2880
W. J. Mortier, M. L. Costenoble, and J. 6.Uytterhoeven
Location of Cations in Synthetic Zeolites X and Y. I 11. Potassium-Alkylammonium Y Zeolites W . J . Mortier,”
M. L. Costenoble, and J. 6.Uytterhoeven
Centrum voor Opperviaktescheikunde en Colloidale Scheikunde, De Croylaan 42, 5-3030 Heveriee, Beigium (Received March 7, 1973) Publication costs assisted by the lnstituut voor Wettenschappelijk onderzoek in Nijverheid en Landbouw
The exchange of monoalkylammonium ions into potassium Y zeolite proceeds to certain limits. X-Ray powder diffraction and structure analysis were preformed on KY samples partially exchanged with ISH4+, CH3NH3+, C Z H ~ N H ~or+ C3H7NH3+. , The samples exhibit a gradual change of physicochemical properties. Variations in the distribution pattern of the K + ions are related to the state of hydration of the samples. Although the alkylammonium ions were not located, some speculations can be made on the exchange mechanism.
Introduction
The exchange of alkylammonium ions in Linde Molecular Sieves X and Y has been studied by Theng, et al.l These authors also review earlier work of other investigations in the same area. They found that each organic ion is characterized by a given maximum exchange limit. This limit decreased linearly with increasing molecular weight of the organic ions and was also influenced by the nature of the outgoing ions. I t was postulated by Vansant, et al., that the maximum exchange limit was not exclusively due to space requirements but was dependent on thermodynamic conditions.2 A satisfactory explanation has not been given thus far. The present structural study has been undertaken to help the understanding of the exchange phenomena by locating the unexchanged inorganic cations (potassium). I t was not our intention to locate the alkylammonium ions, since they hardly fit the symmetry of the faujasite framework. Experimental Section
A Linde Zeolite Y, containing 54.7 exchangeable cations per unit cell, was saturated with K + ions by repeated exchanges with a 0.1 N KC1 solution followed by washing. Aliquots of this sample ( - 3 g) were equilibrated a t room temperature during three days in 200 ml of 0.5 N solutions of ammonium or alkylammonium chlorides. The alkylammonium ions (RNH3+) used were methyl-, ethyl- and propylammonium. With NH4+ a second exchange was performed to ensure maximum exchange. Only one equilibration was made with the alkylammonium ions. It was the intention to replace only an amount of K + ions approximately equal to the “loosely bound” or “unlocated” ions. From previous work3 we know that there are 20.1 such ions per unit cell in the hydrated KY sample. The propyland ethylammonium samples were indeed exchanged to that extent, but the amount of methylammonium ions was significantly higher. The samples were washed free from excess electrolyte, air-dried a t room temperature, and stored in an 80% humidity atmosphere until they reached constant weight. The unit cell composition of the samples was inferred from the weight loss upon calcination a t 800” and from the alkylammonium content deterThe Journal of Physical Chemistry, Vol. 77, No. 24, 7973
mined by the microkjehldahl meth0d.l The unit cell composition of the samples is given in Table I. The symbols quoted in that table contain the initials of the exchangeable cations and are used throughout the text. The X-ray method was essentially the same as that used in previous ~ o r k on ~ .the ~ distribution of the K + ions in the KY samples. Therefore we only mention here the most important definitions and specific modifications used for this work. The spectra were recorded on a Phillips diffractometer a t a scanning rate of 0.02”/min. For the estimation of the peak intensities a