Samuel Krimm

Vibrational spectra in the CH stretching region and the structure of the polymethylene chain

Abstract The CH stretching regions of both the Raman and i.r. spectra of the extended polymethylene chain have broad secondary maxima. In the case of the Raman spectrum, line shapes are dependent on the environment of the chain, a fact which has been previously exploited in the study of biosystems. We have explained this phenomenon in terms of Fermi resonance interaction between the methylene symmetric CH stretching mode and appropriate binary combinations involving the methylene bending mode. It is emphasized that appropriate binary states are to be found throughout the Brillouin zone and not just at its center. It is the resulting continuum of binary states which leads to broad secondary bands. The shapes of these bands depend on the dispersion of the bending mode fundamental. For the isolated chain only parallel dispersion is involved, but in the case of the crystal perpendicular dispersion is equally important and leads to the observed dependence on crystal structure. All secondary bands have been accounted for in these terms. The ratio of the Raman intensities of the symmetric CH stretching fundamentals to the antisymmetric is found to be about 5 and is independent of environment. The relevance of these results to studies on biosystems is briefly discussed.

Infrared Spectra of High Polymers. II. Polyethylene

The infrared spectrum of polyethylene has been obtained between about 3000 cm—1 and 70 cm—1, polarization measurements on oriented specimens having been obtained to about 350 cm—1. Assignments of the fundamentals are made with the help of a group theory analysis. The assignment of the controversial CH2 wagging mode is discussed in detail and especially in terms of new evidence from the spectra of n‐paraffins, both as single crystals and as polycrystalline aggregates. It is shown that this mode is to be assigned to a weak band at 1369 cm—1. A satisfactory determination of the bands which arise from CH3 groups is also made possible by a study of the paraffin spectra. The splitting of bands in the spectrum is conclusively shown to arise from interactions between molecules in the crystalline phase. The nature of this interaction is discussed in terms of recent theories.

Crystal Vibrations of Polyethylene

Several problems involving the internal and external vibrations of the polyethylene crystal have been studied. The splittings of some of the internal vibration bands arising from transition dipole coupling have been evaluated and found to have small but nonnegligible values as compared with the splittings calculated from the intermolecular H···H interaction potential. On the other hand, interactions between permanent CH2 dipole moments in different chains have been shown to make quite insignificant contributions to the translational lattice frequencies. The effects on the vibrational frequencies of cell contraction with decreasing temperature have been calculated, and the experimentally observed upward shift of a lattice frequency is found to be explainable primarily on this basis. The effect caused by the change of the setting angle of each chain in the unit cell has also been examined. The short‐range H···H interaction force constants and the dispersion curves of normal and deuterated polyethylenes have...

Infrared spectra and chain conformation of proteins.

The infrared spectra of various proteins have been analysed in terms of the conformations of their component polypeptide chains. The general method used was that of Miyazawa & Blout (1961) . Several modifications have been made in this method which result in better agreement between observed and calculated frequencies. In addition to the standard structures treated previously, viz. the α -helix and the parallel-chain and antiparallel-chain pleated sheets, the calculations have been extended to the polar chain pleated sheet and the polyglycine II structures. It is shown from an analysis of the infrared spectrum that a polar chain structure is favored for feather keratin. The spectrum of tobacco mosaic virus protein is also discussed in terms of the results of the present theory. The analysis indicates that only a fraction of this protein is in the α -helical form, and that the axes of the α -helices deviate by about 30 ° from being perpendicular to the axis of the virus.

Infrared Spectra of High Polymers. III. Polytetrafluoroethylene and Polychlorotrifluoroethylene

The infrared spectra of polytetrafluoroethylene and polychlorotrifluoroethylene have been obtained between 4000 cm—1 and 70 cm—1. Polarization measurements on oriented samples were obtained in the region of 4000 cm—1 to 300 cm—1. It is known from x‐ray diffraction studies that both polymers have helical chain configurations. From a factor group analysis of the one‐dimensional space group, selection rules and approximate vibrational patterns of the infrared active fundamentals have been derived. A calculation of the normal frequencies of an assumed planar zig‐zag chain model of (CF2)n has been made, resulting in a satisfactory assignment of the bands in the infrared spectra of both polymers.

