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The preparation of four per-O-benzyl-d- or l-glycero-d-galacto and d- or l-glycero-d-gluco heptopyranosyl sulfoxides and the influence of their side-chain conformations on reactivity and stereoselectivity in glycosylation reactions are described. The side-chain conformation in these donors is determined by the relative configuration of its point of attachment to the pyranoside ring and the two flanking centers in agreement with a recent model. In the d- and l-glycero-d-galacto glycosyl donors, the d-glycero-d-galacto isomer with the more electron-withdrawing trans,gauche conformation of its side chain was the more equatorially selective isomer. In the d- and l-glycero-d-gluco glycosyl donors, the l-glycero-d-gluco isomer with the least disarming gauche,gauche side-chain conformation was the most equatorially selective donor. Variable temperature NMR studies, while supporting the formation of intermediate glycosyl triflates at -80 °C in all cases, were inconclusive owing to a change in the decomposition mechanism with the change in configuration. It is suggested that the equatorial selectivity of the l-glycero-d-gluco isomer arises from H-bonding between the glycosyl acceptor and O6 of the donor, which is poised to deliver the acceptor antiperiplanar to the glycosyl triflate, resulting in a high degree of SN2 character in the displacement reaction.

A two-step Pd-catalyzed (3 + 2) cycloaddition/HNO2 elimination reaction sequence has been developed to give novel cyclic 1,3-dien-5-yne systems from Pd-stabilized zwitterionic 1,3-dipoles and 2-nitro-1,3-enyne substrates. The process is highly atom-efficient and tolerates the reaction of 2-vinyloxirane, 1-tosyl-2-vinylaziridine, and diethyl 2-vinylcyclopropane-1,1-dicarboxylate derived 1,3-dipoles with a variety of 2-nitro-1,3-enyne substrates. The stereochemistry of the intermediate (3 + 2) cycloadducts was determined by single crystal X-ray analysis. Furthermore, a selective kinetic elimination of the cycloadduct with an antiperiplanar relationship between the NO2 group and the participating hydrogen was demonstrated, allowing for efficient isolation of a single diastereoisomer of the cycloadduct.

There are only four derivatives of pseudouridine (Ψ) that are known to occur naturally in RNA as post-transcriptional modifications. We have studied the conformational consequences of pseudouridylation and further modifications using replica exchange molecular dynamics simulations at the nucleoside level, and the simulated conformational preferences were compared with the available experimental (NMR) data. We found that the existing AMBER FF99-derived parameters for these nucleosides did not reproduce the observed experimental features and while the recommended bsc0 correction could be combined with these parameters leading to an improvement in the description of sugar pucker distributions, the χOL3 correction could not be applied to these nucleosides as such because of base isomerization. On the other hand, the revised χ torsion parameters (χIDRP) for Ψ developed earlier by us (Deb, I., J. Comput. Chem., 2016, 37, 1576-1588) in combination with the AMBER provided parameters and the revised γ torsion parameters generated conformational distributions, which generally were in better agreement with the experimental data. A significant shift of the distribution of base orientation toward the syn conformation was observed with our revised parameter sets compared to the large excess of anti conformation predicted by the FF99 parameters. Overall, our observations indicated that our revised set of parameters (χIDRP) for Ψ were also able to generate conformational distributions for all of the derivatives of Ψ in better agreement with the experimental data.

Glycosylation reactions were performed on a series of bicyclic C2-substituted pyranoside models to isolate and analyze factors that control the glycosylation stereoselectivities observed in carbohy...

The glycosylation stereoselectivities for a series of bicyclic furanoside models have been carried out in the presence of weak nucleophiles. These results were analyzed through the bent bond/antiperiplanar hypothesis (BBAH) orbital model to test its validity. According to the BBAH, incoming nucleophiles displace one of the two bent bonds of bicyclic oxocarbenium ion intermediates in an antiperiplanar fashion. The glycosylation stereoselectivity is then governed by the displacement of the weaker bent bond as determined by the presence of electron-withdrawing or -donating substituents at C

We have examined the π-facial stereoselectivity in the Diels–Alder reactions of phosphole oxides computationally. The experimentally observed syn-cycloadditions have been rationalized with the Cieplak model and distortion–interaction model. The natural bond orbital analysis suggests that the hyperconjugative interactions are energetically preferred between the antiperiplanar methyl group present in the −P=O unit and the developing incipient (−C–C−) bond in syn-adducts in accordance with the Cieplak model. The distortion–interaction analysis carried out for syn and anti transition states of Diels–Alder reactions of 1-substituted phosphole 1-oxide with different dienophiles reveals that the syn selectivity is favored by distortions and interaction energies compared with the anti selectivity. The formation of a syn adduct is also stabilized by the πCC–σ*PO orbital interaction, and the repulsive n−π interaction destabilizes the anti adduct that leads to the 7.0 kcal/mol thermodynamic preference for the former adduct. Furthermore, the distortion–interaction model rationalizes the formation of stereospecific products in these Diels–Alder reactions, which however is not explicable with the much-debated Cieplak model.

The bent bond/antiperiplanar hypothesis (BBAH) has been applied to the thermal rearrangements of cyclooctatetraene and related C8H8 isomers. This novel orbital model shows that pyramidal singlet diradical intermediates produced from thermal vibrational states of C8H8 isomers account for their chemical reactivity.

L. T. Scott has discovered the 1,2-swapping of carbon and hydrogen atoms which is known to take place on benzenoid aromatics (up to ∼1000 °C range). For example, 13C-1-naphthalene is specifically converted to 13C-2-naphthalene, and there is evidence that this occurs through the formation of benzofulvene and a naphthalene–carbene intermediate. Application of the bent bond/antiperiplanar hypothesis leads to the postulate that higher in energy pyramidal singlet diradical intermediates can be used to propose a mechanism that rationalizes various atom rearrangements on benzenoid aromatics and related isomeric compounds.

The geometries of the two hydroxy groups in 1,2-ethanediol or 2,3-butanediols are more stable in a gauche orientation than those in an anti orientation. This has been generally explained in terms of the gauche effect, which is stabilization due to antiperiplanar electron delocalization between an antibonding orbital of the C–O bond (σCO*) and a bonding orbital of the C–H or C–C bond (σCH or σCC). However, a C–C single bond rotation simultaneously determines the geometries of the six vicinal bonds. Therefore, it is important to understand the effects on conformational stability of other interactions of the bond orbitals adjacent to the rotating C1–C2 bond. Bond model analysis revealed that antiperiplanar bond orbital interactions as a whole contribute to the higher stabilities of hydroxy/hydroxy gauche conformers, where the C–O/C–H or C–O/C–C combination including the σCO*/σCH or σCO*/σCC delocalization is not the dominant interaction stabilizing hydroxy/hydroxy gauche conformers. Rather, our results show ...