least-squares program was used which fitted an experimental profile with a set of theoretical Cu Kal-Cu Kaz combined line profiles. The theoretical profile for KPrY was a Gauss function. For the other samples we used an experimental profile obtained by the method described by G a n g ~ l e e In . ~ this way the intensity ratio of overlapping peaks could be determined accurately. Details of this procedure were published elsewhere.6 The least-squares refinement was performed using a block-diagonal approximation as described earlier.3 This program refined on the basis of structure factors. A modified version of the full-matrix program POWOW,7 refining on the basis of intensities, was used for the statistical treatment explained further. It must be remembered that the standard deviations obtained in the block-diagonal approach are too small. To strengthen the validity of our conclusions we made some statistical comparisons. A subset of q parameters of one structure-is statistically compared to the same set of parameters ( 6 0 ) of a reference structure using the expression
(aE)
&= S1 is the q
(P,-
&)/SI(ii
2qsz
-
io)
=
F , .-,(a)
X q matrix on the coefficients of the normal equations, s2 = Z ( t T & , ) z / ( n - p ) , n is the number of observations (n = 128), and p is the total number of parameters. If Q > F q , n - p ( ~we ) may reject the equality of the two-parameter vectors with probability 01 to take a wrong decision. The test is performed on a conditional basis, i.e., that the remaining p-q parameters are not considered. The same method was applied to the hydrated
2001
Potassium-Alkylammonium Y Zeolites
and dehydrated K zeolites X and Y and described elsewhere in full d.etail.8 The application of liquid scattering functions to unlocated scattering material has been successful in the study of the hydrated K X and KY zeolites.3 Therefore we applied a similar technique in this work. RNH3+ and water molecules were considered as unlocated material. They were supposed to be randomly located in a sphere with a radius of 2.3 A inside the cuboctahedra, or in a shell formed by two concentric spheres with radii R1 and Rz in the large cages. An additional temperature factor of 15 A2 was applied. In fact, the occupancy of the sites I f , 11‘, and I1 must be considered as superimposed on a background, since the sphere of statistically distributed scattering material includes these sites. The residuals were defined as
TABLE I:
Composition of the Samples and Unit Cell Parameter a. Symbols KPrY KEtY KMeY KAmY a
Composition
ao,Aa
Pr1s.sK35.9A154.7Si137.303s4~101 H20 24.754 (3) E t ~ ~ . 4 K ~ ~ . ~ A 1 ~ ~ . 7 S i ~ ~ ~ 24.748 . ~ O ~ (~2 ~) ~ l l l H 2 0 Me27.5K27,3A154.7Si137.30384’1 24H2O 24.742 (3) Am3g.5K15.aA154.7Si137.30384’190H2024.739 (3)
Obtained by extrapolation against cos2 o/Sln 0
ters and the two occupancy factors of the potassium ions on the sites I’ and 11. The matrix S1 was obtained from the second cycle of a full matrix refinement with POWOW. The reference vector Bo is obtained by POWOW. Together with the variance-covariance matrix it defines the confidence region for this vector. The vectors to compare are those given in Table 11. n - p is close to 100, and F g , n - p . ( a ) is not sensitive to small changes in n - p. Therefore we included in Table V a list of F,,loo ( a ) for several values of ( a ) . In Table V we italicized the Q values which indicate that there is no significant difference between a given structure and the selected reference structure, with a probability of 0.995. As can be seen in Table V, each particular sample has been taken as a reference structure for the other samples, which resulted in the most complete statistical comparison between the investigated samples.
