C-H
Glynne J. Boobyer and A. Peter cox
Bristol University Bristol 2, England
Bond Strengths
A physical chemistry experiment
Student laboratory courses in physical chemistry often include experiments in the infrared method; usually these are designed to illustrate its use in the determination of properties of molecular structure besides its routine analytical applications. Detailed determinations may be carried out for small molecules only, usually diatomic gases whose experimental manipulation may provide some difficulty, whereas more conveniently handled liquids provide less tractable theoretical problems. Spectrometers used for these exercises are often of the cheaper analytical variety which are incapable of great resolution. Thus the choice of suitable infrared class experiments is somewhat limited, and necessarily these involve a compromise between experimental convenience and ease of theoretical interpretation. The reported experiment involves a few relatively easy measurements of infrared bands of liquid chloroform, bromoform, trichloroethylene, and propargyl bromide, from which a number of C-H bond parameters are determined while few approximations are made in the molecular model. Theory
Any vibration of a polyatomic molecule may be considered as a superposition of its normal modes, each of which in general involves the oscillation of every atom in the molecule. The observed infrared spectra reflect changes in the vibrational energies, and particular bands may be associated with particular normal modes. In many cases a normal mode can be approximately ascribed to the stretching of a particular bond or the deformation of a particular grouping. Such is the case in the stretching of a C-H bond. The light hydrogen atom undergoes most of the motion in this vibration, and the mode is well considered to be localized in the C H bond. This idea is supported by t>heremarkable constancy of characteristic C-H frequencies, and confirmation has been provided by theoretical work, for example the recent treatment by Bernstein ( 1 ) . Thus, a molecule which has only one C-H bond such as chloroform is an excellent example for quantitative study since the C-H stretching mode may be considered on a simple diatomic basis. The simplest diatomic model is the familiar harmonic oscillator in which the restoring force ( F ) between the atoms is proportional to their displacement from equilibrium (r - re): F
=
-ks(r -
7
