CH2 Protons Research Articles

X-ray crystal structure and nuclear Overhauser effect studies of cerium(IV) complexes with Schiff bases obtained from N, N′-(1 R,2 R)(−)-1,2-cyclohexanediamine and benzaldehyde derivatives

Schiff bases, obtained by condensation of N, N′-(1 R,2 R)-cyclohexanediamine with 2-hydroxy-5-chlorobenzaldehyde or 2-hydroxy-3,5-dichlorobenzaldehyde and of (±)cyclohexanediamine and salicylaldehyde, have been used as ligands for cerium(IV). The complexes were characterized by circular dichroism (CD), 1H NMR, 13C NMR and nuclear Overhauser effect (NOE) studies. X-ray crystal structures determined for: [Ce(IV)((1 R,2 R)(−)chxn(3,5-diClba) 2) 2] and [Ce(IV)(±)(chxn(salal) 2) 2] revealed different orientation of a pair of Schiff base ligands in the coordination sphere. In [Ce(IV)((1 R,2 R)(−)chxn(3,5-diClba) 2) 2] two ligands form a sandwich with the cerium(IV) ion positioned between two layers of dichlorophenolic moieties. CD spectra in acetonitrile solution revealed exciton coupled π → π ∗ and LMCT transitions. CD data are characteristic for distortion of the chelate ring from planarity and λ conformation of the cyclohexane bridge. NOE difference spectroscopy for the Schiff bases (saturated – HC N–) demonstrate correlations of the azomethine protons with the protons of the phenyl ring and the cyclohexane’s CH protons, and a weak correlation with the hydroxyl group. In the case of the cerium(IV) complexes, the strongest NOE (saturated H–C N) was observed with the phenyl ring’s hydrogen atoms, a slightly weaker correlation to the cyclohexane’s CH 2 protons, and a much weaker NOE to the cyclohexyl’s CH 2 protons. Results of spectral analyses suggest central ions in a distorted square antiprism geometry with the two basal N 2O 2 chromophore group. The X-ray crystallography study also confirmed antiprismatic distortion from planarity in obtained complexes.

Quantifying Weak Hydrogen Bonding in Uracil and 4-Cyano-4‘-ethynylbiphenyl: A Combined Computational and Experimental Investigation of NMR Chemical Shifts in the Solid State

Weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl, for which single-crystal diffraction structures reveal close CH...O=C and C[triple bond]CH...N[triple bond]C distances, is investigated in a study that combines the experimental determination of 1H, 13C, and 15N chemical shifts by magic-angle spinning (MAS) solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route, including the isolation and characterization of intermediates, to 4-cyano-4'-ethynylbiphenyl at natural abundance and with 13C[triple bond]13CH and 15N[triple bond]C labeling is described. The difference in chemical shifts calculated, on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends on both intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the two effects are separated computationally by, first, determining the difference in chemical shift between that calculated for a plane (uracil) or an isolated chain (4-cyano-4'-ethynylbiphenyl) and that calculated for an isolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shifts that quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to plane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved in CH...O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involved in conventional NH...O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxed uracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differences of no more than 0.002 A, which corresponds to changes in the calculated 1H chemical shifts of at most 0.1 ppm. For the C[triple bond]CH...N[triple bond]C weak hydrogen bonds in 4-cyano-4'-ethynylbiphenyl, the calculated molecule to chain changes are of similar magnitude but opposite sign for the donor 13C and acceptor 15N nuclei. In uracil and 4-cyano-4'-ethynylbiphenyl, the CH hydrogen-bonding donors are sp2 and sp hybridized, respectively; a comparison of the calculated changes in 1H chemical shift with those for the sp3 hybridized CH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependence on hybridization for weak hydrogen-bonding strength.

