J. Phys. Chem. 1991,95,4601-4610 is a decided falloff in an at 350-450 OC.*I The soot spectra reported here thus represent the volatile components (tar) present in the soot. The spectrum for the soot from a diesel engine (Figure 3b) shows the presence of the unburnt fuel ( m / z 150-250 range) superimposed on a broader envelope of peaks at every mass up to about 600 amu. Apparently, this soot was formed under the conditions that do not defunctionalize the original hydrocarbon structures completely. In the case of the soot from the detonation of RDX-TNT composite (Figure 3c), we can find substantial intensity of peaks at odd masses corresponding to nitrogen-containing compounds. Two series of prominent peaks are evident in this spectrum: (i) due to benzonitrile ( m / z 103) and its benzologues at m / z 153 and 203 and (ii) due to naphthalene ( m / z ____
~~
(21) Ross, D. S.;Hirschon, A. Am. Chem. Soc., Diu. Fuel Chem.. Prepr. Pop. 1990,35,37-45.
4601
128) and its benzologues at m / z 178 (phenanthrene) and 228. Weaker peaks due to alkylated homologues (e.g. at m / z 117 due to methylbenzonitrile) are present. In preliminary work, we have also examined soots from combustion of polymers such as polyurethane, polystyrene, and polyoxazoline. These soots give spectra with features characterized by oligomeric species but, again, no peaks corresponding to fullerenes were observed. In summary, the lack of peaks due to Ca and C,, in the FI mass spectra of most of the common soots is in concert with the conclusion of Frenklach and Ebertm that pyrolysis and combustion of organic materials does not generally lead to soots with fullerene structures.
Acknowledgment. We are grateful to Dr. L. Ebert for encouraging us to examine this question. We thank Dr. P. R. Solomon for the sample of the butane pyrolysis soot and Dr.R. Greiner for the soot from RDX-TNT detonation. This work was supported by SRI's IR&D Fund.
FEATURE ARTICLE Hydrogen Bonds as Deslgn Elements in Organic Chemistry Margaret C.Etter Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: October 2, 1990; In Final Form: December 26, 1990)
This work involves the use of crystal chemistry and crystallographic data base analyses as a way to study hydrogen-bond directed molecular recognition properties of organic molecules. By using the solid phase rather than solution, our studies are not limited to tightly bound dimeric species but encompass aggregate structures with multiple molecules, intricate interlinked patterns, and both strong and weak (often very weak) intermolecular interactions. The point of view taken here is to analyze the consequencesof directed and selective hydrogen-bond interactions on sets of molecules rather than focusing on the energy or geometry of individual hydrogen-bond interactions. The "consequences" are to a solid-state chemist what a new synthesis is to a solution chemist, Le., the formation of new chemical species. Methods for deriving hydrogen-bond rules from large sets of crystal structures are presented here. Ways to prepare organic cocrystals and to use cocrystallization to probe the forces involved in aggregation phenomena are discussed, and examples are given of how molecular aggregation can impart unexpected new properties to organic compounds.
Covalent bonds are the primary design elements in organic molecules. They are stable enough to be detected and directional enough to have predictable structural consequences and occur frequently enough to be of practical interest. Hydrogen bonds play a similar role in organic materials, yet they have escaped attempts at systematization of this role: hence, their use as a tool for carrying out "intermolecular syntheses" has been nearly neglected.' The products of these intermolecular syntheses are hydrogen-bonded molecular aggregates, defined by connectivity patterns arising from hydrogen bonds. These aggregates could form in solution, at interfaces, or in the solid state. Recently, considerable effort has been devoted to the study of molecular association in solution, especially for cases where the aggregate pattern that is formed involves only two species (usually called a host and a guest), where association or binding constants (1) A few notable exceptions include: (a) Ducharmc, Y.; Wuest, J. D. J . Org. Chem. 1988. 53, 5789-5791. (b) Weber, E.; Hecker, M.; Cdregh, I. 1. Am. Chem.Soc. 1989, I l l , 7866-7872. (c) Hart, H.; Lin, L.-T. W.; Ward, D. L. J . Am. Chem. Soc. 1984,106,4043-4045. (d) Zerkowski, J. A.; &to, C. T.; Wierda, D. A.; Whitesides, G. M. J . Am. Chem. Soc. 1990, 112,
9025-9026.
OO22-3654/91/2095-4601$02.50/0
are very high ( IO4 M-'or greater) and where the aggregates are stabilized primarily by topological features such as preorganized cavities? Such systems are important as small molecule models of enzyme activity. Our work addresses the general class of molecular aggregates, including species with weak association constants as well as species with multiple molecules per aggregate structure. Such species are extremely difficult to study in solution, but they are as easy to study in the solid state as are dimers with high solution binding constants. Our work has focused on the role of hydrogen bonds in the absence of preorganized cavities specifically so we could evaluate the independent contributions(such as selectivity or topology) of hydrogen-bonding groups to the structures of aggregates. (2) (a) Rebek, J., Jr. Angew. Chem. 1990, 102, 261. (b) Blake, J. F.; Jorgensen, W. L. J . Am. Chem. Soc. 1990,112,7278-7286. (c) Diederich, F. Angew. Chem., Inf.Ed. Engl. 1988,27,362-386. (d) Friedrichsen, B. P.; Powell, D. R.; Whitlock, H. W. J . Am. Chem. Soc. 1990,112,8931-8941. ( e ) Hamilton, A. D.; Pant, N.; Muehldorf, A. Pure Appl. Chem. 1988, 60, 533-538. (f) Zimmerman, S.C.; Wu. W. J . Am. Chem. Soc. 1989, 111, 8054-8055. (8) Kelly, T. R.; Maguire, M. P. J. Am. Chem. Soc. 1987,109, 6549-6551. (h) Bell, T. W.; Liu, J. J. Am. Chem.Soc. 1988,110,3673-3674.
