J. Phys. Chem. 1992, 96,9039-9043
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Na?' Clusters in Sodium Sodalite V. I. Srdanov,*vt I(. Haug,***H. Metiu,*l+JyOand G.D. Stucky*.tJ Center for Quantized Electronic Structures and Departments of Chemistry and Physics, University of California, Santa Barbara, California 93106 (Received: February 6. 1992; In Final Form: June 8, 1992) We have measured and calculated the absorption spectrum of the Na43+clusters in Na3[A1Si0413sodalite prepared by high-vacuum deposition of sodium atoms. The samples with a Na43+:Na33tcluster ratio up to 1:lO show a single-absorption feature with A, = 628 nm (1.99 eV). The absorption originates from the individual sodalite cages containing the Na>+ cluster. For the Na>+:Na33+cluster ratio larger than l:lO, when some of the Na>+ clusters are likely to interact, the changes in the absorption spectra indicate the onset of insulator-metal transition. Time-dependent quantum mechanical calculations of the photon absorption cross section of Nad3+clusters in sodalite at infinite dilution were carried out. The dependence of the calculated spectra on sodalite framework charges and cluster geometry was used to determine which of the proposed charge distribution models are consistent with the absorption spectra. The best agreement between measurements and calculations is obtained for Na = +1, Si = +1.9, A1 = +0.9, and 0 = -0.95.
I. Introduction The A104and S O 4 tetrahedra are building blocks of all aluminosilicate zeolites described by (M/Al0Jx(SiO2),,. M represents an electron donor, usually an alkali metal, whose stoichiometry is determined by the rule that the number of electrons donated to the framework must correspond to the number of aluminum atoms in a given structure. The Al/Si ratio in zeolites varies as 0 < x/y < 1 and to a large extent determines their structure and properties. The chemical activity of the zeolite surface is strongly influenced by the charge distribution on the framework. Unfortunately, even for the simplest zeolites, this distribution is uncertain. We address this problem for sodalite, a zeolite that consists of identical truncated octahedra cages (Figure 1) of approximately 7 A in diameter. Sodalite has a cubic symmetry,' with the P43n space group. The unit cell consists of two cages with single-cage stoichiometry: Na4[AlSiO4l3X.2H20. X represents a negative ion (e.g., OH or halogen) which occupies the center of the cage and has tetrahedral coordination with four sodium atoms as nearest neighbors. The silicon and aluminum atoms are located at the apexes of the truncated octahedron and are joined by four tetrahedrally coordinated oxygen atoms. Each cage has 6 8-atom rings (4 tetrahedral metal atoms and 4 oxygen atoms) and 8 12-atom rings (6 tetrahedral metal atoms and 6 oxygen atoms), which will be called here the 4 and the 6 rings. The four sodium atoms insii each sodalite cage are coordinated to the three oxygen atoms from the six rings. Sodalites are dielectric materials with a band gap of approximately 6 eV.2 They can be viewed as a heavily "doped)) SiOz (1:l) with both ptype (Al)and n-type (alkali-metal) "impuritiesn3 so that a high probability for trapping charged particles is expected. If an optically transparent halogen sodalite, such as Na4[A1SiO4I3C1.2Hz0,is exposed to high-energy electrons or X-rays, a colored sample is obtained, a phenomenon known as the cathodochromic effect! It is believed that cages with a p-type defect such as negative ion vacancy, Le., (Na4[A1Si04]3.2H20)1+, are responsible for this effect. The electrons formed by the X-rays are trapped by these defects in the place of the absent negative ion and form Na43+clusters with the absorption in the visible region. The 13-peak hyperfine structure of the ESR signalS characteristic of these centers indicates the presence of an unpaired electron delocalized over four equivalent sodium atoms. The number of Na>+ clusters that can be formed in halogen sodalites is limited by the number of negative ion vacancies produced during synthesis. If one chooses to deposit sodium vapor onto the surface of Na3[AlSi04] sodalite, the dehydrated and dehydroxylated form of Na4[AlSi04]30H.2H20,the concentration of the Na>+ clusters 'Center for Quantized Electronic Structures. *Department of Chemistry. Department of Physics.
