1686
J. Phys. Chem. 1980, 8 4 , 1686-1694
(18) (a) K. Luther and J. Troe, Symp. (Int.)Combust., [Proc.],17,(1979); (b) J. Troe, J . Phys. Chern., 83, 114 (1979). (19) A. S.Gordon, J. Chem. Phys., 36,1330 (1962). (20) M. Quack and J. Troe, Ber. Bunsenges. phys. Chem., 81,329(1977).
(21)R. A. Carney, E. A. Piotrowsky, A. G. Meister, J. H. Braun, and F. F. Cleveland, J . Mol. Spectrosc., 7,209 (1961). (22)H.Endo, K.Glanzer, and J. Troe, J. Phys. Chem., 83,2083 (1979). (23)J. R. Dacey and D. M. Young, J. Chem. Phys., 23,1302 (1955).
Infrared Laser Single Photon Absorption Reaction Chemistry in the Solid State. Reactions of Nitrogen Oxides with Sulfur Hexafluoride Edward Catalan0 * Lawrence Liverrnore Laboratory, Livermore, California 94550
and Robert E. Barletta Argonne National Laboratory, Argonne, Illinois 60439 (Received January 2 7, 1980) Publication costs assisted by Lawrence Livermore Laboratory
The infrared laser induced reactions between SF6 as a guest reactant matrix isolated within nitrogen oxides as host reactants at low temperatures are described. These reactions proceed via single-photon excitation of the v3 band of SF6 from the u = 0 1 vibrational states. They are further examples of a process known as single-photon absorption reaction chemistry in the solid state (SPARCSS). The spectral data show that SPARCSS reactions can be very rapid. In the NO(N203)-SF6 system, the primary photoreaction was completed within 0.075 s at a photon flux density of 33 mW cm-2, The activation energy for these SPARCSS reactions, which is 5 photon energy, is dramatically smaller than the gas-phase thermal reaction between NO and SF6. The nitrogen oxides-SF6 SPARCSS reactions provide further evidence of potential energy surfaces for reactions that are drastically different from those for gas-phase reactions because of configurationalaspects of a general matrix phenomenon.
-
Introduction Examples of chemical reactions photoinduced by direct, single-photon excitation of the u = 0 u = 1 vibrational transitions in a guest reactant matrix isolated within a host reactant at cryogenic temperatures have previously been briefly described,l and one has been described in detail2 These reactions were used as experimental examples of the bases for the process of single photon absorption reaction chemistry in the solid state (SPARCSS). The process is itself a manifestation of a more general phenomenon involving photochemistry in which the reactants are in fixed relative spatial configurations. In two cases that have been investigated in detail, SiH42and CH4were the hosts with the UF, and BCl, as their respective guests. The excitations were performed by using very small photon flux densities. These reactant systems are illustrative of several aspects of the SPARCSS process. Experiments on both reactant systems showed that activation energy barriers for infrared laser SPARCSS (IRL-SPARCSS) can be considerably lower than gas-phase thermal activation energy barriers between the same reactant pairs. This is the result of a change in the potential energy surface for reaction which is caused by the host matrix interacting with the guest reactant. Such a change may result in reaction products which are the same as or different from those produced in gas-phase thermal reactions. The SiH4-UF6 IRLSPARCSS reaction yields products that are ostensibly the same as those of the gas-phase reaction. The new pathways may yield products that are quite different from those for gas-phase thermal reaction. The CH4-BC13 system appears to be one in which products are obtained that are different from those obtained in gas-phase thermal reaction. Furthermore, the example CH4-BC1, system shows
-
0022-3654/80/2084-1686$01 .OO/O
that quite different photochemical responses to vibrational excitation under IRL-SPARCSS conditions are obtained which are most probably caused by site differences among the matrix isolated guest molecules. Also, the variation in photochemical response provides evidence for the inhomogeneities in the absorption band of the guest molecules. In this report, IRL-SPARCSS experiments on nitrogen oxides-SF, systems are described. Attempts were made to produce chemical reaction by direct excitation of the u = 0 u = 1transition of the v3 (asymmetric stretching) mode of the SF6guest with small photon flux densities. Four systems were chosen for study: (a) NO-SF,; (b) N,O-SF,; (c) NO(N,O,)-SF,; and (d) NO(N20)-SF6. Systems c and d possess an additional nitrogen oxide as a guest molecule. Gas-phase thermal reactions between any of these reactant sets do not occur at temperatures below 1000 "C. The extent of IRL-SPARCSS reactions ranges from zero for a and b to a large percentage for c. In c and d, the v3 band of SF6appears to be homogeneously broadened. The solid-state chemistry and structures of the oxides of nitrogen are very complex. Pure solid NO consists of a lattice of dimers, and solid deposits of mixtures of oxides of nitrogen are known to form many structural arrangements at cryogenic temperature^.^ Bandwidth studies of SF, isolated in Nz, Ar, Kr, Xe, CH4, CO,NzO, and NO matrices show that the bandwidth is anomalously large for NO and NzO m a t r i ~ e s . ~ Mixtures of NO and SF6have been the subject of gasphase, high-power, multiphoton laser chemistry experiments by Basov et In these experiments the excitation energy was rapidly thermalized. Sensible reactions occur only at temperatures in excess of -1000 "C. The postu-
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0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 13, 1980
Infrared Laser Single Photon Absorption Reactions
1687
TABLE I: Experimental Conditions
--
matrix H-G
--
concn (H):(G)
amount deposited, mmol
irrad no.
