Introduction to Computational Quantum Chemistry
Lesson 06: Frequency Analysis (IR Spectra)
(Prepared by Radek Marek Research Group)
Lesson 06 - Frequency Analysis (IR Spectra) 1
Quick Review, Molecular Motions
despite the typical graphical display of molecular
structures, molecules are highly flexible and undergo
multiple modes of motion over a range of time-frames
motions involve rotations, translations, and changes in
bond lengths, bond angles, dihedral angles, ring flips,
methyl bond rotations.
infrared (IR) spectroscopy: based on IR absorption by
molecules as undergo vibrational and rotational transitions.
(Prepared by Radek Marek Research Group)
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Quick Review, Infrared (IR) spectroscopy
wide range of types of electromagnetic radiation in nature.
(Prepared by Radek Marek Research Group)
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Quick Review, Infrared (IR) spectroscopy
IR radiation is in the range of 12, 800 − 10cm−1
or λ = 0.78 − 1000µm
rotational transitions have small energy differences
≤ 100cm−1
, λ > 100µm
vibrational transitions occur at higher energies
rotational and vibrational transitions often occur together
typical IR spectrum for Organic Molecule
(Prepared by Radek Marek Research Group)
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Quick Review, Infrared (IR) spectroscopy
potential energy resembles classic Harmonic Oscillator
(Prepared by Radek Marek Research Group)
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IR Active Vibrations
in order for molecule to absorb IR radiation:
vibration at same frequency as the source and also, must have a
change in its net dipole moment as a result of the vibration
(Prepared by Radek Marek Research Group)
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Activity I: Calculation of Vibration Frequencies
using Gaussian, performed calculations on these three molecules:
Toluene, 2-Propanone and 2-Propanol
Geometry Optimization and Frequency Calculations using:
HF/6-31G(d) for Opt and Freq
B3LYP/6-31G(d) for Opt and Freq
MP2/6-31G(d) for Opt and Freq
(Prepared by Radek Marek Research Group)
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Activity II: Visualization of In Silico IR Spectra
open Gabedit
under Tools Menu open IR Spectrum and read from Gaussian output
file
you will be redirected to XY GabeditPlot, chose Lorentzian lineshape
and set the wavenumber to cm−1
.
the partial set up should something look like this:
(Prepared by Radek Marek Research Group)
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Activity II: Visualization of In Silico IR Spectra
open the Display Vibration menu
right click and go to Animation > Vibration > Read a
Gaussian output file
you can see the individual Frequencies and IR Intensities
as well as Play the actual vibrations on the screen.
compare the In Silico quantitative values and the IR
spectra to the experimental spectra presented in the
following slides
how are the three methods [HF, DFT(B3LYP), and MP2]
differ when it comes to producing the molecular vibrational
values? relative to the experiement which method is
accurate?
(Prepared by Radek Marek Research Group)
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Toluene IR Spectra
(Prepared by Radek Marek Research Group)
Lesson 06 - Frequency Analysis (IR Spectra) 10
2-Propanone IR Spectra
(Prepared by Radek Marek Research Group)
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2-Propanol IR Spectra
(Prepared by Radek Marek Research Group)
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Assignment - Further Exploration/Meditation ;)
using the each respected optimize geometries of the previous job
performed calculations on these three molecules:
HF/6-311G(d,p) for Opt and Freq
B-LYP/6-311G(df,p) for Opt and Freq
MP2/6-311G(d,p) for Opt and Freq
compare the results with the experimental IR Spectra
is there an improvement?
are the results method dependent or basis set dependent?
(Prepared by Radek Marek Research Group)
Lesson 06 - Frequency Analysis (IR Spectra) 13
Conclusions
it’s been an established fact that the computed quantum chemical
harmonic vibrational frequencies are typically larger than the values
observed experimentally.
a major source of this disagreement is the neglect of
anharmonicity effects in the theoretical treatment.
errors also arise because of incomplete incorporation of electron
correlation and the use of finite basis sets.
thus one be careful when it comes to choosing a method of calculation
and basis sets while balancing with the size of the molecule being
considered.
several studies suggested that a scaling factors for the vibrational
frequencies of a particular theoretical procedures must be applied and it
gave improve results.
(Prepared by Radek Marek Research Group)
Lesson 06 - Frequency Analysis (IR Spectra) 14
END
(Prepared by Radek Marek Research Group)
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