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Chemistry LibreTexts

Solutions 10

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  • Q1:


    The number of degrees of freedom a molecule has is 3N, where N is the number of atoms. Three are translational and for non-linear molecules, three are rotational, leaving 3N-6 vibrational modes. For linear molecules there are only two rotational degrees of freedom, leaving 3N-5 vibrational modes.

    Molecule Vibrational Modes
    \(NH_3\) 6
    \(C_6H_6\) 30
    \(C_{10}H_8\) 48
    \(CH_4\), 9
    \(C_2H_2\) 7


    The one mode of CO stretch is IR active. All but the symmetric stretch of \(CO_2\) are IR active. The one mode of HCl is also active.


    The vibrational frequency is given by \(\omega = \sqrt(\frac{k}{\mu})\), where \(\mu = \frac{m_1 m_2 }{m_1 + m_2}\). The masses are provided.


    The vibrational energy is given by \(E_\nu = \hbar \omega (\nu + \frac{1}{2})\).


    The corresponding wave number of \(D_2 O\) compared with \(H_2 O\) is smaller due to the increased reduced mass, resulting in a lower energy read.

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