J. Phys. Chem. 1985, 89, 4484-4488
4484
Proximity Effects in Low-Temperature Matrices John R. Sodeau* and Robert Withnallt School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, England (Received: April 17, 1985)
Matrix isolation and FTIR techniques have been used to study differential photolysis effects of I4N2l6Oand 15N’4N160 monomers, dimers, and aggregates in argon matrices at 4.2 K. The results are related to recent studies in molecular beams in which the structures of N 2 0 dimers were discussed and flash photolysis experiments in which the intermediacy of (ONNO)+complexes in nitrous oxide photochemistry were postulated. The matrix observations are interpreted on the basis of a proximity effect between neighboring molecules. This results in a greater extinction coefficient for dimeric species compared to that for monomers and leads to a differential rate of photoreactivity
Introduction The assignment of infrared absorptions to isolated molecules, “dimeric species”, and simple polymeric aggregates has been carried out for low-temperature matrix spectra since the earliest reports of the technique.’ In more recent investigations, a fuller understanding of the interaction between guest and host species has been obtained using FTIR spectroscopy and subsequent analysis of line shifts, shapes, and intensities.2 Data of this type are necessary to help answer two recurrent questions which arise in low-temperature matrix studies: (i) Are the guest species under investigation isolated? (To what extent has aggregation occurred?) (ii) How are the guest species perturbed by the host matrix environment? (To what degree are site effects present?) Complete studies of a matrix-isolated molecqle therefore include dilution or annealing experiments to provide information on aggregation and systematic variation of diluent gas-host material to indicate the effect of sites on observed spectra. In situ photoproduction studies of radicals and other reactive species in low-temperature matrices have been pursued by numerous researchers for many years, and it has long been recognized that in rigid matrices the cage effect is of prime importance in determining the efficiency of a particular ph~toprocess.~However, the capacity of weak complex formation to influence observed photochemistry of molecules has only recently been recognized. Examples of such reaction include the production of methanol4 from the low-temperature photolysis of dimeric formaldehyde
and the formation of nitrous oxide5 from X of cis-(N0)2 at 13-20 K: cis-ONNO
< 320 nm photolysis
-
N20+ 0
The present research was initiated with the aim of making a systematic survey of the effect of neighboring molecules on photochemical reactions in matrices at 4 K using FTIR spectroscopy. This report is concerned with differential photolysis effects observed for I4N2Oand lsN14N0monomers and dimers in argon matrices. Nitrous oxide was chosen in this study for the following reasons: (1) The mechanism of its gas-phase photochemistry is well established6 and potentially offers two distinctive photoreactions for monomers and dimers of 15N14N160 in an argon matrix: the former giving 14N’5N’60and the latter giving 14015N14N’60.(2) Infrared spectra of matrix-isolated N20,’,* NO, and (NO)29*’0 have been reported. Indeed the reaction of O(lD) atoms (from the photolysis of ozone) with N 2 0 at 15-20 K has been studied in some detail by both DeMore” and Guillory.lz However, the photochemistry of dimeric NzO at 184.9 nm in a matrix has not been reported before although a reference to unpublished results has been made.I3 ‘Present address: Chemistry Department, University of Virginia, Charlottesville. VA 22901.
