J . Phys. Chem. 1985,89, 827-830
827
for spectroscopic observation. Finally, rearrangement among the many decapentaene cation isomers proceeds following activation by absorption in the green band system; the near-infrared absorptions do not provide sufficient internal energy to initiate the rearrangement.
Scheme I11
/I+. endo-dicyclopentadiene cation, it was not observed as an intermedate in the rearrangement process of the cubane cation. The parent radical cation of bicyclodecatriene was surely produced in this study on the basis of the photochemical appearance of decapentaene cations in bicyclodecatriene experiments as suggested by Scheme 111, which has at least two intermediate cation species in common with Scheme I. The photochemical rearrangement of the bicyclodecatriene molecule to decapentaene has been documented in s o l ~ t i o n ~and ~ * in ' ~ the present matrix studies, and it is no surprise that the radical cation undergoes the same rearrangements. The production of a small amount of decapentaene cation by 45-10 irradiation of the bicyclodecatriene sample is explained by resonance two-photon ionizationlo of the decapentaene photolysis product. The photochemical conversion of decapentaene cations to the all-trans isomer was partially reversed by irradiation in only the t bands using a dielectric filter. Continued photolysis in all of the decapentaene cation band systems favored the all-trans isomer. Similar behavior has been found for hexatriene and octatetraene cations in solid a r g ~ n . ~ Activation ',~~ of a particular isomer by absorption of light unique to that isomer established a dynamic equilibrium among isomeric structures, and those that are not irradiated may then be deactivated by the matrix cage and trapped
Conclusions This investigation has produced dicyclopentadiene cations by matrix photoionization methods and observed weak bands for the endo and ex0 forms. Subsequent photolyses of these parent cations have led to the formation of intermediate cyclic-polyene cations. Additional photolyses produced ring opening to decapentaene cation isomers, with band systems at 19.5,12.5 and 19.2, 11.6 which are lower in energy than similar band systems for octatetraene and hexatriene cations. The decapentaene cation absorption bands were also produced by photochemical rearrangement of bicyclodecatriene cations and by direct photoionization of decapentaene. Prolonged irradiation favored rearrangement to the all-trans-decapentaene cation, the isomer prepared in greatest yield from this neutral molecule. These experiments are complementary to radiolysis in a Freon matrix for the study of cation spectroscopy and photochemistry. Acknowledgment. We gratefully acknowledge financial support from National Science Foundation Grant CHE 82-17749,the gift of samples from and helpful discussions with T. Shida prior to publication of his results, information from R. L. Christensen on decapentaene synthesis, and the loan of apparatus by D. F. Hunt. I.R.D. acknowledges financial support from the University of Strathclyde. Registry No. endo-dicyclopentadiene, 1755-01-7; exo-dicyclopentadiene, 933-60-8; methyldicyclopentadiene, 26472-00-4; 1,3-bishomocubane, 6707-86-4; bicyclo[6.2.0]deca-2,4,6-triene, 36093-14-8; decapentaene, 2423-91-8.
Impurity Perturbations of the Fundamental Band of Carbon Monoxide in Solid Nitrogen Bengt Nelander Thermochemistry Laboratory, University of Lund, Chemical Center, P.O. Box 740, S-220 07 Lund, Sweden (Received: May 23, 1984; In Final Form: October 5, 1984)
The fundamental band of CO in solid nitrogen has been studied at 0.05-cm-' resolution with an FTIR spectrometer. The result indicates that CO is trapped in a substitutional site. Impurities which do not appear to form well-defined complexes with CO (Ar, 02,C02) produce a symmetric pattern of satellites around the CO fundamental, in contrast to impurities which form a well-defined binary complex (H20,HCl). Deposition at 20 K give the sharpest CO band; deposition at lower temperatures gives broader bands. Annealing at 25 K can remove the effects of a low deposition temperature. Surface diffusion during deposition controls the amount of binary complex formed with H 2 0 or HCl.
