importance and limitations of models as ways of
visualizing matter
typical atomic dimensions (van der Waals & covalent radii - Table 1.2)
typical bond lengths (Table 1.1)
molecular surface area & volume (Table 1.3) -- homework problem #4
energetics
heat of formation, heat of hydrogenation, heat of combustion
used to calculate heats of reactions homework problems #5 - 10
calculated heats of formation:
bond increment contributions (Table 1.5)
group increment contributions (Table 1.7)
corrections for gauche interactions (e.g., CH3/CH3 0.8 kcal)
corrections for angle strain (e.g., cyclopropane 27.6, cyclobutane 26.2
kcal)
bond dissociation energies - useful for radical reactions in gas phase
bonding
Lewis structures, ionic bonding, covalent (nonpolar, polar)
dipole moment = charge x distance (units of esu cm = Debye)
partial charges represent partial ionic character homework problem #11
electronegativity - many variations, Pauling's is most common (Table 1.8)
valence bond theory - wavefunctions include covalent contributions only
MO theory - wavefunctions include ionic contributions as well
LCAO-MO approach - pictorial approach to overlap, e.g., s bond from s orbitals
hybrid orbitals - sp, sp2, sp3
- directional (high overlap) with good orientation (minimize repulsions)
VSEPR - maximize bonding (overlap) while minimizing repulsions
bonding in methane
structure is tetrahedral (sp3 hybrids)
PES indicates electron binding energies of ~300 eV (1s), 23.0 eV and 12.7
eV
- electrons are apparently not in 4 equivalent s bonds
symmetry-corrected MOs (Figure 1.18) - 3 equivalent MOs, one different
- the same four MOs would be obtained from the original AOs
bonding in chloromethane
effects of the polar C-Cl bond
- excess e- density leads to a longer C-Cl bond (compared to covalent radii)
- reduced e- density leads to shorter C-H bonds (compared to covalent radii)
- shorter C-H bonds have higher repulsions, greater bond angle ( >109·)
- longer C-Cl bond leads to lower H-C-Cl bond angle (<109·)
variable hybridization
consider CH3Cl uses different hybrids for C-H and C-Cl bonds
- the C-H bonds are sp(2.86) and the C-Cl bond is sp(3.50)
- the C-H bonds are 26% s and 74% p (1/3.86 and 2.86/3.86)
- the C-Cl bond is 22% s and 78% p (1/3.50 and 3.50/4.50)
(note that the sum still accounts for one s orbital plus three p orbitals)
greater s-character in a bonding orbital leads to:
shorter bonds, larger bond angles (compare sp3 , sp2 , sp)
in general, for sp(n) orbitals, %s = 1/n+1 , %p = n/n+1
bond angles can be calculated from hybridization (or vice versa) (eq. 1.40)
homework problems #14 & 15
curved bonds
distinguish between the internuclear bond distance and angles (from x-ray
data)
and the interorbital bond angles (as needed for better overlap, as in cyclopropane)
dichloromethane structure can also be explained with curved bonds, allowing
for repulsions between Cl atoms
correlations with variable hybridization
NMR coupling constant between 13-C and 1-H relates to the hybridization
J = 500/n+1 (eq 1.46) homework problem #21
C-H bond length relates to the coupling constant (eq 1.47) homework problem
#19
kinetic acidity (rate of removal) of the C-H bond relates to the coupling
constant
(eq 1.48) homework problem #21
in general, greater s-character leads to:
shorter bond, larger coupling constant, more acidic H (more stable anion)
bent bonds as a model for multiple bonds
an alternative to the usual sigma,pi formulation of double and triple bonds
consider bonds as tetrahedra joined at a point, an edge, or a face
- calculated bond lengths are very good
- relative acidities can be predicted well using VSEPR
- allylic conformations are predicted well (better than sigma,pi concept)
homework problems #13 & 17
importance of models - know when a given model is useful