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    How do we calculate the exact bond angles and bond lengths in a molecule? i.e. are there any online guides, or introductions to whatever field this is?

    I'm happy to look at computational methods.
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    (Original post by Big-Daddy)
    How do we calculate the exact bond angles and bond lengths in a molecule? i.e. are there any online guides, or introductions to whatever field this is?

    I'm happy to look at computational methods.
    They are not calculated as such. The data is deduced from spectroscopic analysis ...
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    These aren't calculated, these are memorised.
    Although, most are common sense and thereby you only need to remember the more difficult ones (7 sides or more).

    EDIT:
    I just noticed it said "Undergraduate" - COMPLETELY beyond my field of knowledge.
    Sorry.
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    (Original post by Scienceisgood)
    These aren't calculated, these are memorised.
    Although, most are common sense and thereby you only need to remember the more difficult ones (7 sides or more).

    EDIT:
    I just noticed it said "Undergraduate" - COMPLETELY beyond my field of knowledge.
    Sorry.
    From what I've seen you'd be lucky to find what I'm looking for at undergraduate level, at least in the early years ... (but yeah, I did put "exact" in there to throw off A-level people who just want me to memorize VSEPR )

    Although what do you mean by "7 sides"?
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    (Original post by charco)
    They are not calculated as such. The data is deduced from spectroscopic analysis ...

    It makes sense that's how it's done in practice. But let's say we want to predict them for a hypothetical compound.

    I'm beginning to see this may not be undergraduate stuff but I'm still interested to see how it's done. http://en.wikipedia.org/wiki/Density_functional_theory This should do the trick - do you know anywhere I can find a more basic introduction? I can't face the Schrodinger equation being the first thing on the page ...
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    (Original post by Big-Daddy)
    From what I've seen you'd be lucky to find what I'm looking for at undergraduate level, at least in the early years ... (but yeah, I did put "exact" in there to throw off A-level people who just want me to memorize VSEPR )

    Although what do you mean by "7 sides"?
    Seriously, why do you think this chem olympiad is so incredible (it is you that is doing it right?) I looked over some of the past papers and they were way easier than my 1st year exams. Don't forget, a degree is a full time thing, not something you do on the side of your A-levels.

    (Original post by Big-Daddy)
    How do we calculate the exact bond angles and bond lengths in a molecule? i.e. are there any online guides, or introductions to whatever field this is?

    I'm happy to look at computational methods.
    Well the main way is to measure them by X-ray diffraction or neutron diffraction. You can also, in theory, deduce some info about the bonds orientations from rotational spectroscopy.
    Calculations are done by computational programmes which use forms of quantum mechanics. They solve them numerically but make many assumptions to speed up the simulation.
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    (Original post by JMaydom)
    Seriously, why do you think this chem olympiad is so incredible (it is you that is doing it right?) I looked over some of the past papers and they were way easier than my 1st year exams. Don't forget, a degree is a full time thing, not something you do on the side of your A-levels.
    Yes I know. This thread is nothing to do with the Olympiad, they're not going to ask for this - but I've had experience with plenty of undergraduates and even postgraduates and to me this doesn't seem like many first or second years chemistry students would know it. Originally when I mentioned it you said the Olympiad exams looked similar to your first-year stuff which seemed more reasonable, since what they ask from us is mainly to learn from first-year textbooks in preparation.

    Can I see some of your first-year exams then? Essentially I'm just looking for hard and clever problems, if the theory is first-year uni stuff I'm happy to give it a shot.

    I understand it may seem pretentious of me to label my posts "Undergraduate" but I can't think of any other way to avoid responses containing info I already know. The issue is that if I label some of these posts as "Sixth Form" I will get responses that are designed for AS-level board exams and that is a level well below the Olympiad problems. I can tell this most acutely because - I do both!

    (Original post by JMaydom)
    Calculations are done by computational programmes which use forms of quantum mechanics. They solve them numerically but make many assumptions to speed up the simulation.
    This is what I'm looking for. I don't mind if they make approximations; when I said "exact" all I meant was "not VSEPR guesses". If it's quantum mechanics there's going to be some approximation anyway I'm guessing.

