Martin - マーチン
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Why do we use helium in balloons?
49 votes

Actually, hydrogen is the only gas that is lighter than helium. However, it has a very big disadvantage: It is highly flammable. On the other hand, helium is almost completely inert - this is why it ...

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Are the lone pairs in water equivalent?
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46 votes

Water, as simple as it might appear, has quite a few extraordinary things to offer. Most does not seem to be as it appears. Before diving deeper, a few cautionary words about hybridisation. ...

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Ortho-effect in substituted aromatic acids and bases
45 votes

I would like to back up Klaus' answer with some Quantum Theory of Atoms in Molecules (QTAIM) results, based on a DF-BP86/def2-SVP calculation. Note that these are results, obtained without the ...

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Why does cyclopropane react with bromine?
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42 votes

The following ring opening reaction will occour: You are quite right about the angle strain. Because orbital interactions are not optimal in this geometry. Consider p-orbitals, then a natural bond ...

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DFT Functional Selection Criteria
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40 votes

What's up with all that magic? (A chapter formerly known as Introduction) The hunt for for the holy grail of density functional theory (DFT) has come a long way.[1] Becke states in the introduction of ...

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May I treat units (e.g. joules, grams, etc.) in equations as variables?
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39 votes

Yes, you may. It is quite common to convert units into each other. The simplest conversion might be the prefixing of units, e.g. $$\mathrm{\frac{km}{m}}=1000 \Longleftrightarrow \mathrm{1~km = 1000~m}...

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Why is BCl3 a monomer whereas AlCl3 exists as a dimer?
33 votes

Introduction The bonding situation in $\ce{(AlCl3)2}$ and $\ce{(BCl3)2}$ is nothing trivial and the reason why aluminium chloride forms dimers, while boron trichloride does not, cannot only be ...

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How to find the second order perturbation to wave function?
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31 votes

Disclaimer This post is some kind of a legacy post. Find the notation used in the question in the other answer. I added this proof as I was not entirely certain I understood the notation correctly. As ...

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What should be the oxidation state of oxygen in HOF (hypofluorous acid)?
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30 votes

Until the (recent) redefinition of the IUPAC, the concept of oxidation states was not as well defined as one would expect. I have discussed the issues of the old version and outlined the new ...

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What is the perfect definition for chirality?
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30 votes

The correct definition of chirality is given in the IUPAC gold book as follows: chirality The geometric property of a rigid object (or spatial arrangement of points or atoms) of being non-...

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Is the t-butyl carbocation more stable than the benzyl carbocation?
29 votes

I am using a very simplistic quantum chemical approach of the following isodesmic reaction: I have used Gaussian 16 Rev. A.03 and the DF-B97D3/def2-TZVPP level of theory. The summaries of the ...

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Bonding in the phosphate ion
27 votes

First of all, let me state the obvious: Phosphorus is awesome. After we got that out of the way we can focus on why. There are many different modifications of phosphorus in nature. With increasing ...

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Resources for learning Chemistry
26 votes

Video Resources (online) The University of Nottingham's Periodic Videos The periodic table of videos includes introductions to all elements. The molecular videos section focuses on interesting ...

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How can I clean my lab coat?
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26 votes

A lab coat will eventually get dirty, that's why it is a good idea to wear it. I had a lot of students complaining about the condition of their coat and they asked me if they could just wash it in the ...

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Does trans-decalin really have a plane of symmetry?
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26 votes

It's not easy to see from a diagram, because it distorts bonds and angles. I recommend building it with a balls-and-sticks model set. You can also use a molecular viewer to model it; there are a ...

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What is Y-aromaticity? Is the trinitromethanide anion aromatic?
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24 votes

Introduction It is fairly obvious that the statement given by Wikipedia cannot be entirely correct. There is some evidence that the anion (which obeys the 4​n+2 Hückel rule) is aromatic. The ...

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What would follow in the series sigma, pi and delta bonds?
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24 votes

tl;dr The next in the series is called φ bond. There is even a tiny Wikipedia article about it. Nicolau pointed me to the Wikipedia article, that had at the time a tiny section about the φ ...

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How can the dipole moment of carbon monoxide be rationalised by molecular orbital theory?
23 votes

Unfortunately, nothing in the bonding situation in carbon monoxide is easily explained, especially not the dipole moment. According to the electronegativities of the elements, you would expect the ...

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Why are DCM and chloroform so resistant towards nucleophilic substitution?
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23 votes

Introduction (and Abstract, TLDR) In very short words you can say, that the anomeric effect is responsible for the lack of reactiveness. The electronic effect may very well be compensating for the ...

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Is the S–O bond strength in sulfur trioxide or sulfite anion larger?
22 votes

Due to symmetry constraints ($D_\mathrm{3h}$) in $\ce{SO3}$ there 6 electrons in $\pi$ type orbitals. In a wider sense of the term this molecule is Y-aromatic, but the HOMO actually represents two ...

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Does fluorine in FNO3 have +1 oxidation number?
21 votes

tl;dr The ambiguity is due to an unfortunate incompleteness of the previously (prior to 2016) existing rules. The oxidation state of fluorine in $\ce{FNO3}$ is $-1$ according to the present rules. As ...

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What are the important implications to the field of chemistry, if any, of the recently confirmed hexavalent carbon species?
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21 votes

The carbon is not hexavalent, it is hexacoordinated. A covalent bond does not equal to a total of two electrons between the bonding partners and the nature of the chemical bond may lie somewhere ...

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What's so special about chelation?
20 votes

Chelation is an entropy driven reaction, i.e. complexes with polydentate ligands are more stable. Consider what happens when you dissolve copper sulfate in water: \begin{align} \ce{CuSO4 + 6H2O <=&...

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Utility of Bent's Rule - What can Bent's rule explain that other qualitative considerations cannot?
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20 votes

There is a reason for everything. Does Bent's rule have any utility? YES! It wouldn't be there if there was not. But I will get back to this at the end of this post. Let me go through the points ...

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Resources for learning Chemistry
19 votes

Software ChemOffice Professional Commercial software for drawing molecular structures, 3D models and many more. High price but most higher education institutes will provide students/staff with free ...

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Electronegativity Considerations in Assigning Oxidation States
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19 votes

In the IUPAC Recommendations 2016 the definition of oxidation state underwent a significant and comprehensive change. It does now resemble the version wich was proposed be Hans-Peter Loock and is ...

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Atropisomerism of naphthyl alcohol
19 votes

The hindered rotation is due to the hydrogens at the naphthyl moiety. The following shows the rotation calculated at the DF-B97D3/def2-SVP level of theory. To better visualise this, I have chosen a ...

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What is the molecular geometry of NHF₂?
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19 votes

You can look up the molecule on chemspider, where you have a little applet for the 3D structure. Or you can download a coordinate file from NIST and view it in a molecular viewer, like Avogadro. Or ...

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How to evaluate the heat of formation with computational chemistry?
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19 votes

Your approach is quite correct, but as Jan already pointed out, it is incomplete. What you calculated is the difference in the electronic energy of the reaction $$\ce{H2 (g) + 1/2 O2 (g,\,{}^1\Delta_{...

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How to rationalise the planar structre of I2Cl6?
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18 votes

The bonding situation in the compound $\ce{(ICl3)2}$ is by far more complex than what is depicted in this book. The molecule itself has very high symmetry, i.e. $D_\mathrm{2h}$, that needs to be ...

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