J . Phys. Chem. 1990, 94, 6061-6069 to such arrangements in the above crystals are different from the forces leading to an intimate ion pair in water. Nevertheless, observation of close proximities between these groups suggests that electrostatic repulsions can be overcome by environmental effects in both cases. The striking similarity between those unrelated structures is noteworthy. At a C-C distance of 5.95 A the relative orientation of the Gu+ ions differs from that in the intimate ion pair. In typical structures perpendicular arrangements (Figure 10) which facilitate electrostatic quadrupolequadrupole interactions between the two groups are found. Conclusions We have calculated the free energy profile for two Gu+ ions in water and found a clear energy minimum at short distances corresponding to an intimate stacked ion pair. Because of computer limitations, larger separations were not explored and it cannot be concluded whether this intimate pair would be more
6061
stable than a solvent or “infinitely” separated pair. Contrary to the gas-phase electrostatic model, approaching these two cations below approximately 5 A in water led to stabilization. It is stressed that such arrangements will be very sensitive to the molecular environment (e.g., counterions, field of the protein, etc.) and that fluctuations of this environment may either disrupt or enhance this association. Molecular recognition involving reversible binding processes might take advantage of such “counterelectrostatic” attraction. Acknowledgment. Gratitude is expressed to the “groupement scientifique” IBM-CNRS, to the C N R S Computing Center of Strasbourg for support of this research, and to Prof. J. M. Lehn and Dr. Y. Hofflack for helpful discussions. S. Boudon greatly acknowledges fellowship support kindly provided by Rh6nePoulenc and Prof. W. L. Jorgensen for giving the last version of the BOSS computer program. Registry No. Guanidinium, 2521 5-10-5.
Adsorption in Carbon Micropores at Supercritlcal Temperatures Ziming Tan and Keith E. Gubbins* School of Chemical Engineering, Cornell University, Ithaca, New York 14853 (Received: February 12, 1990)
Nonlocal density functional theory and grand canonical Monte Carlo simulations are used to investigate the adsorption behavior of gases of simple spherical molecules in model carbon micropores at temperatures above the critical value for the gas. In most of the calculations the parameters are chosen to model methane as the adsorbed gas, but some calculations are reported for a model of ethylene. The excess adsorption isotherms (which measure the increased density in the pore, relative to that of the bulk fluid) show a maximum at a particular value of the bulk gas density (pressure). Near the capillary critical temperature these maxima have a cusplike nature, similar to that observed experimentally. We investigate the effect of temperature and pore size on the excess adsorption isotherms and on the maximum excess adsorption. Heats of adsorption for an ideal gas phase are also calculated.
1. Introduction Over the past few years powerful theoretical and computer simulation techniques have been brought to bear on the study of adsorption of fluids on single solid surfaces and in micropores of various simple geometries (slits, cylinders, and spherical cavities). Most of these studies have explored low temperatures, below the critical point of the adsorbate, where a wide variety of novel phase transitions (two-dimensional melting and vaporization, orientational transitions, layering transitions, wetting and prewetting transitions, capillary condensation and melting, and so on) and hysteresis effects occur. Less attention has been paid to adsorption effects at higher temperatures. However, many important applications of adsorption involve adsorbate gases near or above their critical temperature, and there is a need to develop a theoretical understanding of the effects of pore size, geometry, nature of surface, and state conditions on the adsorption, density profiles, diffusion rates, and so on. One example of such an application is the storage of methane (the primary constituent of natural gas) at high densities at near-ambient temperatures, well above the critical temperature of the gas (191 K). Such storage methods are needed for both the transportation of natural gas and for its use as a fuel for vehicles; the very high pressures needed to compress the bulk fluid to high densities at ambient temperature are not as economical as storage at low pressure. The use of microporous materials as a storage medium offers the possibility of achieving high methane densities at low pressures of the bulk gas. Questions relevant to such an application are, what are the optimum temperature and pressure of the bulk methane gas in contact with the micropores, and what is the optimum pore size for such an application? In this paper we examine such questions for a simple model of the methane-carbon micropore system, in
