2066
NOTES
2.2
24
2.6 I/T
I
1
3.0
2.8
3.2
io3)
( O K - ~ X
Figure 1. Effect of hydrogen on catalytic activity.
1 1 I 1 2210 2080 2040
1
I
1 2000
1 1960
’
1 1920
I
I 1 I880
1840
FREQUENCY (cm”)
Figure 2 . Effect of hydrogen on spectrum of chemisorbed CO (123”, 11.7 Torr of CO, 12 Torr of 0 % ) .
data (catalyst sample no. l),except point J1, were recorded in the presence of hydrogen, and the H series data (catalyst sample no. 2) and point J1 in the absence of hydrogen. These results were reversible. Following catalyst evacuation at Torr and room temperature for several hours, data taken for the J series catalyst fell on the H series line. The hydrogen did not noticeably react with oxygen, since oxygen in the reactant feed mixture could be accounted for by unreacted oxygen and COzin the product mixture. The presence of hydrogen slightly changed the spectrum of adsorbed carbon monoxide, as shown in Figure 2 (points J1 and J2). Based on band interpretation proposed by Eischens, et U Z . , ~ the two absorption bands have been identified with “linear” and “bridged” surface CO species. In the presence of hydrogen, both bands were shifted to lower frequencies. The shift was greater for the “linear” band center (2076 to 2065 cm-’) than for the “bridged” band center (1982 to 1975 cm-1). Infrared absorption on the low-frequency side of the bridged band was increased.
Discussion In addition to the promoting effect of hydrogen, promotion of the carbon monoxide oxidation has been deThe Journal of Physical Chemistry, Vol. 76, iVo. I S , 2971
scribed for palladium-alloy systems. The activation energy for the reaction on silver-palladium10 and gold-palladium“ alloys suddenly decreases when the palladium content is decreased to the point \There the palladium 4d band is filled. Thus alloying palladium with hydrogen, silver, or gold appears to produce the same change in its catalytic behavior for the carbon monoxide oxidation. An explanation in terms of electrons from dissolved hydrogen filling the metallic 4d band of palladium is proposed. Xechanistically, the promoting effect is attributed to the increased ease of oxygen adsorption. Oxygen, being very electronegative, chemisorbs by partially withdrawing electrons from the metal surface. This electron extraction occurs with difficulty with palladium 4d electrons. However, s or sp hybrid electrons can be much more easily polarized by oxygen molecules at the metal surface, and oxygen chemisorption on metals is thought to involve these electrons.12 Upon hydrogen dissolution in palladium, the Is electrons of hydrogen fill the 4d band. Less energy is required for the subsequent promotion of 4d electrons into the 5s band and correspondingly for oxygen adsorption, which is hypothesized to be the reaction rate limiting step. The spectral results confirm this model. Eischens‘j has concluded that any change which increases the ease of electron donation by the metal will shift both linear and bridged bands to lower frequencies, in accord with the present results. Acknowledgments. This work was supported in part by The National Science Foundation (Grant GP-607). The authors are grateful for the use of the spectrometer provided by The Atlantic Refining Company. The authors are indebted to Drs. R. P. Eischens, C. W. Garland, and R. C. Lord for discussions and advice during the course of the investigation. (9) It. P. Eischens, S. A . Francis, and W. A . Pliskin, J.Phys. Chem., 60,194 (1956). (10) G. A I , Schwab and K. Gossner, Z . Phys. Chem. (Frankfurt am M a i n ) , 16, 39 (1958). (11) 9.G. Daglish and D. D . Eley, Proc. I n t . Congr. Catal., 2nd, 2, 1616 (1961). (12) D. 0 . Haywood and B. M. W. Trapnell, “Chemisorption,” 2nd ed, Butterworth, Washington, D. C.,1964.