A Molecular Shuttle for Driving a Multilevel Fluorescence Switch

A [2]rotaxane-based molecular shuttle comprised a macrocycle mechanically interlocked to a chemical "dumbbell" has been prepared in high yields by a thermodynamically controlled, template-induced clipping procedure. This molecular shuttle has two different recognition sites, namely, -NH2 +- and amide, separated by a phenyl unit. The macrocycle exhibits high selectivity for the -NH2+- recognition sites in the protonated form through noncovalent interactions, which include 1) N+-H...O hydrogen bonds; 2) C-H...O interactions between the CH2NH2+CH2 protons on the thread and the oligo(ethylene glycol) unit in the macrocycle; 3) pi...pi stacking interaction between macrocycle and aromatic unit. Upon deprotonation of the [2]rotaxane the macrocycle glides to the amide recognition site due to the hydrogen bonds between the -CONH- group and the oligo(ethylene glycol) unit in the macrocycle. The deprotonation process requires about 10 equivalents of base (iPr2NEt) in polar acetone, while the amount of base is only 1.2 equivalents in apolar tetrachloroethane. Upon addition of Li+, the conformation of the [2]rotaxane was altered as a result of the collective interactions of 1) hydrogen bonds between pyridine nitrogen and amide hydrogen atoms; 2) coordination between the oligo(ethylene glycol) unit, amide oxygen atom and Li+ cation. Then, when Zn2+ ions are added, the macrocycle returns to the deprotonated -NH- recognition site owing to coordination of the macrocycle and -NH- from the axle with the Zn2+ ion. All the above-mentioned movement processes are reversible through the alternate addition of TFA/iPr2NEt, Li/[12]-crown-4 and Zn2+/ethylenediaminetetraacetate (EDTA), by virtue of hydrogen bonding and metal-ion complexation. Significantly, the three independent movement processes are all accompanied by fluorescent responses: 1) complete repression in the protonated form; 2) low-level expression in the deprotonated form; 3) medium-level expression following addition of Li+; 4) high-level expression on complexation with Zn2+.

Metal−Porphyrin Orbital Interactions in Highly Saddled Low-Spin Iron(III) Porphyrin Complexes

Substituent effects of the meso-aryl (Ar) groups on the 1H and 13C NMR chemical shifts in a series of low-spin highly saddled iron(III) octaethyltetraarylporphyrinates, [Fe(OETArP)L2]+, where axial ligands (L) are imidazole (HIm) and tert-butylisocyanide ((t)BuNC), have been examined to reveal the nature of the interactions between metal and porphyrin orbitals. As for the bis(HIm) complexes, the crystal and molecular structures have been determined by X-ray crystallography. These complexes have shown a nearly pure saddled structure in the crystal, which is further confirmed by the normal-coordinate structural decomposition method. The substituent effects on the CH2 proton as well as meso and CH2 carbon shifts are fairly small in the bis(HIm) complexes. Since these complexes adopt the (d(xy))2(d(xz), d(yz))3 ground state as revealed by the electron paramagnetic resonance (EPR) spectra, the unpaired electron in one of the metal dpi orbitals is delocalized to the porphyrin ring by the interactions with the porphyrin 3e(g)-like orbitals. A fairly small substituent effect is understandable because the 3e(g)-like orbitals have zero coefficients at the meso-carbon atoms. In contrast, a sizable substituent effect is observed when the axial HIm is replaced by (t)BuNC. The Hammett plots exhibit a large negative slope, -220 ppm, for the meso-carbon signals as compared with the corresponding value, +5.4 ppm, in the bis(HIm) complexes. Since the bis((t)BuNC) complexes adopt the (d(xz), d(yz))4(d(xy))1 ground state as revealed by the EPR spectra, the result strongly indicates that the half-filled dxy orbital interacts with the specific porphyrin orbitals that have large coefficients on the meso-carbon atoms. Thus, we have concluded that the major metal-porphyrin orbital interaction in low-spin saddle-shaped complexes with the (d(xz), d(yz))4(d(xy))1 ground state should take place between the d(xy) and a(2u)-like orbital rather than between the dxy and a(1u)-like orbital, though the latter interaction is symmetry-allowed in saddled D(2d) complexes. Fairly weak spin delocalization to the meso-carbon atoms in the complexes with electron-withdrawing groups is then ascribed to the decrease in spin population in the d(xy) orbital due to a smaller energy gap between the d(xy) and dpi orbitals. In fact, the energy levels of the d(xy) and dpi orbitals are completely reversed in the complex carrying a strongly electron-withdrawing substituent, the 3,5-bis(trifluoromethyl)phenyl group, which results in the formation of the low-spin complex with an unprecedented (d(xy))2(d(xz), d(yz))3 ground state despite the coordination of (t)BuNC.

