A method of studying bonds between atoms in which electrons' motions are defined as being all around the molecule, though influenced by nuclei.

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HSAB: alkyl halides & carbonyls LUMO energy flaw

Hard electrophiles have LUMO of higher energy than soft electrophiles. Alkyl halides are considered to be soft electrophiles while carbonyl compounds are considered to be hard electrophiles. The sigma ...
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How should I construct the orbitals of dilithium using only pen and paper?

For a week I have lessons about quantum chemistry and I have a trouble understanding how to arrive at the good answer for the drawing of the orbital $(\ce{III})$ for $\ce{Li2}$ (see the second ...
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How does spin flipping of triplet carbenes occur?

Below is presented a page from Clayden Organic Chemistry 2ed. How and why does the spin flipping during the collision with solvent molecules actually occur?
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Mechanism for the Cleavage of Diborane?

The cleavage of diborane was presented in a recent lecture, and it was said that the borane could be cleaved unsymmetrically if sterics allowed, or symmetrically if they did not, as shown in the image ...
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Why/how is blood red? (colours of hemoglobin)

Oxyhaemoglobin is red, deoxyhaemoglobin is bluish-purple, and carboxyhaemoglobin is a cherry red colour. Evidently in the porphyrin around the iron molecule in heme there is a conjugated pi system ...
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Why does conjugation decrease energy? [duplicate]

Why does an increase in the degree of conjugation in a molecule decrease the HOMO-LUMO gap?
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51 views

How does molecular orbital theory explain CH4?

I have understood the formation of $\ce{CH4}$ by valance bond theory, but I'm having trouble understanding it through molecular orbital theory. The energy level diagram of molecular orbitals of ...
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26 views

“Exactly Equal” and “At Least” in electron excitation

When we examine IR spectra, we see troughs corresponding to absorption at exactly a specific frequency that corresponds to the energy needed to stretch certain bonds (although translational motions ...
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Trends in Single Bond Enthalpy of All Nonmetal Groups

Here I have a list of single bond enthalpies, $\ce{X-X}$, in kJ/mol, for the nonmetals arranged by group: Group 7: F: 155 Cl: 242 Br: 193 I: 151 Group 6: O: 146 S: 266 Se: 172 Te: 126 ...
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29 views

How to find the bond order of molecules like BF3,SO3,NO2+

I know that the formula for bond order is 1/2*(Nb-Na).But is there a quite simpler way than to use this formula?As the use of this formula requires the writing down electronic config according to ...
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Negative hyperconjugation stabilizes tricyclopentane derivate

As far as I know, negative hyperconjugation means, that we have electrons that are transferred from a p- or $\pi$- to a $\sigma^*$-orbital. In the lecture, we analyzed a TMS-substituted ...
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41 views

Why is there only one nodal plane in σ* molecular orbitals for homodiatomic molecules?

I know that there's one nodal plane exactly inbetween the σ* MO however why is there not three nodal planes in total? As the picture shows three areas of zero electron density (one on each P ...
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Does crystal field splitting energy Δ increase going down a group of transition metals if the ligands are weak field?

Going down a group, the metal orbitals are more diffuse so there is greter overlap between the metal and the ligand orbitals. If there are no π-interactions, i.e. with σ-donor ligands like ...
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70 views

Structure of $\ce{HNO3}$ with Molecular Orbital diagram

Take an example, $\ce{HNO3}$ The total valence of $\ce{HNO3}$ must be 24. By using formula $\pi =6\mathrm{n}+2-\mathrm{V}$ where $\pi$ is number of $\pi$ electrons, $\mathrm{n}$ is number of atoms ...
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How are SN2 transition states stabilised by adjacent double bonds and carbonyl groups?

It is my understanding that transition states in an SN2 reaction have some filled and some empty orbital character, as we have the backside of the C-X σ* beginning to fill with electrons from the HOMO ...
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80 views

What does Molecular Orbital Theory get wrong?

I am learning about MO theory in my advanced inorganic chemistry course and am starting to realize that it is truly the most accurate representation of how molecular orbitals look like, where they are ...
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29 views

mechanism in Birch reduction

at first I wondered why 2 H are added to opposite side of benzene like reaction below. because I could imagine 2 H added right next to each other or 1 C away from each other. so, looking for ...
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46 views

What is the criterion for filling up of electrons in molecular orbitals of increasing energy in simple diatomic species?

