31 votes
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What is the exchange interaction?

In quantum chemistry, probably the easiest way to understand the "exchange interaction" is within the context of the Hartree-Fock model. $ \newcommand{\op}{\hat} \newcommand{\el}{_\mathrm{e}} \...
Wildcat's user avatar
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31 votes

Why is the boiling point of fluorine lower than that of oxygen?

It would be tempting to argue that fluorine is so electronegative and holds its electrons so tightly that their polarizability is reduced, thus so are the dispersion forces in $\ce{F2}$. But upon ...
Oscar Lanzi's user avatar
30 votes
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Why is the melting point of PCl3 less than that of PCl5?

Because $\ce{PCl5}$ does something which is not immediately obvious from its molecular formula: it autoionizes and becomes an ionic solid $\ce{PCl4+PCl6-}$. As such, it has much stronger interactions ...
Ivan Neretin's user avatar
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22 votes

Why does graphite have a high melting point even though its intermolecular forces are weak?

It's not usual to consider graphite as a material composed of "molecules" in the typical sense, though it could be viewed as a kind of polymer with two-dimensional macromolecules. Regardless, it is ...
Nicolau Saker Neto's user avatar
21 votes
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Why is octane more volatile than water while having a higher boiling point?

One thing you are missing is that air contains water, but usually does not contain octane. So for water, the process is: $$\ce{H2O(l) <=> H2O(g)}$$ and for octane, it is $$\ce{C8H18(l) <=> ...
Karsten's user avatar
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17 votes
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Help understanding how "steric effects" are distinct from "electronic effects"?

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 ...
matt_black's user avatar
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16 votes
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How to calculate Lennard-Jones potential with quantum mechanical methods

Yes, this is technically possible. A basic tutorial for this is in the excellent Psi4Numpy project, which I'll reproduce here with minor modifications. Their example fits the counterpoise-corrected ...
pentavalentcarbon's user avatar
15 votes
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Why are fluoroalkyl chains hydrophobic/oleophobic?

You have identified a fairly common but counter-intuitive result. At least it's counter-intuitive based on the way the properties of fluorine are described in undergraduate chemistry classes. ...
jheindel's user avatar
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15 votes
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How to identify hydrogen bonds and other non-covalent interactions from structure considerations?

It is safe to say that there will always be intermolecular forces at play. At the time where you will consider these you should already have a good idea about the molecules involved in your system. ...
Martin - マーチン's user avatar
14 votes
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Differences between formulae for dipole–dipole interaction energy

They are both dipole-dipole energies as from the link but their contexts are different. Eq. 3 (as numbered in the LibreTexts link), $$V(r) = - \frac{\mu_{1}\mu_{2}}{4\pi\epsilon_{0}r^{3}} \tag{3}$$ is ...
Unskilled's user avatar
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14 votes

Why is octane more volatile than water while having a higher boiling point?

The are three major factors, affecting observed relative volatility, ordered by assumed importance: The molar mass. The liquid with higher molar mass evaporates faster than a liquid with the same ...
Poutnik's user avatar
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13 votes

Can a long polymer chain interact with itself via van der Waals forces?

You have a possible answer to your question in proteins, an example which includes some long polymer chains. Intramolecular interactions - while not necessarily the driving force for formation of a ...
Buck Thorn's user avatar
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12 votes
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Natural Bond Orbital analysis: Significance of stabilization energy determined by 2nd order perturbation

TL;DR: Lewis $\to$ Non-Lewis $\mathbf{E(2)}$ values have no direct physical significance, are intrinsically un-measurable, and serve only to quantify the extent to which the "real" wavefunction for a ...
hBy2Py's user avatar
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12 votes
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Why does graphite have a high melting point even though its intermolecular forces are weak?

Graphite has got a structure similar to books stacked on top of each other. Multiple layers on top of each other and each layer going by the name graphene. Atoms in each individual layer is ...
abirbhav's user avatar
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11 votes

Why are they called ‘dispersion forces’ if they are an attractice force?

