Quantum chemical investigation of interaction sites in zeolites and

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3318

J. Php. Chem. 1980, 84,3318-3326

Quantum Chemical Investigation of Interaction Sites in Zeolites and Silica Joachim Sauer, Pave1 Hobra, and Rudolf Zahradnik" Central Institute of Physical Chemistry, Academy of Sclences, 1199 fast Berlin, German Democratlc Republic; Institute of Hygiene and Epidemiology, 10042 Prague IO, Czechoslovakia; J. HeyrovskY Instltute of Physical Chemlsfry and Electrochemistry,Czechoslovak Academy of Sciences, 12138 Prague 2, Czechoslovakla (Received: January 14, 1980; In Final Form: June 10, 1980)

Nonempirical SCF calculations with STO-3G and 4-31G basis sets were performed for very simple molecular models of selected interaction sites in zeolites and silica (disiloxane,the aluminasiloxane anion, free metal ions, Li+,Na+,Mg2+,and metal ions coordinated by the aluminasiloxaneanion). A description of charge distribution in silica and aluminasilicateis provided and compared to recent semiempirical results. The optimal structures of complexes formed by interaction between the sites and HzO, CHI, ethylene, and NH4+were found. The basis set extension effect was estimated by the Boys-Bernardi function counterpoise method. Comparison of different interaction sites has been made, and their relation to experimental data concerning adsorption on zeolites and pure microporous silica has been analyzed. Finally, a comment has been made on potentialities of semiempirical methods in zeolite and silica chemistry.

1. Introduction

Because of their great importance for separation and catalytic processes, interactions between zeolites and adsorbed molecules have been of great interest to numerous experimentalists and theoreticians (see, for example, ref 1-3). Knowledge about the nature of interactions of biomolecules with quartz particles is expected to contribute to the understanding of cytotoxic and fibrinogenic actions of quartz (see ref 4 and the literature cited therein). Although, a t first sight, both phenomena seem to be rather different, they exhibit some important common features: (1) Both the adsorption properties of zeolites and the biological action of quartz depend on the electronic structure of samples under study, which is strongly affected by the aluminum content in the framework. For example, pure microporous siIica (silicaIite) is hydrophobic and adsorbs preferably organic molecule^.^ In contrast, common zeolites (A-, X-, or Y-type) are hydrophilic. They possess an aluminasilicate framework and contain metal cations, which compensate for the framework charge (composition of a pseudo unit cell of the Na-A-type zeolite is Na12(A102)12(Si02)12). The cytotoxic and fibrinogenic action of quartz decreases with increasing A1 content.6 (2) The adsorption properties of zeolites depend furthermore on the kind of metal cations, which act as specific adsorption centers and modify the electric field within the cavities. Metal cations located in interstitial positions of quartz influence significantly the biological action. Detailed thermodynamic and spectroscopic data are available for the adsorption of different molecules on many types of zeolites. Their framework structure and cation A significant content have been modified ~ysternatically.~ advance was also achieved in theoretical investigations, reaching from molecular statistical calculations of thermodynamic functions7s8to quantum chemical calculations of sorption c o m p l e x e ~ . ~Reliable -~~ estimates of the interaction energy between the solid and the external partner represent an indispensable requisite for further progress in theoretical interpretation and for deeper understanding of the mechanism of sorption processes. Study of pathogenesis of silicosis clearly shows that primary processes which further lead to lung disease are connected with interactions between the surface of quartz and surface units of cell membranes.12 Among various modifications of the basic theory, the one that has become most popular considers formation of a semiconductor double layer on the surface of In the cited paper4 0022-3654/80/20a4-33 18$01 .oo/o

the electronic structure of a-quartz and dotted quartz has been studied by means of EHT and CND0/2 methods, It is obvious that in all above-mentioned processes van der Waals interactions play an important role. More specifically, ion-dipole, multipole-multipole and dispersion interaction are essential. Unfortunately the choice of a proper model and an adequate quantum chemical method represents a rather difficult task. The reasons are as follows. There is no doubt that it is possible to substitute for an infinite framework of zeolites or silica a cluster consisting of the Si, Al, and 0 atoms. However, a really representative cluster consists of many dozens of atoms which make sophisticated calculations of prohibitive size. The difficulty concerning an adequate method seems to be even more serious, which is due to the van der Waals character of some of the interactions to be studied. Namely, it has been shown that reliable interaction energies for different types of complexes result only when a nonempirical method which includes estimates of correlation energy is a~p1ied.l~ With special types of complexes, the SCF approach (neglecting the correlation energy) se9ms to be sufficient. The situation with semiempirical methods (of the CNDO type) is rather poor: conclusive arguments were presented concerning their inability to describe van der Waals molecule^.^^ Even the promising PCILO method will require a careful examination before being applicable for describing sorption processes. At this point the relative success of empirical potentials has to be mentioned, which is due to their very nature and parametrization. Their region of applicability is limited, however. The outlined difficulties prompted us to adopt the following strategy for our theoretical investigation: In the first exploratory stage the binding properties of interaction sites in zeolites and silica with representatives of different classes of adsorbed molecules are investigated by ab initio quantum chemical methods using small basis sets. The aim is twofold: to obtain a better understanding of interaction processes and to obtain a basis for selection, reparametrization, and perhaps modification of semiempirical methods. In the next part of the study detailed calculations with appropriate basis sets will be carried out and used to fit analytical potential^.'^ In the final step the simpler (semiempirical methods and the analytical potentials will be applied in connection with a model which should be as realistic as possible. In this stage explicit attention will also be paid to procedures in which the 0 1980 American Chemical Society

Interaction Sites in Zeolites and Silica

The Journal 01' Physical Chemistry, Vol. 84, No. 24, 1980

Scheme I: Model Systems Studied

-

interaction site of the solid oxygen atom of the siloxane bond (electron donor, proton acceptor, polarizability of siloxane bond)

prototype for external partners

molecular model for various forms of pure SiO, : disiloxane H3Si \o 2 1 H 3 for aluminasilicates and dotted silicas: alumina siloxane anion H ~ S I \07A"3

