Question

3. Pentafluorobenzene has an IR spectrum with a C-H stretching absorbance at 3050, cm!. Use four significant figures for your answers below. 6 points a. Deuterium (chemical symbol PH or D) is an isotope of hydrogen that contains one neutron in its nucleus in addition to one proton. In nature, deuterium makes up 0.015% of all hydrogen atoms. Predict the stretching frequency in wavenumbers (cm ) of the C-D bond in a sample of pentafluorobenzene-D where 100% of the hydrogens are deuterium atoms. work b. Calculations predict the following bond strengths for the pentafluorobenzene: 410.0 kJ/mol for the C-H bond and 525.0 kJ/mol for the C-F bond. Using the bond strength data above, predict the stretching frequency in wavenumbers (cm) of a C-F bond in a sample of pentafluorobenzene. work

Answer 1

Part A.

$\overline{\nu}$_C-D = 4.12 (K/$\mu$)^1/2

Where 'K' is the force constant of C-D bond = 5*10⁵ dyne/cm

And $\mu$ is the reduced mass = m1m2/(m1 + m2)

Now, $\overline{\nu}$_C-D = 4.12 * {5*10⁵ * (12+2)/(12*2)}^/2 = 2225.054 cm^-1