Introduction Ever since the pioneering work of Pimentel and co-workers,'S2 the matrix isolation technique has been used to study molecular interactions. In this type of work there are one, two, or more impurities in the matrix simultaneously; the presence of binary and higher complexes between these is revealed by the appearance of new bands that cannot be attributed to the monomeric impurities themselves. From the observed spectrum of one such complex, conclusions are then drawn about its shape and binding strength. (1) Thiel, Mathias van; Becker, Edwin D.; Pimentel, George C. J . Chem. Phys. 1957, 27, 95. (2) Thiel, Mathias van; Becker, Edwin D.; Pimentel, George C . J . Chem. Phys. 1957, 27, 486.
However, the presence of impurities in the matrix, in concentrations large enough to give observable amounts of binary complexes, will also influence the spectra of the monomeric molecules. It is common experience in this type of work that the spectrum of a matrix-isolated compound looks better when the compound is alone in the matrix compared to when another compound is also present. There are several mechanisms whereby impurities in the matrix will inhomogeneously broaden absorption bands of matrix-isolated molecules. For instance, if the matrix contains molecules with dipole or quadrupole moments, these will give rise to electric fields which will vary randomly in the matrix and influence the absorption bands of molecules in the matrix. Most molecules differ considerably in size and shape from the matrix-forming atoms or molecules. They will therefore disrupt the matrix sturcture and shift the spectra of nearby molecules to some
0022-3654/85/2089-0827$01.50/00 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 5, 1985
extent. Both these effects may OCCUT simultaneously. For instance, when a nonpolar argon atom substitutes a quadrupolar nitrogen molecule, both the electric field and the matrix structure will change in the neighborhood of the argon atom. The transition dipole moments of not too distant pairs of identical molecules will interact and split their absorption band in a way that depends on the relative orientation of the two molecules. If the matrix contains a distribution of randomly oriented pairs of identical molecules, their absorption bands will be broadened. The inhomogeneous broadening contains information about the matrix structure. It has not been studied to a great extent, since such studies generally require higher resolution than is available with commercial grating spectrometers. Dubost et al? have studied the line shape of the CO fundamental in solid nitrogen as a function of temperature and concentration, using a diode laser spectrometer. The fwhm (full width a t half-maximum) of C O at 9 K and low concentration is only 0.013 cm-'. When the concentration is increased to 1:100, the width reaches 0.18 cm-'. The dipolar interaction between CO molecules of identical isotopic composition is shown to be the major source of the broadening. Giinthard and co-workers4 have used hole-burning to show that the vibration bands of matrix-isolated compounds can be inhomogeneously broadened and to measure their homogeneous line widths. Swanson and Jones5 have studied site splittings and observed intriguing temperature variations in the distribution of sites. This work is a study of carbon monoxide in solid nitrogen, perturbed by different impurities. It has been found that even rather inert impurities such as Ar and O2have a significant effect on the width of the C O fundamental band. Discrete pairs of satellite lines are observed symmetrically around the C O fundamental when Ar, Oz, or COz is present in the matrix. These satellite lines may be attributed to C O molecules, with a single impurity substituting one of the nitrogen molecules in the matrix cage of CO. On the other hand, impurities like H20or HCl, which form well-defined binary complexes with CO, appear to have a much smaller effect on the shape and width of the CO band. Deposition at 20 K gives the sharpest CO band. Deposition at lower temperatures leads to a considerably broader band. In contrast to the impurity-induced broadening, this broadening can be eliminated by annealing of the matrix. If a matrix containing C 0 2 and C O is deposited at a low temperature, the C O band is broadened by both causes. Upon annealing, the line sharpens to the width of the C O band for a 20 K deposited matrix containing the same concentration of C 0 2 .