which the carbon pores are of parallel wall geometry and have surfaces that approximate the basal plane of graphite. Other relevant questions, not addressed here, include the effects of modifying the shape and composition of the solid surface. For our calculations we use density functional theory, together with grand canonical Monte Carlo simulations. The densityfunctional theory treats the fluidsolid system as an inhomogeneous fluid in a static external field exerted by the solid. It involves two major approximations, the smoothed density approximation (SDA),’q2which is used to calculate the contribution to the free energy from the short-range repulsive forces, and the mean field approximation (MFA),’ which is used to estimate the contribution from the longer-range attractive forces. In the SDA the free energy density for the fluid with repulsive forces is given by that for a uniform hard-sphere fluid at a nonlocal smoothed density. We refer to this theory as the nonlocal mean-field density functional theory (NLMFT). The NLMFT has been employed successfully in studies of the structure of confined fluids$J capillary condensation,6-’ layering transitions,6 and the structure and se(1) Tarazona, P. Phys. Rev. A 1985,31,2672; 1985,32,3148. Tarazona, P.; Marini Bettolo Maconi, U.; Evans, R. Mol. Phys. 1987, 60, 573. (2) Tan, Z.; Marini Bettolo Marconi, U.; van Swol, F.; Gubbins, K. E. J . Chem. Phys. 1989, 90, 3704. (3) Sullivan, D. E.; Telo da Gama, M. M. Wetting Transitions and Multilayer Adsorption at Fluid Interfaces. In Fluid Interfacial Phenomena; Croxton, C. A., Ed.; Wiley: New York, 1986, and references therein. (4) Heffelfinger, G.S.;Tan, Z.; Marini Bettolo Marconi, U.; van Swol, F.; Gubbins, K. E. Mol. Simul. 1988, 2, 393. ( 5 ) Ball, P. C.; Evans, R. J . Chem. Phys. 1988, 89, 4412. (6) Tarazona, P.; Marini Bettolo Marconi, U.; Evans, R. Mol. Phys. 1987, 60, 573. (7) Peterson, E. K.; Gubbins, K. E.; Heffelfinger, G.S.;Marini Bettolo Marconi, U.; van Swol, F.J. Chem. Phys. 1988,88,6487.
0022-3654/90/2094-6061%02.50/00 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
lectivity of confined mixture^.^ Experimental studies relevant to this work have included adsorption isotherm measurements for simple fluids adsorbed onto carbon black,lO#llactivated carbon and molecular sieve carbon,12 and zeolite adsorbents.” The excess adsorption isotherms were found to exhibit a pronounced maximum at higher pressures, usually at pressures near that corresponding to the critical density.I4 There have been only a few theoretical investigations. Among early work is that of Steele15and Findenegg and Fischer,I6 who sought analytical expressions for the density profile. Later work by Fischer included the use of a virial expansion for lowpressure adsorption isotherms for a crude model of the argongraphon system,” and an integral equation approach for higher pressures.18 More recently Marini Bettolo Marconi19 examined the adsorption of a simple model fluid at its critical temperature on both a single surface and in slit-shaped pores; he found the adsorption isotherms to be very different in these two cases. Heffelfir~ger~~ studied simple fluids in cylindrical pores at supercritical temperatures. The position of the maximum in the adsorption isotherm was found to shift to lower pressures as the pore width decreased. van Megen and SnookZoused grand canonical Monte Carlo simulation to study a Lennard-Jones (LJ) fluid confined in a slit-shaped pore with a 10-4-3 fluid-wall potential at temperatures just above the critical value; the potential parameters were chosen to model ethylene in carbon pores. Their adsorption isotherms showed the cusplike maxima expected and were in