A DFT analysis of the vibrational spectra of nitrobenzene

Raman and FTIR, spectra of nitrobenzene, nb, and its isotopomers, nb-¹⁵N, nb-¹³C₆ and nb-d₅, were obtained and the fundamental vibrational modes assigned with the aid of a B3LYP/6-311+G** calculation, without the need for scaling of the force constants. The changes in vibrational coupling between the nitro and benzene groups upon certain isotopic substitutions are well modelled by the calculation, which is able to reproduce the isotopic shifts in frequencies for the nitro vibrations, as well as changes in IR intensities.     

Probing site-specific 13C/15N-isotope enrichment of spider silk with liquid-state NMR spectroscopy

Solid-state nuclear magnetic resonance (NMR) has been extensively used to elucidate spider silk protein structure and dynamics. In many of these studies, site-specific isotope enrichment is critical for designing particular NMR methods for silk structure determination. The commonly used isotope analysis techniques, isotope-ratio mass spectroscopy and liquid/gas chromatography-mass spectroscopy, are typically not capable of providing the site-specific isotope information for many systems because an appropriate sample derivatization method is not available. In contrast, NMR does not require any sample derivatization or separation prior to analysis. In this article, conventional liquid-state ¹H NMR was implemented to evaluate incorporation of ¹³C/¹⁵N-labeled amino acids in hydrolyzed spider dragline silk. To determine site-specific ¹³C and ¹⁵N isotope enrichments, an analysis method was developed to fit the ¹H–¹³C and ¹H–¹⁵N J-splitting (J _CH and J _NH) ¹H NMR peak patterns of hydrolyzed silk fiber. This is demonstrated for Nephila clavipes spiders, where [U–¹³C₃,¹⁵N]-Ala and [1-¹³C,¹⁵N]-Gly were dissolved in their water supplies. Overall, contents for Ala and Gly isotopomers are extracted for these silk samples. The current methodology can be applied to many fields where site-specific tracking of isotopes is of interest.      

Solvent-Induced Deuterium Isotope Effects in 13C- and 15N-NMR. Spectra of Enaminones

The effect of deuterium on the ¹³C and ¹⁵N chemical shifts of enaminones has been investigated. D/H isotope shifts are reported for neutral and protonated species, i.e., when the isotope is exchanged on the C(2)-, N-, or O-atoms. In cases of slow exchange the isotope shifts were obtained from solutions containing both isotopomers, whereas for fast exchange (acidic solutions) either separate NMR. sample tubes (¹⁵N-NMR.) or coaxial tubes (¹³C-NMR.) were used. In neutral molecules the isotope effects δ_C(D, H) are intrinsic in nature. In acidic solutions, the enaminocarbonyl cations formed exhibit δ_C(D, H)- and δ_N(D, H)-values which are discussed in terms of the proton transfer. The mesomeric character of the cations is reflected by characteristic features in the δ_C(D, H)- and δ_N(D, H)-values, which can be ascribed to isotopic perturbation of resonance. O-Protonation shifts in the ¹⁵N-resonance, observed for the first time, are large and positive (+60 to +76 ppm), in contrast to amides, where the effects are of the same sign but an order of magnitude smaller. Both protonation shifts and solvent-induced isotope effects are discussed in connection with the nucleophilic character of the reactive centers in the enaminone synthon.     

Molecular Complexes of Fullerenes C60 and C70 with Saturated Amines

New molecular complexes of fullerenes C₆₀ and C₇₀ with leuco crystal violet (LCV, 1-3); leucomalachite green (LMG, 4-6); crystal violet lactone (CVL, 7); N,N,N′,N′-tetrabenzyl-p-phenylenediamine (TBPDA, 8, and 9); N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPDA, 10, and 11); triphenylamine (TPA, 12, and 13); and substituted phenotellurazines (EPTA and TMPTA, 14, and 15) have been synthesized. Crystal structures have been solved for C₆₀ complexes with LMG (5, 6) TBPDA (8), TMPDA (10), and TPA (12). The C₆₀ molecules form closely packed double layers in 5 and 6, hexagonal layers in 10 and quasi-three-dimensional layers in 8 and 12. The substitution of disordered solvent molecules in the complexes with LMG (4, 5) by naphthalene ones results in the ordering of the C₆₀ molecules. According to IR-, UV-visible-NIR and ESR-spectroscopy the complexes have a neutral ground state. The spectra of 1-8, and 10 show intense charge transfer bands in the visible and NIR-range. On photoexcitation by white light (light-induced ESR (LESR) spectroscopy), 1 and 10 were shown to have an excited ionic state. The LESR signals were generated at light energies <2.25 eV indicating that the excited states in the complexes are realized mainly by direct charge transfer from donor to the C₆₀ molecule.     

Transformation of sclareol under ritter reaction conditions

Sclareol under the mild conditions of Ritter reaction is converted into N,N′-[(8R,13)-labdan-14(15)-ene-8,13-diyl]diacetamides stereoisomeric at the C¹³ atom possessing the unrearranged skeleton of the initial diol. One of the minor reaction products is the N-[(8α,13)-epoxylabdan-14-yl]acetamide whose formation requires the intermediate cyclization of the sclareol. Under more severe conditions and at the reversed order of the reagents addition the prevailing components of the reaction mixture are cyclized and rearranged N-[(8α,14)-epoxy-16(13→14)-abeo-labdan-13-yl]acetamides stereoisomeric at the C¹³ atom, and minor compounds, (8α,13)-epoxy-16(13→14)-abeo-labdan-12(13)-ene and (8α,14)-epoxy-16(13→4)-abeo-labdan-12(13)-ene.     

