Home work Physical and Theoretical Chemistry IV

- Molecular Spectroscopy -

Discussion  Fr 17.12.2004 at 10:30,  PK11-2



 

Home work 7            Molecular rotation
 

 

Exercise 1:  Moment of inertia     

a) Show that sulfur hexafluoride SF6  is a spherical top. Calculate the moment of inertia. The S-F bond length is 168 pm

b) Show that methane CH4  is a spherical top. Calculate the moment of inertia. The C-H bond length is 114 pm

Hint: knowing that methane has a tetrahedral structure put the carbon atom to the center of a cube with a side a and the four hydrogen atoms to the appropriate cube corners. Draw three principal axes through the cube center perpendicular to the profile planes.   

c) Show that CH35Cl3 is a symmetric top. Calculate the moment of inertia around the principal axis which contains the C-H bond. The C-Cl bond length is 177 pm and the HCCl angle is 107o.

 

Exercise 2:     Rotational constants and rotational energy terms

a)   Calculate the rotational constants for CO and CO2. Obtain the expression for rotational energy terms. The double C=O bond length is 124 pm 

b)  Which rotational group CH3I  molecule belongs to?  Calculate the rotational constants and obtain the expression for rotational energy terms. The C-I bond length is 210 pm, a C-H bond is 111 pm, and an HCH angle is 110.5o.

c)  Consider CD3Br and CH3Cl molecules. Without calculations show which of them has more closely spaced rotational energy levels. Why?