qualitative agreement with experimental results. In section 2 we describe the models used for the fluid molecules and the fluid-solid interaction. The LJ interaction is used, with a 10-4-3 model for the fluidsolid potential in most calculations; this latter model averages over the lateral structure of the solid. We also report simulation calculations for a wall in which the lateral structure is included and show that at the temperatures considered these structural effects are small except for the smallest pore sizes. The theory and simulation methods are outlined in section 3. In section 4 results for the LJ model of ethylene using the NLMFT are reported and compared with the Monte Carlo results of van Megen and Snook;Zothe theory is found to be quite accurate at the conditions studied. In section 5 we report detailed results based on the NLMFT for methane in carbon pores over a range of temperatures, bulk gas pressures, and pore widths. We also report values of the isosteric heat of adsorption at zero coverage for this model. 2. Intermolecular Potential Models We consider a model pore of slitlike geometry, with parallel walls of infinite extent separated by a pore width H,the walls are assumed to be represented by the basal plane of graphite. We treat both the fluid and carbon molecules using a LJ pair potential. For the fluidsolid interaction, most of our calculations are based on the 10-4-3 potential,15which is obtained by first summing the LJ interactions between the fluid molecule and the carbon molecules in the solid and then averaging over carbon atoms in the various parallel x-y planes of the lattice. This approximation (8) Ball, P. C.;Evans, R. Mol. Phys. 1988, 63, 159. (9) Tan, Z.; Gubbins, K. E.; van Swol, F.; Marini Bettolo Marconi, U. Proc. Third Int. Con/. Fund. Ads. Sonthofen, FRG, in press. (10) Specovius, J.; Findenegg, G. H. Eer. Bunsen-Ges. Phys. Chem. 1978, 82, 174; 1980, 84, 690. (1 I) Findenegg. G. H.; Lbring, R. J. Chem. Phys. 1984,81,3270. BlOmel, S.; Findenegg, G. H. Phys. Reo. Lett. 1985, 54, 447. (12) Ozawa, S.; Kusumi, S . ; Ogino, Y . J . Colloid Interface Sci. 1976,56, 83. (13) Wakasugi, Y.;Ozawa, S.; Ogino, Y . J . Colloid Interface Sri. 1981, 79, 399. (14) Menon, P. G . Chem. Reo. 1968,68,217; Advances in High Pressure Research: Academic Press: New York, 1969; Vol. 3, p 313. ( I 5) Stele, W. A. Sur/. Sei. 1973, 36, 317; The Interaction of Gases with Solid Surfaces; Pergamon: Oxford, 1974. (16) Findenegg, G. H.; Fischer, J. Faraday Discuss. 1975, 59, 38. (17) Fischer, J. Mol. Phys. 1977, 34, 1237. (18) Fischer, J. J . Chem. Phys. 1978, 68, 3947. (19) Marini Bettolo Marconi, U. Phys. Reo. A 1988, 38, 6267. (20) van Megen, W.; Snook,1. K . Mol. Phys. 1981,45,629; 1984.54 741.
Tan and Gubbins neglects the solid structure in the x-y plane but preserves the discrete planes of atoms in the z direction normal to the pore walls. It can lead to errors, particularly at low temperatures and for small pores. By carrying out Monte Carlo simulations for both structured and 10-4-3 walls, we show below that these errors are rather small for supercritical temperatures considered here. Following our earlier p~blications,~ the fluid-fluid pair interaction was cut and shifted at r, = 2.Saf
H* - 0.7 (see Figure 5). Some authors'0v20 have therefore used an effective excess adsorption, defined by
r'*v=
-lH H* - 1.4 1
'-0 7 '
(p*(Z*) - p*b) dz* = p'*p - p*b (21)
0.7
where P ' * ~is given by 1 H* - 1.4
'-0 7
0.7
p*(z*)
dz*
(22)
If one assumes that p * = 0 for z* < 0.7 and z* > H* - 0.7, it is easy to derive the following interrelationships between these various definitions: r*s= rt* - I/2H*p*b = rf*s- 0.7p*b = '/zH*r*,
H*-14 p*p =
*p'*
P
6066 The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
Tan and Gubbins 3.0-
2.0 -
I .o-
r,' 0.05
C
0.'15
0.iO
0.4
0.20
fb' Figure 6. The excess adsorption isotherms of methane in carbon pores for H* = 5 . The temperature T* is indicated in the figure. 2 0 p -
I
0.2
1 0.4
I
0
Figure 9.
0
0.2
0.1
03
pb' for various H* at T* = 1.27.
rtsand 0.0
IO, !5,20
0.6 0.40.2 -
pb' Figure 7 Same as Figure 6 but for H* = 10.
r," I .5
r;l
1.0
H'= 20 0.5
0
3
Figure 10. I'*s and
1
I
0.1
0.2
0.3
pb' Figure 8. r*,and r*v for various H* at T* = 1.35. In the r*,plot, the result for H* = is included as a dashed line. The position of p*bm(m) is given by the circle.