Phase Transitions in Tetraalkylammonium Iodide Salts
by J. Levkov, W. Kohr, and R. A. Mackay* Drexel Uniz‘ersity, Philadelphia, Pennsylcania (Received October 16, 2970)
19104
Publication costs assisted by Drexe2 University
A report appeared recently on the fusion properties of some ionic quaternary ammonium compounds. The
NOTES
2067
Table I : Transition Temperatures, Enthalpies, and Entropies of Tetralkylammonium Iodide Salts. The AH Values Are in Kcal Mole-’, Where the Upper Value Is from the Heating Curve and the Lower Value from the Cooling Curve. The A S Are in eu, Calculated from the Upper AH Values. Temperatures Are in OK 1
Salt RPNI
3
2
AH
Methyl Ethyl
5 . 1 =k 0 . 1 5 . 1 zk 0 . 3 Propyl 0.7 & 0 . 0 3 0 . 2 f 0.1 Butyl 6 . 7 * 0.1 Pentyl 3 . 3 rt 0 . 4 3.9 0 . 1 Hexyl 5.8 & 0.3 1.6 i0.1 Heptyl 2.2 i 0 . 1 0.2 i ? p-Dibromobenzene (reported) Naphthalene (reported)
*
F
AS
T
11.0
465 458 224 214 392 403 399 344 335 358 342
3.1 17.2 8.2 16.9 6.0
5
4
Trans 1-----
7 -
6 Trans--2
---
7
AH
AS
T
3 . 3 1 0.3 3.610.1
8.0
418 398
4.5i0.1 0.610.05 0.6rtO.06
1.4
results were interpreted essentially on the basis of a configurational disordering of the alkyl chains of the cation. In the course of studies on the premelting transitions of tetraalkylammonium salts, we have examined the series of tetra-n-alkylammonium iodides from methyl through heptyl. In addition to the calorimetric studies, we have examined the ir spectra of the methyl, ethyl, and propyl iodides, and our data indicate that a structural change other than a conformational change of the cation occurs in the premelting transition for all of the salts. The enthalpies of transition were measured on a Perliin-Elmer ?Model DSC-lb differential scanning calorimeter.2 At least three separate samples of each salt were run, and each was recycled several t’,imes. The enthalpies and entropies of transition and fusion for the butyl-, pentyl- and heptylammonium iodides are in substantial agreement with those reported by Coker, Ambrose, and Janz,l but their values for the propyl salt are more than twice as large as those reported here. The ir measurements were performed on a Perkin-Elmer 621 spectrophotometer using pressed disks composed of the pure salt. Measurements were made at several temperatures both below and above the temperature corresponding to a transition. The results of the calorimetric study are presented in Table I. The salt is listed in column 1; column 2 lists the transition enthalpies and their average deviation in units of kilocalories per mole. The upper value corresponds to heating, the lower value to cooling. The entropy change in column 3 is calculated from the upper enthalpy value. I n column 4 are the temperatures, in absolute units, a t which the transitions occur. Columns 5-10 list the enthalpies, entropies, and temperatures of succeeding transitions if they occur. The
9
8
AH
341 392 388
10
Trans -3AS
1 . 4 ~ t 0 . 0 2 4.1 1.5rt0.06
T
352 350
--
11
12
13
Fusion-AH
AS
T
2.2 9.0 r t 0 . 1 9.8 r t 0 . 2 4.1rt0.1 4.2 r t O . l 8.7 r t O . 1 9.0 rt? 4.63 10.19 4.84 4.25 f 0.17 4.56
5.2 22.0
418 410 404 378 374 396 392 362
11.0 21.8
355
values for the fusion transition appear in columns 11-13. Also included in the table are the enthalpies of fusion for p-dibromobenzene and naphthalene obtained in this work and reported in the literature. The effect of temperature on the infrared spectra is, as expected, to increase bandwidths and decrease band heights. I n the absence of any physical or chemical changes, the total integrated band intensities should remain constant. Band intensities were estimated by taking the product of the height times the width at half-height, normalized to the room temperature value. Relative absorbances obtained in this manner will decrease with increasing temperature, approximately linearly with 1/T since the tails of the peaks will become increasingly important. The I\/le4NI salt showed no transitions over the temperature range studied. The Etl and Pr4 salts possess bands at 467 and 511 cm-l, respectively, corresponding to the 446-cm-l band in h!te4E\’I. Plots of relative absorbance us. 1 j T for these salts show breaks a t about 459 and 409”K corresponding to observed transitions. Upon cooling, the bands reappear and the temperature dependence of the spectra is “reversible.” Transition one in Et4NI, two in Pr4NI, and one in Pe4n’I all have comparable AH (3-5 kcal/mol) and A S (8-11 eu/mol) values. In addition, the behavior of the ir band in the 450-500-cm-’ region is the same for the two salts examined (Et and Pr). This indicates that the cause of these transitions may be essentially the same for all three salts. At room temperature, Et4NIhas a distorted wurtzite(1)
T.G . Coker, J. Ambrose, and G. J. Jam, J . Amer. Chem. SOC.,
92, 6293 (1970).
