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Showing papers in "Journal of the American Chemical Society in 1977"



Journal ArticleDOI
TL;DR: An improved force field for molecular mechanics calculations of the structures and energies of hydrocarbons is presented in this paper, where the problem of simultaneously obtaining a sufficiently large gauche butane interaction energy while keeping the hydrogens small enough for good structural predictions was solved with the aid of onefold and twofold rotational barriers.
Abstract: An improved force field for molecular mechanics calculations of the structures and energies of hydrocarbons is presented. The problem of simultaneously obtaining a sufficiently large gauche butane interaction energy while keeping the hydrogens small enough for good structural predictions was solved with the aid of onefold and twofold rotational barriers. The structural results are competitive with the best of currently available force fields, while the energy calculations are superior to any previously reported. For a list of 42 selected diverse types of hydrocarbons, the standard deviation between the calculated and experimental heats of formation is 0.42 kcal/mol, compared with an average reported experimental error for the same group of compounds of 0.40 kcal/mol. I t has been now amply demonstrated that force field calculations offer the method of choice for the determination of the structures and energies of molecules under many circums t a n c e ~ . ~ ~ While many previously published force fields are very good, they do contain errors which are sufficiently large as to be worrisome to those wishing to utilize them to the fullest possible extent. While the organic chemist is primarily interested in compounds which contain functional groups, since the fundamental structure of organic molecules in general is hydrocarbon in character, a high degree of accuracy in the hydrocarbon part of the force field is crucial. “First generation” force fields showed that one could indeed calculate accurate structures and energies, although the fit to experiment was in some cases less good than one would desire. There has been some difficulty in ascertaining exactly where the force fields were in error, and in which cases the experimental data were less accurate than the probable errors indicated. This question is still not fully answerable but, clearly, more and better data have become available in the last several years. The best we can do is to utilize the existing data, and point out where we feel that there may be errors. We will discuss herein three of the earlier force fields. These are our earlier force field M M l ( 1973)3 and the most recent force fields by Schleyer (EAS)5b and Bartell (MUB-2).6 For all of their usefulness and accuracy, these force fields contained various flaws which showed up in different ways. In an effort to minimize the discrepancy between calculations and experiment, the van der Waals characteristics of atoms were important quantities to be evaluated. In Figure 1 is shown a graph taken mainly from a recent paper by Bartel16 in which the force exerted by a pair of atoms as a function of distance is plotted for several different force fields including MUB-2, EAS, and M M I . For present purposes we will define a “hard” atom as one for which the plot of the force vs. distance for the repulsive part of the curve shows a steep slope (as the dashed C/C line in the figure), and a “soft” atom as one where this slope is more gentle (as the solid line). We will also define a “bigger” atom as one where the line is slid farther to the right, and a “smaller” atom as one for which it is slid to the left. With this terminology, it is seen from the graph that in M M I we used a hydrogen atom which was both rather hard and large compared to that used by Bartell (and other workers), while we used a carbon atom which was small. The “hardness” of our curves was determined by the Hill equation, which is known to fit well for interactions between rare gases.’ There is no assurance that such curves are ideal for carbon and hydrogen atoms which are covalently bound. However, they seemed like a reasonable choice in the absence of definite information. Bartell, mainly on the basis of theory, chose a much softer hydrogen.8 Most other workers have been inclined to follow Bartell’s lead. Bartell’s more recent choice (bVIUB-2) is based on theoretical calculations by Kochanski9 on the Hz molecule. His new hydrogen is larger but softer than the old one. In our early worki0 we noticed that we could not fit adequately to the axial-equatorial methylcyclohexane energy difference using Bartell’s hydrogen, and varying the other parameters that it seemed one might reasonably vary. We therefore continued to use the hard Hill-type hydrogens. Bartell was less anxious to fit this energy difference, and felt he could do a better overall job with structure using soft hydrogens. In each case, the C /H interaction was taken to be the mean of the H / H and C/C interactions. White has also pointed out in a recent paper that our hydrogens are too hard to explain certain data .” We too regard the cyclodecane case which he discusses as a key case, because of the data now available, and it will be discussed below. We