    Where can I find out about the maths involved?

    Edit: And don't deny it - you're not taught the quantum mechanics of bond angles in the average first year course!
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    (Original post by Big-Daddy)
    Yes I know. This thread is nothing to do with the Olympiad, they're not going to ask for this - but I've had experience with plenty of undergraduates and even postgraduates and to me this doesn't seem like many first or second years chemistry students would know it. Originally when I mentioned it you said the Olympiad exams looked similar to your first-year stuff which seemed more reasonable, since what they ask from us is mainly to learn from first-year textbooks in preparation.

    Can I see some of your first-year exams then? Essentially I'm just looking for hard and clever problems, if the theory is first-year uni stuff I'm happy to give it a shot.

    I understand it may seem pretentious of me to label my posts "Undergraduate" but I can't think of any other way to avoid responses containing info I already know. The issue is that if I label some of these posts as "Sixth Form" I will get responses that are designed for AS-level board exams and that is a level well below the Olympiad problems. I can tell this most acutely because - I do both!

    This is what I'm looking for. I don't mind if they make approximations; when I said "exact" all I meant was "not VSEPR guesses". If it's quantum mechanics there's going to be some approximation anyway I'm guessing.

    Where can I find out about the maths involved?

    Edit: And don't deny it - you're not taught the quantum mechanics of bond angles in the average first year course!
    I have no issue with you labelling your threads undergrad, the level is more like undergrad than A-level. The style of the past paper I looked at was different, much more about factual knowledge than conceptual knowledge, if you get what I mean?

    I have linked the latest prelim exams for chem (1st year)

    To find out about the maths you will need to read a book covering the QM, try atkins molecular quantum mechanics, and then whatever maths book you need to understand it.... Afraid Maths really isn't my strongpoint.
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    (Original post by JMaydom)
    I have no issue with you labelling your threads undergrad, the level is more like undergrad than A-level. The style of the past paper I looked at was different, much more about factual knowledge than conceptual knowledge, if you get what I mean?
    I get what you mean but I think it's the opposite - there is material in your first-year papers which is not in the IChO, but on the whole I'd say the IChO problems are harder/more applied. That's not to say it's easier to answer an IChO paper than one of the papers you linked me to; the wavefunction or early QM material from the first-year paper would not show up in an Olympiad, nor would the calculus you have in the physical paper, but the problems are more applied. I find with the IChO that the approach you take with a question has to be either insightful or very rigorous (e.g. solving the simultaneous equations exactly for a titration of H3PO4 by NaOH), and obviously I tend to go with the rigorous (except for problems like this titration one where it's impractical). However in these first-year papers it is a matter of having a clear, rather than creative, understanding (I would say).

    (Original post by JMaydom)
    I have linked the latest prelim exams for chem (1st year)
    Thank you very much. These are great papers to have, if you ever have any more lying around please send them to me (I'm always happy to get them).

    (Original post by JMaydom)
    To find out about the maths you will need to read a book covering the QM, try atkins molecular quantum mechanics, and then whatever maths book you need to understand it.... Afraid Maths really isn't my strongpoint.
    OK so back on topic now The Atkins Molecular Quantum Mechanics book seems very readily available but the table of contents doesn't mention bond angles or bond lengths. It does mention "calculation of electronic structure" - is that what I'm looking for? (And yes, I will build up the QM from scratch, it's not only for this bond length problem - the first few chapters up to and excluding Group Theory sound familiar anyway)
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    (Original post by Big-Daddy)
    I get what you mean but I think it's the opposite - there is material in your first-year papers which is not in the IChO, but on the whole I'd say the IChO problems are harder/more applied. That's not to say it's easier to answer an IChO paper than one of the papers you linked me to; the wavefunction or early QM material from the first-year paper would not show up in an Olympiad, nor would the calculus you have in the physical paper, but the problems are more applied. I find with the IChO that the approach you take with a question has to be either insightful or very rigorous (e.g. solving the simultaneous equations exactly for a titration of H3PO4 by NaOH), and obviously I tend to go with the rigorous (except for problems like this titration one where it's impractical). However in these first-year papers it is a matter of having a clear, rather than creative, understanding (I would say).