0022-365419212096-9039503.00/0 , I
,
can be varied at will. At low Na43+concentration, the Na3[A1SiO4I3sodalite is pale blue and produces an ESR spectrum6 identical to that reported for the cathodochromichalogen sodalites. Prolonged exposure of the Na3[AlSiO4I3sample to sodium vapor gives rise to a sequence of colors: light blue-bluepurpleblack. This phenomenon has been qualitatively described by Barrcr,6 but no quantitative correlation between the spectroscopic data and the Na43+concentration has been reported. Here we present measurements of the absorption spectra of samples with a known concentration of excess alkali-metal atoms. These are compared to model calculations of the absorption cross section to extract information regarding the sodalite framework charges. The magnitudes of the framework charges in sodalites have been a subject of They vary between the covalent scheme,' which places only a fractional negative charge on oxygen to balance the +1 charge of the Na atoms, and the much stronger ionic model of Skoczyk,'O which places a +3.03 charge on the Si atoms, +2.45 on the A1 atoms, and -1.62 on oxygen. 11. Theoretical Section We outline here how time-dependent quantum mechanics is used to calculate the absorption cross section for a model that mimics the properties of an alkali-metal atom absorbed in the sodalite structure. The details are given in ref 11. In the infinitely dilute limit, the excess alkali metal is a single-site impurity which contains one electron in excess of a closed shell. In the model used here, this electron interacts through a simple pseudopotentia112with the excess Na ion and the framework ions. The observable that we calculate is the absorption spectrum of the excess electron. The calculations connect the framework charges to the absorption spectrum. We assume a rigid Na3[A1SiO4l3sodalite framework surrounding the single Na4[A1SiO4I3sodalite cage. The electron position operator is denoted by r, and the sodalite atoms are located at position (R}with effective charges {Q). The electron momentum is p. The Hamiltonian
H = p2/(2m) + V(r;(R,Ql) (1) contains the kinetic energy of the electron plus its interaction energy with the framework atoms. The formula used to calculate the absorption cross section is similar to the one used by HellerI3 a(o) a w
Re
xm
dt exp(iwt)C(t)
The overlap integral C(t) = exp(iEgt/ h) (glcr exp(-iHt/ h)crlg)
(2)
(3) where lg) is the ground state of the electron of energy E, and c is the polarization vector of the light. In eq 3, the dynamics of the excess electron is treated explicitly and the light is coupled to the system through the electron dipole operator (Le., the electron
0 1992 American Chemical Society
9040 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
Srdanov et al. sodium
shutter
i
/
+-
/
dispenser
zeolite sample /
0
sodium
turbo Pump
halogen
Figure 1. Halogen sodalite cage. Regularly alternating A1 and Si atoms positioned at the vertices of a truncated octahedron are connected by oxygen atoms (not explicitly shown in the figure). The center of the cage is occupied by a halogen ion surrounded by four Na ions forming a tetrahedron.
position operator). This formulation provides the spectrum for all the excited states in one calculation without requiring any quantum chemistry input. This is possible for the present example because, by using pseudopotentialsto describe the electronatom interactions, we have a one-electron problem. In practical calculations, we cannot propagate the wave function for an infinite time. We use a “window” function, exp[-(t/~)], in eq 2, which cuts off the time evolution of the overlap integral for times substantially longer than 7. With this window function, eq 2 generates13J4a low-resolution version of the true spectrum. The absorption lines acquire a width of the order A@ = 2 u / ~ . Conversely, a spectrum taken with a resolution Ab contains information about the motion of the excited electron for the time 7 = 2?r/~0.13 One purpose of these calculations is to examine whether the absorption spectrum is sensitive to the charge distribution on the zeolite framework. The interaction between the excess electron and a zeolite atom at position Ri with charge Qi is given by V(r;(R,Q]) = 2-$Qi exp[-(lr - Ril/X)n]/min [Ir - RiI,Rci] (4) Here e is the electron charge and RCi is a cutoff distance used to truncate the Coulomb potential. We have included the long-range exponential cutoff, exp[-(Jr - RiI/X)n],to simplify the Coulomb potential at large distances. In the calculations presented here, we use X = 15.0-25.0 A and n = 4. In order to calculate the interaction potential, we use a grid that extends out to a maximum of 22.7 A from the grid center and covers three zeolite cages in each direction. The values of RCi are chosen such that the t w c h d y interaction between the electron and ith atom approximates the ionization potential of the corresponding ion with charge Qi. We calculate the absorption spectra for various framework charges proposed in the literature7-10and for several Na-O distances.
