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Questions tagged [quantum-chemistry]

Quantum chemistry is a subfield of quantum mechanics. Like its parent field, quantum chemistry focuses on understanding physical phenomena occuring at the atomic scale. Quantum chemistry however is more focused on providing useful descriptions of electronic structure to aid in understanding chemical problems (e.g. reactions, spectra, dynamics, ...).

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Why is the 2s orbital lower in energy than the 2p orbital when the electrons in 2s are usually farther from the nucleus?

My chemistry book explains that even though electrons in the $\mathrm{2p}$ orbital are closer to the nucleus on average, electrons from the $\mathrm{2s}$ orbital spend a very short time very close to ...
Gordon Gustafson's user avatar
31 votes
3 answers
12k views

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

According to Hund's first rule, a set of degenerate orbitals are singly occupied first, before the second slot in any of the orbitals are populated. This is quite intuitive because electron-electron ...
Aniansh's user avatar
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25 votes
4 answers
39k views

Why does the energy gap for π - π* transitions shrink with the size of the pi-conjugated system?

Quoting from this site: As conjugated pi systems become larger, the energy gap for a π - π* transition becomes increasingly narrow, and the wavelength of light absorbed correspondingly becomes ...
user4717's user avatar
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56 votes
5 answers
10k views

How can antibonding orbitals be more antibonding than bonding orbitals are bonding?

In molecular orbital theory, the fact that a bonding and antibonding molecular orbital pair have different energies is accompanied by the fact that the energy by which the bonding is lowered is less ...
stochastic13's user avatar
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44 votes
3 answers
111k views

What is the inert pair effect?

I was reading about the p-block elements and found that the inert pair effect is mentioned everywhere in this topic. However, the book does not explain it very well. So, what is the inert pair effect? ...
radiantshaw's user avatar
21 votes
1 answer
1k views

What is occuring on the quantum level when a molecule rotates plane polarized light?

What is occuring on the quantum level when a molecule rotates plane polarized light? Also, why do enantiomers then rotate light in opposite directions? I would think that the electromagnetic waves ...
Joe's user avatar
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23 votes
3 answers
14k views

Is Hydrogen Bonding a Type of Dipole Dipole Interaction?

I understand that dipole dipole forces is due to the attraction of the different partials charges of atoms in different molecules due to their different electro-negativities. For hydrogen bonding, ...
Nanoputian's user avatar
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10 votes
2 answers
2k views

What exactly is an orbital?

What exactly is an orbital? Atomic or molecular. Is it the function that describes the behaviour of the electron? Is it the Schroedinger's equation solution, e.g., for Hydrogen atom? Is it the ...
NickyR's user avatar
  • 209
30 votes
3 answers
15k views

Why are noble gases stable

I was recently asked the question "Why are noble gases stable? with the expectation of providing an answer beyond the general explanation of "they have full valence layers" and I couldn't think of one....
Chris's user avatar
  • 409
28 votes
2 answers
38k views

What do the quantum numbers actually signify?

I know how to calculate them and such stuff, but I wanted to know what they actually signify. I have a vague idea that they have something to do with an electron's position in an atom but what do all ...
Jyotinder Singh's user avatar
25 votes
3 answers
2k views

What is charge shift bonding?

Can someone explain to me charge shift bonding? I recognize that it is proposed as another domain of bonding - one different from ionic and covalent bonding. I am also told that $\ce{C-F}$ exhibits ...
Dissenter's user avatar
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8 votes
1 answer
1k views

How does the radial distribution function of Vanadium differ from that of Calcium and how does this affect the ionic electron configurations?

When Vanadium is ionised it loses the 4s electron first, meaning that it's 3+ ion has a different electron configuration to Calcium despite it being isoelectronic. Can it be explained in terms of ...
RobChem's user avatar
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28 votes
2 answers
5k views

How to find the second order perturbation to wave function?

Today, I'm looking for how to find the 2nd perturbation to the base in Rayleigh Schrödinger Perturbation Theory (RSPT). SETUP Starting from the 2nd order perturbation in Dirac's notation: \begin{...
Another.Chemist's user avatar
27 votes
4 answers
8k views

Why don't equivalent hydrogens cause splitting in NMR?

When doing NMR spectroscopy, it is an observed fact that equivalent hydrogens do not split one another. Why don't equivalent hydrogens split each other's signals? For example, why is the NMR spectrum ...
Niels Kornerup's user avatar
20 votes
3 answers
6k views

How to calculate molecular dipole moment from a known wavefunction?

Say I have a molecular wavefunction as a set of molecular orbitals and want to calculate the molecule's dipole moment, but don't know how! I searched a lot but couldn't find any practical example. $$\...
Aug's user avatar
  • 2,141
13 votes
2 answers
11k views

How do Electrons Cross Nodes in Orbitals?

