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Notes: 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 observed in that of the gas. It is noted in this work that the traditional comparison, of the areas under the molar absorption coefficient spectra, Aj, for the gas and liquid through the Polo–Wilson equation, has the drawback that the ratio expected if the dipole moment derivative is unchanged is different for each band as well as for each liquid.A much more convenient ratio, that equals unity for all bands of all liquids under the traditional assumptions, is proposed through the imaginary molar polarizability spectrum of the liquid. The magnitudes of the transition moments and the dipole moment derivatives with respect to the normal coordinates under the double harmonic approximation are calculated from the measured intensities for the gas and liquid phases. It is found that the dipole moment derivative of ν12 is 24% smaller in the liquid than in the gas and that of ν13 is 18% larger. The dipole moment derivative of ν4 is unchanged by condensation. The change in the dipole moment derivative of ν14 is not clear, because of the uncertainty in the gas phase intensity and because of the uncertain origin of the intensity of the additional bands in the liquid.

Notes: This paper reports absolute infrared absorption intensities of liquids methan-d3-ol (CD3OH) and methanol-d4 (CD3OD) at 25 °C between 8000 and 350 cm−1. Measurements were made by multiple attenuated total reflection spectroscopy with the CIRCLE cell, and by transmission spectroscopy with transmission cells fitted with calcium fluoride windows. In both cases, the spectra were converted to infrared real and imaginary refractive index spectra. The refractive indices obtained by these two methods agreed excellently and were combined to yield an imaginary refractive index spectrum k(ν˜) between 7244 and 350 cm−1 for CD3OH and between 5585 and 350 cm−1 for CD3OD. The imaginary refractive index spectrum was arbitrarily set to zero from 8000 to 7244 cm−1 (CD3OH) or 5585 cm−1 (CD3OD), where k is always less than 4×10−6, in order that the real refractive index can be calculated below 8000 cm−1 by Kramers–Krönig transformation. The results are reported as graphs and tables of the refractive indices between 8000 and 350 cm−1, from which all other infrared properties of the two liquids can be calculated. The estimated accuracy, not precision, of the imaginary refractive index is ±3%, except for ±10%, where k is less than 4×10−5. The estimated accuracy of the real refractive index is better than ±0.5%.In order to obtain molecular information from the measurements, the spectra of the imaginary polarizability multiplied by wave number ν˜αm‘ were calculated under the assumption of the Lorentz local field. The area under these ν˜αm‘ spectra was separated into the integrated intensities of different vibrations. The magnitudes of the transition moments were calculated from the integrated intensities, and the double harmonic approximation was used to calculate the magnitudes of the dipole moment derivatives of the liquid-state molecules with respect to the normal coordinates. Dipole moment derivatives with respect to internal coordinates were calculated under the simplest approximations, the validity of which is demonstrated by the experimental data in many cases. The consistency of the dipole moment derivatives with respect to internal coordinates obtained for different isotopomers is shown through their relative rotational corrections. Results are presented for the O–H, O–D, C–H, and C–D stretches; the C–O–H in-plane bending; and the D–C–O–H and D–C–O–D torsion vibrations.

Notes: This paper addresses the separation of the contributions to the visible refractive index of colorless liquids from electronic (ultraviolet) and vibrational (infrared) absorption. The goal is to find the most accurate infrared values of nel(ν˜), the refractive index that results solely from electronic absorption, by fitting and extrapolating currently available visible refractive index data. These values are needed, interalia, to improve the accuracy of infrared real refractive index spectra calculated by the Kramers–Kronig transform of infrared imaginary refractive-index spectra. The electronic molar polarizability αel(ν˜) is calculated from the values of nel(ν˜) at wave numbers between 20 500 and 0 cm−1. The methods are applied to ten liquids: H2O, D2O, CH3OH, CH3COOH, CH3CN (CH3)2CO, CH2Cl2, C6H6, C6H5Cl, and C6H5CH3. The visible refractive indices are expressed as power series in wave number, by expansion of the Kramers–Kronig integral. Terms in ν˜+2m, m=1,2, are due to the electronic contribution and terms in ν˜−2m are due to the vibrational contribution.The vibrational contribution to the visible refractive index is also calculated from experiment by Kramers–Kronig transformation of the known infrared imaginary refractive index spectrum of the liquid. It is shown that the vibrational absorption contributes ≥0.001 to the visible refractive index only for the four hydrogen-bonded liquids, and that, for all ten liquids, at least 25% of the vibrational contribution arises from absorption below 2000 cm−1. If the vibrational intensities are not known, the available visible refractive indices yield the most accurate infrared values of nel for all liquids except H2O if they are fitted to the equation n=a0+a2ν˜2+a4ν˜4. A similar equation, with the additional term a2ν˜−2, is theoretically superior because the latter term adequately describes the vibrational contribution to the visible refractive indices, but only for H2O are the currently available visible refractive indices sufficiently accurate and sufficiently extensive to allow the four coefficients in the equation to be determined with useful accuracy. For H2O, D2O, CH3OH, CH2Cl2, C6H6, C6H5Cl, and C6H5CH3, corrections are given to slightly improve the accuracy of the previously published infrared real refractive-index spectra. © 1995 American Institute of Physics.