III. Experimental Section Sodium hydroxysodalite, Na4[A1Si04]3(OH)-2H20,was synthesized by the standard hydrothermal procedure.15 The NaOH unit was removed from the cages by Soxhlet extraction to obtain the hydrated form of sodalite, Na3[AlSiO,] 34H20. The unit cell size change between the initial hydroxysodalite hydrate, a = 8.903 (1) &la and the fmal hydrosodalite phase, a = 8.848 (1) A,17 was monitored with a Scintag automated powder X-ray diffractometer. Complete conversion of a few grams of hydroxysodalite into the hydrosodalite form requires 7 days of continuous extraction. The thermogravimetric analysis of the hydrosodalite phase gave the expected water loss of 14.5%. After calcination of Na3[AlSi0413~4H20 at 450 OC, the unit cell of Na3[A1SiO4l3was determined. The a = 9.122 A value was in agreement with the literature data.17 The vacuum apparatus1*in which the depition of alkali metals on mlites was made, is schematically shown in Figure 2. In each experiment, 20 mg of hydrosodalite powder was placed at the bottom of the tantalum boat and evacuated at 450 OC for 24 h. The water loss was monitored with the quadrupole mass spec-
I(
0
quartz tube
Figure 2. Apparatus for the deposition of alkali metals on the zeolite surface.
trometer (UTI 1OOC). During sodium deposition, the boat temperature was kept at 250 OC to allow for faster diffusion of the alkali-metal atoms through the sodalite crystallites. The background pressure was maintained at 5 X Torr, and the losses of sodium atoms due to the reaction with residual H 2 0or O2were negligible. A controllable deposition rate of sodium atoms provided by the alkali-metal dispensers19was the main improvement over the standard procedure? The dispenser consists of a small metal container holding 1.2 mg of sodium in the form of sodium chromate mixed with a reducing alloy (16% AI and 84% Zr). The alkali-metal flux produced by the source, when powered by a current-stabilized dc power supply, was calibrated by the mass spectrometric measurements. Once the flux was determined, we could deposit known amounts of sodium onto the zeolite surface by varying the e x p u r e time. The sodium source was placed close to the sample so that only the zeolite surface was exposed to the sodium flux. By assuming that the sodium-sticking coefficient is equal to unity, this approach provides an estimate of the concentration of the excess of sodium atoms in the sodalite sample. After deposition, the sample was transferred, by 180’ rotation of the sample holder, into a 3- X 8- X 25-mm rectangular quartz tube which was immediately sealed. The tube contained 100 mg of dehydrated BaS0,. Diffuse reflectance spectra of the wellhomogenized BaSO,/sodalite mixture were obtained in the region between 240 and 850 nm using a computer-controlled Cary-14 digital spectrometer with the BaSO,-coated integrating sphere attachment. The diffuse reflectance spectra were corrected for instrumental response using BaS0, as reference.
IV. Results and Discussion A. Absorption Spectra of Na4* CIUsters in Sodalite. A series of diffuse reflectance spectra corresponding to different concentrations of Nad3+centers in sodalite are shown in Figure 3, along with the Na?+/Na:+ concentrationratios. For the first 2 samples with an excess of 1 Na atom per 50 and 10 Na3[AlSiO4l3cages, the UV-vis spectrum is dominated by a broad absorption band centered at 628 nm. We ascribe the origin of this band to the electronic transition of a single unpaired electron trapped by four positively charged sodium atoms, forming a Na43+cluster inside the sodalite cage. The shape of this band does not change significantly with an increase of the alkali-metal concentration, except for an increase in intensity. The overall shape of the spectrum is similar to the absorption of the F centers in chlorosodalite (A,,,= = 530 nm) formed upon its exposure to high-energy electrons.20 Spectra of the F centers in halogen sodalite and that of Na-doped Na3[A1SiO4l3sodalite are similar because in both cases the Na:+ clusters are responsible for the absorption. They differ in absorption maxima because
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 9041
Nad3+Clusters in Sodium Sodalite .4
black
.3 c
P
1 -2 4
.I
m;; light b l u e
'whit 8 e-
0.