In this paper, consideration is first given to the assignment of FTIR spectra to isolated, dimeric, and aggregated nitrous oxide species. The observed photochemical reactions are then interpreted in relation to gas-phase results, and finally the proximity effect of neighboring molecules on forbidden transitions in a matrix is discussed. Experimental Section A Heliplex Model CS-308 closed-cycle helium system (Air Products and Chemicals Inc.) was used for refrigeration. The expander module produces a heat capacity of 1.8 W at 4.2 K, the normal operating temperature for the reported experiments. An Air Products APD-H controller was used for temperature measurement above 5 K; below this value a helium vapor bulb pressure gauge was used. The deposition cold window was a cesium iodide disk seated on indium gaskets and enclosed within a vacuum shroud. Infrared radiation was transmitted through potassium bromide outer windows of the shroud and vacuum-UV transmitted through a Spectrosil-B window. Matrices were deposited on the cold window via the pulsed technique utilizing three solenoid valves (Asco Red Hat, 8262C90VH). The system was evacuated with an Edwards Diffstak (Model 100MLN2) backed by a two-stage rotary pump (Edwards E2M8). A Digilab FTS.20V Fourier transform infrared spectrometer was employed for the measurement of matrix spectra using a mercury-cadmium telluride detector cooled to liquid nitrogen temperature. A Perkin-Elmer globar was used as an external infrared source. It was housed with the cryogenic vacuum shroud plus associated optics in an airtight Perspex box, which was linked to the emission port of the FTS.2OV spectrometer. Unwanted interferences from absorption due to atmospheric water vapor and carbon dioxide were minimized by three procedures: (i) evacuating the main chamber and sample chamber of the spectrometer; (ii) flushing the Perspex box with oxygen-free nitrogen; (iii) taking a background spectrum of the detector profile and cesium iodide cold window at base temperature before deposition of the matrix, (1) M. van Thiel, E. D. Becker, and G. C. Pimentel, J . Chem. Phys., 27, 95 (1957). (2) (a) L. H. Jones, B. I. Swanson, and H. A. Fry, Chem. Phys. Left.,87, 397 (1982); (b) L. H. Jones and B. I. Swanson, J . Chem. Phys. 76, 1634 (1982). (3) D. E. Milligan and M. E. Jacox, Adu. High Temp. Chem., 4, 1 (1971). (4) M. Diem and E. K. C. Lee, Chem. Phys., 41, 373 (1979). (5) M. Hawkins and A. J. Downs, J . Phys. Chem., 88, 1527 (1984). (6) H. Okabe, “Photochemistry of Small Molecules”, Wiley-Interscience, New York, 1978, p 219. (7) L. Andrews and G. L. Johnson, J . Chem. Phys., 76, 2875 (1982). (8) D. Foss Smith Jr., J. Overend, R. C. Spiker, and L. Andrews, Spectrochim. Acta, Part A , D A , 87 (1972). (9) W. G . Fateley, H. A. Bent, and B. Crawford Jr., J . Chem. Phys., 31, 204 (1959). (10) S . C. Bhatia and J. H. Hall Jr., J. Phys. Chem., 84, 3255 (1980). (11) W. B. DeMore and N. Davidson, J . Am. Chem. SOC.,81, 5869 (1959). (12) W. A. Guillory and C. E. Hunter, J. Chem. Phys., 50, 3516 (1980). (13) D. E. Milligan and M. E. Jacox, J . Chem. Phys., 55, 3404 (1971).
0022-3654/85/2089-4484%01 SO10 0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4485
Proximity Effects in Low-Temperature Matrices TABLE I: Vibrational Frequencies (cm-') for Isotopic Forms of N20 vib state gas phase Ar matrixo N2 matrix" 14Nl4N I 6 0
2000 0200 11'0 1001 01'1 1200
1284.907 588.767 2223.756 2563.341 1168.134 1880.265 3480.821 2798.290 2461.998
1 000 01'0 0001 2000 0200 1110 1001 01'1 1200
1269.894 585.312 2201.605 2534.533 1159.974 1862.766 3443.652 2712.703 2439.625
1 000 000 1
1280.357 2117.658
1 000 000 1
1265.334 2154.726
1000 0001
1246.885 2216.71 1
1000 0001
1232.854c 2194.045
1 000 01'0 0001
1282.9 (1283) 589.4, 588.7 d (588) 2218.6 (2221) 2559.4 (2559) 1167.6 1879.1, 1878.2 d 3473.8 (3480) 2793.9, 2793.1 d 2459.6
1291.3 589.1 2235.9 2567.0 1166.5 1885.9 3499.9 2809.2 2466.9
1
I5N l4N 1 6 0 1268.1 586.2, 585.3 d 2196.4 2531.0 1159.7 1862.1, 1860.9 d 3436.8 2768.6, 2167.8 d 2437.7
1276.3 585.7 2213.5
1287.1 2189.4
15Nl5Nl60 b 2149.8
1271.8 2166.2
14Nl4Nl80 1245.0 221 1.6
1 , 22279
0
4 0 0 22223 22167 WAVENUMBERS
0 1 22111
0
2240
2220
2200
2180
WAVENUMBERS
Figure 1. Transmittance plots of I4N2l6Oin argon at 4.2 K: (a) MR 1:lOOOO; (b) MR 1:500.