A para-Hydrogen Investigation of Palladium-Catalyzed Alkyne Hydrogenation

The complexes [Pd(bcope)(OTf)2] (1a), where bcope is (C8H14)PCH2-CH2P(C8H14), and [Pd(tbucope)(OTf)2] (1b), where tbucope is (C8H14)PC6H4CH2P(tBu)2, catalyze the conversion of diphenylacetylene to cis- and trans-stilbene and 1,2-diphenylethane. When this reaction was studied with para-hydrogen, the characterization of [Pd(bcope)(CHPhCH2Ph)](OTf) (2a) and [Pd(tbucope)(CHPhCH2Ph)](OTf) (2b) was achieved. Magnetization transfer from the alpha-H of the CHPhCH2Ph ligands in these species proceeds into trans-stilbene. This process has a rate constant of 0.53 s-1 at 300 K in methanol-d4 for 2a, where DeltaH = 42 +/- 9 kJ mol-1 and DeltaS = -107 +/- 31 J mol-1 K-1, but in CD2Cl2 the corresponding rate constant is 0.18 s-1, with DeltaH = 79 +/- 7 kJ mol-1 and DeltaS = 5 +/- 24 J mol-1 K-1. The analogous process for 2b was too fast to monitor in methanol, but in CD2Cl2 the rate constant for trans-stilbene formation is 1.04 s-1 at 300 K, with DeltaH = 94 +/- 6 kJ mol-1 and DeltaS = 69 +/- 22 J mol-1 K-1. Magnetization transfer from one of the two inequivalent beta-H sites of the CHPhCH2Ph moiety proceeds into trans-stilbene, while the other site shows transfer into H2 or, to a lesser extent, cis-stilbene in CD2Cl2, but in methanol it proceeds into the vinyl cations [Pd(bcope)(CPh=CHPh)(MeOD)](OTf) (3a) and [Pd(tbucope)(CPh=CHPh)(MeOD)](OTf) (3b). When the same magnetization transfer processes are monitored for 1a in methanol-d4 containing 5 microL of pyridine, transfer into trans-stilbene is observed for two sites of the alkyl, but the third proton now becomes a hydride ligand in [Pd(bcope)(H)(pyridine)](OTf) (5a) or a vinyl proton in [Pd(bcope)(CPh=CHPh)(pyridine)](OTf) (4a). For 1b, under the same conditions, two isomers of [Pd(tbucope)(H)(pyridine)](OTf) (5b and 5b') and the neutral dihydride [Pd(tbucope)(H)2] (7) are detected. The single vinylic CH proton in 3 and the hydride ligands in 4 and 5 appear as strong emission signals in the corresponding 1H NMR spectra.

Ring Current Effects in Crystals. Evidence from 13C Chemical Shift Tensors for Intermolecular Shielding in 4,7-Di-t-butylacenaphthene versus 4,7-Di-t-butylacenaphthylene

13C chemical shift tensor data from 2D FIREMAT spectra are reported for 4,7-di-t-butylacenaphthene and 4,7-di-t-butylacenaphthylene. In addition, calculations of the chemical shielding tensors were completed at the B3LYP/6-311G** level of theory. While the experimental tensor data on 4,7-di-t-butylacenaphthylene are in agreement with theory and with previous data on polycyclic aromatic hydrocarbons, the experimental and theoretical data on 4,7-di-t-butylacenaphthene lack agreement. Instead, larger than usual differences are observed between the experimental chemical shift components and the chemical shielding tensor components calculated on a single molecule of 4,7-di-t-butylacenaphthene, with a root mean square (rms) error of +/-7.0 ppm. The greatest deviation is concentrated in the component perpendicular to the aromatic plane, with the largest value being a 23 ppm difference between experiment and theory for the 13CH2 carbon delta11 component. These differences are attributed to an intermolecular chemical shift that arises from the graphitelike, stacked arrangement of molecules found in the crystal structure of 4,7-di-t-butylacenaphthene. This conclusion is supported by a calculation on a trimer of molecules, which improves the agreement between experiment and theory for this component by 14 ppm and reduces the overall rms error between experiment and theory to 4.0 ppm. This intermolecular effect may be modeled with the use of nuclei independent chemical shieldings (NICS) calculations and is also observed in the isotropic 1H chemical shift of the CH2 protons as a 4.2 ppm difference between the solution value and the solid-state chemical shift measured via a 13C-1H heteronuclear correlation experiment.

Investigation of the Thermal Decomposition of [biphen(QPiPr)Pt(alk)2]: An Entry to C–C Single Bond Activation?