3,313 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that in the presence of polar solvents, there is a significant enhancement in the intensity of the 0-0 vibronic band at the expense of other bands.
Abstract: The fluorescence intensities for various vibronic fine structures in the pyrene monomer fluorescence show strong solvent dependence. In the presence of polar solvents, there is a significant enhancement in the intensity of the 0--0 vibronic band at the expense of other bands. This strong perturbation in the vibronic band intensities is more dependent on the solvent dipole moment than on the bulk solvent dielectric constant. This suggests the operation of some specific solute--solvent dipole--dipole interaction mechanism. The strong perturbation of the vibronic band intensities has been used as a probe to accurately determine critical micelle concentrations and also to investigate the extent of water penetration in micellar systems.

3,271 citations




Journal ArticleDOI
TL;DR: The photolysis of chemisorbed water on incompletely outgassed TiO/sub 2/ powder yields H 2 and O 2 in the molar ratio of 2 : 1 if conducted under argon in the presence of molecular nitrogen as mentioned in this paper.
Abstract: The photolysis of chemisorbed water on incompletely outgassed TiO/sub 2/ powder yields H/sub 2/ and O/sub 2/ in the molar ratio of 2 : 1 if conducted under argon In the presence of molecular nitrogen, O/sub 2/ is still formed but the evolution of H/sub 2/ is inhibited as chemisorbed nitrogen is reduced to NH/sub 3/ and traces of N/sub 2/H/sub 4/ according to N/sub 2/ + 3H/sub 2/O + nhv yields 2NH/sub 3/ + 150/sub 2/ and N/sub 2/ + 2H/sub 2/O + mhv yields N/sub 2/H/sub 4/ + O/sub 2/ Iron doping enhances the photocatalytic reactivity of rutile and provides prototypes of solar cells for photochemical ammonia synthesis from N/sub 2/ and H/sub 2/O

817 citations


Journal ArticleDOI
TL;DR: In this paper, the exact rate constants for.SO/sub 4/sup -/ with substituted benzenes and benzoates have been determined by pulse radiolysis.
Abstract: Absolute rate constants for reaction of .SO/sub 4//sup -/ with substituted benzenes and benzoates have been determined by pulse radiolysis. The values are found to range from about 5 x 10/sup 9/ M/sup -1/ s/sup -1/ for anisole to less than 10/sup 6/ M/sup -1/ s/sup -1/ for nitrobenzene. A correlation of the rate constants with the Hammett substituent constant sigma gave rho = -2.4 for both series of compounds. It is concluded that the reaction takes place by an electron transfer from the ring to .SO/sub 4//sup -/.

755 citations













Journal ArticleDOI
TL;DR: In this paper, a new approach to aromaticity of conjugated hydrocarbons is described, which is based on the concept of (An + 2) conjugate circuits, which has been recently recognized as an essential structural element for characterization of polycyclic systems.
Abstract: A new approach to aromaticity of conjugated hydrocarbons is described. It is based on the concept of conjugated cir­ cuits, which has been recently recognized as an essential structural element for characterization of conjugated systems (Chem. Phys. Lett., 38, 68 (1976)). Kekule structures of a conjugated hydrocarbon are examined and circuits with an alternation of CC double and single bonds enumerated. Systems having only (An + 2) conjugated circuits are defined as aromatic. Systems having only 4n conjugated circuits are considered antiaromatic, i.e., destabilized by the derealization of T electrons. Finally, systems having both (An + 2) and An conjugated circuits are classified as intermediate, showing partial aromatic nature. The approach represents a logical generalization of the famous Hiickel (An + 2) rule, valid rigorously only for monocyclic struc­ tures, to polycyclic systems. A brief comparison with several alternative schemes is given and their limitations illustrated. Difficulties involved in attempts to characterize aromatic­ ity are well known.1 The simple Hiickel (An + 2) rule, valid only for monocyclic systems,2 and Piatt's perimeter model,3 an attempt to extend the rule to polycyclic systems, remain frequently used for more general situations without a proper justification and despite recognized deficiencies. This perhaps indicates an intuitive appreciation of the significance of the (An + 2) 7r-electron role. We present here an approach to aroma­ ticity in which also a role of (An + 2) tr electrons is dominant. However, it turns out that not the number of tr electrons is the critical factor, but their coupling in conjugated circuits as derived from the Kekule structures of the system. The ap­ proach has lead to a logical generalization of the famous Hiickel (An + 2) rule, valid rigorously only for monocyclic conjugated polyenes, to polycyclic structures. In view of the acclaimed value of the Hiickel rule, believed to have been one of the most successful theoretical predictions made in organic chemistry,4 the approach of a classification of cyclic conjugated ir-electron systems developed here seems to be rather inter­ esting mainly because it encloses the polycyclic systems too. The basis for the approach to aromaticity suggested in this work is the concept of conjugated circuits.5 A polycyclic structure contains various circuits, and an individual Kekule structure assigns a single or a double bond character to bonds in a circuit. Circuits which have an alternation of the CC single and double bonds are called conjugated circuits. They neces­ sarily are even, and are either of a (An + 2) or An type. The notion of conjugate circuits is not so unfamiliar in chemistry; however, it has not been realized that they represent an im­ portant structural element. The analysis consists of the enu­ meration of all distinctive conjugated circuits; hence here we have a typical graph theoretical scheme. We illustrate the approach with azupyrene, two Kekule structures of which are decomposed as shown in Scheme I. The remaining two struc- Scheme I