Experimental Section CO (Aga Specialgas, 99% pure), Ar (L'Air Liquide 99.9995% pure), O2(L'Air Liquide 99.998% pure), and C02 (L'Air Liquide 99.95% pure) were all used as received. HzO was distilled and degassed before use. HCl (Matheson) was degassed and used without further purification. N2 (L'Air Liquide 99.9995%) was passed through a glass spiral immersed in N2 (1) before use. Gas mixtures were prepared by standard manometric techniques on a grease-free vacuum line. The depositon rate was 1.4 mmol/h. Matrix gas was deposited for 10 min before deposition of the sample. Except for a series of experiments with only CO in the matrix, 1 4 0 gmol of nitrogen containing CO and an additional impurity was deposited in each experiment. The molar ratio N 2 / C 0 in the matrix was 810. The cryostat6 and deposition system7 used have been described earlier. The spectra were recorded at 10 K on a Bruker 113V instrument with a nominal resolution of 0.03 cm-' and transformed with box-car apodization. (3) Dubost, H.; Charneau, R.; Harig, M. Chem. Phys. 1982, 69, 389. (4) Dubs, Martin,; Ermanni, L.;Giinthard, Hs.H. J. Mol. Specfrosc. 1982, 92 458 and further references in this paper. Felder, P.; GILnthard, Hs.H. Chem. Phys. Let?. 1982,88,473. (5) Swanson, B. I.; Jones, L H. In 'Vibrational Spectra and Structure"; Durig, J. R.,Ed.; Elsevier: Amsterdam, 1983; Vol. 12. (6) Fredin, L.; Rosengren, Kj.;Sunner, S . Chem. Scr. 1973, 4, 93. (7) Fradin, L. Chem. Scr. 1974, 5, 193.
Nelander
1
YAVENUMBERS CM-1
0.02
n
ii, -0.005
1 2138
1 2139 2140 2141 HAVENUMBERS CM-1
1 2142
2143
Figure 1. Upper panel: N2/C0 = 119, 1.4 mmol of N2 deposited. Lower panel: N2/C0 = 810, N2/C02= 115; 140 pmol of N2deposited. Vertical axis: absorbance units, log &/I).
TABLE I: CO in Solid Nitrogen at 10 K Ddp, A/r, N2/C0 Tdcp,K pmol-' cm-'/pmol 7100 806 806
20 20 20
a
203 806 806
20 10 15
a
806 a
18
3.51 3.69 2.27 2.27 1.95 1.53 2.27 3.10 2.39 2.35
0.189 0.233 0.198 0.198 0.230 0.244 0.206 0.205 0.197 0.204
fwhm, cm-'
(AID,)/ fwhm
0.046 0.050 0.065 0.070 0.082 0.094 0.066 0.054 0.063 0.064
1.17 1.26 1.34 1.24 1.44 1.69 1.38 1.22 1.32 1.36
4Same experiment as on the preceding line after annealing. D, = Maximum absorbance (at 2139.79 cm-I), log (Io/I). A = area of CO band. p = pmol of CO deposited. The real resolution was close to 0.05 cm-'. The figures were reproduced directly from the observed spectra without correction of the wavenumber scale. (To get the corrected wavenumber from the figures, 0.32 cm-' should be subtracted.) The figures given in the text are corrected by using published gas-phase spectra. In most experiments the matrices were annealed by warming to 25 K for 10 min.
Results CO Experiments. Dubost et al. have found the fwhm of CO in nitrogen matrices to be 0.013 cm-' at 9 K and low concent r a t i o n ~ .When ~ the CO concentration increases, the width of the band increases and it reaches 0.18 cm-" at N 2 / C 0 = 100. Insofar as the poorer resolution of the spectrometer used in this work allows a comparison, the bandwidths and band positions observed here agree with ref 3.
