Crystal structures of synthetic analgetics. V. Dextromoramide.

The molecular and crystal structure of dextromoramide has been determined by X-ray methods. The crystals are orthorhombic, space group P212121 with unit cell dimensions a = 9.720(4) A; b = 12.226(3) A; c = 18.381(3) A. The structure was determined by direct methods and the model refined to an R-value of 0.036 for 1788 observed reflections. The mean e.s.d.'s in bond lengths and angles are 0.004 A and 0.3, respectively. The morpholine moiety is nearly in antiposition relative to the quaternary carbon atom C6, the pertinent angle C6 - C7 - C9 - N2 being - 159.4. This conformation is similar to that previously reported for the bitartrate of the title compound. The pyrrolidine ring has the envelope conformation and the amide group is strictly planar. The conformation of some acyclic analgetics are discussed.     

Multicomponent polyanions. 57. Large-angle X-ray scattering study of aqueous molybdophenylphosphonate solutions

The radial distribution functions are calculated from large-angle X-ray scattering (LAXS) measurements for one concentrated aqueous molybdate/heptamolybdate solution and five aqueous molybdophenylphosphonate solutions (lithium chloride medium). Besides water and hydrated lithium, chloride, and molybdate ions, five species in all, having different nuclearities, are postulated to exist in the solutions, according to equilibrium studies using potentiometry and 31P NMR spectroscopy. The structures of the three polymolybdate species Mo7O24(6-), Mo8O26(4-), and (C6H5P)2Mo5O21(4-), for which the structures are determined crystallographically, are confirmed to exist also in aqueous solution. The principal structures of the remaining two complexes, (C6H5P)Mo6O21(OH2)5(2-) and (C6H5P)Mo7O25(OH2)4-, are elucidated with the use of structures of related species. Both anions have one group of four edge-sharing MoO6 octahedra and another group of two MoO6 octahedra connected by sharing corners, forming a bent unsymmetric six-membered ring, with the C6H5PO3 group placed on the crowded side of the ring. In the former, the group of two MoO6 octahedra is edge-shared, while in the latter, the group is face-shared, resulting in a ring small enough to tetrahedrally coordinate to the seventh molybdenum opposite the phenyl group.     

Aspirin. An ab initio quantum-mechanical study of conformational preferences and of neighboring group interactions.

The potential energy surface of acetylsalicylic acid, aspirin, has been explored at the RHF/6-31G* and B3LYP/6-31G* levels, and single-point calculations were performed at levels up to B3LYP/6-311G**//B3LYP/6-31G*. All conformational isomers have been located, the thermochemical functions have been computed, and relative energies and free enthalpies were determined. The conformational space of aspirin is spanned by three internal coordinates, and these are the carboxylic acid C-O conformation (s-trans preferred by about 7 kcal/mol), the C-COOH conformation (Z preferred unless there are H-bonding opportunities), and the ester C-O conformation (s-trans preferred by about 4 kcal/mol). There are nine aspirin isomers since one of the conformers realizes hydrogen-bonding structure isomerism as well. Neighboring group interactions are discussed with reference to the intrinsic properties of benzoic acid and phenyl acetate. The intrinsic conformational preference energies for benzoic acid and phenyl acetate are not additive. The acid s-trans preference energies differ by as much as 9 kcal/mol depending on the Ph-COOH and ester conformations. Similarly, the E-preference energies about the Ph-COOH bond vary by as much as 6 kcal/mol depending on the ester conformation. The structural discussion suggests an overall ortho repulsion between the functional groups in all aspirin isomers including the intramolecularly hydrogen-bonded isomers. The isodesmic reaction between the most stable conformers of benzoic acid and phenyl acetate to form aspirin and benzene is found to be endothermic by 2.7 kcal/mol and provides compelling evidence for and a quantitative measure of ortho repulsion. The ortho repulsion of 2.7 kcal/mol is a lower limit, and the ortho repulsion can increase to as much as 6 kcal/mol in some aspirin isomers.     

Transition-metal-promoted reactions of boron hydrides. 17. Titanium-catalyzed decaborane-olefin hydroborations.

