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The Molecular Modeling Workbook for Organic Chemistry - Hehre J.W.

Hehre J.W., Shusterman J.A. The Molecular Modeling Workbook for Organic Chemistry - Wavefunction, 1998. - 307 p.
Download (direct link): molecularmodelingworkbook1998.djvu
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amines. While the lone pair on nitrogen is better accommodated in an sp3
hybrid than a higher-energy p orbital, conjugation with a carbonyl group
in the case of amides, or with the phenyl ring in the case of arylamines,
should be the most effective when the nitrogen is planar.
Step through the sequence of structures corresponding to inversion of
ammonia. Plot energy (vertical axis) vs. frame number (horizontal axis).
Identify the equilibrium structure. What is the preferred HNH bond angle?
Identify the inversion transition state and calculate the barrier to
inversion. Is it in the same range as single-bond rotation barriers (see
Chapter 5, Problems 1 and 2), or is it significantly larger?
Repeat your analysis for the sequence of structures corresponding to
inversion of trimethylamine. Is the inversion barrier smaller, larger or
about the same as that in ammonia? If significantly different, speculate
on the origin of the difference.
Examine the structure of aniline for evidence of conjugation. Is the CN
bond length a typical single bond distance (as in methylamine), a typical
double bond distance (as in methyleneimine) or somewhere in between? Is
the nitrogen center planar or puckered? Does aniline incorporate a CN ft
bond? Examine the highest-occupied molecular orbital (HOMO). Does it
suggest n bonding involving the amino group?
200 Chapter 14 Nitrogen-Containing Compounds
f
Conformations of Hydrazine and Hydrogen Peroxide
Rotation around the NN bond in hydrazine yields three "staggered"
conformers, one in which the lone pairs are anti and the other two
(equivalent structures) in which they are gauche.
lone pairs anti lone pairs gauche
Step through the sequence of structures corresponding to rotation about
the NN bond in hydrazine. Plot energy (vertical axis) vs. HNNH dihedral
angle (horizontal axis). How many energy minima are there? Do they
correspond to structures in which the hydrogens stagger? What is the
geometry of the lowest-energy structure? What is the energy barrier
separating the minima?
A related molecule is hydrogen peroxide. Again, it might be expected that
rotation (around the oxygen-oxygen bond) would lead to three "all-
staggered" conformers.
hydrogens anti hydrogens gauche
Step through the sequence of structures corresponding to rotation about
the 00 bond in hydrogen peroxide. Plot energy (vertical axis) vs. HOOH
dihedral angle (horizontal axis). How many energy minima are there? Do
they correspond to "staggered" structures? What is the geometry in the
lowest-energy structure? What is the energy barrier separating the
minima?
Summarize your observations on the conformations of molecules with lone
pairs.
Chapter 14 Nitrogen-
Containing Compounds 201
e
Electrostatic potential map for trimethylammonium ion shows most
positively-charged regions (in blue) as likely acidic sites.
The model dealt with here is greatly oversimplified, and is only of
qualitative value. It treats solvation on the ammonium ions in terms of a
single layer (or shell) of solvent, and ignores solvation of neutral
ammonia and trimethylamine.
E(H20) = -75.5860 au
Ammonia or Trimethylamine. Which is the Stronger Base?
Trimethylamine is much more basic than ammonia in the gas phase, whereas
in water, the two are of equal strength.
Me,N + NH4+
Me3NH+ + NH3
AGg;
AGa,
-20 kcal/mol -1 kcal/mol
Examine atomic charges and display electrostatic potential maps for
ammonium and trimethylammonium ions (protonated ammonia and
trimethylamine, respectively). How many acidic hydrogens are there in
each? Assuming that solvent coordinates to acidic hydrogens, how many
solvation sites are there in each?
Are the acidic hydrogens in ammonium ion more or less positively charged
than the corresponding hydrogen(s) in trimethylammonium ion? Would you
expect solvation (on a per site basis) to be greater for ammonium or
trimethylammonium ion? Calculate binding energies for ammonium ion+water
and trimethylammonium ion+water, ions where a single water molecule has
been attached to an acidic hydrogen. (The energy for water is given at
left.) Estimate the total solvation energy in ammonium and
trimethylammonium ions by multiplying this binding energy by the total
number of acidic hydrogens in the two ions. Is the estimated difference
in solvation energies sufficient to account for the observed reordering
of base strengths from the gas to the aqueous phase?
The previous calculation assumed that the solvation energy of ammonium
was equal the solvation energy of a single water molecule times the
number of water binding sites. Is this a valid assumption? Compare the
electrostatic potential maps of ammonium ion and ammonium ion+water. For
which are the exposed hydrogens more acidic? Did the calculation
underestimate or overestimate the difference in solvation energies?
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