= Solution
Use the two-body <reduced mass>, rather than the chlorine mass or hydrogen mass alone:
$$
\mu=\frac{1.0078\times34.9688}{1.0078+34.9688}m_u
=1.62661\times10^{-27}\ \mathrm{kg}.
$$
The <bond force constant> gives
$$
\nu_{\rm harm}=\frac1{2\pi}\sqrt{\frac{481}{\mu}}=8.65467\times10^{13}\ \mathrm{s^{-1}},\qquad
\boxed{\widetilde\nu_{\rm harm}=\frac{\nu_{\rm harm}}{2.99792458\times10^{10}\ \mathrm{cm\,s^{-1}}}=2886.89\ \mathrm{cm^{-1}}.}
$$
The requested “frequency in cm−1” is a <spectroscopic wavenumber>; the ordinary frequency is the value in s−1 above. This calculation uses the question's force constant in the harmonic model, rather than replacing it with a tabulated spectroscopic constant.
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