The Journal of Physical Chemistry, Vol. 83, No. 6, 7979
Communications to the Editor
TABLE I acidity: mequiv/g
pore aperture,
zeolite
exptl
theory
a
offretite L
0.4 0.22
3 1.75
6.3 7.5
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a Acidity measured with dimethylaminoazobenzene as indicator (total acidity).
TABLE I1
A B C
base
indicator
X
Y
size
ZA
ZB ZC
presented shortly in ref 3. At first it has to be said that speaking of accessibility we shall consider only the acid sites in the largest channels and cavities. Looking, for example, at HY zeolite which, among the usual zeolites, has the largest pore appertures, it is easy to see that approximately one indicator molecule may enter a supercage (the smallest indicator described by Kladnig, neutral red, is 6.5 X 12.9 A compared to the 12-13-A diameter of the cavity). Hence eight molecules would fill a unit cell and then 0.66 mmol for 1 g of zeolite. Even if there were two indicator molecules per cavity (1.32 mmol per g) it is quite less than the theoretical 4.66 mequiv of acidity per g. Therefore there are less interacting indicator molecules than there are acid sites. Moreover if so much indicator molecules were in the cavities the base molecules would have no more space to reach the acid sites. Besides these steric effects it is obvious that as for solutions only a small fraction of the acid sites must react with the indicator which is only a probe of the acidity. The number of indicator molecules in cavities has then to be very small, and may be, for instance, one every ten cavities, i.e., 0.06 mmol per g of zeolite or even less. For faujasite type zeolites with rather large cavities, easily accessible through their four windows, the indicator molecules may diffuse in the crystallites and the conclusion of Kladnig on this point is quite reasonable. The problem may be quite different for zeolites with other structures. There may be two types of disturbing effects. First the pore size is large enough to admit indicator molecules but the channels have no connection to each other. This is the case for L zeolite for which the rneasured acidity (Table I)4 is a factor of 8 lower than the theoretical value. The adsorbed indicator and base molecules probably block the channels and stop the titration. The second case is when there is a sieving of the indicator and base from entering the channel. This occurs probably for offretite (Table I)5 where the channel or gmelinite cage apertures are close to 6.3 and 5 A, respectively.6 Only the n-butylamine (4-5-A diameter) may enter easily. Since anyway acidity is titrated it is suggested that the acidic centers close to the crystallite surface and accessible to the indicators molecules may be able to play the role of acidity probe if one assumes that the base neutralizes equally the sites wherever they are located and that the acidity strength distributions on the crystallite interior and exterior are similar in form. This last point has been evidenced for offretite5 by comparing various results. Either with L or offretite zeolites the results of Table I do not reflect the particle surface acidity. This peculiar acidity ?wastitrated with tributylamine which does not enter the pores. It is 0.01 m e q ~ i v / g . ~ Table I1 sums up the various situations. The n-butylamine (size x ) is smaller than the indicator (size y). In 0022-3654/79/2083-0767$01 .OO/O
767
case A the base does not enter the zeolite with pore size zA. No acidity except particle surface acidity is measured (titration with tributylamine). In case B the zeolite (pore size zB)admits only the base (offretite case). The acidity is titrable but probably steric hindrance at the pore apertures seem to disturb the results. In case C the zeolite (pore size zc) admits both the base and the indicator. A disturbing effect may arise if the pores do not allow easy diffusion of the molecules (type L) and the acidity is decreased as in case B. Our own results with H mordenite belong to case B and with faujasite to case C.3 In conclusion when the pore size of the zeolite is small, acidity is still titrable, at least partly, as long as the base enters the pores. Since only a small fraction of the sites has to react with indicators molecules, the size of the base is more critical than that of the indicator.
References and Notes (1) (2) (3) (4)
Kladnig, W. F., J . Phys. Chem. preceeding article in this issue. Drushel, I-I. V.; Sommers, A. L. Anal. Chem. 1966, 38, 1723. Barthomeuf, D. ACS Symp. Ser. 1977, 40, 453. Franco Parra, C.; Ballivet, D.; Barthomeuf, D.J . Catal., 1975, 40, 52. ( 5 ) Mirodatos, C.; Barthomeuf, D. J . Catal , in press. (6) Gard, J. A,; Tait, J. M. Adv. Chem. Ser., 1971, No. 101, 230. (7) Mirodatos, C.; Barthomeuf, D. to be published.
Denise Barthomeuf
Laboratoire de Catalyse Organique L. A. CNRS N. 237 6962 1 Villeurbanne, France Received August 24, 1978
Orbital Interaction in the Cycloaddition Reactions of Tetrasulfur Tetranitride, Tetraarsenic Tetrasulfide, and Tetraarsenic Tetraselenide with Olefins
Sir: Tetrasulfur tetranitride, S4N4,has been of gradually increasing importance as a starting material of sulfurnitrogen chemistry.' Among other reasons, it has recently drawn much attention as the precursor of polymeric sulfur nitride, (SN),,2 which is a low-dimensional metallic conductor, even becoming a superconductor at 0.3 Me3 Meanwhile, the reaction of S4N4with ordinary organic molecules is also a matter of continued i n t e r e ~ t .Several ~ examples of cycloaddition reactions of S4N4with olefins have been originally reported by Becke-Goehring and S ~ h l a f e r .They ~ have naively supposed the products 1 from 1
RHC/S--" I,
CHR
/ N\
2
1
3 an analogy with the usual Diels-Alder reaction, the S4N4 acting as a diene. On the other hand, Gleiter has proposed using the MO concept, structure 2 which would be ob1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
As-
25958
Communications to the Editor
3s
Figure 1. View of the molecular structures of S4N4(A), As4S4(B),and As4Se4(C).