0 1991 American Chemical Society
4602 The Journal of Physical Chemistry, Vol. 9 5 No. 12, 1991
We have also shown that hydrogen-bonded molecular aggregates can have unexpected properties as a result of the collective behavior of these weakly bound molecules. The chemical and physical properties of collective sets of small molecules acting in concert at the surface of or within membranes or near active sites on proteins or on DNA have not been addressed, in part because there have not been tools for either asking or answering these questions effectively. Our crystal chemical approach to such questions offers a way to study such problems and provides information about the role of particular intermolecular effects that occur in weakly bound aggregates as well as in tightly bound host-guest complexes. The strategy of the present work is to focus on the organizational consequences of hydrogen bonds in organic crystals, where the structures of molecular aggregates can be characterized readily. Ideal systems will be sets of neutral molecules with sterically accessible hydrogen-bonding groups where the hydrogen-bond interactions are perturbed minimally by competing steric or ionic interactions. Such systems are represented schematically in i-iv,
- - - - .y-x-
- - - x-y
- - -y e
---
Etter CHART I v
o
*
H
- ---O=P-Ph/Ph
D
‘Ph
*
8
0
i
ii
,
iii
I
iv
showing -X-- -Y-as a hydrogen-bond interaction and different types of molecular aggregate patterns arising from different numbers and orientations of substituents. Note that iii and iv represent cocrystal patterns. From a series of structures that have competitive hydrogenbonding functional groups, as for iv, where Z is a better hydrogen-bond acceptor than Y,selectivity properties of individual functional groups can be determined, and correlations between hydrogen-bond types and aggregate patterns can be made. Competition between hydrogen bonds and other intermolecular interactions can be systematically introduced and evaluated once the intrinsic organizational properties of the hydrogen-bonding functional groups are established. Our approach to systematizing the use of hydrogen bonds is to analyze crystallographic information from small molecule crystal structures, available through the Cambridge Structural Database (CSD)? to determine if there are hydrogen-bond preferences for individual functional groups (preferences in terms of selectivity and preferences in terms of the patterns of molecular aggregates generated by these hydrogen bonds) and then to design organic crystals and cocrystals based on these hydrogen-bond preferences. We have found that cocrystal formation involving hydrogen-bonded host and guest molecules is a remarkably versatile tool for testing hydrogen-bond selectivity. Most of the cocrystals can be prepared either by crystallization from a solvent containing the two reagents or by grinding the two components together in the solid state. Usually the same crystal structures are obtained from the two different methods, indicating that hydrogen-bond connectivity patterns are not idiosyncratic or determined by nonspecific and unmanageable solvent effects or crystallization conditions. These effects are sir iilar to those of solvent, concentration, or temperature in an intrcrmolecular synthesis. They may dictate whether a particular experiment works, but the variability they introduce does not usually preclude the development of useful synthetic methods. Methods for characterizing hydrogen-bond patterns of small organic molecules, for developing hydrogen-bond rules, and for determining hydrogen-bond selectivity preferences of common functional groups will be presented here. Since the solid-state cocrystallization technique is not generally known and since it provides a remarkably easy way to modify the solid-state properties of an organic compound, a short descriptive section about solid-
state cocrystallization is included. Diarylureas and their crystal and cocrystal patterns are discussed as examples of how to use the principles developed here to study molecular recognition properties.
Hydrogen-Bond Pattern Recognition in Molecular Aggregates Hydrogen-bond patterns frequently involve many types of hydrogen bonds with multiple intertwined hydrogen-bond networks. Comparing two or more such patterns, which might even be composed of different kinds of molecules, is a formidable pattern-recognition problem that is usually done, if at all, by visual inspection of stereoviews of crystallographic unit cells. There has been no consistency in what features of the patterns are reported or even detected. Comparing the hydrogen-bond patterns of thousands or possibly tens of thousands of structures from the CSD by visual inspection is a completely unrealistic undertaking. As a step toward simplifying the problem and structuring it so it will be amenable to automatic search methods, a topological decoding scheme is presented that facilitates comparison of hydrogen-bond patterns. This method involves decomposing hydrogen-bond networks into graph sets which are based on the number of proton donors (subscripts) and acceptors (superscripts) used in the motif, on whether the hydrogen bonds are intra- (S) or intermolecular, and in the latter case on whether the intermolecular motifs are finite (R or D) or infinite (C). The size or degree of a pattern, based on the number of atoms in a hydrogen-bonded ring, for example, is shown in parentheses following the graph set descriptor. The details of this procedure have been published el~ewhere;~ examples are given in Chart I. The process for assigning patterns is illustrated in the pnitroaniline figure, Figure la, where three motifs from the crystal structure are highlighted. Patterns A and B contain hydrogen~~
_ _ _ _ _ ~ _ _
(3) Allen, F. H.; Bellard, S.; Brice, M. D.; Cartwright, B. A.; Doubleday, A.; Higgs, H.; Hummelink, T.; Hummelink-Peters, B. G.; Kennard, 0.; Motherwell, W. D. S.; Rodgers, J. R.;Watson, D. G. Acta Crysrullogr.,Sect. B Struct. Sci. 1979, B35, 2331-2339.
Feature Article
The Journal of Physical Chemistry, Vol. 95, No.12, 1991 4603 B
0
0
I I ~~1~--~O-C-C-N-C-C-O-8(1)
O-C-C-N-
I
0
C(0
I I
nm
p) 0 -N-C-C-0
I
Figure 1. Hydrogen-bond patterns for (a) p-nitroaniline and (b) mnitroaniline. It is evident that patterns B and C are present in both
structures.
bonded aggregates made from interactions between either H, or H2and a single nitro oxygen. Similarities between these aggregate structures and those of m-nitroaniline, Figure lb, are immediately evident. The presence of aggregate patterns can also be used, like bond-length or wavelength criteria, as an indicator of, not proof of, hydrogen bonds. When all the structures in a series show the same molecular aggregate patterns, i.e., when their graph sets are the same, then the whole set of structures is classified as hydrogen-bonded. When this procedure was used to study nitroaniline compounds it was found that subsets of the pnitroaniline pattern, in particular, pattern C, had the same graph set as those found in 21 of the 28 structures investigateds (six of the seven that differed had competing intramolecular hydrogen bonds). These patterns were present even though they had very long -NH- - -0 distances (N- - -0= 3.250 A; the sum of the van der Waal's radii is 3.20 or 3.14 A depending on the reference source used: N = 1.70 and 0 = 1.50 Abor 1.60 and 1.54 A'). The -N(H)- - -02N bond lengths for the other compounds had a reasonably uniform distribution from 2.93 to 3.22 A. There is no reason based on crystallographic evidence to categorize the slightly longer contacts as being qualitatively different kinds of interactions from the shorter contacts. Their organizational properties are topologically like those of shorter hydrogen bonds. It is these topological similarities that are within the purview of crystallography and that are the basis for designing "intermolecular syntheses". The graph set method does provide a way to evaluate how molecular aggregate patterns differ as a function of artificially imposed hydrogen-bond cutoff lengths, or included angles, or other measurable parameters. In other words, it can be used to test the consequencesof having different definitions of hydrogen bonds. If van der Waals cutoff distances alone had been used as the defining criteria in the nitroaniline study, then similarities between (4) Etter. M.C.; Machnald, J. C.; Bernstein, J. Acta Crysrallogr., Sect. B Strucr. Sci. 1990, 816, 256-262. (5) Panunto, T.W.; Urballczyk-Lipkowska, Z.; Johnson, R.; Etter, M.C. J. Am. Chem. Soc. 1987, 109, 7786-7797. (6) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
R:(W
Figure 2. Graph set assignments for three polymorphs of iminodiacetic acid showing that polymorph 3 differs from 1 and 2 in the presence of a second C(5) motif, whereas 1 and 2 are differentiated only when com-
binations of different kinds of hydrogen bonds are considered. Their networks consist of rings of different sizes! In the crystal structure, some of the amino and carboxyl groups are charged, whereas others are neutral, as indicated.