The titanium-catalyzed hydroboration reactions of decaborane with a variety of terminal olefins have been found to result in the exclusive, high-yield formation of monosubstituted decaborane 6-R-B(10)H(13) products, arising from anti-Markovnikov addition of the cage B6-H to the olefin. The titanium-catalyzed reactions are slow, often less than one turnover per hour; however, their high selectivities and yields coupled with the fact that they are simple, one-pot reactions give them significant advantages over the previously reported routes to 6-R-B(10)H(13) compounds. The catalyst also has extended activity with reactions carried out for as long as 13 days, showing little decrease in reactivity, thereby allowing for the production of large amounts of 6-R-B(10)H(13). The titanium-catalyzed reactions of decaborane with the nonconjugated diolefins, 1,5-hexadiene and diallylsilane, were found to give, depending upon reaction conditions and stoichiometries, high yields of either alkenyl-substituted 6-(CH(2)=CH(CH(2))(4))-B(10)H(13) (4) and 6-(CH(2)=CHCH(2)SiMe(2)(CH(2))(3))-B(10)H(13) (5) or linked-cage 6,6'-(CH(2))(6)-(B(10)H(13))(2) (6) and Me(2)Si(6-(CH(2))(3)-B(10)H(13))(2) (7) compounds, respectively. The unique tetra-cage product, Si(6-(CH(2))(3)-B(10)H(13))(4) (8), was obtained by the catalyzed reaction of 4 equiv of decaborane with tetraallylsilane. Sequential use of the titanium catalyst and previously reported platinum catalysts (PtBr(2) or H(2)PtCl(6).6H(2)O with an initiator) provides an efficient pathway to asymmetrically substituted 6-R-9-R'-B(10)H(12) species. The structures of compounds 5, 6, and 8, as well as a platinum derivative, (PSH(+))(2)-commo-Pt-[nido-7-Pt-8-(n-C(8)H(17))B(10)H(11)](2)(2-), of 6-(n-octyl)decaborane have been established by single-crystal crystallographic determinations.     

THE U.V. PHOTOCHEMISTRY OF CYTIDYLIC ACID

Abstract The ultraviolet (u.v.) irradiation of the 3' isomer of cytidylic acid (Cp) produces the hydrate (Cp*) with water added across the 5–6 double bond. The yield of this photo-product has been measured by, (a) separating the photoproduct by electrophoresis and (b) by observing the loss in absorbance. When corrections are made for reversal of the hydrate during the experiment, both methods gave the same result. Cross sections and quantum yields for the production of the hydrate were measured over the wavelength range 220 to 290 nm and over a pH range from 1 to 10. The quantum yield is markedly dependent on pH being higher by a factor of 6 to 10 for the neutral form. We have also demonstrated the existence of a very short lived photoproduct (half life 8–9 min) in both Cp3' and Cp5': The nature of this short lived product is not known.     

Theory of damped quantum rotation in nuclear magnetic resonance spectra. II. Numerical simulations for the benzene rotor

In Part I of this series of papers, the damped quantum rotation (DQR) theory, formulated originally for hindered threefold molecular rotors in solids, was generalized to the N-fold case. The stochastic dynamics of such objects, evidenced in NMR line shapes, was shown to be more complicated than in the standard model of classical jumps between the wells of the N-fold torsional potential. Actually, it comprises certain quantum rate (i.e., coherence-damping) processes subject to the requirements of the Pauli principle. The jump picture is recovered only when the quantum rates fit specific patterns. In this work, one of the ways of approaching such a classical limit is identified for the benzene rotor. This is inferred from a quantum mechanical model whose validity was earlier confirmed for a methyl group. Based on that model, theoretical calculations for the benzene ring dynamics in a clathrate crystal, 1-(9-anthryloxy)anthraquinone/benzene-d6, confronted with the pertinent literature data, point to possible deviations from the classical limit. However, the predicted DQR effects are too small to be observed in solid echo 2H NMR spectra of the C6D6 isotopomer. The chances of detecting the effects are improved when Carr-Purcell echo 1H spectra of a single crystal of the isotopomer including C6H6 as a guest are considered. The substantial differences in the sensitivity to the DQR effects of the spectra of protonated and deuterated benzene are concerned with different magnitudes of the intramolecular dipolar spin couplings. The dynamic isotope effect (C6D6 vs C6H6), which is small in this case, is only of secondary importance. Legitimacy of the use of the jump model in 2H NMR line shape studies of benzene-d6 is fully confirmed by the present considerations. However, the physical significance of the dynamic parameters extracted from such studies is shown from a new perspective.     