Methyl vinyl ketone is the second major oxidation product of isoprene, and as such an important volatile organic compound present in the troposphere. In the present study, quantum chemical calculations coupled to high-resolution millimeter-wave spectroscopy have been performed to characterize the ground and first excited vibrational states of the two stable conformers. Equilibrium structures, internal rotation barriers, and relative energies have been calculated at the MP2 and M062X levels of theory. Experimental molecular parameters have been obtained that model the rotational and torsional structures, including splitting patterns due to the internal rotation of the methyl group. For the most stable antiperiplanar (s-trans) conformer, the set of parameters obtained for the ground state should be useful to further model IR spectra up to room temperature. By combining theoretical and experimental data, we obtained a relative energy value of 164 ± 30 cm–1 in the gas phase between the more stable antiperipla...

We describe the synthesis and coordination chemistry of the (1R,2S,5R)-menthyl-substituted N,N′-diaryl-α-diimine ligands N,N′-(2-Men-4-Me-Ph)2-BIAN (L1, Men = menthyl, BIAN = bis(imino)acenaphthene) and N,N′-(2-Men-4,6-Me2-Ph)2-BIAN (L2), the conformational properties of these ligands and their metal complexes, and the ethylene and 1-hexene polymerization behavior of the corresponding (α-diimine)Ni complexes. Free ligands L1 and L2 and square-planar (L1)PdCl2 and (L1,2)Ni(acac)+ complexes exhibit a preference for the syn conformation, in which the two menthyl units are located on the same side of the N═CC═N plane, while tetrahedral (L1,2)MX2 (MX2 = ZnCl2, NiBr2) complexes exhibit a preference for the anti conformation, in which the menthyl units are located on opposite sides of the N═CC═N plane. Both the anti and the syn conformers of [(L2)Ni(acac)][B(C6F5)4] can be activated by Et2AlCl to generate highly active ethylene polymerization catalysts (activity (2.5–6.6) × 106 g of PE/((mol of Ni) h) at 15 psi ...

5- to 6-member ring enlargements of 3-oxa-2-silacyclopentylmethyl to 4-oxa-3-silacyclohexyl radicals were investigated by EPR spectroscopy and QM computations of model indano-oxasilacyclopentane and oxasilinanyl compounds. Both experimental and computational evidence favored a mechanism via a concerted 1,2-migration of the "tethered" Si-group. Thus, the "forbidden" 1,2-Si-group migration from carbon to carbon becomes allowed when the Si-group is "tethered". The EPR data from 3-oxa-2-silacyclopentylmethyl radicals disclosed ground state conformations having semioccupied p-orbitals close to antiperiplanar with respect to their β-Si-C bonds, but indicated Si-hyperconjugation (β-silicon effect) was insignificant in radicals. Kinetic data was obtained by the steady state EPR method for ring enlargement of indano-3-oxa-2-silacyclopentylmethyl radicals. The scope of the novel rearrangement in terms of other ring types and sizes, as well as the analogous 1,2-migration of "tethered" C-centered groups, was explored computationally.

Acyclic αγα-tripeptides derived from fluorinated-furanoid sugar amino acid frameworks act as reverse-turn inducers with a U-shaped conformation, whereas the corresponding nonfluorinated αγα-tripeptides show random peptide conformations. The NMR studies showed the presence of bifurcated weak intramolecular hydrogen bonding (F···HN) and N+···Fδ- charge-dipole attraction compel the amide carbonyl groups to orient antiperiplanar to the C–F bond, thus, demonstrating the role of the fluorine substituent in stabilizing the U-shaped conformation. The NOESY data indicate that the U-shaped tripeptides self-assembly formation is stabilized by the intermolecular hydrogen bonding between C═O···HN with antiparallel orientation. This fact is supported by ESI-MS data, which showed mass peaks up to the pentameric self-assembly, even in the gas phase. The morphological analysis by FE-SEM, on solid samples, showed arrangement of fibers into nanorods. The antiparallel self-assembled pore of the fluorinated tripeptides illust...

1,1″-1,4-Phenylene-bis(methylene)bis-4,4′-bipyridinium cation [C28H24N4]2+ (c), an organic receptor that generally crystallizes in its anti conformation, has recently been shown to be isolated in its syn conformation in an ion-paired compound [C28H24N4][Zn(dmit)2]·2DMF (dmit2– = 1,3-dithiole-2-thione-4,5-dithiolate; DMF = dimethylformamide). In this article, we demonstrated that the same receptor [C28H24N4]2+ (c) can also be stabilized in an unusual intermediate conformation (neither syn nor anti) with PF6– anion in compound [C28H24N4](PF6)2·(1,4-dioxane) (1·(1,4-dioxane)). The energetically favored anti conformation has been described in its nitrate salt [C28H24N4](NO3)2·2H2O (2·2H2O). Compounds 1·(1,4-dioxane) and 2·2H2O, crystallizing in triclinic and monoclinic systems with space groups P1 and P21/n, respectively, were additionally characterized by Hirshfeld surface analysis. The density functional theory calculations are performed to understand the internal mechanism of the stability of various conf...

Polymorphism of four biologically active 3-(4,5-diaryl-4H-1,2,4-triazole-3-yl)propenoic acid derivatives has been investigated. Traditional solution-based solid-state forms screening revealed three anhydrous forms of 3-[4-phenyl-5-(2-pyridyl)-4H-1,2,4-triazole-3-yl]propenoic acid. Noteworthy, two pairs of concomitant polymorphs were detected for this system. Two other compounds were found to be dimorphic. The molecular and crystal structures of all obtained crystal forms were established by single-crystal X-ray diffraction. The resulting crystal structures were analyzed in terms of molecular conformation, intermolecular interaction patterns, and crystal packing motifs. The experimental studies were supported by extended lattice and interaction energy calculations. It was found that the carboxylic group adopts the anti conformation in all studied forms and is involved in the intramolecular O–H···Ntriazole hydrogen bonding. In consequence, the association modes are dominated by the weak C–H···O, C–H···N hyd...

To address the electronic effect in nucleophilic addition, we often use the Felkin-Anh model, in which the most s-electron-withdrawing group (sEWG) at the a-position of the carbonyl should be located anti to the nucleophile approach. The diastereoselectivity is often explained by the antiperiplanar effect of the sEWG. Reetz and Frenking reported that the FMO, the LUMO in the a-substituted ketone, has greater expansion anti the sEWG. Thus, the FMO theory can also apply to this diastereoselectivity. However, there is still no explanation for why the LUMO has greater expansion anti to the sEWG and toward the nucleophile approach with this conformation, as is suggested by the FMO theory. We show here that the phase-continuous cyclic orbital interaction among nNu:--p*C=O-s*C-A/vic-s*CC/gem- controls the diastereoselectivity, which is confirmed with theoretical calculations and the bond model analysis using a conformationally-fixed bicyclic molecule, 2,3-bis(trifluoromethyl)bicyclo[2.2.1]heptan-7-one 1, as a model.