cations Xn+= Li+, Na+, Mg2+

3319

NH,' H, 0 CH, H,C=CH,

screened (coordinated) H3Sl,?/-AIH3

in+ free Xn+

external electric field effect is conisidered in the Hamiltonian (e.g., ref 16--18)of a cluster consisting of the adsorbed molecules and a representative part of the solid. 2. The Model Our model has to satisfy the following conditions: First, it should permit one to investigate basic types of interactions between solids and various small molecules. Second, it should be accessible to quantum chemical ab initio calculations. In pure siilica (silicalite, quartz) there exist two (important) types of interaction sites. The first one is represented by the silanol groups on external surfaces, the second one by the oxygen atoms of the siloxane bridges, =Si-O-Si:=. The former, like the surface and structural OH groups d zeolites, are the subject of a subsequent paper. An attack on the silicon atoms is sterically hindered because each of them is tetrahedrally surrounded by four oxygen atoms. The simplest cluster model of the siloxane bond is represented by disiloxane, H3Si-0-SiH3, which is formed by saturating the dangling bonds of the siloxane bridge by hydrogens (see Scheme I). The model adopted is unable to describe any of the finer effects due to structural features of various quartz modifications. It is true, however, that it would be possible to introduce into the model some structural variability when allowing for variation of the Si-0-Si valence angle. The experimental geometry of diisiloxane used (Rsio = 1.634 A, LSi-O-Si = 144'; ref 19) represents fairly well the structure of elementary units in silicas and zeolites. The model character of disiloxane, including electronic properties, is discussed in more detail in ref 20. Two nonbonding electron pairs of the siloxane bond are expected to act as electron donor, proton acceptor, and polarizable center. In order to be able to study electronic effects connected with introducing A1 atoms into Si positions, we have introduced the aluminasiloxane anion (Scheme I) which possesses the disiloxane geometry as a model for the aluminasilicate framework and for dotted quartz. The presence of metal cations, which compensate for the negative charge of the aluminasilicate framework, creates strong specific interaction sites. The interaction of molecules with free cations alone has been frequently considered as a model of adsorption in zeolites (see, e.g., ref 9 and 10). In this study the complex consisting of a cation and the aluminasiloxaine anion has been used as a further model. It should permit one to draw conclusions concerning the influence of the coordination of the cations by the framework unit on its binding properties. More specific information on the model in which a realistic cation pos-

1 7 H,SI L.

Bo .-a'" (C)

Flgure 1. Complexes between ethylene and the aluminasilicatestructural unit assoclated with the cation X+: (a)six-membered ring, (b) rectangular complex, (c) linear complex.

ition (SI)for the A-type zeolite is used is given in Figure 1. The CHI, C2H4, and H 2 0 molecules were selected as prototypes for nonpolar, nonpolar but polarizable, and polar proton donor adsorbates, respectively. The biopolymers were modeled by their characteristic functional groups. That is, methane simulates a nonpolar part of lipids as well as a side chain of hydrophobic amino acids in albumins; ethylene represents a double bond in unsaturated fatty acids and their derivatives, and, finally, the NH4+ion is a model for the side chain of basic diamino acids as well as for the functional group of phospholipids. For all molecules, included in Scheme I, experimental geometries (disiloxane,lg H ~ 0 , C2H4,22 ~l CH4123 C2H 6,22 NH4+24)have been used and kept unchanged during interaction with the model of the solid. Concluding this section the following statement is necessary. The investigation of interactions with individual, isolated interaction sites is a strong restriction, which excludes, for example, such important structural features as the cavity structure of solids. Extension of the present work in several directions (larger molecular models, inclusion of thermodynamic data, consideration of the cavity structure) is both possible and necessary in the future. 3. Computational Methods The interactioin energy calculated on the SCF level, (AESCF),is determined as the difference between energies of the complex arid the components (molecular model of the solid (s) and adsorbate (a)): USCF

+

= .ESCF(s...a) - ESCF(s) ESCF(a)

(1)

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The Journal of Physical Chemistry, Vol. 84, No. 24, 1980

Sauer et al.

TABLE I: Influence of Basis Set and BSSE' o n the SCF Interaction Energy of Water with Li', Na+, NH,', H,O, and CH,f 1 2 most extended basis complex set available AE AE H,O.. .Li+ - 151' - 190 H,O. . .Na+ -lOOd - 139 H,O. . .NH,+ - 97d - 114 H,O. . .H,O - 21.5' - 33.8 H,O...CH, - 1.4= -4.1

3 4-31G basis

4

5

A€

fAEb

AE

1

-135 - 99 -81 - 24.0 - 2.9

4 4 5.1 4.0

6 STO-3G basis

7 AE

A€

- 335 - 178

-152 -26.8 - 3.3

139 74 43 27.7 8.8

- At.

- 196 - 104 - 109

a Basis set superposition error. See ref 27 for original referReference 36. f = ~Lexp(HzO)/~,.,,c(H,O) = 0.709. ences. e Reference 37. f SCF interaction energies, A E (kJ/mol), and BSSE correction, A e , are calculated for the equilibrium geometry presented in Table 11.

TABLE 11: Equilibrium Geometry for Complexes of Water and Disiloxane with Different Ions and Molecules'

R molecule disiloxane H, 0

a

method STO-3G 4-31G STO-3G 4-31G best SCFb

Li' 0.17 0.18 0.17 0.18 0.19

Na+ 0.20 0.23 0.20 0.22 0.23

Mgz+ 0.18 0.19 0.18 0.19 0.20

Distance, R, in nm; angle, 9,in degree (see Figure 2).

NH,+ 0.24 0.27 0.24 0.27 0.27

HZO R 0.27 0.29 0.27 0.29 0.30

9

17 17 52 37 30

H,C=CH, R v 0.32 8 0.32 0.37

47 57

CH, R 9 0.33 0 0.38 0 0.33 23 0.38 75 0.40 0

See Table I for references.

Throughout the paper the minimum STO-3G and the split valence 4-31G basis sets22(for the 4-31G silicon basis see ref 25) were used and combined with a reoptimized STO3G basis26for the cations.27 The basis set superposition error (BSSE) was estimated by the Boys-Bernardi counterpoise The intersystem correlation energy was approximated by the London dispersion energy (bond polarizabilities modeP1). The C-H, C=C, N-H, and 0-H bond polarizabilities were taken from ref 32 and 33; polarizabilities for Si-H (12.76 X loz5cm3) and Si-0 (7.41 X cm3) were estimated.34 Ionization potentials for CH4,C2H4, and H20were taken from ref 35; for NH4+and disiloxane they were obtained from a 4-31G calculation (Koopmans theorem).

4. Results and Discussion 4.1. Reliability of Computational Procedures Used. SCF interaction energies calculated by means of small (minimum or split valence) basis sets in general suffer from three defects: physical and mathematical effect of the basis set used and neglect of correlation energy. The physical effect manifests itself in one-electron subsystem characteristics (multipole moments, polarizability). The mathematical effect is due to the different extent of A 0 basis set for subsystems and the supersystem. The Boys-Bernardi function counterpoise method allows an estimation of this type of e r r ~ r ~ which ~ - ~ Ooverestimates the interaction energy. Moreover the effect is accompanied by an unrealistic charge transfer from the donor molecule to the acceptor (see section 4.2). The behavior of the basis sets used with the abovementioned types of error is known.14 To enable the estimation of the respective errors, we present in Table I published and our own results for H20.-.H20,H20...CH4, and H20-.-cationcomplexes. STO-3G interaction energies are overestimated with H20.. .X+ (with respect to the reference data obtained in the extended basis set, Table I, column 1). For molecular complexes the agreement is fair. If one investigates the respective BSSE, which is always too large, the agreements prove to be fortuitous. The 4-31G values are overestimated, but in a uniform way. The reason is that this basis set exaggerates the charge separation in molecules; for example, it gives too large a dipole moment for water (4-31G: 2.61, experimentah 1.85 D). This leads to an overestimation of first-order elec-