CO, lines useda
photon flux density, mW cm-,
time, s
168 3600 1 P(32) 583 60 2 P(32) 33 0.075 NO(N,O,)-SF, 900(20)c:1 0.61 1 P(30), P(32) 33 1 2 P(30), P(32) 33 600 3 P(30),P(32) 33 600 lb R(341, R(36) 33 60 NO(N,O)-SF, 900(1.9):l 0.3 1 P(301,P(32) 33 3600 2 P(301, P(32) 33 600 N,O-SF, 1OOO:l 0.3 1 P(30), P(32) 600 600 2 P(30),P(32) a Transitions are those of the (OO"1)-(1O"O) band of CO,. P(30)= 934.9 cm-', P(32)= 933.0 cm-', R(34)= 984.4cm-', Estimated from band R(36)and R(34) lines are not resonant with matrix isolated SF, band. and R(36) = 985.5CHI"'. intensity. NO-SF,
1OOO:l
0.3
lated reaction mechanism included free-radical production and random recombination. Some of the reported reaction products were F2S0 and NzO. The IRL-SPARCSS experiments herein described are a marked contrast to both gas-phase thermal chemistry and conventional multiphoton laser photochemistry. Experimental Section The apparatus used in these experiments has been described earliera2Premixed gas samples were prepared in the desired mole ratios by using standard manometric techniques and utilizing the stainless steel gas handling system. The sampler1 were deposited at 11-12 K on solid Ar (- 3 ymol) which had previously been deposited on a 3-cm2 CsI window. The deposition rate was controlled by flow through a glass capillary 0.055-mm i.d. at a rate of roughly 0.6 pmol min-l. Preparation and irradiation conditions are summarized in Table I. After deposition, the samples were annealed at 25 K for 10-20 min and recooled to 12 K. An initial infrared spectrum was taken of the reactants. Irradiation of the sample was with emission from a grating-tuned, CW C02 laser. Laser emission power was measured with a calibrated thermopile detector. An infrared spectrum restricted to the guest-reactant absorptions was taken after each irradiation. After the final irradiation, a wide-range spectrum (4000-400 cm-') was recorded and was searched for product absorptions. The wide-range spectra required -3 h to record. The narrow-range,high-resolution spectra required -1 h to record. In one series of experiments involving SF6 in NO matrices with N2O3 present, postirradiation thermal treatments were performed to attempt to study the thermal stability of the reaction products. The matrix was kept at 26 and 46 K for 30 and 15 min, respectively. After each thermal treatment, the matrix was again cooled to 12 K, and infrared spectra were taken. Results and Discussion NO-SF, and N20-23FQ Matrix spectra taken on samples fabricated from pure NO and SF6 over the region 400-4000 cm-l had absorptions due only to NO which corresponded to those of Smith et ala6and to the v3 band of SF6. The latter is an extremely intense band. With [NO]/[SF6] = 1000 and the small amount of sample deposited, it is the only band of SF6 intense enough to be observed. The band peak of the v3 band of SF6 is at 933 cm-l with a width at half-maximum of -12 cm-l. It is much broader than the v3 band of SF6 isolated in the rare gases, N2, or C114.4
The N20 formed a poor matrix under the deposition conditions used. A large amount of scatter from the matrix was observed as a rather large background slope having complete opacity for frequencies above 2200 cm-'. Further, the NzO fundamental at 1290 cm-l was complicated by a pronounced Christiansen effect which is indicative of a high degree of crystallinity. The v3 band of SF6in N 2 0 as observed in both low- and high-resolution spectra is considerably altered from that of SF6in NO matrices. The absorption band is characterized by a broad triplet with a full width at half-maximum of 35-40 cm-l. The three components of this band have maxima at 930,935, and 941 cm-l with bandwidths of roughly 8,6, and 4 cm-l, respectively. The individual bands in the triplet are themselves composites of still narrower bands which are observed as shoulders in the band contour. The photon flux densities used for irradiation of both the NO-SF6 and N20-sF6 systems (Table I) were an order of magnitude larger than those that produced photochemical response in the NO(NzO3)SF6system (discussed in the next section). The irradiation frequencies given in Table I are resonant with the v3 bands. Postirradiation infrared spectra of these systems did not reveal any changes in the band contours or the intensities of the v3 fundamentals or the appearance of any additional bands which could be ascribed to reaction products. Hence, it must be concluded that a photoinduced reaction did not occur in either of these systems. NO(N203)-SF6. A matrix spectrum, typical of many recorded for this system, is shown in Figure la. All spectral features in Figure l a are due to SF6,NO, asymmetrical NzO3, and an impurity band due to NzO. The absorption frequencies attributed to NO correspond to those of Smith et al.6 and of Fateley et al.7 The absorption frequency attributed to the NzO impurity band corresponds to the main N20 band reported by Dowsa8 N2O3 spectral properties in and out of matrices have been reported by Fateley et d.,' Hisatsune and co-workersFl0and Varetti and Pimentel.12 The chemistry and properties of NzO3 are described by Jones.3 The v3 band of SF6 has a peak of 937.5 cm-l and is quite broad in this matrix. The data of Varetti and Pimentel12 shown in Table I1 allow an unambiguous assignment of the N2O3 bands in Figure l a to asymmetrical Nz03 In both the Varetti and Pimentel work and our own, the deposition conditions led to the formation of only the asymmetrical N203. This is the form that appears to be favored in reactions between NO and Oz.7J2The correspondenceof our impurity bands to asymmetricalNz03 was indeed excellent. We, therefore, assume that our N2O3impurity was a result of minute leaks
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The Journal of Physical Chemistry, Vol. 84, No. 13, 1980
Catalan0 and Barletta
: " 7 SO"
Ob -
recorded at 2 - ~ r n resolution. -~
TABLE 11: Absorption Bands of N,O," asymmetrical N, 0, w , cm-'
1839.7 1630.4 1302.5 775.7
420.4
optical density, relative 0.60 1.12 0.83 0.26 0.04
symmetrical N,O, w , cm-'
1689.7 1661.0 969.4 704.3 387.4 365.5
optical density, relative 0.85 0.18 0.21