Journal ArticleDOI
TL;DR: In this paper, the structure and state of water in the reversed micellar system, sodium diisooctyl sulfosuccinate/H/sub 2/O/heptane was investigated by both /sup 1/H and /sup 23/Na NMR spectroscopy.
Abstract: The structure and state of water in the reversed micellar system, sodium diisooctyl sulfosuccinate/H/sub 2/O/heptane was investigated by both /sup 1/H and /sup 23/Na NMR spectroscopy. The /sup 1/H NMR spectrum of H/sub 2/O exhibits a downfield chemical shift with increasing water content of the system, gradually approaching that of ordinary water. This suggests a minimal amount of hydrogen bonding present in the micellar water phase at low water content. With increasing water content of this system, both the spin--lattice relaxation rates (1/T/sub 1/) and spin--spin relaxation rates (1/T/sub 2/) of /sup 1/H of water decrease significantly up to 1 percent H/sub 2/O, and then decrease much slower with further addition of water. A simple calculation of the rotational correlation time tau/sub c/ from T/sub 1/ demonstrates that the water molecules are highly immobilized in small water pools due to strong ion--dipole interaction with counter ions. On completion of the solvation shell of the counter ion (i.e., 1 percent H/sub 2/O or H/sub 2/O/Na/sup +/ similarly ordered 6), the rigidity of the micellar core is greatly reduced. /sup 23/Na NMR measurements show an analogous decrease in the 1/T/sub 2/ with increasing H/sub 2/O content. In the largest water pool, viz.more » 6 percent H/sub 2/O, it is estimated that an upper limit of 28 percent of the Na/sup +/ is dissociated from the sulfosuccinate head group. The electronic absorption spectrum of iodide ion in the above systems is quite different from that in bulk water. The intensity of the spectrum increases with increasing water content. These data are also interpreted in terms of a decreased rigidity and increased hydrogen bonding of the micellar water pool with increasing water content« less




