Development of Infrared Electroabsorption Spectroscopy and its Application to Molecular Structural Studies

Abstract

We have constructed an infrared electroabsorption spectroscopic system employing the alternating current (AC) coupled dispersive method developed previously in our laboratory. It is highly sensitive, measuring electric-field-induced infrared absorption changes as small as 6 x 10(-8) in the mid-infrared region of 4000-820 cm(-1). Infrared electroabsorption spectra of liquids and solutions can be measured at room temperature. The system was first applied to liquid acetone to observe the orientational as well as the electronic polarization signals. These signals show distinct behaviors that are exactly predicted by theory. It was then applied to liquid 1,2-dichloroethane, for which electric-field-induced gauche/trans equilibrium changes were detected as infrared absorption changes. The gauche/trans ratio in room-temperature liquid 1,2-dichloroethane has been determined for the first time as 1.4 +/- 0.2. This equilibrium constant has led to the free energy difference deltaG of 0.9 +/- 0.4 kJ mol(-1) (deltaG = G(g) - G(t)) and the entropy difference of deltaS = -3.0 +/- 1.4 J K(-1) mol(-1) with the known enthalpy difference deltaH approximately 0.0 kJ mol(-1). Finally, the system was applied to 1,4-dioxane solutions of N-methylacetamide. The dipole moments of the monomer as well as that of the dimer of N-methylacetamide have been determined separately and a head-to-tail structure of the dimer has been clarified. The instrumentation and the theoretical basis of infrared electroabsorption spectroscopy are described in detail together with the above-mentioned experimental results.

Comparative study of electroabsorption spectra of polar and nonpolar organic molecules in solution and in a polymer film

“…11,12 In solution, on the other hand, molecules are easily reoriented and the fieldinduced reorientation can be detected by the EA measurements. [13][14][15] However, the precise measurements of the EA spectra in solution are not so easy due to the difficulty in application of strong electric fields to solutions.…”

Foreword

“…18 An IR detection technique with very high sensitivity allowed the measurement of the effect of electric field on molecules by infrared spectroscopy. 19 Time-and space-resolved Raman microscopic studies, initially of yeast and of live plant and animal cells, revealed the 'Raman spectroscopic signature of life at 1602 cm 1 ', which clearly reflects the metabolic activity of mitochondria. 20 Hamaguchi thinks that this band serves as a useful marker for the physiological activity of each cell, or even of life itself.…”

Field-effect transistor configuration for the measurement of infrared Stark spectra

“…On the basis of Eqs. (1) and (2), the change in absorbance under the influence of an external field for an isotropic, immobilized sample can be quantitatively expressed as [1,4,5,11] with…”

“…Although infrared Stark spectra have been reported for small molecules [2][3][4][5], proteins [6], and polypeptide [7,8], the number of samples measured is limited. It results from the difficulty in the measurements of infrared Stark spectra.…”