Scope and Content stated in the place of the abstract. / The principal methods of calculating nuclear spin coupling constants
by applying perturbation theory to molecular orbital wavefunctions for the
electronic structure of molecules are discussed. A new method employing a
self-consistent-field perturbation theory (SCFPT) is then presented and compared
with the earlier methods.
In self-consistent-field (SCF) methods, the interaction of an
electron with other electrons in a molecule is accounted for by treating the
other electrons as an average distribution of negative charge. However, this
charge distribution cannot be calculated until the electron-electron interactions
themselves are known. In the SCF method, an initial charge distribution
is assumed and then modified in an iterative calculation until the
desired degree of self-consistency is attained. In most previous perturbation
methods, these electron interactions are not taken into account in a self consistent
manner in calculating the perturbed wavefunction even when SCF
wavefunctions are used to describe the unperturbed molecule.
The main advantage of the new SCFPT approach is that it treats the interactions between electrons with the same degree of self-consistency
in the perturbed wavefunction as in the unperturbed wavefunction. The
SCFPT method offers additional advantages due to its computational
efficiency and the direct manner in which it treats the perturbations.
This permits the theory to be developed for the orbital and dipolar contributions
to nuclear spin coupling as well as for the more commonly
treated contact interaction.
In this study, the SCFPT theory is used with the Intermediate
Neglect of Differential Overlap (INDO) molecular orbital approximation to
calculate a number of coupling constants involving 13c and 19F. The
usually neglected orbital and dipolar terms are found to be very important
in FF and CF coupling. They can play a decisive role in explaining the
experimental trend of JCF among a series of compounds. The orbital interaction
is found to play a significant role in certain CC couplings.
Generally good agreement is obtained between theory and experiment
except for JCF and JFF in oxalyl fluoride and the incorrect signs obtained
for cis JFF in fluorinated ethylenes. The nature of the theory permits
the latter discrepancy to be rationalized in terms of computational details.
The value of JFF in difluoracetjc acid is predicted to be -235 Hz.
The SCFPT method is used with a theory of dπ - pπ bonding to predict
in agreement with experiment that JCH in acetylene will decrease when that
molecule is bound in a transition metal complex. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/15883 |
| Date | 05 1900 |
| Creators | Blizzard, Alan Cyril |
| Contributors | Santry, D.P., Chemistry |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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