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Quantum Chemistry

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Example 1

Find ⟨x⟩, ⟨x2⟩, ⟨px⟩ and ⟨px2⟩ for a quantum harmonic oscillator in the ground state, then determine the uncertainty on the position and momentum. Is the product of the uncertainty on position and momentum consistent with the Heisenberg's Uncertainty Principle?

Heisenberg's Uncertainty Principle

The wavefunction of a quantum harmonic oscillator in the ground state is:

Using this wavefunction the average position and the average of the square of the position can be calculated.


The average position:

use



The average square of the position:

use

use and



The uncertainty on the position:


The average momentum:

use


The average square of the momentum:

use

use

use and



The uncertainty on the momentum:


The product of the uncertainty on the position and the uncertainty on the momentum is:


This is equal to , therefore, a quantum harmonic oscillator in the ground state is consistent with the Heisenberg Uncertainty Principle.


Example 2

Question[edit | edit source]

Starting with the wave equation of a 1D box:

The classical probability of a particle existing in the region of the box [0, 1/L] is,

Where is the length of the 1D box, is the principle quantum number, and is the position of the particle in a 1D box.

Derive an equation that calculates the probability of the particle being in the [0, 1/L] for the quantum mechanical particle in a 1D box. Is this result compatible with the correspondence principle?

Solution[edit | edit source]

Example 3

Question 3[edit | edit source]

Calculate the number of nodes for a particle in a 1D box when n=2 and n=3 when the length of the box is L=5, and give the x-intercept of the node(s).

Wavefunction for a particle in a 1D-Box

Answer[edit | edit source]

Part 1[edit | edit source]

nnodes=n-1

nnodes=2-1=1

We're looking for one x-intercept.





Use the form where





is the x-intercept

x-intercepts are 0, , 5

0 and 5 are the edges of the box and is the only node.

Part 2[edit | edit source]

nnodes=n-1

nnodes=3-1=2

We're looking for two x-intercepts.







x-intercepts are

x-intercepts = 0,

0 and 5 are the edges of the box, and are the nodes.


Example 4

Question[edit | edit source]

An electron in a 1D box emits a photon as the electron transitions to a lower energy level. If the length of the 1D box is equal to 1.0 cm, and the quantum number transition is , what is the electromagnetic radiation frequency of the emitted photon?

Solution:

The energy level of a particle in a 1D box at a specific quantum number () is,

Where is equal to Planck's constant (6.62607015 x 10-34 Js), is equal to the quantum number ( = 1, 2, 3, ...), is equal to the mass of the particle, and is equal to the length of the 1D box. For an electron, the mass is equal to 9.10938356 x 10-31 kg.

Since (assuming the mass and box length are constant), the energy level increases by a factor of 4 as the quantum number increases by a factor of 2. Therefore, if a particle in a 1D box undergoes an energy level transition, there is a difference between the initial and final quantum number energy levels. The energy level difference () of a particle in a 1D box that has undergone an energy level transition is,

Where is equal to the final quantum number, and is equal to the initial quantum number.

If , is a positive value; photon absorbed.

If , is a negative value; photon emitted.

Therefore, the energy level difference of an electron in a 1D box with a length of 1.0 cm, which has undergone a transition is,

The energy level difference for the electron which underwent a transition is equal to -3.01 × 10-33 J. Since , 3.01 × 10-33 J was emitted from the electron. If the electron underwent a transition, the electron would absorb the same amount of energy that was emitted from the transition which was 3.01 × 10-33 J.

Therefore,

The energy of a photon has a specific frequency of electromagnetic (EM) radiation, and the energy is directly proportional to the frequency. The energy of the photon is equal to,

Where is equal to Planck's constant (6.62607015 x 10-34 Js), and is equal to EM radiation frequency.

Rearranging this equation allows for the calculation of the photon EM radiation frequency,

The calculated photon energy was equal to 3.01 × 10-33 J, therefore the EM radiation frequency of the emitted photon from the transition of an electron in a 1D box with a length of 1.0 cm is,


Example 5

For a particle (assume the particle is an electron with 1 quantum number.) in a 1D box of length 5 cm, the equation of energy levels of a particle in a 1D box is,

a. If the length of the 1D box is increased to 10 cm, what is the change in the energy level of this particle in the box?

b. If the length of the 1D box is decreased to 2 cm, what is the change in the energy level of this particle in the box?

c. Explain the effect of length changes on the energy levels of particles in a 1D box.

