Year 12s: what will be the first way you research your future options? >>

Anharmonicity constants

Ill stay to start with this is well above School level, it's second year degree stuff :P

But given a vibrational peak on a spectrum, how do I work out an anharmonicity constant, as Ive been battling with a question through the Easter break :P. I dont want an answer (hence why Im not giving any numbers) but just a way to work it out (before I have a nervous breakdown :frown:

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There is positive rep availiable if any comes up with anything :P

Reply 1

Kaeroll

I'm pretty sure you'll need more than one peak. The harmonic model predicts that vibrational levels are evenly spaced; the anharmonic term corrects for the experimental fact that they become closer in energy with increasing quantum number. To find the anharmonicity term I think you need multiple peaks.

Reply 2

DMed

Okay, Im useless and forget to add I had a fundamental band and a first overtone :P, sorry Kaeroll

Reply 3

Kaeroll

DMed

Okay, Im useless and forget to add I had a fundamental band and a first overtone :P, sorry Kaeroll

No problem. I figured as much. :-) It'd be really handy if this board had LaTeX right about now... (edit- apparently it does. Cheers EVS)

According to my course notes from my second year, the energy of a given level is:

G(V) = (V + \frac{1}{2})\;\nu - (V + \frac{1}{2})^{2}\; \nu \; \chi_{e}

Where v is the quantum number, v is the classical frequency, and Xe is the anharmonicity constant. Do you know v?

If you do it's probably simplest to calculate the energy of each level and work out the difference, which is then equal to:

\Delta\;G(V) = \nu (1 - 2\;\chi_{e})

Reply 4

EierVonSatan

TSR does have latex :smile:

Use the [noparse]

stuff

[/noparse] tags =

stuff

G_v = (v + \frac{1}{2})v - (v + \frac{1}{2})^2vX_e

Reply 5

Kaeroll

Ahh. Didn't see it in the options so figured it wasn't there. I'll go back and edit my post now. Cheers.

Reply 6

DMed

So classical frequency is the IR spectrum wavenumber times the speed of light?

Reply 7

DMed

Also (sorry for posting a double post) but dont you need to know the anharmonicity constant to determing the difference, Im confused :'(.

Reply 8

Percy145

Do yourself a favour. Buy this book and read it:

Fundamentals of Molecular Spectroscopy by Colin Banwell.

Reply 9

DMed

Cheers, Ill have a look in the library for that when Im back in Liverpool :smile:

Reply 10

Percy145

I thought it was required reading for a 1st year undergraduate...What have you been using to teach yourself spectroscopy?

Reply 11

DMed

Percy145

I thought it was required reading for a 1st year undergraduate...What have you been using to teach yourself spectroscopy?

We dont teach ourselves, we have a lecture course in QM and spectroscopy in first year, which I passed at a good standard. Our recommended reading is:

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Housecroft and Constable Chemistry (it's not in there), which was the recommended reading for my spec course

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Atkins Physical Chemistry 8th edn (it's not in there)

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CGW&W Organic Chemistry (deffo not in there)

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Housecroft and Sharpe Inorganic Chemistry (again, not in there)

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Iggo Inorganic Spectroscopy (again more about the methods rather than the theory)

Reply 12

cpchem

Atkins, 8th edn, chapter 13, section 11 (p. 455)

That said, I recommend 'Modern Spectroscopy' by J. M. Hollas above all else.

Reply 13

DMed

Just to update, I still havent got any further (been told to write a lab report out again has kinda put a stop to it), but apparently I need to determing the AC simultaneously :s-smilie: