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@jakebeal's excellent answer to Why do animal cells “mistake” rubidium ions for potassium ions? includes the following passage:

In the case of potassium versus sodium, which are both very important in biochemistry, a recently discovered mechanism in sodium-potassium pumps uses binding properties to grab both, then atomic size (via steric hindrance) to distinguish sodium versus potassium as described nicely in the answers to this question.

Wikipedia's Steric effects says:

Steric effects are nonbonding interactions that influence the shape (conformation) and reactivity of ions and molecules. Steric effects complement electronic effects, which dictate the shape and reactivity of molecules. Steric repulsive forces between overlapping electron clouds result in structured groupings of molecules stabilized by the way that opposites attract and like charges repel.

Since "the way that opposites attract and like charges repel" sounds "electronic" to me, why does one say that "Steric effects complement electronic effects" instead of simply being electronic effects?

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    $\begingroup$ I’ve not the time for a proper answer any time soon. But yes, the waters are slightly murky here: steric effects are because of electrons so fundamentally are also “electronic” effects. One way of phrasing the difference is that steric effects can be fully explained simply by the position of each atom and the repulsion between their associated electron clouds (hence can be explained with molecular models), but electronic effects are more subtle in that they not only depend on atomic positions but also the exact distribution of electron density between different atoms (i.e. orbitals). $\endgroup$ – orthocresol Feb 17 at 9:39
  • $\begingroup$ @orthocresol that actually explains it (to me at least) quite well. Something like the difference between classical Coulomb force analysis and a proper quantum mechanical treatment perhaps? $\endgroup$ – uhoh Feb 17 at 9:53
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    $\begingroup$ I wanted to say what I see what orthocresol already said. You may find the same situation in many case when a general principle or cause is underlying. The same happens when energy of a molecule is decompose in terms of bond length, angle, substituents, etc. This leads to a function that is certainly not analytic, as for all terms are in reality variously interconnected. It makes much sense anyway. Take conformers of CH2X-CH2X, as a simple example. It is another case in which we can consider a term as a function of the angle. $\endgroup$ – Alchimista Feb 17 at 10:11
  • $\begingroup$ If you wish stearic effects are electronic effects themselves, this also somewhat answers you question. $\endgroup$ – Alchimista Feb 17 at 10:13
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    $\begingroup$ @uhoh Yes, in that one could be said to represent a "deeper" layer of analysis than the other, but I'm hesitant to draw any more parallels beyond that. Resonance (an electronic effect) is definitely a QM thing, but the inductive effect (also electronic) doesn't necessarily need QM to be explained, it just needs the basic concept of electronegativity, which can kind of be boiled down to just Coulombic attraction. Though it might be getting a bit philosophical here. :-) I think Matt's answer is clearer than my original comment. $\endgroup$ – orthocresol Feb 17 at 15:18
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The normal distinction between "steric" and "electronic" is based on whether the effect is transmitted through space or through bonds

All the normal physical interactions we experience are arguably electronic. When you touch your desk, you feel force because of interactions between the molecules of the desk and the molecules of your hand because the molecules of one interact "electrostatically" with the molecules of the other.

But your hand and your desk are not bonded together.

When a noble gas condenses into a liquid, it has a finite volume. The atoms become liquid because there are weak attractive forces (not usually described as "bonds") but have a fixed volume because the strong forces between their filled electron shells repelling each other prevent the atoms packing together any more closely. This second effect is very similar to the force stopping you hand penetrating your desk.

Arguably this is all electronic interactions.

But, if you are a chemist, it is worth drawing a line because it makes it much easier to talk about the properties of molecules. Electronic interactions to a chemist are usually interactions transmitted via bonds in a molecule.

So a molecule like fluorescein (shown below) has an extended pi-system so the oxygens at either end of the three fused rings can interact via bonds (and the overall molecule has electronic transitions that make it a useful fluorescent dye).

fluorecein

But in a molecule like this substituted biphenyl the electronic interactions might drive the molecule to be planar (so the two pi-systems can communicate) but the steric size of the 4 methyl groups prevent that because the methyl groups would come too close in 3D space to allow a planar configuration.

biphenyl example

So the molecule will not have two aromatic rings in the same plane but will have significant angles between the rings. A "space filling" view (below) shows the methyl groups take up too much space to allow the rings to be coplanar.

bi 2,6 xylyl example

Chemists think of the forces trying to make this molecule planar as "electronic" as they are transmitted via the bonds but the forces preventing this as being "steric" interactions. They are more like the forces preventing your hand from penetrating your desk than they are alike the forces holding the molecule together.

It is a useful distinction even when the forces are internal to a molecule.