Notes: A new and simple procedure is presented for the calculation of the infrared real, n, and imaginary, k, refractive index spectra from s-polarized attenuated total reflection (ATR) spectra by a modified Kramers–Kronig transform of the reflectance to the phase shift on reflection. The procedure consists of two parts, first a new modified Kramers–Kronig (KK) transform, and second a new, wave number-dependent, correction to the phase shift. The procedure was tested with ATR spectra which were calculated from refractive index spectra that were synthesized under the classical damped harmonic oscillator model. The procedure is far more accurate than previous procedures for the real case of a wave number-dependent refractive index of the incident medium, and yields n and k values that are accurate to ≤0.1% provided that no errors are introduced by the omission of significant reflection bands. This new procedure can be used to obtain optical constants from any ATR experiment that yields the spectrum of Rs, the reflectance polarized perpendicular to the plane of incidence. In this laboratory Rs spectra are obtained from samples held in the Spectra-Tech CIRCLE cell in a Bruker IFS 113 V spectrometer. Accordingly the ATR spectra used to test the new procedure were calculated for the optical configuration of this system, which is m reflections at 45° incidence with equal intensities of s- and p-polarized light and retention of polarization between reflections. For the previously studied [J. S. Plaskett and P. N. Schatz, J. Chem. Phys. 38, 612 (1963); J. A. Bardwell and M. J. Dignan, ibid. 83, 5468 (1985)], but unreal, case of constant refractive index of the incident medium, n0, the new transform gave better results than either of two previously studied procedures. In this case the phase shift at each wave number was corrected by a constant which ensured that the correct phase shift was obtained at the highest wave number in the transform, 7800 or 8000 cm−1. In contrast to a previous study [J. Chem. Phys. 83, 5468 (1985)] it was found that the normal KK transform is inferior for this case to a previous modified KK transform [J. Chem. Phys. 38, 612 (1963)], and it is also inferior to the new modified KK transform. Further, the new transform has only the usual singularity of a KK transform, and this makes it numerically superior to the previous modified KK transform which has an additional singularity at 0 cm−1. For the real case, in which the refractive index of the incident medium changes with wave number, the new transform was used with a new simple wave number-dependent additive correction to the phase shift. This new correction is calculated with the actual value of n0 at each wave number. For molecular liquids such as methanol and benzene the new transform is markedly superior to the previous two transforms. It yields real and imaginary refractive index values that are accurate to better than 0.1% provided the reflection spectrum is known down to 2 cm−1. The latter condition is rarely fulfilled, and the effect of the omission of low wave number bands is illustrated. A method to reduce the impact of missing low-wave number parts of the reflectance spectrum is described, and its effectiveness is illustrated. © 1996 American Institute of Physics.

