3062
Frank K. Chi and Lester Andrews
Argon Matrix Raman Spectra of C120 and Its Photolysis Products CIO and CICIO. Infrared Matrix Spectra of CIO and (C10)2 Frank K. Chi and Lester Andrews" Chemistry Department, University of Virginia, ChariottesvMe, Virginia 22903 (Received July 5, 7973)
The Raman spectrum of argon matrix-isolated ClzO exhibited bands at 638, 298, and 678 cm-l, in agreement with the infrared spectrum. Laser photolysis (4880 A) of the sample was evidenced by intense Raman bands a t 962, 373, and 241 c m - l due to the C1-C10 photoisomerism product and a weak Raman band at 850 cm-I due to the C10 photolysis product; infrared studies confirmed these assignments. The mercury arc photolysis of C120, 0 3 matrix samples produced an intense doublet a t 995 and 986 cm-I which is assigned to Cl=O-Cl=O, another form of C10 dimer.
Introduction The Raman spectrum of argon matrix-isolated OF2 has been recently reported by Andrews using argon ion laser excitati0n.l During laser illumination for Raman scattering, the Raman fundamental of the OF radical appeared in the spectrum; the OF signal increased as a function of laser irradiation time on the sample. Accordingly, the Raman spectra of Cl20 and the C10 free radical were sought using the same techniques. Infrared spectra of matrix-isolated C120 and its mercury arc photolysis products ClClO and ( C l 0 ) ~have been reported by Rochkind and Pimentel (hereafter called RP).2.3 Gardiner4 has recently reported Raman spectra of liquid and solid ClzO and confirmed the RP vibrational assignments. In a study of alkali metal atom matrix reactions with ClzO, Andrews and Raymond5 observed two new infrared bands a t 995 and 850 cm-I which showed proper oxygen18 shifts for the C10 free radical. Following the rationale that the C10 fundamental should be higher than the C10 frequency in C1-ClO (which occurs at 962 cm-l) these workers assigned 995 cm-I to C10 and 850 cm-1 to a perturbed C10 species, although the reverse possibility could not be excluded. The electronic spectrum of the C10 free radical has been investigated e~tensively.6.~ In a very recent study of the vacuum ultraviolet absorption spectrum of C10 by Basco and Morse,s six electronic band systems were assigned to C10; hot bands in three of these systems provided the ground state vibrational frequency of 845 & 4 cm -I. The major aim of this study was to observe the Raman fundamental of C10 and to identify the C10 species absorbing at 995 and 850 cm-1. Infrared spectra of the mercury arc photolysis products of C120 and 0 3 mixtures in solid argon and laser-Raman studies of argon matrix-isolated ClzO are described in a subsequent article. Experimental Section The cryogenic apparatus, vacuum vessel, experimental technique, and instrumentation for the laser-Raman matrix isolation experiments have been described earlier.l,g Dichlorine monoxide and C12lS0 were synthesized using the apparatus and methods described elsewhere.5 The ClzO samples were diluted in argon (Ar/C120 = 50, 100) and deposited from a stainless steel vacuum line for 6 hr The Journal of Physical Chemistry, Vo/. 77, No. 26, 1973
a t the rate of 2 mmol/hr onto a tilted copper block maintained a t 16°K. The 4880- and 5145-A argon ion laser lines were used as sources for Raman excitation. Typical spectra were recorded a t 20 cm-l/min using a 3-sec rise time and the 0.3 x l o u 9 A range. The spectra were calibrated against argon ion emission lines superimposed on the actual scan; frequency accuracy was 1 2 cm -1. The refrigeration system, vacuum vessel, and infrared matrix techniques have been described in earlier papers.10 Ozone was synthesized by tesla coil discharge of 0 2 gas in a Pyrex finger immersed in liquid nitrogen.l2 0 3 and C120 samples were deposited from separate manifolds in one experiment and these reagents were mixed in and deposited from the same stainless steel can in several experiments. Infrared spectra were recorded during and after deposition and also after each photolysis on a Beckman 1R-12 filter-grating infrared spectrophotometer in the 200-2000cm-1 spectral region. Deposition time and rate for infrared experiments were approximately 20 hr and 1.5 mmol/hr, respectively. High-resolution spectra were taken using 8 cm-l/min scan speed and 20 cm-l/in scale expansion; wave number accuracy was 1 0 . 5 cm-I with better than 1-cm-lresolution. Sample irradiations were made using a high-pressure mercury arc (General Electric BH6). A saturated aqueous NiS04 Cos04 solution filter (transmits 90% at 22003600 A and 10% at 5600-6300 A), a water filter, and a water filter plus a Pyrex glass plate were used separately for different experiments.