Most of the results shown below will be in the form of the excess adsorption per unit surface area, I'*srand per unit pore volume, I'*v. taking H as the pore width (Figure 5 ) . In Figures 6 and 7 are displayed the excess adsorption isotherms, r*rand I'*v, for H* = 5 and 10 a t temperatures T* from 1.27, which is near the bulk fluid critical temperature, to 2.0, which is approximately room temperature. Each isotherm exhibits a maximum. For the larger pore an enhanced adsorption and pronounced, almost cusplike, maximum was found at low tem-
0
I
I
0.I
0.2
pb" r*"for various H* at
0.3
T* = 2.0.
peratures; this behavior is characteristic of adsorbed fluids near their critical temperature. For smaller pores, however, the isotherms are very similar in shape for the temperatures shown. This feature indicates that the confined fluid is quite far above its capillary critical temperature. This is as expected, when we recall that the capillary critical temperature falls as H* decreases for confined system^.^.^.^ In Figure 8, we display adsorptions at T* = 1.35 for various integral pore sizes from H* = 2 to 20. For each pore size, both I'*s and I'*" pass through their respective maxima (r*, and r*m); from eq 23 we see that these maxima will occur at the same value of the bulk density, which we call p*bm As the pore size increases, the positions of the maxima are shifted to higher bulk densities. Besides these common features, there are some differences between I'*smand I'*vm.While I'*vm first increases and then decreases as H* increases always increases with H*, and as H* it approaches a point [ ~ * ~ , , , ( m ) ,I'*s,,,(m)], indicated as an open circle in the figure. Moreover, in the r*, plot, at densities smaller than p*bm(m), all I'*smlines cross one another and become indistinguishable when H* is sufficiently large. At densities higher
-
(40) Tan, Z.; Van Swot, F.; Gubbins, K. E. Mol. Phys. 1987, 62, 1213.
Supercritical Gases in Micropores
The Journal of Physical Chemistry, Vol. 94, No. 15, I990 6067 0.6 I
0.5
i
I
1
I1
2.0
A
I
1
4
6
H*
8
-
IO
Figure 11. Excess adsorption of methane in carbon pores shown as a function of pore width for TL = 1.35 and fixed bulk fluid density.
0 5
0
15
10
H"
Figure 13. Maximum excess adsorption of methane in carbon pores, I'*,,,, and r'*",,,,shown as a function of pore width for T* = 1.35 (solid) and 2.0 (dashed).
2'o
7
2.0 1.5
t
i
Gn 1.0
0.5
0.20
0.05
4
H'= 2.5
T* = 1.35
t
1 ,y
01 0
t
I
1
I
I
i
5
IO
15
20
H+
Figure 12. Maximum excess adsorption of methane in carbon pores, r',,,,, shown as a function of pore width for Tz = 1.35 (solid) and 2.0 (dashed). The lower figure displays the bulk density ( P * ~ at ) which the I"*,,,, is obtained.
than P*bm(m), on the other hand, the I'*r lines show simpler behavior, being shifted upward monotonically as the pore size increases. In Figures 9 and 10, additional results for T* = 1.27 and 2.0 are shown. While the above discussion for Figure 8 still holds for Figure 9, the latter shows a new feature. As the pore size increases, the I'*s isotherm shows stronger and stronger near-critical behavior, indicated by a much enhanced cusplike behavior in the shape of the excess adsorption curve. The rSs shown in Figure 10 differs from that in Figure 9 in that at this temperature the curve for H* = 10 is indistinguishable from those for higher H* on the scale of the plot, indicating that the walls may be considered to be isolated from each other. In addition, the I'*,,,,, in contrast to Figure 9, first increases and then decreases as H* increases. In Figure 1 I, we show the adsorption as a function of slit width at fixed bulk density for T* = 1.35. At low densities (p*b < P*bm(m)), I'*sexhibits a global maximum at a certain pore size, flattening out as pore size increases. At high densities (P*b > P*bm(m)), they increase without a maximum, approaching monotonically the limiting value of the amount adsorbed onto a single
I
01 I.2
/
I
r
I
I
I .6
2.0
T* Figure 14. Effect of temperature on I'*sm and P*~,,, for several values of the pore width.
