Macroscopic balls and springs model

Near the equilibrium bond length qe the potential energy/bond length curve for a macroscopic balls-and-spring model or a real molecule is described fairly well by a quadratic equation, that of the simple harmonic oscillator (E = ( /2)K (q — qe)2, where k is the force constant of the spring). However, the potential energy deviates from the quadratic (q ) curve as we move away from qc (Fig. 2.2). The deviations from molecular reality represented by this anharmonicity are not important to our discussion. 

First, the ball and springs model used in molecular mechanics is not completely nonphysical to a fair approximation, molecules really do vibrate and bonds do stretch and bend, as expected from a macroscopic ball and springs model. It is when we want to examine inescapably electronic properties, like, say, UV spectra or the donation of electrons from one species to another to make a bond, that the MM model is completely inadequate. 

X-ray diffraction and spectroscopy have provided the modem chemist with an amazing wealth of stmctural information on organic molecules. Molecules long ago ceased to be just lists of symbols and numbers on a sheet of paper, at best with a few dots and dashes here and there, and nowadays spring into the third dimension with their stereochemical characterization. Modern chemistry is stereochemistry. As a consequence, encouraged by the beautiful models built from balls and sticks or drawn in full color by computers, we now handle molecules as we do ordinary objects of the macroscopic world. We look at them, we weigh them, and in many other ways we size them up. 

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