J. Phys. Che" 1882, 86,107-111
growth studies of Wiechers, Sturrock, and Marais," the rate of reaction was also interpreted in terms of a parabolic rate law with a rate constant, 2.4 dm6 mol-' min-' (mg of seed)-'. Unfortunately, the data cannot be compared with those of the present studies since the specific surface area of the crystals was not reported. The proportionality, demonstrated in Table I, between the rate of growth and amount of inoculating seed crystals confirms the absence of secondary nucleation within this range of supersaturation. Crystal growth was therefore confined to active sites on the added seed crystals. A number of experiments, summarized in Table 11,were made with seed preparation C at pH values in the range 8.25-8.78 and carbonate/bicarbonate ratios varying from 0.20 to 1.08. The constancy of the calculatedrate constant, (17)H.N. S.Wiechers, P. Sturrock, and G. V. R. Marais, Water Res., 9,835 (1975).
107
normalized for seed surface area, 17.8 f 1.2 X lo3 dm6 mol-' min-' m-2, again points to the applicability of eq 1. A series of experiments, summarized in Table 111, were made at 15 and 35 "C with seed preparation C and 0.020 mol dm-3 calcium and carbonate titrant solutions. The marked influence of temperature, illustrated by the values of the rate constants k , corresponds to an activation energy, 39.2 f 3.6 kJ mol-' consistent with the proposed surface controlled mechanism for the crystal growth of ~ a l c i t e . ~ J ~ The constant composition method enables the rates of growth to be studied even at very low supersaturations with a precision hitherto unobtainable by conventional seeding experiments. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Gas Research Institute for the support of this research.
Hydrogen Bonding Interaction Effect on Carbazole Triplet State Photophysics M. M. Martin' and E. BrOhOret Labwatoke de Photophyslque Mol6culelre du CNRS, a t l m n t 213, UniversU Paris-Sud, 91405 Orsay, France (Received: April 30, 1981; In Final Form: August 20, 1981)
Hydrogen bonding interaction between the first triplet state (T,) of carbazole and pyridine was studied by means of the conventional flash photolysis technique. The carbazole triplet lifetime was determined in cyclohexane solutions at low pyridine concentration, that is when hydrogen bonding between carbazole and pyridine occurs neither in the ground state (So)nor in the first excited singlet state (Sl).Kinetics of carbazole triplet quenching by pyridine were examined on the basis of the Mataga kinetic scheme for hydrogen bond formation and dissociation reactions in the excited state. Formation of the hydrogen bonded complex carbazole-pyridine in the triplet state was found to be diffusion controlled. Equilibrium between free and bonded carbazole was shown to be established during the triplet lifetime. Bonded carbazole triplet lifetime was found to be 0.077 of that of free carbazole. Besides triplet-triplet absorption, carbazyl radical absorption was observed and was found to increase with increasing pyridine concentration. The carbazyl radical formation in the presence of pyridine was attributed to hydrogen atom transfer from carbazole to pyridine through the hydrogen bond. Photocomplexation of protonated and deuterated carbazole was compared and the hydrogen atom transfer process was shown to be responsible for the nonradiative deactivation of the hydrogen bonded triplet.
