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M


   
Machine-phase chemistry The chemistry of systems in which all potentially reactive moieties follow controlled trajectories (e.g., guided by molecular machines working in vacuum).
   
Mechanical Pertaining to the positions and motions of atoms, as defined by the positions of their nuclei; see electronic. A purely mechanical device can be described in terms of atomic positions and motions without reference to electronic properties, save through their effect on the potential energy function.
   
Mechanochemistry In this volume, the chemistry of processes in which mechanical systems operating with atomic-scale precision either guide, drive, or are driven by chemical transformations. In general usage, the chemistry of processes in which energy is converted from mechanical to chemical form, or vice versa.
   
Mechanosynthesis Chemical synthesis controlled by mechanical systems operating with atomic-scale precision, enabling direct positional selection of reaction sites; synthetic applications of mechanochemistry. Suitable mechanical systems include AFM mechanisms, molecular manipulators, and molecular mill systems. Processes that fall outside the intended scope of this definition include reactions guided by the incorporation of reactive moieties into a shared covalent framework (i.e., conventional intramolecular reactions), or by the binding of reagents to enzymes or enzymelike catalysts.
   
Metastable A classical system is metastable if it is above its minimum-energy state, but requires an energy input before it can reach a lower-energy state; accordingly, a metastable system can act like a stable system, provided that energy inputs (e.g., thermal fluctuations) remain below some threshold. Systems with strong metastability are commonly described as stable. Quantum mechanical effects can permit metastable states to reach lower energies by tunneling, without an energy input; an associated, broader definition of metastable embraces all systems that have a long lifetime (by some standard) in a state above the minimum-energy state.
   
Misreaction A chemical reaction that fails by yielding an unwanted product.
   
MM2 A molecular mechanics program developed by Norman Allinger and coworkers; the ``MM2 model'' is the molecular potential energy function described by the equations, rules, and parameters embodied in that program.
   
MM2/CSC A molecular mechanics program developed by Cambridge Scientific Computing that closely follows the MM2 model, adding a graphical user interface and other features.
   
Modulus Any of several measures of strain versus applied stress. See shear modulus, Young's modulus.
   
Moiety A portion of a molecular structure having some property of interest.
   
Mole A number of instances of something (typically a molecular species) equaling ~6.022 1023. Mole ordinarily means gram-mole; a kilogram-mole is ~6.022 1026.
   
Molecular machine A mechanical device that performs a useful function using components of nanometer scale and defined molecular structure; includes both artificial nanomachines and naturally occurring devices found in biological systems.
   
Molecular manipulator A programmable device able to position molecular tools with high precision, for example, to direct a sequence of mechanosynthetic steps; a molecular assembler.
   
Molecular manufacturing The production of complex structures via nonbiological mechanosynthesis (and subsequent assembly operations).
   
Molecular mechanics models Many of the properties of molecular systems are determined by the molecular potential energy function. Molecular mechanics models approximate this function as a sum of 2-atom, 3-atom, and 4-atom terms, each determined by the geometries and bonds of the component atoms. The 2-atom and 3-atom terms describing bonded interactions roughly correspond to linear springs.
   
Molecular mill A mechanochemical processing system characterized by limited motions and repetitive operations without programmable flexibility (see molecular manipulator).
   
Molecular nanotechnology See nanotechnology.
   
Molecule A set of atoms linked by covalent bonds. A macroscopic piece of diamond is technically a single molecule. (Sets of atoms linked by bonds of other kinds are sometimes also termed molecules.)

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