Howard D. Stidham

Spectra of C2ν deuterium substituted pyrimidines

Abstract Pyrimidine, pyrimidine-2- d , pyrimidine-2,5- d 2 , pyrimidine-4,6- d 2 , pyrimidine-2,4,6- d 3 and pyrimidine- d 4 have been prepared. Isotopic purity and label specificity were established by proton magnetic resonance and mass spectroscopy. The principal fragmentation pathway in the mass spectrometer ion beam is shown to involve a splitting of HCN from the pyrimidine ring produced by loss of the 4 position carbon. Liquid phase Raman and liquid and vapor phase i.r. spectra are reported. Vibrational assignments consistent with frequency sum rule and product rule criteria are given for most of the 144 fundamentals involved. The two A 2 fundamentals of pyrimidine-4,6- d 2 , pyrimidine-2,4,6- d 3 and pyrimidine- d 4 , and one of the A 2 fundamentals of pyrimidine, pyrimidine-2- d and pyrimidine-2,5- d 2 are identified. Some assignments of earlier authors are altered in order to achieve reasonable frequency sum and product rule agreement.

Vibrational spectra of pyridazine, pyridazine-d4, pyridazine-3,6-d2 and pyridazine-4,5-d2☆

Abstract Infrared spectra of the liquids pyridazine, pyridazine- d 4 and pyridazine-3,6- d 2 are reported from 270 to 4,000 cm −1 . Raman spectra are reported for these liquids with the state of polarization for many lines. The vibrational assignment suggested for these three compounds satisfies the three Teller—Redlich product rules within limitations imposed by evident Fermi resonance perturbation of several of the fundamentals. A method of synthesis of pyridazine-4,5- d 2 was proposed and tested, and preliminary liquid phase infrared data are reported together with a few suggested assignments for this compound.

Far infrared spectrum and conformational potential function of n-butane

Abstract The far i.r. spectrum of gaseous n -butane obtained at 0.06 cm −1 resolution is reported between 80 and 230 cm −1 . Several transitions for the asymmetric torsion of the trans conformer have been identified. Utilizing these data along with the previously reported asymmetric torsional transitions of the gauche conformer from Raman spectroscopic data, the potential function for the conformational change has been obtained. The determined potential parameters were found to be: V 1 = 181, V 2 = 148, V 3 = 1154 and V 6 = −33 cm −1 . The s - trans to gauche , gauche to gauche , and gauche to s - trans barriers in cm −1 were found to be: 1315 (3.76 kcal/mol), 1090 (3.12 kcal/mol) and 1070 (3.06 kcal/mol), respectively. The potential functions obtained from these spectroscopic data are consistent with the trans to gauche energy difference, but not with the high trans / cis potential barrier suggested by recent ab initio calculations.

Spectroscopic characterization of mesogen order in discotic liquid crystal

Infrared and Raman spectra of the discotic liquid crystal molecules benzene–hexa‐hexanoate (BH6), benzene–hexa‐heptanoate (BH7), benzene–hexa‐octanoate (BH8), and benzene–hexa‐nananoate (BH9), and of mixtures of BH7 with BH8, are reported as a function of temperature. The 1615 cm−1 infrared band is strong in the Raman spectrum of BH7 and is taken as a characteristic of and diagnostic for central core disorder in all these molecules and mixtures. The aliphatic side chains are shown to disorder at much lower temperatures than the central core, and order in the central core is shown to remain throughout the liquid crystalline region.

An analysis of poly(lactic acid) with varying regio regularity

Poly(L-lactic acid) samples of varying regio regularity were studied by static and dynamic laser light scattering in conjunction with Raman spectroscopy. Light scattering results allow an estimate of the characteristic ratio, C∞. Experimental values were compared to the characteristic ratio computed from structural parameters determined from Raman spectroscopy. C∞ for poly(L-lactic acid) was determined to be 12.4, a value consistent with an inherently rigid chain. As expected, C∞ declines as regio regularity decreases. The Raman spectrum of solid amorphous samples of poly(lactic acid) of varying D content was compared with the simulated Raman spectrum calculated from an ensemble of 1000 different chains. Structural parameters, force constants and intensity parameters were transferred from similar molecules. The results reinforce the concept that poly(lactic acid) forms stiff chains. Upon introduction of D units, the copolymerization is more consistent with Markovian rather than Bernoullian statistics. Rot...