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 829
Impurity Perturbations of CO 0.5
TABLE 11: CO and an Additional Impurity in Solid Nitrogen" ~
Tdep
K
pmol-'
cm-'/pmol
fwhm
C02 CO2
130 115
20 20
133
10
113 115 122 121
20 20 20 10
1.25 0.92 0.97 0.63 0.90 1.62 1.61 2.60 1.31 2.68
0.245 0.232 0.229 0.224 0.210 0.227 0.211 0.191 0.221 0.212
0.105 0.133 0.123 0.185 0.126 0.098 0.096 0.056 0.102 0.058
CO2 b
Ar 02
H2O H2O a
4
2139.9g96 2140.1993 WAVENUMBERS CM-1
I 2140.399
A/P
N2X
b
-0.05 2139.7999
Dm/p>
X
cm-l
fwhm
cm-'/pmol
111
0.199 0.209 0.184 0.192 0.208 0.152 0.222 0.202 0.216 0.207
0.072 0.070 0.068 0.069 0.116 0.068 0.090 0.075 0.073 0.069
1.35 1.36 1.38 1.32 1.75 1.34 1.89 1.48 1.59 1.51
0.0073 0.0073 0.0080 0.0074 0.0004 0.0019 0.0012 0.0025 0.0066 0.0060
29 1
20
114
10
b 114
15
b 114
2140.399
Figure 2. N2/C0 = 810, N2/H20 = 121; 140 pmol of N2 deposited, deposition temperature, 10 K. Lower panel: before annealing. Upper panel: after annealing. Vertical axis: absorbance units, log (Io/I).
There are some additional features, not reported previously, which seem worth mentioning. In all experiments, regardless of the C O concentrations, a weak satellite band appears at 2140.82 cm-', 1.03 cm-'from the main 12C160band. Since its size relative to the C O fundamental (1:lOOO) appears to be independent of concentration, it is assigned to monomer CO; possibly it is due to combination of the C O fundamental and a local mode or to CO in a different trapping environment. At higher CO concentrations, three new bands appear at 2139.38, 2140.04, and 2140.23 cm-' (Figure 1). These bands are probably due to pairs of C O molecules and will be discussed further below. As is seen from Table I, when the matrix is deposited at temperatures below 18-20 K, the width of the CO band increases significantly. Annealing at 25 K removes the additional broadening due to low-temperature deposition (Figure 2). CO in Ar-, 02-,or C02-Doped Matrices. In the presence of C 0 2 , the C O band gets four satellites at 2138.07, 2138.86, 2140.80, and 2141.57 cm-l, approximately symmetrically around the C O band (Figure 1). One of the components coincides with the C O satellite mentioned above, but its intensity makes it clear that it has a significant C 0 2 contribution. When the matrix is deposited at 10 K, the satellites are observable only after annealing. With 0, or Ar, two very weak satellites are observed in each case, at 2139.24 and 2140.28 cm-I or 2139.05 and 2140.33 cm-', respectively. In addition to inducing satellites, all three impurities broaden the CO band significantly (see Table 11), so that an approximately Lorenz-shaped C O band is observed in each case. Note that the ratio (A/D,)/fwhm is 1.57 for a Lorenz curve and 1.06 for a Gauss curve. CO in H 2 0 or HCI-Doped Matrices. Experiments were camed out with CO in H20-or HC1-doped matrices. Both H 2 0and HCl
AclrH
cm-l/pmol
b
2139.9996 2140.1993 WAVENUMBERS CM-1
1.86 1.91 1.92 1.92 1.85 1.43 1.38 1.30 1.66 1.36
N,/HCl
b
2139.7999
fwhm
'N2/C0 = 806; T = 10 K. bSame experiment as on the preceding line after annealing. Dm = maximum absorbance, log (Io/I),A = area of CO band. p = pmol of CO deposited. TABLE 111: CO and HCI in Solid Nitroeen" ( A + A c ) / p , fwhm, (AID,)/
I
(AID,)/
18
1.72 1.89 1.01 1.53 1.25 1.66 1.46 1.57
N2/C0 = 806; T = 10 K. Same experiment as on preceding line, after annealing. D, = maximum absorbance of CO band, log (Io/I). A = area of CO band. Ac = area of v(OC.HC1) band. Fwhm = full width at half-maximum of CO band. p = pmol of CO deposited. pH = pmol of HCI deposited. form well-defined binary complexes with CO. In this work, v(OC.HC1) is observed at 2152.45 cm-' and v(C0.0H2) at 2147.53 cm-'. (v(OCeHC1) denotes the C O fundamental of a COaHCl complex.) Neither compound induces any satellites in the immediate vicinity of the C O band. After 20 K deposition, the width of the CO band differs little from its width in a pure nitrogen matrix (Table 111). The sum of the areas of the free C O band and the complexed C O band corresponds approximately to the amount of C O in the matrix (Table 111). The amount of complex found depends strongly on the temperature of deposition, as does the width of the (free) CO band. Note in particular the large values for the ratio (A/D,)/fwhm, which signal that a large fraction of the C O absorption is in the wings of the band. Annealing at 25 K removes most of the absorption in the wings.
Discussion A nitrogen molecule in solid nitrogen is surrounded by 12 other nitrogen molecules, 6 around its center and 3 at each end of the molecule.* If a carbon monoxide replaces a nitrogen, the symmetry is lowered and since the molecules around the center have two different orientations relative to the central CO, there are four different ways in which a linear symmetric molecule, or an atom, can replace an N2 next to a CO. The dipole moment of C O in its first vibrationally excited state is 0.026 D larger than in the ground state.9 Therefore the position of the CO fundamental band is shifted by electric fields. If we replace an N, in the first matrix shell around C O with COz, the electric field at the CO will differ, since the quadrupole moment of C 0 2 is ap(8) Jordan, Truman, H.; Smith, H. Warren.;Streib, William E.; Lipscomb, William N. J . Chem. Phys. 1964, 41, 756. Venables, J. A,; English, C. A. Acta Crystallogr., Sect. 8 1974, 830, 929. (9) Meerts, W. L.; de Leeuw, F. H.; Dymanus, A. Chem. Phys. 1977.22, 319.
830 The Journal of Physical Chemistry, Vol. 89, No. 5, I985 proximately 4 times larger than that of N2.10 If we treat both nitrogen and carbon dioxide as point quadrupoles, replace one N z in the first matrix shell with a COz having the same position and orientation as the Nz it replaces and neglect the effect of the matrix rearrangment, we can estimate the shifts of the C O fundamental for the four possible situations to be f1.25 and f0.20 cm-'. When COz is present together with C O in the matrix, four satellites are observed at -1.72, -0.93,0.93,and 1.78 cm-' from the C O fundamental (Figure 1). In agreement with the crude model, two pairs of lines symmetrical around the unperturbed fundamental of CO are observed and the calculated shifts are of the same order of magnitude as the observed ones. The four satellites observed with C 0 2 in the matrix are therefore assigned to C O molecules, with a COz in the first matrix shell. Note that even if the distortion of the matrix is taken into account, four satellites are expected, corresponding to the four different substitutions of C 0 2 for N 2 in the first shell, but it is only when the effect of the electric field determines the C O shift that a symmetric satellite pattern is expected. C 0 2 molecules in the second shell might change both the field at the CO and its exchange repulsion with its neighbors; such interactions should produce a multitude of satellites, corresponding to the large number of relative orientations of C O and COz. They may be the cause of the broadening of the C O band in the presence of C 0 2 (Table 11). With Oz and Ar or with excess C O in the matrix, only two or, with CO, three satellites are observed. These are nearly symmetric around the C O fundamental and much closer to it than the C 0 2 satellites. In analogy with the COz case, these satellites are assigned to C O with a perturber in its first matrix shell. The unobserved second pair of satellites may be too broad so that its maximum intensities do not reach above the noise or too close to the CO fundamental to be observable. The instrument-produced shape of a sharp gas-phase line is asymmetric, which may explain why only one of the second pair of satellites is resolved in the C O case. Water and hydrogen chloride form significantly stronger binary complexes with C O than with N2." In their case, no satellite bands are observed close to the CO fundamental; instead, a single strongly shifted complex band is observed. If the deposition temperature is 10 K, the low intensity of the binary complex band suggests that some CO molecules have a nearest neighbor HCl without forming a complex. At the same time, the intensity in the wings of the C O fundamental becomes quite large (Table 111). Annealing removes the wing intensity and produces an increase in the complex concentration, which approximately corresponds to a situation where all nearest neighbor pairs of C O and HC1 have reacted to form a binary complex. The argument given above suggests that two nearest neighbor impurity molecules in a matrix can choose between two different possibilities. They may either react to form a binary complex, which then dictates a suitable matrix arrangement, or they may take up relative positions dictated by the matrix and not closely related to the structure of the isolated binary complex. Com(10) Buckingham, A. D.; Disch, R. L.; Dunmur, D A. J. Am. Chem. SOC. 1968, 90, 3104. (1 1 ) Perchard, Jean Pierre; Cipriani, Joseph; Silvi, Bernard, Maillard, Daniel J . Mol. Struct. 1983, 100, 317.