trostatic interaction energy (multipole-multipole terms). The relative values of 4-31G interaction energies are reliable. This can be seen from values obtained by simple scaling using a common multiplier f = pex (H@)/p&31G(H20)(cf. ref 38) (see Table I, column 4). %he BSSE for all complexes investigated, except HzO. .CHI, is small. As the 4-31G basis set is not available for all elements involved in our models (Scheme I), we are forced to work also with the less suitable STO-3G basis. From comparison of data in Table I (columns 5, 7, and 2), we learn that AE(ST0-3G) mimics the extended basis values with molecular complexes, whereas for complexes with cations the STO-3G energies are greatly overestimated. On the other hand the corrected interaction energy, AE - Ac, is satisfactory for ion-molecular complexes but fails with molecular complexes. Therefore we decided to use the AE - At values for ion-molecular complexes and uncorrected AB for the other complexes. Interaction correlation energy, which can be approximately identified with dispersion energy, could be neglected for H20. .X+ and similar complexes (for H20** .Li+ it amounts to only 5 kJ/mol x 3%, ref 36); for HzO-.-HzO the dispersion energy equals 18% of AESCF(ref 36), and only for H2O-..CH4and similar weak complexes might the dispersion energy be comparable with AESCF. We have estimated, therefore, the dispersion energy only for molecular complexes (including the molecular ion NH4+). Let us state in conclusion of this paragraph that with the above-mentionedprocedures we are able to assess only the relative values of stabilization energies and not the absolute ones; these relative stabilization energies are sound, however. 4.2. Properties of Models for the Solids. As the models for interaction sites in zeolites and silica have not been treated before, their electronic properties deserve a discussion within this paper. The a s s u r n p t i ~ nmade ~ ~ ~ on charge distribution within the frame of empirical potentials is not realistic enough. Specifically,for pure silica the procedure gives zero atomic charges. The CNDO/ 2 c a l c ~ l a t i o n slead ~ ~ ~to~ results ~ which make a realistic impression, but it is not clear whether they are reliable. Therefore it seems useful to obtain some information (Table 111)on charge distribution from the present calculations. Let us start with the framework models, disiloxane and aluminasiloxane anion.

-

-

The Journal of Physical Chemktty, Vol. 84, No. 24, 1980 3321

Interaction Sites in Zeolites and Silica

TABLE 111: Calculated Charges (Mulliken Population Analysis) for Disiloxane, .Aluminasiloxane, and Complexes with Li', Na+, NH,+, and Mgzt disiloxane 4-31G catiion

4(0)

Lit Na' NH,' Mgz+

-1.373 -1.525 - 1.495 - 1.427 - 1.590

STO-3G

aluminasiloxane anion, STO-3G !I@+) --

4(Xt)

4(0)

+ 0.998

-0.655 -0.580 -0.671

+ 0.542

+1.952

-0.648

t0.989 t 0.940

The oxygen charge is considerable and amounts to -0.655 e with disiloxane and increases by -7% if a silicon atom is replaced by aluminum (-0.701 e). Because of the known overestimation of charge separation with the 4-31G basis (see section 4.1), these STO-3G values are more reliable. CNDOI2 and INDO (using Gordon's reparametrization, cf. ref 20) give for disiloxane -0.566 and -0.544 e, which justifies the use of semiempirical methods for such purposes.11>3g The influence of cations upon the oxygen charge has to be diricussed by using the 4-91G results because of the following reason. An alternation of the oxygen charge, if a cation is added, may arise from two effects. The first one, the polarization of disiloxane or aluminasiloxane itself, increases the oxygen charge, and the second one, charge transfer to the cation, decreases the oxygen charge. The charge transfer to the cations is largely overestimated by STO-3G calculations which are connected with the BSSE (vide supra), Obviously in the present STO-3G calculations the charge transfer outweighs the polarization. A similar defect can be observed with CND0/2 results recently p~blic;hed.ll*~~ The charge transfer to the cations discussed in these papers has to be regarded as an artifact of the method. In contrast, 4-31G calculations indicate an increase of the oxygen charge which is strongly dependent on the kind of the cations. These results allow one to draw the following conclusions: (1) The oxygen charge depends only slightly on the A1 content. Contrary to e x p e c t a t i ~ neven , ~ ~ ~in~pure silica polar binding sites exist. (2) Due to increasing polarizing action an increase in the electrical field gradient can be expected in the following order: H+ < NH4+C Na+ < Lit C Mg2+. (3) The use of charges resulting from INDO or CNDO calculations for the framework is reasonable, but caution is necessary with the decrease of oxygen charges if cations are introduced. In order to avoid the artificial charge transfer to extra framework cations, one should replace the latter by point charges in semiempirical or STO-3G calculations. Next we comment on the binding (abilityof framework oxygen sites toward extra framework cations. In order to separate the overall influence of the electrical field in zeolites, we used disiloxane as a model. Water was chosen as the reference system because its cation binding ability is well-known from 'both experiment and theory.41 As expected the basicity is smaller with disiloxane than with water (cf. Table IV, 4-31G values). Also a comparison with dimethyl etheir may be instructive: its cation binding cnergy is enhanced compared with that of water (experiment Li+ (ref 421, I(? (ref 43)).44 The difference in the cation binding energies (4-31G) by comparing water and disiloxane amounts to 10-30%. This result can be related to heats for binding of gaseous ions by zeolites, which were estimated from ion exchange equilibrium data and compared with the corresponding hydration heats.46 The hydration enthalpy proves to be more negative (-443 kJ/mol for Na+) than the enthalpy for binding to zeolites (-286 and -394 kJ/mol for Na+ to chabazite and to A-type zeolite).46This was attributed to

4(0)

4(X+)

+0.841

-0.701 -0.579 - 0.696

+0.300 t 0.782

t 1.561

- 0.657

t 1.438

__

TABLE IV: Interaction Energy, A E (kJ/mol), for Complexes between Cations and Disiloxane and Water Calculated with the Equilibrium Geometry Presented in Table I1 molecule

-

disiloxane

H2O

method

Li+

Nat

Mgzt

A E , STO-3G AE - AE',~ A E , 4-31G A E - A€,' STO-3G A E , 4-31G

-371 -187 -142 -196 -190

-165 -60 -98 -104 -139

-530 -390 -332 -388 -382

It has a A e = basis set superposition error (BSSE). been shown by calculations that the decisive p u t of A e is obtained if in calculations for disiloxane the ghost orbitals of the respective partner are included.