(BO)
and
The interatomic distances and bond angles were calculated using the ORFFE program.9 We used the cation site notation proposed by Smith.lo The sites I (16 per unit cell) are inside the hexagonal prism. The sites I’ ( 3 2 per unit cell) are inside the cuboctahedron sharing a six ring of oxygen ions with the sites I. Sites I1 and 11’ ( 3 2 per unit cell of each) are inside the supercages and inside the cuboctahedra, respectively, sharing the free six-membered ring of oxygen ions of the cuboctahedra. Sites I11 are inside the supercages, associated with a ifour-membered ring of oxygen ions of the cuboctahedra. In former work3s4 we defined sites 111’ as being inside the supercages and composed of four oxygen ions ( 0 4 - 0 1 - 0 1 - 0 4 ) . Evidence for the location of cations on sites III’ was given by Mikovsky, et al.ll Results The unit cell compositions and parameters are given in Table I. The structure parameters are listed in Table 11, together with the RF and R I values. The interatomic distances, the bond angles, and the distances between the center of the large cage or of the cuboctahedron and the surrounding atoms are collected in Table 111. For the samples KPrY and KEtY an electron density peak was obtained a t site I11 on a difference Fourier map in a plane passing through this site. For the sample KAmY difference Fourier calculations located an electron density peak a t the site 11’. These peaks seemed to refine by least squares. but the uncertainty is large. They will not be discussed further. Table IV is il summary of data characterising the samples. It contains the number of water molecules and the number of K+ and RXH3+ ions per unit cell. Assuming that the electron density a t the ion exchange sites is due to K + or water molecules (at site 11) a cation distribution was calculated as given in Table IV. The data obtained earlier for KY hydrated3 and KY dehydrated4 are included in that tablie for comparison. A list of Io and IC values is available. (See the paragraph of the end of this article regarding supplementary material.) The statistical comparisons are presented in Table V. Two different parameter vectors are considered: (i) the 10 positional framework parameters and (ii) the same framework parameters together with the two positional parame-
Discussion
T h e Framework Parameters. With the exception of KPrY the frameworks of the different samples are not significantly different from each other and from the original hydrated sample KY. The significant difference of KPrY is clearly apparent in Table VA in which the lower row (KPrY compared to all the other samples) and the last vertical column (KPrY taken as a reference) contain high values for Flo,loo (0,005). The similarity between the other samples is confirmed by all the values in the lower left half of Table VA (italicized). The figures marked with an asterisk in the upper right half of that table do not confirm the corresponding figures of the other half, but such exceptions can be due to the size and shape of the confidence region of the reference structure. Differing residuals (Table 11) are an indication of more or less well defined structures and subsequently also of the different size and shape of the confidence region. These observations become especially meaningful when they are compared to the statistical results obtained with the framework parameters of hydrated and dehydrated synthetic faujasites with widely different A1 contents (between 49 and 85 aluminum atoms per unit cell).s It was found for these samples that there was no significant difference between the framework of the hydrated samples. A significant difference was found between the framework of the hydrated and dehydrated zeolites. This was taken as proof that dehydration results in a considerable distortion of the framework. The absence of water molecules or the increased cation population of site I after dehydration were considered as possible reasons for these distortions.8 The difference in framework between KPrY and all the other samples indicates a distortion in the framework of KPrY. These distortions can be ascribed t o the same causes as in the dehydrated synthetic faujasites.8 Indeed, the data in Table IV reveal a marked decrease of the The Journal of Physical Chemistry, Vol. 77, No. 24, 1973
2882
W . J. Mortier, M. L. Costenoble, and J. B. Uytterhoeven
TABLE II: Atomic Parametersa and Residuals for the Samples K - C S H ~ N H ~ - YK-C~HSNHZ-Y, , K-CH3NH3-Y, and K-NH4-Y
X
Y z B x = -y z B x=y z B x = y
z
B x = y z
B x=y occ. B x=y occ. B x=y occ. B x=y
=z
=z
=z
=z
occ. B x = y z occ. B x=y=z R i gRz n molec. B x=y=z
R I , R2 n molec. B
a The
KPrY
KEtY
KMeY
KAmY
0.1245 (2) 0.9445 (2) 0.0346 (2) 1.29 (14) 0.1080 (7) 0.0 0.22 (30) 0.2491 (8) 0.1 397 (6) 1.30 (33) 0.1688 (9) 0.9684 (7) 3.32 (47) 0.1795 (IO) 0.3232 (7) 4.67 (50)
0.1241 (2) 0.9452 (2) 0.0353 (2) 1.68 (12) 0.1062 (9)
0.1245 (2) 0.9446 (2) 0.0354 (2) 2.23 (10) 0.1044 (7)
0.1253 (2) 0.9450 (2) 0.0356 (5) 2.15 (10) 0.1073 (7)
0.0
0.0
0.0
0.70 (38) 0.2502 (11) 0.1389 (6) 2.20 (50) 0.1723 (10) 0.9645 (8) 3.25 (50) 0.1776 (13) 0.3239 (9) 6.52 (65)
1.12 (30) 0.2524 (9) 0.1392 (6) 2.38 (35) 0.1726 (9) 0.9646 (6) 3.27 (37) 0.1785 (11) 0.3213 (8) 6.79 (52)
0.57 (31) 0.2535 (10) 0.1376 (8) 4.72 (53) 0.1 722 (9) 0.9664 (6) 2.23 (36) 0.1792 (10) 0.3216 (7) 4.71 (50)
0.0
...
