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The technique of NMR spectroscopy is employed to study the relationship between the [super 27]Pb NMR chemical shift and the solid-state structure in a series of Pb(II) compounds.

A theoretical basis has been provided for the possibility of effecting conformational control of the products of the photochemical transformations of provitamin D by acting expressly on the conformational equilibrium of the previtamin molecule by causing an interaction with an appropriate reaction medium. The results of the analysis that was carried out are valid for any extended photoreaction with a conformationally fluxional intermediate. [ABSTRACT FROM AUTHOR]

Hypertensive disorders of pregnancy (HDP) are a great danger. A previous GWAS found a relationship between rs259983 of the ZNF831 gene and HDP, such as for chronic hypertension (CHTN) and preeclampsia (PE). We conducted the case-control study to determine the association between rs259983 of the ZNF831 gene and HDP in women with Gestational Diabetes Mellitus (GDM). For target genotyping, we developed primers and TaqMan probes. In analyzing the population, we did not manage to find a relationship between PE and rs259983 of the ZNF831 gene. Additional study of women with PE and PE superimposed on CHTN (SIPE) establishes an association between rs259983 of the ZNF831 gene only with SIPE. Carriers of CC genotypes have been discovered to have a 5.05 times higher risk of SIPE development in women with GDM.

The increasing frequency of isolation of methicillin-resistant Staphylococcus aureus (MRSA) limits the chances for the effective antibacterial therapy of staphylococcal diseases and results in the development of persistent infection such as bacteremia and osteomyelitis. The aim of this study was to identify features of the MRSA ST239 0943-1505-2016 (SA943) genome that contribute to the formation of both acute and chronic musculoskeletal infections. The analysis was performed using comparative genomics data of the dominant epidemic S. aureus lineages, namely ST1, ST8, ST30, ST36, and ST239. The SA943 genome encodes proteins that provide resistance to the host's immune system, suppress immunological memory, and form biofilms. The molecular mechanisms of adaptation responsible for the development of persistent infection were as follows: amino acid substitution in PBP2 and PBP2a, providing resistance to ceftaroline; loss of a large part of prophage DNA and restoration of the nucleotide sequence of beta-hemolysin, that greatly facilitates the escape of phagocytosed bacteria from the phagosome and formation of biofilms; dysfunction of the AgrA system due to the presence of psm-mec and several amino acid substitutions in the AgrC; partial deletion of the nucleotide sequence in genomic island vSAβ resulting in the loss of two proteases of Spl-operon; and deletion of SD repeats in the SdrE amino acid sequence.

Significant differences in the photochemical and photophysical behavior of trans -α-methylstilbene and trans -stilbene have been attributed to structural changes caused by the steric requirements of the methyl group. We present here the X-ray structures of cis - and trans -α-methylstilbene ( c - and t -MeSt). This is the first X-ray structure of a cis -stilbene. Despite the pronounced departure from phenyl group coplanarity, the solid-state packing of t -MeSt resembles that of trans -stilbene in that both exhibit disorder with a bicycle pedal structural relationship, dynamic in t -St but static in t -MeSt. We compare the X-ray structures with calculated structures. We also compare our steady state and transient photochemical and spectroscopic results with predictions in a recent theoretical paper that anticipated some of our experiments. Deviations from planarity imposed by the methyl substitution account for the shorter lifetimes of the trans excited states. The rapid torsional relaxation of 1 t -MeSt* to the twisted intermediate 1 p*, k tp = 2.9 × 10 12 s -1 , observed using fs transient absorption spectroscopy, explains the sharp decrease in the fluorescence quantum yield of t -MeSt. We correct misconceptions that have appeared in the literature concerning the shape of the stilbene potential energy surface in S 1 . The nonplanarity due to methyl substitution leads to chirality issues that are relevant in biological molecules such as the protonated Schiff bases of retinal in the opsins.

Briefly, 25-hydroxyvitamin D (25(OH)D) plays an essential role in embryogenesis and the course of intra- and postnatal periods and is crucially involved in the functioning of the mother-placenta-fetus system. The low quantity of 25(OH)D during pregnancy can lead to an elevated risk for preeclampsia occurrence. Despite the numerous studies on the association of 25(OH)D deficiency and preeclampsia development, the current research on this theme is contradictory. In this review, we summarize and analyze study data on the effects of 25(OH)D deficiency and supplementation on pregnancy, labor, and fetal and neonatal outcomes.

The structure of the novel Zintl phase, Cs 5 NbAs 10 , is reported for the first time. This compound crystallizes in the monoclinic P 2 1 / c space group (no. 14) with eight formula units per cell. The structure represents a unique atomic arrangement, constituting a new structure type with Wyckoff sequence e 32. The most important structural element is the unprecedented [Nb(As 5 ) 2 ] 5- cluster anion, formed by a Nb atom enclosed between two As 5 rings. These nonaromatic cyclic species, formally [As 5 ] 5- , adopt an envelope conformation similar to that of cyclopentane. To date, it is only the second example of an [As 5 ] 5- ring with this conformation, reported in an inorganic solid-state compound. The bonding characteristics of the [Nb(As 5 ) 2 ] 5- cluster and the [As 5 ] 5- rings are thoroughly investigated using first-principles methods and discussed. Electronic band structure calculations on Cs 5 NbAs 10 suggest that this compound is a semiconductor with an estimated band gap of ca. 1.4 eV.

Several independent determinations of the p K a values of trans -resveratrol in water have led to conflicting results. Singular value decomposition analysis of UV absorption spectra of trans -resveratrol ( t- Resv) in N 2 -outgased aqueous solutions buffered to pH values in the 7.0-13.6 range yielded the UV spectra of the three anionic forms and the corresponding p K a values: p K a1 = 9.1 6 , p K a2 = 9.7 7 , and p K a3 = 10.5 5 in very good agreement with calculated theoretical values. The analysis of the absorption spectra guided the assignment of the fluorescence spectrum of each anionic form. With the resolved spectra on hand, we applied the Förster equation to estimate p K Resv anions with O a * values of 2.5 and 0, respectively, for the p - and m -OH substituents of t- Resv in S 1 . Theory supports a proposed mechanism for the reaction of t- Resv anions with O 2 .

Diffusional quenching in isopentane (IP) glass at 77 K is demonstrated by the reduction of triphenylene phosphorescence lifetimes in the presence of 1,3-pentadiene and/or molecular oxygen. Fluorescence spectra and lifetimes of cis - and trans -1,2-di(1-methyl-2-naphthyl)ethene in IP glass at 77 K reveal that the cis → trans photoisomerization leads to the trapping of unstable conformers of the trans isomer. The claim that IP at 77 K is not sufficiently viscous to trap unstable photoproduct conformers is invalidated.

We present here draft genome sequences of five Staphylococcus aureus strains obtained from children suffering from atopic dermatitis. The strains were determined to be of five different sequence types (sequence type 1 [ST1], ST7, ST8, ST15, and ST101) and carried a unique combination of superantigen-like protein (SSL) and serine protease genes.

A photochemical synthesis of AgNO 3 complexes of trans -cycloheptene (TCH) and trans -1-sila-4-cycloheptene (Si-TCH) derivatives is described. A low temperature flow photoreactor was designed to enable the synthesis of carbocyclic TCH derivatives due to their thermal sensitivity in the absence of metal coordination. Unlike the free carbocycles, TCH·AgNO 3 complexes can be handled at rt and stored for weeks in the freezer (-18 °C). Si-TCH·AgNO 3 complexes are especially robust, and are bench stable for days at rt, and for months in the freezer. X-ray crystallography was used to characterize a Si-TCH·AgNO 3 complex for the first time. With decomplexation of AgNO 3 in situ , metal-free TCO and Si-TCH derivatives can engage in a range of cycloaddition reactions as well as dihydroxylation reactions. Computation was used to predict that Si-TCH would engage in bioorthogonal reactions that are more rapid than the most reactive trans -cyclooctenes. Metal-free Si-TCH derivatives were shown to display good stability in solution, and to engage in the fastest bioorthogonal reaction reported to date ( k 2 1.14 × 10 7 M -1 O : MeOH). Utility in bioorthogonal protein labeling in live cells is described, including labeling of GFP with an unnatural tetrazine-containing amino acid. The reactivity and specificity of the Si-TCH reagents with tetrazines in live mammalian cells was also evaluated using the HaloTag platform. The cell labeling experiments show that Si-TCH derivatives are best suited as probe molecules in the cellular environment.-1 in 9 : 1 H 2 O : MeOH). Utility in bioorthogonal protein labeling in live cells is described, including labeling of GFP with an unnatural tetrazine-containing amino acid. The reactivity and specificity of the Si-TCH reagents with tetrazines in live mammalian cells was also evaluated using the HaloTag platform. The cell labeling experiments show that Si-TCH derivatives are best suited as probe molecules in the cellular environment.