AbstractThe reaction of [(COD)PtCl2] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) yields cis‐[{biphen(QPiPr)(Q′PiPr)}PtCl2]. Treatment of trans‐[PtCl2(ethene)(py)] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) gives trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] for Q or Q′ ≠ O. No trans/cis isomerization can be observed for these compounds after 24 h at 105 °C in [D8]toluene. In the case of Q = Q′ = O a trans/cis isomerization takes place at room temperature. X‐ray structures of a molecule in the solid state were determined for the compounds cis/trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with Q or Q′ ≠ O. In the case of Q = O and Q′ = CH2O the closer of the two bridging C atoms in the biphenyl fragment of the ligand is only 3.42 Å away from the Pt center but does not show a coupling to it in the 13C NMR spectrum. Treatment of both cis‐ and trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with two equivalents of ethyl‐ or n‐propylmagnesium chloride gives cis‐[{biphen(QPiPr)(Q′PiPr)}PtAlk2]; the X‐ray structures of cis‐[{biphen(CH2OPiPr)2}PtEt2] and cis‐[{biphen(OPiPr)2}Pt(nPr)2] are discussed. These complexes give the related alkene complexes as a result of β‐H elimination/reductive elimination if heated to temperatures above 90 °C in [D8]toluene, but none of the complexes with Q or Q′ ≠ O is stable at 95 °C. The thus‐formed Pt(alkene) complex loses the alkene (ethene or propene) in a consecutive reaction. For [{biphen(CH2OPiPr)2}PtEt2], selective deuteration of the CH2O group and the Pt‐ethyl group proves that scrambling of deuterium into the iPr methyl groups occurs (by a CH activation/insertion/β‐H elimination/reductive elimination sequence) after the β‐H elimination and before the loss of ethene, which at 95 °C is almost as fast as the loss of ethene. In the final alkene‐free 14 e complex a fast dynamic C–H activation of the iPr CH3 protons by the Pt center (one H at a time) can be detected [an Isotopic Perturbation of Resonance (IPR) is found for the partially deuterated compound], which most probably prevents a further reaction of the complex (e.g. activation of the bridging C–C single bond in the biphenyl backbone or permanent coordination of PPh3). H2, however, is activated by the final complex, and the use of D2 proves that deuterium is incorporated into the iPr groups of the ligand. The reaction of [(COD)PtCl2] with o‐Br‐phen(QPiPr) gives cis‐[PtCl2{o‐Br‐phen(QPiPr)}2] (X‐ray structure reported), which forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. The reaction of trans‐[PtCl2(ethene)(py)] with o‐I‐phen(QPiPr) gives trans‐[PtCl2{o‐I‐phen(QPiPr)}2] (X‐ray structure reported), which also forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. cis‐[Pt{phen(CH2OPiPr)}2] (X‐ray structure reported) slowly isomerizes quantitatively at 95 °C in toluene to the trans complex. No product of reductive elimination can be detected up to 150 °C in mesitylene. cis‐[Pt{phen(OPiPr)}2] (X‐ray structure reported) forms two products ― the trans isomer (X‐ray structure reported) and most likely a dimeric trans‐configured complex ― upon heating to 95 °C. Again, no reductive elimination is observed regardless of whether the reaction is performed under argon or ethene atmosphere up to 150 °C in mesitylene. All materials were characterized by means of 1H, 2H, 13C, 31P, and 195Pt NMR spectroscopy, FAB mass spectrometry, FT‐IR spectroscopy, elemental analysis, and X‐ray crystallography. DFT calculations (B3PW91/LANL2DZ) are also included for the most important compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Synthesis and micropatterning properties of a novel base‐soluble, positive‐working, photosensitive polyimide having an o‐nitrobenzyl ether group