0.338 (37) 9.28 (2.03) 0.0778 (1 7) 0.515 (22) 3.48 (55) 0.2620 (17) 0.470 (19) 3.53 (59)
...
... ...
...
...
...
0.0806 (22) 0.580 (30) 6.91 (83) 0.2590 (24) 0.457 (23) 2.47 (88) ...
... ...
...
-0.125 (0) 0.2383 (259) 0.121 (36) 0.28 (4.54) 0.125 2.3; 0.0 28.24 15 0.375 5.75; 2.0 136.6 15 0.1953 0.1276
-0.125 (0) 0.1221 (240) 0.097 (38) 0.0 (5.23) 0.125 2.3; 0.0 16.0 15.0 0.375 5.7; 2.0 164 15 0.1785 0.1298
...
0.0797 (17) 0.367 (1 7) 1.75 (54) 0.2616 (22) 0.319 (17) 0.68 (77)
... ... ,..
... ... ...
... 0.125 2.3; 0.0 32 15 0.375 5.7; 0.0 160 15 0.1742 0.1265
...
... ... 0.0788 (22) 0.260 (16) 0.12 (84) 0.2635 (48) 0.218 (23) 4.10 (2.01) 0.1999 (168) 0.169 (67) 10.38 (8.88)
... ... ... ...
0.125 2.3; 0.0 32 15 0.375 5.7; 0.0 240 15 0.1625 0.1181
standard deviations are indicated in parentheses.
water content of the sample KPrY and a Kf content of site I which is similar to that of the dehydrated KY sample. In all the other samples site I was found to be empty, as was the case in the hydrated KY sample. T h e Population of Exchange Sites. The incomplete exchange in this type of material has generally been ascribed to ion sieve effects,l but an unambiguous proof for this assumption was never given. Considering the size of the alkylammonium ions it is reasonable to believe that they remain inside the large cavities. The location of the ammonium ions is not unambiguous. NH4+ and K + have the same radius and it is not clear why NH4+ would be excluded from the cuboctahedra while K + ions can freely move through the six-membered ring of oxygen ions. On the basis of these assumptions we can translate the electron density into a population of the different sites by K + ions or HzO molecules as presented in Table IV. Inclusion of the population and positional parameters of the sites I' and I1 into the statistical comparison (Table The Journal of Physical Chemistry, Vol. 77, N o . 24, 1973
VB) reveals that there is a significant difference between the different samples, except maybe between KAmY and KMeY. KPrY is also significantly different from all the others by the high population of site I which is not included in the parameter vector 8. The interatomic distances (Table 111) do not enable us to make a clear distinction between potassium ions and water molecules located on sites 11. From a distance of 3.26 A between site I1 and 0 2 B a u P concluded that this site was occupied by water molecules. The distance K-02 was found to be 2.8 A in the hydrated sample^.