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Valence-Shell Electron Pair Repulsion

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Overview

Molecular Geometries

Overview

The geometries of molecules can be predicted though an electrostatic model known as valence-shell electron pair repulsion (VSEPR).  VSEPR was developed by Ronald Gillespie and Ronald Sydney Nyholm of University College London in 1957.  In VSEPR pairs of electrons, whether they be in a chemical bond or a nonbonding lone pair, move as far apart as possible on a sphere about the central atom to minimize the electron repulsion.  Non-bonding lone pairs create slightly more repulsion than single chemical bonds, leading to bent structures that deviate from higher-symmetry structures.  In this lesson we compare experimental molecular geometries with the expected geometries from VSEPR.

Molecular Geometries

We load the QuantumChemistry package with Maple's with command

withQuantumChemistry;

AOLabels,ActiveSpaceCI,ActiveSpaceSCF,AtomicData,BondAngles,BondDistances,Charges,ChargesPlot,CorrelationEnergy,CoupledCluster,DensityFunctional,DensityPlot3D,Dipole,DipolePlot,Energy,ExcitationEnergies,ExcitationSpectra,ExcitationSpectraPlot,ExcitedStateEnergies,ExcitedStateSpins,FullCI,GeometryOptimization,HartreeFock,Interactive,Isotopes,MOCoefficients,MODiagram,MOEnergies,MOIntegrals,MOOccupations,MOOccupationsPlot,MOSymmetries,MP2,MolecularData,MolecularGeometry,NuclearEnergy,NuclearGradient,OscillatorStrengths,Parametric2RDM,PlotMolecule,Populations,RDM1,RDM2,RTM1,ReadXYZ,Restore,Save,SaveXYZ,SearchBasisSets,SearchFunctionals,SkeletalStructure,Thermodynamics,TransitionDipolePlot,TransitionDipoles,TransitionOrbitalPlot,TransitionOrbitals,Variational2RDM,VibrationalModeAnimation,VibrationalModes,Video

(2.1)

CF2H2

Difluoromethane is a member of the fluorocarbon family that made national headlines in the 1990s for their depletion of the ozone layer.  We define the molecule's geometry from experimental data from the Pubchem database

molF,−1.08900000,−0.26820000,0,F,1.08900000,−0.26820000,0,C,0,0.53630000,0,H,0,1.15100000,0.90240000,H,0,1.15100000,−0.90240000;

molF,−1.08900000,−0.26820000,0,F,1.08900000,−0.26820000,0,C,0,0.53630000,0,H,0,1.15100000,0.90240000,H,0,1.15100000,−0.90240000

(2.1.1)

 

We plot the molecule (click on molecule to rotate)

PlotMoleculemol;

 

(a) Using the Maple plot, provide the name of the geometry of CF2H2 i.e. linear, trigonal planar, tetrahedral, octahedral, etc.  Is the experimental geometry consistent with VSEPR?

 

SF4

Sulfur tetrafluoride, also known as tetrafluorosulfurane, produces toxic fluoride gas in the presence of water or moisture.  We define the molecule's geometry from experimental data from the Pubchem database

molS,0,0,0,F,0.48790000,0.88190000,1.21250000,F,−0.72570000,−1.28890000,0.54600000,F,−1.00400000,0.83310000,−0.88550000,F,1.24190000,−0.42610000,−0.87310000;

molS,0,0,0,F,0.48790000,0.88190000,1.21250000,F,−0.72570000,−1.28890000,0.54600000,F,−1.00400000,0.83310000,−0.88550000,F,1.24190000,−0.42610000,−0.87310000

(2.2.1)

 

We plot the molecule (click on molecule to rotate)

PlotMoleculemol;

(b) Using the Maple plot, provide the name of the geometry of SF4, i.e. linear, trigonal planar, tetrahedral, octahedral, etc.  Is the experimental geometry consistent with VSEPR?

 

SO3

Sulfur oxide is the primary component of acid rain.  We define the molecule's geometry from experimental data from the Pubchem database

molS,0,0,−0.00030000,O,−0.65520000,−1.29490000,0.00010000,O,−0.79390000,1.21480000,0.00010000,O,1.44910000,0.08010000,0.00010000;

molS,0,0,−0.00030000,O,−0.65520000,−1.29490000,0.00010000,O,−0.79390000,1.21480000,0.00010000,O,1.44910000,0.08010000,0.00010000

(2.3.1)

 

We plot the molecule (click on molecule to rotate)

PlotMoleculemol;

 

(c) Using the Maple plot, provide the name of the geometry of SO3 i.e. linear, trigonal planar, tetrahedral, octahedral, etc.  Is the experimental geometry consistent with VSEPR?