Introduction It is known that hydrogen bonding interaction may have considerable effects on deactivation processes of excited ?r-electronicsystems.' In particular, very low fluorescence yield of hydrogen-bonded complexes compared to that of corresponding unbonded molecules has often been observed and is generally explained in terms of chargetransfer interaction through the hydrogen bond. The first direct observation of a transient charge-transfer state of an excited singlet hydrogen-bonded complex has been reported recently for the 2-naphthylaminepyridine system by Ikeda, Okada, and Mataga.2 Excited state quenching by hydrogen bond acceptor has also been observed for triplet states. Hydrogen atom transfer from proton donor to acceptor has been shown to be responsible for triplet quenching of compounds such as 2-naphthol or 1-anthrol by N- heterocycle^.^ In this paper, we report on carbazole triplet quenching due to hydrogen bonding with pyridine and substituted pyridine. Conventional microsecond flash photolysis technique was used to measure carbazole triplet lifetimes Laboratoire associ6 B l'Universit6 Paris-Sud. 0022-3654/82/2086-0107$01.25/0
with increasing pyridine concentration. Low quencher M-were chosen in order to concentrations-below avoid hydrogen bonding complexation in the ground state and in the first excited singlet states4 The kinetics of carbazole triplet quenching by pyridine were studied on the basis of the Mataga scheme for hydrogen bond formation and dissociation reactions in the excited state.' Furthermore, nonradiative processes in the triplet hydrogen bonded complex were investigated. In a previous paper, Martin and Ware4presented a detailed study of the fluorescence quenching kinetics of these systems and examined the nonradiative deactivation of the hydrogen bonded complex in the excited singlet state. The lifetime of the carbazolepyridine complex S1state was estimated to be 28 ps, whereas that of free carbazole is 15 ns. (1) N. Mataga and T. Kubota, 'Molecular Interactions and Electronic Spectra"; Marcel Dekker, New York, 1970. (2)N. Ikeda, T.Okada, and N. Mataga, Chem. Phys. Lett., 69,251 (1980). (3)K. Kikuchi, H.Watarai, and M. Koizumi, Bull. Chem. SOC.Jpn., 46,749(1973);S.A. Yamamoto, K. Kikuchi, and H. Kokubun, J. Photochem.. 6.469 (1976): (1976): . .. Bull. Chem. Jnn.. _ .49.2950 . . .. J. Photochem.. 7, 177 (i977). (4) M. M. Martin and W. R. Ware, J.Phys. Chem., 82, 2770 (1978).
0 1982 American Chemical Society
108
The Journal of Physical Chemistry, Vol. 86, No. 7, 1982
Martin and Br6h6ret
Charge-transfer interaction was suggested for the drastic decrease in S1carbazole lifetime due to hydrogen bonding, although no direct observation of the charge-transfer state has been made. In this paper S1 and T1 complexation reactions are compared and the hydrogen bonding effect on carbazole excited states is discussed.
Experimental Section Carbazole (99%, Aldrich) was purified and deuterated as reported in ref 4. Pyridine and 2,6-dimethylpyridine, purchased from Merck, were distilled several times under reduced pressure. Merck (Uvasol) cyclohexane was passed through a column of basic alumina prior to each series of experiments. In some experiments, Uvasol cyclohexane was used without any further purification. The results obtained under the latter conditions were corrected for solvent impurity quenching. Carbazole concentration was varied in the 0.8-6 X M range. Most of the solutions were 1-2 X 10" M in carbazole. Solutions were deaerated thoroughly and experiments were performed at room temperature. Triplet quenching was studied using the conventional flash-photolysis technique. Light flashes for excitation were produced by discharging a capacitor bank of 23 pF, charged to maximum 18 kV, across six 22 cm long flash lamps.5 For this study, the energy was varied between 400 and 2000 J(6 and 13 kV). The flash duration is 4 pus (independent of the discharge energy). The solutions were illuminated in a cylindrical, optical silica cell, 20 cm long (i.d. 1.4 cm), provided with an annular cylindrical jacket (i.d. 0.4 cm) which contained a filter solution consisting of 5 g of potassium hydrogen phthalate/L of water. The filter transmitted light of wavelengths longer than 300 nm. The flash lamp emission was further filtered by wrapping the cell in a yellow cellophane filter. About 25% of the excitation light was transmitted in the So S1 carbazole absorption wavelength range (300-340 nm) and 2 to 10% in the T1 T, absorption wavelength range (400-500 nm). The transient transmission changes of the flash-exposed solutions were monitored using a xenon lamp (Osram XI30 150W/1) as the analyzing source. Part of the analyzing light was eliminated by means of GG 375 and GG 395 Schott filters. The transmitted xenon light was directed through the sample and through a monochromator (Bausch and Lomb, f = 50 cm, bandwidth 1 nm), onto a photomultiplier (EMI, 9558 BQ). The photomultiplier signal was displayed on a Tektronix 564 oscilloscope and viewed by a video camera (CF 123 V). The data were transmitted to a Tektronix 4051 basic computer through an analog to digital converter (Thomson TSN 1150).