Solutions[edit | edit source]

The energy levels of a particle in 1D box:

where h is Planck's constant equal to

Because this particle is an electron with 1 quantum number:

m is the mass of this particle equal to

n is the quantum number of the particle equal to 1

L is the length of this 1D box equal to

for this question,

a. for part a, the initial length of this 1D box: , the final length of this 1D box:

b. for part b, the initial length of this 1D box: , the final length of this 1D box:

c. Based on the calculations in part a and b, the energy level of the particles in the 1D box is decreased by the increase of this 1D box length, and the energy level of the particles in the 1D box is increased by the decrease of this 1D box length. Therefore, the energy level of the particles in the 1D box is negatively related to the length of this 1D box.


Example 6

Using the following equation calculate the ground state energies of H, He+ and Li2+, , then draw a conclusion on the effect of increasing atomic number on the energy of the ground state.


Solution:

Using the Schrödinger equation (ĤΨ=EΨ) the wave functions of the hydrogen atom have been solved. From the Schrödinger equation, the energy of the quantized states of the hydrogen atom can also be determine, as energy is the eigenvalue of the Schrödinger equation. The equation for energy for each energy level in the hydrogen atom is as follows,

Energy of Energy Levels

In the equation stated above Z is the nuclear charge, me (9.10938356 x 10-31 kg) is the mass of the electron, e (1.60217662 x 10-19 C) is the elementary charge, ε0 (8.8541878128 x 10-12 Fm-1 ) is the vacuum permittivity, ħ is the reduced Planck's constant and n is the principal quantum number.

The wave functions of the hydrogen atom can be extended to any atom with only one electron. Any atom with only one electron will have the electron in the same energy level as the hydrogen atom as electrons must be placed in the lower energy levels before higher ones. The only thing that will differ between hydrogen and other atoms with one electron is the attraction between the nucleus and the electron. As other atoms such as He+ and Li2+ have a greater number of protons in their nucleus resulting in a greater Coulombic attraction of the electron to the nucleus affecting the orbital size. As a result the energy of the ground state will differ between one electron systems with different nuclear charges.

This can be demonstrated by calculating the ground state energies for H, He+ and Li2+ and seeing how the energies differ. Starting with calculating the ground state of hydrogen where Z=1 and n=1,

Then for He+ where Z=2 and n=1,

Finally for Li2+ where Z=3 and n=1,

The ground state energy for the hydrogen atom is equal to -0.5 Hartree, for He+ it is -2 Hartree and for Li2+ it is -4.5 Hartree. The energies are increasing in a non-linear fashion which can be attributed to the Z2 term in the numerator. This means as Z increases more energy will be required to ionize a one electron atom.


Example 7

Example Problem 7[edit | edit source]

Using calculus, derive the most probable radius of an electron 1s orbital.

Solution[edit | edit source]

To determine the equation for the most probable radius of an electron in an 1s orbital we must calculate the value of r at the maximum point on the probability distribution. This is done by setting the derivative of radial probability to zero and solving for radius.

The general formula for the radial probability is.

[1]

After inserting the wavefunction of a 1s orbital we can simplify the radial probability equation as follows.

Radial Component of Wave Function of 1s Orbital

[2]

The derivative of the simplified probability function must then be calculated.

The constants can be excluded from the rest of the equation by moving them to the front of the derivative.

We see that the function is composed of the product of two smaller functions of the radius, multiplied by constants. Therefore we can apply the derivative product rule to solve for the equation of the derivative of the probability function.

Product Rule of Derivatives


Simplifying and moving the constants now will be beneficial to simplifying the final equation


After setting the derivative of radial probability to zero as seen above it can be seen that the only way that the equation can be equal to zero is if the polynomial part of the formula is equal to zero. This is the case because and the first part of the equation is constant which means it cannot be zero. Therefore...

The equation for the exact value of the most probable radius in a 1s orbital is then calculated using the quadradic equation.

Quadratic Equation

Therefore let , , and


The most probable radius cannot be a negative distance or zero meaning the numerator must be negative so that it is cancelled out by the negative value of the denominator. This means that the plus or minus operator in the numerator must be minus.


In conclusion the most probable radius for an electron in an 1s orbital is where that radius is proportional to the bohr radius and inversely proportional to the nuclear charge of the nucleus. This means that for an atom with one proton like hydrogen [H] the most probable radius is while for the helium ion [He+] which also only has one electron in the 1s orbital the most probable radius is .