13C n.m.r. spectra of some pyrylium salts and related compounds

The ¹³C chemical shifts of several alkyl and phenyl substituted pyrylium perchlorates, together with related pyridine and pyridinium salts, are reported. The shifts in the isoelectronic series benzene, pyridine, pyrylium cation correlate well with charge densities calculated by INDO MO theory. Charge densities also account for the shift changes found at C-3, C-4 and C-5 for protonation of pyridine and 2,4,6-trimethylpyridine. The shift changes observed on protonation for C-2 and C-6, along the series pyridine, 2,4,6-trimethylpyridine and 2,4,6-triphenylpyridine can only be rationalized by consideration of both charge density and π-bond order changes. The effects of alkyl substitution on the shifts of the pyrylium cations are not accounted for by charge density changes. Empirical correlations of these shifts with literature data for the alkylbenzenes and the shifts of the phenyl substituted 6-membered heterocycles are discussed.     

Streptonigrin and related compounds. I. Some 2-phenyl- and 2,2-pyridylquinoline-5,8-diones

Methods for the synthesis of 6-amino-7-methoxy- and 7-amino-6-methoxy-2,2-pyridylquinoline- 5,8-diones and the corresponding 2-phenylquinoline-5,8-diones are described. The 6-aminoquinone system was generated by direct amination with sodium azide and the 7-aminoquinone system via the novel 6-hydroxy-7-nitroquinone intermediates. The basic skeleton was derived by the application of the Friedlander quinoline synthesis.     

Jahn—Teller effects in substituted benzene cations. V. Quadratic coupling

A calculation model is derived for taking into account quadratic, in addition to linear, coupling Jahn—Teller effects in determining vibronic energy levels and transitions. Procedures are developed for analysis of Jahn—Teller electronic spectra on this basis and the new features, with respect to the linear coupling approximation, brought about by introduction of quadratic coupling, are discussed. Vibronic analyses of the B?² A″₂-X?²E″ transitions of 1,3,5-C₆F₃H⁺₃, 1,3,5-C₆F₃D⁺₃ and 1,3,5-C₆Cl₃H⁺₃ are carried out, in particular for bands involving excitation of the mode 6 vibration. Experimental evidence for quadratic Jahn—Teller effects is obtained for the sym-trifluorobenzene ions and the linear coupling parameters D₆, ω₆ and quadratic coupling parameter q₆ are derived. Two possible orders of magnitude of the quadratic coupling strength are found to be compatible with the spectra of 1,3,5-C₆Cl₃H⁺₃. The analyses are consistent between the three ions and are not in contradiction with the general findings based on the linear approximation alone.     

Mass spectral fragmentation pattern of 2,2′-Bipyridyls. Part IX. 6-Alkoxy-2,2′-bipyridyls

The mass spectral fragmentation patterns of 6-methoxy-, 6-ethoxy- and 6-propoxy-2,2 ′-bipyridyls are reported. The base peaks in the spectra of both the 6-methoxy and 6-ethoxy compounds are due to the M-lion of 6-methoxy-2,2′-bipyridyl, while the base peak with 6-propoxy-2,2- bipyridyl is due to a species formed by loss of C₃H₆ from the molecular ion.     

Singer polymals. I. Evaluation of matrix elements

Hamiltonian matrix elements between pairs of explicitly correlated Singer polymals are derived. A new method for 1/r₁₂ matrix elements is found which avoids the 6 by 6 matrix inversion required at each quadrature point in the old method. The new formula requires about half the computer time of the old one. The electron repulsion and nuclear attraction matrix elements are shown to be related to incomplete elliptic integrals. Simplified formulas are derived for the special cases of cylindrical symmetry, spherical symmetry, and single-center expansions. An upper bound approximation formula for the purpose of neglecting matrix elements is derived. These bounds may be useful for single-center expansions but are found to be too high for the general case.     