We present a comparative study of the photoisomerizations of lumisterol (Lumi), previtamin (Pre), and provitamin D 3 (Pro) to tachysterol (Tachy) at 77 K in EPA (5:5:2 ether, isopentane, and ethanol by volume) glass. Fluorescence, fluorescence excitation, and UV spectra, measured in the course of these reactions, were analyzed using singular value decomposition with self-modeling (SVD-SM). This represents an extension of previous work that led to the conclusion that in the EPA glass Pre exists as an s-cis,s-cis-conformer (cZc-Pre) which gives, exclusively, an unstable s-cis,s-cis-conformer of Tachy (cEc-Tachy) and Pro gives mainly the tEc-Tachy, that corresponds to a stable s-trans,s-cis-conformer. ( Redwood , C. ; et al. J. Phys. Chem. Lett. 2013 , 4 , 716 - 721 . ) The surprising result was that the major Pre photoproduct from Pro also has a tZc-Pre conformation instead of the expected cZc-Pre conformation. Accordingly, the Pre to Tachy cis-trans photoisomerization proceeds via a conformer specific one-bond-twist (OBT) process as proposed by Havinga ( Maessen , P. A. ; et al. Angew. Chem. Int. Ed. Engl. 1983 , 22 , 718 - 719 . Maessen , P. A. ; et al. Angew. Chem. Int. Ed. Engl. 1983 , 22 , 994 - 1004 . Maessen , P. A. Ph.D. Thesis, State University at Leiden, Leiden, The Netherlands, 1983. ). The role of the EPA glass in controlling conformer distributions and reaction outcomes is further explored by the extension of the studies to Lumi, whose structure differs substantially from that of its stereoisomer, Pro. Initially, the light-induced conrotatory ring openings of Pro and Lumi are expected to give cZc-Pre conformers that differ in the relative orientation of the double bond dihedral angles that define the chiral axis of the triene moiety: (-)cZ(-)c-Pre and (+)cZ(+)c-Pre, respectively. In the case of Pro, much of the cZc-Pre proceeds to tZc-Pre, the precursor of tEc-Tachy. In contrast, we show that under the same conditions most cZc-Pre formed from Lumi retains the cZc-conformation and isomerizes to cEc-Tachy. cZc-Pre from Lumi was not detected by fluorescence, but UV absorption measurements establish its formation as an essential intermediate to Tachy. Aided by theoretical calculations of conformer UV and CD spectra, we conclude that fluorescent thermodynamic Pre and nonfluorescent Pre from Lumi are both (+)cZ(+)c-Pre conformers. They differ in the orientation of the OH in the A ring, pseudoequatorial in the former and pseudoaxial in the latter. The most likely major photochemical sequences starting from Pre and Lumi are (+)cZ(+)c-Pre-eq-OH → (+)cE(+)c-Tachy-eq-OH and Lumi → (+)cZ(+)c-Pre-ax-OH → (+)cE(+)c-Tachy-eq-OH.

Ceftaroline is a unique cephalosporin with activity against methicillin resistant Staphylococcus aureus (MRSA). It was approved for clinical use in the USA, Europe and Russian Federation since 2010 for the treatment of the skin and soft tissue infection and community-acquired pneumoniae. In the present study there was used molecular typing of 24 isolates of MRSA with reduced susceptibility to ceftaroline. For 8 isolates belonging to different genetic lines (ST8, ST239 and ST228) and requiring MICs there were determined antibiotic concentrations preventing formation of resistant mutants (mutant prevention concentration) and the ranges of the mutant selection window (MSW). The last majority of the isolates with reduced susceptibility to ceftaroline (MIC of 2 mcg/ml) belonged to the clonal line ST228. The whole genome sequencing of two isolates of ST228 showed that they belonged to the epidemic South Germany genetic line and were characterized by the presence of mutations in PBP2a (N146K) and PBP2 (C197Y) responsible for reduced susceptibility. The highest rates of MPC (32 mcg/ml) and MSW (2-16 mcg/ml) were observed in the clinical isolates belonging to the genetic line ST8. The isolates of ST239 and ST228 had the selection window within 2-4 mcg/ml. No dependence of the MIC and MPC/MSW levels was detected.