AbstractA novel positive‐working, photosensitive polyimide, poly[1,4‐phenyleneoxy‐1,4‐phenylene‐2,2′‐di(2‐nitrobenzyloxy)benzophenone‐3,3′,4,4′‐tetracarboxdiimide] (OPI‐Nb), developable with an aqueous base was prepared by the o‐nitrobenzylation of a polyimide, poly(1,4‐phenyleneoxy‐1,4‐phenylene‐2,2′‐dihydroxybenzophenone‐3,3′,4,4′‐tetracarboxdiimide) (OPI), derived from 2,2′‐dihydroxy‐3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (DHBA) and 4,4′‐oxydianiline, and it micropatterning properties were investigated. The o‐nitrobenzylation of OPI to OPI‐Nb was conducted with o‐nitrobenzyl bromide in N‐methyl‐2‐pyrrolidinone containing Et3N. The DHBA monomer was synthesized by exhaustive KMnO4 oxidation of bis(2‐dimethoxy‐3,4‐dimethylphenyl)methane obtained by etherification of bis(2‐hydroxy‐3,4‐dimethylphenyl)methane with iodomethane, followed by deprotection of the methoxy groups and cyclodehydration of the obtained 2,2′‐dihydroxy‐3,3′4,4′‐benzophenonetetracarboxylic acid. The intermediate bis(2‐hydroxy‐3,4‐dimethylphenyl)methane was prepared by the condensation of 2,3‐dimethylphenol with paraformaldehyde. The degree of o‐nitrobenzylation was determined to be over 94 mol % from 1H NMR absorption of benzylic CH2 protons. The aromatic OPI was perfectly soluble in a dilute aqueous NaOH solution and tetramethylammonium hydroxide (TMAH), whereas OPI‐Nb was not even swellable in them. In the micropatterning process, OPI‐Nb showed a line‐width resolution of 0.4‐μm and a sensitivity of 5.4 J/cm2 when its thin films were irradiated with 365‐nm light and developed with a 2.38% aqueous TMAH solution at room temperature for 90 s. The thickness loss of OPI‐Nb films measured after postbaking at 350 °C was in the 8–9% range. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 776–788, 2007

Hindered Rotation Leading to Nonequivalence in 2-Substituted Benzyl Cobaloximes: Structure−Property Relationship

Benzyl cobaloximes with substituents at the 2-position having varying electronic and steric properties have been synthesized and characterized. Three different dioximes (dmgH, dpgH, gH) have been used. The dmgH(Me) and Co-bound CH2 protons show nonequivalence in the 1H NMR at subzero temperatures. The nonequivalence has been rationalized in terms of restricted rotation of the Co−C and/or C−Ph bond and is attributed to weak interactions between axial and equatorial ligands. Tc depends upon the nature of the 2-substituent and the dioxime. The molecular structures of 2-Me-C6H4CH2Co(dmgH)2Py, 2-naphthyl-CH2Co(dmgH)2Py, 2-Br-C6H4CH2Co(gH)2Py, and C6H5CH2Co(dpgH)2Py are reported. The activation energies of Co−C and C−Ph bond rotation are calculated from variable-temperature 1H NMR data using line-shape analysis. Also, the theoretical calculations using DFT are performed on 2-Me-C6H4CH2Co(dmgH)2Py and 2-Br-C6H4CH2Co(gH)2Py for the Co−C and C−Ph bond rotation. The conformational energy diagrams of these two molec...

Syntheses, crystal structures and dynamic1H NMR study of diastereotopic CH2protons in several new phosphoric triamides

Several phosphoric triamides with formula C6H5C(O)N(H)P(O)X2, X = N(C2H5)(CH2C6H5) (1), NC4H8 (2), NH-CH2-C6H5 (3), NH-CH2-(2-Cl-C6H4) (4), NH-CH2-CH(C2H5)(C4H9) (5), N(C2H5)2 (6) and P(O)(NH-CH2-C6H5)3 (7) were synthesized and characterized by 1H, 13C, 31P NMR and IR spectroscopy and elemental analysis. The structures were determined for compounds 3, 7 and 36, 4-F-C6H4C(O)NHP(O)(NH-CH2-CH[dbnd]CH2)2. In compound 3, there are intermolecular -P[dbnd]O … H-N- and -C[dbnd]O … H-N- hydrogen bonds which form a one-dimensional polymeric chain. Compound 7 contains intermolecular -P[dbnd]O … H-N- hydrogen bonds leading to a one-dimensional polymeric chain. Compound 36 forms intermolecular -P[dbnd]O … H-N-, -C[dbnd]O … H-N- and -C-F … H-N- hydrogen bonds which produce a three-dimensional polymeric chain in the crystalline network. The effects of rotation, ring inversion, temperature and solvent on the coupling patterns of the CH2 protons in 1H NMR spectra were discussed. The Gibbs free energy for the rotation arou...

