| dc.description.abstract | In the recent past, considerable attention has been focused on the design and synthesis of molecular devices [1]. Molecules that can perform various kinds of interesting motions, such as ratcheting, shuttling, harpooning, etc., are being synthesized and studied. Synthesizing new molecular devices and understanding their behavior could have numerous applications in nanotechnology. Artificial molecular machines built to date have fairly complicated molecular structures. In this respect, smaller molecules that are able to move easily, or that may be useful as movable parts, are very interesting because they may serve as components of molecular machines of the future.
A major part of research in the field of molecular nanotechnology is the design and modeling of various molecular machines and molecular devices. While the ultimate objective must clearly be economical fabrication, the present target is the identification of any such molecular machine structure. The modeling of molecular machines is a cheap and easy way to explore the truly wide range of molecular machines that are possible, allowing rapid evaluation and elimination of obvious dead ends and the retention and intensive analysis of more promising designs.
We have been interested in using fluxionality to design such molecules. Our first example is a molecular roller, which would remain attached (chemisorbed) to a surface but can move on the surface rather easily by rolling. The molecule is hypostrophene. This fascinating molecule, though synthesized long back [2,3] and known to undergo Cope rearrangement [2], has not been studied theoretically in detail. We study the Cope rearrangement of the free molecule (activation barrier of 25.3 kcal/mol) and then examine the molecule adsorbed on an Al(100) surface. We find that the chemisorbed molecule can undergo a “Cope-like” rearrangement on the metal surface with a reasonably low activation barrier (18.02 kcal/mol), resulting in a rolling motion of the molecule on the surface. Our theoretical calculations lead us to believe that the predominant mechanism of movement of adsorbed hypostrophene on the surface is rolling and not sliding [4].
Another fluxional molecule, syn-tricyclooctadiene, on an Al(100) surface was studied, and in this case also we find a fairly low barrier (13.64 kcal/mol) for rolling motion. We have also studied semibullvalene on Al(100), which behaves like a “molecular rocker.”
We have studied the tricarbonyliron complexes of hypostrophene, syn-tricyclooctadiene, semibullvalene, and 1,5-hexadiene (boat form) and found interesting fluxional motion in them. All these molecules have a pair of parallel or nearly parallel double bonds to which the tricarbonyliron fragment is coordinated. Since in these molecules the pair of double bonds shifts during Cope rearrangement, the coordinated tricarbonyliron unit also moves to the new position. In other words, the tricarbonyliron unit bonded to the diene fragment of these fluxional molecules can move from one site to an adjacent site by a “Cope-like” rearrangement process. However, the energy barriers for such motion in these molecules are high (ranging from 21.92 kcal/mol for (1,5-hexadiene)tricarbonyliron to 37.25 kcal/mol for the (semibullvalene)tricarbonyliron complex).
Our next example is a “molecular wheel” on a surface. Here we consider cyclopentadienyl attached to one M atom on an M(111) surface (M = Si, Ge, Sn), with all other possible M sites blocked by coadsorbed hydrogen atoms. The molecule remains attached to the same site, but there can be net rotational motion of the molecule as a whole [5], where the carbon atom bonded to the M atom keeps changing. This kind of motion is well known in organometallic chemistry and is called “ring whizzing.” The barriers for rotation of the cyclopentadienyl ring bonded to Si(111), Ge(111), and Sn(111) are 14.58, 7.86, and 6.38 kcal/mol, respectively.
We have studied some [Ring]Li compounds, and our theoretical results [6] predict cyclononatetraenyl–Li to be a “molecular rattle,” where the Li moves back and forth through the nine-membered anionic ring. The activation energy for such back-and-forth motion is low (11.50 kcal/mol). This motion is very similar to the “umbrella inversion” kind of motion already known in chemistry.
It is important to note that in all the above cases, although we find low activation barriers for rolling, rocking, wheel-like, or rattling motion, we do not have any control over it. Neither is it possible to make these motions unidirectional. This is because we are considering spontaneous motion at room temperature, and it is not possible to extract work from a system at constant temperature.
Finally, we have used the Hellmann–Feynman force approach [7,8] to study the effect of positive charge on the binding of the carbon monoxide molecule [9]. We have investigated CO, (Q–CO), and (CO–Q) (where Q is a unit positive charge) using the force approach to understand the nature of binding in non-classical carbonyls. Our studies reveal how the nature of each molecular orbital changes as the positive charge is brought nearer to the carbon monoxide molecule. Furthermore, the presence of a positive charge on the carbon side of this molecule influences the C–O bond length, making it stronger and shorter compared to the free molecule. | |