(2) E. S. Watson, M. J. O’Neill, J. Justin, and N. Brenner, Anal. Chem., 36, 1233 (1964).
The Journal of Physical Chemistry, Vol. ‘76, N o . 13, 1971
2068
NOTES 22 20
18 16 i
14
4
: 2
+
10
b
8
4
B
2
Figure 1. Form A, Nordic cross formation of the Et4N+ ion, Form B, formation found in the EtaNBr-succinamide lattice compound.
type structure3 where the conformation of the Et4nT+ ion is that which in projection forms a Nordic cross (swastika). This is form A (Figure 1). Similar possibilities exist for the higher homologs. Form B has been found in the Et&Br-succinamide lattice compound, It is possible therefore that the above transitions correspond to a conformational change from forinA to form B. Since form B is of higher symmetry (D2d,neglecting the protons) than form A, the disappearance of some ir bands may be expected. Such a transition may also account for the relatively large AH and AS values observed. A strong proof that this transition is in fact due to such a conformation change could be provided by the absence of the ir band in the 450-cm-l region for a compound in which the E t S + ion is known to be in form B. Unfortunately, in the EtaNBr-succinamide compound, the tail of a strong succinamide band obscures the region of interest. An absorption band at 463 cm-I is present in the spectrum of a concentrated solution of Et4NBr in chloroform. On the basis of steric considerations the energy of form A should be somewhat less than that of form B. However, solvation by chloroform could reverse this and no conclusion can be reached on the basis of the solution spectrum. The Pr4W+ ion is in form B in PrqNBrJ4and this compound has a band at 473 cm-l. A DSC scan of PreNBr revealed two transitions, at 375 and a t 393”1 E t > Pr, while the reverse is found. The fact that the band is observed in solution also shows that it is associated with the R&+ ion itself. The “disappearance” of this band is thus associated with an effect of the premelting transition on the RAY+ ion. Whether this effect is due to a change in unit cell symmetry or to a conformation change accompanied by distortion cannot be determined on the basis of the ir data. Some insight into the problem may be gained by examining the AH and AS for the premelting transitions. Where there are two or three closely spaced transitions, as for hexyl and heptyl, AH = ZAH and similarly for AS. A fundamental difference between n-odd and n-even for the ( C , H 2 n + l ) Nsalts is indicated in Figure 2 where AH and AS are plotted vs. n. It is seen that in each of these plots two separate curves are generated, one for n-odd (the smaller values) and the other for n-even (the larger values). Since the ir band behavior is the same for the E t and P r salts, it is likely that all R 4 S I salts undergo a similar transition with a AH of about 3 kcal/mol and a AS of about 9 eu. I n addition, the even-?a salts undergo another transition which the odd-n do not. The AH and AS for this transition increases with increasing (3) E. Wait and H. M.Powell, J. Chem. SOC.,1872 (1958). (4) A . Zalkin, Acta Crystatlogr., 10, 557 (1957). (5) E. A. V. Ebsworth and N. Sheppard, Spectrochim. Acta, 13, 261 (1959).