of air into part of the sample preparation system. The cold CsI window was exposed to this leak only during sample deposition. It was not exposed to the leak during irradiation, data acquisition, thermal treatment, or storage. Because of this leak, other trace constituents, especially N2, would be present in the matrix. Asymmetrical Nz03undergoes an isomerization reaction to symmetrical N203in N2 matrix at low temperatures with photons of frequency of 14000 cm-l.I2 These frequencies correspond to an electronic excitation. Once in the excited electronic state, the molecule can rearrange. Thermochemical bond-energy arguments favor the symmetrical N203as the more stable s p e c i e ~ . ~Ultraviolet J~ photolysis of the symmetrical N203in N2 matrices converts it back into asymmetrical N2O3.l2 Figure 1,b and c, shows the spectra of the matrix after simultaneous irradiation with P(30) and P(32) COz laser lines (which are resonant with the v3 band of SF6)at a total photon flux density of 33 mW cm-2. Figure l b is typical of several matrix samples irradiated under the same conditions for 0.075 s. Spectra a, b, and c of Figure 1 are all taken from the same matrix sample with irradiation times for b and c of 0.075 s and 600 s, respectively. Several general observations can be made about these spectra: (a) the N 2 0 impurity band remains constant; (b) the SF6band and bands assigned to N203decrease with irradiation; (c) a large number of new (product) bands grow with irradiation (especially prominent are the eight new bands in the 3200-3800 cm-' region); (d) some of the product bands are narrow as if they were matrix-isolated molecular species; some are very broad; and (e) a large amount of photoinduced reaction is complete within 0.075 s, but further spectral changes occur after as long as 600
-
s. All of the spectral features shown in Figure la-c are listed in Table I. In another experiment, the matrix was irradiated with R(36) and R(34) lines of the COz laser (984.4 and 985.4 cm-l, respectively), out of resonance with the SF6band. The photon flux density was 33 mW cm-2. The irradiation time was 600 s. There was no spectral evidence of a reaction. Thus, it is clear that the v3 band of SF6 is intimately involved in the reaction illustrated by the spectra in Figure 1. The excitation of the v3 mode of SF6and the presence of NzO3 are both essential to the IRL-SPARCSS NO(N2o3)-SF6reaction. SF6is directly involved, as evidenced by the decrease in intensity of the v3 band of SF6 with photoirradiation. Further, it is not just SF6by excitation via its v3 band that is involved, since, as will be discussed, only a portion of the SF6in the system was available for reaction. N203 has been shown to be an integral part of the reaction; no reaction is observed in the NO-SF, system. Unlike the work of Varetti and Pimentel,12 the IRLSPARCSS spectral data from this system are not consistent with only a simple isomerization of asymmetrical N203. Our spectra do show that a small amount of the isomerization reaction does occur simply because of the length of time the matrix was in the spectrometer source beam which is consistent with Varetti and Pimentel.12 The bands at 1690 and 1661 cm-' for symmetrical N203in Nz matrix (Table 11) are good indicators of the slow isomerization reaction of asymmetrical N2O3. In an effort to gain more clues as to the identity of the IRL-SPARCSS products, two thermal treatments were made after the final irradiation of the NO(N,OJ-SF, matrix. These spectra are shown in Figure 2. Figure 2a is the spectrum after 10 min of 33 mW cm-2 of P(30) and P(32) irradiation (same spectrum as Figure IC).Figure 2b was taken after keeping the matrix at -26 K for 30 min and recooling to 12 K. Figure 2c was taken after keeping the matrix a t -46 K for 15 min and recooling to 1 2 K. (Figure 2c shows some loss of NO host matrix. This occurred since 46 K is too high a temperature for the Ar protective coating to have remained rigid. Therefore, a quantitative comparison of Figure 2c to Figure la-c and Figure 2b cannot be made, but quantitative comparisons among the others are valid. Figure 2c clearly shows the buildup of amorphous water in the 3000-3600-~m-~ re-
Infrared Laser Single iPhoton Absorption Reactions
The Journal of Physical Chemistry, Vol. 84, No. 13, 1980
1089
80,
WAVE N UM 8 E R
Flgure 2. Matrix spectra of the NO(N20B)-SF, system: (a) spectrum taken after 601-s irradiation; (b) spectrum taken after thermal treatment at 26 K for 30 min and recooling to 12 K; (c) spectrum taken after thermal treatment at 46 K for 15 min and recooling to 12 K. Note that spectra are displayed with respect to each other for ease in viewing. The matrix sample is that for which the spectra in Figure 1 were recorded; a is the same soectrum as in Figure - I C . Portions of the soectra devoid of any matrix absorptions have been deleted. These spectra were recorded at 2-cm-1 iesolution.
TABLE IV: IRL-SPARCSS NO(N,O,)-SF,
TABLE 111: IRL-SIPARCSS NO(N,O,)-SF, Bands freq, cm' ' 3715 3705
C
0.37 0.20
\
0.06
0.05
t 3315 1 3360 3320 3495 3527
0.03 0.04
2235 1930 1832 1685
0.09 0.05 0.16 0.l1 0,ll 0.02 0.02 0.03 0.:12
0.09 0.14
t
O0.04 .l5 0.05 0.03 0.12
assignment
3715 3705
doublet, product band NO product band NO doublet, product band
3680 3630 3590
broad
0.51
1
Oa20 0.55
0.29 0.32 0.08
0.25
0.23
0.11. 0.072
0.1.4 0.1:1 0.065 0.02
0.16} 0.11 0.062 0.03 0.02 0.06 0.04
0.012
0.10
product band,b s-N,O, doublet, product band a-N,O, a-N, 0 , Combination of a-N,O, and product band
SF,
product band,c broad product band a-N,O, product band,= broad doublet, product band
0.013 0.0!2 0.05 0.03 0.05 0.03 0.03 product band,c band 0.03 0.0:3 0.05 a Very intense band; peak height not a good measure of intensity. Product band; but due t o symmetrical N,O,; see text. Broad band; more intensity in band than is apparent from peak height.
1
t
I
gion;13 product bands in the 3000-4000-~m-~ frequency region have disappeared.) Tables I11 and IV summarize these spectral data. The amorphous water buildup has been subtracted in calculating the optical densities. The data cllearly indicate that asymmetrical N203and SF6in NO matrices participate in some complex reaction when the u3 band of SF6 is directly excited. The pho-
a
3315 3320 1930 1685 1630-40 1625) 1605 1590 1305 1270 935 910 900 800 782 680 51 5551 5 a
b
comments
C
0.37 0.20 0.09 0.05
3527t ::E
doublet, product band product band N,O product band: a-N,O,
1
3495 3440
~
0.02
1305
frea. cm-'
doublet, product band
0.09
1605 16251 1605
1260 935 910 800 782 750 700 680
kI
0.a3 0.20 0.:25 0.128
1
3685 3630 3590
optical densitya
peak optical densitiesd a
Bands
0.42 0.03 0.04 0.10
0.06 0.15 0.06 0.05 0.07 0.12 0.12 0.02 0.03 ?
?