Coagulase-negative staphylococci (CoNS) are considered as a reservoir of mobile genetic elements and first of all of the staphylococcal cassette chromosome mec (SCCmec), defining staphylococci resistance to beta-lactams. Types II, IV, IVa, V, VII and VIII SCCmec were detected among 95 staphylococcal strains isolated in different regions of the Russian Federation. Subtypes C1a, C1b, C1c and C1 SCCmec were also identified (class B mec complex and two complexes of ccr1 and ccr2 genes recombinases). Some other cassette types carrying A, C1 and C2 classes of the mec complexes in combination with various recombinase genes were detected. The S.epidermidis isolates mainly formed cassettes carrying mec complex B, while the S. haemolyticus isolates had cassettes carrying classes C1 and C2 mec complex. Out of 9 isolates of S. hominis 5 isolates carried a new type cassette: class A mec complex in combination with the complex of the recombinase ccr1 genes. SCCmec was not identified in S. capitis and S. pasteuri. Their representatives carried either mec complex (1 isolate of S. pasteuri) or the recombinase complexes (2 isolates of S. capitis). The detected SCCmec variants in CoNS could be a source of emergence of new genetic lines of MRSA.

The vibronic structure of the fluorescence spectrum of trans-1,2-di(1-methyl-2-naphthyl)ethene (t-1,1) in methylcyclohexane (MCH) solution at room temperature was expected to become better defined upon cooling of the solution to 77 K. Instead, a broad, λexc-dependent fluorescence spectrum was observed in the glassy medium. Vibronically structured t-1,1 fluorescence spectra were obtained in the MCH glass only upon irradiation at the long-λ onset of the absorption spectrum. The application of singular value decomposition with self-modeling on the fluorescence spectral matrices of t-1,1 allowed their resolution into major and minor pairs of vibronically structured spectra that are assigned to two structural modifications of each of two relative orientations of the 1-methyl-2-naphthyl moieties. The difference between the two structures in each pair lies in the direction of rotation of each naphthyl group away from the plane of the olefinic bond. A complex but different conformer distribution is also responsible for the fluorescence spectra of t-1,1 in 5:5:2 (v/v/v) diethyl ether/isopentane/ethyl alcohol (EPA) glass at 77 K. The conformer distributions are also sensitive to the rate of cooling used in glass formation. Conformer distributions based on predicted small energy differences from gas-phase theoretical calculations are of little value when applied to volume-constraining media. The photophysical and photochemical properties of the analogues of the other two conformers of trans-1,2-di(2-naphthyl)ethene, trans-1-(1-methyl-2-naphthyl)-2-(3-methyl-2-naphthyl)ethene (t-1,3) and trans-1,2-di(3-methyl-2-naphthyl)ethene (t-3,3), were determined in solution. However, it is the calculated geometries and energy differences of the t-1,1 conformers [DFT using B3LYP/6-311+G(d,p)] that are essential guides to the interpretation of the experimental results.

Computation has guided the design of conformationally-strained dioxolane-fused trans -cyclooctene (d-TCO) derivatives that display excellent reactivity in the tetrazine ligation. A water soluble derivative of 3,6-dipyridyl- s -tetrazine reacts with d-TCO with a second order rate k 2 366,000 (+/- 15,000) M -1 s -1 at 25 °C in pure water. Furthermore, d-TCO derivatives can be prepared easily, are accessed through diastereoselective synthesis, and are typically crystalline bench-stable solids that are stable in aqueous solution, blood serum, or in the presence of thiols in buffered solution. GFP with a genetically encoded tetrazine-containing amino acid was site-specifically labelled in vivo by a d-TCO derivative. The fastest bioorthogonal reaction reported to date [ k 2 3,300,000 (+/- 40,000) M -1 O at 25 °C] is described herein with a cyclopropane-fused -1 in H 2 O at 25 °C] is described herein with a cyclopropane-fused trans -cyclooctene. d-TCO derivatives display rates within an order of magnitude of these fastest trans -cyclooctene reagents, and also display enhanced stability and aqueous solubility.

