© 2012 Pearson Education, Inc.
Geometria Molecular e Ligação Química
Atorvastatina cálcica
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
© 2012 Pearson Education, Inc.
Pontos importantes a serem aprendidos:
• As estruturas de Lewis não mostram o tamanho e a forma das moléculas.
• Qual é a relação entre as estruturas 2D e as 3D?
• Desenvolver o senso de geometria molecular e de como esta é governada pelos tipos de ligações químicas
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Geometria molecular é a forma geral de uma molécula, sendo descrita pelas posições relativas dos átomos que a constituem.
O que é geometria molecular?
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Valence-shell electron-pair repulsion (VSEPR):
Prediz a geometria de moléculas e ions considerando que os pares eletrônicos da camada de valência são arranjados ao redor de cada átomo de modo que eles fiquem o mais afastados possível, minimizando a respulsão eletrônica
Teoria VSEPR
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Dois pares: 180° (arranjo linear).
Três pares: 120° em um plano (trigonal planar).
Quatro pares: 109.5° (arranjo tetraédrico).
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5 pares: 3 pares no plano a plane 120° e 2 pares a 90°perpendiculares ao plano (bipirâmide trigonal).
6 pares: 90° entre si (octaedro).
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Modelos com balões de gás
MolecularGeometries
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Domínios Eletrônicos
• Cada par de elétrons é um domínio eletrônico
• Ligações múltiplas entre dois átomos contam com um único domínio eletrônico.
• Quantos domínios há no átomo A?
MolecularGeometries
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Geometrias
• Conta-se o número de domínios eletrônicos na estrutura de Lewis.
A geometria será aquela que corresponde ao número de
domínios eletrônicos
MolecularGeometries
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Geometria Molecular
• Nem sempre a geometria molecular é exatamente aquela prevista pelos domínios eletrônicos.
• Os pares não ligados não fazem parte da geometria molecular: somente os átomos ligados devem ser contabilizados
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MolecularGeometries
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Molecular Geometries
Dentro de cada domínio eletrônico, pode haver mais do que uma geometria molecular.
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Exemplo de impacto dos pares eletrônicos isolados
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Exemplo de impacto dos pares eletrônicos isolados
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Prevendo ângulos de ligação:
Valores padrões: 180°, 120°, 109.5°, quando o átomos central não apresentea pares isolados
O par isolado requer mais espaço (não contribuem para a formação da ligação química), logo estes ângulos padrões são alterados.
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Ligações múltiplas requerem mais espaço que as ligações simples, portanto:
?
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Exercício: preveja as geometrias das moléculas abaixo:
a. AsF3
b. PH4+
c. BCl3
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No AsF3 há 1(5) + 3(7) = 26 elétrons de valência; As é o átomo central:
Há 4 regiões ao redor do As: 3 ligações e um par isolado.
AsF
F
F
Pirâmide Trigonal
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B
Cl
Cl
Cl
BCl3 has 1(3) + 3(7) = 24 valence electrons;B is the central atom.The electron-dot formula is
There are three regions of electrons around B; all are bonding.The electron-pair arrangement is trigonal planar.All of these regions are bonding, so the molecular geometry is trigonal planar.
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P
H
HH
H+
PH4+ há 1(5) + 4(1) – 1 = 8 elétrons de valência; P
é o átomo central atom.
Tetraédrica
?
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Preveja as geometrias de:
a. ICl3b. ICl4-
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I
Cl
Cl
Cl
ICl3 há 1(7) + 3(7) = 28 elétrons de valência. I é o átomo central:
Bipirâmide trigonal
T-shaped
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-
I
Cl
Cl
Cl
Cl
ICl4- há 1(7) + 4(7) + 1 = 36 elétrons de valência e I o átomo central:
octaédrico
Quadrado “planar”
MolecularGeometries
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Moléculas Maiores
Discute-se a geometria de um átomo em particular do que da molécula inteira.
MolecularGeometries
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Sample Exercise 9.3 Predicting Bond Angles
Analyze We are given a Lewis structure and asked to determine two bond angles.Plan To predict a bond angle,we determine the number of electron domains surrounding the middle atom in the bond. The ideal angle corresponds to the electron-domain geometry around the atom. The angle will be compressed somewhat by nonbonding electrons or multiple bonds.Solve In H — O — C , the O atom has four electron domains (two bonding, two nonbonding). The electron-domain geometry around O is therefore tetrahedral, which gives an ideal angle of 109.5°. The H — O — C angle is compressed somewhat by the nonbonding pairs, so we expect this angle to be slightly less than 109.5° . To predict the O — C — C bond angle, we examine the middle atom in the angle. In the molecule, there are three atoms bonded to this C atom and no nonbonding pairs, and so it has three electron domains about it. The predicted electron-domain geometry is trigonal planar, resulting in an ideal bond angle of 120°. Because of the larger size of the C = C domain, the bond angle should be slightly greater than 120°.