Although Danny Rodriguez has already excellently exposed what the dispersion force is in simple terms, the word dispersion still demands a better explanation in my opinion. According to Wikipedia: ...
schneiderfelipe's user avatar
11 votes
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How DFT-D3 incorporates coordination number (CN) into C6

I will quote the paper quite a bit, but I'll try and summarize a bit after the quotes and equations. You might want to start at the bottom and work backward, a lot of this is just for later reference. ...
pentavalentcarbon's user avatar
11 votes

Is a temperature decrease required for hydrogen bonds to form in water?

You are right, in a sense. A more accurate statement would be that as the temperature decreases, the average lifetime of a hydrogen bond increases. Thus, on average the liquid/solid/molecule will be ...
jheindel's user avatar
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11 votes

Differences between formulae for dipole–dipole interaction energy

An equivalent and easier formula when a molecule's coordinates are known is to use vectors. The energy is then $$V=\frac{1}{4\pi\epsilon_0}\left(\frac{\vec\mu_1\cdot\vec\mu_2}{r^3}-3\frac{(\vec\mu_1\...
porphyrin's user avatar
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10 votes

Why are they called ‘dispersion forces’ if they are an attractice force?

It refers to the electrons that are distributed (or dispersed) in the electron cloud which then ensues an attraction between atoms or molecules. Electrons can be dispersed in any number of different ...
Danny Rodriguez's user avatar
10 votes
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Why aren't the Anton computers used more widely?

I myself am a person using MD simulation for my research but one reason I am not so much interested in ANTON is not it's cost, limited resource etc. It's rather because one extremely long simulation ...
Jaehoon Yang's user avatar
10 votes
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Do amines or alcohols have stronger intermolecular hydrogen bonds?

The boiling point of non-ionic compounds are highly depend on their H-bonding abilities. For example, boiling point of water (molar mass: $\pu{18.02 g/mol}$) is $\pu{100 ^\circ C}$ at $\pu{1 atm}$ ...
Mathew Mahindaratne's user avatar
10 votes

Do amines or alcohols have stronger intermolecular hydrogen bonds?

Mathew Mahindaratne has provided analysis based on experimental values of the boiling points of the two compounds. I would like to offer a different view using bonding analysis. Before I begin ...
Tan Yong Boon's user avatar
10 votes

Why does water weaken ion ion attractions?

Electrostatic field caused by ions or any other external source causes partial orientation of molecular electric dipoles along the field vector. Their orientation causes displacements of center of ...
Poutnik's user avatar
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9 votes

Why is the pressure correction in the van der Waals equation proportional to (n/V)^2?

Pressure correction term depends upon: Number of molecules attracting the molecules which comes to strike the wall and as such it is proportional to density of gas i.e. proportional to $n/V$ where $n$...
Yb609's user avatar
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9 votes
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What is a hydrogen bond acceptor and donor?

Dissenter's answer which you linked appears to be wrong (as of now). I have left a comment. The conventional way in which we talk about hydrogen bond acceptors/donors is: in a hydrogen bond $\ce{X-H.....
orthocresol's user avatar
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9 votes
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What is the maximum distance over which van der Waals forces act?

There is no strict upper limit you can place on any type of electromagnetic interaction. That being said, van der Waal's forces are due to the formation of instantaneous dipoles which are a result of ...
jheindel's user avatar
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9 votes

DNA pairs (adenine-thymine, guanine-cytosine)

In Principles of Nucleic Acid Structure, W. Saenger argues that hydrogen-bonded bases contain at least two hydrogen bonds (forming a "cyclic" pattern). Often, there is a tautomeric form possible that ...
Karsten's user avatar
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8 votes
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How does fluorouracil inhibit thymidylate synthase?

The inhibition comes about because: Uracil and $5$-fluorouracil are sufficiently similar that both moieties have a strong binding affinity for the active site of thymidylate synthase. The presence of ...
hBy2Py's user avatar
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8 votes
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Why is the pressure correction in the van der Waals equation proportional to (n/V)^2?

Summing up the argument from the Wikipedia derivation, the reason it is proportional to $(\frac{1}{V_\mathrm m})^2$ where $V_\mathrm m=\frac Vn$ (the molar volume) comes from how the molecules ...
Tyberius's user avatar
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8 votes
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Which is more soluble in water, HF or SO2?

There are a couple different forces at play here. One big difference between the compounds is that $\ce{SO2}$ is much more volatile, with a boiling point of $\pu{-10 ^\circ C}$; in other words it's a ...
airhuff's user avatar
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