The cis-trans photoisomerizations of cis-1-(3-methyl-2-naphthyl)-2-phenylethene (c-3-MPE) was studied in methylcyclohexane (MCH) glass at 77 K. The fluorescence spectra of c- and t-3-MPE are excitation wavelength (λ(exc)) independent because the steric requirement of the methyl group restricts the conformational space of each isomer to a single conformer. Photocyclization, the dominant reaction pathway of c-3-MPE in solution, is entirely suppressed in MCH glass at 77 K. The only reaction on 313 nm irradiation of c-3-MPE in MCH glass is cis-trans isomerization. As the reaction progresses, the structureless fluorescence of c-3-MPE is replaced by the vibronically resolved fluorescence of the stable conformer of the trans isomer. The results are consistent with photoisomerization by the conventional one bond twist (OBT) pathway. Previously reported results on the photoisomerization of cis-1-(2-naphthyl)-2-(o-tolyl)ethene (c-NTE) are reinterpreted. Calculated geometries and energy differences for c- and t-3-MPE and c- and t-NTE [DFT using B3LYP/6-311+G(d,p)] are consistent with the interpretation of the experimental results.

Thiamin diphosphate (ThDP) is a key coenzyme in sugar metabolism. The 4'-aminopyrimidine ring of ThDP cycles through several ionization and tautomeric states during enzyme catalysis, but it is not fully understood which states are adopted during the individual steps of the catalytic cycle. Thiamin has been synthesized with labels selectively inserted into the C2 and C6' positions, as well as into the amino group, creating [C2, C6'-(13)C(2)] thiamin and [N4'-(15)N] thiamin. Magic-angle spinning (MAS) NMR spectroscopy has been employed to record the (13)C and (15)N chemical shift anisotropy (CSA) tensors for C2, C6', and N4' atoms. Our results indicate that the isotropic chemical shifts as well as the principal components of the (13)C and (15)N CSA tensors are very sensitive to the protonation states in these compounds and, therefore, permit differentiating between the two ionization states, 4-aminopyrimidine and 4-aminopyrimidinium. Using density functional theory (DFT), we have calculated the magnetic shielding anisotropy tensors of C2, C6', and N4' and found excellent agreement between the computed and the experimental tensors. Our findings indicate that MAS NMR spectroscopy in conjunction with DFT calculations is a sensitive probe of ionization states in the thiamin cofactor. The results of this study will serve as a guide for characterization of ionization and tautomeric states of thiamin in complexes with thiamin-dependent enzymes.

The 1,6-diphenyl-1,3,5-hexatriene (DPH) triplet decay rate constant in degassed benzene is strongly temperature-dependent. Photostationary states for the fluorenone-sensitized photoisomerization of DPH in degassed methylcyclohexane (MCH) favor the all-trans isomer and become richer in the cis isomers as the temperature is increased. The results are interpreted in terms of competing decays from equilibrated triplet conformers on the basis of the experimentally derived DPH triplet potential energy surface [Saltiel, J.; Crowder, J. M.; Wang, S. J. Am. Chem. Soc. 1999, 121, 895 902; 5352]. Calculated geometries and energy differences [B3LYP/6-311+G(d,p)] are consistent with the experimental model.

Quantum mechanical calculations (DFT) have provided a mechanism for the oxidative C-H bond cleavage step in Fenton-like hydrocarbon hydroxylation. A transition structure for hydrocarbon oxidation by aqueous solvated cationic iron(III) hydroperoxides ((H(2)O)(n)Fe(III)OOH) is presented that involves a novel rearrangement of the hydroperoxide group (FeO-OH --> FeO...HO) in concert with hydrogen abstraction by the incipient HO* radical with activation barriers ranging from 17 to 18 kcal/mol. In every hydroperoxide examined, the activation barrier for FeO-OH isomerization, in the absence of the hydrocarbon, is significantly greater than the overall concerted activation barrier for C-H bond cleavage in support of the concept of O-O bond isomerization in concert with hydrogen abstraction. The transition structure for the oxidation step in simple anionic iron(III) hydroperoxides has been shown to bear a remarkable resemblance to model porphyrin calculations on cytochrome P450 hydroxylation.

This paper describes a multivariate analysis of the fluorescence emission of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) in a series of isotropic solvents of differing polarity and hydrogen-bonding ability. Multivariate methods distill the essential features from spectral data matrices so that the structural details that are embedded within the data are revealed to the analyst. In the aprotic solvents investigated, the analysis reveals a pair of emission components that have emission maxima that scale with the orientational polarizability. In the alcohols, short-lived, polarity-independent blue bands tentatively attributed to neutral hydrogen-bonded solute-solvent complexes form and relax prior to emission from paired bands that have Stokes shifts that scale with the solvent hydrogen-bonding ability rather than the polarity. In water, the short-lived blue bands were not observed, but the shift in the paired bands did scale with the solvent hydrogen-bonding ability.