the aggregate patterns of m- and p-nitroaniline would have been lost. The nitroaniline analysis presented above dealt only with motifs, since the hydrogen-bond patterns contained just one type of hydrogen bond. Networks containing all the combinatorial possibilities of hydrogen bonds formed from two different kinds of hydrogen bonds, or from three different kinds, etc., can also be assigned sequentially. In this manner, complex hydrogen-bond patterns can be described explicitly and pattern similarities become readily recognized as common motifs or combinations of motifs. For example, IMDA, -02CCH2NH2+CH2C02H, has three different hydrogen-bond donors in the molecule and it exists in three polymorphic formsE It is a straightforward process to compare the hydrogen-bond patterns of these polymorphs by using graph sets, as shown in Figure 2.9 Two of the polymorphs (1 and 2) have identical sets of motifs, C(S)R:(lO)C(8), indicating that the connectivity patterns generated by each individual hydrogen donor by itself, independent of any other donor in the system, are the same in these structures. In the third polymorph two of the connectivity patterns match those of 1 and 2 but the third motif is a chain instead of a ring (C(5) instead of R:( IO)). Polymorphs 1 and 2 are differentiable when the hydrogen-bond patterns that contain two different kinds of hydrogen bonds simultaneously are considered. Polymorph 1 has a 14-membered ring with two donors (7) Nyburg, S.C.; Faerman, C. H. Acta Crysrallogr.,Sect. B Srmcr. Sci. 1985, 841, 274-279. (8) Bernstein, J. Acra Crysfallogr., Secr. E Srruct. Sei. 1979, 835,
360-366.
4604 The Journal of Physical Chemistry, Vol. 95, No. 12, 199'1
Etter
TABLE I: HydWW-Boad Rules General Rules (1) all good proton donors and acceptors are used in hydrogen
bonding
(2) if six-membered ring intramolecular hydrogen bonds can form, they will usually do so in preference to forming intermolecular hydrogen bonds (3) the best proton donors and acceptors remaining after
intramolecular hydrogen-bond formation form intermolecular hydrogen bonds to one another Additional Rules for Nitroanilines
v
(4) amino protons will hydrogen bond to nitro groups, if no better
proton acceptors are present
( 5 ) one or more intermolecular amino-nitro hydrogen bonds will form (6) the aggregate patterns formed from intermolecular hydrogen
bonds between substituents in meta and para positions will be acentric (7) the amino-nitro interaction is usually a three-center hydrogen bond, Ri(4)
Figure 3. Hydrogen-bond pattern of a disubstituted urea retained even in the presence of a second hydrogen-bonding functional group.13 These structures illustrate that hydrogen-bond patterns are functional-group dependent and can be used for designing new materials containing their preferred motifs. and two acceptors per ring, while polymorph 2 has an eightmembered ring with four donors and two acceptors per ring.
Hydrogem-Bond Rules The use of graph sets as a tool for analyzing hydrogen-bond patterns of large numbers of structures allows one to determine graph set distributions corresponding to a class of molecules containing one particular functional group, and conversely functional group distributions corresponding to all structures with a particular graph set. Hydrogen-bond rules reflect these relationship and provide useful information about preferred connectivity patterns, hydrogen-bond selectivity, and stereoelectronic properties of hydrogen bonds for a particular functional group or for sets of functional group. There are three very useful rules of thumb that apply to organic hydrogen-bonded structures. The first rule was developed by Donohue during the early days of crystallographywhen there was only a handful of organic crystal structures known. He observed that all acidic hydrogenr available in a molecule will be used in hydrogen bonding in the crystal structure of that compound.'O This is still the most useful of all the hydrogen-bond rules. A second rule, complementary to the first one, is that all good acceptors will be used in hydrogen bonding when there are mailable hydrogen-bond donors.'I This rule is not followed as rigorously as the first one, but it is a useful corollary nonetheless. The third rule, frequently observed in our work on cocrystal design, is that the best hydrogen-bond donor and the best hydrogen-bond acceptor will preferentially form hydrogen bonds to one another.I2 These rules reflect energetically favorable kinds of intermolecular association. What is not intuitively obvious is that crystal packing patterns also reflect these simple rules so frequently. A lovely example of this effect is seen in the urylene and substituted urylene structures studied by Fowler et al. (Figure 3).13 The urea portions of the molecules have a bidentate chain pattern whether or not the carboxylic acid substituents are present. Conversely, the typical carboxylic acid dimer pairs which are found in most acid structures are preserved in the acid-substitutedurylenes. In this example, each functional group retains its preferred hydrogen-bond pattern in the presence of the other functional group ( 9 ) Banstein, J.; Etter, M. C.; MacDonald. J. C. J. Chem. Soc., Perkin Trans. 2 1990,695498. (10) Donohue, J. 1. P h p . Chem. 1952.56, 502-510. (11) Etter, M.C. J . Am. Chem. Soc. 1982, l W , 1095-1096. (12) Etter, M.C. Acc. Chem. Res. 1990, 23, 120-126.
(8) ortho-substituted primary nitroanilines usually form two-center intermolecular hydrogen bonds with graph set Ri(6) rather than three center ,o,,~~~~H -N,
N-
o,,,,,, H
and in a variety of different packing patterns and space groups. Expected or typical hydrogen-bond patterns may not occur for a variety of reasons, including the presence of multiple competitive hydrogen-bond sites, steric overcrowding, or competing dipolar and ionic forces. Part of the purpose of our work is to evaluate how hydrogen-bond patterns adjust to the presence of such perturbations. An extreme example would be the introduction of a hydrogen-bondingguest molecule that competes with and displaces intermolecular hydrogen bonds between host molecules to give a cocrystal. This approach was used to design nitroaniline cocrystals. Hydrogen-bond rules for homomeric nitroanilines were determined from data obtained using the Cambridge Data Base (Table I). Molecules that are better acceptors than the nitro group, such as triphenylphosphine oxide, were introduced as guest molecules during crystallization of p-nitroaniline to specifically perturb the -NH-- -02Ninteraction. A cocry~talwith the aniline hydrogens bonding to the phosphine oxide oxygen atom was o b tained:14
An alternative cocrystal system could be prepared by using hydrogen bonds instead of u bonds to link the nitro and aniline groups together. For example, a nitro-substituted benzoic acid and an amino-substituted benzoic acid form cocrystals of heteromeric dimers. The acids bond to one another to provide the a-bond like link, while the nitro and aniline -NH groups remain to link these dimers into polymer chains according to the nitroaniline hydrogen-bond rules. Several examples of these structures are given in Table 11. Nitroanilines are of interest as organic nonlinear optical materials since they are polarizable and since the electron-donating (13) Zhao, X.;Chang, Y.-L.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 1990, 112,66276634.