So far I have found different ways in different books. For e.g. One says "In MO electronic configuration of diatomic species up to 14 electrons, $\sigma2p_z$ orbital (considering $z$ as the ...
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Orbital and state correlation diagram for ring-closure of pentadienyl to a cyclic allyl

This is an exam question. I am to construct the orbital and state correlation diagram for one of the reaction below (disrotation or conratation). Also, I should explain whether the reaction is ...
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58 views

How do energy levels work in relation to HOMO, LUMO transition?

I've just started a basic quantum course and I'm facing many exercises regarding the transition from HOMO to LUMO. I've already mastered unidimensional problems, but I'm a bit confused in ...
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28 views

What does a molecules color have to do with its bond/orbital energies?

For example, elemental iodine is deep violet. It's sigma bond or perhaps the lone pairs are capable of absorbing all visible light frequencies except violet which is why we see it as that color. ...
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Why is tin(IV) iodide bright orange while elemental iodine is violet?

Does it have anything to with iodines polarizability? Is there a way to explain this using sigma bonding or an MO diagram?
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84 views

Confusion about atomic/molecular orbital terminology

I have come across several different orbital terms: atomic orbitals, natural orbitals, split-localized orbitals, molecule-intrinsic orbitals, quasi-atomic orbitals. I don't know what the difference ...
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56 views

Why is He2- unstable?

$\ce{He2-}$ according to MO theory should be possible with one electron in 2s sigma, but my professor said only at very low temperatures for some reason. Why only at low temperatures? What is the ...
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33 views

Anti-bonding orbitals for ionic bonds

Are there any concept of Bonding and Antibonding orbitals in the case of Ionic bonds, as there are in case of covalent bonds (electron sharing) ? OR: Is it possible to break an ionic bond by pushing ...
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Wavefunction convergence error and keywords. (Spartan)

I am currently working with Spartan software and am calculating MOs. My problem is that when I try to do calculations SCF error occures. And every time I try to put some keywords (like CONVERGE, ...
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How are the hydrogens attached to the nitrogen atom in the ammonium ion?

Naively attaching hydrogens to a nitrogen atom's electron orbitals results in a picture like this: However, there are two problems with this drawing that I can see. First, I have heard that the ...
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Determining the type of electrocyclic via MO theory

I have been working on this particular problem for a while now to determine how this particular reaction plays out. I think it is a a thermo electrocyclic ring transformation involving 6 pi ...
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76 views

Ordinary differential equation for molecule collision

My knowledge is 0 in chemistry. But, I have a good background in mathematics and computer programming. I have planed to simulate the collision of two simple molecules $\ce{Br2}$ and $\ce{H2}$ just ...
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84 views

Why HF is said to be formed from H-1s and F-2pz overlap?

According to Wikipedia, HF is formed by the interaction between hydrogen's $1s$ orbital and fluoride’s $2p_z$ orbital: In hydrogen fluoride HF overlap between the H 1s and F 2s orbitals is ...
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Can you deduce unsaturated fatty acid structure from orbital hybridisation theory?

In biology class we have seen how an unsaturated fatty acid has a double carbon bond which makes the molecule bend. In chemistry class we have seen orbital hybridisation theory and as an exercise I ...
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125 views

Why does AsH5 not exist?

Why does $\ce{AsH5}$ not exist? Is it because the s-orbital of H doesn't and can't overlap with the d-orbital of As after As makes $\rm sp^{3}d$ hybridization?Theoretically the bond can occur because ...
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64 views

How to compare the energies of sigma and pi bonds in molecular orbitals of diatomic heteronuclear species?

There is a rule for diatomic homo-nuclear species that says that if $Z<8$ then $$E(\sigma_{2p})>[E(\pi_{2p_y})=E(\pi_{2p_z})]$$ and the inverse if $Z\geq8$ and in both case we have energy of ...
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Why is conjugation not possible if there is not present alternate $\pi$ molecules according to MO theory?

I was reading this page at Chemwiki; there they were discussing conjugated-$\pi$ bond. In order to show that conjugation requires alternate double bonding, they cited the example of ...
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Jahn-Teller Distortions in Square Planar Complexes?