A combined experimental and theoretical study addresses the concertedness of the thermal Curtius rearrangement. The kinetics of the Curtius rearrangements of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate and methyl 1-azidocarbonyl cyclopropane-1-carboxylate were studied by (1)H NMR spectroscopy, and there is close agreement between calculated and experimental enthalpies and entropies of activation. Density functional theory (DFT) calculations (B3LYP/6-311+G(d,p)) on these same acyl azides suggest gas phase barriers of 27.8 and 25.1 kcal/mol. By comparison, gas phase activation barriers for the rearrangement of acetyl, pivaloyl, and phenyl azides are 27.6, 27.4, and 30.0 kcal/mol, respectively. The barrier for the concerted Curtius reaction of acetyl azide at the CCSD(T)/6-311+G(d,p) level exhibited a comparable activation energy of 26.3 kcal/mol. Intrinsic reaction coordinate (IRC) analyses suggest that all of the rearrangements occur by a concerted pathway with the concomitant loss of N2. The lower activation energy for the rearrangement of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate relative to methyl 1-azidocarbonyl cyclopropane-1-carboxylate was attributed to a weaker bond between the carbonyl carbon and the three-membered ring in the former compound. Calculations on the rearrangement of cycloprop-2-ene-1-oyl azides do not support pi-stabilization of the transition state by the cyclopropene double bond. A comparison of reaction pathways at the CBS-QB3 level for the Curtius rearrangement versus the loss of N2 to form a nitrene intermediate provides strong evidence that the concerted Curtius rearrangement is the dominant process.

Both density functional theory (DFT) (B3LYP) and CCSD ab initio calculations were employed in a theoretical investigation of the mechanism of thiolate-disulfide exchange reactions. The reaction pathway for degenerate thiolate-disulfide exchange reactions with dimethyl disulfide has been shown to proceed through a SN2-like transition structure that is very close in energy to the corresponding trisulfur anionic intermediate ([delta-S-S-Sdelta(-)]). When relatively small substituents are involved, the level of theory must be increased to CCSD to make this rather subtle mechanistic distinction. With the more sterically hindered exchange reaction involving t-butyl mercaptide and di-t-butyl disulfide, the potential energy surface exhibits a distinct preference for the S(N)2 displacement pathway with an activation barrier of 9.8 kcal/mol. When corrections for solvent polarity are included (COSMO), an S(N)2 mechanism is also implicated in both polar and nonpolar solvents. DFT studies on thiolate-disulfide exchange, when the substituent is a model peptide, strongly support the intermediacy of a trisulfur intermediate that lies 10.7 kcal/mol below isolated reactants. A well depth of this magnitude should provide a sufficient lifetime of the intermediate to accommodate the requisite conformational adjustments that accompanies formation of the new disulfide bond.

The B3LYP variant of DFT has been used to study the mechanism of S-S bond scission in dimethyl disulfide by a phosphorus nucleophile, trimethylphospine (TMP). The reaction is highly endothermic in the gas phase and requires significant external stabilization of the charged products. DFT calculations (B3LYP) were performed with explicit (water molecules added) and implicit solvent corrections (COSMO model). The transition structures for this SN2 displacement reaction in a number of model systems have been located and fully characterized. The reaction barriers calculated with different approaches for different systems are quite close (around 11 kcal/mol). Remarkably, the calculations suggest that the reaction is almost barrierless with respect to the preorganized reaction complex and that most of the activation energy is required to rearrange the disulfide and TMP to its most effective orientation for the SMe group transfer way. Different reactivities of different phosphorus nucleophiles were suggested to be the result of steric effects, as manifested largely by varying amounts of hindrance to solvation of the initial product phosphonium ion. These data indicate that the gas-phase addition of a phosphine to the disulfide moiety will most likely form a phosphonium cation-thiolate anion salt, in the presence of four or more water molecules, that provide sufficient H-bonding stabilization to allow displacement of the thiolate anion, a normal uncomplicated SN2 transition state is to be expected.