0.11 0.11 0.23 0.16 0.06 0.03
0.23 0.16 0.06
0.03 0.04
NO band
0.12 0.04 0.01
s-N,O, s-N,O,
0.16 0.27 0.06 0.03
0.04 0.02 0.06
E:;;
0.05 0.04 0.02 0.01 0.03
1
coalescing of bands and shifting SF, coalescing of band and shifting
0.08 0.04
Spectra from Figure 2.
toinduced reaction must be complex because of the large changes in reactant bands and the number, the intensities, the contours, and the frequencies of the product bands. Part of the asymmetric N203disappears during the reaction. (Neither the entire contour of the band at 1605-20 cm-' nor that of the band at 782 cm-' is due exclusively to product formation because some asymmetric N203remains.) The symmetrical N203 that is formed by the near-infrared photoinduced isomerization appears at 1685 and 1635 cm-' in Figure 1, b and c, and Figure 2a-c. The shift of these marker bands from the listed 1690- and 1661-cm-' bands of Table I1 is probably due to a difference in host matrix. The product molecular species which demonstrate the
1690
The Journal of fhyslcal Chemistry, Vol. 84, No. 13, 1980
intense bands (Figure 1)in the 3000-4000-~m-~ region and the intense broad band centered at 1930 cm-I are mysteries. One should note that the appearance and maximum intensity of these high-frequency bands occur only after the first irradiation. They do not appear at all unless the matrix has been irradiated, and they do not change appreciably with either subsequent irradiation or time, behaving exactly as bands clearly assignable to products of the IRL-SPARCSS reaction. The possibility that these bands, particularly those at high frequency, could be caused by vibrations of dimers and higher oligomers of matrix-isolated water may be eliminated. Strictly speaking, in order to rule out assignment of these bands to matrix-isolated water, comparison should be made between these product bands and data for water in NO matrices. In the absence of such data, however, comparison with data for water in nitrogen matrices13J4 should provide information which is sufficiently accurate to determine whether water is indeed the cause of the bands between 4000 and 3000 cm-l. Monomer absorptions of water in N2matrices have been reported at 3725.1, 3632.5, and 1596.9 cm-l.14 Of these bands the 3725.1-cm-' absorption is the most intense, and the 3632.5-cm-' band is the least intense. Table I11 shows that no bands were observed at 3725 cm-'. The closest observed product band was at 3715 cm-l. Assignment of the band to the v3 monomer vibration would require a rather large shift, 10 cm-', to the red relative to its position in nitrogen. This would mean that 3632.5 cm-l in Nz matrices should occur in the region of 3620 cm-' in NO. No bands were observed in this region. Thus assignment of the 3715-cm-' band to a v3 monomer absorption is doubtful. Further, the relative intensity of the bands at 3715 and 3630 cm-' (Table 111) is significantly different from the ratio of intensities for v3 to v1 found in N2 matrices.14 The v3 band is clearly the most intense band in the spectrum regardless of the water concentration. Over a concentration range of Nz:H20of 40:l to 400:1, the v3 band is about a factor of 6 more intense than the v1 band. The ratio between the intensity of the 3715- and 3630-cm-l bands of Table 111 is about 3.7. This ratio is different enough once again to cast serious doubt on the assignment of the 3715-cm-l band to the v3 band of H20 monomers. Indeed, a band at 3715 cm-' is usually assigned to a water dimer absorption.13J4 Therefore, the absence of the H 2 0 monomer absorption bands is strong evidence that these product bands in the 4000-3000-~m-~region cannot be attributed to matrix-isolated water. This is the case, since, in all matrices (that have been thoroughly investigated) in which H20 is a guest molecule no matter whether in small or large concentration, the monomer bands are present. A second argument against these bands being attributable to a matrix-isolated water may be found by considering the relative intensities of the product bands in Table I11 with the relative intensities given in ref 14 for the water oligomer vibrations. In this oligomer case, the ratio of the water polymer absorption at 3688 cm-' to the 3715-cm-' dimer absorption ought to serve as a good indication of the concentration of water in the matrix. The distribution of water oligomers is determined statistically and is independent of the matrix chosen. Therefore, the results of relative intensity measurements of water in nitrogen matrices14 ought to be comparable to those of water in NO matrices. Thus, using Figure 4 of ref 14 as a guide, one can estimate the expected intensities of the other water oligomer vibrations from the intensity of bands at 3715 and 3680-3695 cm-l given in Table 111. The descrepancies between the observed intensities of bands in
Catalan0 and Barletta
Table I11 and those given for corresponding frequencies given in ref 14 again make assignment of the product bands to matrix-isolated water oligomers unlikely. The possibility that these bands correspond to species containing N-H stretches and bends can also be eliminated on the basis that a source of hydrogen must be present. The most logical source of hydrogen in these experiments would be from water. But, to be the source of hydrogen, the water must be present as matrix-isolated water. By the argument given above, there is no evidence for matrix-isolated water in these samples. The water that is present is amorphous water. If the adventitiously introduced water comes in at a very slow rate, then the amount present during the deposition process is so small that it is not observed as a matrixisolated spectrum. With such conditions, the water should accumulate on top of the matrix as amorphous water if the run is long enough. All of our very long runs show evidence of amorphous water (Figure 2, the region 3000-3600 cm-l). If the rate of adventitious water deposition had been large, the spectra would have been complex and concentration dependent.15 The detailed nature of the water spectrum at -3700 cm-' allows one to decide whether the adventitious water was introduced because of an air leak. There is a narrow band at that frequency which is exceptionally concentration dependent in Nz matrices.15 Normal air contains too much water to provide good matrix spectra of water. An air leak into the apparatus can be identified by the presence of this spectral feature. In our apparatus, the spectral evidence favors water desorbing from surfaces as the major source of adventitious water. Amorphous water, from any source, that is deposited on top of the matrix sample simply cannot provide the amount of hydrogen that would be necessary to account for the spectral data. We have tentatively ruled out molecular hindered rotations as sources of the complex structure in the 30004000-cm-' region. Such laser induced molecular motion effects have been observed by Abouaf-Marguin et al. in rare gas-NH3 matrices.16J7 The NO matrix forms much too tight a cage to allow hindered rotation. One may speculate that the intense and complex high-frequency spectral data are indicative of electronic excitation of large ,molecular species which may be electronically excited. To gain additional insight into the nature and extent of the photochemical reaction in this system, the spectral region 840-1100 cm-' which included the v3 mode of SF6 was decomposed into a minimum number of component Lorentzian bands such that the sum reproduced the original data. Spectra taken before irradiation, after 0.075-, 1-,and 600-s irradiations, and after 26 K thermal treatment were sequentially decomposed. The results of these decompositionsare listed in Tables V and VI. The center frequency ( w ) and the width at half-maximum (Aw)of the component bands were fixed after the decomposition of the first spectrum. Additional bands were added as necessary for each sequential spectrum, but always attempting to preserve w, and Awi with a minimum number of components commensurate with the quality of the spectral data. The preirradiated v3 band of SF6was composed of two components: a narrow one, w2 = 937.5 cm-', having a Aw = 9 cm-l and a broad one, w1 = 920 cm-l, having a Aw = 50 cm-l (Table V). Upon irradiation for 0.075 s, the 920cm-l component remains constant, while the 937.5-cm-l component vanishes. The complete disappearance of this broad band upon irradiation with C 0 2 laser line emission that is resonant with only a small portion of this band
Infrared Laser Single Photon Absorption Reactions
The Journal of Physical Chemistry, Vol. 84, No. 13, 1980
1891
TABLE V: Intensity of the Lorenztian Components of the v 3 Band of SF, in the NO(N,O,)-SF, System --
component .-center LCJ , cm-I A w , ~cm-' spectrum preirrad after 0.075 s after I s after 601 s after thermal tr spectrum preirrad after 0.075 s after 1 s after 601 s after thermal tr a
W1
w2
920 937.5 50 9 Peak Height (Absorbivities) 0.018 0.05 0 0.05 0 0.05 0 0.05 0 0.05 Intensities (Areas) 1.26 0.69 0 0.69 0 0.69 0 0.69 0 0.69
Width at half-maximum.