Example 8

Write a question and its solution that quantitatively demonstrates the Heisenberg uncertainty principle holds for the J=0 state of a quantum rigid rotator

Background Information[edit | edit source]

Rigid rotor model[edit | edit source]

Rigid rotor model for the example molecule HCl

In quantum chemistry, the rigid rotor model is used to describe the rotations in molecules, such as HCl. The assumptions used in the rigid rotor model is that the rotating molecules is rigid, and the changes in bond length that naturally occur in a molecule (such as vibrational) is insignificant compared to the bond length re, and thus, negligible. [3]

The total energy of the system is the summation of the potential energy and the kinetic energy. The potential energy of a rigid rotor is 0 given the assumption that the rigid rotor bond length is constant. As such, the total energy of the system is equal to KE, which is equal to the angular momentum. [3] The energy level (EJ) of a linear rigid rotor model (such as HCl) is given by the equation:

[3]

In which I is the inertia, based on the reduced mass of the diatomic and bond length, and J is the quantum energy level.

Rigid rotor model for reduced mass

The rigid rotor model is 3 dimensional, and for ease of calculations, rather than 2 sets of masses that are used (m1 and m2), 1 reduced mass is used instead (μ). [3]


And in a spherical model of the rigid rotor, there are 2 variables that are used to determine position, the angels θ and ϕ, given that r is the constant bond length re.


Thus, the position of the reduced mass is given by the wave function:

Heisenberg Uncertainty Principle[edit | edit source]

The Heisenberg uncertainty principle states that the exact position and momentum of a particle cannot be determines at one given point, and the more precisely either is determined, the less certain the other would be.[4]

However, even thou the exact position and momentum cannot be calculated at a given moment, they can be related, and that relation is: [4]

In which  is the reduced Planck constant,   is the uncertainty of position, and  is the uncertainty of momentum.

However, in the rigid rotor model, this equation does not work. The Heisenberg inequality is recalculated: [4]

Example[edit | edit source]

Question: using the rigid rotor model at J=0, what is the moment of angular momentum and position? Does the rigid rotor follow Heisenberg’s principle at J=0?[edit | edit source]

In the ground state zero-point, J = 0, and thus, the energy level of the rigid rotor is 0.

Given that the energy is 0, the angular momentum is known (Lz = 0).


The probability density of the position can be obtained using the wavefunction:

Average of position = 0, as such, the variance is calculated:

Y =

at m=0 J=0

Y =

if is plotted on a sphere

Where m = integers between J and – J.

|m| = number of longitudinal node

J = number of latitudinal nodes

At ground state, J = 0, |m| = 0. Thus, probability of position is equally spread across the sphere.


at J=0,

thus

is satisfied

Thus, this shows that at ground state, J=0, the rigid rotor model follows the Heisenberg uncertainty.

References[edit | edit source]

[3] Anderson, J.M. Introduction to Quantum Chemistry, 1969, W.A. Benjamin, Inc, pg.91-100.

[4] Fayngold & Fayngold, Quantum Mechanics and Quantum Information, pp 384-388.

  1. Engel, Thomas, and Philip Reid. “Chapter 20: The Hydrogen Atom.” Physical Chemistry, 3rd ed., Pearson, Upper Saddle River, 2018, p. 478.
  2. Engel, Thomas, and Philip Reid. “Chapter 20: The Hydrogen Atom.” Physical Chemistry, 3rd ed., Pearson, Upper Saddle River, 2018, p. 468.
  3. a b c d e Anderson, J.M. Introduction to Quantum Chemistry, 1969, W.A. Benjamin, Inc, pg.91-100.
  4. a b c d Fayngold & Fayngold, Quantum Mechanics and Quantum Information, pp 384-388.


Example 9

Write a question and its solution that calculates the locations of the nodes of an electron in a 2s orbital.

The Question[edit | edit source]

The question is to find the location of the radial node in a 2s electron for a hydrogen atom. To find the node one can start by analyzing, generally, how many nodes one should expect to see in a 2s electron system. There are two equations that give the number of nodes present in an orbital, the angular node equation and radial node equations:

1. Radial Nodes:

2. Angular Nodes:

Therefore ℓ must be determined, and based on the table 1 data one can determine ℓ is equal to 0. And the n is equal to the which is 2, this comes from the number before the orbital type which tells you the principle quantum number.

Table 1: Orbitals and Quantum Number
Orbital Angular

Momentum

Quantum

Number (ℓ)

s 0
p 1
d 2
f 3

So how many nodes are there?

First analyze the number of angular nodes:

Therefore, the number of angular nodes is 0.

