Post on 20-Dec-2015
transcript
Chem 125 Lecture 1410/6/08
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sp1
sp3
There should be a relationship between
Hybridization and Structure
angle spm-spn = cos-1(mn)
1
m n angle
1 1
2 2
3 3
0
*
125.3°
* to avoid net overlap between different e-pairs (Pauli Principle)
1 3 125.3°
? 180° linear
120° trigonal
109.5°tetrahedral
90°
The three X-H Bonds say,“Use 3 sp2 to maximize overlap”
How to Optimize Hybridization
of the X Atom in XH3 ?
The X Atom says, “O.K. make 3 bonds, but Maximize s-orbital occupancy”
B (3 e-) N (5 e-)C (4 e-)3 p2 s
(One X-electron in each of 3 bonding AOs; remainder in the 4th AO)
3 sp2
vacant pWhatever
(all sets of four 1-electron valence AOs use same amount of 2s)
How to Optimize Hybridization
of the X Atom in XH3 ?
The X Atom says, “O.K. make 3 bonds, but Maximize s-orbital occupancy”
B (3 e-) N (5 e-)C (4 e-)
(One X-electron in each of 3 bonding AOs; remainder in the 4th AO)
B (3 e-) N (5 e-)C (4 e-)
BH3
STRONGLY prefers sp2 bonds
(planar)
CH3
Less Stronglyprefers sp2 bonds
(planar)
NH3
must compromisesp
>2 bonds(pyramidal)
3 sp2
1 pWhatever
The three X-H Bonds say,“Use 3 sp2 to maximize overlap”
3 sp2
vacant p3 p2 s
Hybridization Reality Check:
Structure and Dynamics of
XH3BH3 CH3 NH3
valence electrons of X3 4 5
• ••
Competes with bonds for s-character
BH3
STRONGLY prefers sp2 bonds
(planar)
CH3
less stronglyprefers sp2 bonds
(planar)
NH3
must compromisesp
>2 bonds(pyramidal)
Are these Predictions True?
Experiment:X-Ray?
Distortion from plane weakens
bonds and deprives electrons
of s-character.
Distortion from plane weakens
bonds and shifts s-character
to lone electron (not wasted).
Distortion from plane weakens
bonds but shifts s-character from single electrons to pair of electrons.
2 BH3 B2H6 2 CH3 C2H6 GasIR & ESR Spectroscopy
Infrared: Out-of-Plane Bend
X
H
H H
X
H
H H
X
H
HH
X
H
HH
X
H
H H
X
H
H
H
X
H
H
H
X
H
HH
X
H
H
H
This “umbrella” vibration may be treated as a 1-dimensional “Erwin” problem with a fictious “mass”
that reflects the amount of motion of the four atoms.
Infrared: Out-of-Plane Bend
Weaker Planar Preference
Hooke’s Law potential energyadjusted to giveproper energy difference
Amount of deformation
34.2 Terahertz 18.7 THz
34,210,000,000,000vibrations per sec
C
HH
H
C
HH
H
C
HH
H
• • •
1141 cm-1606 cm-1
B
HH
H
B
HH
H
B
HH
H
Strong Planar Preference
3.26 kcal/mole1.73 kcal/moleH HAmount of deformation
Stiffer “Spring”
Two closely-spaced absorptions
Infrared: Out-of-Plane Bend
932 cm-1968 cm-1
37 cm-1
“Tunnel”Splitting:
1 cm-1
Potential Energy“Inversion” Barrier
3 kcal/mole
2 cal/mole
Ground StateTunneling~1011/sec
UmbrellaClock!
Not a Hooke’s Law pattern
Double Minimum potential energy
adjusted to giveproper energies
510-14 sec0.002 (kcal)
0 & 1 node
2 & 3 nodes
N
HH
H
N
HH
H
N
HH
H
• • ••••Lect. 9frame 9
Electron Spin Resonance Spectrummeasures s-orbital character
of the SOMO electron in CH3.•
A line separation due to magnetic interaction between the unpaired electron and the 13C
nucleus occurs only if the
electronspends time ON the
nucleus, which happens only for
s-orbital.
Structural Isotope Effect:
CH3
spendsmore time more bent
than CD3
(thus uses more
s-character for SOMO electron)
C
HH
H
C
HH
H
C
HH
H
• • •
CH3 38 Gauss 2% s
36 Gauss less sCD3
onaverage
CF3•
Repulsion between F atoms? Less Bent (flatter) than •CH3
Since Fluorine holds the lion's share of the bonding electron pair, Carbon has less reason to use its valuable s-character in the bonding orbitals. Uses more for the SOMO.
More Bent than •CH3
OR
Tension!
Differing Goals
Computer Chem 125Student
Minimize kinetic plus
coulomb energies of electrons and nuclei
by “settling down”
Minimize total energy using Schrödinger equation with “realistic” constraints
Understand structure and reactivity with the simplest
“realistic” model
ExperimentalMolecule
e.g. limited set of AOs,SCF, some correlation,
delocalized MOs
e.g. localized bonds,lone pairs; hybridization
E-match/overlapHOMO/LUMO
Qualitative Insight
Validationby
Experiment& Computer
Useful Predictionsof Properties
Validationby Experiment
Structure Total e-Density (X-Ray)
Energies (IR)Nuclear e-Density (ESR)Dipole Moment, etc.