Journal ArticleDOI
TL;DR: In this article, the authors studied the photophysics of fluorescein and three halogenated derivatives (eosin, erythrosin, and rose bengal) in aqueous and simple alcoholic solvents.
Abstract: Subnanosecond lifetime measurements using picosecond pulses from a mode locked Nd3+/glass laser together with conventional absorption and fluorescence yield methods have been used to study the photophysics of fluorescein and three of its halogenated derivatives (eosin, erythrosin, and rose bengal) in aqueous and simple alcoholic solvents. For each of the dye molecules absorption and fluorescence maxima move towards higher energy ("blue shift") as the solvent changes from i-PrOH to HlO. Fluorescence lifetimes and quantum yields are found to decrease markedly with this solvent change and also with in- creased halogenation ("heavy-atom effect") of the fluorescein parent. Published triplet yield data confirm that the variations observed in the nonradiative part of the decay rate can be attributed almost wholly to variations in the rate of SI-TI intersys- tem crossing. A simple and reasonable explanation of the observed effects can be found if for these particular solvent-solute combinations stabilization energies lie in the order AE(TI) < AE(S1) < AE(S0). This idea is consistent with both the increased SI-S0 spectral "blue shifts" and the enhanced intersystem crossing rate, arising from a smaller SI-TI energy gap, when these dye molecules are placed in a more aqueous solvent environment. The studies are relevant to the use of these dyes as fluorescent probes in biologically important molecules. I. Introduction The fluorescence properties of the xanthene dyes have both theoretical and practical interest. The advent of the dye laser' has sparked off renewed interest in the correlation between molecular structure and fluorescence properties, while the use of fluorescent dye probes in structural studies of molecules of biological significance has become an area of considerable a~tivity.~,~ The usefulness of a fluorescent probe relies on variations in the dye's fluorescent properties with alterations in its environment, solvent polarity being an example. Despite a considerable amount of work, no detailed theory explaining the often very dramatic effects of environment and structure on fluorescence has emerged. For example, the effect of solvent and macromolecular environment on the fluorescence lifetime and quantum yield of perhaps the most widely used fluorescent probe, ANS- ( 1 -anilinonaphthalene-8-sulfonate), not well understood although various mechanisms for the fluorescence quenching in polar solvents have been pr~posed.~-~ Until a better understanding of the environmental factors affecting the fluorescence of dye molecules has been achieved, detailed conclusions based on fluorescent probe studies must be treated with caution. Fluorescein and its halogenated derivatives provide an ex- cellent model series for studies of this kind, for not only do the degree and type of halogenation greatly alter the fluorescence yield, but the photophysical properties of these dyes are also very dependent on the nature of the solvent. We have studied absorption and emission spectra, fluorescence lifetimes, and fluorescence yields of eosin, erythrosin, and rose bengal (Figure 1 j in a series of alcohols and in aqueous solution. The data provide a clear picture of the changes in nonradiative decay rate that occur in these molecules as a result of changes in the solvent environment. 11. Experimental Section (a) Chemicals. Eosin (BDH) was purified by recrystallization from acidic solution and rose bengal (BDH) was purified by chromatog- raphy on an alumina-talc column. The erythrosin sample (BDH) was used as supplied. Spectroscopic grade (Merck) methanol, ethanol, 2-propano1, and triply distilled water were used as solvents. All the aqueous solutions were buffered to pH 9.2. Dye concentrations of



Journal ArticleDOI
TL;DR: In this paper, the authors developed and fitted such equations over the range from 0 to 6 M in a system compatible with those for fully dissociated, strong electrolytes, and established the solute standard state and the relationship between the properties of sulfuric acid in that state with those of pure acid.
Abstract: Although the thermodynamic properties of sulfuric acid above 0.1 M and near 25/sup 0/C are well established numerically, they have not been represented accurately by equations which are based upon the ionic species present, H/sup +/, HSO/sub 4//sup -/, and SO/sub 4//sup 2 -/. We have developed and fitted such equations over the range from 0 to 6 M in a system compatible with those for fully dissociated, strong electrolytes. The enthalpy is treated as well as the activity and osmotic coefficients. These equations also establish the solute standard state and the relationship between the properties of sulfuric acid in that state with those for the pure acid. Among the results obtained (for 25/sup 0/C) are the dissociation constant 0.0105 and the heat of dissociation -5.61 kcal mol/sup -1/ for HSO/sub 4//sup -/ and the entropy of SO/sub 4//sup 2 -/, 4.2 +- 0.2, and of HSO/sub 4//sup -/, 32.1 +- 0.3 cal K/sup -1/ mol/sup -1/. Also for the reaction H/sub 2/SO/sub 4/(l) = 2H/sup +/(aq) + SO/sub 4//sup 2 -/(aq), ..delta..H/sup 0/ = -22844, ..delta..G/sup 0/ = -12871 cal mol/sup -1/.