1H NMR spin–lattice relaxation studies in Betaine phosphate (BP) – Transition from classical to quantum rotation

AbstractWe report here the results of our 1H NMR spin–lattice relaxation time (T 1) studies in betaine phosphate which is ferroelectric below 86 K and antiferroelectric below 81 K. The experiments have been carried out in the temperature range 300 to 4 K, at two Larmor frequencies 11.4 and 23.3 MHz. The T 1 data (300–100 K) has been analysed using modified BPP equation based on Ikeda's model. CH3 protons are found to relax other protons via spin diffusion. The low temperature (T < 100 K) T 1 data has been analysed following Lourens model which considers the gradual transition from classical motions to quantum regime. At low temperatures, when classical processes tend to freeze, reorienting groups apparently have minute differences and hence create a distribution in the energy values (E a and E 01) and pre‐exponential factors. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Activation of Methyltetrahydrofolate by Cobalamin-Independent Methionine Synthase

Cobalamin-independent methionine synthase (MetE) catalyzes the final step of de novo methionine synthesis using the triglutamate derivative of methyltetrahydrofolate (CH(3)-H(4)PteGlu(3)) as methyl donor and homocysteine (Hcy) as methyl acceptor. This reaction is challenging because at physiological pH the Hcy thiol is not a strong nucleophile and CH(3)-H(4)PteGlu(3) provides a very poor leaving group. Our laboratory has previously established that Hcy is ligated to a tightly bound zinc ion in the MetE active site. This interaction activates Hcy by lowering its pK(a), such that the thiolate is stabilized at neutral pH. The remaining chemical challenge is the activation of CH(3)-H(4)PteGlu(3). Protonation of N5 of CH(3)-H(4)PteGlu(3) would produce a better leaving group, but occurs with a pK(a) of 5 in solution. We have taken advantage of the sensitivity of the CH(3)-H(4)PteGlu(3) absorption spectrum to probe its protonation state when bound to MetE. Comparison of free and MetE-bound CH(3)-H(4)PteGlu(3) absorbance spectra indicated that the N5 is not protonated in the binary complex. Rapid reaction studies have revealed changes in CH(3)-H(4)PteGlu(3) absorbance that are consistent with protonation at N5. These absorbance changes show saturable dependence on both Hcy and CH(3)-H(4)PteGlu(3), indicating that protonation of CH(3)-H(4)PteGlu(3) occurs upon formation of the ternary complex and prior to methyl transfer. Furthermore, the tetrahydrofolate (H(4)PteGlu(3)) product appears to remain bound to MetE, and in the presence of excess Hcy a MetE.H(4)PteGlu(3).Hcy mixed ternary complex forms, in which H(4)PteGlu(3) is protonated.

Synthesis and characterization of poly(N-vinyl pyrrolidone) initiated by stibonium ylide

1,2,3,4-Tetraphenylcyclopentadiene triphenyl stibonium ylide initiated radical polymerization of N-vinyl pyrrolidone (N-VP) in dioxane, at 60±0.2°C, for 60 min under nitrogen atmosphere has been carried out. The system follows ideal kinetics, i.e., R p ∝ [ylide]0.5 [N-VP]1.0. The values of k 2 p/k t and overall energy of activation have been computed as 0.5 × 10−2 lmol−1s−1 and 33 kJ/mol, respectively. The FT-IR spectrum shows a band at 1692 cm−1 due to the =C=O group of N-vinyl pyrrolidone. The 1H-NMR spectrum shows multiplet at 2.4 δ ppm and a doublet at 3.7 δ ppm due to CH2 protons. The DSC curve shows a glass transition temperature (T g) as 48°C. The presence of six hyperfine lines in the ESR spectrum indicates that the system follows free radical polymerization and the initiation is brought about by phenyl radical.