Feature Article
1
2 3 4
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4605
4-nitropyridine N-oxide 4-aminophenol 4-nitro~vridineN-oxide 4-aminoblnzoic acid 3,5-dinitrobenzoic acid 4-aminbbcnzoic acid 4-aminobenzoic acid 4-chloro-3,5-dinitrobenzoic acid 4-aminobenzoic acid 3,5-dinitrosalicylic acid 4-aminobenzoic acid 4-methyl-3,S-dinitrobenzoic acid 4-aminobenzamide 3,s-dinitrobenzoic acid 3,5-dinitrobenzoic acid 4-aminobenzamide 4-aminobenzoic acid 3,5-dinitrobenzamide 3,5-dinitrobenzamide 4-aminobenzamide
cc 47 P2,2,2, 48 Fdd2 ' 19 49 -
-
49 49
P2,/c P2,/c
49 49 49 49
-
'A is a hydrogen-bond acceptor and -DH is a hydrogen-bond donor. *The space group is given for those systems where single crystals were obtained. A dash in this column indicates that cocrystal phases were obtained and were characterized by noncrystallographic means.
and -accepting properties of the -NH and -NO2 groups contribute to large molecular hyperpolarizabilities.ls Two of the early entries into the field of organic NLO compounds were m-nitroaniline and 2-methyl-p-nitroaniline.I6Many related compounds have been prepared since then, and questions about the role of hydrogen bonds in producing good NLO materials have been addressed." One of the necessary conditions for secondsrder nonlinear optical properties is that the bulk material have no center of symmetry. This property is very hard to control, especially since more than 74% of all organic crystals crystallize in centric space groups. Since nitroanilines were found to form chain structures, it is obvious that each nitroaniline aggregate is acentric. Neighboring molecules in a hydrogen-bonded chain must align so their dipole moments are head-to-tail, unlike dipolar stabilized aggregates where the dipole moments can be arranged in a centric manner. Despite the common Occurrence of non-hydrogen-bonding dipolar groups in organic molecules, close-packing of bulky irregularly shaped organic molecules is usually a more important directing forcc than dipolar stabilization.'* In hydrogen-bonded structures, however, sets of related structures frequently have the same hydrogen-bond patterns regardless of steric irregularities. Their structures look as if the hydrogen bonds had formed first and then were retained during crystallization: 111111111 ___) 1111l1111111
-
111111111
If-Bond Stabilization (acentric)
Thus, nitroanilines can become acentric by way of hydrogenbond directed association. In an effort to capitalize on this self-organization principle, we sought to design an acentric twodimensional aggregate using the cocrystal design ideas presented above. Acentric heteromeric 2-D networks were formed by using multiple nitro groups and -NH donors in appropriateorientations. The nitroaniline cocrystal shown in Figure 4 is such an example. The best donor in the structure is the acid hydrogen of 3,5-dinitrobenzoic acid, and the best acceptor is the carbonyl group of 4aminobenzoic acid. Interactions between these groups promote heterodimer formation. The remaining nitro and amino groups form hydrogen bonds to one another to generate the acentric array. In this particular case the final bulk structure was also found to (14) Etter, M. C.; Huang, K . 4 . Neu Materials for Nonlinear Optics;ACS Symposium Series, in press. (IS) Williams, D. J. An ew. Chem., Inr. Ed. Engl. 1984, 23, 690-703. (16) (a) (i) Shapki, A. Stevenson, J. L. J . Chem. Soc., Perkin Trans. 2 1973, 1197-1200. (ii) Ploug-Sorensen, G.; Andersen, E. K. Acta Crystallogr., Sect. C: Cryst. Stmct. Commun. 1986. C42, 181 3-181 5. (b) Lip sa", 0. F.; Garito, A. F.; Narang, R. S. J . Chem. Phys. 1981, 75, 1509-1 526. (17) Zyss, J.; Nicoud, J. F.; Coquillay, M. J . Chem. Phys. 1984, 81, 4160-4167. (b) Zyss, J.; Berthier, G. J . Chem. Phys. 1982,77,3635-3653.
8.;
0 : 0Q
6
e?
d'?
Flgure 4. Polar hydrogen-bondedcocrystal composed of carboxylic acid heterodimers linked into a polar array by nitroaniline hydrogen The nitro and aniline groups have the same kinds of hydrogen-bond patterns as those found in homomeric nitroanilines.$
be acentric, although the nature of the interplanar interactions is not ~ 1 e a r . I ~ In the simplest hydrogen-bonded crystals, composed of small molecules with a limited number of hydrogen-bonding groups and no other strong intermolecular interactions, stereoelectronic preferences may be critical in determining the packing mode and even the chemical and physical properties of a system. The 1,3-~yclohexanedionesystem is well suited to such a study since it has only one proton donor and one good proton acceptor. Four stereochemically unique modes of association arise from two different planar conformations of the -OHgroup, and the choice of syn vs anti lone pairs of electrons as their acceptor sites (Figure 5 ) . Structure A has been observed for dimedone (5,5-dimethyl- 1,3-~yclohexanedione),~~ structure B for 1,3-cyclohexanedione (CHD),2' and structure C (with a molecule of benzene trapped in the cyclamer cavity) for both 1,3-cyclohexanedione and 5-methyl-1,3-~yclohexanedione.~~ The cyclamer structures are like crown ethers with hydrogen-bonded rather than u-bonded backbones. When crystallized from solutions of mixed benzene and toluene, 1,3-~yclohexanedionehas the ability to selectively trap molecules of benzene in the cyclamer cavities. Solutions of as little as 30% benzene yielded crystals of the cyclamer containing only benzene (structure C), though solutions with less than that gave only crystals of the unsolvated structure B. The ability of CHD to separate benzene from toluene was completely unexpected and unpredictable since the monomer has no apparent complexing ability, though this property is reasonable in light of its crystal structures. Even vapor-phase osmometry indicates that the preferred aggregate form in benzene solution is the same as that in toluene and is a dimer rather than a cyclic hexamer (-20% dimer, 72%monomer, in benzene, 39 OC, 0.025 M). Besides the observation that sets of associated molecules can adopt the properties of macromolecules, these crystals illustrate that stereoelectronic ordering can direct molecular self-assembly.