The diiodobinorsnoutane, bi(5-iodopentacyclo[4.3.0.0(2,4).0(3,8).0(5,7)]non-4-yl) (5), exists in a sterically hindered gauche conformation rather than an anti or an averaged (freely rotating) C2v structure. Density functional theory (DFT) predictions place the gauche conformation 11 kcal/mol more stable than the anti conformation with a barrier of 17 kcal/mol connecting the minima. These are consistent with variable-temperature NMR (17.1 ± 0.8 kcal/mol) estimates and X-ray analysis. Predictions of the torsional profiles of the yet-unsynthesized bromo-, chloro-, and fluoro- analogues show a progressive lowering of the barriers.

3-Nitrobenzanthrone (3-NBA), an environmental mutagen found in diesel exhaust and a suspected carcinogen, undergoes metabolic reduction followed by reaction with DNA to form aminobenzanthrone (ABA) adducts, with the major alkylation product being N-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone (C8-dG-ABA). Site-specific synthesis of the C8-dG-ABA adduct in the oligodeoxynucleotide 5'-d(GTGCXTGTTTGT)-3':5'-d(ACAAACACGCAC)-3'; X = C8-dG-ABA adduct, including codons 272-275 of the p53 gene, has allowed for investigation into the structural and thermodynamic properties of this adduct. The conformation of the C8-dG-ABA adduct was determined using NMR spectroscopy and was refined using molecular dynamics (MD) calculations restrained by experimentally determined interproton distance restraints obtained from NOE experiments. The refined structure revealed that the C8-dG-ABA adduct formed a base-displaced intercalated conformation. The adducted guanine was shifted into the syn conformation about the glycosidic bond. The 5'- and 3'-neighboring base pairs remained intact. While this facilitated π-stacking interactions between the ABA moiety and neighboring bases, the thermal melting temperature (Tm) of the adduct-containing duplex showed a decrease of 11 °C as compared to the corresponding unmodified oligodeoxynucleotide duplex. Overall, in this sequence, the base-displaced intercalated conformation of the C8-dG-ABA lesion bears similarity to structures of other arylamine C8-dG adducts. However, in this sequence, the base-displaced intercalated conformation for the C8-dG-ABA adduct differs from the conformation of the N(2)-dG-ABA adduct reported by de los Santos and co-workers, in which it is oriented in the minor groove toward the 5' end of the duplex, with the modified guanine remaining in the anti conformation about the glyosidic torsion angle, and the complementary base remaining within the duplex. The results are discussed in relationship to differences between the C8-dG-ABA and N(2)-dG-ABA adducts with respect to susceptibility to nucleotide excision repair (NER).

This paper presents the first microwave spectroscopic investigation on hexafluoroisopropanol (HFIP). A pulsed nozzle Fourier transform microwave spectrometer has been used to determine the rotational constants for HFIP as A = 2105.12166(18) MHz, B = 1053.99503(12) MHz, and C = 932.33959(13) MHz. In addition, five isotopologues of HFIP have been observed experimentally to determine the accurate structure of HFIP. The observed spectrum could be assigned to the most stable conformer of HFIP, called antiperiplanar. Available spectroscopic information and ab initio calculations on five prototype molecules helped in exploring the torsional behavior of molecules having a CF3-C-CF3 group. Two-dimensional potential energy surfaces have been analyzed for all molecules, which explained the presence/absence of doubling in the rotational transitions. With the help of natural bond orbital (NBO) analysis, reasons for the conformational preference of HFIP have been explained.

The synthesis of the novel compound cyclopropylethylidynephosphine (C3H5CH2C≡P) and its microwave spectrum are reported together with quantum chemical calculations. The spectrum, which reveals the existence of two conformers, has been recorded in the 38–109 GHz spectral range at room temperature. The H–C–CH2–C chain of atoms is synclinal in one rotamer denoted sc, and antiperiplanar in the second conformer called ap. The spectra of the ground vibrational state and two vibrationally excited states were assigned for each rotamer. The vibrational frequencies of these excited states were determined by relative intensity measurements. Relative intensity measurements were also conducted to determine the energy difference between ap and sc. The latter conformer was found to be the lower-energy form and Eap – Esc was determined to be 0.9(4) kJ/mol. The microwave study has been augmented by quantum chemical calculations at the CCSD/cc-pVQZ and MP2/cc-pVTZ levels of theory. The CCSD predictions were generally in go...

Single crystals of the mononuclear bis-oxamate nickel(II) complex [Ni(bipy)(H2edpba)]·dmso (1) are obtained by reacting [Ni(bipy)Cl2]·H2O and the flexible K2(H2edpba) ligand [bipy = 2,2′-bipyridine; H4edpba = N,N′-2,2′-ethylenediphenylenebis(oxamic acid)]. The reaction of 1 with copper(II) ions resulted in two products in which the replacement of the nickel(II) ion by copper(II) took place: the chain compound [Cu(bipy)(H2edpba)]n·3nH2O·ndmso [dmso = dimethyl sulfoxide] (2) and the analogous chain compound without dmso crystallization molecules [Cu(bipy)(H2edpba)]n·1.5nH2O (3a) in its polycrystalline form. The reaction of [Cu(bipy)Cl2] and K2(H2edpba) yielded single crystals of [Cu(bipy)(H2edpba)]n·1.5nH2O (3b). The H2edpba2– ligand exhibits the anti conformation in 1, 2, and 3b, but it adopts different coordination modes: terminal bis-bidentate (1) and bridging bis-bidentate (2 and 3b) through the two pairs of carbonyl-oxygen atoms of the two oxamate arms. Magnetic susceptibility measurements carried out ...

