T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY Registered in U . S Patent Office
0 Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 4
FEBRUARY 23,1978
Synthesis of Chlorine Nitrate by Photolysis of Nitrosyl Chloride in Solid Oxygen at 10 K David E. Tevault and Richard R. Smardzewski" Chemistry Division, Naval Research Laboratory, Washington,D.C. 20375 (Received September 26, 1977) Publication costs assisted by the Naval Research Laboratory
The photolysis of ClNO in solid oxygen at 10 K produces chlorine nitrate in high yield. The matrix infrared frequencies and intensities of C10N02and C1015N02are in close agreement with gas phase values and the reported nitrogen-15 isotopic shifts. Analogous photolysis studies of oxygen-18-enriched ClNO samples demonstrate that the N=O bond of ClNO remains intact with the oxygen atom of ClNO eventually residing in a terminal position in C10N02. A proposed mechanism to account for these observations involves initial C1-N bond cleavage, followed by incipient ClOO formation via reaction of atomic chlorine with the Oz matrix, and subsequent 0-atom abstraction by a coisolated NO molecule to produce C10 and NO2 radicals which recombine in the same matrix cage to yield C10N02. A similar mechanism is proposed to explain ozone formation when molecular chlorine is photolyzed in solid oxygen.
Introduction The C10, and NO, stratospheric ozone depletion chain reaction mechanisms are now well known. However, only recently have cross combination reactions between the chain carrier species been considered.' In one such process, it has been postulated that chlorine nitrate (C10N02) is produced by the reaction of C10 and NOz in the presence of a third body. The significance of C10N02 in the stratosphere lies in the fact that it may serve as a chain terminator for both the C10, and NO, ozone depletion mechani~ms,'-~ thereby acting as a stratospheric "sink' for these chain carriers. Chlorine nitrate can be prepared by a number of r n e t h o d ~ ~and - ~ is a moderately stable gaseous species a t room temperature. It has been the subject of previous infrared8 and UV-visible2 spectroscopic investigations. In previous work from this laboratory, matrix identification of the halogen-oxygen-nitrogen compounds FON0,9-10 FON,'9''J2 and C10NO13have been reported. As a logical extension of that work, the mechanism of chlorine nitrate formation by oxidation of nitrosyl chloride was investigated via in situ photolysis of solid oxygen matrices (- 10 K) containing ClNO molecules. Reaction products and their nitrogen-15 and oxygen-18 isotopic counterparts were characterized by matrix infrared absorption spectroscopy. This article not subject to U S . Copyright.
Experimental Section ClNO samples were prepared by the gas phase reaction of chlorine (Matheson, 99%) and nitric oxide (Matheson, 99%) in an all-stainless-steel-Teflon vacuum line. Stoichiometric amounts of each reactant were condensed and degassed a t 7 1 K prior to mixing. Cl15N0 was prepared similarly, using isotopically enriched NO (Analytical Supplies Development Corp., 99% I5N). C1NI80 was synthesized by first reducing N1802(Miles Laboratories, Inc., nominally 92.8% l80)with elemental mercury and then reacting it with excess chlorine. Oxygen (Matheson, extra dry, 99.6%) was used as the matrix gas without further purification. The matrix isolation apparatus has been described in a previous publication.'0 02/C1N0 matrix samples were prepared by simultaneously depositing separate oxygen and ClNO samples onto a cold ( 10 K) CsI crystal window a t respective deposition rates of -2 m M / h and -1-4 pM/h over a 2-4-h period. Final 02/C1N0 ratios varied from 500/1 to 2000/1. In some experiments, ClNO disproportionation or incomplete reaction was evidenced by the appearance of NO and Nz03in the infrared spectra. The photolysis source was a 125-W high-pressure mercury arc with all quartz optics (Philips, HPK). The radiation from the arc was focused onto the matrix by a N
Published 1978 by the American Chemical Society
376
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
D. E. Tevault and R. R. Smardzewski
TABLE I: Infrared Frequencies (cm-I ) of Absorptions Observed Following Photolysis of Isotopic ClNO Samples Isolated in Solid Oxygen Matrices ClNO
Cl"NO C1NL80 (99% 15N) (60% " 0 )
1811.9, 1806.7
I
I'
t
a
1812.3, 1806.2
Assignment u,,
1780.4, 1774.7
' 1730.0, 1722.5
~ 1 1 5 ~ 0
1763.9, 1758.3 1730.8
vl,
1715.2b 1288.3
u I , C10N160180
~
1276.3 1263.4 1038.0 808.0
1038.0 807.9 803.3
II 2000
15C3
776.5
1 1000
WAVENUMBER
500
710.9 (702.6) 584.9
Figure 1. (Top) Infrared spectrum (200-2000 cm-') observed following simultaneous deposition of ClNO (-24 pM) and O2 (- 12 mM) at 10 K. (Bottom) Infrared spectrum observed after photolysis; 2 h unfiltered 14 h Pyrex-filtered high pressure mercury arc irradiation.
