ABSTRACT
Results of a search for correlations between typical group mode frequencies and conformation of -OH and =NH substituents is reported. The study is based on quantum chemical data (HF/6-31G (d, p) and MP2 (full)/6-31G (d, p) approximations) of 13 isomers of cytosine. It is closely related to investigations of stability and energetics of all isomers of the more common nucleic acid bases, which revealed correlations between conformation (geometric isomerism) of OH and =NH substituents on the one hand and conversion energies, relaxation of internal structural parameters, electric field gradients, etc. on the other hand. In the majority of cases alteration of conformation of these substituents is accompanied by systematic frequency shifts of stretching, in-plane and out-of-plane bending (torsional) modes conventionally assigned to such groups and by alteration of quantum chemically predicted estimates of harmonic valence force constants and structural parameters dominating the mode frequencies. For identification of group modes 'fractional' mass (other than 'natural' mass) isotope shifts of frequency and normal vectors proved useful. Likewise, estimation of effects of force constant and structural parameter alterations on frequency shifts by first order perturbation theory of the FG problem of partial structures contributed valuable insight into the origin of the shifts.
Subject(s)
Cytosine/analogs & derivatives , Cytosine/chemistry , Adenine/chemistry , Calorimetry , Guanine/chemistry , Isomerism , Models, Molecular , Molecular Conformation , Quantum Theory , Uracil/chemistry , Xanthine/chemistryABSTRACT
A complete spectrometer for recording high resolution ir absorption spectra with diode lasers is presented. The optical assembly has been adapted to the characteristics of diode lasers but can be used as well with other laser sources. With a double-beam technique variations in laser power are eliminated. Problems of optical alignment and electronic signal detection are discussed, and representative spectra are presented.
ABSTRACT
Conventional and modulated excitation infrared spectra of egg phosphatidylcholine are measured in the spectral range of 4000-900 cm-1 by modulation of relative humidity with an amplitude of +/- less than 5% at a mean value of 75% (T=30 degrees C). From the modulated-excitation spectrum, hydration sites are found to be the greater than PO2 group, the greater than C=O group and the choline group.
Subject(s)
Phosphatidylcholines , Binding Sites , Egg Yolk , Female , Molecular Conformation , Spectrophotometry, Infrared , WaterABSTRACT
Results of an investigation of the stability of n-layers of barium stearate, cadmium arachidate and tripalmitin by means of electron microscopy and attenuated total reflection infrared spectroscopy are reported. Odd and even numbered barium starate n-layers with n - 1,2,3,4,5 are found to rearrange spontaneously from a regular film into ultrastructures of irregular, flat islands of varying thickness. The kinetics of the phase transformation of the first layer depends on the substrate, that of n-layers appears to be dependent on n, the temperature, and the surrounding medium. The kinetic behaviour of odd and even numbered layers is distinctly different. Similar studies on cadmium arachidate layers reveal much slower kinetics of the rearrangement process. In the case of tripalmitin n-layers it is shown that electron microscopy and infrared spectroscopy yield valuable complementary information about ultrastructure and molecular structure of the layers in correlation with the rearrangement process, which also occurs with this system. Consequences of the results of this paper for work published in various fields are briefly discussed.
Subject(s)
Eicosanoic Acids , Membranes, Artificial , Palmitates , Palmitic Acids , Stearates , Stearic Acids , Barium , Cadmium , Kinetics , Microscopy, Electron , Spectrophotometry, Infrared , TriglyceridesABSTRACT
The design and construction of a modulated excitation infrared spectrophotometer that allows detection of photoinduced transients in the spectral range 3000-4000 em(-1) is described. The lifetime range of photoproduced species is 10(-1)-10(-5) sec. Information about kinetics may be obtained by amplitude and phase measurements. The sensitivity achievable by single and double modulation is discussed in detail. Performance of the instrument is illustrated by detection of four transient photoproducts of pyrocatechol, distinguishable by phase measurements. Estimated lifetimes are within 1 msec and 40 msec.
ABSTRACT
An ir spectropolarimeter for the 2-8-micro region is described. The instrument features germanium plate polarizers, double modulation by light chopping and an oscillating polarizer, and an optical zero servosystem. The theoretical sensitivity of the instrument is derived in some detail, and the results are comparedwith the actual sensitivity of 15 mdeg.
