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Topic: Molecular Diameter ?  (Read 28540 times)

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atta001

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Molecular Diameter ?
« on: August 27, 2005, 04:50:12 PM »
Hi All,

I want to know if there is any way to calulate the molecular radius of compounds? I will like to apply the known method to calculate the diameter of phosphoryl chloride (POCl3) molecule.

Regards,

Atta.

Offline gregpawin

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Re:Molecular Diameter ?
« Reply #1 on: August 27, 2005, 06:43:52 PM »
You won't be able to get anything like radaii or diameters of any molecules, because wavefunctions are normalized over infinity, but you can get bond distances that are optimized using software packages.
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Offline Borek

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Re:Molecular Diameter ?
« Reply #2 on: August 27, 2005, 07:01:25 PM »
You won't be able to get anything like radaii or diameters of any molecules, because wavefunctions are normalized over infinity, but you can get bond distances that are optimized using software packages.

That's not exactly true. Single atom doesn't have any bond length, yet its diameter is given in tables. Ion diameters are given too. It is all about how you define where the atom or molecule ends. Besides, bond lengths are not enough.
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Offline gregpawin

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Re:Molecular Diameter ?
« Reply #3 on: August 28, 2005, 02:14:25 AM »
Every molecule is going to have different shapes and bond lengths due to the balancing out of charge between the electrons and positively charged nuclei.  So for every molecule, you're going to get different bond lengths/shapes even if they vary only slightly.  Bond lengths are the only semi-exact numbers you can compute... compared to atom/ion "diameters" these are much more meaningful.  Shapes/diameters of bonded or unbonded atoms just have a probability density.  Table numbers probably just mention the approximate size of the valence electron shell where it should be 90% of the time.  There is no true diameter.

To have a true diameter would be to say that the electron does not stray past a certain point, but that's not true because electrons can tunnel through barriers.  There's a certain probability to finding electrons to every point in space but gets exponentially small with distance from the molecule.
« Last Edit: August 28, 2005, 02:17:20 AM by gregpawin »
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Offline Borek

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Re:Molecular Diameter ?
« Reply #4 on: August 28, 2005, 05:16:45 AM »
That's all true - but at the same time theoretical approach disables your ability to see the real world ;)

When you have a bunch of particles (in liquid or solid), each one occupies some space, they don't get to close to each other. So you may assign some kind of diameter to them.

If you have a hole in the crystall lattice it can be occupied by some particle (or atom) that is not part of the lattice - it just have to be small enough to fit (clathrates for example). This size is some kind of diameter too.

You may say it is not possible to calculate diameter for any particle because particle 'never ends' (and theoretically you are absolutely correct), yet you have somehow name the size of the particle observed when it interacts with others.

What about diameter-which-greg-refuses-to-call-diameter? :)

This is similar approximation to the one used when talking about orbital shapes. Orbitals 'don't end' too, yet their shapes are known to every chemist.
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Offline gregpawin

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Re:Molecular Diameter ?
« Reply #5 on: August 28, 2005, 02:13:03 PM »
For me to say that molecules do not collide would be for me to ignore electrostatic forces, and that's not what I'm advocating.

I'm just saying molecular diameters are ill defined.  A ion sitting in a lattice has a different effective diameter than an ion roaming around a dielectric.  What we can talk about is the interaction between two specific things.

I'm not saying there's no usefulness of a molecular diameter.  But this guy is trying to calculate out an diameter.  There's no button that calculates a diameter.  You'd have to arbitrarily define one: 90%?  80%?  95%?

Of course there's shapes.... they're spherical harmonics... or combinations thereof anyways.
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GCT

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Re:Molecular Diameter ?
« Reply #6 on: September 04, 2005, 01:52:44 PM »
There are models which relate very well to molecular diameter, collision frequency, mean free path, as well as topics which can be found in the back portions of Atkins. Not the exact diameter of course, but that's not what the original poster was asking obviously.

Tajul

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Re:Molecular Diameter ?
« Reply #7 on: September 06, 2005, 11:27:34 PM »
what is the molecular diameter of blood and water

Offline Borek

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Re:Molecular Diameter ?
« Reply #8 on: September 07, 2005, 03:25:48 AM »
what is the molecular diameter of blood and water

What do you mean by molecule of blood?
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GCT

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Re:Molecular Diameter ?
« Reply #9 on: September 07, 2005, 11:34:55 AM »
what is the molecular diameter of blood and water

probably referring to the red blood cell

Offline swampie777

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Re:Molecular Diameter ?
« Reply #10 on: February 08, 2012, 02:23:20 PM »
You won't be able to get anything like radaii or diameters of any molecules, because wavefunctions are normalized over infinity, but you can get bond distances that are optimized using software packages.

Sometimes one can take a model too far. If there were NO assignable molecular diameter then molecular sieves could not work or be selective. They DO selectively pick one molecule over another based on a functional size. The physical chemistry folks have done some amazing modelling( involving solutions and reactions at the macro level) . The physics folks have different reasons for what they're doing ( atom fusion with barely enough atoms involved to allow detection). No current model is the end all for everything.

There is nothing to say that the electronic cloud structure does not change during reactions or in physical occurrences like molecular sieve absorption.

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