Results and Discussion When ClNO was deposited into an oxygen matrix, the infrared spectrum shown a t the top of Figure 1 was obtained. Very strong infrared absorptions were observed a t 1811.9, 1806.7, 584.8, and 324.0 cm-l. These bands were assigned to the three infrared active fundamentals of ClNO in agreement with earlier gas phasel"lg and argon matrix" values. The splitting of the high-frequency band was similar to that observed for several other matrix-isolated triatomics and has been discussed elsewhere.'l A strong band was also observed at 1549.2 cm-l with weaker features a t 1592, 1617, 1868, and 1845 cm-l. The first three have been previously assigned to the infrared transitions of solid CY oxygen2* The 1868-cm-l band is assigned to NO impurity while the 1845-cm-l band is due to N203formed by the reaction of NO with the matrix. Photolysis Behavior. Upon commencement of mercury arc irradiation of ClNO samples in solid oxygen, the infrared absorptions of ClNO were observed to decrease in intensity. The 1845-cm-' band disappeared, and several new infrared absorptions were observed as shown in the lower trace of Figure 1. Photolysis (16 h total) was carried out in two steps. First, the use of all quartz optics for 2 h generated new bands at 1730,1722,1288,1038,808,and 777 cm-l, while the ClNO bands decreased to -80% of their original intensity. Additional photolysis for 14 h
585 571.2
+
577.9 560.0 435.0 324.1
0
~
5
~
0
,
ClONO, ~1015~0, C10N160180
u3,
I6O3
uj,
ClONO,
u 3 , C10N160180
ClONO, ~ 1 0 ~ 5 ~ 0 , v 4 , C10N160180 v 8 , ClONO, u4, u4,
768.5
(crn-1)
2 in. diameter, 3 in. focal length quartz lens through a 5-cm water filter fitted with quartz windows. A series of Corning glass filters (no. 3387, 3389, and 3850) was used to determine the wavelength of light responsible for the photolytic behavior. Expanded scale infrared spectra were recorded before and after photolysis and frequency and intensity changes of all bands were monitored. Photolysis was performed for periods of 1-16 h with each of the various cutoff filters. Infrared spectra were recorded on a Beckman IR-12 infrared spectrophotometer. High resolution spectra were recorded at 20 cm-l/in. of chart expansion and 8 cm-l/min scan speed. The spectrometer was calibrated using the gas phase rotation-vibration spectra of standard molecule^.^^
777.1 7 69.8
200
u,, uz, v,,
1
v3, ~ 1 0 1 5 ~ 0 ,
(808.0)c
I
ClN"0
u I , ClONO,
1685.7 1288.3
ClNO
vz, 0 3 v , , ClNO v,, ~ 1 1 5 ~ u , , C1N"O u s , ClONO, u 6 , ClONO, v 3 , ClNO
0
324.0 v3, ~ 1 1 5 ~ 0 Unobserved. Broad. This frequency is indistinguishable from u 3 (Cl-0 stretch) of ClONO,. a
through a Pyrex filter (Acutoff -300 nm) resulted in further destruction of the ClNO features to -20% of their original intensity and increased growth of the new absorptions. In addition, weak new bands were observed at 711,703,560, and 435 cm-l (Table I). The infrared absorptions which increase in intensity with photolysis are in close agreement with the gas-phase spectra of ozone (1038 and 703 cm-l) which is a known product when C12is photolyzed in an oxygen matrix23and C10N02,8the major difference being the matrix splitting of u1 at 1730 and 1722 cm-l. In all probability, this splitting is caused by Fermi resonance interaction with a combination band since it was not observed for isotopic ClONO2 species. In several cases, following extended photolysis, weak product absorption were observed at 1266.1 and 788.0 cm-l (Figure 