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Results and Discussion Raman Spectra. Dichlorine Monoxide, Cl20. Eight new bands were observed in the Raman spectrum of argon matrix-isolated dichlorine monoxide; these are illustrated in Figure l a . Three of these Raman bands, designated a a t 638, 298, and 678 cm-I are respectively due to u l , v 2 , and v3 of ClzO. The stretching modes were observed as split bands at 639.8 and 637.0 cm-I and 677.2 and 675.2 cm-I due to chlorine isotopes in the infrared matrix spectrum. The Raman argon matrix bands are in good agreement with the Raman bands of liquid Cl20: 634, 293, and 673 cm - 1 . 4 The remaining five Raman bands designated by b, c, and d are due to laser photolysis products of C120 formed during the recording of Raman spectra. The intense
3063
Argon Matrix Rarnan Spectra of CIO and ClClO Raman bands a t 962 and 373 cm-I labeled b agree well with the RP3 infrared assignments of 962 and 374 cm-l to ClClO following mercury arc photolysis of Cl20 in solid argon. The very intense b band at 241 cm-1 is associated with the 962- and 373-cm -1 bands by their parallel growth and disappearance behavior during laser photolysis. The new 241-cm-l Raman band, which is assigned to the C1C1-0 bending mode, v3, completes the vibrational spectrum for ClC10. The weak Raman band labeled c at 850 cm-I is most interesting since this frequency is in good agreement with the 850-cm-l infrared band from alkali metal-Cl20 matrix reactions and the 845 f 4 cm-l frequency for C10 deduced from the electronic spectrum.8 In the best of 15 runs, the 850-cm-l band was observed with enough intensity to partially resolve a splitting at 845 cm-I which may be contributed by natural C1-37; this band is shown in the inset block of Figure 1. In the oxygen-18 experiment discussed below, the 850-cm-l band shifted to 818 cm-l. The isotopic frequencies are appropriate for a diatomic harmonic oscillator. Noteworthy is the absence of Raman signal a t 995 cm-1, the frequency of the earlier infrared assignment5 to C10. The Raman observation of 850 cm-I and not 995 cm -1 suggests the reassignment of 850 cm to ClO and 995 cm -1 to a perturbed C10 species. The shoulder labeled d a t 944 cm-1 in the Raman spectrum has an infrared counterpart at 947 cm-I in argon and a nitrogen matrix absorption at 945 cm-l. The frequency has been assigned to ( C l 0 ) ~by Alcock and Pimente1.12 All of the eight labeled bands in Figure l a were observed in the same spectrum scanning a fresh spot of sample with minimum exposure to the laser. However, the intensity behavior of the eight bands under laser irradiation was quite different. During the course of repeating scans of the 200-1000-cm-1 region bands b and d increased in intensity as the intensity of a decreased; then the intensity of d began to decrease while b was still growing. This indicates that (C10)2 is not a stable species, but is subject to further photolysis, perhaps to Cl2 and 0 2 . Due to the lack of appreciable intensity for the 850-cm-l band, the photolytic behavior of C10 relative to ClClO is not quite clear. However, the 850-cm-1 C10 band is one of the photolysis products of ClzO. In the 4 0 0 - 6 0 0 - ~ m -spectral ~ region omitted from Figure l a , a single strong band at 534 cm-1 due to Cl2 was observed; the Cl2 Raman band increased in intensity as a function of laser illumination time. This indicates that Cl2 is also a photolysis product of Cl20. As expected the other photolysis product, 0 2 , was observed at 1552 cm-l. After 1 hr of exposure to 300 mW of 4880-A radiation all signal intensities were reduced; the reddish-brown sample appeared to be bleached by the laser beam. 5145-A excitation produced comparable Raman signals to 4880-A