wall. The oscillating behavior at smaller H* values is due to excluded volume effects in forming the molecular layers between the wallsgand is commonly found for fluids confined in small pores. We are particularly interested in how the maximum excess adsorption varies with temperature and pore width. Such results are shown in Figures 12-15. In Figure 12 the maximum excess adsorption (I'*sm),together with the bulk density (P*bm) at which I'*smis obtained, are plotted. At a fixed temperature, I'lsmincreases rapidly, with oscillations at small pore sizes, then flattens out at large pore sizes, with a weak maximum for the higher temperature (T+ = 2.0 in the figure); P*bm(m) shows a similar pattern but without a maximum. Although I'*m is an increasing function of H*, we must bear in mind that porous materials that contain large pores have small surface-to-volume ratio and hence a small capacity for adsorption. Another factor that is unfavorable to large pores is that P*bm increases rapidly with H*. The I'*vm plot in Figure 13 is more directly relevant to the choice of pore size, and it shows that there exists a global maximum in I'*vmfor a given temperature. The results for I"*vm differ from those for I'*m, the optimum pore size that maximizes the excess adsorption fall to zero at being smaller in that case. Both I'*vmand I"*vm
6068
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
I
Tan and Gubbins
\!i
s',+ I
1
0
i-
2.0
\ 0.6
1.0
-/
I
1.0
1.4
1.82
4
6
8
IO
Tr H' Figure 16. Isosteric heat of adsorption at zero coverage shown as a function of temperature and pore size. The curves are calculated results for the model methane-carbon system with the 10-4-3 wall, and the points in (a) are experimental results.41
~
o l
1.2
I
I
2.0
1.6
T" Figure 15. Effect of temperature on r*vm and r'*vm for several values of the pore width.
H* = 1.64. For pore sizes slightly below this value we found no adsorption in the pore. We hope that this feature could serve as a rough guide in designing a thermodynamically "best" porous material. The temperature dependence of the excess adsorption, keeping the pore size fixed, is shown in Figures 14 and 15. The adsorption decreases as T* increases as expected, with a larger rate at a larger slit width. We note that for large pore size, e.g., H* = 10, unlike for smaller pore sizes, P*bm does not vary monotonically with temperature. The small rate of change with T* found in small pores is expected, since the adsorbed molecules are tightly bound by the fluid-wall forces. Because the variation with T* depends strongly on the pore width, the curves for I'*smand I'*vmfor different H* values may cross. Thus, in Figure 14, the curves for H* = 5 and IO cross. This suggests that at low temperatures I'*smincreases monotonically as H* increases (see Figures 8 and 9). At high temperatures, on the other hand, the I'*sm-H*curve will pass through a global maximum for a given temperature (see Figure 10). As the temperature increases, this global maximum shifts to smaller H*. This interesting feature is, however, not shared by the I'*",,,-H* curve. Since I'*-, 0 as H* m and also as H* 1.64 (see Figure 13), it always exhibits a global maximum at a fixed T as H varies. The crossover in Figure 15, therefore indicates only that the global maximum shifts to smaller
the experiment involves extrapolating the measurements to zero coverage and the theory involves several approximations, we do not expect quantitative agreement. However, the figure suggests that our model gives a reasonable estimation of the zero coverage heat of adsorption. The plot versus H* (Figure 16b) shows the curves cross at about H* = 3-4. Thus, for small pores (H*< 3-4) the heat of adsorption increases with rise in temperature, while for larger pores the reverse is true.
