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IR Active Vibrational Modes

Vibrational modes within a molecule that result in a change in dipole moment, allowing them to absorb IR radiation.

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The statement of the theorem

Consider a molecular vibration described by a normal coordinate

Q

. A vibrational mode is IR active if the derivative of the molecular dipole moment

\boldsymbol{\mu}

with respect to

Q

is non-zero:\n

\frac{\partial \boldsymbol{\mu}}{\partial Q} \bigg|_{Q=0} \neq \mathbf{0}