Ralph C. Dougherty

Temperature and pressure dependence of hydrogen bond strength: A perturbation molecular orbital approach

Lengths and strengths of hydrogen bonds are exquisitely sensitive to temperature and pressure. Temperature and pressure sensitivity is the result of the fact that hydrogen bonds are so weak that the internal energy of the bond is important to bond strength, and the equilibrium bond distance is controlled by a combination of thermodynamics and quantum mechanics, rather than quantum mechanics alone. The importance of thermodynamics in the bond length, and strength, of hydrogen bonds is the result of a breakdown in the Born-Oppenheimer approximation that occurs when the energy of the first vibrational excitation of a bond is of the order of kT. Variation of water–water hydrogen bond length and strength with temperature and pressure is discussed in light of the data for the specific volume of ice Ih, the enthalpy of vaporization of liquid water, and the internal energy of the liquid. In most chemical contexts, correction of covalent bond strength for internal energy is not necessary. For hydrogen bonds this i...

Equilibrium structural model of liquid water: Evidence from heat capacity, spectra, density, and other properties

Hydrogen bond strength depends on both temperature and pressure. The gradient for hydrogen bond strength with temperature, or pressure, depends upon the hydrogen bonded structure. These features create an intimate connection between quantum mechanics and thermodynamics in the structure of liquid water. The equilibrium structural model of liquid water developed from analysis of the heat capacity at constant pressure is complex. The model is based on the assumptions that: (i) the hydrogen bond length and molecular packing density of water both vary with temperature; (ii) the number of different geometries for hydrogen bonding is limited to a small set; (iii) water molecules that possess these hydrogen bonding geometries are in equilibrium with each other under static conditions; (iv) significant changes in the slope of the heat capacity, Cp, and to a lesser extent other properties of the liquid, reflect the onset of significant changes in the chemical structure of the liquid; (v) the partial molal enthalpie...

Effects of Isotopic Substitution on Electron Spin Resonance Signals in Photosynthetic Organisms

Effects of Isotopic Substitution on Electron Spin Resonance Signals in Photosynthetic Organisms

Human Exposure to Pentachlorophenol

Several reports have suggested that people living in industrialized societies will generally be contaminated with pentachlorophenol (PCP) at concentrations of 1 ppb and higher. The most likely source for this contamination appears to be food chain exposure to PCP treated wood products. Another possible source of exposure is metabolism of hexachlorobenzene to pentachlorophenol. Pentachlorophenol has been shown to be a mutagen and commercial preparations of the compound are often contaminated with dioxins. Both of these observations may be significant to the toxicological evaluation of the present hazard from pentachlorophenol exposure.

High-energy fragmentation of chlorophyll a and its fully deuterated analog by californium-252 plasma desorption mass spectrometry

Comparison of the /sup 252/Cf=plasma distortion mass spectrometry (/sup 252/Cf-POMS) spectra of chlorophyll of ordinary isotopic composition((/sup 1/H)Chl a) with those of essentially fully deuterated Chl a ((/sup 2/H)Chl a) led to the establishment of the elemental compositions in the fragmentation pattern and to deduction of plausible structures for the fragment ions produced. The difference in mass between corresponding ions in the spectra gives the number of hydrogens in the molecule. It is necessary to assure that all ions contain 4 nitrogens in order to determine the elemental composition of any ion in question. The /sup 252/Cf-POMS spectra showed an intense molecular ion at 893.6+0.1..mu.., which corresponds to the isotopically averaged molecular weight of (/sup 1/H)Chl a, and 963.3+0.1..mu.., which corresponds to that of (/sup 2/H)Chl a. The mass difference confirms the expected extent of deuteration, 70 of the 72 H atoms in the (/sup 2/H)Chl a molecular ion are D. An intense partially resolved fragmentation pattern was observed from 300 to 700 ..mu... The pattern is mass shifted in the deuterated analog. Some explanation is offered for the fragmentation pattern. (BLM)

Pentachlorophenol in the environment. Evidence for its origin from commercial pentachlorophenol by negative chemical ionization mass spectrometry.