Notes: This paper is concerned with the peak wave number of very strong absorption bands in infrared spectra of molecular liquids. It is well known that the peak wave number can differ depending on how the spectrum is measured. It can be different, for example, in a transmission spectrum and in an attenuated total reflection spectrum. This difference can be removed by transforming both spectra to the real, n, and imaginary, k, refractive index spectra, because both spectra yield the same k spectrum. However, the n and k spectra can be transformed to spectra of any other intensity quantity, and the peak wave numbers of strong bands may differ by up to 6 cm−1 in the spectra of the different quantities. The question which then arises is "which infrared peak wave number is the correct one to use in the comparison of infrared wave numbers of molecular liquids with wave numbers in other spectra?" For example, infrared wave numbers in the gas and liquid phase are compared to observe differences between the two phases. Of equal importance, the wave numbers of peaks in infrared and Raman spectra of liquids are compared to determine whether the infrared-active and Raman-active vibrations coincide, and thus are likely to be the same, or are distinct. This question is explored in this paper by presenting the experimental facts for different intensity quantities. The intensity quantities described are macroscopic properties of the liquid, specifically the absorbance, attenuated total reflectance, imaginary refractive index, k, imaginary dielectric constant, ε″, and molar absorption coefficient, Em, and one microscopic property of a molecule in the liquid, specifically the imaginary molar polarizability, αm″, which is calculated under the approximation of the Lorentz local field. The main experimental observations are presented for the strongest band in the infrared spectrum of each of the liquids methanol, chlorobenzene, dichloromethane, and acetone. Particular care was paid to wave number calibration of both infrared and Raman spectra. Theoretical arguments indicate that the peak wave number in the αm″ spectrum is the correct one to use, because it is the only one that reflects the properties of molecules in their local environment in the liquid free from predictable long-range resonant dielectric effects. However, it is found that the comparison with Raman wave numbers is confused when the anisotropic local intermolecular forces and configuration in the liquid are significant. In these cases, the well known noncoincidence of the isotropic and anisotropic Raman scattering is observed, and the same factors lead to noncoincidence of the infrared and Raman bands. © 1998 American Institute of Physics.

Notes: The infrared spectra of ND3⋅D2O and hydrogen or deuterium impurity in NH3⋅H2O or ND3⋅D2O at 100 K are reported for the first time. Their interpretation is aided by a D2h pseudosymmetry caused by the heavy atoms being nearly coplanar in the P212121, D42, crystal. No evidence of the possible disorder of the hydrogen atoms is observed. Oriented gas model calculations gave the approximate relative intensities of the unit cell components of the molecular vibrations. The site splitting and correlation splitting are comparable for ν3 and, probably, ν4 of ammonia; three unit cell components are observed instead of the six predicted under D2, three components being calculated to have near-zero intensity. The symmetric deformation of ammonia, ν2, yields two unit cell modes with significant intensity, separated by 37 cm−1 for NH3 and 23 cm−1 for ND3. The isotope frequency ratio for ν2 is lower thanfor any other mode, so this large splitting must be due to intermolecular coupling, probably transition dipolar in origin. The two strong νOH (HDO) absorptions are 140 cm−1 further apart than the two strong νOH (H2O) absorptions, a surprising result because intramolecular coupling is negligible because νO–H⋅⋅⋅N is ∼400 cm−1 below νO–H⋅⋅⋅O. In contrast, νOD (D2O) yields four strong absorptions approximately centered with respect to the two strong νOD (HDO) absorptions. The O–D⋅⋅⋅N doublet is due to the B1 and B2 unit cell group components, the B3 component being too weak to see, as is the case for ν2 of ammonia. The corresponding O–H⋅⋅⋅N doublet is unresolved. Use of the O–D⋅⋅⋅O–D'interaction force constant of the ice phases, −0.10 mdyn A(ring)−1, and oriented gas model calculations of the relative intensities shows that the O–D⋅⋅⋅O bands at 2390 and 2459 cm−1 are due to the in-phase (B3) and out-of-phase (B2) motions of the two O–D bonds in each chain of water molecules in each unit cell. In NH3⋅H2O the out-of-phase O–H⋅⋅⋅O vibration interacts, probably with 2ν4 of ammonia, and its intensity is dissipated among several weak features. We are unable to explain the difference between the coupled and uncoupled O–H⋅⋅⋅N frequencies, 2887 and 2825 cm−1, in NH3⋅H2O. The bending mode of water is tentatively assigned at 1696, 1467, and 1243 cm−1 for H2O, HOD, and D2O, respectively. The lattice absorptions are assigned to rotational or translational motion. The rotational modes of ammonia are assigned to the B1, B2, and B3 unit cell modes that have significant intensity.