April 13, 2021, 02:45:07 AM
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Topic: Why isn't bond angle change linear for halogenated methane compounds?  (Read 221 times)

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Offline jonut

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Hi, I was wondering why the change in bond angle isn't linear for halogenated methane compounds (specifically CH3X where X is the halogen). In the chart, I measured the bond angle between the Halogen-Carbon-Hydrogen and it's not a consistent change. Does this have something to do with Bent's rule (which I do not fully understand)? And what could be the cause for the sudden jump in bond angle for Iodine?

I used MolView to measure the bond angles (and yes I used the energy minimization function)

Offline Orcio_Dojek

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Re: Why isn't bond angle change linear for halogenated methane compounds?
« Reply #1 on: March 30, 2021, 04:44:41 AM »
https://en.wikipedia.org/wiki/Bent%27s_rule#Bond_angles
Quote
As one moves down the table, the substituents become more electronegative and the bond angle between them decreases. According to Bent's rule, as the substituent electronegativies increase, orbitals of greater p character will be directed towards those groups. By the above discussion, this will decrease the bond angle.

This explains why bond angle decreases (with CH3Br as an exception).

Why CH3Br is an exception I won't tell you for sure.

Offline sjb

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Re: Why isn't bond angle change linear for halogenated methane compounds?
« Reply #2 on: March 30, 2021, 04:47:03 AM »
What parameter are you using such that the gap between F and Cl; Cl and Br; and Br and I is the same?

Offline Corribus

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Re: Why isn't bond angle change linear for halogenated methane compounds?
« Reply #3 on: March 30, 2021, 06:10:45 PM »
FWIW, the experimental values are considerably different from what you calculated (don't use the word "measure" with a calculation).

https://cccbdb.nist.gov/expgeom1x.asp

According to expt value, CH3-Br and CH3-I have very similar H-C-X bond angles (107.72 and 107.47 deg, respectively).

Still not linear, but I'm not sure why we should expect them to be. Electronegativity isn't the only factor of importance here, there are also sterics and bond length to consider. Degree of covalency of the C-X bond also changes as you go down the series.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline jonut

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Re: Why isn't bond angle change linear for halogenated methane compounds?
« Reply #4 on: April 02, 2021, 03:56:46 PM »
Thanks for the replies, I figured that this was an anomaly and most likely an issue with the MolView software I was using. And the difference in the experimental value further proves that...thanks for sharing that link, I should probably have been using that in the first place!

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