14N15N160 b 2172.6
5
1252.6 2228.9
15Ni4N 1 8 0 1231.3 2189.1
"These are the values of this work. Values in parentheses are from Andrews and Johnson (d denotes doublet). bThe absorptions due to ul of 14N15N'60 and 15N15N160 were obscured by dimer absorptions due and (15N'4N'60)2, respectively. Calculated to uI of (14N14N160)2 value, taken from ref 33. dReference 8.
which was then stored and ratioed against sample spectra. All reported spectra were taken at 0.2-cm-' resolution, boxcar apodized, and computed from the coaddition of 1000 inteferograms. Matrix mixtures were made up on a mercury-free vacuum line fitted with greaseless taps. Guest-host matrix ratios (MR) were calculated from pressure measurements obtained on an MKS baratron unit (Model 310) and a Wallace-Tieman gauge (Model FA 14 1 ). A low-pressure mercury discharge arc (Phillips 93 109E) was used for matrix photolysis at 184.9 nm. The lamp housing was positioned against the Spectrosil-B window and flushed with oxygen-free nitrogen. Argon of stated purity 99.999% was purchased from Messer. Griesheim (GmbH). I4N2I6Owas supplied by BDH with 99.997% stated purity: it was degassed at 77 K and used without further purification. The mass spectra of both gases were taken on a VG gas analysis quadrupole mass spectrometer (Model SX300) and revealed no peaks due to impurities. 15N'4N'60with 96%stated atom purity was used without further purification: its mass spectrum indicated the presence of ca. 5% 14N'4N'80. 15N2160 and i5N14N180 were also present as revealed by FTIR spectra a t M R 1:500. of '5N14N'60:Ar Results The infrared absorption frequencies of 14N2160and 15NL4N160 in argon matrices a t 4.2 K with M R 1:lOOOO and 1:500 were recorded a t a spectral resolution of 0.2 cm-I. Vibrational assignments have been made in accordance with previous matrix results reported for nitrous oxide in argon7 at 12 K (MR 1:300) with 1-cm-' resolution and in nitrogens at 12-15 K (MR = 1:100/200) with 0.9-1.5-cm-' resolution. These data are shown in Table I and compared with the gas-phase values of Foss Smith
et a1.* 1sN14N'60,14N1SN'60, and 14N'4N1s0were present in the 14N14N160 sample in their natural isotopic abundances (0.36% 15N,0.20% lSO). Also, 15N'5N160 and 15N14N180 were present in the 'sN14N'60 sample in trace amounts as well as 4-5% 14N'4N160.Hence, the [00°1] vibrational state for 14N'5N'60 along with the [10°0] and [00°1] states for 14N'4N's0 and 'sN14N'80matrix isolated in argon has been measured for the first time in this work and is reported in Table I. A concentration study was conducted in order to attempt to distinguish absorptions due to monomer from absorptions due to dimer and higher multimer. The chosen range of matrix ratios for nitrous oxide in argon was 150, 1:500, and 1:lOOOO. At M R 1:lOOOO monomers are predominantly formed, absorptions at 2218.6 and 1282.9 cm-' being assigned to v 3 and v 1 of matrix-isolated I4N2l6O,respectively. Absorptions at 2214.3 and 2213.1 cm-I, which have been assigned to the antisymmetric v3 stretch of (N,O), dimers from the higher concentration results reported below, were present even at this dilution although they were very weak. Additionally absorptions at 2221.9 and 2220.7 cm-', which are blue-shifted from the monomer peak, may also be due to (N,O), dimer, since they have approximately the same magnitude of intensity and line width as the dimer absorptions at 2214.3 and 2213.1 cm-I. This would be analogous to the case of 15N'4N'60where photolysis indicated that ('sN'4N'60)2dimers have absorptions both to high and low frequency of v3 for 15N14N'60 monomer, as will be discussed. There were other weak absorptions to higher frequency centered on 2224.6, 2226.3, and 2227.8 cm-l. These are assigned to multimers and N,O perturbed by long-range interactions from N 2 0 . A shift to the blue has been reported by Miller et al.14 for the stretching frequencies of N 2 0 on cluster formation in the vapor phase for vibrational predissociation experiments in molecular beams. They suggested that the shift to the blue for both symmetric and antisymmetric stretches of NzO is due to a decrease in the dipole moment of the molecule when it undergoes the stretch, since the intermolecular forces would become more