Dipyrrolyldiketone Difluoroboron Complexes: Novel Anion Sensors With C‐H⋅⋅⋅X− Interactions

1,3-Dipyrrolyl-1,3-propanediones, synthesized from pyrroles and malonyl chloride, form BF2 complexes, which represent a new class of naked-eye sensors for halide and oxoanions. The association mode for the interactions of both the pyrrolyl NH and bridging CH protons with anions was confirmed by 1H NMR chemical shifts in CD2Cl2 and supported by theoretical studies. The binding constants (Ka) were estimated as 8.1x10(4), 2.0x10(3), 3.3x10(2), 1.3x10(4), and 80 M(-1) for F-, Cl-, Br-, H2PO4(-), and HSO4(-) by UV/Vis absorption spectral changes in CH2Cl2. Augmentation of Ka compared with dipyrrolylquinoxaline for H2PO4(-) is much larger than those for other anions. Contrary to other anions, F- quenches the emission almost completely, which was detected by the fluorescence spectrum as well as the naked-eye. In the case of the chloride anion complex, the formation of Cl(-)-bridged 1D networks, in which anion is associated with two BF2 complexes, is observed in the solid state.

An Investigation of Weak CH···O Hydrogen Bonds in Maltose Anomers by a Combination of Calculation and Experimental Solid-State NMR Spectroscopy

Two-dimensional (1)H-(13)C MAS-J-HMQC solid-state NMR spectra of the two anomeric forms of maltose at natural abundance are presented. The experimental (1)H chemical shifts of the CH and CH(2) protons are assigned using first-principles chemical shift calculations that employ a plane-wave pseudopotential approach. Further calculations show that the calculated change in the (1)H chemical shift when comparing the full crystal and an isolated molecule is a quantitative measure of intermolecular C-H...O weak hydrogen bonding. Notably, a clear correlation between a large chemical shift change (up to 2 ppm) and both a short H...O distance (<2.7 A) and a CHO bond angle greater than 130 degrees is observed, thus showing that directionality is important in C-H...O hydrogen bonding.

Tyrosinemia

In a 5-month-old boy with tyrosinemia, computed tomography revealed diffuse hypodensity in the centrum semiovale. Magnetic resonance (MR) imaging revealed partial signal differences in the white matter and perirolandic regions, and the posterior limbs of the internal capsules revealed higher signal compared with the remainder of the white matter. There were high-signal changes (restricted water diffusion) in the corresponding regions on images of diffusion-weighted MR imaging (b = 1000 s/mm). This likely represented the presence of intramyelinic edema attributable to status spongiosus. Proton MR spectroscopy (repetition time = 1500 milliseconds, echo time = 40 milliseconds) from the lesion sites revealed 2 separate prominent peaks spread between 3.4 and 3.9 ppm. These peaks could represent the CH and CH2 aliphatic protons of the tyrosine molecule.

Interaction of Ionic Biomolecular Building Blocks with Nonpolar Solvents: Acidity of the Imidazole Cation (Im+) Probed by IR Spectra of Im+−Ln Complexes (L = Ar, N2; n ≤ 3)

The intermolecular interaction between the imidazole cation (Im+ = C3N2H4+) and nonpolar ligands is characterized in the ground electronic state by infrared photodissociation (IRPD) spectroscopy of size-selected Im+-Ln complexes (L = Ar, N2) and quantum chemical calculations performed at the UMP2/6-311G(2df,2pd) and UB3LYP/6-311G(2df,2pd) levels of theory. The complexes are created in an electron impact cluster ion source, which predominantly produces the most stable isomers of a given cluster ion. The analysis of the size-dependent frequency shifts of both the N-H and the C-H stretch vibrations and the photofragmentation branching ratios provides valuable information about the stepwise microsolvation of Im+ in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between various intermolecular binding motifs (H-bonding and pi-bonding) and their interaction energies, and the acidity of both the CH and NH protons. In line with the calculations, the IRPD spectra show that the most stable Im+-L dimers feature planar H-bound equilibrium structures with nearly linear H-bonds of L to the acidic NH group of Im+. Further solvation occurs at the aromatic ring of Im+ via the formation of intermolecular pi-bonds. Comparison with neutral Im-Ar demonstrates the drastic effect of ionization on the topology of the intermolecular potential, in particular in the preferred aromatic substrate-nonpolar recognition motif, which changes from pi-bonding to H-bonding. .

