Atomic Force Microscopy

AFM for the elucidation of solid state syntheses which avoid wastes

Prof. Dr. Gerd Kaupp, Dipl.-Chem. Jens Schmeyers,
Dipl.-Chem. Michael Haak, Cand.-Chem. Andreas Herrmann
Labo 1995 6 57

Article installed with permissions of the publisher
WWW-adaption by M. Haak

Imaging of individual atoms is epoch-making. For applied AFM it is important to scan in the nm- and µm-range if the surface structures happen to occupy that range. Both physical properties and chemical reactions may depend on submicroscopic surface structures. The requirements for the scanning of large, steep, densely packed, deeply penetrating nanostructures with AFM have been summarized [4]. In Ref.[4] is also shown how changes of the surface during the scanning are avoided, how artefacts are recognized, and how the images look like if nanoliquids form at the surface or if the limits inherent to the AFM techniques are surpassed, an example being the imaging of membrane pores. Waste-free solid state syntheses which do not use solvents are the future production techniques, undoubtedly. Solid-state photolyses are longer used already [3] however, their large scale technical use poses practical problems.

Much more interesting are gas/solid- [5] and solid/solid-syntheses [6]. Both of these deserve puplic support and publicity. The development of such techniques was hampered, because incomplete theories of the so called topochemistry [7] were not able to "understand" the occurrance of such reactions. Despite the fact that most reactions of the new techniques for syntheses proceed quantitatively and avoid all kinds of waste (several examples have been performed in the kg scale) the opinion still prevails, that one should rather dissolve the reactants even at the expense of usually lowered yield and unnecessary problems with waste. Therefor it is very favorable that the investigation of organic gas/solid- [5] and organic solid/solid-reaktions [8] with the AFM uncovered unexpected new theoretical foundations. Thus primary actions may now be accepted which avoid waste problems because there is not only an experimental basis but also a theoretical one.

Reaction Scheme 1

Fig. 2 excellent quality (GIF 244 K) or Fig. 2 (JPEG 55 K)

Figure 2: AFM surface of hydroxylaminehydrochloride; 
a: main surface after 20 min manipulation in moist air; 
b: after 200 min hydration in moist air; 
d: same as b but inverted image; 
c: after 2 min application of diluted acetone vapor on the surface hydrate 
of hydroxylaminehydrochloride.

Reaction Scheme 2

Fig. 3 excellent quality (GIF 105 K) or Fig. 3 (JPEG 28 K)

Figure 3: Natural (110)-AFM-surface of thiohydantoine;
a: fresh;
b: after repeated treatment with gaseous methylamine

Reaction Scheme 3

Fig. 4 excellent quality (GIF 319 K) or Fig. 4 (JPEG 54 K)

Figure 4: AFM-surface of tetraphenylethylene;
a: fresh natural (10-1)-surface;
b/d: after 10 min exposure to 0.2 bar of NO2;
c: after 20 min exposure to 0.2 bar of NO2;

Upon longer exponere to NO2 gas the circumwalled craters disappear, no doubt, as a consequence of phase transformation into the product lattice. The initially formed cyclic structures are again comprehended by correlation with the crystal structure of the starting material [11]. The tetraphenylethylene molecules which stand vertical under (10-1) have a limited possibility for partial turns around their long axis. They use it if they grow thicker upon nitration. By doing that they collide with next row molecules which then turn in opposite direction and provoke a third and then a forth molecule to the corresponding turns with finally filling the hole left from the first of these molecules. Thus, a concentric process leaving a hole at its center is described. After such initiation the growing molecules (consecutive nitrations) move upward and present their bottom p-positions for nitration with NO2 gas. The crater diameter is enhanced by participation of neighbouring rows of molecules and those processes proceed until the original lattice breaks down [11]. Again, the crystal structure dictates the long range anisotropic molecular movements during phase rebuilding.
Contrary to solution diazotations, gas/solid diazotation may proceed waste-free and allow elegant applications in syntheses [12]. The solid state diazotation of sulfanilic acid to give its diazonium salt, which is needed for the synthesis of widely used pH-indicator methylorange (via azo coupling with N,N-dimethylaniline) provides particularly impressive AFM-images.

Reaction Scheme 4

Fig. 5 excellent quality (GIF 256 K) or Fig. 5 (JPEG 49 K)

Figure 5. (010)-AFM surface of sulfanilic acid monohydrate; 
a: fresh and barely effloresced; 
b/d: island formation after short diazotation with NO2-gas; 
c: after advanced diazotation by several treatments with NO2.

The initial surface in Figure 5a shows traces of efflorescence which may be the reason for the volcano like features. These are swallowed after short application of gaseous NO₂ to the surface in air. Flat islands which may enclose little lakes are formed (Figure 5b, d). The characteristic island features are still recognizable after multiple application of NO₂ (Figure 5c). However, the advanced height-growth is less uniform. A more roughened height scenery with embraced depths is optained. The formation of islands [10] is easily derived from the crystal structure of sulfanilic acid hydrate [13], which is depicted in Figure 6.

Figure 6 (GIF 22K)

Figure 6. Space filling stereoscopic representation of the molecular packing
of sulfanilic acid monohydrate; 
top: view on the (010)-face showing hydrogen bridges between NH3+ and SO3- groups 
with additional H-bridging by H2O molecules; 
bottom: view on (100) which is orthogonal to (010) showing how the molecules 
alternate in their orientation and how they stand long-elongated under 
the (010) face which is on top; 
N: grid; S: shaded; O at S: circles; O of the water: dense circles.