2). These bands correspond closely to two of the strongest infrared absorptions of nitryl chloride, C1N02, isolated in solid argon13 and were accordingly assigned. In order to determine precisely the range of wavelengths responsible for the photolysis reaction, an experiment was performed using colored glass filters. In this study, it was observed that a small amount of C10N02 was generated after long-term ( 15 h) irradiation a t wavelengths 2450 nm (Corning filter no. 3387). On the other hand, ozone did not appear to form a t these wavelengths. Further photolysis for 1 h with only a Pyrex filter effected a considerable growth of the CIONOz absorptions as well as the appearance of the O3 features thereby indicating that near-ultraviolet photolysis greatly enhances the product yield although the reaction does proceed in the presence of visible light (A 1450 nm). ClO + NO2. In one experiment, C10 radicals, generated by the microwave discharge of an argon-chlorine-oxygen (1OO:l:l) mixturez4were codeposited with a separate argon-NO, sample. The infrared spectrum of the products N
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 377
Synthesis of Chlorine Nitrate
I I
60
,
I
I
I
02,h'5N0
/
,
,
I
/
'1300 1260 8iO WAVENUMBER (cm") 1680
,
,
1
750
Figure 2. High resolution infrared spectra observed after photo1 sis of CINO: (top) CIi5NO (99% "N); (middle) and CIN"0 (60% "0); (bottom) in solid oxygen at 10 K.
was similar to that produced by photolysis of ClNO in solid oxygen, although weaker and complicated by other products. In an earlier investigation, a similar radicalradical reaction was used to produce ClONO from C10 and NO in solid argon.13 Isotopic Substitution. As expected, the ClNO bands shifted to lower energies when a 99% nitrogen-15-enriched sample was codeposited with a large excess of 02. The frequencies of the CP5N0 absorptions and the C1015N02 features (produced by photolysis) are listed in Table I. High resolution spectra in the regions of interest are shown in Figure 2. The N-15 shifts of the ClONO2 bands at 1730, 1722,1288,808, and 777 cm-l were measured a t 40.5 (from the average of the 1730- and 1722-cm-l bands of C1014N02) 12.0, 4.6 and 6.7 cm-', which are in close agreement with the reported gas-phase N-15 shifts of 41.4, 11.7, 5.6, and 7.3 cm-la8 Photolysis of an oxygen matrix containing 60% oxygen-18-enriched ClNO produced several infrared absorptions which were observed previously in the naturally abundant isotopic experiment as well as several new features a t 1715.2, 1263.4, and 768.5 cm-l as shown in the lower trace of Figure 2. A comparison of the 1715-cm-' absorption with its oxygen-16 counterpart was difficult since the latter was present as a doublet (1730,1722 cm-l) in the naturally abundant isotopic species, although its 0-18 shift of -11 cm-l (from the average of the 1730,1722 cm-' doublet) is close to that expected for an antisymmetric NO2 stretching vibration upon substitution of one oxygen-18. On the other hand, a comparison of the oxygen isotopic constituents of the symmetric NOz stretch of ClON02, which were present as single components in each isotopic experiment, revealed that the 1263-cm-l absorption was slightly more intense than its 1 2 8 8 - ~ m - ~ oxygen-16 counterpart. The same result was observed for the oxygen isotopic components of the 776-cm-l band which corresponds to the terminal NO2 scissoring vibration. No oxygen-18 isotopic splitting was observed in the C1-0 stretch a t 808 cm-l.