-
-
Results and Discussion A. Carbazole Photolysis in Neat Cyclohexane: Triplet-State Properties. Excitation of carbazole with highintensity microsecond flashes (A > 300 nm) was found to produce two transients: the first triplet state T1and the carbazyl radical RN., characterized by their absorption at 420 nm and around 600 nm, respectively? These transients were first observed by Borovkova et al.?who showed that RN. was formed via carbazole T, states through a biphotonic process. Yamamoto et ala8gave evidence that carbazole two-photon dissociation involves T1 as the in-
5
/
1
Figure 1. Carbazole triplet disappearance examined at 420 nm according to relation 1 in the presence and absence of pyridine. Results are presented here for [Q] = 0, 1.0, 1.5, 3.0, and 5.0 X lo-' M. M in carbazole. Flashed solutions were 2 X
termediate. Martin et al? demonstrated that the carbazyl radical can be formed through stepwise processes involving SI as well as T1 as the intermediate. In the present study, an appreciable decrease in carbazyl radical absorption was obtained by filtering the excitation light in the Tl T, absorption wavelength range as described in the Experimental Section. The carbazole triplet disappearance was examined at 420 nm by subtracting the residual radical absorption. The triplet decay was analyzed assuming that the following reactions were occurring.
-
T1- so (ko) Ti + Ti Ti + So (km) +
The triplet decay rate may then be expressed by the relation -(d(ln C,)/dt),=O = ko
+ km(cT)t=o
(1)
where the subscript t = 0 refers to conditions at time zero and CT to the triplet concentration. Relation 1was used to determine ko and k m . The initial logarithmic triplet decay as a function of C T was obtained by flashing solutions with increasing excitation energy. The rates were plotted as a function of triplet concentration (CT)t=O, extrapolated at time zero (Figure 1, [&] = 0). The curve is linear as required by relation 1. The intercept with the vertical axis gives ko = 3.4 X lo3 s-l and the slope gives krr = 4.1 X lo9 s-l. The first-order rate constant ko corresponds to a 0.29 ms unquenched triplet lifetime. The second-order rate constant krr is, as expected, close to that for diffusion. B. Hydrogen Bonding Interaction with Pyridine: Triplet State Quenching. From previous studies1p4it is well established that carbazole (AH) can form a hydrogen-bonded complex (AH-Q) with pyridine (Q), in the ground state as well as in the first excited singlet state. The equilibrium constant K for complex formation and dissociation reactions is 12 M-' for carbazole in So and becomes as large as 200-400 M-' in S1. As a matter of fact,
I
H
( 5 ) L. Lindqvist, Reo.
Phys. Appl., 3, 15 (1968). (6) M. M. Martin, E. Brdhdret, F. Tfibel, and B. Lacourbas, J.Phys. Chem., 84, 70 (1980). (7) V. A. Borovkova, Kh. S. Bagdasar'Yan, V. F. Nikel'ni, V. A. Koloaov, and I. Kiryukhin, Dokl. Akad. Nauk SSSR,224,616 (1977). (8)S. A. Yamamoto,K. Kikuchi, and H. Kokubun, 2.Phys. Chem., 109,47 (1978).
