Willis B. Person

Infrared Spectra of Charge‐Transfer Complexes. VI. Theory

The general theory of molecular vibrational transition intensities is discussed with emphasis on electronic reorientation contributions to the intensities. The wavefunctions used by Mulliken to represent the electronic states of donor—acceptor complexes are written to include an explicit dependence on the vibrational coordinates. These functions and the general theory are applied to the intensities of halogen vibrations in donor—acceptor complexes. Specific application to actual complexes requires the estimation of the derivative of the vertical electron affinity of the halogen molecule with respect to its internuclear distance, the electronic transition moment of the charge‐transfer band, the coefficients in the donor—acceptor ground‐state wavefunctions and the difference between the energies of the dative‐ and no‐bond states. Evaluation of each of these parameters is discussed for a number of complexes of halogens and relationships between the wavefunction coefficients and the infrared frequency shifts ...

Liquid‐Gas Infrared Intensities, Pressure‐Induced Absorption, and the Temperature Dependence of Infrared Intensities in Liquids

The recent paper by Polo and Wilson eliminates the apparent discrepancy between the equations given for the relationship between intensities of a compound in the gas phase and the intensities in the liquid phase. However, the experimental tests of this equation have not been conclusive. This paper applies their equation to the pressure‐induced absorption of methane, and to the temperature dependence of the integrated intensities of liquid cyclohexanone and liquid benzene. The treatment of Polo and Wilson is extended to give an equation which is valid for solutions. In all cases, qualitative agreement is obtained between experiment and calculation and it would seem that the effect of index of refraction on the intensity accounts for a significant part of the changes observed. The agreement for benzene and for the pressure‐induced absorption of methane is excellent.

Infrared and Raman Spectra, Force Constants, and the Structures of Some Polyhalide Ions: ICl2−, ICl4−, BrCl2−, and Br3−

The infrared and Raman spectra for the polyhalide ions (ICl2−, ICl4−, BrCl2−, and Br3−) are presented. From these data, the X—Y stretching force constant fr, the interaction force constants frr between bond stretching coordinates at 180° to each other, and the interaction force constant frr′ for bond stretching coordinates at an angle of 90° (for ICl4−) have been calculated. The values of fr for the trihalide ions are roughly one‐half the values for the free halogens, and the values of the interaction force constants frr are very large (approximately 35% the value of the stretching constant fr). These rather unusual force constants have been interpreted in terms of the description of the bonding in these ions using p orbitals, which was first suggested by Pimentel. In fact, these results offer rather strong support for the recent evidence from nuclear quadrupole coupling constant measurements of Cornwell and Yamasaki favoring this structure. Attention is drawn to the qualitative similarity between these f...

Absolute Infrared Intensities in Some Crystalline Hydrogen Halides

Absolute intensities of the fundamental vibrational bands of HCl and HBr have been measured in the absorption spectrum of crystalline films. The path length has been determined from the interference fringes of the transmitted light as the film is deposited on a window, following the technique of Hollenberg and Dows. The measured integrated molar absorption coefficients B are 24 000±3000 and 17 600±1200 darks (cm—1cm2/mM), respectively, for HCl and HBr. These values are larger than the gas phase intensities by factors of 6 and 13, respectively. Such large intensification of an X—H stretching vibration is consistent with the large frequency shift and is characteristic of hydrogen‐bonded crystals. The results are compared with those from other hydrogen‐bonded systems. Furthermore, the high intensities are consistent with those required by Hornig and Hiebert using the transition dipole—transition dipole model to explain the observed intermolecular force constants. However, it is likely that other terms contri...

Absolute Infrared Intensities of Some Linear Triatomic Molecules in the Gas Phase

Infrared intensities for the low‐frequency bending vibrations ν3 have been measured by the curve‐of‐growth method for N2O, COS, and CS2 in the gas phase. The intensity of ν3 for gaseous COS has been measured by the same method. These results help to determine which of the values reported previously are to be preferred. These preferred values are listed for the intensities of all the fundamental vibrations.