Structures and stability of medium silicon clusters. II. Ab initio molecular orbital calculations of Si12-Si20

Ab initio all-electron molecular-orbital calculations are carried out to study the structures and relative stability of low-energy silicon clusters (Si(n),n = 12-20). Selected geometric isomers include those predicted by Ho et al. [Nature (London) 392, 582 (1998)] based on an unbiased search with tight-binding/genetic algorithm, as well as those found by Rata et al. [Phys. Rev. Lett. 85, 546 (2000)] based on density-functional tight-binding/single-parent evolution algorithm. These geometric isomers are optimized at the M?ller-Plesset (MP2) MP2/6-31G(d) level. The single-point energy at the coupled-cluster single and double substitutions (including triple excitations) [CCSD(T)] CCSD(T)/6-31G(d) level for several low-lying isomers are further computed. Harmonic vibrational frequency analysis at the MP2/6-31G(d) level of theory is also undertaken to assure that the optimized geometries are stable. For Si12-Si17 and Si19 the isomer with the lowest-energy at the CCSD(T)/6-31G(d) level is the same as that predicted by Ho et al., whereas for Si18 and Si20, the same as predicted by Rata et al. However, for Si14 and Si15, the vibrational frequency analysis indicates that the isomer with the lowest CCSD(T)/6-31G(d) single-point energy gives rise to imaginary frequencies. Small structural perturbation onto the Si14 and Si15 isomers can remove the imaginary frequencies and results in new isomers with slightly lower MP2/6-31G(d) energy; however the new isomers have a higher single-point energy at the CCSD(T)/6-31G(d) level. For most Si(n) (n = 12-18,20) the low-lying isomers are prolate in shape, whereas for Si19 a spherical-like isomer is slightly lower in energy at the CCSD(T)/6-31G(d) level than low-lying prolate isomers.     

Electron transfer. 144. Reductions with germanium(II)

Solutions 0.2-0.4 M in Ge(II) and 6 M in HCl, generated by reaction of Ge(IV) with H3PO2, are stable for more than 3 weeks and can be diluted 200-fold with dilute HCl to give GeCl3- preparations to be used in redox studies. Kinetic profiles for the reduction of Fe(III) by Ge(II), as catalyzed by Cu(II), implicate the odd-electron intermediate, Ge(III), which is formed from Cu(II) and Ge(II) (k = 30 M-1 s-1 in 0.5 M HCl at 24 degrees C) and which is consumed by reaction with Fe(III) (k = 6 x 10(2) M-1 s-1). A slower direct reaction between Ge(II) and Fe(III) (k = 0.66 M-1 s-1) can be detected in 1.0 M HCl. The reaction of Ge(II) with I3- in 0.01-0.50 M iodide is zero order in oxidant and appears to proceed via a rate-determining heterolysis of a Ge(II)-OH2 species (k = 0.045 s-1) which is subject to H(+)-catalysis. Reductions of IrCl6(2-) and PtCl6(2-) by Ge(II) are strongly Cl(-)-catalyzed. The Ir(IV) reaction proceeds through a pair of 1e- changes, of which the initial conversion to Ge(III) is rate-determining, whereas the Pt(IV) oxidant probably utilizes (at least in part) an inner-sphere PtIV-Cl-GeII bridge in which chlorine is transferred (as Cl+) from oxidant to reductant. The 2e- reagent, Ge(II), like its 5s2 counterpart, In(I), can partake in 1e- transactions, but requires more severe constraints: the coreagent must be more powerfully oxidizing and the reaction medium more halide-rich.     