Radial nodes:

Therefore there is one radial node present in a 2s orbital, resulting in the question becoming where is the location of that radial node?

The Wavefunction

Now the next main step is to determine what wavefunction describes the wavefunction this scenario of the 2s electron. The wavefunction can be found online which is

In the equation the is the wavefunction for the 2s electron, 𝒓 is the radius, and is the Bohr radius.

Based on the equation we can then solve for the position of the electron. The best way to do this is to find where is the equation going to be equal to zero and what term that contains the position causes this. The first part is a constant thus won't change with the radius, the position, of the electron. So the only place that will change with the position of the electron are the 𝒓 terms. With the 𝒓 term can be any number and the term won't be zero unless the 𝒓 is approaching infinity, while the can potentially be equal to zero since it has 2 subtracted by the position term. Therefore one can set this term equal to zero and solve for 𝒓.

Therefore, we get the solution to the position of the radial node which is 𝒓 , so when 𝒓 the probability of the electron being there is 0 all around the nucleus creating a node. The has a length of 52.9 pm which means the node is 105.8 pm in radius away from the nucleus.



In Conclusion

In conclusion the approach to the problem of finding nodes for an electron in an orbital boils down to first finding the number of theoretical nodes, then determining the wavefunction, analyzing the wavefunction's variables and solving for 0. After this is all done you will have the solution to the radial node locations. Further problems can be solved as well, because they are follow up questions that are made easier to solve after finding the nodes, like the position of the electron in it's most probable state. This problems follows the solution of arranging the P= then finding the derivative of the wavefunction and then simplifying it. The final step is to find the zero points, where the 𝒓 is equal to zero which will give the most probable locations of the electron. The practical applications of finding the nodal locations can help with understanding how orbitals work which can help with making molecular orbital diagrams and SALCs that can be used to determine the way atoms and molecules bond. Other applications include understanding the energy levels of the bonds and orbitals to predict possible interactions between molecules and atoms, for research purposes and chemical engineering.


Reference

1) Branson, J. The Radial Wavefunction Solutions. https://quantummechanics.ucsd.edu/ph130a/130_notes/node233.html (accessed November 16, 2021).


By Dmitry Ivanov


Example 10

Question[edit | edit source]

Use Planck's radiation law to find the surface temperature of the Sun when its maximum intensity of EM radiation is emitted at 504 nm. Ensure the temperature units are in Kelvins.

Getting the correct function[edit | edit source]

A blackbody material is defined by its ability to absorb ALL radiation that falls onto it.[1] When the blackbody is at constant temperature, the distribution of its emission frequency can be determined by assuming its only direct relation is to temperature.[1] Furthermore, the frequency of the electromagnetic (EM) radiation can also be measured in units of wavenumber .

Planck's approach to defining was deriving for a closed form Harmonic Oscillator was based on Boltzmann's distribution.[2][3] The resulting form of Planck's law can then be applied to the question now that all parameters are known except for the parameter in question. The model below shows the radiation intensity distribution (i.e., area of the curve) of EM with respect to frequency (in Hertz; Hz) and temperature (in Kelvins; K).

Planck's law for a blackbody material predicts the behaviour of its quantitative properties (such as frequency, wavelength, and temperature) when the environment are in extreme conditions.[3]

Planck's Law of BlackBody Radiation

Solving for the temperature[edit | edit source]

The maximum value of the independent variable of any function can be found by equating the function's derivative to zero. Only is unknown and is with respect to the independent variable, . Thus, Planck's law will be derived with respect to .

First, for simplicity, let .

The first term consists only of non-zero constants so that leaves:

Subbing back in gives:

At max intensity, . Subbing in all known values and constants (in SI units) then rearranging to solve for T will determine the surface temperature of the Sun under these specific conditions.

Known Values[3][4]
Variable Value Units

Therefore, the surface temperature of the Sun at a maximum wavelength of 504 nm is just above 5709 Kelvins.

References[edit | edit source]

  1. a b Quantum Hypothesis Used for Blackbody Radiation Law https://chem.libretexts.org/@go/page/210776 (accessed Nov 20, 2021).
  2. "Rotations and Vibrations of Polyatomic Molecules", Molecular Physics, Weinheim, Germany: Wiley-VCH Verlag GmbH, pp. 203–236, retrieved 2021-11-20
  3. a b c "1.1: Blackbody Radiation Cannot Be Explained Classically". Chemistry LibreTexts. 2020-03-18. Retrieved 2021-11-20.
  4. "Boltzmann constant | Value, Dimensions, Symbol, & Facts | Britannica". www.britannica.com. Retrieved 2021-11-21.