Characteristic infrared frequencies of rotational isomers of alkyl chlorides

Abstract A study of the infrared spectra of primary, secondary and tertiary alkyl chlorides reveals that the frequency of the so-called C-Cl stretching mode is a function of the detailed structure in the vicinity of the C-Cl bond. In primary chlorides the frequency region depends on whether a hydrogen atom or a carbon atom is trans to the Cl atom across the common C-C bond. In secondary chlorides it is found that the trans substituents on both C-C bonds adjacent to the C-Cl bond are important in determining the frequency region, and a similar result is observed for the tertiary chlorides. Frequency ranges for the various rotationally isomeric structures of primary, secondary and tertiary chlorides are given.

Infrared Spectra of High Polymers. I. Experimental Methods and General Theory

The problems involved in obtaining and interpreting the infrared spectra of high polymers are discussed. Experimental methods are described for recording the infrared spectrum, with particular emphasis on the techniques used in the region between 30 and 140 μ. The method for obtaining selection rules for high polymer spectra is described and the use of selection rules and other techniques for assigning fundamental frequencies is evaluated. The calculation of the skeletal frequencies of a polymer molecule is discussed in detail.

Lattice‐Frequency Studies of Crystalline and Fold Structure in Polyethylene

The assignment of the 71 cm−1 band in the infrared spectrum of polyethylene to the B1u translational lattice vibration has been confirmed by dichroism studies on an a‐axis oriented sample. This permits confident use of the Tasumi‐Krimm calculations for the dependence of this frequency of unit‐cell parameters. The results of these calculations have been applied to the analysis of the observed lattice frequency differences between odd and even n‐paraffins, and it is shown that the frequency shifts are interpretable in terms of changes in unit cell parameters resulting from different methyl end‐group packing. The variation of the lattice frequency with degree of branching observed in a series of low‐density polyethylenes can be understood on the basis of incorporation of branches in the lattice. The dependence of the lattice frequency in high‐density polyethylene on the physical state of the specimen can be correlated with different constraints imposed by the fold regions on the chain packing. These results ...

Ab initio-based vibrational analysis of α-poly(L-alanine)

Polarized ir and Raman spectra have been obtained on oriented films of α-helical poly(L-alanine) (α-PLA) and its N-deuterated derivative. These improved spectra permit a more complete assignment of observed bands to A-, E1-, and E2-species modes. A new empirical force field has been refined, based on ab initio force fields of N-methylacetamide and L-alanyl-L-alanine, which reproduces observed frequencies above 200 cm−1 to less than 5 cm−1. A new transition dipole coupling treatment avoids the weak coupling and perturbation approximations, and can now account for the newly observed and reassigned amide I (E2) mode. As a result of this improved force field, several other observed bands have also been reassigned.

Infrared intensities of amide modes in N‐methylacetamide and poly(glycine I) from ab initio calculations of dipole moment derivatives of N‐methylacetamide

The infrared intensities of the amide modes in N‐methylacetamide (NMA) and poly(glycine I) (PGI) have been studied using ab initio dipole moment derivatives obtained for the peptide group in NMA and an empirical force field refined for PGI. Good agreement is found between the calculated transition moment magnitudes and directions of the amide I and II modes and experimental intensity and dichroism data. By analyzing the separate contributions of each internal coordinate to the total intensity, we are able to understand in detail the origins of the IR intensities of the amide modes. Besides demonstrating one approach by which IR intensities can be studied in complex molecules and polymers, our results also provide a basis for using IR intensities in structural studies of peptides and polypeptides.