Silabutadienes. Internal rotations and pi-conjugation. A density functional theory study

The potential energy surfaces (PESs) for internal rotation around the central single bond of nine silabutadienes, which include all possible mono-, di-, tri-, and tetrasilabutadienes, are investigated computationally by using DFT with the B3LYP functional and the 6-311+G(d,p) basis set. For 1-silabutadiene (3), 2-silabutadiene (4), 1,4-disilabutadiene (5), 2,3-disilabutadiene (6), and 1,3-disilabutadiene (7), the s-trans rotamer is the most stable. For 1,2-disilabutadiene (8), 1,2,3-trisilabutadiene (9), and 1,2,4-trisilabutadiene (10), all having a trans-bent SiSi double bond, the most stable conformers are those having an antiperiplanar (ap) structure. For tetrasilabutadiene (11), the global minimum is the gauche rotamer. The internal rotation barriers (RB) (relative to the global minimum) follow the order (kcal/mol) 5 (10.0) > 3 (7.4) > 1,3-butadiene (12, (6.6)) > 10 (4.9) > or = 7 (4.4) > or = 4 (4.0) approximately = 8 (3.9) > 9 (2.7) approximately = 6 (2.6) > 11 (2.4). The barriers are slightly smaller at CCSD(T)/cc-PVTZ, but the trend remains the same. The size of the rotation barrier is mainly dictated by the length of the central single bond; that is, it is the largest for dienes with the shorter C-C central bond (5, 3, and 12), and it is smaller for dienes with the longer Si-C and Si-Si central bonds. The strength of pi-conjugation in the s-trans conformers of silabutadienes was estimated by resonance stabilization energies (RE) calculated by using the Natural Bond Orbital (NBO) and Block Localized Wave function (BLW) methods and bond separation equations. A linear correlation is found between the barrier heights for internal rotation and pi-conjugation energies. The calculated RBs are significantly smaller than the corresponding REs, indicating that pi-resonance energies are not the only factor that dictate the RB, and therefore, RBs, although suitable for estimating trends in pi-conjugation in a series of compounds, cannot be used for estimating absolute resonance energies.     

Triaziridine. II. Erste Beispiele: 2, 3-Dialkyl-triaziridin-1-carbonsäurealkylester

Triaziridines. II. First Examples: Alkyl 2,3-Dialkyl-triaziridine-1-carboxylates The first examples of substituted triaziridines 2 are described; they carry an alkoxycarbonyl and two alkyl groups (as in 4 ). The preparation of these novel threemembered nitrogen homocycles was achieved by photolysis of 1-alkoxycarbonylazimines 3c. In this way, methyl 2, 3-(cis-1, 3-cyclopentylene)triaziridine-1-carboxylate ( 6a ), methyl trans-2, 3-diisopropyl-triaziridine- 1 -carboxylate ( 8a ) and their ethyl ester analogues 6b and 8b were obtained in 50, 18, 65 and 21 % yield, respectively. The structure of the triaziridines 6 and 8 was deduced from their spectroscopic properties which reveal several interesting features: 1) N (2) and N (3), carrying alkyl groups, are pyramidal and invert slowly; 2) the isopropyl groups of 8 are situated trans to each other on the three-membered ring, whereas the two alkyl groups of 6 are cis as forced by the C-ring system; 3) N (1) is also pyramidal, despite its «amidic» nature; it inverts with an activation energy of 62 (± 4) kJ/mol; 4) the alkoxycarbonyl, group does not conjugate with N (1) and rotates rapidly. The triaziridines 6 and 8 are thermally labile, isomerizing slowly at room temperature into the corresponding azimines 5 and 7 by cleavage of one of the bonds to N (1 ). The velocity of this ring opening reaction is almost the same for 6 and 8 , so that a dependence on the relative configuration at N (2) and N (3) is not evident. The Arrhenius activation energy for the isornerization of 6a to the corresponding azimine 5a and the enthalpy difference between 6a and 5a were both determined as 100 (± 4) kJ/mol. The photolysis of 1-alkoxycarbonyl-2, 3-diisopropyl-azimines ( 7 ) in diethyl ether was accompanied by a side reaction leading to methyl and ethyl N-(1-ethoxyethyl)carbamate ( 9a and 9b , resp.), presumably by insertion of the alkoxycarbonylnitrene, generated by photofragmentation of the azimines, into the ethereal solvent.     

