C. Dale Keefe

Infrared intensities of liquids XVI. Accurate determination of molecular band intensities from infrared refractive index and dielectric constant spectra

Abstract Absorption spectra of liquids in which the intensities are believed to be absolute rather than relative can be described by several different absorption quantities. The most important of these are the molar absorption coefficient, E m ( ν ), the imaginary refractive index or absorption index, k ( ν ), and the imaginary dielectric constant ϵ″( ν ). These are phenomenological properties of the liquid, and are not independent. With the assumption of a model for the local field which acts on the molecules in the liquid, they can be converted to a molecular quantity, the complex molar polarizability, α ^ m ( ν ). The imaginary molar polarizability, α″ m ( ν ), also describes the absorption spectrum. The lineshapes and peak positions in these different absorption spectra differ in a way that seems not to be fully recognized. Vibrational intensities of the molecules in the liquid can be calculated from any of these spectra as the magnitudes of the transition moments or of the dipole moment derivatives with respect to the normal coordinates, always under an assumption about the local field but also under other approximations for the E m , k and ϵ″ spectra. These intensities can also be calculated, under the same approximations as for the ϵ″( ν ) spectra, from the peak wavenumbers in the ϵ″ and α″ m spectra. This paper illustrates the differences between the lineshapes and peak positions in the different spectra, and explores the accuracy of the vibrational intensities calculated from them. The exploration uses the Lorentz local field, and uses both experimental spectra and spectra calculated from the classical damped harmonic oscillator model. The results show that the α″ m spectrum most reliably gives the molecular properties, but it does impose the Lorentz local field model on the experimental spectrum. Symmetric α″ m and ϵ″ bands correspond to asymmetric k and E m bands, particularly at low wavenumbers, so the α″ m or ϵ″ spectrum should always be used when the lineshape is relevant. Accurate calculation of vibrational intensities can only be done reliably from the α″ m spectrum. Sometimes high accuracy may be obtained for separated bands from the E m , k and ϵ″ spectra, but the anomalous dispersion in the real dielectric constant introduces an uncertainty that increases with band strength and is difficult to assess for any but well separated weak bonds. The consequent errors in the intensities range from 0% to over 20%.

Measurement and use of absolute infrared absorption intensities of neat liquids

Abstract This paper describes the methods used in this laboratory to measure absolute infrared absorption intensities of liquids and their applications in analytical and physical chemistry. The applications include the measurement of 43 bands of benzene, toluene, chlorobenzene and dichloromethane as secondary standards of infrared absorption for the International Union of Pure and Applied Chemistry, the use of these standards to calibrate transmission cells, measurement of the absolute intensity spectra of liquid benzene, the comparison of absorption intensities and transition moments in liquid and gaseous benzene, the separation of the integrated intensity of benzene into contributions from different transitions by curve fitting, and the determination of intensities and transition moments of the OH, OD, CH and CD stretching bands of CH 3 OH, CH 3 OD, CD 3 OH and CD 3 OD. The accuracy of the absolute intensities is impressively shown by the close similarity of the intensities of corresponding bands in the spectra of different isotopomers of methanol. One key to the measurement of accurate intensities is the determination of the baseline absorption.

Infrared Intensities of Liquids XIII: Accurate Optical Constants and Molar Absorption Coefficients between 6500 and 435 cm−1 of Toluene at 25°C, from Spectra Recorded in Several Laboratories