excitation although 4880 A produced a slightly better yield of photolysis. products. The 5682-A line of a krypton plasma laser produced weak signals for v 1 and v 3 of Cl20, a weak band a t 940 cm-1, and no detectable C1-C10 or C10 photolysis products. Oxygen-18 Dichlorine Monoxide, C 1 ~ ~ The ~ 0 . Raman spectrum of an equimolar C12160-C12180 argon matrix sample was examined in three experiments; Figure 1b presents the best spectrum obtained. New oxygen-18 species are labeled with a prime corresponding to the oxygen-18 counter-part of a band observed previously: aC12l60 and a’-C12180. The VI and v3 Raman bands of C 1 P O labeled a’ were observed at 612 and 647 cm-1 in
’u’ 5 l
1
1000
1
900
1
1 I 1 I 1 800 700 600% FREQUENCY S H I F T
I 400
I
1 ‘ 1 300
I 200
m”) Figure 1. Raman spectra of argon matrix-isolated C120 and its laser hotolysis products at 15°K. Approximately 300 m W of 4880-1 excitation at the sample: (spectrum a) Ar/CI20 = 100, inset box contains best Raman spectrum of 850-cm-’ band: (spectrum b) Ar/C120 = 100, CI2 1sO/C12180 3/2. good agreement with chlorine isotopic infrared bands5 at 615.8, 613.0 cm-I and 649.3, 647.5 cm-l. The bending mode band near 298 cm-1 was not resolved into oxygen isotopic components. The b’ band at 927 cm-I corresponds to the oxygen-18 counterpart of the 962-cm-l ClClO v1 band. The intense bands at 373 and 238 cm-1 b’ contain both C1C1l60 and C1C1l80 isotopic labeled b bands for v2 and ~3 of C1-C10. A weak band a t 818 cm-I labeled c’ is the oxygen-18 counterpart of the 850-cm-l Raman C10 band. The weak d’ shoulder at 908 cm-I corresponds to (C11’30)~observed in the infrared a t 909 cm-1 in solid N23 and a t 910 cm in solid argon. Infrared Spectra. In order to correctly identify the 995cm band assigned earlier5 (and now we believe incorrectly) to C10, it was desired to produce as large a yield of the 995-cm-1 band as possible. Arkell and Schwager have shown that i n situ photolysis of C120-O3 argon matrix samples produced an extraordinarily good yield of the 995cm- band.13 Accordingly the photolysis of C120-O3 matrix samples was investigated. First, the photolysis of Cl20 in argon was studied to seek the infrared counterpart of the 241-cm-l Raman band assigned above to v3 of ClClO. Second, the photolysis of ozone-dichlorine monoxide mixtures will be discussed. Lastly, two new experimental techniques for producing C10 species, microwave discharge and proton beam irradiation, will be briefly reported. Dichlorine Monoxide Photolysis. Figure 2 contrasts the spectral regions of interest; trace a illustrates the 200260-, 340-420-, and 900-1000-cm-1 spectral regions for an Ar/C120 = 100 sample deposited for 20 hr; trace b shows the effect of mercury arc photolysis through a saturated NiS04-CoS04 filter for 10 min. The intense doublet a t 961.8 and 953.7 cm-1 is in excellent agreement with the RP3 result for v1 of ClC10, the C1-0 stretch. Note the well-resolved chlorine isotopic splitting. Arkell and Schwagerl3 have also observed this intense doublet a t 961.0 and 952.7 cm-I after mercury arc photolysis of Cl2,
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The Journal of Physical Chemistry, Vol 77, No. 26, 1973
3064
Frank K. Chi and Lester Andrews
I
80-
' ac
900
940
900"
400
FREQUENCY
360
(om")
Infrared spectra of in situ mercury arc photolysis of CI2O and C120-03 mixtures in solid argon at 15°K: (a) Ar/Ci20 = 100 sample before photolysis; (b) after photolysis for 10 min using ultraviolet radiation (saturated CoS04-NiS04 filter); (c) Ar/CioO/O1 = lOO/l/l samDle after 33-min ultraviolet and 3-min- uitraviolet and visible (water bath filter) photolysis; (d) Ar/Ci?O C11180/'603 = 120/1/1, C12160/C12180 2/3 , sampik after 15-min'ultraviolet phoiolysis.
Figure 2.