1,3-Butadiene (BD) is an industrial and environmental chemical present in urban air and cigarette smoke, and is classified as a human carcinogen. It is oxidized by cytochrome P450 to form 1,2,3,4-diepoxybutane (DEB); DEB bis-alkylates the N(6) position of adenine in DNA. Two enantiomers of bis-N(6)-dA adducts of DEB have been identified: R,R-N(6),N(6)-(2,3-dihydroxybutan-1,4-diyl)-2'-deoxyadenosine (R,R-DHB-dA), and S,S-N(6),N(6)-(2,3-dihydroxybutan-1,4-diyl)-2'-deoxyadenosine (S,S-DHB-dA) [ Seneviratne , U. , Antsypovich , S. , Dorr , D. Q. , Dissanayake , T. , Kotapati , S. , and Tretyakova , N. ( 2010 ) Chem. Res. Toxicol. 23 , 1556 -1567 ]. Herein, the R,R-DHB-dA and S,S-DHB-dA adducts have been incorporated into the 5'-d(C(1)G(2)G(3)A(4)C(5)X(6)A(7)G(8)A(9)A(10)G(11))-3':5'-d(C(12)T(13)T(14)C(15)T(16)T(17)G(18)T(19)C(20)C(21)G(22))-3' duplex [X(6) = R,R-DHB-dA (R(6)) or S,S-DHB-dA (S(6))]. The structures of the duplexes were determined by molecular dynamics calculations, which were restrained by experimental distances obtained from NMR data. Both the R,R- and S,S-DHB-dA adducts are positioned in the major groove of DNA. In both instances, the bulky 3,4-dihydroxypyrrolidine rings are accommodated by an out-of-plane rotation about the C6-N(6) bond of the bis-alkylated adenine. In both instances, the directionality of the dihydroxypyrrolidine ring is evidenced by the pattern of NOEs between the 3,4-dihydroxypyrrolidine protons and DNA. Also in both instances, the anti conformation of the glycosyl bond is maintained, which combined with the out-of-plane rotation about the C6-N(6) bond, allows the complementary thymine, T(17), to remain stacked within the duplex, and form one hydrogen bond with the modified base, between the imine nitrogen of the modified base and the T(17) N3H imino proton. The loss of the second Watson-Crick hydrogen bonding interaction at the lesion sites correlates with the lower thermal stabilities of the R,R- and S,S-DHB-dA duplexes, as compared to the corresponding unmodified duplex. The reduced base stacking at the adduct sites may also contribute to the thermal instability.

Previous efforts to model the diiron(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron(IV) TPA complex, 2, with an O=Fe(IV)-O-Fe(IV)-OH core that has two ferromagnetically coupled Sloc = 1 sites. Addition of base to 2 at -85 °C elicits its conjugate base 6 with a novel O═Fe(IV)-O-Fe(IV)═O core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (Sloc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6a assumes an anti conformation with a dihedral O═Fe-Fe═O angle of 180°. Mössbauer and EPR analyses show that 6b is a diiron(IV) complex with ferromagnetically coupled Sloc = 1 and Sloc = 2 sites to give total spin St = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral O═Fe-Fe═O angle of 6b is ∼90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear Fe(IV)═O complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.

One-dimensional (1-D) zigzag chains of {[AgL](ClO4)·0.5Et2O}n (1), {[AgL](OTf)·Et2O}n (2), and {[AgL](BF4)·0.5CH3OH}n (3) were obtained from the reaction of AgClO4, AgCF3SO3, and AgBF4 with 1,4-bis(pyridine-3-ylethynyl)benzene (L), respectively, in similar reaction conditions, whereas it led to the formation of a 1-D sinusoidal chain of {[AgL(NO3)]·H2O}n (4) and a metallolocycle of [Ag2L6](BF4)2 (5) in a different reaction procedure for the AgNO3 and AgBF4 cases. Complexes 1–3 form 1-D zigzag chains, and every two chains are further stacked to give a pair of chains assisted by weak Ag(I)···Ag(I) and π···π interactions, whereas a single 1-D sinusoidal chain of 4 can only be obtained. Although the reaction is carried out in a similar reaction as 4 except that BF4– was used instead of NO3–, a metallocycle of 5 was obtained instead. It is noted that only the bridging L ligands in 5 adopt a syn conformation, whereas those in 1–4 are in an anti conformation. In this regard, this work has demonstrated the delica...

The microwave spectrum of cyclopropylmethyl isocyanide, C3H5CH2NC, has been investigated in the 25-75 GHz spectral range. The spectra of two conformers were assigned. The H-C-C-N chain of atoms is antiperiplanar in the conformer denoted ap and synclinal in the sc rotamer. The sc conformer tends to be slightly more stable than the ap form. The internal energy difference was determined to be Eap - Esc = 0.2(7) kJ/mol from relative intensity measurements. The spectra of the ground vibrational state and six vibrationally excited states belonging to two different normal vibrations were assigned for sc. The frequencies of these two modes were determined by relative intensity measurements. The dipole moment of this conformer was determined to be μa = 12.16(6), μb = 5.91(4), μc = 0 (preset), and μtot = 13.52(6) × 10(-30) C m [4.05 (2) debye]. The spectra of the ground and of two vibrationally excited states belonging to the torsion and lowest bending vibration were assigned for ap. The microwave work was supported by quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory. Most, but not all, of the theoretical predictions are in good agreement with experiment.

The diastereoselectivity of the addition of (E)-2-butenyltrimethylsilane to acetaldehyde under electrophilic (BF3, H3O(+)) and nucleophilic (F(-)) activation is investigated using density functional theory (M06-2X). The interaction-distortion/activation-strain model of reactivity is used to rationalize the origin of the selectivity. Consistent with experimental model systems, the synclinal transition states are determined to be preferred over the antiperiplanar transition states in the electrophilic-activated manifolds and vice versa for the fluoride-activated manifold. The selectivity for the syn diastereomer in the electrophilic activation manifolds is accounted for by increased electrostatic and orbital interactions for a synclinal transition state (syn-T3) at the expense of increased steric interactions relative to antiperiplanar transition states. The enhanced orbital interactions for the synclinal (syn-T3) versus antiperiplanar transition states can be attributed to increased π→π* interactions. The selectivity for the anti diastereomer in the nucleophilic manifold is explained by the lesser electrostatic repulsion in the antiperiplanar transition states which are favored relative to the synclinal transition states. Additionally, the diastereoselectivity is partly attributed to variation in the distortion of the crotylsilane.

HexSiH3, PhSiH3, and PhSiClH2 reacted with [Pd(PCy3)2] to yield dipalladium complexes with bridging silyl ligands: [{Pd(PCy3)}2(μ-HSiXR)2] (1, R = Hex, X = H; 2, R = Ph, X = H; 3, R = Ph, X = Cl). The X-ray crystallographic results displayed a typical bis(silyl)-bridged dinuclear structure with an anti conformation of the substituents on the Si atom in the solid state. Temperature-dependent NMR spectroscopic analyses of 1 and 2 revealed a dynamic syn–anti isomerization of the complex via exchange of the bridging and nonbridging Si–H hydrogens in solution. Complex 3 with bridging chloro(phenyl)silyl ligands did not show such a dynamic behavior.

