Aspects of Comp Chem

8/7/2019 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

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Aspects of Comp Chem