would indicate that it is homogeneously broadened. Thus, it appears that these are at least two typesof matrix-isolated SF6molecules. The first, with a quasi-v, mode band, w i = 920 cm-l with A01 = 50 cm-l, we speculate corresponds to the vibrational mode of a weak complex between host and guest molecule that is stable at low temperatures, This speculation is based on the anomalously large bandwidth of the v3 band of SF6 isolated in NO.4 The second has a narrower u3 mode band (w2 = 937.5 cm-l with Awz = 9 cm-l). The fact that the SF6molecules, whose absorptions correspond to the latter band, remain unreactive upon irradiation is an indication of the selectivity of the SPARCSS process. An estimate of the amount of SF6which reacted during the course of the laser-induced reaction was made by comparing the intensity change between the unirradiated and post-0.075-s-irradiated 920-cm-l band of SF6 to the total intensities of the 920- and 937.5-~rn-~ unirradiated components. Data from Table V indicate that 65% of the SF, had reacted via the laser-induced reaction. Numerous additional bands appear in the postirradiation spectra in the 840-1100-~m-~ region. Some of these bands are quite broad (Aw,in the range of 55-60 cm-l) and have been tentatively assigned as product bands. Table VI lists wl, Aw,, and the peak absorbances and total intensities of these bands. One can speculate about the fate of SF6 in this reaction on the basis of spectral data after photoreaction and thermal reaction. 'The photoreaction products involve species which have strong bands whose centers are very near the v3 band of 13F6and shifted to the red. Much of the product bands overlap the SF6 bands as is deduced from the band contour in Figure 2c and the data in Tables V and VI. Note particularly components w7, us,and cog of Table VI. A product such as an R-SF, species would be
t
I 2400
2100
1 4 0 0 1300 1 2 0 0 WAVENUMBER
1000
900
800
Flgure 3. Matrix spectra of the NO(N20)-SF, [(H):(G)] system: (a) spectra before irradiation but after annealing; (b) spectra after irradiation of the v3 band of SF,. The spectra on the right are of the u3 band of SF,. Those on the left and center are of N 2 0 bands. These spectra are displayed for clarity in vlewing. The (H):(G) = 900; 0.3 mmol of sample deposited on an Ar-coated CsI window maintained at 12 K. The P(30) and P(32) C 0 2 laser irradiation lines used are at 934.9 and 933.0 cm-', respectively. Irradiations were for 60 and 3600 s at a total photon flux density of 33 cm-~,Both lasedsimultaneousb. These spectra were recorded at 2-cm-I resolution.
,,
consistent with this behavior, but these bands could be due to more than one species. The data in Tables V and VI imply a complex photochemical response as a function of irradiation time. The SF6band changes, as noted above, occur within the first 0.075-s irradiation and then remain constant. The data in Table VI (and also in Table 111) suggest that there may be more than one photochemical reaction. The behavior of w7, an intense product component band of the first 0.075-s photochemical reaction upon irradiation (P(30) and P(32) lines), indicates that the species having this absorption band is a probable reactant in the secondary photoreaction. The spectra taken after the thermal treatments show that whatever some of the primary products are they are not stable at -46 K. Upon heating, a smaller number of bands survive, and all of the primary product bands in the high-frequency region disappear. NO(N20)-SF6. The u3 band contour of SF6 in this system is broad and similar to the corresponding band contour of the NO(Nz03)-SF6matrices. Figure 3 shows the band contours of this band and those of NzO bands. Two irradiations at 33 mW cm-z resonant with the v3 band of SF6were made (Table I). The extent of reaction was very small. In this case, no product bands were observed growing in. There were decreases both in the NzO bands and in the SF6 v3 band. All apparent spectral changes were completed during the 1-min irradiation period. N 2 0 matrix-isolated bands were assigned as in Smith et aL6 Table VI1 summarizes the
TABLE VI: Intensities of Lorenztian Components in the 840-llOO-cm-' Spectral Region of the NO(N,O,)-SF6 System component w3 w4 WS w6 w7 W 8 w9 w 10 -center w , cm-' 845 890 900 915 940 955 1040 1080 A w , cm-' ~ 10 10 10 15 55 10 60 60 spectrum Peak Heights (Absorbivities) preirrad 0 0 0 0 0 0 0.007 0.008 after 0.075 s 0 0 0.028 0.03 0.038 0 0.02 0 after 1 s 0 0.025 0.028 0.03 0.048 0.01 0.015 0 after 601 s 0.001 0.025 0.037 0.035 0.038 0.01 0.009 0 after thermal tr 0.015 0.025 0.032 0.023 0.038 0.01 0 0 spectrum Intensities (Areas) preirrad 0 0 0 0 0 0 0.54 0.49 after 0.075 s 0 0 0.43 0.68 2.90 0 1.53 0 after 1 s 0 0.38 0.43 0.68 3.67 1.70 1.15 0 after 601 s 0.15 0.38 0.57 0.80 2.90 1.70 0.69 0 after thermal tr 0.23 0.38 0.49 0.35 2.90 1.70 0 0 a Width at half-rnaxirnum.