The microwave spectra of a mixture of cis- and trans-H-N-C-O forms of N-vinylformamide, (H(2)C═CHNHC(═O)H), have been measured at room temperature in the 18-75 GHz spectral range. The spectra of two forms were assigned. The first of these forms has a cis arrangement for the H-N-C-O chain of atoms, whereas the second form has a trans arrangement. The C-C-N-C chain of atoms is antiperiplanar (180°) in both forms. The inertial defect of the ground vibrational state of cis is -0.142(5) × 10(-20) u m(2), whereas this parameter is -0.087098(26) × 10(-20) u m(2) for trans. It is concluded that the equilibrium structures of both cis and trans are completely planar. The dipole moment determined from Stark effect measurements is μ(a) = 9.96(8), μ(b) = 2.22(3), μ(c) = 0 (by symmetry), and μ(tot) = 10.20(8) × 10(-30) C m [3.06(2) D], for cis, and μ(a) = 7.64(16), μ(b) = 9.24(10), μ(c) = 0 (by symmetry), and μ(tot) = 12.0(2) × 10(-30) C m [3.59(5) D] for trans. The spectrum of one vibrationally excited state, presumably the first excited state of the torsion about the C-N bond of cis, was assigned and the frequency of this state was determined to be 76(15) cm(-1) by relative intensity measurements. The spectra of two vibrationally excited states of trans were assigned. These states are assumed to be the first excited state of the torsion about the C-N bond, and a low bending vibration. Relative intensity measurements yielded 101(20) and ca. 300 cm(-1), respectively, for the frequencies of these normal vibrations. Accurate values of the quartic centrifugal distortion constants, the dipole moments, and the vibration-rotation constants have been obtained for both cis and trans. The experimental work has been augmented by high-level quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD(T)/cc-pVTZ levels of theory. The theoretical calculation performed without symmetry restrictions correctly predict that cis and trans are both planar. The CCSD(T) rotational constants are in excellent agreement with their experimental counterparts, whereas the B3LYP quartic centrifugal distortion constants and the vibration-rotation constants are in fairly good agreement with experiments. The CCSD(T) dipole moments deviate more than expected from the experimental dipole moments. It is estimated that further conformers of cis and trans must be at least 4 kJ/mol higher in energy.

Reactions of a tripodal ligand, N,N',N″-tris(3-pyridinyl)phosphoric triamide (TPPA), and a series of transition-metal ions result in the assembly of five discrete M(6)L(8) coordination cages [M(6)(TPPA)(8)(H(2)O)(12)](ClO(4))(12)·57H(2)O [M = Ni(2+) (1), Co(2+) (2), Zn(2+) (3), Cd(2+) (4)] and [Pd(6)(TPPA)(8)]Cl(12)·22H(2)O (5). X-ray structural analyses reveal that the cages have large internal cavities and flexible windows. The flexible ligand TPPA adopts the syn conformation in cages 1-4, but it transforms to the anti conformation in cage 5. Because of the conformational transformation, the sizes of the windows and the volume of the internal cavity of cage 5 are increased. (1)H NMR and electrospray mass spectrometric studies show that cage 5 maintains its structural integrity in solution. Additionally, compounds 3 and 4 exhibit strong blue fluorescent emissions, which are 1 order of magnitude higher than that of the free ligand.

The mechanism of the acid-catalyzed intramolecular Schmidt reaction of 2-azidopropylcyclohexanones was studied using density functional theory (primarily M06-2X). The reaction was found to proceed through rapid formation of azidohydrin intermediates followed by rate-determining concerted N(2)-loss/shift of the alkyl group antiperiplanar to the N(2) leaving group. For cases where steric, lone pair-cation, and cation-π effects have been invoked previously as regiocontrol elements, the origins and magnitudes of these effects have been examined theoretically.

Three lanthanide–organic frameworks have been solvothermally synthesized and characterized. Complex 1 {[Pr2(bbda)(OH)2(H2O)6]·12H2O}n (H4bbda = 5,5′-(2,4,6-trimethyl-1,3-phenylene)bis(methylene)bis(oxy)diisophthalic acid) is a 3D porous framework belonging to a 2-nodal net with the ssa topology. The coordinated formate ligand from in situ hydrolyzation of dmf binds metal centers to generate rigid rod-shaped SBUs (secondary building units) in both complexes 2 and 3, [Ln(HCOO)(L1)(dmf)]n (Ln = Er (2), Tm (3), H2L1 = 2,2′-(2,3,5,6-tetramethyl-1,4-phenylene) bis(methylene)bis(sulfanediyl)dibenzoic acid, dmf = N,N′-dimethylformamide). The flexible ligand H4bbda in complex 1 exhibits syn conformation, while the flexible ligand H2L1 in complexes 2 and 3 adopts anti conformation. The anti L1 ligand links the rod-shaped SBUs to give 2 and 3 3D network with the rtl topology.

Alkyl analogues of methylphenidate (Ritalin) salts are slow onset, long duration dopamine reuptake inhibitors with a potential use as a cocaine abuse pharmacotherapy. X-ray crystallographic studies and nuclear magnetic resonance (NMR) investigations strongly suggest that avoidance of sterically unfavorable gauche(-)gauche(+) orientations effectively influences both the C(α)-alkyl side chain conformation and the formation of a predominant rotamer about the CH-CH bond ligating piperidine and C(Ar)R moieties. The favored CH-CH rotamer in D(2)O and in CD(2)Cl(2) of the pharmacologically interesting i-Bu and CH(2)-cyc-Pnt (RS,RS)-salts has the same antiperiplanar arrangement that was found in the crystal structures, although there clearly is a fast equilibrium involving smaller amounts of synclinal partners. While the rotamer in the (RS,SR)-i-Bu HCl crystal structure exhibits a synclinal orientation for the vicinal pair of adjacent methine protons, the weighted time-averaged arrangement for these protons becomes almost completely antiperiplanar when the crystals are dissolved in D(2)O. Increased steric congestion around the CH-CH bond in the analogous N-methyl tertiary ammonium salts seems to augment the quantity of the preferred rotamer within the mixture. The stereochemistry of the species observed via NMR seems to arise from specific combinations of N-methyl orientation and avoidance of sterically unfavorable gauche(-)gauche(+) arrangements.

Orange dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate (1) and blue (Z)-2,3-bis(ferrocenyl)maleimide (2) were prepared and characterized using NMR, IR, and UV–vis spectroscopy as well as X-ray crystallography. X-ray crystallographic studies of 1 and 2 revealed a locked anti conformation of the ferrocene substituents, which was explained on the basis of the steric properties of these groups. Electronic structures and solvatochromic properties of 1 and 2 were investigated by UV–vis spectroscopy and polarized continuum model (PCM) density functional theory (DFT-PCM) as well as time-dependent DFT (TDDFT-PCM) approaches. The calculated vertical excitation energies are consistent with the experimental data and clearly suggest the dominance of metal-to-ligand charge-transfer bands in the visible region of the UV–vis spectra for 1 and 2. Redox properties of 1 and 2 were investigated using cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemical and chemical oxidation approaches. In a DCM/(NB...

