Frank Weinhold

1
Natural Bond Orbital Picture of Valency, Bonding,
and Torsional Phenomena

• Frank Weinhold
• Theoretical Chemistry Institute and Department of
  Chemistry, University of Wisconsin, Madison, WI
  53706
• (weinhold_at_chem.wisc.edu)

Lise Meitner Lecture I, Hebrew University,
Jerusalem, June 17, 2007
2
Outline

• How NBO Works
• Why Concepts/Analysis?
• NAO/NBO/NLMO/NRT Overview
• How Chemical Bonding Works
• Donor-Acceptor Superposition
• Some Exotic Species
• How Rotation Barriers Work
• Ethane
• Proteins

3
The Nature of the Chemical Bond is the problem
at the heart of all chemistry.
Bryce
Crawford, Jr. (1953)
If you look at the great controversies in
organic chemistry, They were interesting because
in most of them you had to decide what it is that
you have to write down on paper to
represent chemical structuresThey had problems
with benzene and resonance hybrids of various
kinds. John D. Roberts, Chem.
Intelligencer, Apr. 1998, p. 29
4
The more accurate the calculations become, the
more the concepts tend to vanish into thin
air. R. S. Mulliken, J. Chem. Phys.
S2, 43 (1965)
It is at least arguable that, from the point of
view of quantum chemistry as usually practiced,
the supercomputer has dissolved the bond. B.
T. Sutcliffe, Int. J. Quantum Chem. 58, 645
(1996)
5
It is nice to know that the computer understands
the problem. But I would like to understand it
too. E. P. Wigner (quoted in Physics Today,
July 1993, p. 38)
6
How Does NBO Work?
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
www.chem.wisc.edu/nbo5
12
What is Chemical Bonding?
(A1) Thermally robust atom-atom attraction
(A2) Exchange-type valence attraction (Pauli
overlap)
vs.
(A3) Donor-Acceptor Superposition (1-e and
composites)
F. Weinhold, Chemical Bonding as a
Superposition Phenomenon, J. Chem. Educ.76, 1141
(1999)
13
2-Center QM Superposition
14
Pauli Restrictions
Donor-Acceptor
Donor-Donor
1-e donor-acceptor superposition
General mechanism
15
Unit (half-bond)
Covalent reciprocal complementary
2-c symmetry types
Dative/Coordinate uni-directional
competitive
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
3-center/2-electron Hypovalent Tau
bonds (Lipscomb, Longuet-Higgins) e.g., diborane
20
3-center/4-electron Hypervalent Omega
bond (Pimentel-Rundle-Coulson) e.g., bifluoride
ion
21
2-center/2-electron Nu bonds e.g., N2 radical
cation
22
Rydberg Bonds e.g., 3T4 CH3CHO
23
High-Order Metal-Metal Bonds W-W Quintuple Bond
24
(No Transcript)
25
(No Transcript)
26
Ethane Rotation Barrier Sterics or
Hyperconjugation?
27
Sterics?
Hyperconjugation?
28
T. K. Brunck FW, J. Am. Chem. Soc. 101, 1700
(1979) FW, Nature 411, 539 (2001)
29
PMO Theory (Dewar) Mathematical Consistency?

• Q1. Whats the unperturbed H(0)?
• non-Hermitian (unphysical!)
• imaginary eigenvalues
• non-orthogonal eigenstates
• (non-conservation of probability
• and other overlap pathologies)

VB, pp. 229-234
The PMO unperturbed problem is more
mysterious than the effect it purports to explain!
30
Q2. Why were the s (acceptor) orbitals ignored?
FW, Angew. Chem. 115, 4320 (2003)
Conclusion s-s hyperconjugation better
explains the PMO diagram!
31
Sterics Common Sense?
Steric size increases but DEbarrier
decreases!
E. B. Wilson, Adv. Chem. Phys. 2, 367 (1959)
32
Sterics Common Sense?
H---H distances increase but DEbarrier
increases!
H.R. Duebal F. F. Crim, JCP 83, 3863 (1985)
33
Protein Folding Torsional Barriers at Double
Bonds
34
Eclipsing propensity at double bonds?
35
(No Transcript)
36
Resonance Control of Torsional Stiffness?
37
Coordinative H-Bond Modulation of Amide Rotation
Barriers
38
Torsional Steering of AcetamideX Complexes
39
Residue(R-X)-Modulated Folding Code Mechanism?
(Current MM/MD potentials oblivious to these
effects!)
40

• Summary
• Only the hyperconjugative s-s picture
• of torsional barriers satisfies reasonable
• criteria for
• mathematical consistency
• common sense
• predictive utility with respect to
• conjugative and H-bonding phenomena

Many thanks!