Structural study of poly(beta-benzyl-L-aspartate) monolayers at air-liquid interfaces

As normally studied, in the solid state or in solution, poly(beta-benzyl-L-aspartate) (PBLA) differs from the other helical polyamino acids in that its alpha-helical conformation is most stable in the left-handed rather than in the right-handed form. The slightly lower energy per residue for the left-handed form in PBLA is easily perturbed, however. The helical screw sense can be inverted in a polar environment and, upon heating above 100 degrees C, a distorted left-handed helix or omega-helix is irreversibly formed. From external reflectance Fourier transform infrared measurements at the air-water interface, the conformation of PBLA in the monolayer state is directly established for the first time. The infrared frequencies of the amide bands suggest that right-handed alpha-helices are formed on the surface of water immediately after spreading the monolayers and independently of the polypeptide conformational distribution in the spreading solution. The right-handed helical form prevails throughout the slow compression of the Langmuir monolayers to collapsed films. The helical screw sense can be reversed by lowering the polarity of the aqueous phase. In addition, an alternate conformation similar to the omega-helix forms on addition of small amounts of isopropanol to the aqueous subphase, and appears to be an intermediate in the helix-helix transition.

Librational Amplitudes: Raman and Nuclear Quadrupole Resonance Spectra of p‐Dichlorobenzene and p‐Dichlorobenzene‐d4

Mean‐squared angular amplitudes of vibration are calculated for a one‐dimensional crystal model with torsional nearest‐neighbor interaction. Vibrational modes with nonzero wave vector are all included in the averaging, and the effect is shown to be amplification of amplitude of magnitude dependent on the extent of interaction. Raman spectra for the 0–100‐cm−1 region of crystalline p‐dichlorobenzene and p‐dichlorobenzene‐d4 are reported and an assignment suggested for these librational fundamentals. Raman frequencies so assigned are used in an approximate calculation of the temperature dependence of nuclear quadrupole resonance frequencies of p‐dichlorobenzene and the results compared with experiment. An interpretation is given of the result of Dinesh and Narasimhan for the nuclear quadrupole resonance frequency of HgCl2.

A spectroscopic study of polymers containing long flexible side chains at an air–liquid interface

The structures, in particular the amount of conformational order in terms of the trans/gauche ratio of the methylene units, associated with monolayer films at the air-water interface, of a phospholipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, a vinyl comblike polymer, poly(octadecyl acrylate) and a hairy-rod co-polyglutamate with long alkyl side chains, all have been studied by external reflectance Fourier transform infrared spectroscopy. The structures of these monolayers were monitored as a function of molecular area/surface pressure and subphase temperature using a microcomputer-controlled Langmuir trough interfaced to an infrared spectrometer. The unique features of this combination of techniques permit the direct evaluation of the molecular dynamics associated with the formation and collapse of spread monolayer films at the air-liquid interface, as well as the description of the adsorption kinetics of amphiphile molecules at an air-solution interface. The conformation sensitive methylene stretching vibrations of the hydrocarbon side chains proved to be extremely sensitive indicators of conformational change during slow dynamic compression of the Langmuir films changing from expanded to more condensed monolayer states up to their collapse region. The three systems exhibit significantly different spectroscopic features that can be directly associated with differences in the intermolecular packing. The effect of temperature (from +8 to +27°C) on the molecular order for compressed monolayers of poly(octadecyl acrylate) was also studied. The frequencies of the methylene stretching bands for the liquid crystalline co-polyglutamate showed that at high compression, the long flexible hydrocarbon side chains were mostly in an all-trans configuration. At low compression, the side chains became disordered and contained a large fraction of gauche configurations.

The lattice vibrations of the diazines

Abstract The laser Raman polarization spectra of the lattice vibrational region of the diazines pyrazine, pyrazine- d 4 , pyrimidine and pyrimidine- d 4 are reported for single crystals. The lattice region for polycrystalline pyridazine is also reported. Assignments are suggested based on results of the polarization and deuterium substitution studies.

Polymer Dynamics: Boson Representation of Gyration Radius

The expansion parameter aR2 for the gyration radius of a linear polymer molecule exhibiting excluded‐volume effects and dissolved in a theta solvent is approximated by the boson operator representation technique. The approximation scheme gives the known coefficient in the expansion of aR2 as a function of expansion parameter z exactly. Agreement of the approximate theory with a recently reported exact value for the coefficient of the term quadratic in z in this expansion is poor. Comparison with light‐scattering results shows that the qualitative features of the experiment are accurately reproduced by the approximate theory for all values of z.