Molecular Geometry: Experiment vs Modeling

[FCMS34]

All general chemistry textbooks have sections about molecular geometry, explaining that H2O is “bent”, NH3 is “trigonal pyramidal” and CH4 is “tetrahedral”, etc. But in a typical introductory textbook, it’s not easy to figure out how much is from experimental (observed) facts and how much is derived from modeling (to rationalize observations). I presume that one important piece of data is bond length, which can be somehow determined by analyzing crystals in various ways.

Are there any instruments / experimental methods that can more or less directly address the issue of molecular geometry? For example, if a student asks how do we know that the shape of NH3 is trigonal pyramidal, what should be the answer?

 

[DrDu]

In crystals you can determine the whole structure using x-ray crystallography. In the gas phase, for these simple molecules basically all information can be inferred e. g. from microwave spectroscopy which yields the moments of inertia of the molecules. Given the masses of the atoms, they allow to deduce the bond lengths and angles.

 

[FCMS34]

Dear DrDu,

Thank you so much. You answer was very helpful. Maybe I should study the basics of microwave spectroscopy.

If I may ask one related question about analyzing crystals…. If you look at the diagram of a fully-analyzed and interpreted ice crystal, a number of water molecules appear to form an overall “star” shape. Can such a picture be a source from which the “bond angle” of the H2O molecule can be inferred? If such a picture is to used to calculate the bond angle, wouldn’t it be necessary to figure out which connections/interactions (in the crystal) are covalent and which are merely hydrogen bonding? If so, how can they be distinguished? In other words, in a crystal, which is a solid aggregation of matter, how can one demarcate a molecule? (Please ignore my question if this is totally off the mark and unrelated to research reality/ history.) Thanks in advance.

 

[DrDu]

That's a very difficult question which has several layers.
The first problem is that with x-rays it is very difficult to determine the position of hydrogen atoms as they only have one electron which can scatter the x-rays and this electron is not always locacted at the hydrogen while heavier atoms tend to have core electrons which are always near the nucleus. Hence the position of hydrogens is often better determined with other methods like neutron scattering which has a high sensitivity for hydrogen.
The second problem is that hydrogen atoms are not completely ordered in ice. Hence the best one can see is that there are two minima over which the hydrogen atoms are distributed statistically, the shorter distance corresponding to a covalent bond and the longer one to hydrogen bonding.
E.g. the entropy of ice resulting from this disorder was analyzed by nobel laureate Linus Pauling.

Btw, many of the questions you are interested in should be treated in a book on physical chemistry, e.g.Walter John Moore, Physical Chemistry.

 

[FCMS34]

Thanks for guiding me in the right direction. I appreciate your help.