repulsive. At MR 1:500 absorptions assigned to dimeric species are more intense than for M R 1:lOOOO as can be seen in Figure 1, which shows transmittance plots in the v3 region for 14N,160. The dimeric features are sharp (AijlI2 < 0.5 cm-I), which indicates a precise structure for (N20)2: potential candidates for such locally stable van der Waals complexes have been discussed and include crossed configurations, offset T-shaped structures, or geometries in which molecular axes are parallel with staggered centers.I4 At MR 1:50 the v3 absorption of I4N2l6Ois broad and extends up to ca. 2240 cm-'. DowsIS has reported v 3 of solid N 2 0 to be centered on 2238 cm-' so it is likely at this low dilution that the environment of some NzO molecules is similar to a solid aggregation. Intense absorptions at 2215.5, 2214.3, and 2213.1 cm-I lie to lower frequency than the u3 fundamental of monomeric (14) R. E. Miller, R. 0.Watts, and A. Ding, Chem. Phys., 83,155 (1984). (15) D. A. Dows, J . Chem. Phys., 26, 745 (1957).
4486 The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 TABLE 11: u, and vib state 1000 0001
1000 0001 95
85
uj
\f
Absorption Frequencies of (N20)2Dimers dimer abs frea/cm-' ('4N2160)2 1280.7, 1278.8, 1277.7 2221.9, 2220.7, 2215.5, 2214.3, 2213.1 ( I 5N I4Nl60), 1265.1, 1264.1, 1263.1 2200.3, 2193.4, 2192.4, 2191.4
N
e
40
,
,
19OC
1875
1850
1825
1803 1775 WAVENUMBERS
'900
1875
18%
1825
1800 1775 WAVENUMBERS
I
1
1750
1725
1700
1725
1700
1750
Figure 2. Product absorptions on photolysis at 184.9 nm: (a) I4N2l6O in argon, MR 1:50;(b) lsNI4Ni60in argon, MR 1:500.
I4N2l6Oand have been assigned to v3 of the corresponding dimer due to their concentration-dependent behavior. Similarly, absorptions at 1280.7, 1278.8, and 1277.7 cm-' lie to lower frequency than the ul fundamental of monomeric I4N2l6Oand have been assigned to v1 of dimer species. Dimers are also produced in '5N'4N160/Armatrices, and measured v 1 and u3 absorption frequencies for these species are summarized in Table I1 along with associated data for ('4N2'60)2. The infrared spectra in 1700-1800-~m-~region after 3.00-h photolysis of I4N2l60/Ar (MR 1:50) and 16.00-h photolysis of l5NI4Nl60/Ar(MR 1500) at 184.9 nm are shown in Figure 2, a and b, respectively. In Figure 2a, product peaks are centered on 1863.6 and 1775.9 cm-I, and the doublet at 1879.1 and 1878.2 cm-' is assigned to the u1 + v21 combination of I4N2l6O(see Table I). In Figure 2b, product peaks are centered on 1849.7 and 1757.5 cm-', and the doublet at 1862.1 and 1860.9 cm-I is assigned to the v l v2I combination of parent 15N14N'60.Miscanceled water vapor lines are marked with an asterisk. In the 16.00-h photolysis experiments two weak but broad absorption peaks centered on ca. 2235 and 1290 cm-', respectively, were recorded. These are assigned to the formation of I4N2Oas a secondary photolytic product of surface films of air (even through the cryostat shroud mbar). Hence, the photolysis was routinely evacuable to 5.0 X of solid oxygen at 184.9 nm results in the formation of ozone which may be subsequently photolyzed to produce an O('D) atom. This can attack the molecular nitrogen component of the surface film and form nitrous oxide in its characteristic aggregate environment at low temperature. On account of this problem the effect of photolysis on the u3 absorption bands of (14N2160)2 to higher frequency than the monomer absorption at 2218.6 cm-' could not be determined. However, this complication did not arise for the photolysis of l5NI4Nl6Osince the v3 absorption bands of 15Ni4Ni60 and I4N2l6Odo not overlap: these experiments showed that a differential photolysis rate for dimers vs. monomers was measurable. After 3.00- and 16.00-h photolysis at 184.9 nm of a 15N14N160:Ar matrix with MR 1:500 an absorbance plot of the u3 absorptions of dimeric (15N'4Ni60)2 showed that bands at 2192.4, 2191.4, 2193.4, and 2200.3 cm-I had decreased at approximately twice the rate of the monomer v3 absorption of l5N2I6O at 2149.8 cm-' and '5N14N'80at 2189.1 cm-]. Furthermore, absorptions assigned to multimeric i5N14N'60were little affected by the photolysis. A new feature a t 2172.6-cm-' wavenumbers was observed and assigned to I4Nl5Ni60,but no band assignable
+
m
Ell E
A
t
a
!
