What is Molecular orbital theory: Definition and 31 Discussions

In chemistry, molecular orbital theory (MO theory or MOT) is a method for describing the electronic structure of molecules using quantum mechanics. It was proposed early in the 20th century.
In molecular orbital theory, electrons in a molecule are not assigned to individual chemical bonds between atoms, but are treated as moving under the influence of the atomic nuclei in the whole molecule. Quantum mechanics describes the spatial and energetic properties of electrons as molecular orbitals that surround two or more atoms in a molecule and contain valence electrons between atoms.
Molecular orbital theory revolutionized the study of chemical bonding by approximating the states of bonded electrons—the molecular orbitals—as linear combinations of atomic orbitals (LCAO). These approximations are made by applying the density functional theory (DFT) or Hartree–Fock (HF) models to the Schrödinger equation.
Molecular orbital theory and valence bond theory are the foundational theories of quantum chemistry.

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  1. E

    Chemistry Molecular orbital theory book reccomenations

    Hey guys, I decide I need to learn the mathematics of molecular-orbital theory, to build on the qualitative approach of my chemistry coursee. To do this I also first need to study single-electron systems and then many-electron systems, the Born-Oppenheimer approximation, and relevant topics...
  2. WMDhamnekar

    MHB Molecular orbital theory question about energy level diagrams

    Hello, The sigma $(\sigma)$ molecular orbitals are symmetrical around the bond-axis while pi $(\pi)$ molecular orbitals are not symmetrical. For example, the linear combination of 1s orbitals centered on two nuclei produces two molecular orbitals which are symmetrical around the bond-axis. Such...
  3. Amitkumarr

    Chemistry HOMO-LUMO energy gap of compounds

    According to me Option A is a correct option because as the energy difference between two atomic orbitals increases, their interaction, and thus the value of E (or E*) decreases. Here, E is the energy difference between the bonding molecular orbital and the atomic orbital of lower energy , while...
  4. cookiemnstr510510

    Frontier Molecular Orbital Theory Problem

    Summary: How do I draw the HOMO of HO-. The problem asks about the HOMO of HO- interacting in different ways with the LUMO of H-Cl. I cannot proceed with the question without figuring out how to draw the HOMO of HO-. I have attached pictures. Hello All, I have been struggling with this problem...
  5. sams

    Calculating the Molecular Orbitals of a Molecular State

    I am performing ab initio calculations on a heteronuclear diatomic compound using the MRCI method on the MOLPRO quantum chemistry package. I obtained the molecular states of the compound but I was wondering how to find the molecular orbital configuration of each electronic state in addition to...
  6. prakhargupta3301

    Is Molecular Orbital Theory really helpful?

    Who am I kidding? Of course it is. But, everywhere I look, the series of increasing levels of orbitals is till σ*2pz only That's all. So if I need to find bond order of , say, some molecule with greater number of e- like BF3 with 24 electrons, how do I proceed?
  7. A

    Term symbol of a molecule in an excited state

    Homework Statement Considering the molecule of ##C_2^+## in an excited state with valence configuration ##2\sigma_g^2## ##2\sigma_u^2## ##1\pi_u^2## ##3\sigma_g^1##. Finding all the possible term symbol. Homework Equations Term symbol ##^{2S+1}\Lambda## ##L=0 \rightarrow \Sigma## ##L=1...
  8. Wrichik Basu

    Atomic Books on Molecular Orbital Theory

    I need good books on Molecular Orbital Theory. The book(s) should have the basics, and the level should not be less than graduation. I have Quantum mechanics books, but they don't throw any light on this. My course books are rubbish, only mentioning the name of the theory and some illogical...
  9. A

    Hybridization, Antibonding Orbitals and All that Jazz

    This going to be a long post ! Orbital Dilemma: i) Why there is opposite phase orbital for s-orbital? [ Couldn't find any representation on the web except for a pic in Ian Fleming's Molecular Orbitals and Organic Chemical Reactions,pg3) *I assert no right over the image,image is referenced...
  10. T

    Energy gap between orbitals and stability

    We know that the lower the energy gap between bonding and antibonding orbital the easier it is to break the bond making it unstable. However, in a conjugated double bond, the overlapping of wavefunctions cause the energy gap between the bonding and antibonding orbital go down. Then why...
  11. Ron19932017

    I Ionic bonding from a quantum mechanics point of view

    Hi all, I have a question about the quantum nature of ionic bonds. I know some ideas about molecular bonding which can combine two ground state of H atom to get a bonding state and an anti-bonding state which in the case of H2 molecules it correspond to the covalent bonding (sharing of...
  12. sams

    Coefficients of Secular Equations

    Dear Everyone, I would like to ask a question regarding the evaluation of the coefficients in Linear Combination of Atomic Orbitals (LCAO). In Molecular Quantum Mechanics book authored by Peter Atkins and Ronald Friedman (Fourth Edition ISBN 0199274983), we are trying to calculate the...
  13. HelloCthulhu

    Breaking van der Waals molecules with magnetic fields

    I was researching the relationship between magnetic dipoles and chemical bonding and I came across a very interesting paper. I'm hoping some of you can shed some light on how magnetic fields are making dissociation of a molecule possible and how to measure it...
  14. G

    Problem in basic equalities for Huckel energy

    http://vallance.chem.ox.ac.uk/pdfs/VariationPrincipleNotes.pdf In the proof above I need to understand why: $$S_{ij}=S_{ji}$$. Which is the same as proving $$\int f_i f_j dg=\int f_j f_i dg$$ (I) Not sure about what I should call the variable for so I called it g. Can someone prove this...
  15. jdawg

    What Does Molecular Orbital Theory Say About the Overlap of Atomic Orbitals?

