porphyrin
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Understanding group theory easily and quickly
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29 votes

The figures below show you how to navigate your way round point group tables. The irreducible representations (irreps) are shown as the row of characters. A reducible representation is a collection of ...

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Why does bond angle decrease in the order H2O, H2S, H2Se?
19 votes

The question asks why water has a larger angle than other hydrides of the form $\ce{XH2}$ in particular $\ce{H2S}$ and $\ce{H2Se}$. There have been other similar questions, so an attempt at a general ...

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Is it possible for a substance to absorb a longer wavelength of EM wave and emit a shorter wavelength?
18 votes

Yes it is possible for molecules to absorb a single photon at a longer wavelengths than some of those that they emit at. If the molecules are thermally isolated from their surroundings then the result ...

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What is resonance, and are resonance structures real?
15 votes

Bon has a very nice example of motion potentially restricted because of a barrier, and although it turns out that this is not the case for the norbornyl cation, there are simpler examples that do show ...

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Can magnetic fields affect a chemical reaction?
14 votes

Yes, magnetic fields can affect chemical reactions. These are usually reactions in which two radicals are produced. The reaction of a molecule R-R can form two radicals and is written generically as R-...

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Is there any empirical upper limit for order of reaction?
13 votes

The order of a reaction is an experimentally determined quantity and can be positive, negative or fractional. The order need not be related to the stoichiometric coefficients, although sometimes it ...

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Uv vis and fluorescence spectroscopy: sensitivity
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13 votes

Fluorescence is a 'zero background' or absolute type of measurement meaning that single photons can be measured against a 'dark' background so the sensitivity is huge, and limited by the fraction of ...

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Why is the magnesium(II) ion preferred over other ions in chlorophyll?
13 votes

Chlorophyll in plants has two main functions. First, it facilitates energy transfer of an absorbed photon's energy via the many other chlorophylls that constitute the antenna pigment-protein complex ...

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How exactly does UV light break bonds?
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12 votes

Briefly an electronically excited state is produced (distinct from the ground state) and if the uv energy is sufficient it is produced above its dissociation energy, which is less than in the ground ...

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Basics of wave-mechanical model of atom
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12 votes

These orbitals represent the angular part of the wavefunction. The solution obtained directly from solving the Schrödinger equation produces equations containing complex numbers so cannot be drawn on ...

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Why do spin isomers of hydrogen (ortho and para hydrogen) change their nuclear spin with temperature variance?
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12 votes

The effect is due to the symmetry properties of the rotational energy levels and those of the nuclear spin. The change with temperature is, as usual, traced back to the Boltzmann distribution. In ...

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Won't the net effect of a catalyst be zero if it creates a new path with lower activation energy?
12 votes

Yes, one expects both forwards and backward reactions to speed up as you suggest; there seems to be no reason why microscopic reversibility would be suspended. The point of the catalyst will be to ...

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Differences between formulae for dipole–dipole interaction energy
11 votes

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\...

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Terminology of atomic spectroscopy: Difference Among Term, States and Level
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11 votes

The figure below shows the situation between configuration for a $p^2$ configuration, terms, levels and states. The word 'state' tends to be used colloquially to mean any of Term, Level or State. ...

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Why would a ~1 cm thick layer of argon be a significantly poorer conductor of heat than air?
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11 votes

The thermal conductivity coefficients of some gasses in units of W/(m•K) are helium 0.142, argon 0.016, air 0.026, methane 0.03, propane 0.015, bromine 0.004, and steam 0.018. These are all rather ...

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Kinetic energy of molecules in liquid state?
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11 votes

The average translational kinetic energy of a molecule is $3kT/2$ irrespective of whether the molecule is in the gas, liquid, or solid phase. In the liquid the motion giving rise to kinetic energy is ...

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Does deuterated water slow down the overall metabolism of a cell?
11 votes

Yes, the kinetic isotope effect is the main reason due to the differential lowering of the zero point energy in reactants and transition state, which has the effect of increasing, slightly, the ...

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the chemical mechanism for deviations from Raoult's law
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11 votes

First some general comments about Raoults law before discussing the particular solutions in question. Raoult’s law suggests that the partial pressure of each substance above a solution is ...

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Pi electron stacking, how does it work?
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11 votes

You going along the right track. The stacking attraction occurs when two molecules with $\pi$ orbitals come face to face with one another, typically their separation is 0.34 nm. However, for the ...

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Will entropy increase in adiabatic expansion?
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10 votes

The answer to your question is yes and no. You are correct in supposing that as an ideal gas expands the entropy will increase as it has more space to occupy and so the number of ways the molecules ...

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Dominant contribution of v = 0 → v = 1 transition in vibrational spectroscopy
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10 votes

The reference should really be to the Boltzmann distribution where the population is proportional to $\exp(-E_n/(k_\mathrm BT))$ where $E_n$ is the energy of the $n^{th}$ energy level. This shows ...

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Why is entropy favorable?
10 votes

Do not think of entropy as 'disorder' as this is misleading, better is that it is a 'measure of disorder' but this is equally vague. It is better to think of entropy as the number of ways that '...

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Two reactions which speed each other up
10 votes

Have a look at some non-linear reaction schemes such as gas phase radical chain reactions that may lead to explosion. The propagation steps in the hydrogen/bromine gas phase reaction do what you ...

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Pi molecular orbitals of polyenes
10 votes

I should like to extend @orthocresol's clear answer and so obtain results for any size of conjugated system. Starting with the determinant in $\alpha -E $ and $\beta$. $$\begin{vmatrix} \alpha - E &...

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Overall effective activation energy for parallel reactions
10 votes

The effective activation energy you quote is the weighted sum of the two individual ones to P and Q. The fraction of molecules going to P is $k_1/(k_1+k_2)$ and so $k_2/(k_1+k_2)$ go to Q. Thus the ...

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What is the seventh plane of symmetry in benzene?
10 votes

Have a look at this picture which shows some of the symmetry axes and planes. You should be able to see other similar ones. Taken from www.molecule-viewer.com.(You can also use this site to practice ...

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What is the physical basis for Hund's first rule?
10 votes

Some understanding can be gained by looking at the symmetry of the orbital parts of the wavefunctions involved. The total wavefunction for electrons must be anti-symmetric with respect to exchanging ...

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Rate Constant Units and Eyring Equation
9 votes

You are correct in your analysis that the rate constant is first order with units $\ce s^{-1}$. The problem you have is that the Eyring equation you use is not complete as I try to show below after ...

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Is it "common practice in Fourier transform spectroscopy to multiply the measured interferogram by an apodizing function"? If so, why?
9 votes

If there is a noisy signal that decays away, such as the FID in an NMR experiment, the signal to noise ratio is larger at shorter times than at longer ones where the noise remains but the signal is ...

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What is the physical significance of molecular partition function?
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9 votes

The partition function $q=\sum_i\exp(-E_i/k_BT)$ in your question can be regarded as the effective number of levels accessible to the molecule at a given temperature. It also means that in the ...

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