Sodeau and Withnall
00 ! 2195
I
2187
I
I
I
2178 2170 2162 WAVENUMBERS
I
2153
1 2145
Figure 3. Absorbance plots of 2195-2145-cm-I region of 15N'4N'60 in argon (MR 1:500): (a) after 16.00-h photolysis at 184.9 nm; (b) after 3.00-h photolysis at 184.9 nm; (c) before photolysis.
to l4NI5Nl8Owas produced. These results are summarized in Figure 3.
Discussion The Infrared Spectrum of Nitrous Oxide in an Argon Matrix at 4.2 K . Some infrared absorptions of I4N2l6Oin an argon matrix at 12 K have been reported by Andrews and J ~ h n s o n .They ~ used a spectral resolution of 1 cm-I, and their values which are listed in Table I show good agreement with the results obtained in this study at 4.2 K. It should be noted that the higher resolution used in the present experiments has shown that a doublet at 589.4 and 588.7 cm-I is observed and is due to a lifting of the degeneracy of the u21 vibration. Therefore, the v 1 + ut' and v2' + v 3 combinations are also doublets. Similarly the doublet at 586.2 and 585.3 cm-' in the spectrum of 1sN14N'60represents a split in the two fold degeneracy of the vzl vibration of 15N14N'60.It follows that the u1 + v21 and v2' + vj combinations of 15N14N'60 are also doublets since their twofold degeneracies are lifted. The 2uZ0overtone of both l4NZl6O and 15N'4N'60has a single absorption since it is nondegenerate. The 2v2 overtone would be expected to appear as a doublet since it has a twofold degeneracy which would be removed, but it is too weak to be observed. Fermi resonance of the [02OO] and [ 1001 vibrational states is responsible for the strong intensity of the 2v20 overtone compared to the 2u22 overtone. There is no Fermi resonance of the [0220] and [ 1001 vibrational states since they have different symmetry. It is suggested that the lifting of the twofold degeneracy, described above, is due to a lack of spherical symmetry of the argon matrix sites in which nitrous oxide is trapped. N 2 0 has a linear structure with a nitrogen-nitrogen bond length of 1.128 8, and a nitrogen-oxygen bond length of 1.184 Therefore, it might be too small to occupy, with any stability, a substitutional 0, symmetry site which has a diameter of 3.75 8, in solid argon.I7 A lifting of the degeneracies of a vibrational band by matrix sites of low symmetry has been reported before, for example the v3 band of SF6 in solid argon.2 (16) J. Laane and J. R. Ohlsen, Prog. Inorg. Chem., 27, 465 (1980). (17) M.T. Bowers, G . I. Kerley, and W. H. Flygare, J . Chem. Phys., 45, 3399 (1966).