    Hey! So I'm a little fuzzy on my understanding of MO theory. One question I had on my study guide said that according to MO theory, overlap of two s atomic orbitals produces _________. I know the answer is one bonding molecular orbitals and one antibonding molecular orbital. Why is this? Also I...
  16. Robsta

    Why Does the Matrix Element <2 0 0|z|2 1 0> Equal -3a0 in Hydrogen?

    Homework Statement Show that for hydrogen the matrix element <2 0 0|z|2 1 0> = -3a0 where a0 is the Bohr Radius. On account of the non-zero value of this matrix element, when an electric field is applied to a hydrogen atom in its first excited state, the atom's energy is linear in the field...
  17. N

    Molecular Orbitals: Understanding & Rules

    My general understanding of Molecular Orbitals is as follows: N atoms come together to form N molecular orbitals. N/2 of the orbitals will be spatially asymmetric with symmetric spins.This corresponds to the antibonding orbital. N/2 will be spatially symmetric with antisymmetric spins. This...
  18. R

    Chemistry Molecular Orbital Diagram for Tetrahedral Compound

    Homework Statement I'm trying to construct a molecular orbital correlation diagram for a tetrahedral compound [NiX4]2- considering the ligand pi-orbital basis. I've already constructed a diagram wherein the only the sigma-orbital basis is considered. In that diagram, I had 12 electrons for the...
  19. K

    QM questions: MO theory orbital combination

    1) When we learn about MO theory, we learn that there are different ways to combine atomic orbitals to find the molecular orbitals for a molecule. However, going back to the fundamental/physical meaning of orbitals, orbitals are the wavefunctions. Wavefunctions do not have any physical meaning...
  20. M

    Molecular Orbital Theory Question about bonding/antibonding orbitals

    Hi I spotted on an MCAT book I am studying off of that the shading of the lobes(from a figure of p and sp3 orbitals) in an orbital represents the direction of spin for the electron and that in order for the electron density to overlap, the electrons must have the same spin. Firstly, I thought...
  21. M

    Chemistry Molecular orbital theory: assigning electrons

    Hello, Is it true that according to molecular orbital theory valence electrons in molecules are not assigned to individual atoms? Does this mean that even lone pairs are not assigned to inidividual atoms in a molecule , but rather assigned to a specified area across the molecule? Thanks!
  22. R

    Chemistry Chemistry: Molecular Orbital Theory and Conjugation

    I am working my way through Ian Fleming's 'Molecular Orbtials and Organic Chemical Reactions' and I am having trouble with one of the exercise questions. (Ch2 Q1) It reads: Given that two pi bonds conjugated together have a lower energy than two separate pi bonds and a C-H bond conjugated with...
  23. 1

    Molecular Orbital Theory: Explaining a "Dot" Representing No Interaction

    Question on Molecular Orbital Theory (and the "dot" that represents no interaction) I have a question about the "dot" that represents no interaction between orbitals. For example, in the molecular orbitals of H3, there is the lowest energy molecular orbital that has two bonding interactions...
  24. R

    Question about molecular orbital theory

    I know that the sigma 2p bonding orbital could be less/higher in energy than the pi 2p bonding (based on if it is C2,N2,B2 etc), but Why is the sigma 2p antibonding orbital always higher in energy than the pi 2p antibonding one?
  25. T

    A question on Molecular Orbital theory

    My question is as follows. When (qualitatively at least) constructing molecular orbitals for a given molecule, a few "rules" are used to construct them. I'm wondering why the "rules" exist at all. For example, it is given that orbitals must have "similar energies" (however one quantifies...
  26. J

    Chemistry Molecular orbital theory of heterodiatomics?

    Homework Statement What determines the order of the Molecular orbitals in heterodiatomics? The Attempt at a Solution In my book, I have NO with an order of 1s 2s 3s 1p 2p 4s Where s means sigma and p means pi. Also sometimes I see in the book the ordering being 1sigma...
  27. M

    Difference between Valence bond theory and molecular orbital theory?

    I am having trouble identifying the difference between VB theory and molecular orbital theory. To me, they seem to be one and the same. In VB theory, two atomic orbitals overlap and share electrons forming a bond... Wouldn't this combination of two Atomic orbitals make a molecular orbital?
  28. A

    Asking to make sure I understand: Valence bond vs. Molecular Orbital theory.

    Hello. I am currently a freshman in college and finding the chapter on Quantum Mechanics incredibly interesting. Whether I decide to delve into this field is still up in the air, but I am having trouble fully understanding the concepts behind Molecular Orbital Theory and how it relates to the...
  29. U

    Chemistry Molecular Orbital Theory problem

    According to molecular orbital theory, which of the following species is the most likely to exist? a) H_{2}^{-2} b) He_{2} c) Li_{2} d) Li_{2}^{-2} e) Be_{2} I think the answer is c), due to it having a bond order of 1, but I just wanted to make sure.
  30. B

    Understanding Molecular Orbital Theory

    Yo, I don't understand the concept of the M.O. Theory and I don't know how to draw these diagrams. Are these closely related to Hybridization drawings and resonance structures? Please help me, I'm confused with all the bonding/anti bonding symbols etc. Thank you,