New base-soluble positive-working photosensitive polyimides havingo-nitrobenzyl ester group

A new positive-working polyimide having photosensitive o-nitrobenzyl ester group as side substituent, poly{1,4-phenyleneoxy-1,4-phenylene-3,6-di[4-(o-nitrobenzyloxy)carbonylphenyl]pyromellitimide} (ODA-PI-Nb), was prepared and its aqueous alkali-developability and photosensitivity were investigated. ODA-PI-Nb was synthesized by the esterification reaction of poly[1,4-phenyleneoxy-1.4-phenylene-3,6-di(4-carboxylphenyl)pyromellit imide] (ODA-PI) with o-nitrobenzyl bromide in the presence of triethylamine (Et3N). ODA-PI-Nb obtained was characterized by FT-IR and 1H-NMR spectroscopy. The degree of esterification reaction was found from 1H-NMR absorption of CH2 proton to be over 95 mol%. Upon photo-irradiation ODA-PI-Nb transformed to the freely aqueous alkali-soluble ODA-PI under formation of o-nitrosobenzaldehyde. The thickness loss of thin ODA-PI-Nb films upon post-baking at 400°C was in the 10–15% range. ODA-PI-Nb showed positive-tone behavior in characteristic sensitivity curve and positive patterns were obtained using a typical lithographic process using aqueous tetramethylammonium hydroxide developer. The patterns with excellent resolution were observed and evaluated by optical microscopy and scanning electron microscopy. Copyright © 2005 John Wiley & Sons, Ltd.

Complete assignment of the 1H and 13C NMR spectra and conformational analysis of bonellin dimethyl ester

The use of a high-field NMR instrument (ν(1 H ) = 500 MHz ) and 2-dimensional NMR techniques (HMQC, HMBC, ROESY) enabled us to fully assign the 1 H and 13 C chemical shifts of bonellin dimethyl ester. The β-pyrrolic proton of C -3 appeared as a broad singlet at δ = 8.93, whereas that of C -8 gave a quartet with δ = 8.69 and 4J H - H = |1.28| Hz . The C -21 methyl protons appeared as a doublet with δ = 3.55 and 4J H - H = |1.07| Hz , while the C -71 methyl protons afforded a doublet with δ = 3.51 and 4J H - H = |1.28| Hz . These results suggest that the β-pyrrolic carbons of ring A belong to the aromatic 18 π-electron [18]diazaannulene delocalization pathway, whereas those of ring B remain outside the aromatic pathway. The broadening of the C -3 β-pyrrolic proton signal can be attributed to the allylic 3- CH - 21- CH 3 coupling and the 3- CH - 21- NH coupling. At 330 K, the tautomeric exchange 21- NH a ⇌ 23- NH b is fast and only one broad signal at δ = -2.49 is seen for these protons. The ROESY spectrum showed clear correlation signals between the 182- CH 3 and 171- CH 2 protons, the 182- CH 3 and 174- CH 3 protons, as well as between the 181- CH 3 and 17- CH protons. These results are compatible with the previous assignment that the absolute configuration at C-17 is S. Application of spin simulation enabled us to determine the chemical shifts and the 3J H - H coupling constants of the 17-propionate side-chain. The 3J H - H -values were used to calculate the populations for the 171-17 and 172-171 rotamers. A relatively high population value of 0.41 was found for the 171-17 g--rotamer, whose methoxycarbonylmethyl group points to the C -15 methine-bridge. This was interpreted as explaining the high tendency of bonellin to form anhydrobonellin. The rotational freedoms in the 13-propionate side-chain were studied by measuring the 1 H NMR spectra of the side-chain at temperatures between 300 and 195 K. At 300 K, the 131- and 132- CH 2 proton signals appeared as deceptively simple triplets, which at 195 K were split into complex multiplets. At 195 K, the signal arising from the 131- CH 2 protons exhibited more splitting, which indicates that these protons have less rotational freedom than the 132- CH 2 protons.

Local Measures of Intermolecular Free Energies in Solution

Proton spin-lattice relaxation rate changes induced by freely diffusing oxygen in aqueous and mixed solvents are reported for representative amino acids and glucose. The local oxygen concentration at each spectrally resolved proton was deduced from the paramagnetic contribution to the relaxation rate. The measured relaxation increment is compared to that of the force-free diffusion relaxation model, and the differences are related to a free energy for the oxygen association with different portions of the solute molecules. The free energy differences are small, on the order of -800 to -2000 J/mol, but are uniformly negative for all proton positions measured on the amino acids in water and reflect the energetic benefit of weak association of hydrophobic cosolutes. For glucose, CH proton positions report negative free energies for oxygen association, the magnitude of which depends on the solvent; however, the hydroxyl positions report positive free energy differences relative to the force-free diffusion model, which is consistent with partial occupancy in the OH region by a solvent hydrogen bond.