I
change in carbazole ground state absorption is observed
The Journal of Physical Chemistry, Vol. 86, No. 1, 1982
Hydrogen Bonding Interaction in Trlplet States
I09
TABLE I
k, or 7 i 1 x carbazole
quencher
NH NH
pyridine 2,6-dimethylpyridine pyridine
ND Scheme I
+
AH(So)
hu
t Q
AH*(T,)
& k2
s-1
k,, 2106 s-1
k , or 7c-1 x 104 s-1
1350 1000 3500
0.75 1.0 0.30
4.4 t 0.4 3.6 t 0.4 2.3 t 0.3
AH*(S,i
NH
20-
(AH*-Q)(T,)
products
(C1
5". AH(S0)
K ,M-*
kl,
104 s-I 2109 M-I 0.34 t 0.03 1 0.34 t 0.03 1 0.34 f 0.03 1
Ot
(AH-Qi(Soi
for pyridine concentrations above M, whereas fluorescence quenching occurs for pyridine concentrations as low as M. In the present experiments, hydrogen bonding interaction in the triplet state was thus studied for pyridine concentrations below M to avoid complexation in the So state (less than 1%)and to minimize complexation in the S1state as much as possible. Under these conditions, according to Mataga kinetic scheme,' the proposed light-induced reactions are given in Scheme I with ko and k, the triplet decay rate constants of free and bonded carbazole, and kl and k2 the rate constants for complex formation and diasocition reactions, respectively. If reaction C occurs, one expects from exciplex photophysics4p9a two-component triplet decay for AH* and a growth followed by a one-component decay for (AH*Q), that is [AH*] = cle-Xlt ~ ~ e - ~ 2 ~
.
Flave 2. Photocomplexatbn in the triplet state. Fit obtained between experimental 0)and calculated (-, ---,eq 2) data, for the change in the carbazole triplet decay constant k ( [ Q ] )wlth increasing pyridine concentration: protonated carbazole; ( 0 )deuterated carbazole.
m
+
[(AH*-&)] = c3(e-'lt - e-X2t) with Xi,z =
f/2[k0
+ ki[Q1 + k2 + kc
((k2 + k c - ko -
~I[QI)+ ~ 4~ik2[Q11"21 (2) However, if reaction C reaches equilibrium rapidly compared to the triplet lifetime, that is if kl[Q] and k 2 >> ko and k, (3) One expects a single exponential triplet decay for both AH* and (AH*-Q) with the same decay constant A. Equation 4 gives the relationship between A, ko,k,, and the equilibrium constant K, for a given pyridine concentration. (X - k&'
(k,- ko)-'[l
+ (K[Q])-']
(4) Experimentally, the carbazole triplet disappearance was examined at 420 nm with increasing pyridine concentration, in the same way as described in part A. Parallel straight lines were obtained for relation 1 as shown in Figure 1. The intercept with the vertical axis k([Q]), k([QI = 0) = ko,is shifted toward higher values when pyridine concentration is increased, indicating that carbazole (AH) triplet state is quenched by pyridine. The change in triplet decay constant k([Q]) is reported in Figure 2. Triplet quenching is attributed to the formation of the hydrogen-bonded complex carbazolepyridine (AHQ) (reaction C). On the other hand, the triplet decay constants were found to verify eq 4 (Figure 3), showing that the equilib(9) W. R. Ware, D. Watt, and J. D. Holmes, J . Am. Chem. SOC.,96, 7853 (1974).
Flgure 3. Rapid equllibrlum between free and bonded carbazole in the triplet state. The change In k ( [ Q ] )with Increasing pyridine concentration is analyzed by using eq 4: (e) protonated carbazole: (A) deuterated carbazole.
r i m between free and bonded carbazole has been reached during the triplet lifetime. The equilibrium constant K and the bonded triplet decay constant k, were determined from the intercept with the vertical axis and from the slope of the straight line given in Figure 3. The values are reported in Table I. A fit between the experimental k([Q]) values and X were searched for by using the general eq 2. As shown in Figure 2, good agreement was found between measured decay constants and hl calculated by using K and k, values deduced from Figure 3 (A2 -). The fit could be obtained only for kl 2 lo9 s-l X M-l, which fulfills equilibrium conditions (eq 3). It is seen from the results that hydrogen bonding complexation of carbazole in the triplet state is a diffusioncontrolled process and that the equilibrium between free and bonded species establishes during the triplet lifetime. The equilibrium constant is found to be considerably larger than that obtained in the S1state.4 This is surprising because the hydrogen bonding power of carbazole in T1 is probably smaller than that in S1,as it is usually observed for proton donating molecules, in view of the larger electron density around the nitrogen atom in the triplet state than in the singlet state. Smaller K values cannot explain the triplet quenching reported in Figure 2 and illustrated by reaction C. ' The reason for the discrepancy between S1and T1equilibrium constants does not come out clearly from the present study and it would be interesting to
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110
The Journal of Physical Chemistty, Vol. 86,No. 1, 1982
Martin and Br6h6ret
Figure 4. Excitation energy effect on carbazyl radical absorption at 600 nm; V , capacitor voltage given in kV/2 (see text): (0)pyridine free solution; (e)8 X lo-' M pyridine solution.
