Robert G. Snyder

Vibrational Study of the Chain Conformation of the Liquid n‐Paraffins and Molten Polyethylene

A vibrational and attendant conformational analysis of the liquid n‐paraffins and molten polyethylene is presented. For the purposes of the analysis a valence force field was derived which is applicable to both planar and nonplanar chains. The force field was evaluated from observed frequencies of trans (T) and gauche (G) n‐C4H10; TT and GT n‐C5H12; TTT, GTT, and TGT n‐C6H14; and (T)∞ polyethylene, all of whose infrared spectra were assigned in detail. Infrared spectra of the liquid‐n‐paraffins n‐C4H10 through n‐C17H36 were measured at room temperature and n‐C4H10 through n‐C12H26 also at a temperature just above their melting point. Frequencies and normal coordinates were calculated for the extended forms and for forms having one gauche bond of n‐C4H10 through n‐C8H18. These quantities were also calculated for the conformations of n‐C5H12 through n‐C7H16 having two gauche bonds and for the nonplanar but regular conformations (TG)∞ and (G)∞ of polyethylene. Some bands attributable to forms of n‐C5H12 and ...

Vibrational spectra of crystalline n-paraffins: II. Intermolecular effects

Abstract Intermolecular effects in the infrared spectra of crystalline n-paraffins, n-C20H42 through n-C30H62 are reported. Absorption bands in the spectra of the triclinic form are singlets. Extensive doubling of bands occurs for the mono clinic and orthorhombic structures in accordance with selection rules based on the factor group symmetry. In particular, the methylene rocking mode bands of the orthorhombic structure display a striking pattern of splitting. Frequency separations of the components can be expressed by a simple function of a single parameter, kπ (m + 1) , where m is the number of methylene groups of the n-paraffin and k is an integer which characterizes a particular intramolecular mode. From these data several intermolecular force constants have been evaluated. In relating these force constants to specific methylene interactions, methylenes in adjacent planes were found to interact more strongly than those in the same plane. By extending Steins theory that the splitting arises from short range repulsive forces, to include additional interactions, the force constants are explained satisfactorily. However, interactions which involve only dipole-dipole forces are found to be inadequate.

Vibrational spectra of crystalline n-paraffins: Part I. Methylene rocking and wagging modes☆

Abstract Infrared spectra of crystalline n-paraffins from n-C20H42 through n-C30H62 are reported. It is found that to an excellent approximation, the frequencies of methylene rocking and wagging modes are a function of a single parameter. This parameter is related to the phase difference, ϕ, in the motion between two adjacent methylene groups of a given chain. From the infrared frequencies, explicit equations have been derived which give methylene rocking and wagging frequencies as a function of ϕ. Extrapolation to infinite chain length indicates some revisions in earlier infrared and Raman assignments in the vibrational spectrum of polyethylene.

Vibrational analysis of a series of n-alkyl chlorides in the solid state: Ethyl chloride through n-hexyl chloride

Abstract The infrared spectra of a series of n-alkyl chlorides, ethyl through n-hexyl, have been measured in the solid state at low temperature. Only two conformations of each molecule (for ethyl chloride only one) were found to be present. In both forms the carbons are coplanar. In one case (trans) the chloride is also in the plane and in the other (gauche) it is not. A nearly complete vibrational assignment for each form is given for the region 1500-290 cm−1. The analysis is based on normal coordinate calculations using force constants most of which were transferred from the n-paraffins. Twelve of the constants associated with the -CH2Cl group were adjusted until the average error between the calculated and observed frequencies of some 210 fundamentals reduced to 3.9 cm−1. These results support our recent interpretation of the vibrations of the extended n-paraffins.

Group Moment Interpretation of the Infrared Intensities of Crystalline n‐Paraffins

The infrared intensities of bands in the spectra (3000–700 cm−1) of the crystalline n‐paraffins n‐C4H10 through n‐C8H18 have been measured and interpreted using a group moment model. In this model intensities are expressed in terms of dipole moment derivatives with respect to group coordinates, i.e., coordinates describing the motions of CH3 and CH2 groups. In spite of a severe limitation in the accuracy of the measured intensities caused by preferred orientation in the polycrystalline films, a good correlation between calculated and observed intensities has been obtained.