tained by the attack of two olefins on transannular S-S bonds.6 Recent crystallographic analyses concerned with the bis adduct of S4N4with norbornadienes using X-ray diffraction7 or with trans-cyclooctenes using a NMR technique,8 however, have definitely shown a different structure, 3. In this case, the C=C bonds of two olefins add across two S-N-S units to give two five-membered C-S-N-S-C rings retaining the eight-membered (SN), ring. Here we attempt to reexamine the orientation resulting from the cycloaddition of S4N4with olefins based on a knowledge of the frontier orbital patternsg of S4N4with the structure in Figure lA.1° It is also of interest to study the reactions of analogous compounds such as tetraarsenic tetrasulfide, As4S4(realgar), and tetraarsenic tetraselenide, As4Se4,which have similar geometries to S4N4as shown in Figure 1B and 1C,I1 although there have been no experimental reports concerning cycloaddition reactions of these compounds. The MO calculations are performed with a semiempirical INDO-type ASMO-SCF method.12 According to the calculation, the lowest unoccupied (LU) MO’s of S4N4,As4S4,and As4Se4are all doubly degenerate and their levels are negative (-2.689, -0.942, and -1.871 eV, respectively). The patterns of the LUMO’s are shown in Figure 2. These energetically low-lying LUMO’s imply electron deficient properties for these compounds as the present authors have pointed 0 ~ t . lTherefore, ~ the interaction between the highest occupied (HO) MO of olefin and the LUMO’s of S4N4,As4S4,or As4Se4is dominant. Although there are two degenerate LUMO’s in these species, one of them, e.g., LUMO (a) in Figure 2, will become slightly lower than the other, LUMO (b), according to the initial charge transfer from the approaching olefins. For S4N4,considering the phase and spatial distribution of the LUMO of S-S pa* type, one of the olefins will attack concertedly SI and S3,and the other Szand S4as illustrated in Figure 3A, and vice versa for the other LUMO (b),giving structure 3 observed in crystallographic analyses. Another a 1 structure of two five-membered C-N-S-N-C rings proposed earlier on the basis of NMR and other spectroscopic
Figure 2. Patterns for the LUMO’s of S4N4(A), As4S4(B), and As4Se4 respectively. (C). X designates S or Se in As4S4 or As,Se,,
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Figure 3. Orbital interactions of olefin HOMO with S4N4LUMO (A), As4X4 1) MO (C). LUMO (B), and with As4X4 (LU
+
methods14 is thus ruled out from the present orbital interaction treatment, as has been rejected in recent experimental works.'^^ On the other hand, As4X, (X = S or Se), an olefin will attack either of two LUMO’s of As-As p?r types, affording a four-membered C-As-As-C ring as shown in Figure 3B, where % denotes 7r-type MO parallel to the four S (or Se) plane. The spatial distribution of the LUMO, however, may not be necessarily effective for cycloaddition cornpared with that of the (LU + 1)MO of As-As p~ type, the level of which is also negative (-0.152 eV for X = S and -1.251 eV for X = Se). Therefore, there will be another possibility that two olefins attack the (LU + 1)MO to yield two four-membered C-As-As-C rings as in Figure 3C. Anyway, for the reactions of As4X4 with olefins, the
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The Journal of Physical Chemistry, Vol. 83, No. 6,
Communications to the Editor
nonspecific solvent-solute interactions. The treatment proposed by Litt and Wellinghoff has several undesirable features. Mathematically, the correlation of five-seven experimental observations with four adjustable parameters is a poor practice, even with the restriction that the extinction coefficient of the complex ( E ) must be the same in all solutions. In the case of carbon tetrachloride as solvent, the conventional treatment (of all the data with two parameters ( K = 0.7 L/mol, E = 387) gives better agreement with the reported values of the normalized absorbance (in terms of root-mean-square deviations and the maximum deviation) than does the proposed treatment with four parameters. The obvious redundancy in the four-parameter fit must cast doubts on the physical significance of the parameters. This redundancy is clearly illustrated in the application of their equation with one of their parameters fixed (a = 1, eq 3, ref l),giving a linear relationship between the reciprocal of the normalized absorbancy and the ratio of concentrations of inert solvent and donor. From this linear relationship requiring only two independent parameters, they extract three parameters, each with stated limits of uncertainty. On a theoretical basis, their assumption of simple exchange equilibria between solvent and donor in the first coordination shell of the acceptor (an assumption which is not supported by any experimental evidence) should require the use of activity coefficients for the solvent and donor and for the various solvated fornis of the acceptor. Cancellation of these activity coefficients is at best equivalent to similar assumptions which have been made in the usual treatments. In the study of weak charge-transfer complexes (or hydrogen bonds), it is often necessary to make measurements over the entire composition range from almost pure solvent (S)to pure donor (D), with the acceptor (A) and the complex (C) very dilute.' The NIBS (nearly ideal binary solvent) treatment3$4has been shown to be quite dependable for estimating thermodynamic properties of simple solutes at high dilution in binary solvent pairs which are free of association. The form of the NIBS equation which has been most successful for describing the excess chemical potential of solutes is based on the simple mixing model of a multicomponent ~ y s t e m ~ , ~
structure >As-C-C-As