Solid-State Preparation of Cocrystals The cocrystals discussed above can be made by evaporating solutions containing stoichiometric mole ratios of the components. We have found that almost all of them can also be prepared simply by grinding the two solid reagents together. Depending on the rate and vigor of grinding, particle size, and vapor pressures of the reagents, a complete conversion to the new hydrogen-bonded (18) (a) Whitesell, J. K.;Davis, R. E.; Saunders, L. L.; Wilson, R. J.; Feagins, J. P. J. Am. Chem. Soc.. in press. (b) Gavewtti. A. J . Phys. Chem. 1990, 94, 4319-4325. (19) Etter, M. C.; Frankenbach, G. M. Chem. Mater. 19S!J, I , 10. (20) (a) Semmingsen, D. Acta Chem. Scan. 1974, 828, 169-174. (b) Singh, I.; Calvo, C. Can. J . Chem. 1975, 53, 10461050. ( 2 1 ) Etter. M. C.; Urbailczyk-Lipkowska,2.; Jahn, D. A.; Frye, J. S. J. Am. Chem. Soc. 1986, 108, 5871-5876.
4606 The Journal of Physical Chemistry, Vol. 95, No. 12, I991 SYn
Etter
SYn
A. syn-syn
4N-0
P b
N H-
N - 0 I 2.593(4) A N-11-0 175(4)'
B. anti-anti
Figure 6. ORTEP pattern showing the strongly hydrogen-bonded pyridine- - -acid dimer found in cocrystals formed selectively in the presence of a second weaker acid, p(dimethy1amino)benzoic acid.
sistent feature in the solid-state cocrystallization we have carried out is that cocrystallizations are mast likely to be successful when the two components form stronger hydrogen bonds to one another than to themselves. C. syn-anti
D. anti-syn
F i p e 5. Four stereoelectronicallydifferent hydrogen-bond patterns of 1,3-cyclohexanedioneare shown. Structures A-C have been found for
differently substituted cyclohexanedione Crown-ether-like type C structures wcur with molecules of benzene included in their cavities. cocrystal phase can be accomplished in the solid state. More often than not, the phase that is obtained is identical with that obtained from solution crystal growth, implying that soluent is not necessary to direct molecules with strong directional intermolecular interactions into their preferred crystal form. Solvation and desolvation mechanisms certainly are important in controlling rates of formation and stabilities of aggregates in solution, and they can determine as well which of the several polymorphs is preferred. Solvent also promotes mobility, so different species can diffuse together more readily to initiate and propagate crystal nucleation. To our surprise, however, diffusion of small molecules through solids can take place rapidly a t room temperature and at temperatures well below the melting point of either component or of their eutectics. Elevated temperatures promote these reactions, as does continuous grinding to free clean surfaces for reaction. The details of these mechanisms are not understood, although there are precedents for solid-state ~ocrystallization.~~ The one con(22) Etter. M. C.; Parker. D. L.; Rukru, S.R.; Panunto, T. W.;Britton, D.1. Inclusion Phenom. Mol. Recognir. Chem. 1990,8, 395-407.
Hydrogen-Bond Selectivity Determining relative hydrogen-bonding abilities of organic functional groups has been a subject of intensive research for decades. Most of these studies have been based on measurements of solution association constants under equilibrium conditions2' or microwave studies of molecules with competitive intramolecular hydrogen bonds.25 While it is difficult to model or measure the detailed thermodynamic and kinetic contributions to formation of a particular crystal structure, one can determine whether or not functional groups show consistent selectivity patterns in the solid state and can compare observed solid-state selectivities to solution and gas-phase selectivities. The degree to which the solid-state patterns match solution patterns is indicative of the degree to which equilibrium hydrogen-bond strengths determine association patterns in the solid state and in ordered media. Our methods for ranking solid-state hydrogen-bond preferences are based on functional group competitions in homomeric crystals or heteromeric cocrystals. The procedure involves analyzing which donors are selected by a limited number of acceptors or vice versa during crystallization. Three general schemes are shown in v-vii, D l p - - A - Q
A
4
or
SDIA . . - D l q D l
A
where -Di are hydrogen-bond donors and -A is a hydrogen-bond acceptor. The choices that the donor group has for bonding to an acceptor in each different type of experiment are indicated. Scheme v is a crystallization of a homomeric system where there are two donors and only one acceptor on each molecule. Scheme vi is a cocrystallization containing two components, a molecule with two different donors, and a second molecule with one acceptor. Scheme vii is also a cocrystallization. This experiment (23) (a) Rastogi, R. P.; Bassi, P. S.;Chadha, S.L. J. Phys. Chem. 1963, 67, 2569-2573. (b) Rastogi, R. P.; Singh, N. B. J. Phys. Chem. 1966, 70, 3315-3324. (c) Rastogi, R. P.; Singh, N. B. J. Phys. Chem. 1968, 72, 4446-4449. (d) (i) Patil, A. 0.;Pennington. W. T.; Desiraju, G. R.; Curtin, D. Y.; Paul. I. C. Mol. Crysf.Liq. Cryst. 1986,134,279-304. (ii) Patil, A. 0.;Curtin, D. Y.; Paul, 1. C. J. Am. Chem. SOC.1984, 106, 348-353. (24) Abraham, M. H.; Ducc, P. P.; Prior, D. V.; Barratt, D. G.; Morris, J. J.; Taylor, P. J. J. Chem. SOC.,Perkin Trans. 2 1989, 1355-1375.
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4607
Feature Article TABLE III: Selectivity in Cocrystalliution of Pyridines with Mixtures of Carboxylic Acids
1:l eoeryrlnl
SA
2:1 and 1:l
cocryslal
2AP
Pyridines in Cocrystal
Acid in Cocrystal
Uncomplexed Acid
roo-I.,,
,
0
,
.
,
.
I
,
.
.
50
I
.
,
. . .
66
. I 100
Mole 96 ZAP
I
Figure 7. Nonequilibrium phase diagram for 2-aminopyrimidine and succinic acid indicating that the 2:l cocrystal composition is unstable relative to 1:l. This result is consistent with our observation that solidstate grinding of the two components in concentrationsof 50-80% 2AP frequently yields pure 1:l cocrystals, but 2:l cocrystalsare always contaminated with the 1:l phase.
