This preview shows page 1. Sign up to view the full content.
Unformatted text preview: quare of the bond displacement is placed on the xaxis.
The slope of the line is related to the OH stretching force constant (refer to Section 4.3 of your text for an
equation). Calculate the stretching force constant and report its value in units of kcal mol–1 Å–2. Include a table
of your data and both graphs when you turn in your assignment. 3
4. Using the relationship between the harmonic frequency ν o and the force constant k,
1 ȹ k ȹ
νo =
ȹ ȹ
2π ȹ µ Ⱥ
€ 1/ 2 , determine the harmonic frequency in s–1 corresponding to the stretching force constant that you calculated in step
3. The quantity µ is referred to as the reduced mass and is defined in terms of the masses of the atoms in the
€
bond. For an AB bond, the reduced mass is µ= mA mB
.
mA + mB Note that the masses in this equation refer to the mass of a single atom of A or B, not the molar mass.
Finally, convert the result to units€ f cm–1 and discuss whether or not this value is what you would expect for
o
the typical IR stretching frequency of an OH group. Determination o f B ending F orce C onstant
1. Starting from scratch, build and determine the equilibrium geometry of a new copy of the water molecule using
the MMFF94 force field. Measure and record the total energy in kcal/mol and the equilibrium bond angle in
degrees.
2. The next step is to vary the HOH bond angle from 90 to 120 degrees in 7 steps (5 degree increments). To do
this, follow the instructions in Appendix 4 to use the Bond Manipulation tool,
, to adjust the bond angle
to each of the suggested values (the OH bond distances should not be changed). Compute single point energies
for each bond angle and record these values in kJ/mol (see Appendix 3 for instructions on computing single
point energies). 3. Follow a similar procedure to that described above for the stretching force constant to make graphs of your
results and calculate the HOH bending force constant. Report the value that you obtained for the bending force
constant in kcal m...
View
Full
Document
This document was uploaded on 03/04/2014 for the course CHEM 380.37 at Illinois State.
 Fall '13
 JeanStandard
 Mole

Click to edit the document details