Example 11

Example 11[edit | edit source]

Use the 1D particle in a box model to estimate the wavelength of light required to excite an electron from a pi to pi* MO in ethene.


Solution:

The energy levels of a particle in a 1D box with a specific quantum number , are as follows.

In this equation represents Planck's constant, is the mass of the particle, and is the length of the box.

The pi electron in the double bond between the carbon atoms in ethene can be approximated to the particle in a 1D box model. This means that the mass of the particle in this question will be the mass of an electron, and the length of the box corresponds to the bond length between the carbon atoms in the molecule ethene.

Additionally, the energy equation above needs to be transformed into a equation for since the electron is moving from one energy level to another.

The change in energy between the pi and pi* MO in ethene can now be calculated knowing that the bond length between doubly bonded carbon atoms is 133pm and the mass of an electron is 9.1093856x10-31 kg. Moving from the ground state n=1 to an excited state of n=2 :

Now that the energy required to excite the electron to the pi* orbital is known, the wavelength of light can be calculated through the following equation, where c is the speed of light in a vacuum and is the wavelength of light.

The equation can then be re-arranged to solve for the wavelength of light.

By plugging in the known constants and the value for that has been calculated above, the wavelength can be found.

Therefore the wavelength of light required to excite an electron from a pi to pi* molecular orbital in ethene is 19nm.


Example 12

Write a question and its solution that shows the specific selection rule for a quantum rigid rotor.


Consider an N2 molecule with a bond length of 1.09 Å.

(a) Calculate the energy at the quantum number 3 using the specific selection rule for a rigid rotor.

Solution: This bond length is given (1.09 Å), and the reduced mass (μ) and the inertia (I) must be calculated to determine the energy at the angular momentum quantum number . The bond length and reduced mass must also be changed to SI units.

[1]

Now, calculating the moment of inertia ():

[2]

Since the molecule is linear (N2), the energy level () can be calculated with the linear rigid rotor equation:

[3]


(b) Calculate the quantum number if the transition energy is 2.4176 × 10-22J. In reference to part (a), does this value adhere to the specific selection rule? Why or why not?


The linear rigid rotor equation must be rearranged into linear form to solve for .

To solve this relationship, the quadratic formula must be utilized:

Rigid rotor quantum numbers cannot be negative, ∴ .

This transition adheres to the specific selection rule for a quantum rigid rotor because the change in rotational quantum number is .




(c) Calculate the quantum number if the transition energy is 1.2073 × 10-21J. In reference to part (a), does this value adhere to the specific selection rule? Why or why not?

The linear rigid rotor equation must be rearranged into linear form to solve for .

To solve this relationship, the quadratic formula must be utilized:

Rigid rotor quantum numbers cannot be negative, ∴ .This transition does not adhere to the specific selection rule for a quantum rigid rotor, because the change in rotational quantum number is not within the . Compared to part (a), it has a change of .

References[edit | edit source]

  1. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Exercises%3A_Physical_and_Theoretical_Chemistry/Exercises%3A_Aktins_et_al./12.E%3A_Rotational_and_Vibrational_Spectra_(Exercises)
  2. https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/10%3A_Fixed-Axis_Rotation__Introduction/10.06%3A_Calculating_Moments_of_Inertia
  3. Anderson, J.M. Introduction to Quantum Chemistry, 1969, W.A. Benjamin, Inc, pg.91-100.


Example 13

Write a question and its solution that shows the specific selection rule for a quantum harmonic oscillator

Calculate the energy for the vibrational transitions and . If they have the same energy gaps comment on why?

The energy of the vibrational transitions from from and have the same energy gap . This is because of the specific selection rule for the quantum harmonic oscillator. The rule states that the molecule is only allowed to move up or down one vibrational energy level for the transition to occur. If the molecule diverges from the rule then it is considered an overtone, and these are unlikely to occur.


Example 14

Show using calculus the most probable position of a quantum harmonic oscillator in the ground state (n=0)

Question:


What is the most probable position of a quantum harmonic oscillator at the ground state? Calculate this using the probability density equation to find the most probable position at n=0.

