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Physical Chemistry
A Very Brief Introduction
Aleksey Kocherzhenko
March 14, 2013
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What Is Physical Chemistry?
All chemistryis a branch of physical science
Physical Scienceis that department of knowledge which relates to the order of nature, or in other words, to the regular succession of events.
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What Is Physical Chemistry?The name of physical science, however, is often applied in a more or less restricted manner to those branches of science in which the phenomena considered are of the simplest and most abstract kind, excluding the consideration of the more complex phenomena.
James Clerk Maxwell, “Matter and Motion”, 1876
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What Is Physical Chemistry?
• Physical chemists:– often look at phenomena at an atomic and even subatomic level– describe phenomena in an abstract mathematical way
Chemistry started out as an empirical science
• Synthetic chemists: think in terms of functional groups
• Biochemists: concerned with the functionality of large molecules
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What Is Physical Chemistry?
Traditionally included are:
• Thermodynamics• Kinetics• Quantum mechanics
as applied to molecular-level phenomena
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A Great Textbook
D. A. McQuarrie and J. D. SimonPhysical Chemistry: A Molecular Approach University Science Books, 1997
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Quantum Mechanics and Chemistry
Reactivity is determined by orbital symmetry…
R
ortho
meta
para
… in many cases, so are functional properties!
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Orbits vs. Orbitals
Ernest Rutherford J.J. Thomson
Planetary atomic model Plum pudding atomic model
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Orbits vs. Orbitals
The absorption/emission spectra of atoms are discrete, not continuous!
Rydberg’s formula:
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Orbits vs. Orbitals
Niels Bohr
Model of an H-like atom
Coulomb force
Centripetal force:
Angular momentum quantization:
Principal quantum numbern = 1, 2, 3, …
ReducedPlanck’s constant
where
Bohr radius
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Orbits vs. Orbitals
n = 1
n = 2n = 3
n = ∞ E = 0
n = 4
E = –13.6
E = –3.40E = –1.51
–0.85
E (eV)
+
Energy levels of a hydrogen-like atom:
Rydberg constant, R
Planck constant:
Correct absorption frequency:
to satisfy Rydberg’s formula:
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Orbits vs. Orbitals
Louis de BroglieInterference: light is a wavePhotoelectric effect:light consists of particles?
Light (and everything else) has both particle-like and wave-like properties
Or: (angular wavenumber)
(angular frequency)
De Broglie wavelength:
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Particle-wave duality
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Orbits vs. Orbitals
n = 1
n = 2n = 3
n = ∞ E = 0
n = 4
E = –13.6
E = –3.40E = –1.51
–0.85
E (eV)
+
Particle in centrosymmetric potential:“Old” quantum theory
Wave in centrosymmetric potential:“New” quantum theory
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Orbits vs. Orbitals
n = 1
n = 2n = 3
n = ∞ E = 0
n = 4
E = –13.6
E = –3.40E = –1.51
–0.85
E (eV)
+
1) Electrons in atoms are found in stable (time-independent) states2) Electrons behave as waves
Stable electronic states (orbitals) are standing waves
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What a Typical PChem/QMech Course Covers:
0) Mathematical background and notation
1) Foundations of quantum mechanics Postulates, equation of motion, stationary states
2) Simple model systems Particle-in-a-box, harmonic oscillator, hydrogen atom, barriers
3) Multi-electron atoms Approximate methods, spin
5) Spectroscopy Degrees of freedom, types of spectroscopy to probe them
6) The paradoxes of quantum mechanics and its relationship to classical mechanics
4) Molecules Orbital hybridization, chemical bonding
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