undertake further investigation to clarify this aspect of carbazole photocomplexation. Since the equilibrium between free and bonded carbazole is reached during the triplet lifetime, the bonded carbazole triplet could not be discriminated from that of free carbazole. On the other hand, at high pyridine concentration the bonded triplet absorption could not be determined either because hydrogen bonding complexation occurs in the first excited singlet state leading to a drastic triplet yield de~rease.~JOThe spectrum of the triplet complex is expected to be slightly red shifted compared to that of free carbaz~le.~ Furthermore, as the absorption band centered at 420 nm is fairly broad, the extinction coefficient at this wavelength probably remains almost unchanged., The bonded triplet lifetime (7,) is found to be 0.077 of that of free triplet ( T ~ giving ) evidence for nonradiative deactivation induced by hydrogen bonding. In Table I, results are compared for pyridine and 2,6-dimethylpyridine. N o appreciable change could be observed. The bonded triplet lifetime is almost the same for both quenchers, in contrast with the bonded singlet (S,) lifetime which previously had been found to increase upon pyridine methylation? The nonradiative deactivation of the excited complex thus depends on the nature of the excited state involved in the complexation. C. Nonradiative Deactivation of Hydrogen-Bonded Carbazole T1State. Carbazyl Radical Formation. As mentioned in part A of the results section, it is known that the carbazyl radical (RN.) can be formed by carbazole photodissociation, through a biphotonic process involving carbazole S1 and T1 states as the intermediates. In the present study, the radical concentration was studied as a function of pyridine concentration and of excitation energy, by measuring the transient optical density OD,(RN-) at 600 nm. The radical concentration was found to increase when pyridine was added to the solution, provided that [Q] < M. Furthermore, the excitation energy ( E ) effect on radical formation was shown to be different in the presence and absence of pyridine. As a matter of fact, if both mono- and biphotonic reactions are occurring OD,,(RN.) should vary with E according to relation 5.
OD,,(RN.)/E
=
+ QE
(5)
where a1and a2are proportional to the quantum yield of mono- and biphotonic radical formation, respectively. In the reported experiments, E was assumed to be proportional to the square of the discharge voltage V to produce the excitation flashes and OD,(RN.)/ V was therefore plotted against V. In figure 4,results are compared for carbazyl alone and for a 8 X lo4 M pyridine solution. It is seen that only the biphotonic reaction is occuring in the absence of pyridine whereas the curve obtained for the (10) N. Mataga, Y. Torihashi, and Y . Kaifu, 2.Phys. Chem. (Frankfurt am Main), 34, 379 (1962).