Nelander promises between these two extremes may also be possible. Here it is interesting to compare with the work of Dubost,Izwho observes single satellite bands for C O in argon matrices containing COz or Nz and only a single band corresponding to pairs of CO. Probably the nondirectional Ar-Ar interaction makes it possible for the interacting pairs of impurities to find their optimum relative orientation, while in the nitrogen matrix, the interaction energy of the impurity pair is too small to overcome the directing influence of the matrix. Assume that the matrix contains, in addition to the molecule we are studying, a polar impurity with dipole moment po. The distribution of dipoles will give rise to a position-dependent electric field in the matrix; the electric field distribution can be obtained from a formula of HoltsmarkI3 if we assume that the dipoles have random positions and orientations. Using this formula we get the following expression for the line shape (the transition moment is assumed to be unperturbed by the field):
-1
PA All (AV)~+ ( p ~ ~ p ) 2
A = 4.54~0 p is the number density of the perturber with dipole moment po, Av is (v - vo), where vo is the unperturbed line position, and
Ap the dipole moment change between the upper and lower states we are studying. Note that the shape of the inhomogeneously broadened band is Lorenzian. This example shows that inhomogeneous broadening des not always produce a Gaussian band shape. Using the formula above, we estimate the fwhm of the C O fundamental to be 0.2 cm-' at a HCl/Nz ratio of 1:120. The major contribution to the width should come from molecules 8-10 A from a CO. The distribution of HCl around a C O is not random, and complex formation decreases the probability of finding a HCl in the first two matrix shells. The HCKO complex is regarded as a separate entity, since its C O fundamental is far outside the region of interest. The depletion of the first two shells shold not affect the estimated width by a large amount, since the major contribution to the half-width comes from molecules outside them, if the conditions for using Holtsmark's formula are fulfilled. The observed line width of C O in the presence of 1% HCl after 20 K deposition or after low-temperature deposition and annealing is much smaller than the estimated width, perhaps an indication that the orientation of HCl molecules within 10 A from a C O are correlated with the C O orientation. It is significant that annelaing after low-temperature deposition of matrices with H 2 0 or HCl as additional impurity produces a C O fundamental which is as sharp as after 20 K deposition (Table 111). The amount of binary complex formed in the 20 K deposited matrix is at least 3 times larger than after annealing of a 10 K deposited matrix. This shows that depletion of the neighborhood of free CO molecules of H 2 0 or HC1 due to complex formation cannot explain the small width of the C O fundamental. It also underlines the importance of surface diffusion for the formation of binary complexes. Registry No. CO, 630-08-0; Nz, 7727-37-9; Ar, 7440-37-1; Oz, 7782-44-7; CO2, 124-38-9; HZO, 7732-18-5; HCI, 7647-01-0. (12) Dubost, H. Chem. Phys. 1976, 12, 139. (13) Holtsmark, J. Ann. Phys. 1919, 58, 577.
