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We know an individual molecule of $\ce{H2O}$ has a bent structure, but if we consider a volume of water, then there are many molecules having hydrogen bonding with each other. Due to this, does the structure of a single molecule, on average, deviate from bent? How severe are the deviations?

I took water as a simple example, but I mean my question can be considered more generally for other compounds as well.


This question is mainly inspired by the following question from the (2014) JEE advanced paper:

The correct statement(s) for orthoboric acid is/are (Multi correct)

(a) It behaves as a weak acid in water due to self ionisation

(b) Acidity of its aqueous solution increases upon addition of ethylene glycol

(c) It has a three-dimensional structure due to hydrogen bonding

(d) It is a weak electrolyte in water

Option 'c' in particular is what made me start thinking about this.

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  • $\begingroup$ It looks as if the five answers are correct. And the deviations of the angle H-O-H must be rather weak. $\endgroup$
    – Maurice
    Aug 24 at 8:12
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    $\begingroup$ Compare water bond angles in vapour and ice. $\endgroup$
    – Poutnik
    Aug 24 at 11:22
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When a molecule hydrogen bonds, intramolecular bonds tend to be weakened, and such bonds become longer. Hydrogen bonds can also result in distortion of internal bond angles. However, H-bonds are typically weaker than covalent bonds by an order of magnitude or so, so H-bonds will perturb but hardly alter the conformation of rigid molecules, exceptions including rotational (conformational) potentials with low energy states differing in energy by an amount comparable to the H-bond energy.

In some macromolecules such as proteins, H-bonds occur extensively and often cooperatively (synergistically), both internally and with the solvent, altering (possibly even "flattening") the potential energy landscape, and resulting in 3D structures that would not be stable in their absence. H-bonding is one contributor to the acquisition of specific secondary to quaternary protein structures (another complementary one being hydrophobic interactions).

Note that molecules are in general inherently dynamic. You can explain this using quantum mechanical (QM) arguments, but you might simply think of molecules somewhat roughly as mechanical arrangements of atoms bonded by spring-like bonds. The vibrational state of a rigid molecule usually occupies a single energy minimum at room temperature. The QM wavefunction describes a distribution of possible observable instances (conformations) of the molecule, from which we may compute an average and other statistics such as a most likely state. Hydrogen bonding perturbs this distribution by altering the underlying potential. The perturbation due to a single H-bond, however, is relatively small.

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