Solution
Eyedrops for dry eyes usually contain a water-soluble polymer called poly(vinyl alcohol), whichis based on the unstable organic molecule vinyl alcohol:
Predict the approximate values for the H — O — C and O — C — C bond angles in vinyl alcohol.
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MolecularGeometries
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Momento de dipolo
• Há dipolos de ligação:• Não necessariamente
que dizer que a molécula seja polar
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Polarity
Adição vetorial de dipolos de ligação
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Polaridade
© 2012 Pearson Education, Inc.Chemistry, The Central Science, 12th EditionTheodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
Sample Exercise 9.4 Polarity of Molecules
Analyze We are given three molecular formulas and asked to predict whether the molecules are polar.Plan A molecule containing only two atoms is polar if the atoms differ in electronegativity. The polarity of a
molecule containing three or more atoms depends on both the molecular geometry and the individual bond polarities. Thus, we must draw a Lewis structure for each molecule containing three or more atoms and determine its molecular geometry.We then use electronegativity values to determine the direction of the bond dipoles. Finally, we see whether the bond dipoles cancel to give a nonpolar molecule or reinforce each other to give a polar one.
Solve (a) Chlorine is more electronegative than bromine. All diatomic molecules with polar bonds are polar
molecules. Consequently, BrCl is polar, with chlorine carrying the partial negative charge:
The measured dipole moment of BrCl is µ = 0.57 D.(b) Because oxygen is more electronegative than sulfur, SO2 has polar bonds. Three resonanceforms can be written:
For each of these, the VSEPR model predicts a bent molecular geometry. Because the molecule is bent, the bond
dipoles do not cancel, and the molecule is polar:
Solution
Predict whether these molecules are polar or nonpolar: (a) BrCl, (b) SO2, (c) SF6.
© 2012 Pearson Education, Inc.Chemistry, The Central Science, 12th EditionTheodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward
Sample Exercise 9.4 Polarity of Molecules
Experimentally, the dipole moment of SO2is µ = 1.63 D.
(c) Fluorine is more electronegative than sulfur, so the bond dipoles point toward fluorine. For clarity, only oneS — F dipole is shown. The six S — F bonds are arranged octahedrally around the central sulfur:
Because the octahedral molecular geometry is symmetrical, the bond dipoles cancel, and the molecule isnonpolar, meaning that µ = 0.
Practice ExerciseDetermine whether the following molecules are polar or nonpolar: (a) NF3, (b) BCl3.Answers: (a) polar because polar bonds are arranged in a trigonal-pyramidal geometry, (b) nonpolar becausepolar bonds are arranged in a trigonal-planar geometry
Continued
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Uma ligação se forma quando:
• Há overlap de dois orbitais de dois átomos vizinhos.
• Há no máximo dois elétrons por orbital.• Quanto maior o overlap, mais forte é a ligação.
Teoria da Ligação de valência: ligação de covalente através da mecânica quântica.
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Overlap and Bonding
• We think of covalent bonds forming through the sharing of electrons by adjacent atoms.
• In such an approach this can only occur when orbitals on the two atoms overlap.
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Overlap e Ligação
• Efeito do aumento do overlap
• Efeito de overlap excessivo
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Hibridização: usa-se uma combinação de orbitais atômicos (de um mesmo átomo) para descrever-se a ligação química)
O número de orbitais híbridos formados é sempre igual ao número de orbitais atômicos que são combinados.
MolecularGeometries
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Orbitais Híbridos
• Considere berílio:
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Se energia é absorvida: 2s --> 2p
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Hybrid Orbitals
• Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals.– These sp hybrid orbitals have two lobes like a p orbital.– One of the lobes is larger and more rounded, as is the
s orbital.
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Hybrid Orbitals• These two degenerate orbitals would align
themselves 180 from each other.• This is consistent with the observed geometry of
beryllium compounds: linear.
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Hybrid Orbitals
• With hybrid orbitals, the orbital diagram for beryllium would look like this (Fig. 9.15).
• The sp orbitals are higher in energy than the 1s orbital, but lower than the 2p.
MolecularGeometries
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Hybrid Orbitals
Using a similar model for boron leads to three degenerate sp2 orbitals.