^ The distance site 11-02 of 3 A found in this work indicates that at least an important fraction of this site is occupied by K + ions. The distance site I'-& in all the samples is in agreement with an occupancy by K + ions. The KPrY sample has a population of K + inside the small cavities which is comparable to that of the dehydrated sample KY. It was stated for the hydrated samples KY and K X as well, that one-fourth of the cations were
Potassium-Alkylammonium Y Zeolites
2883
TABLE I l l : Interatomic Distancesa (A) and Bond Anglesa (deg) for the Samples KPrY, KMeY, KEtY, and KAmY
-
KPrY
KEtY
KMeY
KAmY
1.61 (2) 1.67 (2) 1.68 (2) 1.68 (3) 1.66 133.9 (1.2) 141.8 (1.2) 138.6 (1.1) 141.8 (1.3) 139.0 11 7.8 (8) 119.2 (9) 106.0 (9) 101.1 (9) 107.4 (9) 104.2 (1.1) 109.3 2.81 (3) 2.84 (2) 2.63 (3) 2.59 (3) 2.70 (3) 2.66 (3) 2.71
1.61 (2) 1.65 (3) 1.63 (3) 1.68 (4) 1.64 137.2 (1.8) 145.4 (2.5) 145.6 (2.8) 141.5 (3.1) 142.4 114.8 (1.3) 112.8 (1.6) 107.0 (1.9) 104.9 (1.9) 110.4 (2.2) 106.7 (1.6) 109.4 2.74 (4) 2.69 (4) 2.64 (4) 2.74 (4) 2.74 (5) 2.65 (4) 2.70
1.58 (2) 1.65 (2) 1.64 (2) 1.65 (3) 1.63 142.6 (1.2) 148.3 (1.2) 144.2 (1.0) 144.6 (1.5) 144.4 11 1.4 (0.8) 111.1 (0.9) 111.8 (1.0) 106.0 (0.9) 108.5 (1.0) 107.7 (1.2) 109.4 2.67 (3) 2.65 (2) 2.68 (3) 2.63 (3) 2.68 (3) 2.66 (3) 2.66
1.63 (2) 1.63 (2) 1.66 (2) 1.64 (2) 1.64 136.7 (1.1) 153.5 (1.5) 142.2 (1.O) 142.8 (1.4) 143.8 110.6 (1.0) 113.6 (0.9) 108.1 (0.9) 104.9 ( I .O) 111.0 (1.1) 108.7 (1.2) 109.8 2.68 (3) 2.75 (2) 2.65 (3) 2.61 (3) 2.70 (3) 2.68 (3) 2.68
-
Tetrahedron T-01 T-02
T-Q3 T-04
Mean T-01-T T-02-T T-Q3-T T-04-T
Mean
01- - T - 0 2 01-T-03 01--T-04 02-T-03 02-T-04 Os-T-04
Mean 01-02 03-03 01-04 02--03
02-04 03-04
Mean Cations K(I)
-03
-K(I‘) -02
01
K(I’)
-03
-02
K(II)
-02 -04
w~0(111)-01 -04
-02
(5) (4) (5) (46) (58) (31)
-T -04
-K(I) -K(II’) Center large cage -site( I I I ) -K(II) -04
-02 -01
-T The standard deviations
2.85 3.42 3.65 3.45 2.85 3.23 3.05 3.26
(2) (4) (2) (1) (4)
(5) (6) (6)
...
... ...
...
...
...
2.02 (4) 4.17 (2) 4.36 (2) 5.00 (1) 5.26 (2) 5.36 (0)
1.88 (3) 4.30 (4) 4.39 (3) 4.97 (1) 5.26 (5)
...
...
3.38 4.84 6.96 7.30 7.31 7.55
(64) (4) (2) (2) (2) (1)
.
I
6.26 4.97 7.02 7.30 7.35 7.57
.
(60)
(5) (3) (4) (2) (1)
2.85 3.38 3.41 3.75 2.78 3.23 3.13 3.28
...
...
...
-02
(3) (3) (4) (2) (5) (4) (6) (6) (7) (3) (41) (47)
... ...
...
