HCl Vibrational-Rotational Spectroscopy

(3/26/20, bes)

Adapted from Experiment #38 (Shoemaker, Garland, Nibler, 1989)

See Chapter 8 (Engel), more will be presented in lecture...

Sample Preparation

a) Pull the HCl (g) off the headspace of a bottle of concentrated HCl using a 60 mL syringe.

b) deliver the HCl (g) to the gas-sampling IR cell.

IR Gas-Phase Data Collection

a) Collect data using the highest resolution.

b) average 32 scans (both background and sample)

c) Save-As...

(A complete set of data can be found here.)

Spectral Analysis

a) load IR data into Igor or Excel.

b) Use the cursor tool to tabulate the H³⁵Cl and H³⁷Cl data into separate columns,

c) Assign an "m-value" to each transition (note the 'm=0' forbidden transition) with the help of the following graphic (click to make bigger).

WRONG -->

RIGHT -->

Data Analysis

Okay, so you should now have a table, probably in Excel but if you know how to do it in Igor then great (Igor does not offer any advantages here) with heading like this:

m-value	H35Cl	H37Cl
-12	XXXX	YYYY
-11	XXXX	YYYY
-10	XXXX	YYYY
-9	XXXX	YYYY
...	...	...
+1	XXXX	YYYY
+12	XXXX	YYYY
+13	XXXX	YYYY

a) Make a plot, m-value verses freq (in 1/cm) for H³⁵Cl data; this will NOT be a straight line but a polynomial curve.

b) fit a second order polynomial to this data... ie y= ax² + bx +c, and note the values for a, b, and c in your notes (lab notebook preferred).

c) During our last lab period you were given a Shoemaker, et al. handout (???) that discussed this lab activity. Eq. 9 was:

ṽ = ṽ_o + (2B_e - 2 α_e) m - α_e m², which has the form y = c + bx + ax²

a = α_e

b = 2B_e - 2 α_e

so you can solve for B_e (this is called the rotational constant and has units of 1/cm)...

and Be =

d) Now solve for the radius (r), the average internuclear separation of the H³⁵Cl...

e) repeat above for H³⁷Cl.

Reduced mass calculations

https://en.wikipedia.org/wiki/Isotopes_of_chlorine

atomic masses in amu

Reduced mass, μ = (m1*m2)/(m1+m2)

¹H 1.007825 amu (99.9885%)

³⁵Cl 34.968853 amu (75.78%)

³⁷Cl 36.965903 amu (24.22%)

conversion factor: 1.66054e-27 kg/amu

WebMO/Gaussian Calculation

WebMo/Gaussian calculation can be done to determine the B_e, the rotational constant. It is usually required to specify the specific isotope for the atoms. This can be done by editing the input file (the z-matrix) within WebMO.

- choose "preview", then "generate" when submitting your job...

- add the text (Iso=35) or (Iso=37) after the Cl in the input file; for completeness add (Iso=1) after the H as well:

REPORTING

Please send email with the following information:

H³⁵Cl data

ṽ₀ =

Be =

μ (H³⁵Cl) =

r =

H³⁷Cl data

ṽ₀ =

Be =

μ (H³⁷Cl) =

r =