I
i
n
0.41
H
involves three different molecules, with the acceptor selecting one of two donor molecules during cocrystallization. An example of the third scheme is the cocrystallization of pyridines with pairs of carboxylic acids. We have found that 4-phenylpyridine and ethyl isonicotinate are both capable of completely separating the acid of lower pKa value from the one with the higher pK, by cocrystallization (Table 111). Neutral hydrogen-bonded cocrystals form with a single hydrogen bond between the acid -OH group and the nitrogen of the pyridine component (Figure 6).26 We were surprised to find that even acids with very similar pKa values, such as 3,4-dinitro- and 3,5-dinitrobenzoic acids, could still be cleanly separated by this technique. The implications are that the strength of the hydrogen bond in solution, to the extent that pK, reflects this parameter, has determined which species will nucleate crystal growth. Experiments are underway now to test this hypothesis by detecting and comparing aggregates formed in solution with those observed in the solid state. Cocrystal selectivity can be used to monitor subtle changes in proton-accepting abilities that arise from intramolecular substituent effects. Carboxylic acids readily cocrystallize with 2aminopyrimidines (2AP) to form either 2:l or 1:l complexes.*' Interestingly, the preferred pattern is not a function just of the stoichiometries of the starting reagents. Sometimes the 1:l pattern is preferred even if the solution or solid-state mixture that is used as the starting material is a 2: 1 mixture. Phase diagrams of several of these systems have shown that both patterns may be possible for a particular combination of acid and pyrimidine, with one of them being the kinetically favored product. The phase diagram for 2-aminopyrimidine and succinic acid is shown in Figure 7. This phase diagram is unusual since a metastable 2:l cocrystal phase is evident to the right of the diagram. We have not been able to make a sample of pure 2:1 cocrystal since it is always contaminated with the I:] phase. Also the 2:l phase will revert even in the solid state (with heating and grinding, over a period of days) to give a mixture of the 1:l phase and excess 2-amino~yrimidine.~~ ~~
Legon, A. C.; Millen, D. J. Acc. Chem. Res. 1987,20,39-46. (b) Wilson, E. B.; Smith, 2.Acc. Chem. Res. 1985, 20,257-262. (25) (a)
(26) Etter. M. C.; MacDonald, J. C., unpublished results. (27) Etter, M.C.; Adsmond, D. J . Chem. Soc., Chcm. Commun. 1990,8, 589-591.
In the 1:l hydrogen-bond pattern, pattern B, the pyrimidine ring nitrogens have become inequivalent. One explanation for
/
RCOzH (Addition of another acid to LIE ridPYrimWpeir)
A
B
A
the inequivalence is to consider the crystallization process as a series of steps, each one involving addition of another molecule to a preformed aggregate. The first complexation step might consist of one acid bonding to 2AP, causing the accepting ability of the as-yet unbonded N(2) ring nitrogen to decrease due to conjugative effects. Reduced accepting ability would make this nitrogen a less attractive site for further acid hydrogen bonding, so rather than pick up an additional acid molecule to form pattern A, it associates with another dimer to give pattern B. A-type patterns have been observed for many cocrystals of 2-aminopyrimidine with small difunctional acids like succinic acid.27 B-type patterns are more likely to occur for unsymmetrically substituted 2-aminopyrimidines with acids. For example, in 2amino-4-chloro-6-methylpyrimidine (CMAP), a neighboring
CMAP
CMAP:Bcnzoic acid
1:l
methyl group (an electron-donating group) should increase the electron density on N( l ) , and the chlorine atom should decrease it on N(2). It is predicted that cocrystallization with a carboxylic acid should proceed by preferentially binding to the N( 1) side of the molecule, thus reducing the accepting ability of N(2) even further. The B-type pattern should have the acid groups only at the N( 1) sites while the pyrimidine- - -pyrimidine interactions should m u r only on the N(2) side of the molecule. Subsequent
4608 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 crystallographic analyses of cocrystals of CMAP with benzoic acid and of CMAP with glutaric acid have confirmed these predictions.28 While these studies give only relative rankings, not quantitative rankings (e.g., N(1) is a better acceptor than N(2), but it is not known how much better), they are generally consistent with the quantitativerankings that Taft et al. have developed from solution solvatochromic studies.29 The study of multiple equilibrating aggregate species in solution is an extremely difficult problem since multiply hydrogen-bonded species involving three or more molecules will be present in low concentrationsin the presence of the more dominant dimers. Solid-state measurements provide a convenient way to rank the relative hydrogen-bonding abilities of multiply bonded functional groups, allowing a free carbonyl, a once hydrogen-bonded carbonyl, and a twice hydrogen-bonded carbonyl, for example, to be ranked independently. Diaryhwur as a Model System for the Study of Hydrogen-Bond Mrected Host-Guest Complexation Bis(m-nitropheny1)urea(BmNU) is a host molecule that adapts its conformation and its hydrogen-bonding properties to the presence of proton-accepting guest molecules. Guests usually bind to both -NH protons of the urea. Unlike crown ethers, or specially designed rigid acceptor molecules,” there is no pteorganized pocket in BmNU for trapping acceptors and promoting strong binding interactions. Our interest is not in optimizing binding constants but rather in determining how selective the binding process can be even when there are no preorganized binding sites. There are two features of this chemistry that are noteworthy. One is that BmNU, unlike virtually all other known neutral proton donor molecules, behaves primarily as a proton donor, a property that is even more remarkable given the presence of a strong accepting group (the urea carbonyl) in the molecule.” Also, other closely related analogues of BmNU show very limited or no complexation ability. Both of these features appear to arise from an unusual weak intramolecularproperty of the host, which was evident only by inspection of a large number of crystal structures and by the use of selective cocrystallization studies. When BmNU is cocrystallized with small molecules containing strong acceptors such as phosphoryl or sulfoxide groups or with molecules that are much weaker acceptors such as ketones, ethers, and nitro compounds, it forms 1:l cocrystals with the following typical hydrogen-bond pattern:
Etter guest, triphenylphosphineoxide. In most instance8 these urea hosts
separate as pure phases during cocrystallization attempts. The clue to the special complexation properties of BmNU appears to be that the ability of the urea carbonyl group to form intermolecular hydrogen bonds has been reduced to nearly zero by the presence of the m-nitro groups.32 The origin of this effect is not clear, but the consistent planarity of the urea molecule in its cocrystals compared to the nonplanar structure of BmNU and other diary1 ureas in their homomeric crystals and the fact that nitro groups are good electron-withdrawinggroups suggest, that there may be special stabilizing interactions between the aromatic ring hydrogen atoms and the carbonyl group of the BmNU molecules. To test this idea, -CF3 groups were substituted for the -NOz groups to give a derivative that readily formed cocry~tals with the two strongest acceptors (triphenylphosphineoxide and dimethyl sulfoxide) only. The significance of intramolecular -CH- - -O=C hydrogen bonds to the special cocrystallization properties of BmNU has been further supported by ab initio self-consistent field molecular orbital analysis of these structure^.'^ It was found that the calculated atomic charge of the aromatic -CH hydrogen adjacent to the -NOz group in BmNU is larger than the analogous charge in diphenylurea (+O. 130 vs +0.095). The negative charges on the urea oxygens are about the same in both structures (-0.295 in BmNU vs -0.305 in diphenylurea). Analyses of surface electrostatic potentials of these ureas by Politzer et al.33 also provide new insight into selectivity questions. Diphenylurea has a surprisingly extended negative surface potential on the carbonyl side of the molecule. It extends over the carbonyl group as well as over the near-neighbor -CH hydrogens. The possibility that self-association rather than cocrystallizationis promoted by this property implies that the cocrystallization differences between BmNU and diphenylurea are amplified due to differences in their self-association abilities. We have recently begun a study of imide cocrystallization properties since they appear at first glance to be complementary to the ureas.34 They could in principle serve as bidentate acceptors, just as the ureas have served as bidentate hydrogen-bond donors. Thus, we sought hydrogen-bonddonating molecules, such as phenols, which could donate one proton to both of the imide carbonyl groups. To our surprise, diphenylimide does not easily form cocrystals with any proton donors; rather, it prefers to act as a proton donor itself and bind to triphenylphosphine oxide through a hydrogen bond with the -NH hydrogen. It prefers to leave its two carbonyl groups free rather than leave an -NH hydrogen free.