    Thank you very much. These are great papers to have, if you ever have any more lying around please send them to me (I'm always happy to get them).



    OK so back on topic now The Atkins Molecular Quantum Mechanics book seems very readily available but the table of contents doesn't mention bond angles or bond lengths. It does mention "calculation of electronic structure" - is that what I'm looking for? (And yes, I will build up the QM from scratch, it's not only for this bond length problem - the first few chapters up to and excluding Group Theory sound familiar anyway)
    I imagine calculating bond angles (without using a computer) cannot be found in a textbook. There will be perhaps a theoretical discussion of the relevant factors somewhere (MO theory most likely), but the calculations are done by a computer. It would take you years to solve as the schroedinger equation has to be solved numerically.

    I don't think you realise just how advanced this stuff is. It would never be expected of 3rd years to be actually able to calculate bond angles, just rationalisation, and my uni is very keen on advanced theory.
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    (Original post by JMaydom)
    I imagine calculating bond angles (without using a computer) cannot be found in a textbook. There will be perhaps a theoretical discussion of the relevant factors somewhere (MO theory most likely), but the calculations are done by a computer. It would take you years to solve as the schroedinger equation has to be solved numerically.
    But the formulae must be documented somewhere ... presumably besides just obscure journals :confused:

    (Original post by JMaydom)
    I don't think you realise just how advanced this stuff is. It would never be expected of 3rd years to be actually able to calculate bond angles, just rationalisation, and my uni is very keen on advanced theory.
    I have a vague idea, I'm not sure how far into MO theory it is but from what I've seen of MO theory so far I couldn't begin to calculate bond angles. Don't worry, I don't plan to sit in an exam when they say "estimate the bond angle of ClO2" and start calculating it. (I guessed 117 degrees, central atom has 2 substituents and 1 lone pair and 1 radical - and apparently the calculated value was 117.4 ) I'm just interested to see how it's done.

    Do you think it might be worth asking on a physics forum?
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    (Original post by Big-Daddy)
    But the formulae must be documented somewhere ... presumably besides just obscure journals :confused:



    I have a vague idea, I'm not sure how far into MO theory it is but from what I've seen of MO theory so far I couldn't begin to calculate bond angles. Don't worry, I don't plan to sit in an exam when they say "estimate the bond angle of ClO2" and start calculating it. (I guessed 117 degrees, central atom has 2 substituents and 1 lone pair and 1 radical - and apparently the calculated value was 117.4 ) I'm just interested to see how it's done.

    Do you think it might be worth asking on a physics forum?
    OK, quick rundown of how QM works..... Schrodinger equation describes the energy.... Lowest energy of the system will be what is adopted (in terms of orientations)
    You then need to find the wavefunction, VERY difficult, or rather a wavefunction that describes the system correctly. Then you have the problem that the equation cannot be solved, which is why you need a computer to solve it numerically (throw loads of numbers at it and find which works best, sort of just trial and error)
    I know one of the main QM theories used in these problems, to simplify things so that it doesn't take a supercomputer, is the Hartree-fock method. I only really know this from a lab last year which involved running simulations to find the bond angles and bond vibrational modes of different molecules. I do not understand the theory.

    I'm sorry to be a pessimistic but i think you'll need to be studying physics/physical chem to a high level before you understand this/can start to learn about how it is actually done. I believe the advanced QM option they offer at my uni may cover some of this. This option is way past the core QM studied however, and only the students that are VERY good at this sort of stuff actually see this option through.
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    (Original post by JMaydom)
    OK, quick rundown of how QM works..... Schrodinger equation describes the energy.... Lowest energy of the system will be what is adopted (in terms of orientations)
    You then need to find the wavefunction, VERY difficult, or rather a wavefunction that describes the system correctly. Then you have the problem that the equation cannot be solved, which is why you need a computer to solve it numerically (throw loads of numbers at it and find which works best, sort of just trial and error)
    I know one of the main QM theories used in these problems, to simplify things so that it doesn't take a supercomputer, is the Hartree-fock method. I only really know this from a lab last year which involved running simulations to find the bond angles and bond vibrational modes of different molecules. I do not understand the theory.

    I'm sorry to be a pessimistic but i think you'll need to be studying physics/physical chem to a high level before you understand this/can start to learn about how it is actually done. I believe the advanced QM option they offer at my uni may cover some of this. This option is way past the core QM studied however, and only the students that are VERY good at this sort of stuff actually see this option through.
    Well, the Atkins book you mentioned does indeed cover this and I will have a look in it

    By the way, when you say the Schrodinger equation cannot be solved, do you mean no solution is known or that it's been proved unsolvable?
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    (Original post by Big-Daddy)
    Well, the Atkins book you mentioned does indeed cover this and I will have a look in it

    By the way, when you say the Schrodinger equation cannot be solved, do you mean no solution is known or that it's been proved unsolvable?
    It is unsolvable. It's a mathematical impossibility to solve analytically.
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    (Original post by JMaydom)
    It is unsolvable. It's a mathematical impossibility to solve analytically.
    Ah ok, I see. Something to do with the n-body problem being impossible to solve exactly?

    By the way, could you let me know which textbooks you used for your first-year physical chem and inorganic chem? I find the papers you sent me ask some questions (e.g. on the Clausius-Clapeyron equation) for which the theory is not covered in my current textbook but which are similar theory-wise to those that could come in the IChO. I'm ok with the organic but I'd appreciate the names of your inorganic and physical textbooks at that stage, so I could have a look at them. (Or whichever inorganic/physical textbooks you base the first-year course on at your uni)
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    (Original post by Big-Daddy)
    Ah ok, I see. Something to do with the n-body problem being impossible to solve exactly?

    By the way, could you let me know which textbooks you used for your first-year physical chem and inorganic chem? I find the papers you sent me ask some questions (e.g. on the Clausius-Clapeyron equation) for which the theory is not covered in my current textbook but which are similar theory-wise to those that could come in the IChO. I'm ok with the organic but I'd appreciate the names of your inorganic and physical textbooks at that stage, so I could have a look at them. (Or whichever inorganic/physical textbooks you base the first-year course on at your uni)
    The textbooks, except the organic one were pretty naff for 1st year. They were atkins inorganic and physical chem. I feel a bit bad saying his books were bad considering he essentially gave them to me for free.
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    (Original post by Big-Daddy)
    Ah ok, I see. Something to do with the n-body problem being impossible to solve exactly?
    Most quantum problems are impossible to solve analytically. Not just the n-body problem.
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    (Original post by JMaydom)
    The textbooks, except the organic one were pretty naff for 1st year. They were atkins inorganic and physical chem. I feel a bit bad saying his books were bad considering he essentially gave them to me for free.
    Well they got you through your first year didn't they?

    At first glance Atkins' Physical Chemistry does seem exactly the right level. You mean this book right: http://www.amazon.co.uk/Atkins-Physi.../dp/0198700725. Originally I had it pegged as too advanced(told it was a mass of differential equations!) but now I look it should be fine.

    It looks like it covers everything that could come up in the first year and maybe marginally more? "Physical Chemistry" looks great for IChO, at least in terms of topics covered. I'll take a look at inorganic, but thanks
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    (Original post by Big-Daddy)
    Well they got you through your first year didn't they?

    At first glance Atkins' Physical Chemistry does seem exactly the right level. You mean this book right: http://www.amazon.co.uk/Atkins-Physi.../dp/0198700725. Originally I had it pegged as too advanced(told it was a mass of differential equations!) but now I look it should be fine.

    It looks like it covers everything that could come up in the first year and maybe marginally more? "Physical Chemistry" looks great for IChO, at least in terms of topics covered. I'll take a look at inorganic, but thanks
    That's the one, well the 9th edition anyway.

    No it didn't get me through. I started the year off badly as I assumed these textbooks would be sufficient. Physical is definitely better than inorganic though (the book, not the subject :P)
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    (Original post by JMaydom)
    That's the one, well the 9th edition anyway.

    No it didn't get me through. I started the year off badly as I assumed these textbooks would be sufficient. Physical is definitely better than inorganic though (the book, not the subject :P)
    Oh - so what did you do to supplement it?

    I'm just trying to get my hands on all the resources I can up to that level
 
 
 
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