Electrogenerated chemiluminescence: Part XXVII. E.C.L. and electrochemical studies of selected laser dyes

The electrogenerated chemiluminescence (e.c.l.) and electrochemistry of the laser dyes, coumarin-2, coumarin-30, rhodamine-6G (perchlorate), rhodamine-B (perchlorate), oxazine-1 (perchlorate), and Nile Blue (perchlorate) were studied in acetonitrile using 0.1 M tetra-n-butylammonium perchlorate (TBAP) as a supporting electrolyte. Rather low intensity e.c.l. was obtained for all dyes except Nile Blue. A study of the electrochemical oxidation and reduction of coumarin-30, oxazine-1 and rhodamine-6G using cyclic voltammetry and controlled potential coulometry demonstrated that chemical side reactions of the electrogenerated reactants are responsible for the low e.c.l. efficiency. In several cases the one-electron transfer reaction at the electrode is followed by a dimerization reaction. The neutral free radical formed on reduction of oxazine-1 was investigated by electron spin resonance spectroscopy and coupling constants for it are reported. Some experiments in which the e.c.l. of mixtures of the dyes with rubrene or 9,10-diphenylanthracene were determined are also described.     

Adsorption of butane isomers and SF6 on Kureha activated carbon: 1. Equilibrium

Adsorption equilibria of butane isomers and SF6 on Kureha activated carbon were investigated using the volumetric method and the tapered element oscillating microbalance (TEOM) technique. The isotherm data of the butane isomers measured by the TEOM technique are in good agreement with those determined by the volumetric method. Single-component adsorption isotherms are reported at temperatures in the range from 298 to 393 K and at pressures up to 120 kPa. SF6 molecules are mainly adsorbed in the larger micropores, resulting in a lower adsorption capacity. The amount adsorbed for n-butane is slightly higher than that for isobutane in the whole range investigated. This is attributed to the fact that the linear n-butane molecule can adsorb in the smaller micropores. The T6th model appropriately describes the equilibrium data of the butane isomers, while the isotherm data of SF6 can be fitted by the Langmuir model. The isosteric heats associated with adsorption for these three adsorptives show different loading dependences. The present study indicates that the activated carbon can be well characterized by the probe molecules having different molecular sizes.     

Synthetic analogues of Peganum alkaloids. III. Pentamethylenequinazolones

Monosubstituted 5-, 6-, and 8-methoxy-3,4-dihydro-2,3-pentamethylenequinazolones (1–3) have been syntehsized by the condensation of monosubstituted methoxyanthranilic acids with caprolactam. Demethylation with hydrobromic acid gave the corresponding hydroxy compounds [4–6]. When the 6- and 8-methoxy- and 6- and 8-hydroxy-3,4-dihydro-2,3-pentamethylenequinazolones (2, 3, 5, and 6) were reduced with zinc in hydrochloric acid, the corresponding quinazoline derivatives (7–10) were obtained. The melting points of the basis and their hydrochlorides are given. Some features of their UV, mass, and PMR spectra are reported.     

Pyrimidines. XIII. Novel pyrimidine to pyrimidine transformation reactions

Novel pyrimidine to pyrimidine transformation reactions are described. 1,3-Dimethyl-(or diethyl)-uracil(1) are converted into isocytosine, 2-thiouracil or uracil derivatives by treatment with guanidine, thiourea or urea, respectively. The latter two cases require base catalysis. The effects of some substituents at C-5 and C-6 of 1,3-dialkylated uracils (1a → 1e) on this transformation were examined and a plausible mechanism is offered for their reaction. The utility of this reaction is exemplified by the facile two-step conversion of pseudouridine into the anlileukemic agent, pseudoisocytidine, in good overall yield.     

A BSSE-free SCF algorithm for intermolecular interactions. II. Sample calculations on hydrogen-bonded complexes

The usefulness and reliability of the recent BSSE -free SCF algorithm based on the “chemical Hamiltonian approach” (CHA /F ) is demonstrated by calculating potential curves for several hydrogen-bonded complexes with 4-31G , 6-31G , and 6-31G ** basis sets. It is concluded that the CHA /F scheme gives results that are numerically close to those of the Boys–Bernardi a posteriori correction scheme but are free from the “overcompensation” characteristic of the latter at smaller distances and given basis sets. © 1992 John Wiley & Sons, Inc.