Aspects of Comp Chem
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Transcript of Aspects of Comp Chem
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SOME ASPECTS OF
COMPUTATIONAL CHEMISTRY
MOHAMED IMRAN P K
Lecturer (SG) in Chemistry
Islamiah College, VANIYAMBADI(Affiliated to Thiruvalluvar University)
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CONCEPTS
CLASSICAL NEWTONIAN
NON-CLASSICAL- QUANTUM
MECHANICS
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Only a Non-classical concept can allow
Carbon to form 5 bonds (PlasmaChemistry Thermosphere)
Heisenbergs Uncertainty Principle and
Einstein's Special Theory of Relativity arethe concepts in Non-classical Form
Schrodingers Wave Equation, of course
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2 times 3 is 3 times 2
BUT
A times B is not B times A
of a MATRIX
This is one of the many thingsthat is addressed by
Quantum Mechanics
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Orbitals are defined by
a set of wave functionsand each Molecule has
Hundreds of Orbitals
that are classified as
sets (matrix)
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Orbitals of SYMMETRY are considered rather
than just Sigma or Pi Orbitals
The wave functions for localised structure isconstructed based on assumptions and
approximations
The MOs belonging to the same subgroup are
constrained to be mutually orthogonal while
those belonging to different subgroups are free
to overlap
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The following approximations are considered
# CNDO
# CINDO
# MNDO
# MINDO
# LCAO
# Hartree-Fock SCF
# Moller-Plasset
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Molecular orbital energies
Total Electron and Nuclear Energies
Heat of Formation Partial atomic charges (Mulliken Population analysis)
Electrostatic Potentials
Dipole moments
Bond orders and valencies
Vibrational Frequencies
Atomic Electron Densities pi and sigma
Exact stable geometry under a set of conditions
The computer does the calculations
Here are some important properties
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NEGATIVE HYPERCONJUGATION - The effect of donation of electron density from
filled Tor nT-orbitals to the symmetry adapted
W*-orbital(s) ofW-bond(s) of a molecular entityresulting in building T-character into bonds thatnominally possess only W-character. Theconsequences of the effect are, for example, inelongation of the C-F bond in the F-fluoroethyl
anion and stabilization of the antiperiplanarconformation of fluoromethylamine.
Pure Appl. Chem., Vol. 71, No. 10, pp. 1919-1981, 1999
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Hyperconjugation is T p W orT p pC
Negative HC is T or nT
p W* (C---X)
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Some of the important consequences of the negative hyperconjugation are
The shorteningof the AY bond in XnAY, where X is an
electronegative element, A is any hypervalent atom and
Y is any other atom or group attached to A
The increase in pi-orbital population of the X-atom and
hence an increase on charge of that atom
Wideningof the XAY angle e.g. 113 deg in place of 109
deg.
Lengtheningof the AX bond
And some changes in the geometrical properties
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The calculations for this system were done under the Cs symmetry
and the total charge on the Fluorine atoms are tabulated as below:
___________________________________________________
Total Charge Si-F bond length C-Si-F angle __________________________________________________
1. F -0.349 1.597 A 113.86 deg
2. F -0.358 1.595 A 110.40 deg
3. F -0.358 1.595 A 110.40 deg
__________________________________________________
Si-F bond length (theoretical) = 1.57 A
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Our present picture of Physical reality,
particularly in relation to the nature oftime,
is due for a grand shake-up evengreater, perhaps, than that which has
already been provided by present-day
relativity and quantum mechanics.Roger Penrose