-04
-03
2.86 3.48 3.43 3.72 2.89 3.18 2.99 3.27 3.15 2.19 4.34 2.71
... ... ...
-02
-K(I‘) Framework Center cuboctahedron -K(I‘)
a
(2) (4) (2) (2) (5)
...
-03
K(I1’)
2.95 3.34 3.46 3.78 2.71 3.10 3.06 3.26 4.48 4.51 5.04
1.94 (4) 4.30 (2) 4.47 (2) 4.98 (1) 5.21 ( 2 ) 5.36 (0)
... ... 4.86 7.00 7.24 7.39 7.56
(5) (3) (2)
(2) (1)
(2) (5) (2) (2) (6) (6) (12) (12)
... ... 2.43 (41) 3.10 (41) 3.16 (42)
1.98 4.26 4.51 4.97 5.22 5.36 3.21
(5) (2) (2) (1) (2)
(0) (41)
... 4.78 6.98 7.25 7.32 7.55
(12) (2) (2) (2) (1)
are indicated in parentheses.
located in the small cavities in order to obtain a n equal statistical neutralization of the negative charges supposed to be spread statistically over the different types of oxygen ions.3 This would amount to 13.7 ions per unit cell for the present samples. This value is reached and even exceeded
for the samples KPrY and KEtY, indicating that not only KPrY but also KEtY has a tendency to behave like a dehydrated KY. In the samples KMeY and KAmY the population of site I’ is definitely lower than 13.7. The rule of the statistically uniform neutralization of charges seems The Journal of Physical Chemistry, Vol. 77, No. 24, 1973
2884
W. J. Mortier, M. L. Costenoble, and J. 5. Uytterhoeven
TABLE I V : Summary of Data on the K-Alkylammonium Y Samples, the Hydrated KY Samples, and t h e Dehydrated KY Sample KY Deh
KPrY
KEtY
KMeY
KAmY
KY Hydr
K+/U.C.
54.7
H2Q/U.C. NH4+ or
...
35.9 101
31.3 111
27.2 124
15.2 190
54.7 240
18.8 5.4 16.5
23.4
27.5
39.5
...
...
...
...
18.6
11.8
8.4
1.3 13.3
21.9 15 30 -0.5
18.6 14.6 29.2 -1.9
11.8 10.2 20.5 5.2
8.4 7.0 14.1 -0.2
...
RNH3+/U.C.
Site I ( K ) Site I ’ ( K ) Sites i
5.4 18.1
+ I’
(all K + ) Site I I ( K )
23.5 26.8
...
or (HzQ)
Unloc. K +
4.4
TABLE V: Statistical Comparison of Some Framework and Cation Parameters A. q = 10;
Positional Framework Parameters
Reference
K Y hydr KAmY
KMeY KEtY K PrY
KY hydr
KAmY
KMeY
KEtY
0.93 2.81* 2.27
6.62* 2.32
4.03*
1.66
1.15
2.24 6.33
0.39 1.14 3.65
5.34* 5.15* 2.82”
2.78
1.51
0.10 0.05 0.025 0.01 0.005
1.18 6.14
KPrY
4.40 4.31 4.88 2.66 2.62
1.67 1.94 2.20 2.46 2.77
5. q = 14; Positional Framework Parameters C Positional Parameters and Occupancy Factors of t h e Cations in Site I ’ and I I
Reference
KY hydr
KAmY
KY hydr KAmY
0.84 8.14 4.16 2.25 2.32
36.14 2.49 5.16 15.4 21.1
KMeY KEtY KPrY
0.1 0 0.05 0.025 0.01 0.005
KMeY 13.3 1.10 0.49 3.90 6.77
KEtY
KPrY
8.20 13.2 4.87 1.22 4.86
8.89 6.03 4.23 3.83 7.73
1.58 1.81 2.02 2.29 2.52
not to hold for these systems with large cations, although the total amount of K- ions in the sample is sufficient to satisfy that rule. Relevance of These Data for t h e Understanding of t h e Ion-Exchange Behavior. Theng, e t al.,1 and Vansant13 have stated that the limited exchange of the alkylammonium ions in zeolites X and Y cannot be understood only on the basis of space requirements of the bulky ions. The present data show that ion sieve effects, Le., the inaccessibility of the small cavities for large ions, cannot be considered either as the exclusive factor responsible for incomplete exchange. Indeed, the sample KAmY was saturated The Journal of Physical Chemistry, Vol. 77, No. 24, 7973
14.6 20.0
... 20.1