the fact that the framework atoms are more rigid in the lattice than are the oxygen atoms in water. The comparison of the computed binding energies for the individual oxygen binding sites (i-e., comparison of H 2 0 and disiloxane) suggests another reason, namely, an electronic one, which may be additionally operating: these values are lower for oxygen binding sites in zeolite framework than in water (cf. Table IV). 4.3. Interaction between External Partners and Models for Interaction Sites in Solids. In this section we start with the interaction of individual sites with external partners. Free metal cations are used as the most simple models for interaction sites in zeolites. Since not enough reliable data on complexes of cations with hydrocarbons exist in the literature (in contrast to cation water comp l e x e ~ )we , ~ investigated ~ the interaction of Li+, Na', and Mg2+with CH4,C2H6,and CzH4 The results are presented in the first paragraph of this section. The second paragraph deals with the interaction of disiloxane as a framework model with external partners. After that it is necessary to consider the fact that external molecules and ions interact not only with single sites of silica or zeolites. The simultaneous interaction of external molecules and NH4+ with a cation and an aluminasilicate fragment is considered in the third paragraph by using the (Xt-.aluminasiloxane anion) model. In this paragraph the action of quartz in biological medium is also discussed. Interaction, of Lit, Nat, and MgZtwith Hydrocarbons. The 4-31G equilibrium distances and interaction energies obtained for the most favorable approach are given in Table V. For ethylene, obviously, the perpendicular approach (structure 1, cf. Figure 3) is preferred, A similar result has been obtained for the isobutene.. -Na+complex."' Opposite information (preference of structure 2, cf. Figure 3) resulted from I'CILO for Nat-. eisobutene, which indicates the failure of this method in such a case.48 Finally, the CNDOIB prediction agrees with the 4-31G one.47148 The governing energy contribution originates from ionquadrupole interaction which prefers the approach to be perpendicular to the molecular plane (structure 1, cf. Figure 3). This becomes obvious if these energy contributions are calculated by means of classical formulas47

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The Journal of Physical Chemistry, Vol. 84, No. 24, 1980

Sauer et al.

TABLE V: Interaction between Hydrocarbons and Li', Na+, and Mgz+(4-31G calculation^)^ C,H,. . .X+ CH,. . .X+ cation X+ Lit Na+ MgZ a R in nm and AE +

R

C,H,. . .X+

-AE

approach 1

-AE

R

0.234 25.2 0.227 38.8 0.276 16.7 0.270 25.9 0.232 95.9 in kJ/mol; for the definition of R see Figure 3.

R

-AE

R

-AE

0.227 0.267 0.227

63.3 48.5 194.0

0.241 0.282

21.3 13.5

TABLE VI: SCF Interaction Energy, A E (kJ/mol), and Dispersion Energy, E D (kJ/mol), for Complexes of Disiloxane and Water with External Partners Calculated with the Equilibrium Geometry Presented in Table I1 molecule disiloxane water

method A E , STO-3G A E , 4-31G ED A E , STO-3G A E , 4-31G

NH,' -92a -84 -39.6 -109' -114

H,O -22.7 -22.4 -ll.gb -26.8 -33.8

C,H, -3.1 -8.4 -5.1 -6.8

CH, -2.2 - 2.4 -5.6 -3.3 -4.1

approach 2

R

\

o,

' ' '

x+= L,', ai, M~'',NH:

X+

Flgure 2. Geometry of models used for disiloxane and water complexes.

a In case of the NH,' cation A E - A e is given for the STO-3G basis. Corresponding value for (H,O),: - 3.0 kJ/mol. 5 2

using polarizabilities and quadrupole moments for ethylene (ref 49) and the distance in Table V. The basis-set effect on the quoted molecular properties50 suggests that the relative stability of the two structures will not change when passing to more extensive basis sets. Experimental data for comparison with calculated energies are rare. From the binding enthalpies of Li+ to isobutene and propene42the binding enthalpy for Li+... ethylene of about 75 kJ/mol has been obtained by extrapolation. This value is somewhat larger than the calculated one, which might be due to poor description of the polarizability within the 4-31G basis set. For CzHs the interaction energy with cations is only about one half that of C2H4. This is understandable because C2HGhas a nearly vanishing quadrupole moment.51 The further reduction in interaction energy with CH4runs parallel to the decreased polarizability of this molecule. Interaction of Disiloxane with CH,, C2H4,H 2 0 , and NH4+. The electronic structure of oxygen in disiloxane resembles to some extent oxygen in dimethyl ether and water. The behavior of water in many different environments is well-known from both experiment and theory. Thus a comparison of interaction energies between water and disiloxane is expected to provide useful information about the binding properties of disiloxane, e.g., the relative basicity. As expected the SCF interaction energy (cf. Table VI, 4-31G values) clearly predicts disiloxane to have a lower proton acceptor ability than water. When passing from water to methanol and dimethyl ether the proton affinity is also reduced (STO-3G calculation for methanol... water).53 The estimated dispersion energy is considerably large for all complexes with disiloxane, however, and this will decrease the differences in stabilization energy found between complexes of water and disiloxane. A quantitative energy estimate for the hydrogen bond between water and disiloxane would be useful in discussion of adsorption phenomena (vide infra). It is possible to base this estimate by comparing the disiloxane. .H20 and HzO. .HzO complexes. Adopting the experimental hydrogen-bond energy for the water dimer (21.7 kJ/rnollS4 and assuming comparable correlation contributions in both cases (4-5 k J / m 0 1 ) ~and ~ reliability of relative 4-31G energies, AE - At, we arrive at 13-14 kJ/mol for the disiloxane..-H,O bond. This value is considered as a lower limit because the correlation contribution for disiloxane

-

-

I

x+

R

L

I

Flgure 3. Geometry of model complexes between hydrocarbons (CH4, CpHe,C2H4)and cations.

TABLE VII: STO-3G Distances (in nm) for X+. .Yand Aluminasiloxane-. . .X+Complexesa

X'

Y H,O

Li+ Na+ Mgz+ a

0.17 0.20

0.18

CZH, approach approach 1 2 0.23 0.24 0.22

0.22 0.27

CH,

aluminasiloxane-

0.23 0.26 0.22

0.16 0.19 0.17

Cf. Table VI11 and Figures 2 and 3.

is indicated by the dispersion energy to exceed that of water (cf. Table VI). Interaction of Alurninasiloxane Anion- -X+with CH,, C2H4,H 2 0 ,and NH4+. In the next step we shall consider the fact that cations in zeolites or dotted silica are bound to the aluminasilicate framework. Figure l a shows the environment of the Naf ion occupying an SI site in the A-type zeolite as well as the approach of ethylene (cf. ref 55 for X-ray data concerning Na-A X 6CzHZ). Ab initio treatment of the whole six-membered aluminasiloxane ring would be very costly. Therefore the approach of ethylene from above to the cation which is bound in-plane to the aluminasiloxane anion (rectangular structure) was studied (Figure lb). Clearly, a linear structure (Figure IC)would be more favorable because of minimal repulsion, but it is the rectangular model which includes the essential feature of the real structure (cf. Figure la). In this manner the interactions of H20, CH4, and CzH4with aluminasiloxane-...X+ were investigated by assuming that the geometry of aluminasiloxane-. .X+ and X+. * .H20, CHI, C2H4is the same as the optimized one for the binary complexes (cf. Table VII, Figures 2 and 3). For ethylene both approaches, 1and 2 (Figure 3), were studied in order to see whether their relative convenience is influenced by the framework. a

The Journal of Physical Chemisfry, Vol. 84, No. 24, 1980

Interaction Sites in Zeolites and Silica

3323

TABLE VIII: Interaction Energy between H,O, C,H,, CH,, NH,', and Different Interaction Centerse system t'3A1\0/0s

energya

H3d

.*

b

L

C

t'3A1\f5

a b

H3d

C

wl * C i- 3AI \ / 5

H3d

&2+

C,H, -approach 1 approach 2 -20.5 -20.5

CH4 - 14.4

NH,'