solution containing pyridine indicates partly monophotonic, partly biphotonic radical formation (a1# 0). It is also seen that the biphotonic process (az)for carbazole alone is 0.7 of that of carbazole with pyridine. Since in the solution containing pyridine only part of the triplets formed are hydrogen bonded, biphotonic radical formation must be due to both free and bonded carbazole with a higher yield for the bonded species. On the other hand, monophotonic process may be attributed to bonded carbazole, which implies that hydrogen atom transfer occurs from carbazole to pyridine, through the hydrogen bond of the complex in the triplet state Tl. When pyridine concentration is increased above M, singlet S1 complexation occurs and radical concentration decremes. For [Q] = 1M, that is when carbazole is totally hydrogen bonded in the ground state as well as in the excited state, neither triplet nor radical absorption could be detected. This observation is consistent with previous statements that nonradiative deactivation of bonded carbazole in S1 leads neither to intersystem crossing nor to carbazyl radical f ~ r m a t i o n . ~ JTherefore, ~J~ hydrogen atom transfer occurs exclusively in the bonded triplet state. Carbazole Deuteration Effect. Triplet quenching by pyridine was compared for deuterated ( N D ) and protonated ( N H ) carbazole (Figures 2 and 3). From the results, reported in Table I, it is seen that free carbazole triplet lifetime is not influenced by deuteration whereas that of bonded carbazole is about twice longer for the deuterated molecule. The equilibrium constant is also increased by deuteration. The results give evidence for hydrogen atom transfer from carbazole to pyridine in the hydrogen-bonded complex formed in the triplet state. Complex dissociation and carbazyl radical formation (RN.) are both slowed down by carbazole N H bond deuteration. N o isotope effect had been observed on the excited singlet complex lifetime.4
Conclusion We have provided evidence that hydrogen bonding complexation of carbazole with pyridine occurs in the first triplet state and leads to carbazyl radical formation through a hydrogen atom transfer process. We have thus shown that the nonradiative deactivation process induced by hydrogen bonding is different for carbazole S1and T, states. The photocomplexation is a diffusion-controlled reaction in both states but the equilibrium constant for the complex formation and dissociation reactions is about five times larger in the triplet state. The equilibrium betweeen free and bonded carbazole cannot be established in the first excited singlet state because of the very rapid deactivation of the complex,* whereas equilibrium is reached in the triplet state. The lifetime of the excited singlet hydrogen bonded complex is about 500 times shorter than that of free carbazole. The triplet lifetime is changed by a factor 13 only. Charge-transfer interaction between partners of the complex has been proposed to explain the fact that bonded carbazole is almost unfluorescent.'r4 Although no direct observation of the carbazole-pyridine charge-transfer state has been reported so far, results similar to those obtained for the 2-naphthylaminepyridine system are expected.2 The hydrogen atom transfer from carbazole to pyridine is responsible for the bonded triplet state deactivation since carbazyl radical formation competes with T1 So electronic relaxation. In reaction C , lz, is 13 times larger than ko because of k,. The electronic relaxation rate constants k , and k3 are probably identical. As a matter of fact, the Franck-Condon factors are not expected to vary upon hydrogen bonding since the
-
(11) H. Masuhara, Y. Tohgo, and N. Mataga, Chem. Lett., 59, (1975).
J. phys. Chem. 1982, 86, 111-114
spectra are just slightly red shifted and broadened. Hydrogen atom transfer has been reported for systems such as 2-naphthol-pyridine and 1-anthrol-pyridine in the excited singlet state as well as in the triplet state.3 For these systems radical formation is shown to occur from the encounter complex in competition with hydrogen-bonded complex formation. The hydrogen-bonded complex leads to radical formation only in the triplet state. The hydrogen atom transfer from carbazole to pyridine does not occur
111
in the first excited singlet state. The carbazole deuteration effect on the bonded triplet lifetime gives evidence for carbazyl radical formation from the hydrogen-bonded complex in the triplet state.
Acknowledgment. The authors acknowledge much help and advice from B. Fourmann regarding the data analysis setup and from Dr. L. Lindqvist regarding the present paper.
Quantum Chemical Study of the Physical Characteristics of Calclum-Faujasites and Their Interactions with Water S. Beran The J. HeyroWkf Instmite of phvsicel Chemlsby and Electrochemlsby, Czedwsbvak Acedmy of Sclences, 121 38 Rague 2, Czechoslovakk fRecelv&: May 18, 1881: I n Final Form: July 28, 1881)
The CNDO/2 method is used to study the physical characteristics of Ca- and Ca(0H)-faujasites, modeled by T60&OH)12clusters, and their interactions with water molecules. It has been shown that the properties of the skeleton of the Ca zeolite are practically identical with those of the faujasites of the alkali metals and that the cationakeleton bond has marked ionic character. It was also shown that the cations of the hydroxyl group are unstable and tend to exhibit basic character. The interaction of water molecules with the Ca cation of the zeolite, which did not appear particularly advantageous, resulta in donation of electron density from the water molecule to the Ca cation and polarizationof the water molecule by the electrostaticfield of the cluster.
