An ab initio study of low-frequency, large-amplitude molecular vibrations

Persistent Link:
http://hdl.handle.net/10150/290692
Title:
An ab initio study of low-frequency, large-amplitude molecular vibrations
Author:
McCarthy, William James, 1964-
Issue Date:
1996
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
The ab initio treatments of molecular vibrational motion often invoke only the harmonic oscillator approximation. For vibrational modes whose amplitudes access anharmonic regions of the potential energy surface, the harmonic oscillator approximation fails. Low-frequency large-amplitude vibrations, in particular, can access anharmonic regions in addition to other minima of the potential energy surface. Ab initio harmonic frequencies are often scaled to lower values by empirical factors which presumably account for anharmonicity effects as well as an incomplete basis set and account of electron correlation. However, the scaling of those ab initio harmonic frequencies corresponding to low-frequency large-amplitude vibrations results in theoretical values that are still typically several times larger than the experimental values. It is demonstrated in this dissertation that transforming the nuclear motion Hamiltonian to internal coordinates facilitates construction of ab initio potential energy curves, or surfaces, pertaining to low-frequency large-amplitude molecular vibrational modes. The use of internal coordinates complicates the expression of the kinetic energy in the Hamiltonian, and makes it difficult to obtain. Six different methods for determining the kinetic energy expression in internal coordinates are presented and reviewed. The computational implementation of these six methods was performed to allow their critique. Several example calculations of the presented methodology are given. The solution for the vibrational expectation values of the modes expressed by the developed Hamiltonian was also computationally implemented. The resultant theoretical transition frequencies of the molecular systems of 2-sulpholene and 2-aminopyrimidine are combined with experimental studies, and demonstrate the practical usefulness of the presented methodology.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Chemistry, Physical.; Biophysics, General.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Chemistry
Degree Grantor:
University of Arizona
Advisor:
Adamowicz, Ludwik

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleAn ab initio study of low-frequency, large-amplitude molecular vibrationsen_US
dc.creatorMcCarthy, William James, 1964-en_US
dc.contributor.authorMcCarthy, William James, 1964-en_US
dc.date.issued1996en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractThe ab initio treatments of molecular vibrational motion often invoke only the harmonic oscillator approximation. For vibrational modes whose amplitudes access anharmonic regions of the potential energy surface, the harmonic oscillator approximation fails. Low-frequency large-amplitude vibrations, in particular, can access anharmonic regions in addition to other minima of the potential energy surface. Ab initio harmonic frequencies are often scaled to lower values by empirical factors which presumably account for anharmonicity effects as well as an incomplete basis set and account of electron correlation. However, the scaling of those ab initio harmonic frequencies corresponding to low-frequency large-amplitude vibrations results in theoretical values that are still typically several times larger than the experimental values. It is demonstrated in this dissertation that transforming the nuclear motion Hamiltonian to internal coordinates facilitates construction of ab initio potential energy curves, or surfaces, pertaining to low-frequency large-amplitude molecular vibrational modes. The use of internal coordinates complicates the expression of the kinetic energy in the Hamiltonian, and makes it difficult to obtain. Six different methods for determining the kinetic energy expression in internal coordinates are presented and reviewed. The computational implementation of these six methods was performed to allow their critique. Several example calculations of the presented methodology are given. The solution for the vibrational expectation values of the modes expressed by the developed Hamiltonian was also computationally implemented. The resultant theoretical transition frequencies of the molecular systems of 2-sulpholene and 2-aminopyrimidine are combined with experimental studies, and demonstrate the practical usefulness of the presented methodology.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectChemistry, Physical.en_US
dc.subjectBiophysics, General.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineChemistryen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorAdamowicz, Ludwiken_US
dc.identifier.proquest9720668en_US
dc.identifier.bibrecord.b3458027xen_US
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