- 23.9

-9.1C

H3b

a

W j A I \&S

H,O - 68.6

a b C

- 112 - 197

- 60.0 - 106

39.8 -73.3 - 106 21.8 - 259

-393 29.4

31.6

- 10.3 - 32.9

11.4

9.8 -43.1 11.1 4.3 -- 14.4 -1.5

-98.4 - 268 34.0

- 38.3 16.6 - 5.0 - 13.6

-107.1'

-0.4 -14.6 - 115 11.5

repulsive

a a : AE( [A-X']...Y) = A E T - ~AE(A--.X'), ~ ~ ~ A E T is~the~ stabilization ~ ~ energy related to the three noninteracting constituents, A- (aluminasiloxane anion), X' (cation) and y ; b : AE(X+...Y);c : AE(A-...Y); AEtotal, aE(A-.-X'), and AE(X+*..Y)are corrected for BSSE estirnated by means of the Boys-Bernardi counterpoise method as the difference between the energies of the systems A-...Y, A-, and Y, respectively, with and without the functions of the cation being present in the The same geobasis set. The three-body contribution is defined as E ( 3 )= A E ( [A-X+]*..Y)- AE(Xf...Y) - AE(A-...Y) . metrical model (cf. Figure 2) as with disiloxane is used; the respective distances ( R , in nm) and angles ( 9 , in degrees) are Cf. Figure 4; distances R are taken from the alumi0.25, 12; 0.519, 5; and 0.29, 0 with H,O, C,H,, and CH,, respectively. nasiloxane-...X' complexes (cf. Table VII); angle 01 optimized for Lit equals 120" and is used also with other complexes. The approach of H,O, C,H,, and CH, t o the cation is The optimized distance R (in nm) is 0.25 with both Li' and Na+. depicted in Figure 1b; the geometrical parameters are the same as the values optimized for the aluminasiloxane-...Xt and X+.*.H,O,C,H,, CH, complexes (cf. Table VI1 and Figures 2, 3). e For comparison interaction energy, b, with free cations and pair interaction energy, c , between H,O, C,H,, CH,, and the aluminasiloxane anion are given (STO-3G values in kJ/mol).

The calculations were performed with the STO-3G basis set, and the results are presented in Table VIII. From the values 21 and from the entries of Table V, we may conclude that the STO-3G energies, which are corrected for the BSSlE (vide supra), reasonalbly parallel those with the 4-31G. The data presented in Table VIII show that the coordination of cations by the alumiriasiloxane framework manifests itself in a very pronounced reduction of the interaction energy. With saturated hydrocarbons it amounts to only a few kJ/mol. This effect can be partly ascribed to the repulsion between the aluminasiloxane anion and the' molecule (HzO, CW4, C2H4) which approaches the cation. Hence binding energies for CHI to aluminasiloxane-*.-X+(X+ = Li', Na+) are larger (27.9 and 8.6 kJ/mol) for the linear structure due to vanishing repulsion (cf. Figure IC).It is known from calculations on complexes of the HzO. .X+. * .HzO that the three-body effect decreases the interaction energy. It strongly depends on the geometry and can be explained, a t least partially, by the repulsion of induced dipoles in both of the ligands.56 For the H20.. -Li+.-.HzOcomplex56 the three-body contribution is only 2.4% of the total interaction energy, but, its absolute value is nearly as large as the direct repulsion between the two water molecules. In the HzO.*nBe2+..-Hz0complex57the three-body contributions are equal to 6.3 and 152 kJ/mol with linear and rectangular structures, respectively. Two-body repulsion amounts to 28.5 and 102 kJ/mol. For the cases under study we calculated the two-body repulsion energy, AE(aluminasiloxr~e-...U),and hence are able to estimate the three-body contribution. Caution is necessary because minimal basis (setsare known to overestimate such This manifests itself in unrealistically high charge transfer from ligands to the cation which diminishes its polarizing action. We hope to minimize this error by estimating the three-body coiitribu1,ion using interaction energies from which the main part of the BSSE is eliminated (cf. footnote d in Table VIII). From the entries for Li+ and Na+ of Table VIII, it rnay be concluded that the three-body effect is of the same order of magnitude as the direct repulsion and in cases where the direct interaction with alumina-

-

siloxane is attractive (aluminasiloxane-. CzH4, CH4) it forms the only repulsive part. For Mg2+this effect is several times larger than the two-body repulsion. The approach 1of CzH4 to the cations is the more convenient one in spite of the fact that the framework interaction favors the approach 2 (Table VIII). This result does not permit a definite conclusion about the real situation because the model contains only a very limited part of the framework. Further investigation using CNDOI2, in which the whole six-membered ring is considered, is in progress.47 We proceed now to comparison with the other kind of interaction sites accessible for adsorbed molecules in zeolites: the oxygen sites of the aluminasilicate framework which are represented by oxygen in the aluminasiloxane anion. The results show that strong hydrogen bonds are formed with water and moderately strong ones with CzH4 and CH4 These energies are overestimated because other charged centers in the neighborhood diminish in reality the electrical field created by the anion. From a comparison with both kinds of interaction sites in zeolites (cationic ones and framework oxygens), the following conclusion can ble drawn: Because of strong specific interaction in cases of HzO and CzH4 the cationic interaction sites are stronger than the framework oxygen sites except CHI where the situation is reversed. For the adsorption of HzO on Na+ containing zeolites there is some evidence that the H20 molecules both are bonded to Na+ with their oxygen site and form hydrogen bonds to the framework oxygen (bidentate adsorption),58,69 To be as realistic as possible we investigated the aluminasiloxane. .Na+. .HzO complex with the 0-. .Na+ distance (0.236 nm) and the O-~..Na+..~OH, angle (38O) derived from the cr:ystal structure for hydrated Na-A zeolite.6g The O H bond of water points toward the aluminasiloxane oxygen (O.-.O distance of 0.362 nm, Na+. .OHz distance of 0.228 nm). For this complex the following energies have been obtained: a = -104; b = 40.5; and c = -20.6 kJ/mol (cf. Table VIII). For the attack of NH4+on the aluminasiloxane-. .X+ complexes, a different model has been used which accounts for the fact that ion exchange is possible between NH4+ and other cations in aluminasiloxane. Figure 4 shows an

-

.

-

-

3324

The Journal of Physical Chemistry, Vol. 84,No. 24, 1980

H

Sauer et al.

