Measuring Dipole Moments
Name: Andrew T.
How do you measure the dipole moment of a new
(solid)compound? Is there an instrument that measures it directly (a
diople meter?) If so, were are these commercially available? Or do you
have to determine several properties (such as density, with a
pycnometer) and dielectric constant, and then use an equation - if so, are there
simplifying assumptions that you make? Are there other properties that
need to be measured?
A dipole moment (I am assuming you mean a "permanent electric" dipole
moment (vs. an induced dipole moment and electric vs. magnetic) is a
property of a gas or a molecule in solution. In the case of a gas it can be
measured by several methods. The most accurate is the Stark effect the shift
of rotational energy levels caused by an electric field. The deflection of a
molecular beam of a molecules by an electric and/or inhomogeneous magnetic
field can also be used. In solution the dipole moment is related to the
dielectric constant of the molecule in solution.
Solids do not usually have a permanent asymmetry in electrical charge,
although a few solids (usually high molecular weight "cigar-shaped" ones)
can. The measurement of permanent electric dipole moments can get rather
messy mathematically. I suggest a Google search for resources. You can find
most any level of complexity (messy-ness) you desire.
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Update: June 2012