NOTES
2069
chain length. The observed enthalpies and entropies of fusion are consistent with this idea, the odd-n salts having the higher A H and A S of fusion. The solid-state transition just preceding melting in tetraamylammonium thiocyanate was largely accounted for by a kink-block type of rearrangement of the alkyl groups within the quarternary ammonium cation.6 Here, however, it would seem more reasonable t o ascribe the “first” transition, which we postulate as being common to all the salts, to a crystal structure change. The magnitude and relative uniformity of the A H and AS for this transition as well as the crystal structures of Et4NIa and Pr4NBr4r5are in accord with such an assignment. Both salts crystallize in the trigonal space group 13, each ion having four nearest neighbors of the opposite charge approximately at the corners of a tetrahedron. Thus, the salts may have similar crystal structures while still having different cation conformations, and it is conceivable that a conformation change and crystal structure change could occur either separately or in conjunction. This may explain how more than one transition could give rise to only one observed DSC peak or why sometimes two or three closely spaced peaks of constant total area but variable individual area may be observed. As the chain length increases the number of different conformations increase. The differences in energy (at least due to intramolecular interactions) between conformations would tend to decrease with increasing chain length. This would explain the increasing difference between AH and A S values for both the odd and even-n salts, and the fact that the even-n A H values are beginning to level off a t higher n while the A S values are still increasing. If these considerations are correct, then the conformation of the even-n ions in the solid just below the melting point should be essentially the same as in the melt, while this should not be true of the odd-n ions. The Et4N’ ion has conformation A (% a t room temperature, and we propose that the propyl and higher homologs, due to their greater Chain lengths which would enable them to “wrap around” the iodide ion, have conformation €3 The first transition, which may be a change in crystal structure, is designated as
(x).
I, and the second transition, postulated to be a conformation change, as 11. The fusion transition is 111. This scheme is shown below.’ I
Et
A +A +decomposes
Pr
B -+B
BU
B - B ~ A - A
Pe
I B+BIII-A
I
---f
decomposes
I
I
I11
I1
Hex B + B - A - % A Hep B L B Z A While we have not directly considered it, there may be a configuration disordering involved in transition
I.* The fact that no transition is observed for the Me4NI salt is also consistent with the postulated nature of the transitions. There is, of course, no possibility of a conformational change for the R/Ie4N+ion, and on the basis of radius ratio it is conceivable that the crystal structure of R!le4NI is different from that of the higher homologs. In any event, due to the smaller cation size, it is not unreasonable to expect that any structure change could occur a t a considerably higher temperature which may be above the decomposition point. In summary, we conclude that all of the R4NI salts may undergo a similar transition accompanied by the disappearance (or large decrease in intensity) of an ir band around 500 cm-‘. The even-n salts undergo an additional transition, close to the first, which is probably associated with a conformational change of the R4N+ ion as discussed by Coker, Ambrose and Janz.’ This is reflected in the lower enthalpies and entropies of fusion of the even-n salts. (6) T. G. Coker, B. Wunderlich, and G. J. Janz, Trans. Faraday Soc., 65, 3361 (1969).
(7) We wish to note that a transition designated as X + X (or B B), etc., does not necessarily imply no change in conformation +
a t all, since for the longer chains a change in conformation a t the end of a chain will produce only a small change in (intramolecular) energy. We consider, however, that a t least the “core” conformation (the first two carbons in each chain attached to the nitrogen) does not undergo any change in conformation. (8) w. Wong and E. Westrum, Jr., J. Phys. Chem., 74,1303 (1970).
The Journal of Physical Chemistry, Vol. 76, N o . 29, 2971