TABLE IX: Summary of Experimental Data Used in the Discussion adsorbed molecule CH4

Figure 4. Approach of NH',

-

to the aluminasiloxane. .X+ complexes.

arrangement in which both cations interact as closely as possible with oxygen but have minimum repulsion between themselves. The a angle was optimized for X+ = Li+ and the distance R of the NH4+ion for both X+ = Li+ and X+ = Na+. The STO-3G basis set was used and results are presented in Table VIII. From the entries of Table VI we can learn that interaction energy of disiloxane with NH4+ is considerably larger than that with H20,CzH4, and CH4. When passing to models for dotted silica significant changes are observed (Table VIII). The interaction energy with NH4+ is decreased; with aluminasiloxane--. -Mg2+--.NH4+ even repulsion results. However, the interaction energy with HzO and C2H4 is considerably increased. It is worth mentioning that the energy of interaction for CH4 and CzH4 with aluminasiloxane--.-X+ does not achieve the value for disiloxane...NH4+ . These results seem to correlate with known fibrogenic activity of quartz and dotted quartz. This point will be discussed in a subsequent paper where also proper attention will be paid to the surface silanol groups. 4.4. Comparison of Interaction Sites Investigated and Relation to Experimental Information from Adsorption. A comparison of different interaction sites should help to elucidate differences in adsorption ability of zeolites containing different cations and pure microporous silica. The relevant experimental data are collected for convenience in Table IX. From comparison with the STO-3G energies (Table VI) the following conclusions can be drawn: (1) Saturated Hydrocarbons. The differences are expected to be small because their interaction with cations in zeolites is reduced by the aluminasilicate framework and hence becomes comparable to their direct interaction with the framework oxygen. The interaction with oxygen in pure silica (disiloxane.CH4) is somewhat smaller. Because of close contact of hydrocarbons with the framework oxygens, the dispersion energy is important in all cases. These conclusions are in agreement with the nonspecific character of the adsorption isotherms and with the dependence of the heats of adsorption of the pore filling known from experiments.62The small increase of the heat of adsorption with increasing coverage of the cavities is due to adsorbate-adsorbate interactions. Furthermore the initial heat of adsorption62for C2H6shows very little dependence on cation exchange (Na, K, Rb, Cs), which means that the cation binding sites are not dominating. The only exception is the Li+-exchanged zeolite, which exhibits a larger initial heat of adsorption, but if more and more molecules are adsorbed the value typical for other zeolites (Na-X) is approached. This behavior is understandable in view of the data of Table VIII: only for this cation binding site is the interaction energy with CH4larger than that for the framework oxygen. Specifically, the heat of adsorption for CHI on Na-X is comparable with the calculated (about 17 kJ/mo1)60*61 interaction energy aluminasiloxane-. .-CHI (Table VIII). The corresponding value on US-Ex is 14 kJ/mo160 (extrapolated from the n-alkane series starting with C3Ha). N

heat of adsorption, kJ/mol silica

-A H

US-Ex

14.0 * 1.5a

zeolite Na-Y

-AH

15.9 i 1.5a

Na-X 16.9 i 1.5a Na-X 17.6b C2H6 Li-X 32.lC Na-Y 24.3 * 1.5a Na-X 26.3 i 1.5a Na-X 25.gC cyclohexane US-Ex 47d Na-Y 5 6d n-hexane silicalite 67-75e Na-Y 63.6f US-EX 47.3s C,-H,. Li-X 5O.gc9 Na-X 37.3c 6' H6 US-Ex 45d Na-Y. 80d H, 0 Na-A' 10Ojl~ Li-X, SIII 90gim Li-X, SII 65g Na-X, SJII 80s Na-X, SII 65g silicalite 25e framework 55-63gaj US-Ex 45h oxygen a Extrapolated from the n-alkane series6"starting with C,H,. Reference 61. Reference 62. Reference Reference 60. g Reference 63; the 40. e Reference 5. data for different adsorption sites were estimated from dependence of differential heat of adsorption on pore filling. h Reference 64. Nonlocalized cation, J Reference 65. Gas-phase binding enthalpy Li'. . -C,H, of about 75 kJ/ mol extrapolated from data for isobutene and propene.42 Gas-phase binding enthalpy for Na'. . .H,O amounts to 100 kJ/moL4' Gas-phase binding enthalpy for Li'. . .H,O amounts to 142 kJ/moL4I

The heats of adsorption for n-hexane on Na-X,GOU S - E X , ~ and silicalite5 are 63.6,47.3, and 67-75 kJ/mol. The difference between the two silica modifications may be explained by a higher dispersion energy contribution in silicalite due to the all-round contact of an n-hydrocarbon with the walls of the channels. Finally the surprisingly good agreement between the calculated free ion binding energies for CHI and C2H6 (Tables V and VIII) and the heats of adsorption (cf. Table IX) seems to be, in the light of the preceding discussion, fortuitous. (2) Unsaturated Hydrocarbons. Despite the screening of the cations, specific interactions are involved in the adsorption. That means first that a clear dependence of the heat of adsorption on the cation exists, which is in agreement with experiment,B2and second that considerably smaller heats for silicalite and US-Ex than for zeolites are expected. For the former, olefins show interaction energies similar to those of saturated hydrocarbons (compare disiloxane.. CZH4 and disiloxane..-CH4 in Table VI). Experimental information can be obtained from the heat of adsorption for benzene and c y c l ~ h e x a n e .On ~ ~US-Ex it equals 45 and 47 kJ/mol, whereas in the presence of cations (Na-Y) the corresponding values are 80 and 56 kJ/ mol. (3) Water. Most pronounced differences for adsorption properties of various silica and zeolites are predicted for water. Even for the screened cations a very large interaction energy is obtained in agreement with experiments for H,O on Li+ and Na+ containing zeolites (cf. Table 1x1. The dependence of heats of adsorption on the degree of pore filling permits one to make conclusions about interaction energies with different sites. The range of enthalpies for the cation binding sites screened to a different extent (cf. Table IX) becomes plausible on the basis of the

The Journal of Physical Chemistry, Vol. 84, No. 24, 1980

Interaction Sites in Zeolites and Silica

calculated data (Table VIII). Even the fact that the energy for the most screened position (SIIin Na-X and SI in Na-A) is almost equal to the energy for the interaction with a framework oxygen is reproduced by the calculations. The highest, value of -100 kJ/mol for Na-A approaches the gas-phase binding enthalpy fOr Na+...H20 (100 kJ/ mol) and the calculated value (106 kJ/mol, compare Tables VI11 and I). This could be explained by interaction with a nearly zero coordinated cation as detected in dehydrated Na-A by SefP6 or by a simultaneous interaction of HzO with Na+ and the framework oxygen within the eight-ring windows of Na-A,6b In order to decide this question it is necessary to make calculations which take into account the external field created by the net charges of all the atoms within the calculation of the Na+.. .HzO complex and to consider complexes of the aluminasiloxane-. .HzO...Na+ type, too. At this point however it should be mentioned that such an assignment of djfferential heats of adsorption to the individual types of interactions is somewhat arbitrary because, if a further molecule is added, all water molecules already present in the cavity rearrange to give the lowest total energy of the system. In this process water-water interactions may also play a role, which is nicely illustrated by the crystal structure of the hydrated Na-A zeolite.6g The main result derived from the calculations is summarized by the following ordering for the strength of interactions: free cation. .HzO > screened cation.. -H20 > framework ogygen.. .HzO > Hz0...lH20. ( 4 ) Hydrophobicity. The preferable adsorption of organic molecules by silicalite compared with water, which was attributed to the very weak basicity of the siloxane bond (“hydrophobic” properties of the siloxane b o n d 6 ~ 9 is mainly a coinsequence of the strong interactions in liquid water itself. The sequence of strength of pair interactions from the calculations (cf. Table VI) is as follows: HzO. * .HzO > disiloxane.. -HzO> disiloxane.. .CH, > CH,. CH,. That means if water “can choose” between water and internal silica surfaces (US-Ex, silicalite) as an interaction partner, it prefers water, whereas CH4 favors the silica surface. Thus the different behavior for water, which condenses outside the solid, and organic molecules, which fill the pores: is understandable. A possible influence of the surface silanol groups is discussed in the next paper. The different experimental values for the initial heats of adsorption on silicalite and US-Ex (25 and 45 kJ/mol, cf. Table IX) are tentatively explained as follows. The geometry of the channels in silicalite permits the water molecules to form at most two hydrogen bonds to siloxane oxygens; that nieans the interaction energy should fall into the range between one and two hydrogen-bond energies which amounts to 14-28 kJ/mol (lower limit). In contrast, the macropores present in US-Ex permit condensation thereinMand therefore the measured heat is very close to the heat of condensation for water. Acknowledgment. Our sincere thanks are due to Professor w. Schirmer (Berlin) and to Professor F. Janda (Prague) for support and for continuous interest in this work.