Stereospecific intramolecular antarafacial epoxidation of a double bond via an early Pummerer reaction intermediate has been demonstrated. The intermediate is presumably generated via trifluoroacetylation of a sulfoxide precursor. Ionization of trifluoroacetate would generate, formally, a di-positive “sulfenium” equivalent. This specie attacks an otherwise unactivated, proximal olefinic linkage in an anti-periplanar fashion, with trifluoroacetate serving as the nucleophile. Proposed mechanistic intermediates have been characterized structurally (in several cases by crystallographic means) and shown to serve as precursors en route to the final antarafacial epoxides. The sense of the cyclization seems to be driven by principles inherent in Markovnikov’s Rule.

A 1,3,5-substituted benzene platform has been widely used in the fields of supramolecular chemistry and molecular recognition. Here, we show that 1,3,5-tris(2-benzyl-o-carboran-1-yl)benzene 6 exhibits solvent-dependent conformation in the crystalline state. Recrystallization from dichloromethane-n-pentane gave the anti conformation 6-anti, while recrystallization from methanol-acetone gave the syn conformation 6-syn, in which the three benzyl-o-carboranyl moieties are located to one side of the central benzene ring. Interestingly, one acetone molecule is captured in the π-rich space of 6-syn and two complexes facing each other encapsulate two acetone molecules in a π-rich container formed by the eight benzene rings. The inclusion involves several weak interactions, that is, T-shaped C-H···π interactions, and C-H···O and C-H···π interactions. Two C-H···O interactions involving benzylic C-H hydrogens activated by the electron-withdrawing character of the o-carborane cage and the oxygen atom of the acetone seem to be the most important. DFT calculations indicate that the binding energy for entrapment of acetone is 6.6 kcal/mol. Inclusion of acetone is achieved through not only multiple C-H···O interactions but also a number of C-H···π interactions. The third benzyl-o-carborane moiety is fixed in the syn conformation by intramolecular and intermolecular C-H···π interactions.

In the present systematic study, the solid-state conformational features of widely utilized Frechet-type dendrimers and dendrons derived from 3,5-di(benzyloxy)benzene are discussed on the basis of X-ray structures reported up now. Their conformations are described in terms of the six-torsion-angle sequences u1-u2-u3-u3′-u2′-u1′ of the 3,5-di(benzyloxy)benzene moiety, and of the conformational descriptors Ap (antiperiplanar), Ac (anticlinal), Sc (synclinal), and Sp (synperiplanar). Of the 10 possible conformational types of u3-u3′ sequences, only six types were observed in single crystals, and out of 136 possible types of conformations of u2-u3-u3′-u2′ sequences, only 14. Ap-Sp and Ap-Ap are the preferred conformations of sequences u3-u3′, and Ap-Ap-Sp-Ap and Ap-Ap-Ap-Ap those of the sequences u2-u3-u3′-u2′. The presence of a substituent at position 2 of the focal phenyl ring generally induces a preference for the Ap-Sp conformation of u3-u3′ sequences and for the Ap-Ap-Sp-Ap conformation of u2-u3-u3′-u2′ ...

We have investigated new folding pathways of human telomeric type-1 and type-2 G-quadruplex conformations via intermediate hairpin and triplex structures. The stabilization energies calculated by ab initio methods evidenced the formation of a hairpin structure with Hoogsteen GG base pairs. Further calculations revealed that the G-triplet is more stable than the hairpin conformation and equally stable when compared to the G-tetrad. This indicated the possibility of a triplex intermediate. The overall folding is facilitated by K(+) association in each step, as it decreases the electrostatic repulsion. The K(+) binding site was identified by molecular dynamics simulations. We then focused on the syn/anti arrangement and found that the anti conformation of deoxyguanosine is more stable than the syn conformation, which indicated that folding would increase the number of anti conformations. The K(+) binding to a hairpin near the second lateral TTA loop was found to be preferable, considering entropic effects. Stacking of G-tetrads with the same conformation (anti/anti or syn/syn) is more stable than mixed stacking (anti/syn and vice versa). These results suggest the formation of type-1 and type-2 G-quadruplex structures with the possibility of hairpin and triplex intermediates.

The MW spectrum of propargyl thiocyanate (HC≡CCH 2 SC≡N) has been investigated for the first time in the 25-80 GHz spectral region at room temperature or at 0 °C. The spectra of the ground vibrational state and of the first excited state of the C-S torsional vibration have been assigned for one conformer. This rotamer, denoted ap, has a symmetry plane (C, symmetry) and an antiperiplanar arrangement for the C-C-S-C link of atoms. It has previously been claimed that a conformer that has a synclinal conformation for this chain of atoms is present in the gas in approximately the same concentration as ap (∼50% of the gas), but this is not supported by the present experiments, where it is shown that the synclinal rotamer, denoted sc, cannot be present in a concentration exceeding 1/3 of the total. It is therefore concluded that ap must be al least 3.0 kJ/mol more stable than sc. The spectroscopic work has been augmented by quantum chemical calculations at advanced B3LYP/aug-cc-pVQZ, B3LYP/6-31 1++G(3df,3pd), and MP2/aug-cc-pVTZ levels of theory. These theoretical calculations underestimate the energy difference between ap and sc and predict values for the conformationally important C—C—S—C dihedral angle of the hypothetical synclinal form that deviates by ∼10°.

Using ligands BITMB and IPA or TMIPA (BITMB = 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene, IPA = isophthalic acid, TMIPA = 2,4,6-trimethylisophthalic acid), in which the BITMB can adopt syn or anti conformations, three metal−organic frameworks (MOFs) have been synthesized. These MOFs contain a variety of structures: a one-dimensional (1D) chain (1), two-dimensional (2D) interpenetrating layer (2), and three-dimensional (3D) porous framework (3). In MOFs 1 and 2, the BITMB ligand adopts a syn conformation, connecting metal ions to form a macrometallocycle or chain as the subunit, which are further connected by the rigid carboxylate ligands to generate the final 1D chain or 2D interpenetrating layer. The BITMB ligand in 3 adopts an anti conformation, which connects metal ions to form a 1D zigzag chain as the subunit. The 1D chain subunits are further connected by the carboxylate ligand to form the 3D porous framework. The conformation of the BITMB and thus the dimensionality of the MOF can be contro...

