Infrared spectra of C2H4BR and C2H4I in solid para-hydrogen: Bridged or open structure?

Chen, Yu-Hsuan

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https://hdl.handle.net/2142/100487 Copy

Description

Title

Infrared spectra of C2H4BR and C2H4I in solid para-hydrogen: Bridged or open structure?

Author(s)

Chen, Yu-Hsuan

Contributor(s)

Lee, Yuan-Pern

Issue Date

2018-06-22

Keyword(s)

Vibrational structure/frequencies

Abstract

The electrophilic addition of halogens to alkenes is important in organic synthesis. The structure of the reaction intermediate, haloalkyl radical, plays an important role in the stereo specificity of the addition reaction. The simplest intermediate, haloethyl radical C2H4X, has two possible geometries: an open (classical) structure and a bridged structure. However, which structure is more stable is controversial because several calculations yield varied results. Experiments using time-resolved X-ray diffractiona and UV absorptionb indicated that C2H4I has a briedged structure, but our results of Cl + C2H4 in solid para-hydrogen (p-H2) indicated that C2H4Cl has an open structure.c Even though experiments in noble-gas matrices suggest that C2H4Br is open,d whereas C2H4I is bridged,e the spectral evidence remains uncertain. We took advantage of the diminished cage effect of p-H2 to investigate the infrared (IR) spectra of C2H4Br and C2H4I by irradiating p-H2 matrices containing C2H4 and Br2 or I2 with light at varied wavelengths. New spectral lines were grouped according to their behavior upon subsequent annealing and secondary photolysis. The assignments were derived on comparison with scaled vibrational wavenumbers and IR intensities calculated with the B3LYP/aug-cc-pVTZpp method. Our preliminary results indicate that lines at 676.9, 776.7, 1068.1, 1148.0, and 3126.8 cm−1 are assigned to the 2-bromoethyl radical (C2H4Br) in an open form, whereas those at 933.7, 1139.0, 1436.8, and 1609.0 cm−1 to the iodoethyl radical (C2H4I) in the bridged form. A small amount of C2H4Br2 and C2H4I2 was also observed; their stereochemistry will also be discussed.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/100487

DOI

10.15278/isms.2018.FE03

Copyright and License Information

Copyright 2018 Yu-Hsuan Chen

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