by NH4f to the maximum possible limit,l and there is still an appreciable number of K+ ions in the large cavities. The samples KMeY, KEtY, and KPrY were not exchanged to the maximum possible level. The two latter samples already have a population of the sites (I 1’) which is comparable to that in dehydrated KY (see Table IV). Obviously the increase of the number of K ions in the small cavities is due to the decrease in the hydration level. In the hexagonal prisms and inside the cuboctahedra the K + ions can still realize optimal coordination with framework oxygen ions and water molecules. In extensively exchanged samples the number of unexchanged K + ions was found to be 20.3, 22.7, 24.3, and 28.1 for samples KAmY, KMeY, KEtY, and KPrY, respective1y.l Even for a maximum occupancy of the sites inside the small cavities (23.1 as in KY dehydrated) there would still be, a t least for the sample KPrY, unexchanged potassium ions in the large cavities. Since the present samples were never heated or outgassed we assume that the sample compositions and water contents given in Table I are close to that of the samples in the ion-exchange suspension. This is in line with the interpretation of the limited exchange behavior given by Vansant.13 When an RNH3+ ion enters the zeolite cage it replaces an inorganic ion but also a number of water molecules sufficient to allow accomodation of the large incoming ion. Vansant considered the possibility that the exchange limit is reached when a further introduction of organic ions would require the removal of the first hydration shell of the remaining inorganic cations. This hypothesis seems to be justified for the larger alkylammonium ions, but it is not applicable to the KMeY sample, which behaves more like the ammonium sample.
+
Acknowledgment. W. 9. M. is grateful to the Relgisch Xationaal Fonds voor Wetenschappelijk Onderzoek for a research grant as “aspirant.” M. L. C. thanks the Instituut voor Wetenschappelijk onderzoek in Nijverheid en Landbouw for a research grant. Financial support by the Belgian Government (Wetenschapsbeleid) and the gift of samples by the Linde Division of Union Carbide are gratefully acknowledged. The calculations were performed in the computer center of the Katholieke Universiteit te Leuven. Supplementary Material Available. A listing of I,, and I,, values will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 20X reduction, negatives) contain-
Potential Energy Curves and Dissociation Energy of Ti0 ing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. c. 20036. Remit check Or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-73-2880. References and Notes
2885 (3) W. J. Mortier and H. J. Bosmans, J. PhYs. Chem., 75,3327 (1971). (4) W.J. Mortier, H. J. Bosmans, and J. B. Uytterhoeven, J. Phys. Chem.. 76. 650 i1972). (5) A . Ganguiee, J. kppl. Cryst, 3, 272 (1970). (6) W. J. Mortier and M. L. Costenoble, J. A w l . Cryst., in press. (7) W. C. Hamilton, POWOW, Brookhaven National Laboratory, Brookhaven, N. Y . , 1962. (8) W. J. Mortier, Acta Cryst., A29, 473 (1973). (9) W.R. Busing, K. 0. Martin, and H. A . Levy, ORFFE, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1964. (101 > - , - J. V. Smith, Advan. Chem. Ser., No. 101,171 (1971). (11) R. J. Mikovsky, A. J. Sylvestri, E. Dempsey, and D. H . Oison, J . Catai., 22, 371 (1971). (12) W. H. Baur, Amer. Mineral., 49, 697 (1964). (13) E. Vansant, Ph.D. Thesis, Kathoiieke Universiteit te Leuven, 1971 ~
(1) B. K. G. Theng, E. Vansant, and J. B. Uytterhoeven, Trans. Faraday SOC., 64, 3370 (1968). (2) E. Vansant and J. B. Uytterhoeven, Trans. Faraday Soc., 67, 2961 (1971).
