semi-empirical methods: where is the rest? matthew grandbois chem 381 spring 2004
TRANSCRIPT
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Semi-Empirical Methods:Where is the Rest?
Matthew GrandboisCHEM 381Spring 2004
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IntroductionIntroductionComputational chemistry is one of the fastest growing
areas of chemistry
Used to computationally determine vital information Geometry
Bond Angles, Bond Distances, Dihedral AnglesDipole MomentEnthalpy of FormationIonization Potentials
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Electronic StructureElectronic Structure
Calculations based on determining electronic structures
Different Approaches:Density Functional TheoryAb InitioSemi-empirical
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Semi-Empirical MethodsSemi-Empirical Methods
Some of the electrons are considered explicitly
Reduces computational demand of the problem
Pilar, F.L. Elementary Quantum Mechanics. Second Edition. Dover Publications, Inc. Mineola, New York, 1990, 454.
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Semi-Empirical ApproximationsSemi-Empirical Approximations
“Ignoring” of core electronsCore electrons reduce nuclear chargeIntroduction of function to model combined repulsion
due to nuclei and core electronsMinimum basis set of functions to account for
valence electronsMajority of basis functions are taken to be STOs
Jensen, F. Introduction to Computational Chemistry. John Wiley & Sons, England 1999, 81.
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Huckel ApproximationsHuckel Approximations
1931, E. Huckel showed depiction of conjugated hydrocarbons by use of quantum mechanical model which only considered pi electrons.
Applicable to chain and cyclic conjugated systems
- C = C – C = C – C = C -
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Central AssumptionCentral Assumption
Zero Differential Overlap approximationNeglects all products of basis functions depending on
the same electron coordinates when located on different atoms
How many integrals are neglected is which determines the various Semi-Empirical methods
Jensen, pg 81
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Neglect of Diatomic Differential Overlap (NDDO)
Neglect of Diatomic Differential Overlap (NDDO)
Only uses the previously mentioned central assumption
Overlap Integral
S = < A| B > = vAB
Jensen, pg 82
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Intermediate Neglect of differential Overlap (INDO)
Intermediate Neglect of differential Overlap (INDO)
In addition to NDDO:Neglects all two-centre 2 electron integrals which are
not of the Coulomb typeTo preserve rotational invariance, some integrals
must be made independent of orbital type
Jensen, pg 83
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Complete Neglect of Differntial Overlap (CNDO)
Complete Neglect of Differntial Overlap (CNDO)
Only the Coulomb one-centre and two-centre 2 electron integrals remain:
< AB | CD> = ACBD < AB | AB>
< µAB | µAB> is independent of orbital type (s or p)
Jensen, pg. 83
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ParameterizationParameterization Direct use of ZDO approximations is not useful
due to only qualitative picture of MOs 3 Methods to transform NDO approximations into
useful computational models1) Remaining integrals calculated from functional form of AO’s2) Remaining integrals made into parameters, assigned
values based on experimental data3) Remaining integrals made into parameters, assigned
values based on fitting to experimental data
Jensen, pg. 84
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Modified NDDO ModelsModified NDDO Models
MNDO
AM1
PM3
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Modified Neglect of Diatomic Overlap (MNDO)
Modified Neglect of Diatomic Overlap (MNDO)
One of first paramaterization models usedParameterizes: H, B, C, N, O, F, Al, Si, P, S, Cl, Zn,
Ge, Br, Sn, I, Hg, and Pb. Some LimitationsSucceeded by AM1 and PM3 Models
Jensen, pg. 87
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Austin Model 1 (AM1)Austin Model 1 (AM1)
Developed by Dewar at the University of Texas at Austin, 1985.
Came from systematics errors of MNDOToo high repulsion between atoms 2-3 Å apart
Parameterized for:H, B, C, N, O, F, Al, SI< P, S, Cl, Zn, Ge, Br, I, and
Hg.Some Limitations
Jensen, pg. 87
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AM1AM1
- Heats of Formation (kcal/mol )- 1,4-pentadiene: 25.2 (expt), 25.0 (Dewar), 25.2
(CAChe)- MNDOd calculations yielded 26.0- 2-propyl cation: 192 (expt), 192 (Dewar), 208
(CAChe)- Ammonium: 155 (expt), 151 (Dewar), 151 (CAChe)
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AM1 GeometriesAM1 Geometries
EtheneCC 1.339
1.326CH 1.086
0.964HCC 121.2 114.64
NitrogenNN 1.094 1.106
FuranOC1 1.362 1.431C2C3 1.361 1.526C3C4 1.431 1.522C2H 1.075 1.121C3H 1.077 1.117HC2O 115.9 107.2HC3C4 128.0 111.2
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Modified Neglect of Diatomic Overlap, Parametric Method
Number 3 (PM3)
Modified Neglect of Diatomic Overlap, Parametric Method
Number 3 (PM3)MNDO and AM1 parameters were done by hand,
limiting number of reference compoundsEssentially, AM1 with all parameters fully optimizedStill needs some human interventionParameterized for:
H, Li, C, N, O, F, Mg, Al, Si, P, S, Cl, Zn, Ga, Ge, As, Se, Br, Cd, In, Sn, Sb, Te, I, Hg, Tl, Pb, Bi, Po, and At
Additional transition metals are being developed to include d orbitals
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LimitationsLimitations
~1000 atoms, due to diagonzalization of Fock matrix
Calculations are extremely close, but not exactlyUnable to predict unknown compound typesNo guarantee to trust calculations
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AdvantagesAdvantages
Once atom has been parameterized, all possible compounds can be calculated
Ability to describe bond breaking and forming reactions
Provide methods for calculating electronic wave functions
Save on amount of time for calculations
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SummarySummary
Use of Semi-Empirical methods provides relatively-reliable, time-efficient calculations of chemical systems via minimal basis sets
Several different methods have been discussed:NDO’s, MINDO, AM1, PM3
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Thank YouThank You
Dr. Brian Moore
Augustana College