.

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References and Notes (1) J. A. Rabo, “Zeolite Chemistry and Catalysis”, American Chemical Society, Waslhington, 1976. (2) J. R. Katzer, Ed., ACS Symp. Ser., No. 40 (1977). (3) D. W. Breck, “Zeolite Molecular Sieves-Structure, Chemistry, and Use”, Wiley-Interscience, New York, 1974. (4) P. Hobza and J. Hurych, Environ. Res., 16, 432 (1978). (5) E. M. Flanigan, J. M. Bennett, R. W. Grose, J. P. Cohen, R. L. Patton, R. M. Kirchner, and J. V. Smith, Nature(London), 271, 512 (1978). (6) 6.G. Beck, R Holusa, D. JirEtkov.5, B. KyseiEt, K. Robock, and V. Skoda, Staub-Reinhait. Luft, 33, 3 (1973).

3325

(7) R. G. Kretschmer and K. Fledier, Z . Phys. Chem. (Leipzig), 258, 1045 (1977). (8) A. G. Bezus, A. V. Kiselov, A. A. Lopatkin, and Pham Quang Du, J. Chem. SIX., Faraday Trans. 2 , 74, 367 (1978). (9) H. H. Dunkeri and V. I. Lygin, ”Quantenchemie der Adsorption an FestkbperoMchen”, VEB Deutscher Verlag f i Gntndstoffindusble, Leipzig, 1978. (10) D. Geschke, W.-D. Hoffmann, and D. Deininger, Surf. Sci., 57, 559 11976). ( 1 1) W. J. b e r , P. Geerlings, C. Van Alsenoy, and H. P. Figeys, J. Phys. Chem., 83, 855 (1979). (12) J. DeDasse aind J. Warlus, J. CoiioaInterface Sci., 56, 616 (1976). . . (13) K. Robock, Befir. Silkose-Forsch., 26, 112 (1974). (14) P. Hobza and R. Zahradnlk, “Weak Intermolecular Interactlons In Chemistry and Bloiogy”, Eisevier, Amsterdam, 1980. E. Ciementi. “Lecture Notes in Chemistrv”. Voi. 2, Springer-Veriaa. . West Berlin, 1976. M. R. Hayns and L. Dissado, Theor. Chim. Acta, 37, 147 (1975). J. 0. Noell arid K. Morokuma, J . Phys. Chem., 80, 2675 (1976). J. Pan& and R. Zahraddk, Helv. Chim. Acta, 61, 59 (1978). A. Aimenningen, 0. Bastiansen, V. Ewlng, K. Hedberg, and M. Traetteberg, Acta Chem. Scand., 17, 2455 (1963). J. Sauer and B. Zurawski, Chem. Phys. Lett., 85, 587 (1979). W. S. Benedict, N. Gallar, and E. K. Piyler, J. Chem. Phys., 24, 1139 (1956). J. A. Pople in “Applications of Electronic Structure Theory”, H. F. Schaefer, 111, Ed., Pienurn Press, New York, 1977, p 1 . H. C. Alien and E. K. Piyler, J . Chem. Phys., 26, 972 (1957). “Tables of Interatomic Distances and Configuration in Molecules and Ions”, Special Pubiicatlon No. 1 1 , The Chemical Society, London, 1958. L. C. Snyder and Z. Wasserman, Chem. Phys. Left., 51, 349 (1977). All the valence orbitals are Included In our calculations, which corresponds to the C 111method of Pullman et al?’ To be cmsistertt, we reoptimized also the STO-3G basis for Li’ and arrived at the following reoptimlzed exponents: 1s 3.76, 2sp 2.95, which yields for LI’ a total energy of -7.183809 au. A. Pullman, H. Berthod, and N. Gresh, Int. J . Ouanfum Chem., S 10, 59 (1976). A. Johanssen, P.Kollman, and S. Rothenberg, Theor. Chim. Acta, 29, 167 (19733. M. Urban and F’. Hobza, Theor. Chim. Acta, 36, 207, 215 (1975). S. F. Boys and F. Bernardl, Mol. Phys., 19, 553 (1970). P. Claverie in “Intermolecular Interactions: From Diatomics to Blopoiymers”, 13. Pullman, Ed., Wiley, New York, 1978, p 69. M. J. Mantlone and J. P. Daudey, Chem. Phys. Lett., 6, 93 (1970). R. J. W. Le FBvre, Adv. Phys. Org. Chem., 3, 1 (1965). J. Sauer, unpublished result. W. J. Wedenejeiw, L. W. Curwitsch, W. H. Kondratew, W. A. Medwedew, and E. L. Frankewitch, ”Energien Chemischer Bindungen, Ionizatbnspoteniialeund ElektronenaffMiten”, VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1971. G. H. F. Dlercksen, W. P. Kraemer, and B. 0. Roos, Theor. Chim. Acta, 36, 249 (1975). S. R. Ungernach and H. F. Schaefer 111, J. Am. Chem. Soc., 96, 7898 (1974). H. Umeyama and K. Morokuma, J. Am. Chem. Sac., g9, 1316 (1977). S.Beran and J. Dubskg, J. Phys. Chem., 83, 2538 (1979). W. Schirmer, H. Thamm, H. Stach, and U. Lohse, “The Influence of Dealumlnatiori of Synthetic Y Zeolltes on the Equilibrium of Adsorption of C,-Hyidrocarbons”, Central Institute of Physlcal Chemistry, Academy of Sciences of the German Democratlc Republic, preprint, 1979. P. Schuster, W. Jakubetz, and W. Marius, Top. Curr. Chem., 60, l(1975). R. H. Staley and J. L. Beauchamp, J . Am. Chem. Soc., 97, 5920 (1975). W. R. Davidson arld P. Kebarle, J. Am. Chem. Sot,, 98,6133 (1976). It Is worth mentioning that STO-3G energies predlct a reverse order in U+ affinity, namely: disiloxane (-371 kJ/moi), water (-335 kJ/mol), dimethyl ether (-326 kJ/moi).‘* A. Pullman, C. Gicsssner-Prettre, and Yu. V. Kruglyak, Chem. Phys. Lett., 35, 156 (1!375). R. M. Barrer and J. A. Davies, J. phys. Chem. Sol&, 30, 1921 (1969). J. Sauer and D. Delninger, In preparatlon. R. Lochmann, W. Meiler, and K. Muller, Z. Phys. Chem. (Leiprig), in press. F. Mulder and C. Huiszoon, Mol. Phys., 34, 1215 (1977). F. Muider, M. van Hemert, P. E. S. Wormer, and A. van der Avoird, Theor. Chim. Acta, 46, 39 (1977). A. D. Bucklngham, R. L. Disch, and D. A. Dunmur, J . Am. Chem. Soc., 90, 3104 (’1968). W. Kolos, Theor. Chim. Acta, 51, 219 (1979). J. E. Del Bene, J . Chem. Phys., 55, 4633 (1971). R. W. Boiander, J. L. Kassner, and J. T. Zung, J. Chem. Phys., 50, 4402 (1969). A. A. Amaro and K. Seff, J. Phys. Chem., 77, 906 (1973). H. Kistenmacher, ti. Popkie, and E. Clementi, J . Chem. Phys., 61, 799 (1974).