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The Journal of Physical Chemistry, Vol. 84, No. 13, 1980
TABLE VII: Intensity Changes of Reactant Bands of NO(N,O)-SF, System upon Irradiation total change in absorbivity, % band
N, 0 doublet N, 0
SF, a
band freq, cm-'
bada
2230 1303 1285 933
-17 - 11 - 49 - 28
1st
2nd irrada - 17 - 11
- 49 - 28
See Table I.
spectral data. It is clear from these data that both the N20 and the SF6take part in a photochemical reaction, but the extent of reaction is quite small. Since no product bands were observed in postirradiated spectra, the products must have been formed in minute quantities or their absorption band intensities are much smaller than either N20 or SFG. Existence of Photochemical Reaction. The criteria chosen for deciding whether a reaction occurred were spectroscopic. Reaction was inferred from the reproducible growth of bands attributed to products as well as from the diminishment of absorption bands due to the reactant species. In the latter case, the bands which should be the most sensitive are those which are due to absorptions of the guest molecule. In applying these criteria to photochemical experiments performed with the reactants in low-temperature matrices one finds that they work quite well. In the laser-induced photochemistry of Fe(C0)4,1"21in two nonlaser infraredinduced isomerizations,12p22and in the IRL-SPARCSS experiments,'V2 both criteria are completely satisfied. In this paper, all cases in which reaction was claimed showed a reproducible reduction in the intensity of reactant bands and particularly those due to the guest species. Further, in the NO(N203)-SF6system, product bands were reproducibly observed in the infrared spectrum only after irradiation of the u3 band of SF6. The product bands did not appear, or grow, in the spectra with time or with irradiation that was nonresonant with the u3 band of SF6. The differences between the infrared laser SPARCSS experiments and other infrared-induced matrix photochemistry go beyond these simple criteria. The most important involves product identification. Unfortunately, the IRL-SPARCSS work contains incomplete product identification. In some experiments (SiH4-UF6)?tentative product identification could be made. In others, only one of the products could be identified (HC1in the CH4-BC13 experiments).l In the NO(N203)-SF6experiments, no product assignments could be made. Yet the lack of conclusive product identification merely indicates an incomplete understanding of what appear to be complex reactions rather than an indication that no reaction has occurred. This is true only if the observed spectral changes are not the result of photophysical processes. The evidence for reaction in the NO(N203)-SF6system is conclusive. However, there remains a paucity of understanding since (a) detailed knowledge of the molecular reactants is not fully known, (b) the products of the primary photochemistry are stable only at low temperatures, (c) there may be more than one photoinduced reaction, and (d) the products have not been identified. Evidence for reaction in the NO(N20)-SF6system is less conclusive in that the reactant bands diminish with irradiation, but product bands do not appear in post irradiation spectra. The spectral data showing the bands of two reactants diminishing in intensity with irradiation in several separate experiments is, however, quite convincing. The extent of reaction based upon the reduction of reac-
Catalan0 and Barletta
tant band intensity is small. This makes product identification extremely difficult. Photophysics. The general discussion of intramolecular energy transfer and competitive dissipative energy transfer in ref 2 also applies to these systems. Single-Photon Excitation. The photon flux densities in these IRL-SPARCSS experiments in which reaction occurred were 33 mW cm-2. In all of these systems, the observed bandwidths of the u3 band of SF, that imply lifetimes of the u = 1vibrationally excited state are of the order of 10-13-10-14s. Thus, to even populate the ( l , l , O ) state of the triply degenerate u = 1 level would require photon flux densities in the range of megawatts to gigawatts cm-2. Clearly, a multiphoton process is extremely improbable. Band Homogeneity. No band-homogeneityinformation is obtainable from the data on the NO-SF6 and N20-SF, systems since neither exhibited a photochemical response. The natural bandwidths of the CW C02laser lines used for irradiation sources in these experiments are -0.001 cm-l. Two lines resonant within the v3 band of SF, sources were used for study of the NO(N203)-SF6and NO(N2O)-SF6 systems. The contour of the u3 band examined as a function of irradiation time did not exhibit diminishment in intensity correlated with source frequency. Rather, the diminishments observed were independent of source frequency as long as the source was resonant with some part of the band. (The off-resonance experiment in NO(N203)-SF6had no photochemical response.) These data support a homogeneous band structure of the v3 band of SF6 in these two systems. Photophysical Modes of Reactant Band Diminishment. The photoirradiation of a sample can cause a temperature rise in the bulk sample (bulk photoannealing). This may cause annealing of the matrix to occur with concomitant shifting of guest bands. The extent of this effect depends upon the thermal history of the sample. Bulk annealing was performed on our matrix samples before irradiation to determine whether the concentration of guest molecules in the matrix was low enough to prevent dimer formation and to obtain quasi-equilibrium site distributions of the guest in the matrix. The annealing was carried out at a higher temperature than the sample would achieve during irradiation. Certainly no shifting of bands or band contours was observed in the NO-SF6 and N20-SF6 systems with irradiation of the v3 band of SF6. This was also the case in the off-resonance irradiation of NO(N203)-SF6. Since band shifting or contour changes were not evident, and since the samples had been thermally bulk annealed before irradiation, photobulk annealing is not a reasonable reactant band removal mechanism in these experiments. Local heating at the irradiated molecule site may cause a thermal sublimation of the irradiated guest molecule and thus a loss of band intensity. However, in IRL-SPARCSS experiments, the host molecules envelop the guest being irradiated at very low temperature (12 K). Since the vapor pressure is an exponential function of temperature, the rate of sublimation of the pure guest is proportional to the same exponential function. It is further reduced by having the effective sublimation area reduced by the host. Thus, the rate of sublimation in a well-executed IRL-SPARCSS experiment should be negligibly slow. The argument, based on the experiments, given for the photobulk annealing also applies to local site sublimation. A similar argument would hold for a strictly annealing phenomenon on a local site basis. Local site heating does not account for the spectral data for the NO(N203)-SF6 and NO(N2-
Infrared Laser Single Photon Absorption Reactions