6. Conclusions For the temperatures studied here, the NLMFT appears to be quite accurate for adsorption calculations (Figures 2-4) except perhaps for the very smallest pore sizes. For the density profiles the theory is somewhat less accurate, as expected, but still gives generally good results and follows the qualitative trends (Figure 1); the discrepancies will be most noticeable for small pore sizes. The choice of d, the hard-sphere diameter, is important. The use of d2 given by eq 15 gives good results; it is likely the results could be improved slightly by the use of a more accurate description of d, e.g., the WCA prescription. Both the theoretical and simulation results show the excess adsorption isotherms to have a maximum (usually expressed here as I'*vmor I'*sm)at some bulk gas density P*bm, after which the excess adsorption slowly falls off with increasing bulk density. For temperatures near the capillary critical temperature these maxima have a cusplike nature. These features are very similar to those observed experimentally. As the temperature is raised, the maximum becomes weaker and the excess adsorption smaller (Figures 6 and 7). Both pore size and temperature have a proH*. nounced effect on the maximum excess adsorption that can be 2. Isosteric Enthalpy at Zero Cooerage. The isosteric heat achieved, as shown in Figures 8-10 and 12-15. Using the defof adsorption (the enthalpy change on transferring a molecule of inition of pore volume corresponding to H in Figure 5, there is adsorbate from the gas to the adsorbed phase at constant pressure) an optimum pore size that maximizes the excess adsorption at at zero coverage, qst, is given byi5 any given temperature. For T* = 1.35 this is in the range H* = 2.9-3.9. I'*",,, is a complex function of temperature and pore Jdz Ve,t(z) exP(-Vext(z)/m qst size (see Figure 15) but generally decreases with temperature for q*st 1 - I Pa given pore size. It should be noted that for small pores the cff I d 2 exP(-Vext(Z)/W conclusions are quite strongly modified according to how the pore T* - ( Vext(2)) (24) in place of size is defined (Figure 5). If one examines I"*m the optimum pore size that maximizes excess adsorption is smaller where ( Vat@)) means the average energy of an isolated molecule than in the latter case (Figure 13). Which definition of adsorption in the pore. In Figure 16 we display this zero coverage heat of is most appropriate will depend on the application and on the adsorption as a function of P and H*. It is seen (Figure 16a) experimental method used to determine the pore size. that a t small H*,say 2.5, q*st increases slightly as T* increases. The model system studied here neglects many features of real At large H*, on the other hand, q*st passes through a weak micropores, including the lateral structure of the carbon walls, maximum and then drops off quite rapidly. The experimental various terms in the solid-fluid potential (see section 2), wall values shown in Figure 16a were obtained from the slope of the logarithm of the Henry's law constant versus inverse t e m p e r a t ~ r e . ~ ~ roughness, variations in the shape and size of pores, and interconnectivity of the pores. Nevertheless, we believe these results They should be compared with the H* = line in the plot. As provide a useful and not too unrealistic model with which to interpret and compare the behavior of methane-carbon systems. (41) Kiselev, A. V.;Poshkus, D. P. J . Chem. Soc., Farday. Trans. 2 1976, In applying these results to the methane storage problem, we note 72. 950. (42) Heffelfinger, G.S.Ph.D. Thesis, Cornell University, 1988. that the porosity will play a key role; it will be necessary to combine
-
-
-
J . Phys. Chem. 1990, 94, 6069-6073 high porosity with a high excess adsorption. Other factors will also be important in the design of an optimum material, including the heat of adsorption and diffusion rates, which will strongly affect the ease of introducing and extracting the adsorbate molecules from the pores. We plan to report a study of these effects in a
6069
future paper. Acknowledgment. This work was supported by a grant from the Gas Research Institute and in part by a National Science Foundation US-UK grant (INT-8913150).
Thermally Induced 0 to N Acyl Migration In Salicylamides. Thermal Motion Analysis of the Reactants K. Vyas,? H. Manohar,*vt and K. Venkatesant Department of Inorganic and Physical Chemistry and Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India (Received: September 18. 1989; In Final Form: February 5, 1990) Analysis of thermal motion, using the anisotropic displacement parameters obtained from X-ray crystal structure analysis, has been employed to understand the thermally induced 0 to N acyl migration reaction in salicylamides. Application of rigid-bond criterion reveals considerable internal motion in the molecule. The large mean square displacement amplitude between the amide nitrogen and acyl carbon atom and the significant libration of the acyloxy group are consistent with the intramolecular mechanism proposed earlier from X-ray crystallographic studies.