Raman Intensity Studies on CCl4 in Various Systems

Solution measurements are presented of the Raman intensity changes occurring in the four fundamental vibrations of CCl4 upon changing from “inert” solvents to ones which can act as electron donors. Donor solvents containing oxygen have little or no effect on the Raman vibrational intensities of CCl4, while significant changes are found in the intensities of ν1, ν3, and ν4 in solutions containing aromatic and amine donors. The nature and magnitude of the observed Raman intensity changes are similar in the latter solutions suggesting that they are caused by a common effect. The observed changes in the intensities of the CCl4 fundamentals are analyzed in terms of changes in bond polarizability parameters. It is concluded that the observed intensity changes are due to specific intermolecular interactions, such as weak charge transfer complex formation.

Absolute infrared intensifies of CS2 fundamentals in gas and liquid phases. An interpretation of the bond moments of CO2 and CS2

Abstract Infrared intensities have been measured for ν 2 of CS 2 in the gas and liquid phases. The values (500 darks in the gas phase and 800 darks in the liquid) are approximately as expected from the Polo—Wilson equation, but both are somewhat lower than previous estimates had suggested. The bond moments are e (= d μ/ dr ) = 5·6 D/A and μ = 0·60 D, or roughly comparable to the values for other linear triatomic molecules. The explanation for the anomalously high value of e seems almost certainly to be the large contribution from the “delocalization moment” as a result of a change in the electronic wave function during the vibration. An attempt to estimate the magnitude of the “delocalization moment” is presented; the results confirm the interpretation, although the quantitative agreement could be better.

Absolute infrared intensities of the fundamental absorption bands in solid CCl4

Abstract The absolute infrared absorption intensifies of the two active fundamental vibrations of CCl4 have been measured for the solid at —170°C. The thickness was determined by interference fringes. Intensity measurements are also reported for ν4 in the gas and liquid phases. The strong CCl stretching vibration, ν3, in Fermi resonance with ν1 + ν4, has an intensity in the solid of 45.3 ± 6.5 kdarks, almost the same as in the liquid state, but somewhat higher than in the gas phase. An increase in intensity from the gas phase is also observed for the weak bending mode, ν4, which has an intensity of 0.08 ± 0.02 kdarks in the solid state. These values suggest that the CCl4 solid shows approximately “ideal” behavior, as judged by the infrared intensities, since they are predicted fairly well from the gas phase intensities by the field effect. In an attempt to display infrared intensity data from condensed phase in a more meaningful way, the “delocalisation moment” is defined and the results for CCl4 and for some linear triatonmic molecules are presented.

Infrared Studies of Crystal Benzene. I. The Resolution and Assignment of v20, and the Relative Magnitudes of Crystal Fields in Benzene

Under the higher resolution available with double pass prism and small grating infrared spectrometers, it has finally been possible to observe splitting of the degenerate fundamentals of benzene in the solid phase. Three components were found for v20 at 1032.6, 1034.7, and 1039.4 cm—1, as predicted by Zwerdling and Halford. These are assigned to the absorptions along the b, c, and a axes, respectively, by a mixed crystal study of benzene‐d6 in benzene combined with the polarized work of Zwerdling and Halford. The crystal splitting of v20 was also observed in the benzene‐d6 crystal, showing components at 806.5, 808.5, and 812.2 cm—1 assigned to absorptions along the b, c, and a axes, respectively. The magnitudes of the static and correlation fields are thus found to be similar, contrary to the ideas advanced by Zwerdling and Halford, and long accepted intuitively. The effect of temperature on the appearance of the spectrum was studied. An explanation for the changes observed is advanced on the basis of the...

Infrared Studies of Crystal Benzene. II. Relative Intensities

Intensities of the absorption bands observed with a thin sample of polycrystalline benzene have been measured relative to the absorption of v20 at 1036 cm—1. The gas‐phase‐allowed fundamentals are much the strongest bands in the spectrum. The relative intensities of these fundamentals are considerably different from the relative intensities in the gas or liquid phase. The experimental errors are discussed in detail with the conclusion that the observed difference in relative intensities are well outside any conceivable experimental errors. As a result, it is concluded that existing theories of spectra in condensed phases must be modified to predict different behaviour for each fundamental vibration. Finally, attention is drawn towards some of the anomalies still existing in the assignment of vibrational frequencies in the benzene molecule.