BmNU : Cyclohexanone
The common features of these cocrystals are that the urea carbonyl is not hydrogen bonded by any intermolecular hydrogen bonds, both -NH protons are used in hydrogen bonding, the conformation of the molecule places the m-nitro groups syn to the carbonyl group, and the torsion angles of the phenyl rings relative to the urea groups are ,-lose to zero. ~f the nitro groups are with -H, X H ~ ,-OCH~,or if the nitro groups are moved to the para positions, the derivatives are no longer good complexing agents. It is difficult to form cocrystals with these compounds at all, and the only cocrystals that have been isolated arc for the -CHI and p N 0 2derivatives with the strongest acceptor (28) Etter, M.C.; Adsmond, D., unpublished results. (29) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J . Org. Chem. 1983,48. 2811-2881. (30) Cram. D. J.; Trueblood. K. N. In Host-Guesr Chemistry, Macrocycles; V6gtk F., Weber, E., Eds.; Springer-Verlag: New York, 1985; pp 125-188.
postlllalcdImide Analog
cocryslal
Observed Bauimide Calystal
Diacetamide, the simplest imide in the set, proved to be a very versatile cocrystallizingagent, in contrast to diphenyl imide. It crystallizes in two crystal forms.35 The metastable form has a trans-trans conformation and forms hydrogen-bonded chains, as expected. The cis-trans conformation forms diacetamide dimers leaving One group unbonded. (31) Etter, M. C.;Panunto, T. W. J . Am. Chem. Soc. 1988, 110. sa96-5a9i. (32) Etter, M. C.;Urbaficzyk-Lipkowska,Z.; Zia-Ebrahimi, M.; Panunto, T. W. J . Am. Chem. Soc. 1990, 112, 8415-8426. (33) Murray, J. S.;Grice, M. E.; Politzer, P. J . Mol. Eng., in press. (34) Etter. M.C.;Reutzel, S. M. J . Am. Chem. Soc., in press. (35) (a) Kurcda, Y.; Taira, A.; Uno,T.; Osski, K. Cryst. Strut. Commun. 1975, 4, 321-324. (b) Matias, P. M.; Jeffrey, G. A,; Ruble, J. R. Acra Crystallogr., Sect. E Struct. Sei. 1988, B44, 516-522.
Feature Article
The Journal of Physical Chemistry, Vol. 95, No. 12, I991 4609 'H CRAMPS Selld-Stalr NMR
OYpjYCHJ HJC 0 trans-tram
e cbnlnr
cis-trans
e dimers
Diacetamide forms cocrystals with many different proton donors, including carboxylic acids, phenols, amides, and ureas. The hydrogen-bond patterns in these structures are varied and are currently being investigated. One pattern that has emerged is that diacetamide cocrystallizes with phenols by preserving the stable dimer pattern shown above for the cis-trans conformation, while using its lone carbonyl group as an appendage for attaching to phenol -OHgroups. Thus diacetamide can be converted from a dimer into a complex with four molecules and into an infinite hydrogen-bonded chain by cocrystallizing with pnitrophenol or with hydroquinone, respectively:
20.0
15.0
10.0 5:O 0:O PPM Figure 8. 'H CRAMPS solid-state NMR spectrum of acetyldibcnzoylmethane showing the unusual highly shielded resonance at 5.0 ppm due to a close intermolecular CH- -x interaction.
-
form, crystal form 3 might be the only one obtained if aggregate 2 nucleated crystal growth faster than aggregate 1 did. Crystal Form 1
Molecul;
The pattern shown below, from the crystal structure of diacetamide with phydroxybenzoic acid, shows that the diacetamide dimer is not preserved here, but that a heterodimer is formed instead. Clearly, there are many challenges awaiting crystal chemists!
Weak Hydrogen-BodInteractions as Design Elements in organic crystals It is not unreasonable to consider that strong hydrogen-bond interactions can direct molecular aggregation in organic crystals since only one or a few specific intermolecular contacts provide a large proportion of the stabilization energy of the crystals.36 When arc hydrogen bonds or other less well-defined intermolecular contacts also able to control the organization of molecules in a crystal? The answer depends not only on the enthalpic properties arising from a particular kind of interaction but also on all the processes that contribute to crystal nucleation, which, we propose, are very similar to those that contribute to any form of molecular association. It is important to recognize that crystals obtained in the lab may not be the lowest free energy forms, even if they arc the only forms that are isolable. Polymorphism is ubiquitous among organic compounds."' Metastable crystal forms are often easy to obtain and are often indefinitely stable once they have been isolated. Another important point is that the hydrogen-bond aggregate found in a crystal is not necessarily the most stable aggregate pattern. Once nucleation of one crystal type occurs, all the solution equilibria can be displaced to favor the aggregate being trapped in the fastest growing nuclei. For example, in the scheme shown below, even if crystal form 1 were the most stable (36) Dauber. P.; Hagler, A. T. Acc. Chcm. Res. 1980, 13, 105-1 12.
