Vibrational energy flow in substituted benzenes

Pein, Brandt

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https://hdl.handle.net/2142/49347 Copy

Description

Title

Vibrational energy flow in substituted benzenes

Author(s)

Pein, Brandt

Issue Date

2014-05-30T16:39:21Z

Director of Research (if dissertation) or Advisor (if thesis)

Dlott, Dana D.

Doctoral Committee Chair(s)

Dlott, Dana D.

Committee Member(s)

• Gruebele, Martin
• McCall, Benjamin J.
• Suslick, Kenneth S.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Vibrational Energy Flow
• intramolecular vibrational energy redistribution (IVR)
• Nitrobenzene
• Halogenated Benzenes
• Nitromethane
• Acetonitrile
• Isopropylbenzene
• Toluene
• Tertbutylbenzene
• o-Nitrotoluene
• o-Fluoronitrobenzene
• IR-Raman (Infrared)
• Picosecond

Abstract

Using ultrafast infrared (IR) Raman spectroscopy, vibrational energy flow was monitored in several liquid-state substituted benzenes at ambient temperature. In a series of mono-halogenated benzenes, X-C6H5 (X = F, Cl, Br, I), a similar CH-stretch at 3068 cm-1 was excited using picosecond IR pulses and the resulting vibrational relaxation and overall vibrational cooling processes were monitored with anti-Stokes spectroscopy. In the molecules with a heavier halide substituent the CH-stretch decayed slower while midrange vibrations decayed faster. This result was logical if the density of states (DOS) in the first few tiers, which is the DOS composed of vibrations with smaller quantum number, is what primarily determines energy flow. For tiers 1-4, the DOS was nearly identical in the CH-stretch region while it increased in the midrange region for heavier halide mass. Excitation spectroscopy, an extension of 3D IR-Raman spectroscopy, was developed and used to selectively pump vibrations localized to the substituent or the phenyl group in nitrobenzene (NB), o-fluoronitrobenzene (OFNB) and o-nitrotoluene (ONT) and in the alkylbenzene series toluene, isopropylbenzene (IPB), and t-butylbenzene (TBB). Using quantum chemical calculations, each Raman active vibration was sorted, according to their atomic displacements, into three classifications: substituent, phenyl, or global. Using IR pump wavenumbers that initially excited substituent or phenyl vibrations, IR-Raman spectroscopy was used to monitor energy flowing from the substituent to phenyl vibrations and vice versa. In NB nitro-to-phenyl and nitro-to-global energy flow was almost nonexistent while phenyl-to-nitro and phenyl-to-global was weak. When ortho substituents (-CH3, -F) were introduced, energy flow from nitro-to-phenyl and nitro-to-global was activated. In ONT, phenyl-to-nitro energy flow ceased possibly due to the added methyl group diverting energy from entering the nitro vibrations. Energy flow is therefore unidirectional in the phenyl-to-nitro direction in NB while in ONT it is unidirectional in the nitro-to-phenyl direction. In the alkylbenzenes phenyl-to-substituent energy flow was about the same in each while substituent-to-phenyl energy was accelerated for larger alkyl substituents. If the DOS controls energy flow this is opposite of what would be expected and gives a possible route to control energy flow from an attached alkyl substituent to a phenyl group.

Graduation Semester

2014-05

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http://hdl.handle.net/2142/49347

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Copyright 2014 Brandt Pein

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