# Chemical Forums

## Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Beefsteak on December 07, 2020, 06:34:09 PM

Title: Vibrational-Rotational Spectra of HCl and DCl
Post by: Beefsteak on December 07, 2020, 06:34:09 PM
Hello I am trying to finish an incomplete lab by Dec 14 for Pchem 2. I am having a large difficulty understanding what my professor want in part 3 of the calculations. I tried to catalog my work for this assignment in a pdf and see if anyone can provide any helpful guidence.

For my work I indexed the line with the corresponding frequencies of ṽ(m) which was provided from the spectra as C35 and C37. Δṽ(m) was then calculated by taking the difference between those peak side by side on both the R and P branches, respectively which gives me my Δṽ(m) values. Then I plotted Δṽ(m) vs m on the graph in excel and preformed a linear fit to then determine my values of v0, Be,and αe. I feel like I may have mess up somewhere as the theoretical values do not match up.
Title: Re: Vibrational-Rotational Spectra of HCl and DCl
Post by: mjc123 on December 08, 2020, 07:12:12 AM
Δν(m) is not the difference between the 35Cl and 37Cl lines. Read the instructions carefully.
Also, you have got your P and R branches the wrong way round, perhaps due to Fig. 3 being plotted the opposite way to your graph. R branch is higher frequency.
Title: Re: Vibrational-Rotational Spectra of HCl and DCl
Post by: Beefsteak on December 09, 2020, 04:47:14 PM
Thank you very much for the response! I manage to go over worksheet again and with some help from a video online I was able to correctly interpret the data and made some progress in determining αe, ν0 and Be. Although I am kinda lost on determining v(e) and veχe for H35Cl. From the instructions I have to manipulate from eq 8 and insert into eq 16. Although I am kinda lost in the math trying to understand what are my known values.
Title: Re: Vibrational-Rotational Spectra of HCl and DCl
Post by: mjc123 on December 10, 2020, 05:55:24 AM
You seem to be making good progress. (Why did you miss out the first R line? I don't suppose it will make a huge difference to your regression analysis, but you ought to include it. And P8 is 2703, not 2705.)
Combining equations 8 and 16, and writing Y for (μ/μ*)1/2, which is known, we have
ν0 = νe - 2νexe
ν0* = νeY - 2νexeY2
so you have 2 simultaneous linear equations  for 2 unknowns.