Probability distribution



Solution:

The Hermite polynomial at n=0 is:

The normalization factor at n=0 is:

α is a constant and is equal to:

The probability distribution at n=0:

The most probable position is when the maximum probability distribution is:

Applying this partial derivative to the probability distribution gives:

The constants can be taken out of the derivative:

The derivative gives:

Since it is equal to zero the constants can be divided out leaving:

Since all of the parts are multiplied they can be divided out leaving:

The point where the probability distribution is at a maximum for the ground state of n=0 for the quantum harmonic oscillator is 0.


Example 15

Write an example question showing the determination of the bond length of CO using microwave spectroscopy

Example 15:[edit | edit source]

Deriving the Required Equations[edit | edit source]

When a photon is absorbed by a polar diatomic molecule, such as carbon monoxide, the molecule can be excited rotationally. The energy levels of these excited states are quantized to be evenly spaced. The distance between each rotational absorption lines is defined as twice the rotational constant which can be measured via the following equation:

[1]

h = plank's constant = 6.626 ᛫10−34 J ᛫ s

c = speed of light = 2.998᛫108 m ᛫ s-1

I = moment of inertia


The energy to required to rotate a molecule around its axis is the moment of inertia . It can be calculated as the sum of the products the masses of the component atoms and their distance from the axis of rotation squared:

[2]

Working it out for an heterogeneous diatomic molecule:


The distance from the atom to the center of mass cannot easily be measure, however, by setting the origin at the center of mass equation can be derive for the two values that uses the bond length as a variable:

Substituting these equations into the moment of inertia equation:


This equation can be simplified further if we imagine the rigid rotor as a single particle rotating around a fixed point a bond length away. The mass of this particle is the reduced mass of the two atoms that make up the diatomic molecule:

[3]

Simplifying the previous moment of inertia equation, we get:

Solving for Bond Length[edit | edit source]

From here we have everything we need to be able to determine the bond length of a polar diatomic molecule such as carbon monoxide.

First, we must solve for the moment of inertia using the rotation constant:


As explained earlier, the rotational constant can be determined by measuring the distance between the rotational absorption lines and halfling it. In the case of the rotational constant is m-1 [4]. Plugging this value in we can determine the moment of inertia:

kg ᛫ m2


Now that we know inertia, we can rearrange the equation we derived earlier in order to determine the bond length:


The exact atomic mass of is 12.011 amu and is 15.9994 amu [5]. As such the reduce mass is calculated to be:

amu

amu ᛫


Plugging in the reduce mass back into our equation we can finally solve for the bond length of a carbon monoxide molecule:

References[edit | edit source]

  1. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Exercises%3A_Physical_and_Theoretical_Chemistry/Exercises%3A_Aktins_et_al./12.E%3A_Rotational_and_Vibrational_Spectra_(Exercises)
  2. https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/10%3A_Fixed-Axis_Rotation__Introduction/10.06%3A_Calculating_Moments_of_Inertia
  3. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Exercises%3A_Physical_and_Theoretical_Chemistry/Exercises%3A_Aktins_et_al./12.E%3A_Rotational_and_Vibrational_Spectra_(Exercises)
  4. https://webbook.nist.gov/cgi/cbook.cgi?ID=C630080&Mask=1000
  5. https://www.angelo.edu/faculty/kboudrea/periodic/structure_mass.htm


Example 16

Write an example question showing the calculation of the frequency of EM radiation emitted when a HCl molecule transitions from the J=1–>0 rotational state.


Example 16: Giving the bond length re=1.27Å, find the frequency of the EM radiation emitted when a HCl molecule transitions from the J=1→0 rotational state

Given the reduced mass (μ) and inertia (I), the energy can be found and subsequently be used to find frequency of the transition.

Note that for rotational states for J=1→0, ΔE is not required as the transition states are in the ground state.

Inertia can be found by using reduced mass of the molecule and their bond length(re = 1.27Å):

where reduced mass can be calculated by the following:

, where and

Convert to SI units:

, thus

Note that the bond length must also be in SI units,

1Å= 1×10-10m

Bond length = 1.27×10-10m


Inertia can now be calculated:


After calculating the inertia, energy can be found:

and , where h is the Planck's constant.

Frequency of the transmission can therefore be found:


Example 17

The force constant of HCl is determined computationally to be 480 N/m. Given this information find the frequency of EM radiation required to excite the HCl molecule from its ground state to its first excited state.

Solution[edit | edit source]

Given the bond constant (k) of HCl , we use the relationship between fundamental frequency and bond constant to find the bond constant.

where is the bond constant.

is the reduced mass of HCl.

To find the reduced mass of HCl the masses of H and Cl are multiplied and divided by the sum of the masses.