This paper presents accurate infrared absorption intensities of liquid toluene at 25°C. The accuracy is estimated from the agreement between the intensities measured by different spectroscopists using the same instrument in the same laboratory and also by different spectroscopists in different laboratories using instruments made by different manufacturers. The average agreement between integrated intensities over specific wavenumber ranges is about ±1.8%. The spectra from the different laboratories have been averaged, unweighted, to give intensity spectra of toluene that are presented as the best available. The use of data from different instruments in different laboratories has included the influence of systematic instrumental errors, so that the precision of the intensity data presented should be a better approximation to its accuracy than would be the case from an extensive study by one person on one instrument. The results obtained agree with the only measurements that have been made against a primary standard, the estimated accuracy of which is about 6%. The results are presented as graphs and tables of the molar absorption coefficient spectrum and the real and imaginary refractive index spectra between 6500 and 435 cm−1. The peak heights and the areas under the bands in the imaginary refractive index (i.e., absorption index) and molar absorption coefficient spectra are reported. The absorption index, k(ν˜), and molar absorption coefficient, Em(ν˜), values are believed to be accurate to an average ±2.5% at the peaks of 39 strong, medium, and weak bands and ±1.9% at the peaks of 51 very weak bands below 4100 cm−1. Above 4100 cm−1, 11 very weak bands have an average accuracy of ±1.3%. The baseline k(ν˜) values are accurate to between ±3 and ±10%. The areas under bands or band groups in k(ν˜) and εm(ν˜) spectra are accurate to 2.4% on average, or 1.2% for strong, medium, and weak band groups between 3150 and 775 cm−1 with 0.002 < kmax < 0.112. The real refractive index, n(ν˜), values are believed to be accurate to 0.2%.

Comparison of infrared absorption intensities of benzene in the liquid and gas phases

This paper presents a comparison of the absolute infrared absorption intensities in the liquid and gas phases for the four infrared active fundamentals of benzene. In Herzberg’s notation these are ν12 (∼3070 cm−1), and ν4 (∼675 cm−1). Published data are used, including the recently published spectra of liquid benzene that have been accepted by the International Union of Pure and Applied Chemistry as secondary intensity standards. The present results agree qualitatively with the conclusions drawn in 1970 that the intensity Aj of ν12 is much smaller for the liquid than for the gas, and those of ν13, ν14, and ν4 are all larger for the liquid. The inclusion of measurements made since 1970 should make the quantitative results reported here the most reliable. However, two quite different values have been reported in the 1980’s for the intensity of ν14 in the gas phase, and both are considered. The comparison for ν14 is also complicated by the existence of weak bands in the spectrum of the liquid that are not ob...

Computer programs for the determination of optical constants from transmission spectra and the study of absolute absorption intensities

Abstract FORTRAN computer programs used in the determination of optical constants and other physical properties from transmission measurements have been written by the Jones and Bertie research groups. Over the last several years, C++ has emerged as the standard programming language and it has become desirable to convert these programs. In this paper, C++ versions are presented and their accuracy is confirmed. In addition, some new features have been added to some of the programs.

Improper Hydrogen-Bonding CH·Y Interactions in Binary Methanol Systems As Studied by FTIR and Raman Spectroscopy

Fourier Transform infrared spectroscopy and Raman spectroscopy have been used to investigate hydrogen bonding of methanol in different solvents with an aim to explore potential experimental evidence for improper hydrogen bonding involving the methyl group of methanol as suggested by various computational studies. Pure methanol and solutions of methanol in water, acetonitrile, carbon tetrachloride, deuterium oxide, and deuterated acetonitrile have been studied over a range of concentrations. Wavenumber shifts of the CH stretching vibrations were examined to determine if the CH from methanol participates in hydrogen bonding. New concepts of the vibrational wavenumber and integrated intensity at infinite dilution are proposed and given the respective symbols nu(CH(o)) and C(j,CH)*(o). Using the results obtained for methanol in carbon tetrachloride as a reference, shifts in nu(CH(o)) of methanol to higher wavenumbers (blue shifts) were observed in each of the other solvents studied, with the shifts being greatest for the methanol-water interactions. The shifts in vibrational wavenumber suggest possible improper hydrogen bonding, although at this stage a definitive conclusion is not possible. The C(j,CH)*(o) results show that there is no distinguishable change in the methanol CH stretch integrated intensity in carbon tetrachloride and acetonitrile, while there is a significant decrease in the methanol CH stretch integrated intensity in the water solutions.