These results are consistent with the conclusion that all of the oxygen atoms from the original ClNO end up in the terminal position of C10N02 rather than the bridging position between the chlorine and nitrogen. If, for example, the ClNO oxygen atom randomized (as might be expected if symmetrical NO3 were the intermediate), the 1263-cm-' absorption would be weaker than the 1288-cm-l band in a 60% l8O-enriched experiment at worst, and, a t best, three absorptions would appear due to (a) the molecule with all 0-16 (do%), (b) the molecule with a bridging 0-18 (20%), and (c) the molecule with a terminal 0-18 (40%). Most likely, the original two would have overlapped, since the vibration in question is the symmetrical NOz stretch involving both terminal 0 atoms (Le., the bridging 0 atom is expected to experience very little displacement in this normal mode). In such a case, the higher frequency band of the isotopic doublet would have been the more intense (60%) absorber. Mechanism. The fact that the original oxygen of ClNO occupies a terminal position in C1ONOZ,coupled with the absence of any oxygen-oxygen bonds in the product, places very narrow restrictions on a possible mechanism for the photolytic reaction. In the overall scheme, it becomes obvious that an oxygen molecule must undergo bond rupture with one of the atoms becoming the "bridging" oxygen of chlorine nitrate while the other occupies a terminal position. The first step in the proposed photolytic mechanism is rupture of the C1-N bond of ClNO to produce the C1 and NO species in the same matrix cage. The analogous gas-phase photodecomposition reaction is known to proceed with light of wavelength 1640 nm (reaction l).25 ClNO t h u
--t
{Cl t NO}cage
(1)
Atomic chlorine may then proceed to react with a nearby oxygen molecule to produce the ClOO species (reaction 2). CI t 0 , (matrix) t NO + {ClOO + NO }cage (2) Other investigatorsz3have demonstrated that the ClOO radical is produced when chlorine atoms are generated in an oxygen matrix. The next proposed step in the overall scheme involves abstraction of an oxygen atom from ClOO by the coisolated NO molecule in the same cage. Reaction {ClOO f NO Icage
-
+
{ClO -t NO,
Icage
(3)
3 is exothermic by 10-15 kcal/mol. Finally, the C10 and NOz radical fragments can combine in the same matrix cage to form chlorine nitrate (reaction 4). The possible {CIO t NO,}cage -+ CIONO,
(4)
existence of a metastable species such as ClOONO cannot be ruled out by our observations. However, if this species does exist, it is apparently unstable toward C10N02 formation in the presence of visible radiation (A 2450 nm). The possibility of secondary photocleavage of C10N02 was considered, although, as pointed out by Birks et al.,26 there is considerable uncertainty regarding the exact mechanism of photolysis. The observation of several infrared absorptions of nitryl chloride, C1N02, in the absence of any NO2 (a strong infrared absorber near 1620 cm-l) would seem to lend credence to one mechanism of photolysis where ClONO and 0 atoms are formed as products of the secondary photolysis of C1ONOZ. The ClONO species may then photoisomerize to the more stable ClNO2 form while some of the oxygen atoms are scavenged by the oxygen matrix to produce ozone. Supportive evidence for such a process was provided by a recent argon matrix study from this laboratory13 when
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378
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
M. Ogasawara and L. Kevan
it was demonstrated that ClONO does isomerize to CINOz in the presence of near-ultraviolet light. Ozone Formation, When molecular chlorine is photolyzed in solid oxygen, ozone has been identified as one of the reaction products.23 A mechanism, similar to that postulated for C10N02 formation from ClNO, may be invoked for this process (reactions 5 and 6). Unlike re-
-
a,
h
hv
< 500 n m
hv
a00 t o,+
0 matrix
2c1 -L---+ ClOO
c10 t 0,
(6)
action 3, however, which is mildly exothermic, the 0-atom abstraction reaction (6), which may in fact occur in two steps, is -40 kcal/mol endothermic and would therefore require the presence of visible or ultraviolet radiation. This latter mechanism is basically equivalent to that proposed earlier,23the chief difference being the absence of higher oxides of chlorine (i.e., C103 or C104) which shduld be observable in the infrared. The absence of any detectable absorption of the C10 radical near -850 cm-' is not surprising considering its extremely low extinction coefficient in the infrared.24,27
Conclusions The infrared spectrum of chlorine nitrate, C10N02,has been observed following visible or ultraviolet irradiation of nitrosyl chloride, ClNO, in solid oxygen matrices a t 10 K. The CIONOz features increase in intensity with extended photolysis while the ClNO bands diminish markedly. Nitrogen-15 isotopic data are in agreement with the reported gas-phase frequency shifts. Photolysis of oxygen-18-enriched ClNO samples in unenriched oxygen matrices revealed that the oxygen in the ClNO reactant occupies a terminal position in the C10N02 photolysis product. This result, coupled with thermodynamic considerations, is used in support of a proposed mechanism for the observed reaction.