For HCl the reduced mass is calculated as

convert to the SI unit of Kg

To find the fundamental frequency

After finding the fundamental frequency, the Energy at different quantum levels can be found by

For the ground state i.e.

For the first excited state i.e.

The difference in energy between the two states is

and Energy is defined as Planck's constant multiplied by frequency


Example 18

Write an example question showing the determination of the force constant of CO using IR spectroscopy.

Question[edit | edit source]

Using IR spectroscopy, calculate the force constant of carbon monoxide (CO) and report the value in kN/m.

Solution[edit | edit source]

To solve this question, the IR spectrum of CO is required. The website, webbook.nist.gov/chemistry/ is a database that is available to the public for access to IR spectra. If you are unfamiliar on acquiring an IR spectrum, follow these steps: with the nist website open, go to Search Option -> Name -> Enter: Carbon Monoxide -> Select: IR Spectrum -> Change Transmittance to Absorbance -> Finally, take a screen shot.

The CO IR spectrum earlier acquired is a low resolution rotational-vibrational spectrum. In IR spectrum's, the fundamental frequency can be determined by utilizing the P-branch, Q-branch, and R-branch. Remember that the Q-branch is pure vibrational, which is forbidden, therefore the peak does not exist and is located between the P-branch and R-branch.

Using the website, apps.automeris.io/wpd/ will allow for an accurate collection of data points from the image of the CO IR spectrum taken earlier. For low resolution spectrums, locate the R-branch and P-branch, use the peak maxima as the data point. If you are unfamiliar on acquiring the data points from a image, follow these steps: Open the website -> Load Image -> Choose File -> Open -> 2D(X-Y) Plot -> Align Axes -> Plot Known -> Complete -> Insert Known Values -> Ok -> Add Points -> View Data.


Equation 1: Using the data points from the image, the fundamental frequency can be determined by the following relationship.

Where,

is the fundamental frequency

is the highest wavenumber

is the lowest wavenumber


Equation 2: The following equation is used to relate the fundamental frequency to the force constant.

Where,

is the fundamental frequency

is the speed of light

is the force constant

is the reduced mass


Equation 3: The reduced mass equation is required for relating the fundamental frequency to the force constant. Remember to use exact masses, and since the isotopes were not specified, that means we assume the most abundant version is to be used.

For oxygen:

For carbon:

Where,

is the reduced mass

is the exact mass 1

is the exact mass 2


Equation 4: Rearrange equation 2 to solve for the force constant.


The fundamental frequency of CO from IR spectra is determined using equation 1:


The reduced mass of CO is calculated using equation 3:

The force constant of CO is calculated using equation 4:

Answer: Using IR spectroscopy, the force constant of carbon monoxide was determined to be 1.87 kN/m.


Example 19

Write an example question showing the calculation of the frequency of EM radiation required to excite an H atom from ground to n=2 electronic state.


Example 20

An electronic transition of H atom from an energy level to the ground state is observed with a corresponding wavelength of 102.5 nm. Determine the initial state of the electron from which transition has occurred.

The Rydberg's phenomenological equation will be used to solve the energy transition problem

Rydberg's phenomenological equation for hydrogen

where the is Rydberg constant for hydrogen and is equal to 109737 . is the final energy level, is the initial energy level for hydrogen transition. Both and are integers and .


The wavelength is given as 103nm

Concert to SI units:


The H atom's transition is from unknown energy level to the ground state.

So we can know the final state is the ground state which means


The now can be calculated:


Because the energy level always

Therefore, the initial state of the electron from which transition has occurred is energy level 3.


Example 21

Write a question and its solution that shows the calculation of the vibrational force constant of H19F. Show what the force constant would have to be for HF to absorb visible red light instead of IR light.

If H19F has a vibrational fundamental wavenumber of 4141.3 cm, What is the bond force constant? What would the force constant need to be to absorb visible red light instead of IR light? H has a n exact mass of 1.00782 u, and 19F has an exact mass of 18.998403 u.

Solution:

First, calculate the reduced mass for H19F.

Now, solve for the force constant:

Therefore, the bond force constant is 967 N m-1.

Red light is around 700 nm, or 700×10-9 m. Converted to wavenumber, that is 14285.71 cm-1. Substituting this in,

Therefore, the bond force constant would have to be 11506 N m-1 to absorb visible light.


Example 22

Make a table showing all the absorption transitions for the first 5 rotational states of HCl. Include a column that shows which are allowed and which are forbidden.