all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence in solution consists of emissions from the S1 (2(1)A(g)) and S2 (1(1)B(u)) states of the s-trans,s-trans conformer (s-t-DPH) and emission from the S1 state of the s-cis,s-trans conformer (s-c-DPH). The contribution of s-c-DPH fluorescence increases upon excitation at longer wavelengths, and both minor emissions, s-c-DPH and 1(1)B(u) s-t-DPH fluorescence, contribute more at higher temperatures (Ts). Resolution of a spectrothermal matrix of DPH fluorescence spectra by principal component analysis with self-modeling (PCA-SM) is hampered by T-dependent changes in the spectra of the individual components. We avoided differential polarizability-dependent spectral shifts by measuring the spectra in n-alkanes (Cn, C8 to C16 with n even) at T values selected to keep the index of refraction constant, hence under isopolarizability conditions. Compensation of the spectra for T-induced broadening allowed resolution of the spectral matrix into its three components. The optimum van't Hoff plot gives Delta H = 2.83 kcal/mol for s-c-DPH/s-t-DPH equilibration, somewhat smaller than the 3.4 kcal/mol calculated value, and the optimum Boltzmann distribution law plot gives Delta E(ab) = 4.09 kcal/mol for 1(1)B(u)/2(1)A(g) equilibration. The 1(1)B(u) fluorescence spectrum bears mirror-image symmetry with the DPH absorption spectrum, and the energy gap, 1431 cm(-1), is consistent with the 1615 cm(-1) difference between the lowest energy bands in the 1(1)B(u) and 2(1)A(g) fluorescence spectra. The results give V(ab) = 198 +/- 12 cm(-1) for the vibronic matrix coupling element between the 2(1)A(g) and 1(1)B(u) states. Fluorescence quantum yields and lifetimes under isopolarizability conditions reveal an increase in the effective radiative rate constant of s-t-DPH with increasing T.

A theoretical study specifically addresses the question of whether nucleophilic addition to the carbonyl groups of acid chlorides, esters, and anhydrides involves an addition-elimination pathway or proceeds by a concerted S(N)2-like mechanism in the absence of the generally assumed tetrahedral intermediate. Density functional calculations [B3LYP/6-31+G(d,p)] establish that chloride ion exchange reactions with both formyl and acetyl chloride proceed by a pi attack on the C=O bond. No discernible tetrahedral intermediate typical of an addition-elimination pathway was found in either case. While a tetrahedral intermediate does exist for the addition of fluoride ion to (Cl)(2)C=O, halide exchange of LiCl with both ClFC=O and (Cl)(2)C=O also proceeds by a concerted S(N)2-like pathway. The formation of a tetrahedral intermediate from the addition of methanol to acetyl chloride is slightly exothermic (4.4 kcal/mol). The ion-dipole complex of methanol weakly bonded to the carbonyl carbon of protonated acetyl chloride is stabilized by 13.8 kcal/mol but does not collapse to a tetrahedral intermediate. When four CH(3)OH molecules are H-bonded to protonated acetyl chloride, a tetrahedral intermediate is not completely formed and this solvated complex more closely resembles the precursor to an S(N)1-type ionization of Cl(-). With six H-bonding methanol molecules, a methanol adds to the carbonyl carbon and a proton relay occurs with formation of a tetrahedral-like structure that immediately loses chloride ion in an S(N)1-like solvolysis. These results corroborate earlier suggestions (Bentley et al. J. Org. Chem. 1996, 61, 7927) that the methanolysis of acetyl chloride does not proceed through the generally assumed addition-elimination pathway with a discrete tetrahedral intermediate but is consistent with ionization of Cl(-). The reaction of methoxide ion with methyl acetate proceeds via a multiple-well energy surface and involves the intermediacy of an asymmetrical species with differing C-OMe bond lengths. Models of synthetic applications of acyl transfer reactions involving anhydrides that form N-acyloxazolidinones also proceed by a concerted S(N)2-type pathway even with the carboxylate leaving group. Concerted transition states were observed for the reactions of each enantiomer of a 1,3-diphenylcycloprop-2-ene carboxylic anhydride by S-3-lithio-4-phenyloxazolidinone. Despite close structural similarities between the diastereomeric transition states, the relative energies correlated closely with the experimental results.

