J. Phys. Chem. 1984, 88, 1373-1376 to couple internal degree of freedom between the ion and the collision partner.
Acknowledgment. We are indebted to the Natural Sciences and Engineering Research Council of Canada for financial support.
1373
K. Nagase gratefully acknowledges the leave of absence provided by Tohoku University, while J. Herman is indebted to Dr. W. Forst for helpful discussion. Registry No. Isobutene, 115-1 1-7.
Infrared Spectra of Natural Sodalite S . C. Zilio* and V. S. Bagnato Instituto de F h c a e Quimica de Siio Carlos, Universidade de Siio Paulo. Siio Carlos, SP Brasil CEP 13560 (Received: June 14, 1983)
Infrared spectra ace reported for Brazilian natural sodalite in the frequency intervals 4000-1 100 cm-' and 260-10 cm-I. Measurements of k = 0 phonons below 260 cm-' allow the comparison to Raman measurements and to the superlattice approximation, which enables the classification of Raman- and infrared-active modes as either strong or weak. Through the chemical substitution of K+ for Na' and Br- for C1-, we were able to qualitatively identify modes in the external mode region which are related to the vibrations of the Na4Cl tetrahedron. Modes in the 2100-1300-crn-' region, corresponding to the two-phonon process, were also observed.
Introduction Sodalite, [Na4(A1Si04)3C1]2,is an aluminosilicate which has received considerable attention during the last decade due to the photochromic and cathodochromic properties associated with F centers lying at halide vacancy sites. These properties give to the sodalite technological importance as dark-trace screen material in storage cathode ray tube (CRT) display devices.*-3 With the purpose of understanding its coloration-bleaching characteristics, several techniques have been employed such as optical and luminescence spectroscopy, dielectric relaxation, and electron spin resonance. In particular, infrared and Raman spectra have been studied in synthetic and natural sodalites in an attempt to understand their phonon modeses Since its structure is complicated, with 46 atoms in the unitary cell, this understanding is difficult and yet uncomplete. Ariai and Smiths used a superlattice approximation to calculate the number and symmetries of the zone-center phonon modes and classify the infrared- and Raman-active modes as either strong or weak. This procedure is fairly successful in explaining qualitatively their own Raman spectra and the available infrared data5 above 300 cm-'. The comparison of the spectra obtained by means of these two techniques is possible because in the sodalite lattice (space group G) the Tz modes are both Raman and infrared active. In the internal mode region, which lies in the frequency range 300-1 100 cm-', the modes are assigned to the internal vibrations of the TO4complex (where T = Si or A1).5s6 The origins of the infrared bands in this region have been identified by observing spectral changes as Ga is chemically substituted for A1 and Ge for Si.6 For frequencies below 300 cm-' (external mode region), where N a and C1 (or Br for bromosodalite) play an important (1) M. J. Taylor, D. J. Marshall, P. A. Forrester, and S. D. McLaughlan, Radio Electron. Eng., 40, 17 (1970). (2) R. C. Duncan, B. W. Faughnan, and W. Phillips, Appl. Opt., 9,2236 (1970). ( 3 ) B. W.Faughnan, I. Gorog, P. M. Heyman, and I. Shidlovsky, Proc. IEEE, 61,927 (1973). (4) C. L. Angel, J . Phys. Chem., 77,222 (1973). (5) C.M. B. Henderson and D. Taylor, Spectrochim. Acta, Part A , 33A, 283 (1977). (6) C . E. Stroud, J. M. Stencel, and L. T. Todd Jr., J. Phys. Chem., 83, 2378 (1979). (7) M. K. Badrinarayan, J. M. Stencel, and L. T. Todd Jr., J. Phys. Chem., 84, 456 (1980). (8) J. Ariai and S . R. P. Smith, J . Phys. C, 14, 1193 (1981).
0022-3654/84/2088-1373$01.50/0
role in calculating the phonon modes, infrared measurements6 extend to only 150 cm-I. Below this, there are no available infrared data to be compared to the previous Raman measurements and group-theoretical analysis.8 This analysis predicts for this region five strong infrared lines (IRS) and three weak infrared lines (IRW). Reference 6 shows two IRS absorptions at 294 and 200 cm-' and two IRW absorptions at 250 and 170 cm-I; so, it must be expected the occurrence of three IRS lines and one IRW line below 150 cm-'. We have therefore obtained and discussed the spectrum in this region. The spectral changes associated to chemical substitution of K+ for Nat and Br- for C1- are discussed in terms of the vibrations of the Na4Cl tetrahedron and librations of the TO4complex. Since sodalite is, structurally, closely related to zeolites, such a study is also interesting in order to understand some of the vibrational modes of zeolites containing monovalent cations. Spectra in the mid-infrared region were also obtained with the purpose of sample characterization.
