In: Chemistry
Calculate the absorption frequency corresponding to the –C-H stretching vibration treating the group as a simple diatomic C-H molecule with a force constant of k= 5.0 102 N/m. (Hint: use the actual mass of the two atoms to determine the reduced mass. Note that N is the unit of a force.) Repeat the calculation for the deuterated bond.
1) as we know that
= reduced mass = m1 ( mass of hydrogen ) =1 amu = 1.66 * 10-27 kg
mass of carbon = 12 amu = 12 * 1.66 * 10-27 Kg = 19.92 * 10-27 Kg
m1m2/m1+m2 = 1.66 * 10-27 * 19.92 * 10-27 / 1.66 * 10-27 + 19.92 * 10-27
= 33.067 * 10 -54 / 21.58 * 10-27 = 1.53 * 10-27
vibrational spectra of diatomic molecule --> = 1/2c (k / )1/2
where = adsorption frequency , c = speed of light in (cm) , k = force constant
= 1/2 * 3.142* 3 1010 ( 5.0* 102 / 15.3 10-28)1/2
= 0.053 * 10-10 ( 0.33 1030 ) 1/2 = 0.053 * 0.57 * 10-10 * 1015 = 3021 cm-1
2) for deutrium = 2 amu = 2 * 1.66 * 10-27 Kg = 3.32 * 10-27 kg
= reduced mass = m1m2/m1+m2 = 3.32 * 10-27 * 19.92 * 10-27 / 3.32 * 10-27 + 19.92 * 10-27
= 66.13 * 10 -54 / 23.24 * 10-27 = 2.845 * 10-27
vibrational spectra of diatomic molecule --> = 1/2c (k / )1/2
= 1/2 * 3.142* 3 1010 ( 5.0* 102 / 28.45 10-28)1/2
= 0.053 * 10-10 ( 0.176 * 1030 ) 1/2 = 0.053 *0.419 * 10-10 * 1015 = 2220.7 cm-1