A Jahn-Teller distortion is predicted whenever a non-linear symmetric molecule has degenerate orbitals and has unequal electron occupation in those degenerate orbitals. Of course this most often is ...
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61 views

Is it possible to have accumulation of electron density in the inter-nuclear region even if the overlap integral is zero?

Recap: Lately, I've been studying Hückel theory where I learned about the approximation of neglecting all overlaps and making overlap matrix an identity matrix that is $S_{AB}= 0\;_; \;S_{AA}= 1$ or ...
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How can off-diagonal Hamiltonian matrix elements be nonzero when the overlap matrix is diagonal?

While studying about Hückel theory, I got accustomed to the approximation of making the overlap matrix an identity matrix; that is making the off-diagonal elements zero as $S_{AB}= S_{BA}= 0\;;$ this ...
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What is the reason behind neglecting & putting the overlap integrals $0$ in Simple Hückel Theory?

This is an excerpt about Simple Hückel Theory from Elements of Physical Chemistry by Peter Atkins: The first step that Hückel took was to ignore the $\sigma$-bonding framework and focus solely on ...
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How to determine gerade & ungerade symmetry of a MO orbital?

J.D.Lee writes in his book Concise Inorganic Chemistry: [...] An alternative method for determining the symmetry of the molecular orbital is to rotate the orbital about the line joining the two ...
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'In MO theory, electrons are treated as spreading throughout the entire molecule'- isn't it exhibited in VB theory also?

First statement: In the molecular orbital theory, the valency electrons are considered to be associated with all the nuclei in the molecule. - Concise Inorganic Chemistry by J.D.Lee. Second ...
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218 views

Why does the PH3 geometry deviate more from the trigonal planar one, than does NH3?

$\ce{PH3}$ has a more bent structure than $\ce{NH3}$. The HOMO-LUMO gap for $\ce{PH3}$ is smaller than for $\ce{NH3}$, and so the distortion from the trigonal planar geometry is said to be larger. ...
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Successful predictions of MO software?

There have been many successful predictions of MO software. Is there a website or journal dedicated to recording them all?
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56 views

Molecular Orbital Diagram for dioxygen

In the MO diagram for $O_{2}$ : Shouldn't there be 3 different $3\sigma_{g}$ Bonding MO's as each p orbital can form a $\sigma_{g}$ Bonding MO along its respective axes? In which case the ...
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How derive g and u symmetry labels for orbitals?

When asked whether a molecule has an inversion center, we "invert" the coordinates of all atoms; i.e. move each atom from its position through the center of symmetry and to a new position equidistant ...
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What will occur after excitation of electron from HOMO to LUMO?

After an electron absorbs the sufficient energy it will get excited from the HOMO to LUMO state. Does this mean that after excitation, molecules will be $50\%$ with LUMO (have one electron) and ...
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Transition of electron from HOMO to LUMO?

Molecules usually have certain number of (doubly) occupied molecular orbitals and an infinite number of unoccupied molecular orbitals but the probability of electron makes the orbitals shapes so, how ...
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254 views

Why does the mixing of sigma 2s and 2p orbitals lower the energy of the sigma 2s orbital and raise the energy of the sigma 2p orbital?

My general chemistry textbook (General Chemistry Principles and Modern Applications, Tenth Edition) says that for bonding in diatomic molecules with Z ≤ 7, orbital mixing occurs between the ...
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How does one calculate chemical shift in a gauge independent manner?

The literature on ab initio calculations of chemical shift in NMR experiments usually provide Lamb's and Ramsey's formulae as the solution. Yet the expressions explicitly depend on vector potential ...
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Geometries of 4-coordinate d8 transition metal complexes

The molecule $\ce{[PdCl4]^2-}$ is diamagnetic. Since $\ce{Pd^2+}$ is a $\mathrm{d^8}$ ion, there should be two lone pairs around $\ce{Pd}$. The molecule would therefore belong to the $\ce{AX4E2}$ ...
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MO shapes for dx2-y2 sigma bond

How do the Bonding and Anti-bonding MOs of dx2-y2 sigma bonding with another dx2-y2 look like ? I mean to ask that if the sigma bond is along the x-axis then is the shape of the lobes along y-axis ...