Potential Energy Curves and Dissociation Energy of Titanium Monoxide V. S. Kushawaha Department of Physics, Banaras Hindu University, Varanasi-221005, lndia (Received May 24, 1973)
Potential energy curves for Ti-0 interactions corresponding to the X3&, A”, and C3A states of the T i 0 molecule have been calculated using the method of Rydberg-Klein-Rees as modified by Vanderslice, et al. The ground-state dissociation energy has been estimated using Lippincott’s three-parameter form of the potential function and also using the chemical energy available in a chemiluminescent reaction of Tic14 + 0 2 in the presence of potassium vapor. From the chemiluminescent reaction its value was found to be 167.82 f 2 kcal/mol.
Introduction As T i 0 is observed very strongly in the spectra of M and S type stars, its spectroscopic study has been the subject of interest of many ~ o r k e r s , l -and ~ ~ seven electronic band systems are known in the region 3000-1000 A. High-resolution studies of the T i 0 bands have confirmed two singlet and three triplet states in the energy region 10000-2000 cm-l. The ground state of T i 0 is well confirmedg to be the 3Arr with a very low lying state at about 581 cm-I and a slightly higher state a t about 2295 cm-l. Uhler’sg study of the electronic spectrum of T i 0 has revised the previous designation of the molecular , and C 3 7 r to X3Ar, A34 and C3A, states from X 3 ~ rB3Z and has given a new set of molecular constants for these states only. The molecular constants of the two singlet states are st ill uncertain. The present communication deals with th,e experimental potential energy curves of these electronic states using the molecular constants reported by Uhler.9 The ground-state dissociation energy of T i 0 has been determined by a number of workers using different methods, but the values are different from each other. Do(Ti0) is 156.9 f 2.2 kcal/mol by Hampson, e t a1.,I5 167.38 f 2.30 kcal/mol by Wahlbeck, e t a1.,16 159.9 kcal/mol by Groves, e t al.,I7 157 kcal/mol by Berkowitz, e t al.,ls 158.62 kcal/mol by Wheatley,lg 157 kcal/mol by In the Herzberg,20 and 129 kcal/mol by Carlson, e t present communication an attempt has been made to clarify the exact value of Do(Ti0) using the potential
energy curve of the ground state of T i 0 and the chemical energy available in a chemiluminescent reaction. Experimental Section Construction of t h e Potential Energy Curves. The potential energy curves have been calculated from the experimental energy levels, using the method of RydbergK l e i n - R e e ~ , ~ las - ~ modified ~ by V a n d e r ~ l i c e ,known ~~ as the RKRV method. This is a WKB method where one starts with the observed energy level E and from this calculates the maximum and minimum points of vibration. This method gives the potential function very accurately but it is restricted to known energy levels. Ginter and Battino25 have suggested an extrapolation of the RKRV curves which are best for the region of the potential curve immediately beyond the last point determined from the experimental data, but they often can be extended meaningfully to as many as twice the number of vibrational levels known experimentally or to the dissociation limit, whichever is reached first energetically. The spectroscopic data used in the calculation are given in Table I and the results of the calculation are given in Table 11. Determination o f t h e Ground-State Dissociation Energy. ( I ) Curue-Fitting Method. The method of curve fitting has been used to estimate the ground-state dissociation energy of a number of molecules. This method involves the comparison of the RKRV curves for the ground state of the molecule to an empirical potential function with different values of dissociation energies. The value of the The Journal ot Physical Chemistry, Vol. 77, No. 24, 1973