8
150
2
3
4
d S
Figure 3. Diffuse reflectance spectra of the Na-doped Na3[A1SiO4I3 sodalite in the 220-850-nm region. The ordinate is -(log (Illo)) where I is the intensity of the light reflected from the sample and Io is the intensity of the light reflected from the BaS04 reference powder.
in halogen sodalite the Na:+ clusters are surrounded by the cages containing the (Na4Cl)3+clusters, while in the sodalite case the surrounding cages contain Na33+clusters. This, together with the substantial difference in the unit cell size between these two structures (0.24 A), is likely to be responsible for the 100-nm shift observed in the absorption maxima. A question that has not been addressed previously is the origin of the large width of the absorption band of the alkali-metal-doped sodalite which is on the order of 1.5 eV (FWHM).In Figure 4a, we show the calculated absorption spectrum of an isolated Nad3+ cluster, consisting of a narrow absorption band centered at 388 nm (3.2 eV). The computed spectrum of the same cluster inside of the sodalite is shown in Figure 4b. The spectrum is calculated" for a particular polarization of light with respect to the oriented single crystal of Na3[AlSiO4I3.The additional absorption features in Figure 4b appear because of interaction of the electron with the Na3[A1SiO4l3lattice. Since the spectral measurementswere made on a polycrystalline powder consisting of randomly oriented microcrystallites,we must average the computed spectra over all orientations of the electric field polarization. We must also average over the orientations of the Na3 groups inside the sodalite cages. This was done by a Monte Carlo procedure with the final cross section calculated as U(W) = (l/N)Zun(u). The individual cross sections, un(u), correspond to a specific Na3orientation. The orientation-averaged spectrum for the particular framework charge distribution (Si = 1.9, A1 = 0.9,O = -0.95, and Na = 1.0) is shown in Figure 4c. The spectral width of the orientationally averaged spectrum is in qualitative agreement with the experimentally obtained one shown in Figure 3. The theoretical model used here does not match the experimental width of the spectrum for two reasons. First, we neglect the nuclear motion which, if taken into account, would cause a more rapid decay of the overlap integral eq 3. A more rapid decay will result in a broader spectrum. Second, it is very likely that the experimental spectrum is inhomogeneously broadened by the thermal motion and by considerable fluctuations in the local environment expected for the small-size crystallites. The blue sample, corresponding to a concentration ratio of 1:4 in Figure 3, has an additional absorption band at the high-energy end of the spectrum. The onset of a weak UV absorption band at -250 nm (-5 eV) can be observed in the absorption curve corresponding to the -light blue" sample with a Na>+/Na?+ ratio of 1:lO. The band expands toward the IR region as the concentration of the Naz+ centers increases and continuous absorption throughout the entire 200-850-nm region quickly sets in.
0
1
2
3
Figure 4. Calculated absorption cross section (in arbitrary units) of the isolated (hypothetical) Na': cluster (a). The absorption cross section of the same cluster inside a sodalite cage surrounded by the "empty" Na3[A1SiO4l3 sodalite cages. The spectrum was calculated for particular light polarization with respect to the sodalite single crystal (b). The absorption cross section averaged over all orientations (c). In all calculations, 2.401 A for the Na atom-center cage distance was used. The cage potential was computed using the model d charges listed in Table I,
: e black
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0.2 0'3!
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9042 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
0
12
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12
(1,
Figure 6. One-dimensional cuts through the potential energy surface felt by the excess electron. Two different cut directions are shown: (a) the cut along the 2-fold rotation axis (through the four rings) and (b) the cut along the 3-fold rotation axis (through the six rings). The asymmetry in b is due to the orientation of the Na4 group in conjunction with the six ring. The ground-state energy in this case is -5.00 eV.
