March 28, 2024, 09:20:42 PM
Forum Rules: Read This Before Posting


Topic: anharmonicity  (Read 3542 times)

0 Members and 1 Guest are viewing this topic.

Offline tulip11

  • Regular Member
  • ***
  • Posts: 12
  • Mole Snacks: +0/-0
anharmonicity
« on: April 09, 2018, 01:23:09 PM »
I was thinking about how well the harmonic oscillator approximation represents the real potential of a molecule and was wondering it there was any dependence on mass of the molecule, i.e. if anharmonicity would be expected to increase down a group?

Although I can't find any mass dependence in the equations it feels like anharmonicity would be bigger in Po2 rather than in O2. I can't find any grounds as to why this may be true, but was hoping someone could tell me if it is true either way! 

Offline Corribus

  • Chemist
  • Sr. Member
  • *
  • Posts: 3471
  • Mole Snacks: +526/-23
  • Gender: Male
  • A lover of spectroscopy and chocolate.
Re: anharmonicity
« Reply #1 on: April 10, 2018, 10:18:47 AM »
Sorry you did not receive an answer yet. This is an interesting question that I'd have to think about.
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 mjc123

  • Chemist
  • Sr. Member
  • *
  • Posts: 2048
  • Mole Snacks: +296/-12
Re: anharmonicity
« Reply #2 on: April 10, 2018, 11:52:08 AM »
Couldn't find anything about Po2, but got these data from NIST:
Molecule      ωe (cm-1)     xeωe (cm-1)
O2               1580.2          11.98
S2                 725.7          2.844
Se2               385.3          0.964
Te2               247.1          0.515
Thus it seems for these molecules, relative to the fundamental frequency, the anharmonic correction is larger for the lighter molecules.

Offline tulip11

  • Regular Member
  • ***
  • Posts: 12
  • Mole Snacks: +0/-0
Re: anharmonicity
« Reply #3 on: April 11, 2018, 09:50:46 AM »
Thankyou! I find this really interesting although the trend of anharmonicity to molecular weight appears to be the opposite way around to what I originally thought. Do you have any ideas as to what may cause greater deviations from harmonic potential in lighter molecules?

Could it be that in the case below it is due to
 1. Increasing size down the group tends to make the bond weaker and weaker therefore a quicker deviation from the harmonic potential as it tends to dissociation quicker
2. Smaller atoms have more repulsion, due to presence of lone pair causing there by at shorter bond lengths the repulsive forces are stronger again deviating from harmonic potential quicker?

Sponsored Links