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(57) P. A. Koiiman and I. D. Kuntz, J. Am. Chem. Soc., 94,9236 (1972). (58) L. Bertsch and H. W. Habgood, J . Phys. Chem., 67, 1621 (1963). (59) V. Grarniich and W. M. Meier, Z. Kristallogr., 133, 134 (1971). (60) W. Schirmer, H. Stach, H. Thamm, M. M. Dubinin, A. A. Isirikjan, N. I. Regent, and E. Ch. Anaktschjan, Z. Phys. Chem. (Leiprig), in press.

(61) R. J. Neddenriep, J. Colloid Interface Sci., 26, 293 (1968). (62) A. G. Bezus, A. V. Kiseiev, Z. SediBEek, and Pham Quang Du, Trans. Faraday Soc., 67,468 (1971).

(63) 0.M. Dzhigit, A. V. Kiseiev, K. N. Mlkos, G. G. Muttik, and T. A. Rahmanova, Trans. Faraday SOC.,67,458 (1971). (64) U. Lohse, H. Thamm, and H. Stach, "Adsorption of Hydrocarbons in Zeolites", Reprints of the Workshop, Academy of Sciences of the German Democratic Republic, East Berlin (1979). (65) M. M. Dubinin, A. A. Isirikjan, G. U. Rachmatkariev, and V. V. Serpinskij, Izv. Akad. Nauk SSSR, Ser. Khim., 1269 (1972). (66) V. Subramanian and K. Seff, J. Phys. Chem., 81, 2249 (1977). (67) N. Y. Chen, J. Phys. Chem., 60,60 (1976).

Gas-Phase ,6 Decay of Multitritiated Methane in Methyl Chloride and Bromide, and in Binary Mixtures of Methyl Fluoride, Chloride, and Bromide M. Coloslmo" and R. Bucci Istltuto dl Chimica Nucleare de/ C.N.R., C.P. IO, 00016 Monterotondo Stazlone, Rome, Ita& (Received: January 21, 1980)

The mechanisms of the gas-phase CX3+(X = H, T) electrophilic attack to methyl chloride and bromide are discussed on the basis of the radioactive end products. The observed chloride and bromide ion transfer processes support the formation of dimethylchloroniumand dimethylbromonium ions. CX3+attack in binary mixtures of CH3Y (Y = F, C1, and Br) is discussed in terms of the observed relative rates and the methyl cation transfer processes among the halomethanes. The existence of long-lived gaseous dimethylhalonium ions is supported.

Introduction The disagreement between the results obtained in the condensedl and in the gaseous2phase about the reactivity of methyl halides toward methylating agents led us to investigate the reactions between CX3+(H, T), produced by tritium 0decay,3p4eq 1,and CH3F at near atmospheric CX4

82%

+

CX3+ 3He + p-

+v

(1) pressure^.^ Labeled methyl fluoride was recovered as the only tritiated organic fluoro compound; thus, we concluded beyond doubt that the methyl ion attacked the fluorine n electrons and that the observed fluoride ion transfer involved participation of the symmetrical complex postuaccording to lated by Henis et

CX3+ + CH3F + CX3FCH3+ F= CX3F +CH3+ (2)

+

CX3+ CH3F

-

--

CX3FCH3+

CHX3 + CH2F+

(3a)

CHBX+ CX2F+ (3b)

Furthermore, we excluded that, if CX3+attack occurred on u electrons, it led to the formation of stable neutral fluoro compounds found in superacidic solutions.laVd The results agreed with those obtained in gas-phase y-radiolysis experiments on methyl fluoride: which supported the formation of ethyl fluoride in ion-molecule processes from the rearrangement of excited dimethylfluoroniumions and not from direct methylation of CH3F to the u bonds. In this article, we extend our investigation to methyl chloride and bromide and report on the behavior of these compounds toward CX3+ions during the reactions: CX3++ CH3Cl ... radioactive neutral compounds CX3++ CH3Br

--.,.

(4)

radioactive neutral compounds (5)

Then, the results are compared with those obtained in the CH3F system. In addition, we discuss the relative rates of CX3+attack in binary mixtures of CH3F, CH3C1, and 0022-3654/80/2084-3326$0 1 .OO/O

CH3Br, taking into account other data from the literat ~ r e . ~ ~ ~ The approach used in this work represents a sensitive method for determining the site of the attack of methyl ions. Moreover, the use of labeled ions at pressures higher than those commonly employed in conventional ionmolecule experiments provides a unique tool for the study of electrophilic reactions, filling the gap between solution chemistry and mass spectrometric methods.

Experimental Section Materials. Multitritiated methane, from stock solution of CT4 in CH4,578v9was freed from impurities through a well-established procedurea8Methyl fluoride and methyl chloride (Matheson Co., USA), methyl bromide (MerckSchuchardt, FRG), and oxygen (SI0 Co., Italy), added as a thermal radical scavenger, were used as received; gas chromatographic analysis confirmed the absence of interferring impurities in the reacting gases. Procedure and Analysis. Sample preparation was described in detail elsewhere? Duplicate experiments were carried out and at least seven analyses were performed. The total quantity of methane was 1.9 torr in every sample; thus, ion-molecule processes between CX3+ ions and methane were regarded as highly unfavored, and their contribution was disregarded. A previous work5 showed that radiolytic effects did not play a significant role during the formation of products. The flow-radiogas-chromatographiclo analyses through a Porapak Q column were carried out under the following conditions. (a) Pure methyl chloride and bromide systems were analyzed at 160 "C with 1.2 L h-l nitrogen flow; (b) the mixed methyl fluoride/methyl chloride and methyl fluoride/methyl bromide systems were analyzed at 60 "C with 1.2 L h-l nitrogen flow for 30 min, then the oven temperature was raised to 160 "C manually; (c) the mixed methyl chloride/methyl bromide systems were analyzed at 160 "C with a 1.2 L h-l nitrogen flow. Make-up nitrogen was added to the effluent gas from the gas chromatograph to produce a total flow of 10 L h-l at the exit of the ionization chamber. In ad@ 1980 American Chemical Society