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4487
Proximity Effects in Low-Temperature Matrices The infrared absorptions of i5N14N160,''N15N160, 15N15N'60, 14N14N180, and 15N14N1s0have not been reported in an Ar matrix before. The [00°1] vibrational state is red-shifted by 4-5 cm-I in each case from corresponding gas-phase values. The [ 10°0] states of 14N15N'60and I5Nl5Nl6Oare obscured by dimer aband (15N14N'60)2,respectively, sorptions due to v 1 of (14N14N160)2 in this study. It can be seen from Table I that the v3 antisymmetric stretch of N 2 0 displays an upward shift of ca. 17 cm-' between argon and nitrogen matrices. The v3 vibrational absorption of N 2 0 in solid argon is closer to the gas-phase value than it is in solid nitrogen which suggests that there is a stronger guest-host matrix interaction in nitrogen. The Photochemistry of Nitrous Oxide in an Argon Matrix at 4.2 K . The gas-phase mechanism of photolysis for nitrous oxide is relatively well established and is one of the most commonly used actinometers in the vacuum-UV. Excitation of N 2 0 at 184.9 nm into its first excited IB2(lA) level is believed to produce O('D) atoms with a quantum yield of unity via the reaction'* N,O('A)
+
N,('2,+)
+ O('D)
(3)
The O(lD) atoms thereby produced react with N 2 0via reaction 4a or 4b.
O('D)
+ NzO(lZ,+)
-
+ + NO(,II)
NZ(lZg+) 0,(singlet)
(4a)
NO(,II)
(4b)
In a static system NO, is formed, probably exclusively via the slow, termolecular reaction of nitric oxide with oxygen:
NO
+ NO + 0 2
+
NO2 + NO2
(5)
The quantum yield of the N, photoproduct at 184.9 nm, @N2184'9, is 1.44 f 0.1 l,I9 but perhaps due to differing methods of analysis, between the reported literature values there is poor agreement20*21 for Nevertheless, the energetics of the product N O molecules of reaction 4b have been analyzed by kinetic spectroscopy22and laser ionization s p e c t r o ~ c o p y where , ~ ~ it was observed that the vibrational distribution of the N O molecules is quite broad. Since no single level appeared to dominate, it was suggested that an (ONNO)+ complex acted as an intermediate in the photochemistry. The findings of the present study for the photolysis of nitrous oxide in an argon matrix at 4 K are in agreement with this conclusion. In Figure 2 product peaks are shown to be observed at 1863.6 and 1775.9 cm-' for l4N2I6Oin Ar and at 1849.7 and 1757.5 cm-' for 15N14N160 in Ar. It is concluded that the nitric oxide dimer is symmetric because two forms of O=N-O=N would result from the reaction of 15N14N160 with O('D). Ab initio calculations indicate that the nitric oxide dimer has two preferred conformations: planar cis and planar t r a m z 4 In the trans form the symmetric stretch of the nitrogen-oxygen bonds would be infrared inactive because it would involve no change of dipole moment in the molecule. It would be infrared active in the cis form, however. The two absorptions in the nitrogen-oxygen stretching region indicate that both the symmetric and antisymmetric nitrogenoxygen stretching vibrations are infrared active. Therefore, the absorptions at 1863.6 and 1775.9 cm-l in Figure 2a can be assigned to cis-planar 16014N'4N160 dimer. Likewise, the absorptions at 1849.7 and 1757.5 cm-' in Figure 2b can be assigned to cis-planar (18) CODATA Task Group on Chemical Kinetics: D. L. Baulch, R. A. Cox, P. J. Crutzen, R. F. Hampson Jr., J. A. Kerr, J. Troe, and R. T. Watson, J. Phys. Chem. Ref. Data, 11, 327 (1982). (19) M. Zelikoff and L. M. Aschenbrand, J. Chem. Phys., 22, 1680 (1954). (20) (a) G. A. Castellion and W. A. Noyes Jr., J. Am. Chem. SOC.,79, 290 (1957); (b) N. R. Greiner, J . Chem. Phys., 47, 4373 (1967). (21) R. Simonaitis and J. Heicklen, J . Phys. Chem., 80, 1 (1976). (22) G. A. Chamberlain and J. P.Simons, J. Chem. Soc., Faraday Trans. 2, 71, 402 (1975). (23) N. Goldstein, G. D. Greenblatt, and. J. R. Wiesenfeld, Chem. Phys. Lett., 96, 410 (1983). (24) S . Skaarup, P. N. Skanke, and J. E. Boggs, J . Am. Chem. SOC.,98, 6106 (1976).
TABLE 111: Observed Infrared Absorption Frequencies of Cis-Planar 16014N'4N160 in Argon v(N-o)/cm-' sym
method of preparation of cis-planar
antisym
ref
16014Nl4N160
irradiation of an argon matrix containing I4N2l6Oat 184.9 nm 1866 1776 deposition of gas mixtures of I4Nl6O in argon on a cesium iodide window at liquid helium temperature 1868.75 1778.12 deposition of gas mixtures of I4Nl6O (and 0,) in argon on a cesium iodide window at 10 K 1868 1778 irradiation of an argon matrix containing l4N2I6Oand O3 using a medium-pressure mercury discharge arc 1865 1777 photolysis of an argon matrix containing l4N2I6O cesium or sodium atoms using a medium-pressure mercury arc
1863.6
1775.9
+
this work 9 10 12
13
16015N'4N160 dimer. Furthermore, Raman studiesz5have shown that the nitrogen-oxygen vibration which has the higher infrared frequency is due to the symmetric stretch, because its absorption line is polarized. The frequencies of the infrared absorptions of cis-planar 16014N14N160 and their assignments are listed in Table I11 along with the observed frequencies of other reports. There was no evidence for the production of any other forms of nitric oxide dimer in this work. Other research groups have reported evidence for a trans-planar form of nitric oxide in nitrogen', and in carbon dioxideg exhibiting a single infrared-active stretching frequency near 1764 and 1740 cm-', respectively. Fately et aL9 have suggested the existence of trans dimer in argon, but its exact frequency of absorption was not given. Smith and GuilloryZ6found no evidence for the formation of trans dimer in oxygen matrices. The method of preparation of the nitric oxide dimer probably determines whether or not the trans form is produced in observable quantities. In this study when O(lD) atoms react with N,O, the transition state of minimum potential energy might be expected to yield exclusively the cis dimer. Indeed transition state I (TSI) below would be expected to have a lower potential energy than transition state I1 (TSII) because it could be stabilized by electron orbital overlap of the oxygen atoms.
TSII
trans
The structure of the gas-phase nitric oxide dimer has been determined recently from microwave studiesz7 It was reported, in agreement with this report, that the molecule is in a cis-planar structure with a bond between the nitrogen atoms. The cis-planar O N N O dimer is formed via the following reactions in the argon matrix. X = 184.9 nm N 2 0 + hv Nz + O('D) O('D)
+ N,O
-
ONNO
(7) Thus, the dimer is produced in the same cage as a molecule of nitrogen. The perturbing influence of N, on the O N N O dimer ~
(25) A. L. Smith, W. E. Keller, and H. L. Johnston, J. Chem. Phys., 19, 189 (1951). (26) G . R. Smith and W. A. Guillory, J . Mol. Spectrosc., 68,223 (1977). (27) S . G. Kukolich, J . Mol. Spectrosc., 98, 80 (1983).