Acknowledgment. We thank Dr. W. B. Fox for many helpful discussions and Professor F. S. Rowland for an informative personal communication. One of us (D.E.T.) also gratefully acknowledges the NRC for support of this research through an NRC-NRL Resident Research Associateship. References and Notes F. S. Rowland, J. E. Spencer, and M. J. Molina, J. fhys. Chem., 80, 2713 (1976). F. S. Rowland, J. E. SDencer, and M. J. Molina. J . fhvs. Chem., 80, 2711 (1976). F. S. Rowland and M. J. Molina, Rev. Geophys. Space fhys., 13, 1 (1975). M. T. Leu, C. L. Lin, and W. B. DeMore, J . fhys. Chem., 81, 190 (1977). H. Martin, Angew. Chem., 67, 524 (1955). M. Schmeisser, Inorg. Syn., 9, 127 (1967). C. J. Schack, Inorg. Chem., 6, 1938 (1967). R. H. Miller, D. L. Bernitt, and I. C. Hisatsune, Spectrochim. Acta, Part A , 23, 223 (1967). R. R. Smardzewski and W. B. Fox, J. Chem. SOC.,Chem. Commun. 241 (1974). R. R. Smardzewski and W. B. Fox, J. Chem. fhys., 60, 2980 (1974). R. R. Smardzewski and W. B. Fox, J . Am. Chem. SOC.,96, 304 (1974). R. R. Smardzewski and W. B. Fox, J. Chem. fhys., 60, 2104 (1974). D. E. Tevault and R. R. Smardzewski, J . Chem. fhys., 67, 3777 (1977). E. K. Plyler, A. Danti, L. P. Blaine, and E. D. Tidwell, J . Res. Natl. Bur. Stand., 64, 29 (1960). W. G. Burns and H. J. Bernstein, J. Chem. fhys., 18, 1669 (1950). A. G. Putford and A. Walsh, Trans. Faraday Soc., 47, 347 (1951). W. H.Eberhardt and T. G. Burks, J . Chem. fhys., 20, 529 (1952). L. Landau and W. H. Fletcher, J. Mol. Spectrosc., 4, 276 (1960). L. H. Jones, R. R. Ryan, and L. B. Asprey, J . Chem. fhys., 49, 581 (1968). M. E. Jacox and D. E. Milligan, J . Mol. Spectrosc., 48, 536 (1973). D. W. Green, S. D. Gablenick, and G. T. Reedy, J . Chem. Phys., 64, 1697 (1976), and references therein. B. R. Cairns and G. C. Pimentel, J . Chem. fhys., 43, 3432 (1965). A. Arkell and I. Schwager, J . Am. Chem. SOC.,89, 5999 (1967). F. K. Chi and L. Andrews, J . fhys. Chem., 77, 3062 (1973). G. B. Kistiakowski, J . Am. Chem. SOC.,52, 102 (1930). J. W. Birks, B. Shoemaker, T. J. Leck, R. A. Borders, and C. J. Hart, J . Chem. fhys., 66, 4591 (1977). L. Andrews and J. I. Raymond, J . Chem. fhys., 55, 3087 (1971).
Laser Photoionization Study of Time Dependent Spectral Shifts of Localized Electrons in Ethanol Glass. Temperature Effects Masaaki Ogasawarat and Larry Kevan" Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received September 27, 1977)
Electron solvation kinetics in ethanol glass have been studied from 83 to 123 K by ruby laser photoionization of 6-naphtholate. A t 123 K first-order kinetics are observed but at lower temperatures the kinetics deviate from first order and can be represented as composite first order or linear in electron concentration vs. log time. It is concluded that a hindered molecular reorientation mechanism for electron solvation is most probable. The temperature dependence of this mechanism is expected and found to be weaker than that of the bulk viscosity or dielectric relaxation.
Introduction Electrons produced in condensed phase alcohols by radiolysis' or by photoionization2 of a suitable solute are rapidly localized. They are Characterized by optical spectra with maxima in the near infrared3 and by a narrow ( - 5 G) electron spin resonance line.4 These electrons can be t On leave from the Faculty of Engineering, Hokkaido University, Sapporo, Japan.
0022-3654/78/2082-0378$0 1.OO/O
considered as presolvated electrons which have not yet reached an equilibrium configuration with the alcohol solvent molecules. When generated a t 4 K some of these presolvated electrons can be stably t r a ~ p e dbut , ~ a t 77 K and above they undergo a time dependent change to achieve an equilibrium solvent configuration and become so1vated.l These solvated electrons in alcohols are characterized by an optical spectrum in the visible and a wider (-12 G) electron spin resonance line. 0 1978 American
Chemical Society
