The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.

Artificial Molecular Machines.

Artificial Molecular Rotors

An inductive method for a systematic selection of energy functions of interatomic interactions in large families of molecules is suggested and is applied to the family of cycloalkane and n‐alkane molecules. Equilibrium conformations, vibrational frequencies, and excess enthalpies, including strain energies and vibrational enthalpies, are all derived from the same set of energy functions. The energy‐function parameters are optimized by a least‐squares algorithm to give the best possible agreement with a large amount and variety of observed data. Analytical derivatives of the various calculated quantities with respect to the energy parameters help to facilitate the computational procedures. The resulting agreement with experiment is used as a measure of success of the energy functions with optimized parameters, referred to as “consistent force field” (CFF). Different CFF's are compared and selected according to their relative success. Energy functions commonly used in conformational analysis are examined in...

Consistent Force Field for Calculations of Conformations, Vibrational Spectra, and Enthalpies of Cycloalkane and n‐Alkane Molecules

An empirical potential based on permanent atomic multipoles and atomic induced dipoles is reported for alkanes, alcohols, amines, sulfides, aldehydes, carboxylic acids, amides, aromatics, and other small organic molecules. Permanent atomic multipole moments through quadrupole moments have been derived from gas phase ab initio molecular orbital calculations. The van der Waals parameters are obtained by fitting to gas phase homodimer QM energies and structures, as well as experimental densities and heats of vaporization of neat liquids. As a validation, the hydrogen bonding energies and structures of gas phase heterodimers with water are evaluated using the resulting potential. For 32 homo- and heterodimers, the association energy agrees with ab initio results to within 0.4 kcal/mol. The RMS deviation of the hydrogen bond distance from QM optimized geometry is less than 0.06 A. In addition, liquid self-diffusion and static dielectric constants computed from a molecular dynamics simulation are consistent wit...

Polarizable Atomic Multipole-based Molecular Mechanics for Organic Molecules

Die zum Bau kleiner Maschinen notwendige Miniaturisierung von Komponenten erfolgt derzeit nach dem Verkleinerungsprinzip (top-down approach). Diesem Ansatz, der Festkorperphysiker und Elektronikingenieure zwingt, mit immer kleineren Materialbausteinen zu arbeiten, sind allerdings Grenzen gesetzt. Eine Alternative besteht im Vergroserungsprinzip (bottom-up approach), bei dem man von den kleinsten Teilen der Materie mit eindeutiger Form und definierten Eigenschaften, den Molekulen, ausgeht. Vor dem Hintergrund dieser Herausforderung haben Chemiker das Konzept der makroskopischen Maschine auf die molekulare Ebene ubertragen. Eine molekulare Maschine kann als eine Anordnung einer definierten Anzahl von molekularen Komponenten definiert werden, die so konzipiert wurden, dass sie als Reaktion auf geeignete externe Stimulation (input) maschinenahnliche Bewegungen ausfuhren (output). Genau wie ihr makroskopisches Gegenstuck ist eine molekulare Maschine durch folgende Merkmale charakterisiert: 1) die Art der Energie, die ihr zugefuhrt werden muss, damit sie funktioniert, 2) die Art der Bewegungen ihrer Komponenten, 3) die Methoden, durch die ihre Funktionen verfolgt und gesteuert werden konnen, 4) die Moglichkeit der cyclischen Wiederholung, 5) die Zeit, die fur die Durchfuhrung eines vollstandigen Arbeitscyclus benotigt wird, und 6) der Zweck ihrer Funktion. Zweifellos sind Photonen oder Elektronen die besten Energielieferanten fur molekulare Maschinen. So ist es moglich, mit sorgfaltig ausgewahlten photochemischen oder elektrochemischen Reaktionen, funktionierende molekulare Maschinen zu entwerfen und zu synthetisieren. Daruber hinaus hat unser rasch angewachsenes, fundamentales Verstandnis uber die Selbstorganisation und die ihr zugrunde liegenden Prozesse in der chemischen Synthese zum Aufbau kunstlicher molekularer Maschinen beigetragen. Dies geschah vor allem durch die Entwicklung neuer Methoden in der nichtkovalenten Synthese und das Aufkommen der supramolekular unterstutzten kovalenten Synthese als ausgesprochen leistungsfahiges Syntheseprinzip. Ziel dieses Ubersichtsartikels ist eine einheitliche Darstellung des Gebiets der molekularen Maschinen, wobei besonderes Augenmerk auf das in der Vergangenheit Erreichte, auf gegenwartig bestehende Grenzen und auf Zukunftsperspektiven gelegt werden soll. Nach der Beschreibung einiger naturlicher molekularer Maschinen werden die wichtigsten Entwicklungen auf dem Gebiet der kunstlichen molekularen Maschinen vorgestellt. Dabei wird auf folgende Systeme naher eingegangen: 1) chemische Rotoren, 2) photochemisch und elektrochemisch induzierte molekulare (konformative) Umlagerungen und 3) chemisch, photochemisch und elektrochemisch steuerbare (cokonformative) Bewegungen in ineinander greifenden (interlocked) Molekulen (Catenanen und Rotaxanen) sowie in Koordinationsverbindungen und supramolekularen Komplexen (darunter Pseudorotaxanen). Kunstliche, auf Biomolekulen basierende molekulare Maschinen und kunstliche molekulare Maschinen, die auf Oberflachen oder festen Tragern aufgebracht wurden, sind zwei der spannenden Entwicklungen, die besprochen werden. Die Erweiterung des Konzepts einer Maschine auf die molekulare Ebene ist nicht nur fur die Grundlagenforschung von Interesse, sondern auch fur die Weiterentwicklung der Nanowissenschaften und der daraus erwachsenden Nanotechnologie.