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O3 argon matrix samples. The partially resolved quartet a t 375.1, 371.2, 369.3, and 365.7 cm-I in Figure 2b agrees with the R P finding;3 the chlorine isotopic structure is appropriate for a C1-C1 stretch of two nonequivalent chlorine atoms. The new band a t 239.4 cm-I parallels the other two bands in photolysis growth behavior; this feature was not observed by R P due to the 250-cm low-frequency limit of their spectrophotometer. The weak 239.4-cm infrared band is associated with the intense 241-cm-l Raman band. Dichlorine Monoxide-Ozone Photolysis. An Ar/C120/03 = 100/1/1 sample was subjected to a detailed mercury arc photolysis study. First, a saturated NiS04-CoS04 filter was used (transmits 2200 8, < X < 3600 A) for ultraviolet photolysis without appreciable visible light; four spectra were recorded a t regular intervals during a 33-min photolysis period. The 995-, 986-cm-l doublet and the 962-, 954-cm -1 doublet uniformly appeared during photolysis reaching intensities of 0.16 OD for the 995-cm-l band and 0.09 OD for the 962-cm-l band after 33 min of uv photolysis. Clearly, uv photolysis favored the production of the 995-, 986-cm-l doublet relative to the 962-, 954-cm-l The Journal of Physical Chemistry, Vol. 77, No. 26, 1973
doublet. Second, a water filter only was used (transmits 2200 A < h < 10,000 A) for an additional 3 min of photolysis; this spectrum is shown in Figure 2c. The 995-cm-1 band increased to 0.38 OD while the 962-cm-l feature markedly grew to 1.3 OD; no band was observed a t 850 cm-l. Note the parallel intensity behavior of the 962-, 375-, and 240-cm-l C1-ClO bands. An additional 20 min of full uv and visible photolysis further increased the 962cm-I band but the 995-cm-I band decreased to 0.14 OD. The 986-cm-l band maintained a constant one-third intensity relative to the 995-cm band during photolysis studies; the 986-cm-l band is clearly the C1-37 counterpart of the 995-cm-l C1-35 band. This was verified by a harmonic diatomic calculation with the two frequencies measured a t 994.3 and 985.6 cm on expanded scale. The 994.3-, 985.6-cm photolysis doublet agrees well with the 994.8-, 985.8-cm-l doublet observed by Andrews and Raymond5 following lithium atom-Cl20 matrix reactions. These workers also observed the oxygen-18 isotopic species a t 957.3 cm-l. Clearly these isotopic shifts are appropriate for a harmonic isolated C1-0 vibration. The observation of the greatest yield of the 995-, 986-cm-' doublet with 0 3 and C l ~ 0photolysis and no detectable quantity with Cl20 photolysis indicates that these bands are a photolytic reaction product of C120 and 0 3 . Furthermore, these bands were favored by ultraviolet photolysis which dissociates ozone to 0 2 and an oxygen atom which is free to diffuse through the argon matrix and react with a suitable molecule. This evidence strongly points to the identification of the 9 9 5 , 986-cm-l doublet as Cl=O-Cl=O, another structural isomeric ( C l 0 ) ~species. In the lithium -Cl20 matrix reaction work,5 this species was produced by dimerization of C10 radicals, evidenced by the 850cm-I absorption, now correctly assigned as isolated C10 monomer. The ultraviolet photolysis of a C121eO/C12180 = 2/3 sample with 'GO3 using the CoS04-NiS04 filter is shown in Figure 2d. Oxygen-18 counterparts for the C1-C10 bands are immediately obvious. The intense doublet a t 925.5 and 916.8 cm-I is the C1-0 stretch for the Cl35Cl18O and Cl37Cl18O isotopic species. Again, the isotopic structure of the 374-cm-1 band was not completely resolved; components a t 374.1, 370.1, 368.8, and 364.5 cm-I were observed under high resolution. The oxygen-18 shift for the Cl-C1 mode of C1-C10 is on the order of 1 cm-l. In the low-frequency region, the oxygen-18 counterpart of the 239.4-cm-l band was resolved a t 231.8 cm-l. This doublet confirms the bending mode assignment of
c1-c10. Microwave Discharge of Ar-(212-02 Mixtures. It was desired to produce the 995- and 850-cm infrared bands with as many physical techniques as possible. Three experiments were conducted passing argon, chlorine, oxygen mixtures through a microwave discharge and condensing the effusing gases a t 15°K; the apparatus has been described by Smith and Andrews.14 Best results were obtained using Ar/C12/02 = 100/2/1. In one particularly productive experiment, discharged sample was deposited for 28 hr and a number of familiar new bands were observed. The most intense feature was a doublet 1039 and 1033 cm-1 due to ozone; C102 bands were observed a t 1100 and 952 cm-1. ClClO was evidenced by an intense doublet a t 962 and 954 cm-1 and a single feature a t 375 cm-I with low-frequency shoulders. Of most interest were sharp reproducible bands a t 849.5 (0.025 OD) and 994.6
