X l O I 5 cm-, as the HN3 density (in substantial excess) was varied over a wide range. Figure 3 shows the time profile X intensity, recorded for the 0,O band near 226 of the N O A nm, for two different H N 3 densities. The profiles show an intense initial pulse whose width varies inversely with the HN, density followed by a much slower and considerably less intense decay. For a fixed density of HN3, variation of the O3density had little effect on the time profile but dramatically affected the intensity, which increased with increasing O3 density. Time profiles of the N H A --* X and OH A X emissions were also recorded but were not thoroughly investigated. The N H time profile was substantially different from that of the N O emission, exhibiting a more rapid rise (
