10-ESTRUTURA MOLECULAR

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Representações de probabilidade (ψ2)
ESTRUTURA MOLECULAR
Observar que orbitais são tridimensionais!!
Fases dos orbitais
1
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TEORIA DO ORBITAL MOLECULAR
LCAO: Linear Combination of Molecular Orbitals
Exemplo 1: H2
2
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There are several points to notice about this diagram.
• Two atomic orbitals (AOs) combine to give two molecular orbitals (MOs)
• By LCAO we add the two AOs to make the bonding orbital and subtract them to
make the antibonding orbital
• Since the two atoms are the same, each AO contributes the same amount to the
MOs
• The bonding MO is lower in energy than the AOs
• The antibonding MO is higher in energy than the AOs
• Each hydrogen atom initially had one electron. The spin of these electrons is
unimportant
• The two electrons end up in the MO lowest in energy. This is the bonding MO
• Just as with AOs, each MO can hold two electrons as long as the electrons are spin
paired
• The two electrons between the two nuclei in the bonding MO hold the molecule
together—they are the chemical bond
• Since these two electrons are lower in energy in the MO than in the AOs, energy is
given out when the atoms combine
• Or, if you prefer, we must put in energy to separate the two atoms again and to break
the bond
Exemplo 2: He2
Simetria σ
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Simetria σ
Simetria π
Exemplo: NaF
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Outros fatores que afetam a superposição dos orbitais
1- Tamanho do orbital
2- Simetria dos orbitais
TEORIA DA LIGAÇÃO DE VALÊNCIA
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TEORIA DA HIBRIDIZAÇÃO
sp3
Etano: C2H6
Metano:
109,5º
C-H
Sigma sp3-s
1,1Å; 98kcal/mol
C-C
Sigma sp3-sp3
1,54Å; 88 kcal/mol
4 ligações sigma sp3-s
C-H 1,1Å; 415KJ/mol
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sp2
H
Etileno: C2H4
H
Combinação:
C=C
H
Etileno
H
2p2
2s2
1s2
2s + 2px + 2pz
3 sp2
C=C 1,33Å; 152 kcal/mol
Ligação sigma: sp3-sp3
Ligação pi: 2p-2p
C-H 1,1Å; 103 kcal/mol
Ligação sigma: Csp3-Hs
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sp
Acetileno
H–C≡C-H
Combinação:
2p2
2s2
1s2
2px + 2s
Acetileno:
2 sp
FORÇA DE LIGAÇÃO
LIGAÇÃO
Cl – Cl
C – Cl
H–H
kcal/mol
40
80
104
C≡C 1,20Å; 200 kcal/mol
1 Ligação sigma: Csp-Csp
2 Ligações pi: C2p-C2p
C-H 1,1Å; 125 kcal/mol
Ligação sigma: Csp-Hs
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COMPRIMENTO DA LIGAÇÃO
ligação
H 2C – H
=C – H
≡C–H
C–C
C=C
C≡C
ÂNGULO DE LIGAÇÃO
λ (Å)
1,10
1,07
1,06
1,54
1,33
1,20
TEORIA DA REPULSÃO DOS
PARES DE ELÉTRONS NA
CAMADA DE VALÊNCIA
109.5°
107°
100°
104.5°
P
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POLARIDADE DE MOLÉCULAS
POLARIDADE DA LIGAÇÃO
MOLÉCULAS APOLARES
§  Eletronegatividade
§  Polaridade de ligações
momento de dipolo
distância entre as cargas
MOLÉCULAS POLARES
magnitude da carga
Da Física: vetor momento de dipolo elétrico
vetor deslocamento de carga
Debye: 1D = 3,33564 x 10-30 C . m
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Table 2-1 Dielectric Constants and Permanent Molecular
Dipole Moments of Some Common Solvents
Substance
Formamide
composto comprimento diferença de momento
da ligação (Å) eletronegativi de dipolo
dade
(D)
HF
0,92
1,9
1,82
HCl
1,27
0,9
1,08
HBr
1,41
0,7
0,82
HI
1,61
0,4
0,44
Dielectric
Constant
Dipole Moment
(debye)
110.0
3.37
Water
78.5
1.85
Dimethyl sulfoxide
48.9
3.96
Methanol
32.6
1.66
Ethanol
24.3
1.68
Acetone
20.7
2.72
Ammonia
16.9
1.47
Chloroform
4.8
1.15
Diethyl ether
4.3
1.15
Benzene
2.3
0.00
Carbon tetrachloride
2.2
0.00
Hexane
1.9
0.00
Source: Brey, W.S., Physical Chemistry and Its Biological Applications,
p. 26, Academic Press (1978).
a. Amphiphiles Form
Most biological mole
charged) and nonpolar
neously hydrophilic and
10
(a)
R
O
H
..
of a solvent is a measure of its ability to keep opposite
charges apart. In a vacuum, D is unity and in air, it is only
negligibly larger.The dielectric constants of several common
solvents, together with their permanent molecular dipole
moments, are listed in Table 2-1. Note that these quantities
tend to increase together, although not in a regular way.
The dielectric constant of water is among the highest of
any pure liquid, whereas those of nonpolar substances, such
as hydrocarbons, are relatively small. The force between
two ions separated by a given distance in nonpolar liquids
such as hexane or benzene is therefore 30 to 40 times
greater than that in water. Consequently, in nonpolar solvents (low D), ions of opposite charge attract each other
so strongly that they coalesce to form a salt, whereas the
much weaker forces between ions in water solution (high
spread over the volum
arrangement greatly at
tween ions, which is wh
electric constants.
The orienting effect
cules is opposed by th
tend to randomly reori
solvated complex are th
reason why the dielect
greater than that of oth
moments is that liquid w
permits it to form orie
randomization, thereby
charges. Indeed, ice un
dielectric constant of 3 b
reorient in response to a
The bond dipoles of
them soluble in aqueo
that ionic substances ar
polar and ionic substanc
tional groups, such as h
carboxyl (¬CO2H or
groups, that can form h
lustrated in Fig. 2-6. In
such as proteins, nuclei
with just such groups.
lack both hydrogen bon
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FORÇAS INTERMOLECULARES
DIPOLO-DIPOLO
LIGAÇÕES DE HIDROGÊNIO
FORÇAS DE DISPERSÃO DE LONDON
PONTOS DE EBULIÇÃO
COMPOSTO
butano
P.E. (°C)
0
metil etil éter
acetona
8
54
propanol
ácido acético
98
118
COMPOSTO
P.E. (°C)
éter dimetílico
etanol
- 24
78
COMPOSTO
pentano
isopentano
P.E. (°C)
36
28
COMPOSTO
metano
butano
P.E. (°C)
- 132
0
neopentano
9.5
pentano
36
SOLUBILIDADE
11