Sometimes multiple polymorphic forms are all nucleated in the same solution and mixtures of polymorphs crystallize simultaneo~sly.~* More often only a single crystal type is isolated, and it is not always the most stable form. It may be that very weak intermolecular interactions have a disproportionately large effect on the crystal growth process compared to their interaction energies.39 Weak interactions like 4 H -- -?r or -CH--?r contacts do appear in crystal They may be just chance occurrences. They might even be repulsive interactions. Crystallographic data cannot answer those questions directly, but they can show that these interactions are present and that the structures appear to have been organized as though that interaction was the determining factor in establishing connectivity patterns during the aggregation and nucleation process. During the course of investigating the tautomeric properties of P-diketomethanes in the solid state, we found that one of our samples, 2-methyl-1,3-diphenylpropanedione,had an unusual 'H solid-state NMR spectrum." There was a strong well-resolved peak at about 5 ppm, a region of the spectrum where very few hydrogen resonances occur. In solution, there was no peak in that region. We had previously solved the crystal structure of this compound and had not noticed any inter- or intramolecular interactions that looked unusual. The solid-state NMR results prompted us to reinvestigate the crystal structure, and we found that there was a close contact between an aromatic -CHhydrogen of one molecule and the aromatic ring in a neighboring molecule (2.73 A, displaced by 0.27 A from the hexad axis). Such effects are postulated as being important stabilizing interactions for side-chain phenyl rings of amino acids in protein^.'^ Analogous
-
(37) Haleblian. J. K. J . Pharm. Scf. 1975, 64, 1269-1288. (38) Etter, M. C.; Krm, R. B.; Bernstein. J.; Cash, D. J. J. Am. Chcm. Soc. 1984,106,6921-6921. (39) Desiraju, G. R.; Murty, B. N.; Kishan, K. V. R. Chcm. Mater. 1990, 2,447-449. (40)(a) Traetteberg, M.;Bakken, P.; Scip, R.; LQttke,W.; Knieriem, E. Acra Chcm. S c a d . 1988, A42, 584-593. (b) Nakatsu, K.; Yoahioka, H.; Kunimoto, K. Acta Crystallogr.,Sect. B Stmct. Sci. 1918,834,2357-2359 and references therein. (41) Etter. M.C.; Vojta, G. M.; Bronnimann, C. E. J . Magn. Rcson., in press.
J. Phys. Chem. 1991, 95,4610-4618
4610
intramolecular 4 H - --T interactions have been known and studied by solution NMR spectroscopy for many years.43 A close -CH- -r interaction is known to cause the hydrogen to become highly shielded with a resultant upfield shift in its resonance position, which is precisely the effect we found in our solid-state NMR spectrum (Figure 8). These results do not prove that this interaction is a stabilizing one or even that it is decisive in controlling the packing pattern. There are, however, no other intermolecular interactions in the crystal structure that are even suggestive of hydrogen bonds, charge-transfer interactions, or other stabilizing interactions besides van der Waals' contacts. This crystal structure and its solid-state NMR pattern show that -CH---r interactions may be important in organizing sets of molecules in crystal structures. It alerts crystallographers and theoreticians to be aware that such interactions may be important determinants in controlling molecular aggregation and that the possibility of such weak interactions should not be overlooked in interpreting known crystal structures or in designing new ones.
-
Conclusions Intermolecular hydrogen-bond interactions have enormous potential as design tools in both solution and solid-state chemistry. The procedures presented here, where hydrogen bonds are thought of as synthetic tools similar to Q bonds in classical organic chemistry, should be helpful in studying a wide range of chemical problems. Structureproperty relations in pharmaceutical, electrooptical, and energetic solid-state materials might be studied by correlating graph sets of hydrogen-bonded or otherwise strongly (42) Burley, S.
K.;Petsko, G. A. Science 1985, 229, 23-28.
associated sets of molecules with one or more of their solid-state properties. Cocrystallization and the study of cocrystal structures holds promise for modeling molecular recognition events taking place in nonrigid media. Postulated structures of small molecule host-guest complexes in solution might be observable in cocrystal forms even if they are not present in very high concentrations in solution,4 Site-specific recognition in protein-DNA complexes4s or interchain association in polymeric matrices& might also be amenable to cocrystallization of components of these systems. Hydrogen-bond rules developed with small molecule studies are functional-group specific and as such should be useful as initial models of the independent role of hydrogen-bonding functional groups in complex systems. Acknowledgment. A special thank you to Diana Parker, a graduate student at the University of Minnesota (UM), for technical and scientific assistance with preparation of this paper, to Professor Doyle Britton, (UM), for providing expert crystallographic assistance, and to Professor Jim Loehlin, Wellesley College, for helpful discussions about graph sets. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to NIH (GM42148-Ol), NSF (CHE8600383-02), and ONR (N0001489-K-1301) for partial support of this research. (43) Bovey, F. A. Nuclear Magmric Resonance Speciroscopy; Academic: New York, 1969; pp 64-71. (44) Rebek, J. Science 1987, 235, 1478-1484. (45) Jordan, S.R.; Pabo, C. 0. Science 1988,242, 893-907. (46) Stadler, R. Prog. Colloid Polym. Sci. 1987, 75. 140-145. (47) Lechat, J. R.; de A. Santos, R. H.; Bueno, W. A. Acia Crysiaflogr., Seci. B Siruci. Sci. 1981, 837, 1468. (48) Lechat, J. R., private communication. (49) Etter, M. C.; Frankenbach, G. M.,unpublished results.
ARTICLES Excltatlon of IF(Bsn(O+)) by Metastable 02. 1. Studies Involving IF(X, v ) S.J. Davis* and A. M. Woodward Physical Sciences Inc., 20 New England Business Center, Andouer, Massachusetts 01810 (Receioed: June 12, 1990; In Final Form: December 14, 1990)
A mechanistic study of the energy transfer from singlet molecular oxygen 02(a1A,b22) to the B3n(O+) state of iodine
monofluoride is reported. Chemiluminescence and laser-induced fluorescencetechniques were used to probe various aspects of this reaction scheme. By using several precursor reactions as the source of the IF(X), we have shown that vibrationally excited IF(X,o>9) is a n active participant in the excitation of IF(B) by O,(lA).
1. Introduction The excitation of the lower lying triplet states (A'IIl, B3&) of the halogens and interhalogens via energy transfer from metastable collision partners has been the subject of numerous studies over the past two decades. A pioneering study was completed by Arnold, Finlayson, and Ogryzlo,' who reported that I2 was both electronically excited and dissociated in the presence of electronically excited 02(a'A,b'E). Later, Derwent and (1) Arnold, S.J.; Finlayson, N.;Ogryzlo, E. A. J. Chem. Phys. 1966,44, 2529.
0022-3654/91/2095-4610302.50/0
Thrush2" completed a series of detailed experiments that showed that the B311(0,+) state in I2 was excited when molecular iodine was reacted with O2('4'2). They also observed that the I2 was dissociated by the 02,and as a result of this study, Derwent and Thrush correctly predicted that the near resonant energy transfer from 02('A) to the atomic iodine 2Pl/2state could produce an atomic iodine electronic transition laser operating on the 2PI 7 2P3/2transition at 1.315 pm. This pioneering study was the 6asn ( 2 ) Derwent,
(3) Derwent,
R.G.; Thrush, B. A. Trans. Foraday Soc. 1971,67, 2036. R.G.; Thrush, B. A. Chem. Phys. Lcrr. 1971, 9, 591.
0 1991 American Chemical Society