The synthesis of a 2-halogenoethylphosphine, (2-chloroethyl)phosphine (ClCH(2)CH(2)PH(2)), is reported for the first time. This compound was prepared by a chemoselective reduction of diethylchloroethylphosphonate with dichloroalane using a vacuum line. (2-Chloroethyl)phosphine has been studied by microwave spectroscopy at room temperature, or at -20 degrees C, in the 22-80 GHz spectral interval. The experimental study has been augmented by quantum chemical calculations at the MP2/6-311++(3df,3pd) and B3LYP/6-311++(3df,3pd) levels of theory. The spectra of two rotameric forms, denoted I and II have been assigned. These conformers both have an antiperiplanar arrangement for the Cl-C-C-P chain of atoms, but with different orientation of the phosphine group. Conformer I was found to be 5.2(6) kJ/mol more stable than II by relative intensity measurements. The spectra of the first excited states of the C-C torsional vibration of both I and II were assigned. The torsional frequency was determined to be 63(20) cm(-1) for conformer I by relative intensity measurements, and it has a similar value in II. It was found that this normal vibration deviates from a near-harmonic behavior. The quantum chemical calculations produce rotational and centrifugal distortion constants that are in satisfactory agreement with observations, but fail to predict correctly low-frequency fundamental frequencies. The quantum chemical calculations indicate that three additional conformers characterized by a synclinal orientation for the Cl-C-C-P link of atoms and with different orientation of the phosphine group are high-energy forms of ClCH(2)CH(2)PH(2). Searches for these forms were unsuccessful.

The MW spectrum of propargyl thiocyanate (HC[triple bond]CCH(2)SC[triple bond]N) has been investigated for the first time in the 25-80 GHz spectral region at room temperature or at 0 degrees C. The spectra of the ground vibrational state and of the first excited state of the C-S torsional vibration have been assigned for one conformer. This rotamer, denoted ap, has a symmetry plane (C(s) symmetry) and an antiperiplanar arrangement for the C-C-S-C link of atoms. It has previously been claimed that a conformer that has a synclinal conformation for this chain of atoms is present in the gas in approximately the same concentration as ap (approximately 50% of the gas), but this is not supported by the present experiments, where it is shown that the synclinal rotamer, denoted sc, cannot be present in a concentration exceeding 1/3 of the total. It is therefore concluded that ap must be at least 3.0 kJ/mol more stable than sc. The spectroscopic work has been augmented by quantum chemical calculations at advanced B3LYP/aug-cc-pVQZ, B3LYP/6-311++G(3df,3pd), and MP2/aug-cc-pVTZ levels of theory. These theoretical calculations underestimate the energy difference between ap and sc and predict values for the conformationally important C-C-S-C dihedral angle of the hypothetical synclinal form that deviates by approximately 10 degrees.

Secondary deuterium isotope effects (IEs) on basicities of isotopologues of trimethylamine have been accurately measured by an NMR titration method applicable to a mixture. Deuteration definitely increases the basicity, by approximately 0.021 in the Delta pK per D. The IE is attributed to the lowering of the CH stretching frequency and zero-point energy by delocalization of the nitrogen lone pair into the C-H antibonding orbital. Because this depends on the dihedral angle between the lone pair and the C-H, a further consequence is a preference for conformations with H antiperiplanar to the lone pair and D gauche. This leads to a predicted nonadditivity of IEs, which is confirmed experimentally. It is found that the decrease in basicity, per deuterium, increases with the number of deuteriums. The nonadditivity of IEs is a violation of the widely assumed Rule of the Geometric Mean.

The microwave spectrum of 2,2,2-trifluoroethanethiol, CF3CH2SH, and of one deuterated species, CF3CH2SD, has been investigated in the 7-80 GHz spectral interval. The microwave spectra of the ground and three vibrationally excited states belonging to three different normal modes of one conformer were assigned for the parent species, and the vibrational frequencies of these fundamentals were determined by relative intensity measurements. Only the ground vibrational state was assigned for the deuterated species. The identified form has a synclinal arrangement for the H-S-C-C chain of atoms and the corresponding dihedral angle is 68(5) degrees from synperiplanar (0 degrees). A weak intramolecular hydrogen bond formed between the thiol (SH) group and one of the fluorine atoms is stabilizing this conformer. There is no evidence in the microwave spectrum for the H-S-C-C antiperiplanar form. The hydrogen atom of the thiol group should have the ability to tunnel between two equivalent synclinal potential wells, but no splittings of spectral lines due to tunneling were observed. The microwave work was augmented by quantum chemical calculations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory.

The reactions of the unsymmetrical ligand N-phenyl-N'-cyano-formamidine (L) with AgX (X = NO 3 - , SO 3 CF 3 - , and BF 4 - ) afforded the complexes [AgL 3 NO 3 ] (1), [AgLSO 3 CF 3 ] ∞ (2) and [AgLBF 4 ] ∞ (3), whereas the reactions of L with AgClO 4 in tetrahydrofuran at room temperature and at refluxing temperature afforded two supramolecular isomers of [AgLClO 4 ] ∞ , (4) and (5), respectively. These complexes were characterized by X-ray crystallography, which confirmed that complexes 2-5 are coordination polymers. Complex 1 is a triangular planar molecule with three L's coordinating to the Ag(I) metal center through the cyano nitrogen atoms. While the cations of complexes 2-4 form one-dimensional zigzag chains, those of 5 show helical chains that are interlinked through η 3 π-donor behavior of the unsymmetrical ligand L to form two-dimensional polymeric sheets. The L ligands in complex 1 adopt the syn conformation, and those in 2-5 adopt the anti conformation.

The tautomeric equilibrium and hydrogen bonding in nucleotide 2'-deoxyguanosine monophosphate that interacts with hydrated Mg2+ cation (4H2O.Mg[dGMP]) were studied at the MP2/cc-pVDZ//B3LYP/cc-pVDZ and B3LYP/aug-cc-pVTZ//B3LYP/cc-pVDZ levels of theory. The Mg2+ ion forms two inner-shell contacts with the nucleotide, similar to small phosphorylated molecules under physiological conditions. The presence of the phosphate group and the hydrated magnesium cation leads to a change in guanine tautomeric equilibrium of 4H2O.Mg[dGMP] in comparison to free guanine. The influence of the phosphate group and the magnesium cation on tautomeric equilibrium is larger in the anti conformation where the P=O--Mg and Mg--N7 coordinate bonds are formed. The canonical oxo form of guanine is more stable (by 6-8 kcal/mol) than the O6-hydroxo form in anti conformation. Thus, the interaction with Mg2+ ion is capable of further suppressing the likelihood of a spontaneous transient formation of the rare tautomer. In the syn conformation of 4H2O.Mg[dGMP], the interaction of the guanine nucleobase with the phosphate group and the magnesium cation is not as strong as in the anti conformation, and the relative stability of guanine tautomers is close to those in free guanine.