Künstliche molekulare Maschinen

The heats of combustion, melting points, heats of fusion, boiling points, refractive indices and in four cases the densities have been determined on selected samples of the cyclanes with 4 to 9 C-atoms; some hitherto unknown transition points were discovered and the heats of transition measured. The heats of vaporisation at 25° are estimated. Results of the measurements are discussed.

Thermal quantities of some cycloparaffins. Part III. results of measurements

The series of the normal saturated fatty acids and their methyl esters with 5 to 20 carbon chain atoms was synthesized and careful purified, mainly by fractional distillation of the methyl esters in columns with up to 58 theoretical plates.



The purity was checked with the aid of melting curves determined in a microcalorimeter and proved to be 96.6% or better.



Refractive indices were determined and can be represented by










with an accuracy of 5 units in the fifth decimal place (c = number of carbon chain atoms).



Densities, accurate to 1 unit in the fourth decimal place, can be represented by










Boiling points can be given by



b.pt. 20 mm 






for the methyl esters (c> 5), accurate to 0.1 °.



From our data, combined with literature values, the molecular refractivity of the CH2 group in an aliphatic chain calculated according to Lorentz-Lorenz. was determined at 4.64 at room temperature.



In addition, melting and transition points, and heats of fusion and transition were determined.

Some physical constants of normal saturated fatty acids and their methyl esters

Diarylmethane derivatives VIII: Methyl Substitution Products of Tetraphenylethanes

Accurate heats of combustion of the title compounds were measured in an electrically calibrated calorimeter; the liquid samples were burned in Pyrex ampoules; the water content in the samples was titrated by the Karl Fischer method.

The increment of the standard enthalpy of combustion, -ΔHR25liq. (=-ΔHC0liq.) for one CH2 group in the chain proved to be 653.8 kj/mole for both the acids and the methyl esters studied, which is the same value as determined for the liquid paraffins. The following formulae, in which c = the number of carbon chain atoms (c > 5) can be used to calculate the standard enthalpies of combustion (based on the 1951 atomic weights):


-ΔHliq. = 653.8 c − 430.2 kj/mole for the acids, accurate to 0.03 − 0.04%,


-ΔHliq. = 653.8 c + 292.6 kj/mole for the methyl esters, accurate to 0.02%.



The theoretical heat of esterification of a fatty acid with methanol at 25 °C, disregarding heats of solution and dilution, was calculated to be 3.8 ° 0.6 kj/mole, which can be compared with the value of 4.2 ° 0.3 kj/mole for the theoretical heat of esterification of acetic acid with ethanol at about 20 °C, determined directly by Mme Berenger-Calvet.

Heats of combustion of normal saturated fatty acids and their methyl esters

The integral heats of solution of some samples of potassium chloride have been measured carefully with an adiabatic calorimeter. The results indicate that it is necessary to revise the value of the heat of solution of potassium chloride in water.

J. Coops

Papers

Thermal quantities of some cycloparaffins. Part III. results of measurements

Some physical constants of normal saturated fatty acids and their methyl esters

Diarylmethane derivatives VIII: Methyl Substitution Products of Tetraphenylethanes

Heats of combustion of normal saturated fatty acids and their methyl esters

The use of potassium chloride as a standard substance in solution calorimetry