MolecularGeometries
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Hybrid Orbitals
With carbon, we get four degenerate sp3 orbitals.
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Hibridizações sp3d e sp3d2
• Examplos:
• PCl5• SF6
Quais são as geometrias?
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• A hibridização maximiza a ligação:– Mais ligações = mais orbitais ocupados = maior
estabilidade
• Para um mesmo átomo, há diferentes hibridizações:– C = sp, sp2, sp3
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Carbon HybridizationsUnhybridized
2s 2p
sp hybridized
2sp
sp2 hybridized
2p
sp3 hybridized
2p
2sp2
2sp3
MolecularGeometries
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Teoria da Ligação de Valência
• Hybridização é importante!!
• Há várias possibilidades para o overlap de orbitais:
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Sigma () Bonds
• Sigma bonds are characterized by– Head-to-head overlap.– Cylindrical symmetry of electron density about the
internuclear axis.
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Pi () Bonds
• Pi bonds are characterized by– Side-to-side overlap.– Electron density above and below the internuclear
axis.
MolecularGeometries
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Ligações Delta
Os 4 lóbulos de orbitais d interagem frontalmente
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Single BondsSingle bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.
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Multiple Bonds
In a multiple bond, one of the bonds is a bond and the rest are bonds.
MolecularGeometries
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Multiple Bonds
• In a molecule like formaldehyde (shown at left), an sp2 orbital on carbon overlaps in fashion with the corresponding orbital on the oxygen.
• The unhybridized p orbitals overlap in fashion.
MolecularGeometries
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Multiple Bonds
In triple bonds, as in acetylene, two sp orbitals form a bond between the carbons, and two pairs of p orbitals overlap in fashion to form the two bonds.
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ResonanceThe organic molecule benzene has six bonds and a p orbital on each carbon atom.
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Resonance• In reality the electrons in benzene are not
localized, but delocalized.• The even distribution of the electrons in benzene
makes the molecule unusually stable.
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Teoria do Orbital Molecular (MO)• Aplica-se a equação de Schrödinger (ondulatória)
em uma molecule para se calcular um conjunto de orbitais moleculares .
• Os electrons pertencem à molécula como um todo, logo há delocalização
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LCAO• Combinação Linear de Orbitais Atômicos:
orbitais de átomos diferentes são somados ou subtraídos para originar uma nova função de onda.
• A combinação pode ser constrututiva ou destrutiva
MolecularGeometries
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Orbitais Moleculares• bonding molecular orbital (ligante):
construtiva , – Característica?
• antibonding molecular orbital (anti-ligante): *, *– Característica?
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Interaction of 1s Orbitals
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Molecular Orbital Theory
• Electrons in bonding MOs are stabilizing.– lower energy than the atomic orbitals
• Electrons in antibonding MOs are destabilizing.– higher in energy than atomic orbitals– electron density located outside the
internuclear axis– electrons in antibonding orbitals cancel
stability gained by electrons in bonding orbitals
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1s 1s
*
hydrogen atomicorbital
hydrogen atomicorbital
Dihydrogen, H2 molecularorbitals
Since more electrons are in bonding orbitals than in antibonding orbitals,
there is a net bonding interaction.
MolecularGeometries
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MO and Propriedades• Bond order (Ordem de ligação): diferença entreo
número de electrons ligantes e anti-ligantes.– Considerar sobmente os elétrons de valência
– Pode ser fracionário
– Maior ordem de ligação: maior força e menor comprimento.
– Se B.O. = 0, a ligação é instável se comparada com os átomos individuais
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H2
* Antibonding MOLUMO
bonding MOHOMO
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1s 1s
*
helium atomicorbital
helium atomicorbital
Dihelium, He2 molecular
orbitals
Since there are as many electrons in antibonding orbitals as in bonding orbitals,
there is no net bonding interaction.
BO = ½(2 − 2) = 0
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1s 1s
lithium atomicorbitals
lithium atomicorbitals
Dilithium, Li2 molecular
orbitals
Since more electrons are in bonding orbitals than in
antibonding orbitals, there is a net bonding interaction.
2s 2s
Any filled energy level will generate filled bonding and antibonding MOs;therefore, only need to
consider the valence shell.BO = ½(4 − 2) = 1
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bonding MOHOMO
* Antibonding MOLUMO
Li2
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Interaction of p Orbitals
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Interaction of p Orbitals
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MO Theory
• The smaller p-block elements in the second period have a sizable interaction between the s and p orbitals.
• This flips the order of the and molecular orbitals in these elements.
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MolecularGeometries
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Second-Row MO Diagrams
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