O)-SF6 systems. Further, local site heating should be correlated with the absorptivity of the irradiated species. Since the contours of the v3 bands of SF6 isolated within pure NO, N0(N2O8),and NO(N20)matrices are the same, the lack of band contour change with irradiation in the NO-SF6 system reinforces the conclusion that local heating does not occur to a measurable extent. In addition to the selective evaporation due to local site heating, it is possible that a local temperature rise could produce thermal chemical reactions in the matrix. Such a process is more likely to occur under multiphoton conditions of high photon flux density. Under the singlephoton infrared irradiation conditions used in the IRLSPARCSS experiments, however, both the photon energy and the flux density are very low. This fact coupled with the small, but nonzero, thermal conductivity of the host precludes large loctd temperature rises to the degree which would be necessary to induce simple thermal reactions between reactants. It should be recalled that the temperatures required for the gas-phase thermal reaction would be of the order of 1000 "C. Another possible explanation for the reduction of reactant band intensity upon irradiation is photoorientation in which some degree of transient random motion of absorbing molecules in a rigid environment is produced. Should the molecules orient themselves with their absorbing axis out of view of the incident light, the net result would be a reduction of the observed band intensity. The theory of photoorientation processes has been reported by A l b r e ~ h t The . ~ ~ basic ~ ~ ~requirements are that the guest molecular species be optically anisotropic and that the photon irradiation source be intense, although not necessarily in the multiphoton absorption regime. For the isotropic guests, photoorientation is impossible. The effect should be most pronounced with polarized irradiating light, although polarization is not a necessity. The SF6 guest molecules are optically isotropic. Indeed, as of the review by Burdett and there were no known examples of a strictly physical photoorientation effect, although examples of induced orientation in optically anisotropic molecules produced in matrices by photolysis have been observed. We know no photophysical mechanisms that are reasonably probable in matrices for the growth of apparent product bands which are relatively far removed in frequency from the irradiated guest molecule band. Thus, for the NO(N203)-SF6and NO(N20)-SF6 systems, there appear to be no photophysical modes to account for the spectral data. Potential Energy Surfaces for Reaction. Thermal gas-phase reactionri between the nitrogen oxides and SF6 are reactions that have very large activation energy barriers. Gas-phase thermal reaction, whether carried out by means of multiphoton laser pyrolysis5 or by conventional heating techniques, is a random energy input process. Those reactions with very high activation energy barriers proceed by nonselective bond breaking followed by radical recombination. The products and their concentration distribution of thisi set of reactants reflect this process. The activation energy of the IRL-SPARCSS reactions proceeding via single-photon vibrational excitation must be equal to or less than the photon energy-for these experiments, 2.7 kcal mol-'. For the NO(N203)-SF6and NO(N20)-SF6 systems, the activation energy for the IRL-SPARCSS reaction is dramatically lower than that of the gas-phase reaction. Furthermore, the products of the IRL-SPARCSS experiments are different from those found for the gas-phase thermal r e a ~ t i o n .In~ these IRL-
The Journal of Physkal Chemistry, Vol. 84,
No. 13, 1980 1693
SPARCSS experiments, the effect of localization of energy in a single vibrational mode of SF6reactant, as well as the effect of fixing the relative configurations of the reactants by matrix isolation, was to drastically lower the activation energy for the reaction and also to produce different sets of products. Such dramatic changes can only be the result of a significant modification of the potential energy surface for reaction available to the reactants. As yet, some of the information regarding important parameters of the SPARCSS process is unavailable or unrecognized. The selectivity of the process must be related to modifications or restrictions in the potential energy surface, but the nature of these modifications is unknown. An additional complication is the effect of secondary guest molecules, e.g., Nz03,in the matrix upon the course of the reaction.
Conclusions The nitrogen oxides-SF, systems are examples of systems in which the SPARCSS reactions have activation energies that are dramatically lower than those for gasphase reactions and for which the products of the reaction are also different from those for the gas-phase reaction. The configurational aspects of the matrix modify the potential energy surfaces for reaction to allow new reaction paths to become accessible. The SPARCSS process enables selective excitation of systems via low energy and low photon flux density vibrational excitation in low-temperature matrices. In contrast to conventional multiphoton laser photochemistry,26SPARCSS offers the possibility of localization of the excitation energy in a limited number of energy states-in some cases, to a single energy state. As with any photochemical process, inter- and intramolecular relaxation processes are competitive with reaction. When viewed in toto, these experiments, along with those reported the matrix photochemistry of Fe(C0)4,18-21 and the isomerizations of HON0,22DON0,22 and a-N20312in matrices suggest the existence of a general matrix phenomenon. SPARCSS represents only one manifestation of a general matrix phenomenon. A detailed description of this matrix phenomenon would have to include both photoexcitation and configurational aspects which are manifested in all of these experiments. The general phenomenon should apply to either vibrational or electronic excitation, single-photon or multiphoton processes, and coherent or noncoherent excitation. But, we believe that single-photon processes (SPARCSS), particularly in the infrared, are those which will yield the maximum information regarding this hitherto unrecognized matrix phenomenon. A recent publication by Lucas and Pimente127is cogent to these conclusions. In the investigation of the reaction between NO and O3 in solid nitrogen, they report the observation of a thermal reaction having an activation energy of 106 cal mol-l between 11and 20 K. This reaction channel has not previously been observed in gas-phase thermal reactions which have activation energies of 2.3 or 4.2 kcal mol-', depending on whether ground-state or electronically excited NOz is formed. Unlike the SPARCSS reactions described herein, this matrix reaction is not induced by infrared radiation. However, the general matrix phenomenon, of which we propose that SPARCSS is but one manifestation, is applicable. We quote from Lucas and Pimentel: "Perhaps, more likely, the matrix environment may exert specific influence on the energetics of the activated complex. This influence could be connected with the dielectric environment,access to the matrix phonon modes,
J. Phys. Cbem. 1980, 84. 1694-1698
1894
and/or the absence of reactant and product rotational freedom. We are presently focusing attention on the last-named factor, which implies that the matrix acts by constraining the reactants and the activated complex dictated by the cage geometry." The seeds of such concepts of matrix effects on chemical reactions were proposed by PimentelB in 1958. We believe that the physical reality behind such concepts is strongly suggested by the results of the SPARCSS experiments. Acknowledgment. We particularly acknowledge the effects of the work of Dr. G. C. Pimentel on the development of our ideas. We thank Dr. M. J. Steindler for his continued strong encouragement, Dr. G. C. Pimentel for his very helpful discussion on the matrix spectroscopy of nitrogen oxides, and Dr. R. N. Zare for his comments. Thanks also go to Dr. L. W. Hrubesh and Mr. D. C. Johnson for the construction of the CO, laser. Finally, we especially thank Dr. B. B. Saunders, as well as Dr. J. R. Kolb, for their comments on the manuscript. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore Laboratory under contract No. W-7405-Eng-48and by the Argonne National Laboratory under contract No. W-31-109-Eng-38. References and Notes (1) E. Catalano and R. E. Barletta, J . Chem. Phys., 88, 4706 (1977). (2) E. Catalano and R. E. Barletta, J . Chem. Phys., 70, 3291 (1979). (3) K. Jones in "Comprehensive Inorganic Chemistry", Vol. 2, J. C. Bailar et al., Eds., Pergamon Press, Oxford, 1973, p 147ff.