1.oo
3.00
5.00
E (eV)
TABLE I: Various Charge Distribution Scbemes for the Zeolite Framework Used To Calculate tbe Absorption Cross Section of the Ned3+Clusters in NaJABiOAl.rSodalite model b C d e a +1.0 +1.0 +1.0 +1.0 +1.0 Na +2.2 +1.5 +1.9 0.0 +1.0 Si +1.3 +0.85 +0.9 0.0 +0.8 AI 0 -0.25 -0.7 -0.8375 -0.95 -1.125 P 0.99 0.98 0.97 0.95 0.05
-
the blue to the black color is caused by superposition of the UV IR expanding absorption band which appears at a higher concentration of the excess alkali-metal atoms. In the sodalite crystal lattice, each cage is surrounded by 14 neighboring cages. Eight surrounding cages are connected through the larger six-ring openings, while the other six cages share smaller four-ring openings. In Figure 6, we plot a cut through the potential energy surface felt by the CXC~SSelectron. The framework charges used to calculate the pseudopotential correspond to model d in Table I. Two different cut directions are shown: curve a represents the cut along the 2-fold axis (through the four rings), and b is the cut along the 3-fold axis (through the six rings). We note a significant difference in potential barriers along the two axes, implying that the interaction between the electrons in two neighboring Na>+ clusters takes place mostly through the six rings. The early onset of the UV absorption at -5 eV, for the Na2+/Na33+cluster ratio of 1:10, coincides with the formation of the first neighboring cages containing the Na2+ clusters. For the Nat+/Na?+ ratio of 1:4, the absorption band expends to -3.5 eV. Here, each Na43+cage is on the average connected to an adjacent Na43+cage through the six ring. Our calculations show that the wave functions of the excess electrons in the cages containing Nad3+clusters overlap favorably in the direction of the six rings. On the basis of the experimental evidences and the considerations ahwe, we conclude that overlapping wave functions of the exms electrons in the neighboring cages form an extended band which is responsible for the appearance of the additional absorption in alkali-metal-doped sodalite. The experimentally found 1:4 Na:+/Na:+ concentration threshold for the appearance of this band compares favorably to the site percolation probability calculated for the various types of 3D cubic lattices.2' The observations described above also suggest the possibility for the existence of an insulator-metal transitionU induced by an increase in the alkali-metal concentration in sodalite. Although the observed changes in the ESR spectrum of alkali-metal-doped zeolitesz3are in qualitative agreement with this hypothesis, the final proof requires measurements of actual conductivity. We are planning to continue investigations of these interesting systems in the near future.
Figure 7. High-resolution absorption spectra of the b-e model sodalite with the corresponding charges of the framework ions listed in Table I. The fixed value of d = 2.401 A corresponding to the distance of the sodium atoms from the middle of the cage was used in all calculations.
B. Influence of the Framework Charges a d Nad3+Cluster Geometry on Calculated Spectra. It is important to realize that the coordinates of the Na ions in an isolated cage containing the Na43+cluster cannot be obtained by structural analysis. It is assumed that the sodium ions have tetrahedral geometry and that each ion is coordinated to the three oxygen atoms from the six ring in the way similar to the hydroxysodalite and halogen sodalite structures. The effect of the framework charge distribution and the Na-Na distances in Na:+ on the calculated spectra is examined in detail in ref 11. Here we outline the main results only. The approximation of the interaction potentials for ions in a crystal as being purely electrostatic perturbations due to point dipoles goes back at least to the crystal field theory of Bethe in the 1 9 2 0 ~ The .