Example 23

Create a table showing all the possible absorption transitions between the first 5 vibrational states of HCl. Include columns for the energies and frequencies. The last column should indicate if the transition is allowed or forbidden.

The simplified diatomic quantum harmonic oscillator models the vibration of a chemical bond as the displacement of a reduced mass on a spring fixed to an origin point. The displacement of the reduced mass is measured with respect to the equilibrium bond length (x=0).

Solution:

The vibrational frequency of a linear diatomic molecule depends on its reduced mass and vibrational spring constant (k). The spring constant for 1H35Cl was measured and reported as 515.825 N/m.[1]

The reduced mass was calculated as follows:

The reduced mass was converted to SI units as follows:

The fundamental vibrational frequency was calculated as follows:

The fundamental vibrational frequency was converted to wavenumbers as follows:

Vibrational energy levels for the quantum harmonic oscillator (where h is Planck's constant, and ω is frequency).

The energy for a given vibrational state can be calculated as follows:

For example, the energy of the vibrational ground state:

The transition energy between the ground state and the first excited vibrational state was calculated as follows:


According to the specific selection rule, vibrational transitions are only allowed between adjacent energy levels (where ). Consequently, due to the quantization of energy levels, vibrational transitions allowed by the specific selection rule are equivalent in energy and frequency, such that . Conversely, overtone transitions are those for which . As such, overtone frequencies can be calculated as integer multiples of the fundamental frequency. For example, the vibrational frequency for the transition between the ground state and the second excited vibrational state was calculated as follows:

Similarly, the wavenumber for the transition between the ground state and the third excited vibrational state was calculated as follows:

Finally, the energy for the overtone transition between the first and fourth excited vibrational states was calculated as follows:

However, because , this transition is forbidden .

Frequency, wavenumber, and energy values for transitions between the first five vibrational energy levels of 1H35Cl.
Transition Frequency (s-1) Wavenumber (cm-1) Energy (J) Allowed/Forbidden
1←0 8.96237·1013 2989.52 5.93853·10-20 Allowed
2←0 1.79247·1014 5979.05 2.37541·10-19 Forbidden
2←1 8.96237·1013 2989.52 5.93853·10-20 Allowed
3←0 2.68871·1014 8968.57 5.34468·10-19 Forbidden
3←1 1.79247·1014 5979.05 2.37541·10-19 Forbidden
3←2 8.96237·1013 2989.52 5.93853·10-20 Allowed
4←0 3.58495·1014 11958.1 1.18771·10-18 Forbidden
4←1 2.68871·1014 8968.57 5.34468·10-19 Forbidden
4←2 1.79247·1014 5979.05 2.37541·10-19 Forbidden
4←3 8.96237·1013 2989.52 5.93853·10-20 Allowed
5←0 4.48119·1014 14947.6 1.48463·10-18 Forbidden
5←1 3.58495·1014 11958.1 1.18771·10-18 Forbidden
5←2 2.68871·1014 8968.57 5.34468·10-19 Forbidden
5←3 1.79247·1014 5979.05 2.37541·10-19 Forbidden
5←4 8.96237·1013 2989.52 5.93853·10-20 Allowed

References:


Example 24

Write a question and its solution showing the probability of a particle being located in the bottom left quadrant of the ground state of the particle in a square.

Background Information[edit | edit source]

Particle In a Square (2D)[edit | edit source]

The particle in a square model is where a particle in confined to a restricted

region inside a 2D box. For this model we consider a particle which is free to move

anywhere inside the x,y dimensions but cannot exist outside of the 2D box where it would

have infinite potential energy.

The conditions for the potential energy are meaning that the particle spends 100% of its time within the walls of the box.

Since the particle in a square is a 2D model the wave function must take into account the x and y direction ;

The wave function for a particle in a square

The states the quantum system is in are defined by the quantum numbers

In quantum energy levels only discrete energy levels are possible. In this model the particle cannot be motionless meaning the lowest possible energy level must be non-zero (n=1). Since the particle is always moving even in the ground state the particle has a a non- zero energy level. Therefore the ground state of a particle in a box is . In quantum mechanical systems there is a presence of nodes where in states above the ground state for the 2D model there are nodal lines that divide the square. As the quantum numbers increase so does the amount of nodal lines as . The sign of the wavefunction changes between the nodes and has a value of zero on the nodes.

Probability Distributions[edit | edit source]

The probability (P) of a particle being found in an interval can be found by integrating the probability distribution (P(x))

The probability of finding a particle in the range