w Commercial pentachlorophenol (PCP) contains significant quantities of te trachlorophenol (TCP). The occurrence of TCP in environinental samples provides a chemical marker for PCP originating from commercial formulations. Negative chemical ionization mass spectrometry has been used to examine a commercial PCP formulation and a series of environmental and human samples. Tetrachlorophenol was determined by the ion current at m / z 229, tetrachlorophenoxide, and PCP was determined by the ion current a t m / z 267, pentachlorophenoxide. The ion current a t m / z 267 may include contributions from the oxygen/chloride exchange product of hexachlorobenzene, an environmental precursor of PCP. The ratio of PCP to TCP in Dowcide G-ST, a commercial PCP formulation, was 2.5 f 0.1. The ratio of m / z 267 to mlz 229 in a jellyfish, Mnemiopsis macrydi, from the Gulf of Mexico was 2.7 * 0.1, in human semen it was 4.1 f 0.1, and in human adipose tissue it was 15.5 f 0.1. PCP in the semen samples was concentrated in the sperm cells by a factor of 9.

Application of californium-252 plasma desorption mass spectroscopy to chlorophyll studies☆

Applications of 252Cf-PDMS to a variety of chlorophyll problems are described. This form of heavy particle induced desorption mass spectroscopy is highly suitable to the mass measurement of nonvolatile, thermally unstable chlorophylIs and chlorophyll derivatives. 252Cf-PDMS has been applied to the characterization of the products of the classical chlorophyll allomerization reaction, and to the synthesis of linked chlorophyll special pairs designed to mimic the in vivo P700 photoreaction center. The applications of 252Cf-PDMS to the characterization of chlorophyll aggregates and to the examination of the redox properties of the chlorophylls are also described.

Chemical geometrodynamics: Physical fields can cause asymmetric synthesis

This paper presents theoretical arguments that suggest that chiral combinations of physical fields which can induce motion of elementary particles or molecular systems can in principle cause asymmetric synthesis. The theory is founded on the application of parity and time reversal operators to chiral dynamical systems. The motion of these systems induced by a chiral set of physical fields is shown not to be invariant on parity and time reflection. The relationship between tetrahedral dissymmetry and belical dissymmetry is analyzed in terms of the moments of inertin of a tetrahedrally dissymmetric rotor rotating around each of the four bond axes. The magnitude of anticipated enantiomeric excess which would result from conducting a prochiral chemical reaction in a chiral set of physical fields is estimated to be very small, parts per million or less, for virtually all sets of readily accessible physical fields. The results of experiments in which prochiral chemical reactions were conducted in a sealed tube which was spinning perpendicular or paralled to the earths surface, are reviewed as are experiments in which prochiral chemical reactions were conducted in intense oriented magnetic fields. Enantiomeric recognition may have been one of the principal mechanisms for amplifying small differences in the rates of a given prochiral chemical reaction.

Magnetic and other field effects on prochiral chemical reactions

Experiments are reported in which three prochiral organic reactions were conducted in the presence of a ca. 1T magnetic field which was oriented with reference to the earths geometric axes. The sign and magnitude of the rotation varied with the macroscopic orientation of the magnetic field and the time that the reaction was performed. Control measurements were in accord with expectations. The fact that the sign of the observed optical rotation of the product was reversed for all three reported reactions when the magnetic field was reversed for reactions conducted on the same day suggests that either the observed asymmetric synthesis was due to the reactions being conducted in a chiral physical field or extremely unusual stochastic processes were involved.

Reduction of organochlorine emissions from municipal and hazardous waste incinerators

The authors have modeled some chemical processes that occur in municipal incinerators that result in the emission of organochlorine compounds by studying the high-temperature destruction of noncombustible polymers, specifically poly(vinylidene chloride), in a stream of heated, flowing, moist air. The dominant volatile products form the thermal destruction of poly(vinylidene chloride) were polychloroaromatic hydrocarbons. The thermochemistry for the reaction of calcium, oxide, chloroacetylene, and oxygen to give calcium chloride, carbon dioxide, and water is very favorable (-571 kcal/mol). Calcium oxide is available by the heat treatment of crushed limestone. Fluidized-bed technology involving calcium oxide is already in place for reduction of acidic emissions (SO/sub 2/) from coal-fired power plants. Preliminary experiments in which a fluidized bed of calcium oxide was placed above the thermal destruction zone for poly(vinyldene chloride) in a 1000/sup 0/C tube furnace results in a decrease in aromatic polychloride effluents of a factor of 3 x 10/sup 2/. 18 references, 4 figures, 2 tables.