Yoram Apelblat

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%.

Differential cross sections for rotationally state‐resolved inelastic scattering of HF by argon

We present differential cross section (DCS) measurements for scattering of HF by Ar. These crossed‐beam experiments employ rotational state sensitivity, allowing determination of the DCS as a function of the scattered HF rotational state. The initial HF rotational distribution is generated by nozzle expansion, without further state selection. Its composition is mostly J=0 and J=1, with small admixtures for J>1. The DCS for each final state J’ is measured using a stabilized cw HF chemical laser, in conjunction with a rotatable liquid He‐cooled bolometer. Measurable signals are obtained for scattering into 0≤J’≤5, where J’=6 is the thermodynamic limit for our collision energy of 120 meV. The measured DCS’s show a strong forward peak, largely from elastic scattering. In addition, the DCS’s evolve from a broad shoulder in the θ≊25°–40° region for J’=0—through a flattening of the wide‐angle scattering for J’=2 and J’=3—to an increase in the scattering beyond ∼40° for J’=4. The DCS for scattering into J’=5 also...

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.

Compact Table for the Publication of Infrared Spectra That are Quantitative on Both Intensity and Wavenumber Axes

A Compact Table format is presented for the publication of infrared spectra that are quantitative on both intensity and wavenumber axes. The format is illustrated with a molar absorption coefficient spectrum, Em(ν˜) vs. ν˜, and with infrared real and imaginary refractive index spectra, n(ν˜) vs. ν˜ and k(ν˜) vs. ν˜, respectively. The algorithm consists of two steps: first, the number of spectral points is reduced by using larger wavenumber spacings than appear in the original spectrum; second, the resulting spectral points are presented in a compressed table format. The Compact Table is about one tenth the size required for the original spectrum to be presented in a conventional XY table. The essential criterion for increasing the wavenumber spacing is that it must be possible to recover the original spectrum by interpolation to an accuracy better than that of the original spectrum. Nearly all the recovered imaginary refractive index and molar absorption coefficient values are within 1% of the original values, and for each quantity the average of the magnitudes of the accuracies of recovery is 0.2%. The real refractive index spectrum is most accurately recovered by Kramers-Kronig transformation of the recovered imaginary refractive index spectrum. Nearly all the recovered real refractive index values are within 0.02% of the original values, and the average of the magnitudes of the accuracies of recovery is 0.005%. The real and imaginary infrared dielectric constant spectra, ɛ′(ν˜) vs. ν˜ and ɛ″(ν˜) vs. ν˜, can be calculated from the recovered data with an accuracy in ɛ′ that is about one half of that of the real refractive index and an accuracy in ɛ″ that is approximately that of the imaginary refractive index. The detailed method is outlined and is applied to infrared intensities of chlorobenzene. Computer programs are presented for the construction of the Compact Table and for the recovery of the full spectrum from the tabulated information.

Infrared Intensities of Liquids XIX: A Simple and Effective Approximate Method for the Calculation of Infrared Optical Constant Spectra of Liquids from Transmission Measurements

A simple and effective approximate method is presented for the calculation of the optical constants of neat liquids from transmission measurements. The method calculates the apparent absorbance due to reflection losses by treating the liquid cell as a single slab of the window material. This approach makes the method far simpler than the exact iterative method that has been used to develop secondary infrared intensity standards and that applies Fresnels equations to each interface in the cell. However, for all but the strongest absorption bands, the approximate method gives imaginary refractive indices that are within ∼1% of those from the exact method. The method is, thus, useful for nearly all common liquids in cells with alkali halide windows for all but the strongest bands. The effect of the size of the mismatch between the real refractive indices of sample and windows has been explored to some extent. It is recommended that results from the approximate method be regarded with caution if the refractive indices of the sample and windows differ by more than 0.15.

Fluoride salts as supersonic nozzle materials for hot fluorine

An intense supersonic beam of atomic fluorine has been generated using nozzles fabricated from single‐crystal CaF2 and MgF2. The latter material has been tested up to 1000 °C with no observable damage. This is ≳250 °C hotter than previously achieved, increasing the atomic beam intensity by ≳5×.

Absolute Infrared Intensities of Binary Mixtures of Liquid Chlorobenzene and Toluene

Absolute infrared intensities of binary mixtures of liquid chlorobenzene and toluene were determined from transmission measurements. In general, the integrated intensities between isosbestic points of the ṽα m ″ spectra were linear with respect to the mole fraction of the pure components. The α m ″ spectra of the pure liquids were curve-fitted with bands of classical damped harmonic oscillator shape. For most fundamentals, the experimental spectra of the mixtures and synthetic spectra obtained by mixing data from the fitted bands of the pure components agreed to within 2–4%. Larger deviations (7–15%) occurred for some of the bands, indicating molecular interaction between chlorobenzene and toluene.

An Approximate Method for the Calculation of the Infrared Molar Absorption Coefficient and Absorption Index Spectra of Liquids from Transmission Measurements

A simple and effective approximate method for the calculation of infrared molar absorption coefficient spectra, E m(Ž), and absorption index spectra, k(Ž), of liquids from transmission measurements is presented. In the approximate method, the apparent absorbance due to reflection losses is calculated by assuming the cell is a single window. This simplifies the correction of the experimental absorbance for the contribution of reflection and eliminates the need for an iterative calculation. Although simpler, the method gives k and E m values accurate to 1 % for all but the strongest infrared absorptions. Areas under these spectra are accurate to 0.5%. The method is demonstrated on a mixture of 90 mole% toluene and 10 mole% chlorobenzene.

Absolute absorption intensities of liquids: the determination of secondary infrared absorption intensity standards; absorption intensities of benzene, chlorobenzene,m and toluene

The presentation of absorption intensities in infrared spectra is usually limited to relative intensities, instead of absolute intensities. The measurement of absolute intensities would be facilitated by the existence of accepted secondary intensity standards that could be used to calibrate the path length of a transmission cell or the number of reflections in an attenuated total reflection cell. Under the auspices of the Molecular Spectroscopy Commission of the International Union of Pure and Applied Chemistry, we have developed proposals for such secondary standards based on the absolute intensities of benzene, chlorobenzene, and toluene.

Absolute absorption intensities of liquids: the use of secondary infrared absorption intensity standards to calibrate transmission cells

As described in another paper in this conference proceedings, we have developed secondary infrared absorption intensity standards based on the complex refractive index and molar absorption coefficient spectra of benzene, toluene and chlorobenzene. As a distinct part of the same project, we have developed a computer program that enables the routine use of these intensity standards to calibrate the pathlength of a transmission cell. Results obtained from cells of independently known pathlengths are presented.