Experimental Section The sodalite samples were obtained from Itabuna in the state of Bahia, Brasil. They have a deep blue color due to a color center localized on the aluminosilicate f r a m e ~ o r k . ~A Laue X-ray diffraction pattern shows discrete but somewhat diffuse points, suggesting that the samples are aggregates of microcrystals with a preferential direction of orientation. An X-ray powder spectrum shows the same result as obtained by Stroud et a1.,6 with a = 8.90 A. Atomic absorption and gravimetric analysis provided the composition 33.2% A1203,37.0% S O 2 , 20.6% NazO, 0.1% KzO, 0.22% MgO, 0.12% CaO, and 0.09% FezO,. X-ray fluorescence and a microprobe indicated the presence of traces of iron and copper, and the latter technique detected chlorine. The doping with K+ and Br- was accomplished by heating the sodalite in an electric furnace together with KCl and NaBr, respectively, until the alkali halide fusion. With this procedure we were able to replace about 45% of chlorine by bromine and 75% of sodium by potassium. Several sample thicknesses were used for spectroscopic measurements in different spectral regions: between 4000 and 1200 cm-l slabs 0.2 mm thick were used, while in the lO-lOO-cm-' range the slabs were 1 mm thick. In the frequency interval 70-260 cm-', ~
~~~
(9) W.E.van den Brom, J. Kersen, and J. Volger, Physica (Amsterdam), 77, l(1974).
0 1984 American Chemical Society
1374 The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 0
-
I
Zilio and Bagnato TABLE I: Proposed Assignments of the Overtonesa
pred, cm-' AS- A 2065 B+ B 1955 a Basic frequencies: A and C = 725 cm-' . overtone
/
I
obsd, pred, obsd, cm-' overtone cm-l cm-' 2065 B+ C 1702 1700 1955 1450 1450 C+ C = 1032.5 cm-I, B = 977.5 cm-' ,
FREQUENCY [crri')
Figure 1. Infrared spectra (4000-1200 cm-l) of natural sodalite before (a) and after heating at 900 "C for 4 h (b).
the measurements were carried out with ground sodalite in a Nujol mull deposited on a 6-wm-thick Mylar sheet. Far-infrared spectra were obtained with a Michelson Fourier transform spectrometer and a composite semiconductor bolometer,I0 both of our own construction. Radiation from the interferometer enters the sample and detector Dewar from below, and an externally rotatable sample holder can be moved to bring up to four samples into the light path. Sample temperature fpr measurements in this region is typically 2 K. Measurements in the mid-infrared region were carried out with a Perkin-Elmer 180 grating spectrometer, with the samples at liquid-nitrogen temperature and resolution of about 3 cm-I.
Results and Discussion Figure l a shows the infrared spectrum of natural sodalite for the 4000-1200-~m-~region. Bands a t 3620 and 3525 cm-I are due to the stretching of OH- and H,O, respectively." By comparing the area of the absorption at 3620 cm-' to that of a synthetic sarnple,l2 we estimated that about 0.1% of the halide sites are occupied by hydroxyl ions against 10% for the synthetic sodalite. The water content is also very small. The weak bands around 3000 cm-' are due to organic radicals located among sodalite grains and shall not be discussed further. Although not explained, the absorptions at 2065, 1955, 1700, and 1450 cm-' have firstly been observed by Taylor et al." in high-purity sodalites. Their strengths are much smaller than those of the internal modes. We noticed that each of thqse frequencies is approximately the frequency sum of pairs of k = .Oghonons of the internal mode region. According to this, these absorptions are tentatively assigned to the two-phonon process, where the fundamental phonons are related to the three highest frequency modes in the internal mode region, namely, A = 1033 cm-I, B = 980 cm-I, and C = 735 cm-'. In order to confirm this assignment, measurements were carried out in K-doped samples. In this case, it is well-known5that the internal modes change their frequencies and intensities when compared to those of the Nasodalite. Our results show a behavior of the overtones which is in agreement with these changes. It should be pointed out, however, that infrared-active overtones are usually dye to combinations of phonons with k located at the various critical points (symmetry points) but not at the center of a Brillouin 20ne.l~ This is a consequence of the higher density of states at the zoqy edges. For sodalite, the overtones are combinations of phonon%with frequencies close (but somewhat lower) to those of the k = 0 phonons. This suggests that the dispersion relation w ( k ) for this material must present T O branches which are nearly constant, decreasing slowly as k increases. In fact, our proposed assignment (10) N. S.Nishioka, M.S. Thesis, University of California, Berkeley, CA, 1976. (1 1) M. J. Taylor, D. J. Marshall, and H. Evans, J. Phys. Chem. Solids, 32, 2021 (1971). (12) R. C. Alig and A. T. Fiory, J . Phys. Chem. Solids,36,695 (1975). (13)%. Burnstein, F. A. Johnson, and R. Loudon, Phys. Rev. [Sect.]A , 139, 1239 (1965).
Figure 2. Absorption coefficient between 300 and 80 cm-' of natural sodalite before (a) and after heating at 900 OC (b).
I
I\;
l0OC
6 G ~ 0 . cm'l 8
/-
O L
60
1 100
I
I
I
I
I
I
140 180 FREQUENCY (crn-1)
Figure 3. Absorption coefficient (160-70 cm-I) of natural sodalite before (a) and after heat treatment at 900 O C for 4 h (b).
works better when the fundamental phonons are taken at the CriticaLpoints, with frequencies just a little bit lower than those of the k = 0 phonons, as shown in Table I. The smaller features of the spectrum in this region could be due to overtones with fundamentals at lower frequencies than A , B, and C. It seems that modes due to stretching and bending of the Si-0 linkages ( A , B, and C) couple stronger than those due to A1-0, located at lower frequencies. Figure l b shows the infrared spectrum of the same sample after heating in vacuo at 900 O C for 4 h. The band at 1335 cm-I disappears and is attributed to the stretching of a molecular impurity which decomposes at high temperatures. This point, and also the appearance of tee absorption at 2345 cm-I, will be discussed elsewhere. The desgption of water and hydroxyl groups crazes the samples to the extent that they become partially opaque, and in this way, the overall transmission is reduced. This opacity is smaller for larger wavelengths, which can be seen by the increasing transmission for smaller wavenumbers. The desorption also increases the lattice parameter ( ~ 0 . 8 % ) ,and ~ as a consequence, there are small frequency shifts of overtones and fundamentals toward smaller wavenumbers. Below 1100 cm-' the
Infrared Spectra of Natural Sodalite
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1375
TABLE 11: Comparison of Raman and IR Phonon Modes ref Ariai and Smith*
obsd modes, cm-'
strengths RS RW IRS
Stroud et aL6
295
23 2
294 25 0 244
IRW
IRS IRW
this work
132
k 4 p / / ' . , l
I
___---30
40
50 60 70 FREQUENCY ( c d )
1
114
104 62
243.8 244 243.8 232
900°C Br-doped K+doped
00
Figure 4. Absorption coefficient (80-10 cm-I) of natural sodalite before (a) and after heating at 900 OC for 4 h (b).
t- 90
-
0
-
z w
(14) C. Peuker and D. Kunath, J. Chem. SOC., Faraday Trans. 2,77,2079
205.5 208 208 119
190.6 191 191.4 152
133.6 134 133.6
114.8 104 115 103 114.8 101 101 94
I
LL W
absorption is complete for the thickness used. In the 1100300-cm-I region the spectra are the same as reported earlier.4" The far-infrared absorption coefficient (a = (-l/d) In (Z/Zo)) for the 260-10-cm-' region is shown in Figures 2a, 3a, and 4a, with resolutions givqn respectively by 1.6, 0.8, and 0.4 cm-'. The vertical bars represent the results of Raman measurements.8 Shifts of infrared bands to higher frequencies are a consequence of a decrease in the lattice parameter due to the low temperature of the sample. The peaks at 219.5 and 98.0 cm-' have no corre: spondent in the Raman spectrum. They disappear after the sample is heated at 900 O C , and so, they are attributed to translational modes of the molecular impurity responsible for the peak at 1335 cm-]. The same behavior is found for the bands at 56.5 and 27.5 cm-l, but in this case, the tentative assignment is for librational modes of the same impurity. Figures 2b, 3b, and 4b show the absorption coefficient after the samples were heated in vacuo at 900 O C for 4 h. Besides the absence of the bands mentioned above, we may also observe an oscillator strength increase of the peak at 205.5 cm-' and a simultaneous frequency shift to 208 cm-'. This is unexpected since the lattice parameter increase should shift the bands toward smaller wavenumbers. van den Brom et aL9 proposed a breaking of the AI-0 band due to the X-ray incidence in order to explain the deep blue color acquired by the natural sodalite. We carried out a separate measurement in the internal mode region for X-ray-exposed and heat-treated samples, and as a result, we found, in both cases, a new band at 545 cm-'. Since modes in this region are related to the aluminosilicate framework, we identify this absorption as due to the A1-0 bond breaking. In this way, we assign the peak at 205.5 cm-' to the librations of the tetrahedra A104 and SO4. The behavior of this band concerning the strength and position is explained by the rearrangement of the electronic cloud around the aluminosilicate cage as result of the bond breaking. This assignment finds support from the far-infrared spectra of zeolites and dealuminated, ultrastable molecular sieve US-EX.14 Na-X zeolite, which is structurally closer to sodalite than Na-Y, Li-X, and US-EX materials, presents modes at 227, 266,294, and 322 cm-', which are attributed to the aluminosilicate framework. According to our results, the Na-X mode at 227 cm-' corresponds to the libration of the oxygen group. Group-theory
(1981).
59
natural
m
20
190
99
treatment
*'
10
205
110
TABLE 111: Mode Frequencies for Different Sample Treatments sample obsd modes, cm-'
.
0
179 170 134
f/ a
198 200
60 -
2
9
-
I-
n
8 300
m
a
6 G 10.8 cm-l
O60L
I
l
100
l
,
140
I
I
I
I
2 20
180
I
I
-
260
FREQUENCY I c d )
Figure 5. Absorption coefficient (260-70 cm-I) of Br-doped sodalite.
60
IO0
140
I80
220
FREQUENCY
(cm-0
260
0
Figure 6. Absorption coefficient (260-70 cm-I) of K-doped sodalite.
analysis8 predicts three strong infrared absorptions due to cage vibrations in this region. One of them is attributed to the oxygen's libration and must be Raman weak. This requirement is satisfied by the band at 205.5 cm-', as shown in Table 11. The doping of sodalite with K+ and Br- changes the peak positions as shown in Figures 5 and 6 and in Table 111. The band at 243.8 cm-' does not depend very much on the alkali halide substitution in the sense that it has a relatively small frequency shift, and for this reason it must also be assigned to the aluminosilicate framework. This band has already been observed by Stroud et a1.,6 but their spectra show a much weaker intensity. They also measured a very strong absorption at 294 cm-', which
J. Phys. Chem. 1984, 88, 1376-1379
1376
can be attributed to the aluminosilicate cage by comparison to the Na-X spectrum. The superlattice approximation states that, for the sodalite strong Raman modes (RS) correspond to IRW structure modes,while weak Raman modes (RW) correspond to IRS modes, except for translational (TR) modes (modes in which all the atoms of a particular species undergo an identical translation) which are both Raman and infrared strong. One such mode is expected for the framework. We assign it to the band at 294 cm-I, which is very strong both in Raman and infrared spectra. On the other hand, the intensity of the absorption at 243.8 does not fit the superlattice scheme since it is R S and should also be IRW. The band at 190.6 cm-I is R W and IRS. Without any obvious reason, we will attribute it to the aluminosilicate framework, too. In this way, we have the best correlation between our results and the superlattice aproximation. With these assignments, all bands between 150 and 300 cm-' are due to cage vibrations, except for the impurity mode at 219.5 cm-'. Modes below 150 cm-I are due to the Na4Cl tetrahedron. The far-infrared spectra of Br-doped sodalite show no obvious differences when compared to those of the undoped heat-treated samples. A similar behavior was found in Raman spectra of chloro- and bramosodalite.* This doping results in a decrease of the absorption at 103 cm-' with the simultaneous appearance of a new band at 101 cm-l, which is then attributed to the bromine mode. Accordingly, the absorption at 103 cm-' is due to the chlorine T R mode, since it is both R S and IRS. The Br- mode lies at a lower frequency than the C1- mode because of its larger mass. The most significant spectral change is achieved when K+ substitutes for Na'. In this case, the spectra are quite different from that of the undoped sample and the assignment is more difficult. The absorptions at 133.6 and 114.8 cm-I are the most affected by this substitution; therefore, we assign them, together with the mode at 62 crn-', to Na+ vibrations. For the Na-X zeolite,I4the Na+ ion inside the cage absorbs at 156 and 110 cm-',
(c),
which are close to our assignment. For the K-X zeolite, these modes are found at lower frequencies (106 and 73 cm-'), following the same behavior of the K-doped sodalite. Na+ does not fit into the superlattice C# used to predict the other modes, and as a consequence, the rule presented above cannot be applied in this case. For Na' contribution, IR and Raman activities are the same. The bands at 62, 114.8, and 133.6 cm-' satisfy this requirement; however, the one at 133.6 cm-' is not as intense as it should be.
Conclusions The origins of phonons It = 0 in the sodalite external mode region have been discussed. Br- and K-doped samples were used for the classification of the types of atomic vibrations involved in each mode. This analysis must, however, be treated with some reservation because a substantial mode mixing will always occur. The spectra between 150 and 300 cm-l are related to the aluminosilicate framework, while below 150 cm-' the modes are assigned to the Na4C1 tetrahedron. Our data agree with the predictions of the superlattice approximation although modes at 133.6 and 243.8 cm-I have uncorrected intensities. At this point we are not in a condition to make any firm statement concerning the absorptions at 42, 150, and 162 cm-I. Impurity modes were observed at 27.5, 56.5,98,219.5, and 1335 cm-', but they will be discussed elsewhere. Two-phonon processes were observed in the 2100-1300-~m-~region. The fundamental phonon frequencies are close to those of the center of the Brillouin zone, which indicates that unlike alkali halides, _sodalite has maxima in the density of states at frequencies near k = 0. Thus, its dispersion relation presents branches which are nearly constant. Acknowledgment. This work has been supported by CNPq, FAPESP, and FINEP. We are grateful to Prof. J. C. Castro for helpful suggestions and to Prof. P. S. Pizani for supplying the samples.
,
Registry No. Sodalite, 1302-90-5.
-
-
Picosecond Fluorescence from the Second Excited Singlet State of Cycl[ 3.3.3]azine, a So and S2 S, Bridged 12~-PerimeterAnnulene. Spectra and Kinetics for S2 Transitlons Werner Leupin,*+Sylvia J. Berens, Douglas Magde,* Department of Chemistry, University of California at San Diego, La Jolla, California 92093
and Jakob Wirz Physikalisch-chemisches Institut der Universitiit Basel, CH-4056 Basel, Switzerland (Received: March 7, 1983)
-
The lifetime of the second excited singlet state Sz is 85 f 25 ps in the title compound and less than 30 ps in the derivative 1,3-di-tert-butylcyc1[3.3.3]azine-1,3-dicar~xylate,as determined by measuring the decay of the Sz Soblue fluorescence. Single-photon correlation was used along with a CW mode-locked argon ion laser. Furthermore, emissions in the near-IR were detected from both compounds and assigned to S, S1 fluorescence on the basis of their spectral distributions and their decay behavior. Both the energy of the Sz SI fluorescences and the lifetimes of the Sz states agreed with previously published predictions.
- -
Introduction The first successful synthesis of the compound cyc1[3.3,3]azine (1) was reported in 1969 by Farquhar and L ~ ~ Together ~ ~ ~ +Present address: Institut fur Molekularbiologie, Honggerberg, CH-8093 Ziirich, Switzerland.
with its derivatives, this novel and interesting structure has proa testing ground for chemical understanding in several areas, . asI described recently.2 The first excited singlet state of 1 lies (1) (a) Farquhar, D.; Leaver, D. Chem. Commun. 1969,24. (b) Farquhar, D.; Gough, T. T.; Leaver, D. J . Chem. SOC.,Perkin Trans. 1 1976, 341.
0022-3654/84/2088-1376$01.50/0 0 1984 American Chemical Society