Introduction The ever-increasing interest in zeolites in various areas in the chemical industry has led in recent years to intense study of their properties and factors which affect these properties. A factor which has a marked effect on the physical, chemical, and catalytic properties of a particular type of zeolite is the kind of cation compensating the negative charge on the zeolite skeleton. Consequently, the properties of (primarily faujasite type) zeolites were studied with various exchange cations using different ex~erimentall-~ and also theoreticalh21 methods. A reason (1)Breck, D. W. "Zeolite Molecular Sieves"; Wiley, New York, 1974. (2) Jacobs, P. A. 'Carboniogenic Activity of Zeolitee";Elsevier: New York, 1977. (3) Haynes, H. W., Jr. Catal. Rev. Sci. Eng. 1978,17, 273. (4) Mikheikin, I. D.; Abronin, I. A.; Zhidomirov, G. M, Kazanskii, V. B. J. Mol. Catal. 1978,3,435. (5) Kezanekii, V. B.; Gritscov, A. M.; Andreev, V. M.; Zhidomirov, G. M. J. Mol. Catal. 1980,4, 135. (6) Lygin, V. I.; Smolikov, V. V. Zh. Fiz. Khim. 1978,49,1626. (7) Mortier, W. J.; Geerlings, P.; Van Alsenoy, C.; Figeys, H. P. J. Phya. Chem. 1979,83,855. (8) Mortier, W. J.; Geerlings, P. J. Phya. Chem. 1980, 84, 1982. (9) Sauer, J.; Hobza, P.; Zahraddk, R. J. Phys. Chem. 1980,84,3318. (10)Gibbs, G. V.; Meacher, E. P.; Smith, J. V.; Pluth, J. ACS Symp. Ser. 1977, No. 40, 19. (11) Toesel, J. A.; Gibbs, G. V. Phya. Chem. Miner. 1977,2, 21. (12) Mortier, W. J.; Pluth, J. J.; Smith, J. V. J. Catal. 1976,46,367. (13) Grabowski, W.; Moisono, M.; Yoneda,Y. J. Catal. 1980,61,103. (14) Dubs&, J.; Beran, S.;BosHEek, V. J. Mol. Catal. 1979, 6, 321. (15) Beran, S.; Dubs&, J.; BosBEek, V.; J'h, P. React. Kinet. Catal. Lett. 1980, 13, 151. (16)Beran, S. J. Mol. Catal. 1980, 10, 177. 0022-3654/82/2086-0111$01.25/0
was an effort to modify the properties of presently available zeolites to obtain the most optimal properties of these zeolites for their use for various hydrocarbon conversions. On the other hand, knowledge of the physicochemical characteristics and changes in these characteristics in dependence on various factors can also be useful in the synthesis of new types of zeolites. A method yielding direct information (at a molecular level) on the physicochemical properties of zeolites involves calculation of the electron structure and various physical characteristics of models of these zeolites using quantum chemical methods. Thus the aim of this work has, in connection with an earlier work dealing with the quantum chemical study of H, Na, Li, K, and A1 zeolite^,'^-^^ to perform a theoretical study of the physicochemical characteristics of Ca-faujasites and their interactions with water molecules.
Model and Methods The zeolite skeleton was modeled using T606(OH)12 clusters (where T designates a Si or Al atom) representing a sixfold window leading into a large cavity (cluster A, cf. Figure 1)or into a prism (cluster B, Figure l), i.e., a model which has already been successfully used to model zeolites ' J ~ ~ ~cluster with Li, Na, K, Mg, and Al c a t i o n ~ . ~ * ~Similar models of the solid phase, however, describe only short(17) Beran, s.;Dubs&, J. J. Phys. Chem. 1979,83, 2538. (18) Beran, S.; Dubs&, J. Chem. Phys. Lett. 1980, 71, 300. (19) Beran, S. Z. Phys. Chem. (Frankfurt am Main),1980,123,129. (20) Beran, S.; J'h, P.; WichterlovH,B. J.Phys. Chem. 1981,85,1951. (21) Beran, S. J. Phys. Chem. Solids In press.
0 1982 American Chemical Society