4488
Sodeau and Withnall
The Journal of Physical Chemistry, Vol. 89, No. 21, 198 5
could explain the slightly lower absorption frequencies compared to those observed for the species being prepared in other ways (Table 111). Proximity Effects in Low-Temperature Matrices. One effect of neighboring molecules on observed photochemistry in lowtemperature matrices has long been recognized: reactive intermediate recombination or reaction promoted by the matrix "cage". In fact, for rigid matrices the cage effect is of prime importance in determining the efficiency of a particular photoprocess. A much less well-recognized and understood effect is the capacity of weak complex formation to influence observed photochemistry in a matrix. The present study shows that a differential photoactivity exists between monomers and dimers of nitrous oxide in an argon matrix at 4.2 K. The absorbance plots shown in Figure 3 for a 1sN14N'60/Ar matrix at MR 1:500 provide evidence for the preferential photolysis of dimeric nitrous oxide over monomeric species. After both 3.00and 16.00-h photolysis of the matrix at 184.9 nm, absorptions assigned to u3 of dimers had decreased at twice the rate of absorptions assigned to u3 of monomers. Furthermore, no photowas observed. isomerization of 14N15N1s0 The results can be explained, in part, by the temperature dependence of the electronic absorption spectrum of nitrous oxide measured by Selwyn et aL2* At 302 K the cross section, u, for N 2 0at 185 nm is 1.43 X 1O-I' cm2 while at 194 K u takes a value of 1.22 X cm2. The observed decrease in absorption cross section is interpreted on the basis that the 'A X'Z' transition ftir nitrous oxide is orbitally forbidden but vibronically allowed by incorporation of the u2 bending mode at 589 cm-1.29 Hence at 296 K, 5.7% (ca. exp[-(589/206)]) of the N 2 0 molecules are excited to the [OlO] vibrational state. This fraction is reduced to 1.3% at 194 K and effectively to zero at 4.2 K. The effect of temperature on u was then explained by assuming that the excited bending states of nitrous oxide have much larger Franck-Condon overlap with the upper state than does the [OOO] mode of the ground-state molecule. This is a reasonable assumption in view of the established bent geometry of the lA excited state of N20. Therefore, 4.2 K matrix photochemistry of monomeric nitrous oxide might be expected to be absent. At M R 1:500 dimers and "monomersn were in fact photolyzed at 184.9 nm, albeit with different efficiencies. The observation of a faster rate of photochemical reaction for dimers may be explained in terms of an effect similar to that observed over 30 years ago in infrared studies on compressed gases3" Here a
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(28) G. Selwyn, J. Podolske, and H . S. Johnston, Geophys. Res. Lett., 4, 427 (1977). (29) K. M. Monahan and W. C. Walker, J . Chem. Phys., 63, 1676 (1975).
collision-induced interaction between two atoms or molecules can result in one-photon excitation of a collision partner because a transient dipole moment is induced during an encounter between partner moel~ules.~'More recently the principles of quantum electrodynamics have been used to derive expressions for such cooperative absorptions involving transitions in the UV region, which are forbidden in the absence of any pairwise i n t e r a ~ t i o n . ~ ~ The results were applied to van der Waals molecules and any other system involving two independently oriented molecules including matrix-isolated species. It is suggested that the photochemical results presented in this paper depend on the formation of weak van der Waals complexes (as either dimers or "monomers" perturbed by longer range interactions with other guest species in relatively high concentration matrices, e.g. MR 1:50 or 1:500). Thus, in a matrix, interaction between neighboring nitrous oxide molecules results in a greater extinction coefficient for dimers than for monomers leading to differential photolysis rates: we term this a proximity effect. The primary photochemical process is the same in both cases.
Conclusion Matrix isolation and FTIR techniques have been used to study differential photolysis effects of I4N2l6Oand ISNI4NI6Omonomers, dimers, and aggregates in argon matrices at 4.2 K. The results have been related to recent theoretical studies in cooperative one-photon excitation. Weak complex formation has been shown to modify observed photochemistry in other matrix experiments, and the findings of this study may be of direct relevance to recent work on f ~ r m a l d e h y d e . ~ The relatively high resolution FTIR spectra obtained in this work have shown that the [OlO] vibrational state of N 2 0 in argon is a doublet: the lifting of this degeneracy is interpreted on the basis of a site effect. Acknowledgment. We thank the SERC for a grant to enable purchase of equipment, the University of East Anglia for a Studentship (R.W.), Dr. David Andrews for helpful discussions, and John Crowley for some experimental assistance. Registry No.
I4N2I6O, 10024-97-2; 15N'4N160, 20509-24-4;
'5N15Ni60, 20621-02-7; I4Ni4Ni8O, 21296-89-9; 15N'4N180, 56289-83-9; NOz, 16824-89-8; Ar, 7440-37-1. (30) For a review, see J. A. Ketelaar, Spectrochim. Acta, 14, 237 (1959). (31) B. L. BIaney and G. E. Ewing, Annu. Rev. Phys. Chem., 27, 553 (1976). (32) D. L. Andrews and M. J. Harlow, J . Chem. Phys., 78, 1088 (1983). (33) A. Chedin, C. Amiot, and 2. Cihla, J. Mol. Spectrosc., 63, 348 (1976).