~~ quest for finding the appropriate charges has been considerable. On the basis of the suggestions in the literature, we have examined several charge distributions which are shown in Table I and are denoted as models a-e. The a model has the weakest framework charge that we consider. We consider it since it was obtained by a widely used procedure to evaluate the electric fields in mlites.' The Si02 framework is regarded as being neutral, and formal negative charges are distributed on the oxygen atoms upon the substitution of A1 for Si. The alkali-metal counterion takes on a + I charge to maintain charge balance with the -0.25 fractional charge on the oxygens. The b model is our interpolation between the a model and the c model from Leherte et a1.,8 who use Mulliken ab initio STO-3G atomic net charges to suggest that in ferrierite zeolite there is a +l.5 charge on the Si atoms. The d model is from Van Genechten et al.? who use electronegativity equalization methods to obtain a +1.9 charge on the Si atoms in sodalite along with a -0.95 charge on oxygen as noted in Table I. The e model is our interpolation between the d model and a much stronger charge model suggested by Skoczyk'o which places a +3.03 charge on the Si atoms, +2.45 on the Al atoms, and -1.62 on the oxygen. Figure 7 shows the calculated spectra for charge distributions b-e (Table I) for a selected polarization direction. This of charge distributions corresponds to increasing the magnitude of the charge on the Si, Al, and 0 atoms while maintaining overall charge neutrality as well as Na = +1 charge. A change in the sodalite framework charge alters the energy levels, the oscillator strengths, and the spatial distribution of the electron. Here we used d = 2.401 A for the distance between Na and the center of the cage, a fixed Na3 group orientation, and a fixed polarization direction of the monochromatic light. Our calculation shows that there is virtually no difference in calculated spectra between models a and b. For model c, some
J. Phys. Chem. 1992,96,9043-9048 of the absorption features are being shifted toward the lower energy end of the spectrum for a fraction of electronvolts. From this point on (Si = +1.5, A1 = +0.85), the calculated spectra became strongly dependent upon a slight increase of the framework charges and quickly reached the point of a complete spread of the absorption features over the entire spectral region (model e). An instructive parameter to follow is probability P of finding the electron within the volume that approximately corresponds to the size of the sodalite cage (see data in Table I). With the increase of the framework charges, the excess electron becomes increasingly delocalized, but the probability of finding it within the sodalite cage for model d (Si = +1.9, A1 = +0.9) are still very large; P = 0.95. Any further charge increase, however, leads to a quick delocalization of the electron over the neighboring framework ions so that for model e (Si = +2.2, A1 = +1.3) P is only 0.05. This allows us to set the upper l i t of the charge distribution in sodalite as Si < +2 and A1 < + 1 , 0 > -1, Na = +l. As pointed out earlier, the actual Na distance from the center of an isolated Naz+ cluster in the Na3[A1Si0.J3 sodalite matrix cannot be obtained by structural analysis. Because of that, all our preliminary calculations used a value of d = 2.401 A obtained from the crystal structure of dehydrated hydroxysodalite.” In the Nad3+cluster, the negatively charged OH ion is replaced by a single electron, and it is expected that the Na distances will change. We calculated the Na43+spectra in the 2.65 < d < 2.40 A range and found a significant shift of the spectral features which amounts to approximately 1 eV. The overall trend is that an increase in cluster size red shifts the spectrum. Using the model d charge distribution, we optimized the sodium distances to d = 2.60 A to obtain the closest agreement with the experimental spectrum. Absorption measurements on single crystals using polarized light would provide more detailed data that will make it possible to refine the theoretical model. Acknowledgment. This research was supported by the NSF Science and Technology Center for Quantized Electronic
9043
Structures, Grant No. DMR 91-20007, and by the Office of Naval Research.
References and Notes (1) Pauling, L. Z . Krist. 1930, 74, 213. LBns, J.; Schulz, H. Acta Crysrallogr. 1967, 23, 434. (2) Van Doorn, C. Z.; Schipper, D. J.; Bolwijn, P. T. J . Electrochem. Soc. ..
1972, 119, 85.