3065
Argon Matrix Rainan Spectra of CIO and ClClO cm-1 (0.015 OD), These features are assigned here to the C10 radical and a C10 dimer, respectively. Proton B e a m Irradiation of Ar-Cl2O Mixtures. An apparatus has been developed in this laboratoryls to deposit a 2-keV proton beam into matrix samples. The gross effect is the production of bombarding electrons through the matrix from ionizing argon atoms as the proton kinetic energy is absorbed. Sample deposition of Ar/C120 = 100 with simultaneous proton beam irradiation was conducted for 24 hr. The ozone band a t 1039, 1033 cm-I and the C102 band a t 1100 cm-1 were observed. Interestingly ClClO was produced in large yields as evidenced by bands a t 962 and 954, 375, and 240 cm-l. Also observed was the 696-cm feature assigned by Noble and Pimentel16 to HC12, and a doublet a t 984, 975 cm-I which was assigned by RP3 to another (C10)2 dimer. Lastly, a good yield of the 994.8-cm-l feature (0.09 OD) and a typical yield of the 849.3-cm-l band (0.02 OD) were recorded.16b Sample warming t o approximately 38°K and recooling to 15°K essentially eliminated the weak 850-cm band and reduced the 995-cm-l band to 75% of its original intensity while the 984-, 975-cm-1 doublet increased to 180% of its original intensity. This diffusion experiment reduced the ClClO features by 30%. Normal Coordinate Analysis. With a complete set of Cl-C10 fundamental frequencies in hand, a normal coordinate analysis was done using the Wilson FG matrix method17 and the Schachtschneider18 programs GMAT and FADJ. Since no structural data are available for the C1-C10 intermediate species, preliminary force constant estimates and Badger’s rulelS were used to estimate the Cl-0 bond length of 1.587 A and the C1-C1 bond length of 2.263 A for C1-C10. Valence angles of 180, 120, and 90” were used; 120” gave the best correlation between calculated and observed frequencies; thus, we estimate the C1C10 bond angle is in the 120” vicinity, near the 110” bond angle for the isoelectronic C120 molecule.2 Twenty-four frequencies (fourteen of them were independent) for the isotopes 35C135C1160, 35C137C1160, 37C135C1160, 37C137C1160, 35C135C1180, 35C137C1180, 37C135C1180,and 37C137C1180were used as input data for the force constant calculations. Three calculations were done for the 120” geometry: first, the best diagonal force constants were determined holding the of€-diagonal constants a t zero; second, the diagonal and C1-C1 stretchbend interaction force constants were calculated; third, the diagonal and three off-diagonal potential constants were determined. All three of these calculations produced an excellent frequency fit (average error 0.4 cm-I). More importantly, the diagonal force constants changed by only *0.06 units when one or three off-diagonal potential constants were determined. The interaction constants were small with errors of the same magnitude, but the principal constants were well defined with very small errors. The diagonal potential function listed in Table I1 together with the potential energy distribution for the 35C135C1160 isotopic frequencies adequately defines the vibrational frequencies of C1-ClO which are listed in Table I. The excellent frequency fit without interaction force constants and the lack of appreciable potential energy distribution to interaction force constants when calculated show that the normal modes of C1-C10 are well described, respectively, by the internal coordinates C1-0 and C1-C1 bond stretching and Cl-C1-0 valence angle bending. The C1-0 force constant of C1-C10, 5.94 mdyn/A, is
TABLE I: Observed and Calculated Frequenciesa from Normal Coordinate Analysis of CI-CIOb Isotopic frequencies, cm-‘
isotopic frequencies, cm-’
obsd
calcd
obsd
calcd
961.8 375.1 239.4
962.1 375.1 240.8
925.5 374.1 231.8
925.4 374.1 233.5
953.7 369.3 239.4
953.7 369.7 239.6
916.8 368.8 231.8
916.6 368.6 232.3
961.8 371.2 239.4
962.1 371.2 238.9
925.5 370.1 231.8
925.4 370.0 231.5
953.7 365.7 239.4
953.6 365.6 237.7
916.8 364.5 231.8
916.6 364.4 230.4
Average difference between calculated and observed frequencies, 0.4 cm-’. OGeometry: CI-0 distance 1.587 A, Ci-CI distance 2.263 A, Ci-CI-0 angle 120’. a
Av =
TABLE I I: Potential Constants and Potential Energy Distribution
from Normal Coordinate Analysis of CI-CIOa
Force constants Error Potential energy distribution for 35C135C1’60
5.94
0.05 v1 ~2 ~3
99.1 0.4 0.6
1.37 0.03 0.7 83.1 16.1
0.78 0.03 0.2 16.5 83.3
Interaction force constants fixed at zero Fi, and f 2 2 are CI-0 and CI-CI stretching force constants, respectively, in mdyn/b, units f 3 3 Is the CI-CI-0 bending force constant in mdyn &‘rad2 units a
