An Ab initio study on the conformations of protonated,neutral, and deprotonated amidine

Ab initio SCF molecular orbital calculations have been performed to ascertain the conformational preferences of protonated, neutral, and deprotonated amidine [HC(?NH)NH₂], using the 3-21G split valence basis set. The states of eight stable species, eight transition states, and four higher-order saddle points have been determined by complete geometry optimization utilizing analytic energy gradient techniques. Protonation at the amidine ?NH is preferred over the –NH₂ site by 37.1 kcal/mol. Neutral amidine has rotational barriers of 9.6 and 11.7 kcal/mol for the HN?CN cis and trans isomers, respectively, while all the stable HC(NH₂)₂⁺ and HC(NH)₂^? species possess torsional barriers larger than 23 kcal/mol. There is, however, essentially free C—N single-bond rotation in HC(?NH)NH₃⁺, the calculated barriers being 0.7 and 1.8 kcal/mol for the cis and trans HN?CN isomers, respectively.     

Optimum geometries and relative energies for cytosine,thymine, uracil,the imino tautomer of cytosine,the enol tautomer of thymine,and the enol tautomer of uracil by the MINDO/2 SCF MO method

A MINDO /2 SCF MO geometry optimization of cytosine (C), thymine (T), uracil (U), the imino tautomer of cytosine (C*), the enol tautomer of thymine (T*), and the enol tautomer of uracil (U*)was made. The optimized geometries for cytosine, thymine, and uracil agree well with crystallographic data. The optimized geometries for the tautomers show the correct trends in bond lengthening and bond angle except for the C4—O4 length and C4—O4—H angle of T* and U*. The energies of tautomerization were found to be 10.3, ?9.0, and ?14.2 kcal/mol for C?C*, T?T*, and U?U*, respectively, when optimized geometries are used. The overestimation of the C4—O4—H angle is speculated to arise because of an inadequacy in the parametrization of the one-center integrals in MINDO /2.     

The quantitative effect of microextractor cell geometry on the analytical supercritical fluid extraction efficiencies of environmentally important compounds

Summary The quantitative effect of microextractor cell geometries on supercritical fluid extraction (SFE) efficiencies of polycyclic aromatic hydrocarbons (PAHs) and methoxychlor from octadecyl-bonded sorbents has been evaluated and compared to similar effects seen upon increasing the supercritical fluid density. For the PAHs studied, correlations between the fused ring number and the relative increase in recoveries have been established. SFE recoveries can be increased by greater than a factor of two by decreasing the diameter to length ratio from 120 to 11. The relative recovery increase upon decreasing the diameter to length ratio of the extraction vessel is dependent on the analyte extractability, increasing in proportion to the fused ring number for the PAHs. Recoveries increased linearly as a function of supercritical fluid density for the PAHs studied. The change in the relative recovery upon increasing the supercritical carbon dioxide density again was dependent on the analyte type, decreasing linearly with fused ring number. Although fluid density generally had the greatest effect on achievable SFE recoveries, the cell geometry had effects of a similar order of magnitude, highly dependent on the initial extractability of the analyte.     

A versatile stereocontrolled approach to chiral tetrahydrofuran and tetrahydropyran derivatives by use of sequential asymmetric Horner-Wadsworth-Emmons and ring-closure reactions

An approach to chiral tetrahydrofuran and tetrahydropyran derivatives based on the sequential use of an asymmetric Horner-Wadsworth-Emmons reaction and a cyclization step is presented. The approach is both stereochemically and structurally versatile since three different cyclization methods can be employed starting from the same HWE product: (i) palladium-catalyzed substitution, (ii) hetero-Michael addition, or (iii) epoxide opening. The asymmetric HWE reaction controls the absolute configuration of the ultimate product, whereas the relative configuration is controlled by the combined influence of the geometric selectivity in the HWE reaction and the stereochemistry of the respective cyclization method.     

Point charge representation of multicenter multipole moments in calculation of electrostatic properties

Summary Distributed Point Charge Models (PCM) for CO, (H₂O)₂, and HS-SH molecules have been computed from analytical expressions using multicenter multipole moments. The point charges (set of charges including both atomic and non-atomic positions) exactly reproduce both molecular and segmental multipole moments, thus constituting an accurate representation of the local anisotropy of electrostatic properties. In contrast to other known point charge models, PCM can be used to calculate not only intermolecular, but also intramolecular interactions. Comparison of these results with more accurate calculations demonstrated that PCM can correctly represent both weak and strong (intramolecular) interactions, thus indicating the merit of extending PCM to obtain improved potentials for molecular mechanics and molecular dynamics computational methods.Dedicated to Prof. Alberte PullmanPacific Northwest Laboratory is operated for the US Department of Energy by Battelle Memorial Institute under contract DE-ACO6-76RLO 1830