1. Introduction and Background In recent years, X-ray crystallography has been one of the important tools in understanding reactions in the solid state.la,2 The dynamic behavior of molecules, from static crystal structure results, can be obtained in two ways. In the first method, one looks for the correlation among the relevant structural parameters in the molecular fragment of interest present in many crystal structures. The correlations, if any, will indicate the reaction path, and hence an idea about the reaction mechanism can be gained. This method is referred to as the structure-correlation method.la The second method involves the analysis of the thermal motion of the molecules using the anisotropic displacement parameters (ADPs) and determination of the mean square amplitudes of m ~ t i o n . l ~Both * ~ methods have been employed in our attempts to understand the thermally induced 0 to N acyl migration in salicylamides (Scheme I). The crystal structures of three 0-acylsalicylamides viz., the acetyl (OAC): benzoyl (OBZ): and propionyl (OPR)5 derivatives, have been determined. The reaction, in essence, involves bond formation between the N( 1) and C(8) atoms. The interaction between the N( 1) atom and the acyl carbonyl group, C(8)=0(3), is considered as a nucleophile-electrophile interaction, the amide nitrogen N( 1) being the nucleophile with a lone pair of electrons. Based on structural parameters of 0-acylsalicylamides (OAC, OBZ, and OPR) relevant to N--C=O interactions,'* such as the N ( 1 ) 4 8 ) contact and the relative orientations of the amide group and the acyloxy groups, an intramolecular, as opposed to an intermolecular, mechanism has been proposed.5 Another reaction with such an interaction is the thermally induced cyclodehydration of 0-acetamidobenzamide (OAB in Scheme II).6 The results of thermal motion analysis of OAC, OPR, OBZ, and OAB are presented in this paper, and their relevance to the reaction mechanism is examined in the following sections. Thermal motion analysis has been employed for elucidating phase transitions involving inversion of pyramidal nitrogen,' changes in hydrogen bonding,* and changes in conformation9 The present study, to our knowledge, is the first example of the application of thermal motion analysis to a chemical reaction involving a change in covalent bonding. 2. Results and Discussion 2.1. Details of Computation. The ADPs of OAC, OPR, OBZ, and OAB obtained from X-ray structure analysis were analyzed
f
Department of Inorganic and Physical Chemistry. Department of Organic Chemistry.
SCHEME I: Solid-state 0 to N Acyl Migration in Salicylamides 0 (1) II
CW
O K 'R OAC, R :CH, ( t
-
100 OC 1
NAC, R
= CH,
OBZ, R C6H5(t-100 "C)
NBZ, R
CsH5
OPR, R=C2H5(t- 65'C)
NPR, R
C2H5
SCHEME 11: Solid-state Cyclodehydration of @-Form of 0-Acetamidobenzamide (OAB)
QZD OA B
for thermal motion by using the program ~ ~ ~ v The 9 .program l ~ (i) calculates the quantities AA,B = Z A B * - ZBA2 along any interatomic vector AB ( Z A B 2 is the mean square displacement am( 1 I Dunitz. J. D. X-rav Analvsis and the Structure of Ormnic Molecules: Co;nkll University Pres: London, 1979; (a) pp 366-j84, ib) pp 244-261: (2) Paul, I. C.; Curtin, D. Y. Arc. Chem. Res. 1973, 6, 217-225. (3) (a) Dunitz, J. D. Bull. Chem. Soc. Jpn. 1988,61, 1-1 1. (b) Dunitz, J. D. Angew. Chem., Inr. Ed. Engl. 1988, 27, 880-895. (c) Dunitz, J. D.; Schomaker, V.; Trueblood, K.N. J . Phys. Chem. 1988, 92, 856-867. (4) Vyas, K.;Mohan Rao, V.; Manohar, H. Acta Crysrallogr., Sect. C 1987, 43, 1197-1200. (5) Vyas, K.; Manohar, H. Mol. Crysr. Liq. Crysr. 1986, 137, 37-43. (6) Etter, M. C. J . Chem. SOC.,Perkin Tram. 2 1983, 115-121. (7) Chakrabarti, P Seiler, P.; Dunitz, J. D.; Schluter, A.-D.; Szeimies, G. J . Am. Chem. SOC.1981, 103, 7378-7380. (8) (a) Yang, Q.-C.; Richardson, M. F.; Dunitz, J. D. J . Am. Chem. Soc. 1985, 107, 5535-5537. (b) Yang. Q.-C.; Richardson. M. F.; Dunitz, J. D. Acta Crysrallogr., Secr. B 1989, 45, 312-323. (9) Murthy, G. S.; Guru Row, T. N.; Venkatesan, K.XIV Inrl. Congr. Crysr. Collecr. Absrr. 1987, 04.4.5. (10) (a) Trueblood, K. N. THMVO. Thermal Vibration Analysis Computer Program, University of California, Los Angeles, 1984. (b) Schomaker, V.; Trueblood, K. N. Acra Crysrallogr., Sect. A 1984, 40, Suppl. C-339.
0022-3654/90/2094-6069%02.50/0 0 1990 American Chemical Society