lower than this constant for C102 (7.02 mdyn/A)20 but it is higher than the C10 force constants for C10 radical (850 cm-1, 4.66 mdyn/A), HOC1 (3.9 mdyn/A),2i and Cl20 (2.75 mdyn/A).2 The C1-C1 force constant for C1-C10, 1.37 mdyn/A, is lower than the diatomic chlorine force constant (554 cm-1, 3.16 mdyn/A).22 The C1-(21-0 valence angle bending force constant, 0.78 mdyn A/rad2, is near the bending force constant for C120 (1.32 mdyn A/rad2)2 which has a slightly higher frequency. Bonding i n ClO Species. The observation of the C10 free radical a t a frequency (850 cm-l) lower than its chlorine bonded counterpart C1-C10 (962 cm -I) in marked contrast to the second row species NO (1875 ~ m - ~ and ) ~ 3 C1-NO (1799 invites further consideration of the bonding to third row diatomics as compared to second row diatomics. Notice the parallel frequency behavior for the X-SN and X-C10 species in Table I11 which differ from the X-NO and X-00 molecules. Furthermore, the (C10)2 frequencies (995, 982, and 945 cm -I, antisymmetric modes, symmetric frequencies are not known), which are probably due to three different isomeric structures, are above C10 monomer whereas the ( N 0 ) z frequencies (1776, 1866 cm-1)23 fall below the monomer (1875 cm-I). It appears, therefore, that the bonding to third row diatomics does not follow the Spratley-Pimentel arguments for second row diatomics.25 The Journal of Physical Chemistry, Vol. 77, No. 26, 1973
3066
Frank K. Chi a n d Lester Andrews
TABLE II I: Frequencies for Second and Third Row Diatomics Bonded to Halogen Atoms
Molecule
NO F-NO GI-NO 02
F-00 GI-00
Molecule
YN-0 or v0-0
FN-oor
Fo-0
Second Row 1875 15.5 1844 14.7 1799 14.1 1552 11.4 1500 10.5 1441 9.7 U S - N or YCI-0
Fs-N or Fa-0
Ref (v, F)
a b, c d, c e f
9 Ref ( U >F )
Third R o w
SN F-SN CI-SN GI0 F-GI0 GI-GI0
1204 1372 1325
850 1038 962
8.30 10.71
10.03 4.66 6.85 5.94
h i, i k
e I e
a Reference 23. R. J. H. Woltz, E. A. Jones, and A. H. Nielsen, J. Chem. Phys., 20, 378 (1952). J. P. Devlin and I. C. Hisatsune, Spectrochim. Acta, 17,206 (1961). Reference 24. e This work. f P. N. Noble and G. C. Pimentel, J. Chem. Phys., 44, 3641 (1966 g A. Arkeli and I. Schwager, J. Amer. Chem.,Soc., 89, 5999 (1967). P. 8.Zeeman, Can. J. Phys., 29, 174 (1951). H. Richert and 0. Glemser, Z. Anorg. Allg. Chem., 307, 328 (1961). W. Sawodny, A. Fadini, and K. Ballein, Spectrochim. Acta, 21, 995 (1965). A. Muller, G. Nagarajan, 0. Glemser, S. F. Cyrin, and J. Wegener, ibid., 23, 2683 (1967). L. Andrews, F. K. Chi, and A. Arkell, to be submitted for publication.
Here we seek a difference between 0 and N on the one hand and S and C1 on the other. The possible use of 3d orbitals26 on S and C1 to form additional P bonding between C1-0 or S-N is a controversial issue. Certainly, the 3d orbitals are very high in energy for the atom itself, but the bonding of an electronegative X atom may make the 3d orbitals more accessible for R bond formation. The amount of d orbital participation is not insignificant in C10 itself.27 The data in Table I11 tempts one to suggest that the C1-0 bond in CI-ClO has additional T character due to C13d-Ozp P bond formation enhanced by the C1 atom bonded to C10. However, this is a relatively weak C1-C1 bond; the weak C1 interaction with the C10 is evidenced by the diatomic like character of the C10 part of C1-C10. The case for (Cl0)z is complicated by the observation of antisymmetric (out-of-phase) C1-0 frequencies only, but presumably the symmetric counterparts are not very different. Again the intermolecular C10-C10 bonding is weak and the diatomic-like character of these modes is retained. The relatively weak interaction between C1 and C10 in C1-C10 and the two C10 parts of ( C 1 0 ) ~is probably insufficient to involve the C1 3d orbital as a possible means of strengthening the C10 bonds in Cl-C10 and (Cl0)Z. Returning to the Spratley-Pimentel (p-x*) picture,25 one possible explanation of the data for third row diatomics requires that electron density be withdrawn from the C10 (p-p) P* orbitals. In the case of (ClO)z, this could be accomplished in forming a bond between the two C10 species, Cl=O-Cl=O. For the F-C10 and Cl-ClO species, the electronegativity of fluorine and chlorine atoms may be sufficiently greater than the electronegativity of C10 to remove antibonding electron density from C10 leaving a stronger C10 bond. Table I11 shows that a fluorine atom The Journal of Physical Chemistry, Vol. 77, No. 26, 1973
produces a greater frequency shift for the third row diatomics than for the second row diatomics. This interpretation suggests that C10 and S N are less electronegative than 0 2 and NO, which is, of course, the relationship between the electronegativities of second and third row atoms.