I was wondering in orbitals that have nodes such as the p orbital, how does the electron move from one lobe to the other? I know that people say it is because of the wave/particle nature of electrons ...
Nanoputian's user avatar
  • 7,412
43 votes
2 answers
289k views

Difference between shells, subshells and orbitals

What are the definitions of these three things and how are they related? I've tried looking online but there is no concrete answer online for this question.
Amuna's user avatar
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26 votes
5 answers
18k views

Physical intuition behind negative values for wave function?

So a positive and a positive wave function create a bonding orbital where the probability of finding an electron is summed while a positive and a negative create an anti-bonding orbital with a lower ...
norlesh's user avatar
  • 543
23 votes
2 answers
25k views

Derivation of the Heisenberg uncertainty principle

The Heisenberg uncertainty principle states that $$\Delta x \Delta p \geq \frac{\hbar}{2}$$ where $\Delta x$ is the uncertainty in the position, $\Delta p$ is the uncertainty in linear momentum, and ...
orthocresol's user avatar
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15 votes
3 answers
14k views

Most probable point for finding an electron in the 1s orbital of a Hydrogen atom

My professor says that the most probable point for finding an electron in a 1s orbital of a hydrogen atom is at its origin. He explains this by citing the fact that the square of the wave function ...
Anjan's user avatar
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9 votes
1 answer
8k views

Energetic Placement of Atomic Orbitals in the HCl Molecular Orbital Diagram

How are the 3p orbitals of chlorine lower in energy than the 1s orbital of hydrogen?
EJC's user avatar
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9 votes
1 answer
3k views

What is the exact definition of the radial distribution function?

I have been very confused by the radial distribution function which is often used in chemistry to predict the probability of finding an electron at a distance from the nucleus. From Atkins' Physical ...
S R Maiti's user avatar
  • 5,685
6 votes
1 answer
1k views

Physical reason for why delocalisation leads to stability?

Why does delocalization of electrons generally make compounds more stable (e.g. in carboxylate anions, where the lone pair on the negatively charged oxygen is delocalised into the C=O π* orbital)? Is ...
user avatar
-1 votes
1 answer
489 views

A question regarding excitation of electrons in atomic orbital [closed]

In Bohr's model of an atom, the formula used to find the energy between the 2 orbits and wavelength of emitted photon was valid only for single electron species like hydrogen.In the case of a multi-...
AJknight's user avatar
35 votes
1 answer
59k views

Acidity of substituted phenols

Phenol has a $\mathrm pK_\mathrm a$ approximately equal to $9.9$. When one studies the acidity of chlorophenols, one notices the following: First of all, chlorophenols are more acidic than phenol, ...
mannaia's user avatar
  • 1,756
33 votes
2 answers
16k views

Sp5 hybridization in cyclopropane?

I have never before heard/read about something as a $sp^5$ hybridization. Today, Henry Rzepa's blog post made me aware of the existance of such a bonding system. That made me search a little bit and I ...
pH13 - Yet another Philipp's user avatar
30 votes
2 answers
42k views

Why do atoms "want" to have a full outer shell?

Okay, so I know that this is about filling the orbitals of the atom, and I understand that. What I don't understand is why? For example, an Oxygen atom has 8 protons and 8 electrons spinning around it....
Mertcan Ekiz's user avatar
18 votes
1 answer
788 views

Analytical solution of the Schrödinger Equation for AB^+ systems

According to Wikipedia, there are not many problems for which the Schrödinger equation can be solved analytically. [1] The $\ce{H2+}$ ion is probably the most complex molecule that can be treated this ...
pH13 - Yet another Philipp's user avatar
17 votes
2 answers
1k views

After a unitary transformation, is Koopmans' theorem still valid?

Koopmans' theorem says that the energy of the HOMO of the Hartree-Fock orbitals is equal to the first ionization energy of whatever system is being studied. This is only approximate because it assumes ...
jheindel's user avatar
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16 votes
2 answers
26k views

Why are full and half filled orbitals the most stable?

Why are degenerate orbitals (restricted to a single spin) less stable when neither fully filled nor completely empty? Why, in most molecules, are half-filled and fully-filled shells more stable than ...
Niels Kornerup's user avatar
13 votes
3 answers
15k views

What is the difference between ψ, |ψ|², radial probability, and radial distribution of electrons?

I am very confused regarding these four terms- ψ,|ψ|², radial probability and radial distribution I know that ψ is called the wave function but is it the same as radial probability? I also know ...
Aura Sartori's user avatar
13 votes
1 answer
6k views

The "rules" for LCAOs in Molecular Orbital Theory

In our course on physical chemistry, which involves MOT, we have been taught that in the LCAO approach, the wave function for a molecule … say hydrogen ion ($\ce{H2+}$), can be approximated by a ...
transistor's user avatar
11 votes
5 answers
4k views

What actually is the Wavefunction?

I am aware that the square of the Wavefunction gives the probability density of finding an electron at a particular point in space. I have also heard that it's a complex number but since it's a ...
RobChem's user avatar
  • 9,794
10 votes
1 answer
3k views

How can two orbitals constructively and destructively interfere simultaneously?