The molecules stand vertically under (010) and their orientation alternates. Thus, a maximum of hydrogen bridges is obtained and the water molecules add further H-bridging. While the upmost layer on (010) is open for attack of the NH2 groups by NO₂ of half of the molecules (Figure 6, top), the proceeding of the reaction into the crystal requires, that long elongated molecules exit from their H-bonded lattice positions and move upward in order to make the amino groups available for diazotation which pointed downward from the first layer and upward from the second layer etc. Clearly the reaction is hindered by this particular packing. Therefore the reaction spreads around the initial successfull invasions into row 2 etc. and thus from flat islands. 2 planes with a distance of 15 nm are present in Figure 5b/d.

Reaction Scheme 5

Fig. 7 excellent quality (GIF 292 K) or Fig. 7 (JPEG 57 K)

Figure 7: AFM-surface of benzopinacol (100);
a: fresh in a distance of 0.5 mm from an onlaying crystal of p-toluenesulfonic acid monohydrate
b/d: after 12 h at 50 °C, at a distance of 0.5 mm;
c: after 12 h at 50 °C, at a distance of 0.1 mm;

The crystals are heated to 50°C for 12 h to initiate the reaction. They are remounted to the AFM and then scanned. One obtaines well-formed craters in 0.5 mm distance from the proton source (Figure 7b,d). Those are 5 mm deep and 150 mm wide. Importantly the reaction has much more proceeded it 0.1 mm distance (Figure 7c). The craters found there are about 11 nm deep and are mostly circumwalled with heights of typically 4 nm. Only at distances of more than 2 mm one finds surface images similar to the one in Figure 7a. The distance dependent reaction and crater formation can be understand again in terms of crystal data [8]. Here catalytic quantities of protons move over 2 mm from the small acid crystal and produce a charge separation, causing the crystals to stick together firmly. Mechanistically the protons combine with OH groups at the front side of molecular layers and water is liberated. Therefor, new protons are formed at the backside of cations after their rearrangement to yield triphenylacetophenone. These newly formed protons react with the frontside OH groups of the next layer etc. [8]. Interestingly, the morphologically dominant (001)-face does not react correspondingly, according to the AFM investigation. That surface remains unchanged after the same treatment and the acid crystal does not stick to that surface of benzopinacol. The reason for that is to be found in the orientation of the OH groups in the upmost layer of benzopinacol. On the (001)-face the hydroxyl-H (but not the free electron pairs) point outward. Therefor, no protonation can occur, inasmuch as there is also steric hindrance [8]. Conversely, on the reactive (100)-face, the free electron pairs are pointing outward and may thus be protonated [8]. This first mechanistic investigation of a crystal/crystal-reaction shows the potential of AFM and indicates a mechanistic understanding of the many further waste-free solid/solid-reactions which occur in most diverse types [6].

[1]  G. Binning, C. Quate, C. Gerber: Atomic force microscope,
Phys. Rev. Lett. 56 (1986) 930.
[2]  J. Frommer: Scanning Tunneling and Force Microscopy in 
Organic Chemistry, Angew. Chem. Int. Ed. Engl. 104 (1992) 1324.
[3]  G. Kaupp: AFM and STM in photochemistry including photon
tunneling, Advances in Photochemistry 19 (1995) 119.
[4]  G. Kaupp, J. Schmeyers, U. Pogodda, M. Haak, T. Marquardt, M. Plagmann:
AFM for the imaging of large and steep submicroscopic features,
artifacts and scraping with asymmetric cantilever tips,
Thin Solid Films, SXM1 special issue, 1995, in press.
[5]  G. Kaupp: Atomic force microscopy in organic gas/solid-reactions:
How do the new phases build up?, Mol. Cryst. Liq. Cryst. 211 (1992) 1, cit. lit.
[6]  F. Toda in Y Ohashi (Hrsg.): Reactivity in Molecular Crystals,
Kap. 4: Solid-to-solid organic reactions, Kodansha/VCH, Tokyo/Weinheim, 1993, S. 177.
[7]  M. D. Cohen, G. M. J. Schmidt, F. I. Sonntag: J. Chem. Soc.
(1964) 2000; G. M. J. Schmidt: ibid. (1964) 2014;
M. D. Cohen: Angew. Chem. 87 (1975) 439; Mol. Cryst. Liq. Cryst. 50 (1979) 1.
[8]  G. Kaupp, M. Haak, F. Toda: Atomic force microscopy and solid
state rearrangement of benzopinacol,
J. Phys. Org. Chem. S 95 H 12, in press.
[9]  G. Kaupp, U. Pogodda, J. Schmeyers: Gas/solid-reactions with
acetone, Chem. Ber. 127 (1994) 2249; cit. lit.
[10] G. Kaupp, J. Schmeyers: Gas/Solid-Reactions of Aliphatic Amines
with Thiohydantoines: Atomic Force Microscopy and New Mechanisms,
Angew. Chem. Int. Ed. Engl. 105 (1993) 1656.
[11] G. Kaupp, J. Schmeyers: Gas/solid-reactions with nitrogen
dioxide, J. Org. Chem. 60 (1995) Heft 16, in press.
[12] A. Herrmann: Diploma Thesis, University of Oldenburg, 1995.
[13] A. I. M. Rae, E. N. Maslen: The crystal structure of
 sulfanilic acid mono-hydrate, Acta Cryst. 15 (1962) 1285.

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