Conclusions The Raman spectrum of matrix-isolated Cl20 agrees well with the infrared matrix spectrum. Laser illumination photolyses some of the ClzO to ClClO as observed by intense Raman bands a t 962, 373, and 241 cm-I in excellent agreement with infrared spectra following mercuryarc photolysis. The observation of a Raman band a t 850 c m - l with an oxygen-18 counterpart a t 818 cm -I confirms the ultraviolet assignment of the ground-state C10 fundamental. The vibrationa1 fundamental of C10 a t 850 cm-1 has been directly observed in three different systems by infrared matrix techniques and in the Raman spectrum of matrix-isolated ClzO as produced by laser photodissociation of ClzO. Infrared studies have been impeded by the apparent very low infrared extinction coefficient of C10; in situ photolysis studies have been hindered by the competing photolytic rearrangement of ClzO to C1-ClO. The cage effect inhibits the photolytic yield of C10 by trapping the C1 atom adjacent to C10 leading to the formation of C1-C10. The 995-, 986-cm -1 doublet observed in infrared matrix studies is reassigned to another (Cl0)z species, perhaps CI-0-C1-0, based on the production of extraordinarily high yields by ultraviolet photolysis of 0 3 , C120 matrix samples. Unlike the second row diatomics whose fundamentals are lowered upon bonding to a halogen atom, the C10 fundamental is increased upon dimerization or bonding to a halogen atom. It is suggested that this may be due to the removal of antibonding electron density by the more electronegative halogen atom or in forming the intermolecular dimer bond.
Acknowledgments. The authors gratefully acknowledge financial support for this research by the National Science Foundation under Grant No, GP-28582 and partial support for the laser-Raman instrument under Grant NO. GP-18251. We also acknowledge 'helpful discussions with Dr. Alfred Arkell on his unpublished work. References and Notes L. Andrews, J. Chem. Phys., 57, 51 (1972). M. M. Rochkind and G. C. Pimentel, J. Chem. Phys., 42, 1361 (1965). M. M. Rochkind and G. C. Pimentel, J. Chem. Phys., 46, 4481 ( 1967). D.J . Gardiner, J. Mol. Spectrosc., 38, 476 (1971). L. Andrewsand J. Raymond, J . Chem. Phys., 55,3087 (1971). G. Porter, Discuss. Faraday Soc., 9, 60 (1950). R. A. Durie and D. A. Ramsey, Can. J. Phys., 36, 25 (1958). N. Basco and R. D,Morse, J. Mol. Spectrosc., 45, 35 (1973). D. A. Hatzenbuhler and L. Andrews, J. Chem. Phys., 56, 3398 (1972). L. Andrews, J. Chem. Phys., 48, 972 (1968); 54, 4935 (1971). L. Andrews and R. C. Spiker, Jr., J. Phys. Chem., 76, 3208 (1972). (a) W. G. Alcock and G. C. Plmentel, J. Chem. Phys., 48, 2373 (1968). (b) Depolarization ratio ( p ) measurements for the Y I Raman band of CI2O yielded 0.85 f 0.07. Clearly this high value does not represent the true depolarization ratio for the molecule. Presumably the high matrix p is due to scrambling of the scattered light by the polycrystalline matrix. Similar high p values were determined for the three Raman bands of CI-CIO. A. Arkell and I . Schwager, unpublished results.