The molecular orbital theory dictates that when two atomic orbitals form molecular orbitals, then two molecular orbitals must form (i.e number of atomic orbitals = number of molecular orbitals). For ...
RobChem's user avatar
  • 9,794
9 votes
1 answer
2k views

Why is no dynamical correlation recovered in the CASSCF method?

Configuration interaction is the only certain way to get a calculation to converge toward the exact solution of the many-body Schrödinger equation (within the approximations used for the framework of ...
Yoda's user avatar
  • 4,733
8 votes
2 answers
1k views

Trying to understand the statement: 'Resonance is not a flickering between the contributing states. '

So, the famous resonance definition: it is the weighted average of different Lewis structures.... Well, this is purely wrong. Resonance is nothing but quantum 'superposition of the wavefunctions ...
user avatar
7 votes
4 answers
1k views

On Heisenberg's uncertainty principle

Why can't we find the exact position and velocity of a particle? And why is it that if the uncertainty in position is very, very large then the velocity can be determined, and vice versa? Please ...
geek101's user avatar
  • 1,051
7 votes
2 answers
756 views

How are London Dispersion Forces generated?

While talking about the gaseous state of matter we came to the topic of London Dispersion Forces caused by the generation of a dipole in one atom which induces a dipole in another. While talking about ...
user avatar
25 votes
2 answers
7k views

What exactly is meant by 'multi-configurational' and 'multireference'?

Some topics here have touched on this before (see 1, 2, 3), but I haven't found a clear definition yet. I would like to know what exact property of the wave function these terms refer to. It would ...
snurden's user avatar
  • 1,459
21 votes
2 answers
3k views

How to calculate wavenumbers of normal modes from the eigenvalues of the Cartesian force constant matrix?

I would like to get the wavenumbers (Frequencies -- in Gaussian) from the eigenvalues of the force constant matrix (...
Roloka's user avatar
  • 339
19 votes
3 answers
3k views

Computing two-electron integrals with an STO-3G basis set

I am trying to implement a restricted Hartree-Fock calculation using an STO-3G basis set, for fun. I managed to perform this calculation where only $\mathrm{1s}$ orbitals are present ($\ce{H2}$ and $\...
user avatar
17 votes
1 answer
3k views

Born–Oppenheimer adiabaticity

The Born–Oppenheimer aproximation is said to be adiabatic. What does "adiabatic" mean? What would be a non-adiabatic approximation to the Schrödinger equation?
Another.Chemist's user avatar
16 votes
3 answers
2k views

Are there any full worked examples of DFT calculations?

I just started learning DFT and now I am totally confused. Assuming I want to use B3LYP: \begin{align} v_s\left(\textbf{r}\right) &= v_\text{ext}\left(\textbf{r}\right) + \int d^3r\frac{n\left( \...
Aug's user avatar
  • 2,141
15 votes
2 answers
459 views

What constraints are imposed on a wavefunction by the symmetry of the system?

As a follow-up to my answer here, I'd like to ask what exactly does it mean for a wavefunction to "respect the symmetry" of the system. The original context is: immediately after ionisation of $\ce{...
orthocresol's user avatar
  • 71.4k
14 votes
6 answers
10k views

What is a "hydrogen-like" or "hydrogenic" atom?

I'm studying some chemistry on my own in anticipation of the new school year and in my book, I came across the Rydberg equation for the first time. I worked through some examples and everything was ...
Melanie Shebel's user avatar
13 votes
1 answer
1k views

Why is it possible to image LUMO if these orbitals are, by definition, unoccupied?

LUMO stands for lowest unoccupied molecular orbital. If it's unoccupied, how is it possible to image it? See for example in this paper ("Pentacene imaged with STM and NC-AFM").
Sparkler's user avatar
  • 4,275
12 votes
1 answer
24k views

Is the d-block ever part of the valence electrons?

The d-block sub-shell is always one shell lower than the s-block and p-block sub-shells. Assuming only the electrons in the highest energy shells count toward the set of valence electrons (is that ...
Hal's user avatar
  • 2,325
11 votes
2 answers
5k views

The energy gap between a pi-conjugated system with (2 bonding and 1 anti-bonding orbital) and (1 bonding and 2 anti-bonding) orbitals

I asked a question previously about "why" it is the case the expanding the size of pi-conjugated systems decreases the required energy to excite an electron from a HOMO to a LUMO band: Why does the ...
user4717's user avatar
  • 487
9 votes
1 answer
3k views

Quantum Chemistry: Small imaginary frequencies

I am running a freq calculation on a geometry optimized structure using the ORCA quantum chemistry program. The output of the freq. calculation contains small neg. frequencies. Normally, these are ...
logical x 2's user avatar
  • 2,794
8 votes
1 answer
2k views

Letter codes in molecular term symbols

I'm trying to find an explanation for the letter codes (X, A, B, C, etc) when you get a term symbol like this $$X\,{}^3{\Sigma}^-_g$$ Can anyone point me to some literature that explains these ...
Arno van der Weijden's user avatar

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