5.2: Introduction
- Page ID
- 373230
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)This module uses a simple homodiatomic molecule, \(\ce{I2}\), to demonstrate that molecules have quantized vibrational and electronic states, and that the transitions between electronic and vibrational states can be coupled.
- Collection and analysis of the Absorption Spectrum of Iodine Vapor
- Collection and analysis of the Emission Spectrum of Iodine Vapor
Chemists use spectroscopy to determine fundamental properties of molecules, and to monitor interesting chemical and physical changes. In this module, you will use linear spectroscopy to monitor the absorption and emission of iodine vapor. This is a demonstration of how scientists use sensitive spectroscopic methods to determine the structure and properties of molecules.
Iodine is purple in color. At room temperature, it is in equilibrium between a solid and gaseous state (see Figure). You can probably draw the Lewis structure and molecular orbital diagram of \(\ce{I2}\).
![Screenshot 2023-10-09 at 10.56.31 AM.png](https://chem.libretexts.org/@api/deki/files/462888/Screenshot_2023-10-09_at_10.56.31_AM.png?revision=1)
To know what a molecule is at the atomic scale we must use instruments that are more sensitive than our eyes. We know from the Lewis dot structure of I2 that it consists of two iodine atoms with a single bond between them. But the simple Lewis structure, and rigid dumbbell model that comes to mind, does not describe the dynamics of the iodine molecule.
Consider this for example:
- Using the simple models you learned in Gen. Chem (Lewis dot stuctures or simple MO diagrams), how many electronic transitions should we expect between the HOMO and LUMO of iodine?
You might expect just one electronic absorption from the HOMO to the LUMO. However, the UV-vis absorption spectrum shows many different closely spaced absorptions, and the emission spectrum also shows several wavelengths of light upon excitation.
The fine structure of the electronic absorption and emission spectra are due to the many vibrational states that occur in \(\ce{I2}\) when it is in its ground and excited electronic states. In other words, the iodine atoms move in space with respect to each other, which effects the energy of the initial state. Furthermore, the electron excitation alters the bonding in the molecule, effecting vibrational states.
The vibronic structure in a UV-Visible absorption spectrum reveals the vibrational energy levels in the excited electronic states of iodine. From these levels a detailed description of the inter-atomic potential between the iodine atoms in these excited states can be obtained. And, the vibronic structure in the emission spectrum from the excited states of iodine reveals the ground state vibrational levels, from which the ground state potential energy curve can be constructed. Finally, comparison of the ground and excited state potentials determined in this way reveals how the bond length changes and how the vibrational potential softens (or stiffens) on electronic excitation.