The microwave spectra of 4-pentyn-1-ol, HO(CH2)3C triple bond CH, and one deuterated species (DO(CH2)3C triple bond CH) have been investigated in a Stark spectrometer in the 17.5-80 GHz spectral region at about 0 degrees C, as well as in a pulsed-nozzle Fourier transform spectrometer in the 2.5-14 GHz range. A total of 14 spectroscopically different all-staggered rotameric forms are possible for this compound. It has previously been assumed that a conformer stabilized by intramolecular hydrogen bonding predominates in the gas phase, but the microwave spectrum of this rotamer was not assigned and it is concluded that this form is not present in high concentrations. However, the microwave spectrum indicates that several forms are present, two of which denoted ag+g+ and ag+a were assigned in this work. In these two forms, the H-O-C-C chains of atoms have an antiperiplanar conformation and the O-C-C-C links are synclinal ("gauche"). The C-C-C-C triple bond CH link is synclinal in ag+g+ but antiperiplanar in ag+a. The ag+g+ form is determined to be 1.5(6) kJ/mol more stable than ag+a by relative intensity measurements. The microwave study was augmented by quantum chemical calculations at the MP2/6-311++G** and G3 levels of theory. Both these quantum chemical procedures indicate that there are small energy differences between several rotametric forms, in agreement with the microwave findings. Both methods predict that ag+g+ is the global minimum.

The chemical names of a pair of recently synthesized antitumor drugs are given in the present study as 1',2'-didehydro-3',4'-deoxycytidine and 3',4'-didehydro-2',4'-deoxycytidine. The order of stabilities, geometries, and ionization potentials of the unsaturated sugar-modified cytidine derivatives is investigated quantum mechanically. Our density functional theory calculations based on the B3LYP/6-311++G** model reveal that 3',4'-didehydro-2',4'-deoxycytidine (SD-C2) is slightly more stable than its isomer, 1',2'-didehydro-3',4'-deoxycytidine, by an energy of 5.28 kJ x mol(-1) in isolation. The isomers structurally differ by only the C=C location in the sugar ring. However, the compounds exhibit an unusual orientation with a less puckered sugar ring; that is, 3',4'-didehydro-2',4'-deoxycytidine is determined to be a beta-nucleoside, which is a C1'-endo, north conformer with an anticlinal sugar ring, whereas 1',2'-didehydro-3',4'-deoxycytidine is neither an alpha-nucleoside nor a beta-nucleoside but is a C4'-endo, south conformer with an antiperiplanar sugar ring. The present study further indicates that the C=C double bond location imposes significant effects on their ionization potentials (IPs) and other important molecular properties such as molecular electrostatic potential (MEP). In addition, inner shell binding energy spectral variations with respect to the C=C bond exhibit more site dependence. The valence shell binding energy spectral changes are, on the other hand, significant and delocalized. The latter indicates that such changes in valence space are not isolated effects but are within the entire nucleoside. Finally, the present study suggests that the nearly 0.6 eV difference in the first ionization potentials (highest occupied molecular orbital) of the isomers is sufficiently large to identify them by further spectroscopic measures.

The structural and conformational properties of etheneselenocyanate (H2C=CHSeC[triple bond]N) have been explored by microwave spectroscopy and quantum chemical calculations performed at the MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ levels of theory. The spectra of two rotameric forms were assigned. The more stable form has a synperiplanar conformation, whereas the less stable form has an anticlinal conformation characterized by a C-C-Se-C dihedral angle of 163(3) degrees from the synperiplanar position (0 degrees). The synperiplanar form was found to be 4.5(4) kJ/mol more stable than the anticlinal form by relative intensity measurements performed on microwave transitions. The spectra of several isotopologues and two vibrationally excited states were assigned for the synperiplanar conformer. The anticlinal rotamer displays a complicated pattern of low-frequency vibrational states, which is assumed to reflect the existence of a small potential hump at the antiperiplanar (180 degrees) conformation. The predictions made in the MP2 and B3LYP calculations are in reasonably good agreement with the experimental results in some cases, whereas rather large differences are seen for other molecular properties.

X-ray crystallography shows the gold atoms in [R3PAu{SC(OMe)=NC6H4NO2-4}] (R = Et, Cy, Ph; 1-3, respectively) and [(Ph2P-R-PPh2){AuSC(OMe)=NC6H4NO2-4}(2)] (R = CH2, (CH2)2, (CH2)3, (CH2)4, Fc; 4-8, respectively) are linearly coordinated by phosphorus and thiolate-sulfur; weak intramolecular Au...O interactions are featured in all structures. The smaller ethyl substituents in 1 allow for supramolecular association via Au...S and Au...Au interactions that are not found in 2 and 3, which contain larger phosphorus-bound Cy and Ph groups, respectively. Intramolecular Au...Au interactions are found in the dppm, dppe, dppp, and Fc structures but not in the dppp analogue, for which an anti conformation was found. The structures have been correlated with the results from photophysical study conducted in the solid state. Thus, photoexcitation of 1-7 with lambda > 350 in the solid state and in solution produces green and blue luminescence, respectively. The spectra in each medium are remarkably similar to each other, and so the emission energy and excitation maxima observed for 1-7 appear to be independent of the nature of the ancillary phosphines, as well as the presence or absence of Au...Au interactions, either intermolecularly or intramolecularly.

The influence of conformation and aggregation on the hydrogen bond donor ability of fluorinated alcohol solvents [1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 1-phenyl-2,2,2-trifluoroethanol (PhTFE)] was explored theoretically (DFT) and experimentally (NMR, kinetics, crystal structure analyses). The detailed DFT analysis revealed a pronounced dependence of the H-bond donor ability on the conformation along the CO-bond of the monomeric alcohols. The donor orbital energy (sigma*(OH)) decreases and the molecular dipole moment (mu) increases drastically from the antiperiplanar (ap) to the synperiplanar (sp) H(C)COH conformation. The kinetics of olefin epoxidation with H(2)O(2) in HFIP indicate higher order solvent aggregates (2-3 monomers) to be responsible for the activation of the oxidant. Single-crystal X-ray analyses of HFIP and PhTFE confirmed the existence of H-bonded aggregates (infinite helices, ribbons, and cyclic oligomers) and the predominance of sc to sp conformations of the fluoroalcohol monomers. These aggregate structures served as the basis for a DFT analysis of the H-bond donor ability at the terminal hydroxyl group of HFIP mono- to pentamers. Both the LUMO energy and the natural charge of the terminal hydroxyl proton indicated a substantial cooperative influence of dimerization and trimerization on the H-bond donor ability. We therefore conclude that dimers and trimers, with the individual monomers in their sc to sp conformation, play a crucial role for the solvolytic and catalytic effects exerted by HFIP, rather than monomers.