(3) Kasai, P. H. J . Chem. Phys. 1965, 43, 3322. (4) See, for instance: Faughnan, B. W.; Gorog, I.; Heyman, P. M.; Shidlovsky, I. Proc. IEEE 1973, 61, 927. (5) Smeulders, J. B. A. F.; Hefni, M. A.; Klaassen, A. A. K.; de Boer, E.; Westphal, U.;Geismar, G. Zeolites 1987, 7, 347. (6) Barrer, R. M.; Cole, J. F. J . Phys. Chem. Solids 1968, 29, 1755. (7) Vigne-Maeder, F.; Auroux, A. J . Phys. Chem. 1990, 94, 316. (8) Leherte, L.; Lie, G. C.; Swamy, K. N.; Clementy, E.; Derouane, E. G.; Andre, J. M. Chem. Phys. Lett. 1988, 145, 237. (9) Van Genechten, K. A.; Mortier, W. J.; Geerlings, P. J . Chem. Phys. 1987,86, 5063. (10) Skorczyk, R. Acta Crystahogr. 1976, A32, 447. (1 1) Haug, K.; Srdanov, V. I.; Stucky, G.D.; Metiu, H. J . Chem. Phys. 1992, 96, 3495. (1 2) See,for instance: (a) Harrison, W. A. Pseudopotentials in the theory ofmetals; W. A. Benjamin Inc.: New York, 1966. (b) Shaw, R. W., Jr. Phys. Reo. 1968, 174, 769. (13) Heller, E. J. J. G e m . Phys. 1968, 68, 3891. (14) Engel, V.; Schinke, R.; Hennig, S.;Metiu, H. J . Chem. Phys. 1990, 92, 1. (1 5) Breck, D. W. Zeolite Molecular Sieves; Krieger: Malabar, FL, 1986. (16) Felsche, J.; Luger, S . Thermochim. Acta 1987, 118, 35. (17) Felsche, J.; Luger, S . Baerlocher, Ch. Zeolites 1986, 6, 367. (18) Srdanov, V. I.; Margolese, D.; Saab, A.; Stucky, G.D. In prepara-
tion.
(19) Succi, M. R.; Canino, M.; Ferrario, B. Vacuum 1985, 35, 579. (20) Medved, D. B. Am. Mineral. 1954,39,615. Hodgson, W. G.; Brinen, J. S.;Williams, E. F. J . Chem. Phys. 1967, 47, 3719. (21) Kirkpatrick, S.Reo. Mod. Phys. 1973, 45, 574. (22) Mott, F. N. Metal-hulator Transitions; Taylor & Francis: London, 1990. (23) Harrison, M. R.; Edwards, P. P.; Klinowski, J.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J . Solid Stare Chem. 1984, 54, 330. (24) Cotton, F. A.; Wilkinson, G.Advanced Inorganic Chemistry; Wiley: New York, 1980; p 63s.
Methylamines Basicity Calculations. I n Vacuo and in Solution Comparative Analysis Iiiaki Tuibn, Estanislao Silla,* Departamento de Quimica Fhica, Universidad de Valencia, Dr. Moliner 50, 46100 Burjasot, Valencia, Spain
and Jacopo Tomasi Dipartamento di Chimica e Chimica Industriale, Universitci di Pisa, Via Risorgimento 35, 56126 Pisa, Italy (Received: February 11, 1992)
A purely electrostaticmethod that does not explicitly take account of specific interactions such as hydrogen bonds has been used to calculate the basicity ordering of methylamines in aqueous solution. The influence of in vacuo energies and solvation energies is discussed by means of quantum calculations and the polarizable continuum model of the solvent. Our results, which reproduce experimental ordering and magnitude quite well, seem to indicate that electron correlation is very important in order to have a good estimation of the inductive effect produced by methyl substitution and that the continuum model is quite suitable for dealing with the solvation of methylamines. Finally a simple electrostatic explanation can be employed to rationalize the behavior of some homologous series in solution.
Introduction The irregular ordering of the basicity of methylamines in SOlution has been the object of many experimenta1’q2 and theoretica13-7studies. Basicities of methylamines in aqueous solution have been found to be8 NH, < NH2Me NHMe2 > NMe, while experimental studies9 have shown that the basicities of
methylamines in the gas phase increase with successive methyl substitution in agreement with the inductive effect order of the increased electron releasing ability of the methyl W U P a m p r e d with that of hydrogen. In many of them it is considered that this anomaly is caused by solvation effects and not internal characteristics of the molecules or accumulative effects of the substituents like the inductive Solvent effects have been discussed in terms of both electrostatic and hydrogen bonding interactions
0022-3654/92/2096-9043%03.00/00 1992 American Chemical Society