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  • $\begingroup$ +1 immediately because even though I will read through more carefully later, I know an excellent answer when I see it ;-) $\endgroup$ – uhoh Feb 17 at 11:18
  • $\begingroup$ That's better than my comment. $\endgroup$ – orthocresol Feb 17 at 15:11
  • $\begingroup$ It is easy to find papers that distinguish "steric" and "electronic" this way (for example, doi.org/10.1021/cs501813v and doi.org/10.1016/j.poly.2011.09.035 ). So, that's what people do. Personally, I feel like "steric" is a subset of "electronic" and find this restricted usage of "electronic" objectionable. $\endgroup$ – WaterMolecule Feb 17 at 18:23
  • $\begingroup$ I observe that "steric" effects would still hold if we think of the space filling model as made of neutral pieces. It's that atoms take up space that matters. Sure, Pauli exclusion (of charged particles) prevents interpenetration, and the charges soften the potential barriers we might imagine from the space filling model, but those are details -- the core idea is that parts of the configuration space are excluded because atoms have volume. $\endgroup$ – Eric Towers Feb 18 at 20:34
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    $\begingroup$ @WaterMolecule you might find it objectionable, but it is highly practical. Unless you have some magic way to completely solve the wavefunction for all the electrons in every complex molecule. The practical difference is clear and easy to understand. $\endgroup$ – matt_black Feb 23 at 13:14
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tl;dr Effects are:

  • steric (or non-steric) if they're related (or not) to physical shape/configuration;

  • electronic (or non-electronic) if they're related (or not) to electronic charge.

I would suggest seeing these as distinct concepts; effects can be qualified with respect to sterics or/and electronics, as appropriate (or just called an "effect" if no qualification is needed).


Basically:

  • "Steric" basically just means "related to shape". Steric effects are those related to shape/configuration.

  • "Electronic" basically just means "related to electric charge". Electronic effects are those related to electric charges.

Chemistry students often learn about chemical reactions in the context of simple molecules. For example, a substitution reaction: $$ \text{R-Br} + \text{OH}^{−} ~\Rightarrow~ \text{R-OH} + \text{Br}^{−} \,, $$ where $`` \text{R} "$ is a placeholder for a generic substrate. Reactions like this are understood through theories about reaction mechanisms, which focus on how the electrons move around and just generally why the reaction happens as it does.

The situation can get more involved when we're talking about reactions with larger molecules, such as proteins in Biochemistry, where the substrate, $`` \text{R} " ,$ can be massive and wrapped around the reaction site, significantly obstructing it. Or another chemical component (another functional group) could be maintained in a certain configuration in close proximity, significantly affecting reactivity. These are discussed as steric effects.


Note: "Steric" and "electronic" aren't alternatives.

With respect to electronics:

  • "Electronic" clarifies that an effect is related to electronics.

  • "Non-electronic" clarifies that an effect isn't related to electronics.

With respect to sterics:

  • "Steric" clarifies that an effect is related to sterics.

  • "Non-steric" clarifies that an effect isn't related to sterics.

An effect can be qualified with respect to electronics, sterics, both, or neither, as appropriate.

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  • $\begingroup$ Mostly just trying to clarify some confusion I perceived as possibly being reflected in the Wikipedia articles, where "electronic effect" seemed to be presented as a synonym for "non-steric effect". $\endgroup$ – Nat Feb 17 at 13:24
  • $\begingroup$ @uhoh: Generally we just say "electronic" when we're referring to electronic effects as opposed to non-electronic effects, e.g. a kinetic-isotope effect. If we don't need to draw such a distinction, we merely don't say either "electronic" or "non-electric". The Wikipedia articles attempt to make a case for a system in which sterics and electronics are dichotomous, rather than just using the "non-" prefix, which is just.. well, honestly it's just really silly. $\endgroup$ – Nat Feb 17 at 13:33
  • $\begingroup$ "electronic... if they're related... to electronic charge." So are you saying that steric effects are not related to charge? If not by the Coulomb force, by what effect can one molecule respond to the shape of another? $\endgroup$ – uhoh Feb 17 at 13:43
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    $\begingroup$ @uhoh: I'm saying that sterics and electronics should be spoken of separately rather than assumed related in specific ways. For example, yes, there are non-electronic steric effects, e.g. steric isotope effects. $\endgroup$ – Nat Feb 17 at 13:45
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    $\begingroup$ Arguably, if we had the computational power to fully solve the complete Schrodinger equation for molecules, there would be no distinction between "electronic" and "steric" effects as the solution would consider all interactions and produce the full configuration space of the molecule. But we can't. So chemists need to make the pragmatic distinction between electronic and steric effects to make any sense of their data. $\endgroup$ – matt_black Feb 18 at 14:48

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