Raman scattering activities for partially oriented molecules

Abstract Raman scattering activities have been derived for two cases of uniaxially oriented molecules. In the first case, one principal axis of the molecule is held in fixed orientation while the other two axes are randomly oriented. In the second case, one principal axis is confined to a plane while, as before, the other two axes are randomly oriented. Expressions for scattering activities in terms of elements of the derived polarizability tensor are tabulated for three orthogonal sample orientations and for both right-angle and back viewing. These expressions should be useful in interpreting Raman spectra of samples such as stretched polymers, partially ordered polycrystallites, molecules or macro-molecules partially ordered by shear flow, or molecules contained in liquid crystals or clathrates.

Vibrational Spectrum of Crystalline Potassium Hydroxide

The infrared spectrum of KOH has been obtained in the region 4000–300 cm—1. At 23°C the spectrum shows two broad bands, one at 3600 cm—1 and the other at 643 cm—1. At —180°C the O–H stretching band near 3600 cm—1 appears as a doublet. Splitting is shown to arise from coupling between the two OH— ions in the unit cell. Evidence is presented that indicates a small difference in the strengths of the hydrogen bonds in KOH and KOD. The first overtone of the O–H stretching band is found at 6868 cm—1 and from this a value of ωexe of 110±11 cm—1 is derived for KOH at —180°C. The 643 cm—1 band of KOH at 23°C is identified as a librational fundamental. At —180°C this band appears at 709 cm—1 together with bands at 840 and 671 cm—1; these latter two bands are also identified as librational modes. These librational frequencies lead to a considerably higher barrier to rotation of the OH— ions in KOH than in Mg(OH)2 or Ca(OH)2. This higher barrier is probably the result of the weak hydrogen bonding in KOH. Spectra of K...

Structure of Potassium Hydroxide: An X‐Ray and Infrared Study

Infrared evidence has been obtained which indicates that the crystal structure of KOH reported in the literature is incorrect. A subsequent x‐ray examination has shown that this is the case and has led to a structure compatible with the infrared spectrum. KOH crystallizes in the monoclinic space group C 2 2–P21 (chosen over P21/m on the basis of the infrared evidence), with two molecules in a cell of dimensions a=3.95, b=4.00, c=5.73 A, β=103.6°. Each potassium atom is surrounded by a distorted octahedron of oxygen atoms. There is an oxygen‐oxygen zigzag chain parallel to the b axis. Stereochemical arguments are advanced to suggest that the hydrogen atoms are near to or lie in the plane of the zigzag oxygen chain; infrared data obtained at —180°C for KOH and KOD are interpreted as supporting the placement of the hydrogen atoms in ordered positions along the chain with the O–H···O bonds (of length 3.35 A) nearly or exactly linear. It is argued that breaking of these hydrogen bonds will lead to the high‐temperature cubic structure of KOH reported in the literature.

Interpretation of the Raman spectrum of polyethylene and deuteropolyethylene

Abstract The Raman spectrum of polyethylene and deuteropolyethylene have been calculated using polarizability parameters from cyclohexane. It is shown that the spectrum of the stretched polymer measured by Hendra and Willis is not in conflict with the previously established assignments. From this spectrum the sign relationship between the spherical part and the anisotropy of the derived polarizability tensor can be determined. Polarizability components for CH and CC bonds are reported. It is concluded that the quantity Γ CC (= β β CC − 2 β β CH ) is negative, contrary to general opinion.

A bond polarizability interpretation of the Raman intensities of cyclohexane and cyclohexane-d12

Abstract The Raman intensities of cyclohexane and cyclohexane- d 12 are interpreted in terms of a modified bond polarizability model. The main emphasis is on the vibrations below 1500 cm −1 . Both the spherical and anisotropic parts of the derived polarizability tensors are considered. The observed values of the derived mean polarizabilities were used to evaluate three parameters α ′ CC , α ′ HCH , and α ′ CCC while in a less satisfactory but nonetheless illuminating calculation the derived anisotropies were used to evaluate three more parameters β ′ CC , β ′ CH , and β ′ CC . Uncertainties in the data, the normal coordinate the model are discussed.