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Ir Spectra of Matrix Isolated M + N 0 3 - Ion Pairs (14) D. W. Smith and L. Andrews, J. Chem. Phys., in press. (15) R. 0. Allen, I-. Andrews, and J. M. Grzybowski, to be submitted for publication, (16) (a) P. N. Noble and G. C. Pimentei, J. Chem. Phys., 49, 3165 (1968). (b) A similar proton beam experiment using C102 produced the 8 4 9 . 6 - c n r ' band (0.05 OD) and the 995.0-cm-' band (0.10 OD). (17) E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, "Molecular Vibrations," McGraw-Hili, New York, N. Y., 1955. (18) J. H. Schachtschneider, Shell Development Co. Technical Report No. 57-65. (19) D. R. Herschbach and V. W. Laurie, J. Chem. Phys., 35, 458
(1961). (20) M. G. K. Pillai and R. F. Curl, Jr.. J. Chem. Phys., 37, 2921 (1962). (21) I . Schwager and A. Arkell, J. Amer. Chem. SOC., 89,6006 (1967). (22) W. Hoizer, W. F. Murphy, and H. J. Bernstein, J. Chem. Phys., 52, 399 (1970). (23) W. G. Fateley, H. A. Bent, and B. Crawford, Jr., J. Chem. Phys., 31, 204 (1959). (24) W. G. Burns and H. J. Bernstein, J. Chem. Phys., 18, 1669 (1950). (25) R. D. Spratiey and G. C. Pimentei, J. Amer. Chem. SOC., 88 2394 (1966). (26) See, forexample, E, L. Wagner, J. Chem. Phys., 37, 751 (1962). (27) P. A. G. O'Hare and A. C. Wahl, J. Chem. Phys., 54,3770 (1971).
Matrix Isolated M+N03- Ion Pairs in Argon, Glassy Water, and Ammonia Norman Smyrl and J. Paul Devlin" Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74074 (Received August 8, 7973) Publication costs assisted by the National Science Foundation
The infrared spectrum of lithium nitrate and potassium nitrate ion pairs (M+N03-) matrix isolated in argon, glassy water, and glassy ammonia at 12°K have been measured. These spectra show a drastic reduction in the splitting of the v3(e) nitrate asymmetric stretch mode in the H20 and NH3 matrices when compared to the argon case. This effect apparently results from solvation of the cation of the contact ion pairs with the solvent-cation interaction through the oxygen and nitrogen lone pair electrons. This result is consistent with the place exchange mechanism for aqueous nitrate solutions. The new data, when combined with published dilute solution spectra, complete the spectra for the two nitrate environments required by the place exchange view and are used here to argue for the importance of cooperative effects in the spectra of molten nitrates and their concentrated aqueous solutions.
Introduction The relatively recent recognition that alkali metal nitrate salts volatilize in U ~ C U Onear their melting points without dissociation1-2 has made possible a number of informative studies. In particular, the spectra of the metal nitrate monomers (ion pairs), M+N03-, isolated in inert matrices, such as argon, have been reported with the emphasis placed on the splitting of the degenerate v3 asymmetric stretching mode as a useful measure of the extent of cation distortion of the anion. More recently it has also been shown that the salt volatility enables the study of pure simple nitrate glasses for the first time.3 Since a glass structure is believed to be essentially the same as that of the corresponding melt, preparation of the vapordeposited filrns has greatly simplified the infrared spectroscopic study of such structures as well as the measurement of the glass transition temperature^.^ It seems, however, that possibly the chemically most informative study to be facilitated by nitrate volatility has been overlooked. It is now possible to deliberately place a metal nitrate ion pair in a particular environment for comparison with the abundance of published spectra which have been interpreted by invoking the presence of such ion pairs. The most obvious medium for a comparison is that of aqueous solutions for which contact ion pairs have been invoked and place exchange equilibrium constants have been e ~ a l u a t e d . If ~ . it ~ is assumed that the ion pair environment in glassy water resembles that for
the aqueous ~ o l u t i o n then , ~ the matrix isolation of the ion pair in a glassy water deposit permits a rather direct test of the assumptions made in interpreting the aqueous solution spectra and should help establish the degree of importance of contact ion pairs in such systems. This paper reports the results of a study of the lithium and potassium nitrate ion pairs in glassy water and glassy ammonia matrice prepared by deposition a t 12°K. New spectra have also been obtained for crystalline argon matrices with the metal-oxygen stretch for the (M+N03-) species now observed. The argon matrix spectra are presented here in detail for comparison with the glassy-matrix spectra.
Experimental Section The volatilization of the metal nitrates was from the melts in Pyrex glass Knudsen cells as described earlier.3 The matrix gases (argon, water, and ammonia) were metered through a Fisher-Porter Ihs-in. glass flow meter and co-condensed with the nitrate molecular beam within a standard low-temperature infrared cell fitted to an Air Products CS202 closed-cycle helium refrigerator. Since precise control of the nitrate beam density is not possible, optimum matrix ratios were established by trial and error. In the case of water and ammonia, the useful ratio is limited by the absorption spectrum of the matrix. The substrate deposition temperature was 12"K, guaranteeing that, for the deposition rates used, the water condenses as an amorphous (glassy) solid.* The Journalof Physical Chemistry, Vol. 77, No. 26, 7973