Determination and Use of Secondary Infrared Intensity Standards

The presentation of absorption intensities in infrared spectra is usually limited to relative intensities instead of absolute intensities. The measurement of absolute intensities can be facilitated by the use of secondary intensity standards. Such standards have been accepted by the Commission on Molecular Structure and Spectroscopy and the Physical Chemistry Division of the International Union of Pure and Applied Chemistry and were published recently. The secondary standards are based on the complex refractive index and molar absorption coefficient spectra of benzene, chlorobenzene, toluene, and dichloromethane. They have been used in this laboratory to calibrate the effective pathlength of a transmission cell and the effective number of reflections in a Circle® multiple attenuated total reflection cell. A computer program, IRYTRUE, has been developed to standardize the routine use of these intensity standards to calibrate the effective pathlength of a transmission cell. The program has been used to calibrate three transmission cells. The agreement between the calibrated values of the effective pathlength obtained from the use of different standard band groups was determined. The calibrated cell pathlength agrees with that calculated from the interference fringe pattern of the empty cell within 3% for very thin cells and within 1% for cells thicker than 100 μm. We propose that the effective pathlength evaluated in this manner be called the cell constant, and that this cell constant be used in place of the pathlength in quantitative infrared analysis. The calibration of multiple attenuated total reflection measurements in the Circle cell has been achieved in two ways: by the use of peak heights and by the use of areas. Programs PCCALC and CIRCLCAL and its associated program RSCALC are described for this purpose. The intensity standards allow one to measure absolute infrared absorption intensities of liquids with confidence to an estimated accuracy of 2–3% by either transmission or calibrated ATR methods.

Infrared optical constants, dielectric constants, molar polarizabilities, transition moments, dipole moment derivatives and Raman spectrum of liquid cyclohexane.

Previous studies have been done in this laboratory focusing on the optical properties of several liquid aromatic and aliphatic hydrocarbons in the infrared. The current study reports the infrared and absorption Raman spectra of liquid cyclohexane. Infrared spectra were recorded at 25 degrees C over a wavenumber range of 7400-490 cm(-1). Infrared measurements were taken using transmission cells with pathlengths ranging from 3 to 5000 microm. Raman spectra were recorded between 3700 and 100 cm(-1) at 25 degrees C using a 180 degrees reflection geometry. Ab initio calculations of the vibrational wavenumbers at the B3LYP/6311G level of theory were performed and used to help assign the observed IR and Raman spectra. Extensive assignments of the fundamentals and binary combinations observed in the infrared imaginary molar polarizability spectrum are reported. The imaginary molar polarizability spectrum was curve fitted to separate the intensity from the various transitions and used to determine the transition moments and magnitudes of the derivatives of the dipole moment with respect to the normal coordinates for the fundamentals.

OPTICAL CONSTANTS AND DIPOLE MOMENT DERIVATIVES OF BROMOBENZENE BETWEEN 3200 AND 400 CM-1 AT 25 C

The optical constants (real and imaginary refractive indices) of bromobenzene were determined at 25 °C via transmission measurements. Experimental absorbance spectra measured on a Nicolet Impact 410 FT-IR were converted to imaginary refractive indices by using methods described in the literature. The real refractive indices were obtained by Kramers-Kronig transformation of the imaginary refractive indices. The complex refractive indices were used to calculate the molar absorption coefficient (Em) and complex molar polarizability (m) spectra. The integrated intensities and dipole moment derivatives with respect to normal coordinates for the fundamentals were obtained from the areas under the bands in the α“m spectrum. These dipole moment derivatives were compared to those obtained from the spectra of chlorobenzene in the literature. It was found that, in general, the dipole moment derivatives displayed very little dependence on the substituent, even for some of the vibrations for which the wavenumber is substituent sensitive.

Infrared Intensities of Liquids. Part VII: Progress towards Calibration Standards

In 1961 the International Union of Pure and Applied Chemistry (IUPAC) formulated calibration standards for wavenumber measurements in the infrared, but comparable standards for infrared intensity measurements are still not available. In 1985 IUPAC established a Working Party on Secondary Standards for Intensity Measurements in Infrared Spectroscopy. This project is now at a stage where it is desirable to solicit advice and experimental support from a wider circle of spectroscopists in anticipation that a specific set of infrared intensity standards and calibration procedures could be formulated for measurements in the liquid phase. This paper outlines the background of these studies and reviews the present status of the project.