E. Catalano and I?. E. Barletta, unpublished work. N. G. Basov, E. P. Markln, A. N. Oraevskii, and A. V. Pankratov, Sov. Phys. Dokl. (Engl. Trans/.), 18, 445 (1971). A. L. Smith, W. E. Keller, and H. L. Johnston, J . Cbem. Phys., 19, 189 (1951). W. 0 . Fateley, H. A. Bent, and B. Crawford, J . Cbem. Pbys., 31, 204 (1959). D. A. Dows, J . Cbem. Pbys., 28, 745 (1957). I. C. Hisatsume, J. P. Devlln, and Y. Wada, J. Chem. phvs., 33, 714 (1960). I. C. Hisatsume and J. P. Devlln, Speckochlm. Acta, 18, 401 (1960). J. P. Devlin and I. C. Hisatsume, Spectrocbim. Acta, 17, 218 (1960). E. L. Varetti and G. C. Pimentel, J . Chem. Pbys., 55, 3813 (1972). R. E. Barletta, Ph.D. Thesis, Brown University, Providence, RI, June 1977. A. J. Tursi and E. R. Nixon, J . Cbem. Pbys., 52, 1521 (1970). G. Ritzhaupt, N. Smyrl, and J. P. Devlin, J . Cbem. Pbys., 84, 435 (1976). L. Abouaf-Marguin, H. Dubost, and F. Legay, Chem. Phys. Lett., 7 , 61 (1970). L. Abouaf-Marguln and M. Dubost, Chem. Phys. Lett., 15,445 (1972). A. McNelsh, M. Poliakoff, K. P. Smith, and J. J. Turner, J . Chem. Soc., Cbem. Commun., 859 (1976). M. Pollakoff, B. Davies, A. McNeish, and J. J. Turner, Conference "Lasers in Chemistry", London, May 1977; also, "Conference on Matrix Isolation", Berlin, June 1977. B. Davies, A. McNeish, and M. Poliakoff, J . Am. Cbem. Soc., 99, 7573 (1977). B. Davles, A. McNeish, M. Poliakoff, M. Tranquille, and J. J. Turner, J . Chem. Soc., Chem. Commun., 36 (1978). R. T. Hall and 0. C. Pimentel, J . Chem. Pbys., 38, 1889 (1963). A. C. Albrecht, J . Chem. Pbys., 27, 1413 (1957). A. C. Albrecht, J . Mol. Specfrosc., 8, 84 (1961). J. K. Burdett and J. J. Turner in "Cryochemlstry", M. Moskovits and G. A. Ozln, Eds., Wiley, New York, 1976, p 493. N. Bloembergen and E. Yablonvltch, Pbys. Today, 31, 23 (1978). D. Lucas and G. C. Pimentel, J. Phys. Chem., 83, 2311 (1979). G. C. Pimentel, J . Am. Cbem. Soc., 80, 62 (1958).
Solubilities of Fifteen Solvents in Copolymers of Poly(viny1 acetate) and Poly(vlnyl chloride) from Gas-Liquid Chromatography. Estimation of Polymer Solubility Parameters Werner Merk, R. N. Lichtenthaler, and J. M. Prausnltz" Chemical Engineering Department, University of Calfornia, Berkeley, California 94720 (Received November 19, 1979)
Solubilities of 15 polar and nonpolar solvents were measured at 125 and 140 "C using a standard gas-liquid chromatograph. Solubilities were obtained in the two homopolymers and in copolymers containing 3,10, and 17 wt % poly(viny1acetate). Flory x parameters were calculated for all binary systems. Special care in data reduction was required for acetic acid because of extensive vapor-phase dimerization. The Flory x parameters yield solubility parameters for the polymers; these are only weak functions of polymer composition. A t 125 " C , the solubility parameter of poly(viny1 acetate) is 17.9; that of poly(viny1 chloride) is 16.2 ( J / C ~ ~ ) ' / ~ .
Gas-liquid chromatography provides a useful technique for measuring solubilities of volatile solvents in p~lymers.l-~ Such data are of practical importance because polymeric products, especially films, are often used for packaging of food and other consumer items. Therefore, solvents which often are toxic must be removed from the polymer prior to use. In recent years, solubilities have been reported for solvents in a variety of homopolymers.4-s However, only a few studies report solubilities in copolymer^.^^^ Moreover, only few data are available for solubilities of polar solvents in polymers14J5 although these are often of practical interest. This work reports solubilities for 15 nonpolar and polar solvents in poly(viny1 acetate) (PVA),
poly(viny1chloride) (PVC), and their random copolymers, containing 3, 10, and 17 wt % poly(viny1 acetate). Experimental Method The experimental apparatus is shown in Figure 1; it is essentially the same as that used previou~ly.'~~'~ The column is packed with a polymer-coated support (Chromosorb P, AW-DMCS, 30/60 mesh size or Fluoropak 80, 40/60 mesh size). A small amount of solvent (0.02 pL) and a small amount of air (1.0 pL) are injected with a l.0-pL syringe. The solvent is vaporized in the injection block. Air and vaporized solvent are swept through the column by the carrier gas, helium. Air is essentially insoluble in the polymer and passes through the column with no re-
0022-3654/80/2084-1694$01.00100 1980 American Chemical Society