Theoretical calculations at the B3LYP/6-31+G(d,p) level have been used to study the oxidation of dimethyl sulfide by a series of bicyclic and tricyclic model C4a-flavin hydroperoxides. The intrinsic gas-phase reactivity of tricyclic C4a-hydroperoxyflavin 4 is ca. 10(9) greater than t-BuOOH but is ca. 10(7) less reactive toward the oxidation of dimethyl sulfide than peroxyformic acid. The SN2-like attack of the nucleophile on the distal oxygen of the hydroperoxide and the relative reactivity of the peracid are in excellent agreement with the earlier experimental data of Bruice. The effect of N1 or N5 hydrogen-bonding interactions on the activation barriers for oxygen atom transfer have been examined. Classical energy barriers for oxygen atom transfer from neutral and ion-paired forms of C4a-hydroperoxyflavin to dimethyl sulfide are predicted to differ by a small margin, suggesting that proton distribution exerts a relatively small influence on the reactivity of alkyl hydroperoxides. Isolated N1- and N5-protonated cations exhibit artificially low barriers as a consequence of their location in a high energy region of the potential energy surface domain.

The strain energies (SE) for dioxirane (DO) dimethyldioxirane (DMDO) and related dioxiranes have been examined by several methods using high-level computational schemes (G2, G2(MP2), CBS-Q). A series of calculated O-O, C-O, and O-H bond dissociation energies (G2) point to special problems associated with classical homodesmotic reactions involving peroxides. The relative SEs of DO, DMDO, methyl(trifluoromethyl)dioxirane (TFDO), and difluorodioxirane (DFDO) have been estimated by combination of the dioxirane with cyclopropane to form the corresponding 1,3-dioxacyclohexane. The relative SE predicted for DMDO (2) is 7 kcal/mol lower than that of DO, while the SE of 1,1-difluorodioxirane (4) is 8 kcal/mol higher. The most reactive dioxirane, methyl (trifluoromethyl)dioxirane (3), has an estimated SE just 1 kcal/mol greater than that of DO but 8 kcal/mol greater than that of DMDO. Six independent methods support the proposed SE for DO of 18 kcal/mol. The SE of the parent dioxirane (DO) has been estimated relative to six-membered ring reference compounds by dimerization of dioxirane and or its combination with cyclopropane. The relative SE of cyclic hydrocarbons, ethers and peroxides have been predicted by the insertion/extrusion of -CH(2)- and -O- fragments into their respective lower and next higher homologues. The moderated SE of DMDO (approximately equal to 11 kcal/mol) has also been estimated on the basis of group equivalent reactions. The unusual thermodynamic stability of DMDO is largely a consequence of combined geminal dimethyl and dioxa substitution effects and its associated strong C-H bonds and C-CH(3) bonds. The data clearly demonstrate that the reference compounds used to estimate the SE for highly substituted small ring cyclic compounds should reflect their molecular architecture having the same substitutents on carbon.

The effect of substitutents on the strain energy (SE) of cyclic molecules is examined at the CBS, G2, and G2(MP2) levels of theory. Alkyl substitutents have a meaningful effect upon the SE of small ring compounds. gem-Dimethyl substitution lowers the strain energy of cyclopropanes, cyclobutanes, epoxides, and dimethyldioxirane (DMDO) by 6-10 kcal/mol relative to an unbranched acyclic reference molecule. The choice of the reference compound is especially important for geminal electronegative substitutents. The SE of 1,1-difluorocyclopropane is estimated to be 20.5 kcal/mol relative to acyclic reference molecule 1,3-difluoropropane but is 40.7 kcal/mol with respect to the thermodynamically more stable (DeltaE = -20.2 kcal/mol) isomeric reference compound 2,2-difluoropropane. The SE of dioxirane (DO) is estimated to be approximately 18 kcal/mol while the SE of DMDO is predicted to be approximately equal to 11 kcal/mol by using homodesmotic reactions that maintain a balanced group equivalency. The total energy (CBS-APNO) of DMDO is 2.6 kcal/mol lower than that of isomeric 1,2-dioxacyclopentane that has an estimated SE of 5 kcal/mol. The thermodynamic stability of DMDO is a consequence of its relatively strong C-H (BDE = 102.7 kcal/mol) and C-CH(3) (BDE = 98.9 kcal/mol) bonds. By comparison, the calculated sec-C-H and -C-CH(3) G2 bond dissociation energies in propane are 100.3 and 90.5 kcal/mol.

The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms.