Water molecule dipole moment

  1. a water monomer has HOH angle of 104.5 and OH length of 0.98 , partial charges are +0.4 on H and -0.8 on O. The reported electric dipole moment for this monomer is 1.86D. I tried to calculate it and got 2.3 D. The way I do it is by multiplying the distance between the center of the H-H and the oxygen atom times the partial chrge. Any clues why I get different result?

    Thanks in advance
     
  2. jcsd
  3. Borek

    Staff: Mentor

    Wrong approach. You have two vectors - add them.
     
  4. It gives the same result because the OH lengthes are the same. This can be proven geometrially. Any other clues?
     
  5. Would you mind showing your calculations step by step?
    I have the nagging suspicion that this is one of those "too obvious to look for" problems that we all make from time to time.
     
  6. Sure, let's follow the vectors summation approach:
    Let the molecule be in x-y plane with the origin on the center of the oxygen atom and the +ve part of y-axis divides the angle 104.5 into two halves.

    now the dipole between one hydrogen and oxygen is 0.98x0.4 =0.392 (e.A) in direction from H to O.
    Similarly we get the other moment. Now the x-components will cancel each other and only the components in -ve y-axis will add algebrically to:
    2X0.392xcos(52.25)= 0.47998 e.A

    using this table of unit conversions:
    http://mccammon.ucsd.edu/~blu/Research-Handbook/physical-constant.html

    1D = 0.20822678 e.A.
    So the result that I get is 2.3 D. While this gas phase geometry of the water molecule has a dipole moment of 1.86D. The question is why ? (No Quantum mechanics should be involved in this simple calculation)
     
  7. Ygggdrasil

    Ygggdrasil 1,713
    Science Advisor

    The difference may be due to the simplification of water into three point charges. In reality, you would have to consider the continuous charge distribution across the entire molecule.

    Also, are you sure the numbers are correct? A -0.8 charge on the oxygen and a +0.4 charge on each hydrogen does not give an electrically neutral molecule.
     
  8. The charge is neutral -0.8+ 2x(+0.4) =0.
    This is a classical model for water molecule similar to the famous ones used in classical molecular dynamics simulations( TIP4P,TIP5P,SPC,...) All these models are point charges and they all have the dipole value of 1.86 D which is the experimental gas phase cluster for water. But I don't understand how they calculate these dipoles from point charges.
     
  9. chemisttree

    chemisttree 3,721
    Science Advisor
    Homework Helper
    Gold Member

    How did you determine that the partial charges were +0.4 and -0.8?
     
  10. These partial charges are fitted to reproduce other known properties such as HOH angle, O-O distance for dimer...etc. So you can say that they are part of the given model.
     
  11. I can't help but notice they very nearly work out, if someone were to mistakenly use a bond angle of 120º instead of 104.5º.
    I'm wondering if they weren't "fitted to reproduce" erroneous information.
     
  12. your calculation is ok. The 1.86D is an experimental result, and the calculation you did is the first approximation. Remember that in nearly all Forces fields partial charges do not reproduces exactly the dipole moment. If you want a better result you will have to take into account a spherical charge distribution over each atom but however it is also a classical calculation the result is a little bit better. Finally you will reproduce this valur with a DFT calculation before a optimzation geometry or if you want with the equilibrium values for the bending and the bond of 0.98
     
  13. 1.85 D is the gas phase dipole moment. Models such as SPC are designed for bulk liquid phase simulations, and liquid water has a higher dipole.
     
  14. water monomer has HOH angle of 104.5 and OH length of 0.98 , partial charges are +0.4 on H and -0.8 on O. The reported electric dipole moment for this monomer is 1.86D. I tried to calculate it and got 2.3 D.I have donecalculated by Gaussian it , Is it correct? why is different between 1.85 and 2.3 D?
    Thanks in advance
     
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