Multinuclear NMR Characterization of Cyanuric Fluoride (2,4,6‐Trifluoro‐1,3,5‐triazine)

Although 2,4,6‐trifluoro‐1,3,5‐triazine, C₃F₃N₃, is a highly symmetrical molecule, its NMR parameters can be obtained by reducing its symmetry through the introduction of ¹⁴N/¹⁵N and ¹²C/¹³C isotopomers. Experimental and computed chemical shifts of cyanuric fluoride have been obtained for ¹³C, ¹⁵N, and ¹⁹F. Spin‐spin coupling constants have been measured and compared with previous experimental data and with the complete set of computed EOM‐CCSD coupling constants.     

The microwave spectrum of isoselenocyanic acid,HNCSe

A new molecular species has been detected by passing HBr gas through dry AgNCSe, which is shown by consideration of the rotational constants of the four isotopic species H¹⁴N¹²C⁸⁰Se, H¹⁴N¹²C⁷⁸Se, H¹⁴N¹³C⁸⁰Se and D¹⁴N¹²C⁸⁰Se to be HNCSe. The spectrum is consistent with that of a quasilinear molecule. A preliminary structure has been determined having r(HN) = 0.99 A, r(NC) = 1.95 A, r(CSe) = 1.717 A and a mean HNC angle of 143°. The quadrupole interaction parameter eQq = 1.17 MHz and the dipole moment component μ_a = 1.98 D have also been determined.     

Microwave spectra of tetrahydroselenophene-α13C, -β13C and molecular ring structure

Microwave spectra of isotopic species α-¹³C and β-¹³C of tetrahydroselenophene molecules have been investigated and rotational constants determined: A = 5608.98 Mc, B = 2819.532 Mc, C = 2022.624 Mc forα-¹³C isotopic species and A = 5695.94 Mc, B = 2770.714 Mc, C = 2009.166 Mc for β-¹³C isotopic species. The r_s-ring structure was found to be Se-C₂ = 1.963 Å, C₂-C₃ = 1.549 Å, C₃-C₄ = 1.527 Å, ∠C₅SeC₂ = 90° 44', ∠SeC₂C₃ = 104° 58', ∠C₂C₃C₄ = 106° 52', the angle of twist = 29° 44'.     

Potential energy surfaces and vibrational spectra for isotopomers of N2O

Potential energy surfaces and vibrational spectra for the four isotopomers (^l5N¹⁴N¹⁶O,^l4N^I5N¹⁶O,¹⁵N₂ ¹⁶O and¹⁵N₂ ¹⁸O) of N₂O have been investigated with the vibrational self-consistent field-configuration interaction method. It is shown that the isotopomers with the same end atom have similar values of the potential parameters, and that substitution with different end atoms can affect the potential obviously. The calculated vibrational levels are in good agreement with the observed values by the optimization of several potential parameters (f ₁ ⁽¹⁾,f ₁₃ ⁽⁰⁾,f ₃ ⁽¹⁾ which are sensitive to isotopic substitutions. Project supported by the National Natural Science Foundation of China (Grant No. 29673029).     

Experimental separation of torsional and charge redistribution effects in rotational spectra of HCN dimers

B_oand D^jhave been determined for H¹³C¹⁵N-HC¹⁵N, HC¹⁵N-H¹³C¹⁵N, HC¹⁵N-DC¹⁴N and DC¹³N-HC¹⁵N. From them and other, previous results a full substitution structure has been obtained for HCN(1)-HCN(2). It leads to torsional amplitudes θ₁ and θ₂ of 13.6 and 9.3° for the two monomers in the dimer. A determination by fitting B_o for six isotopic species gives 13.7 and 8.7°. These values are used to separate torsional and charge redistribution effects upon the hyperfine interactions of ¹⁴N and D in the dimers. For ¹⁴N, about 40% of the difference in χ_a between HCN monomer and dimer is caused by charge redistribution. The C-D bond length in the dimer is considered.     

Analyse isotopique de l'azote par spectrometrie optique,pour de faibles teneurs en 15n

For mixtures of ¹⁴N₂ and ¹⁴N¹⁵N low in ¹⁵N (isotopic abundances less than 2%), previous optical spectrometric methods do not produce rapid results of sufficient precision. In this paper, operating conditions are described for the determination of the isotopic content to ±3% near the natural abundance and to ±1% when the sample contains an atom ratio of ¹⁵N: ¹⁴N of 1.5:100. The time required for a determination is ca. 10 min. A spectrometer is modified by the substitution of an exit slit and a photomultiplier in place of a photographic plate. The excitation of the electrodeless discharge tube is made by a high-frequency field. The method has been applied to samples of ammonium salts, fertilisers and plant material. In all cases, the precision of the isotopic measurements was greater than that required for most agricultural studies (5–6%).     

Application of 13C-n.m.r. spectroscopy to study the mechanism of N-demethylation of [NMe13C] codeine by cell-free extracts of Cunninghamella bainieri

Codeine was synthesized with 90% enrichment of the N-methyl group with carbon-13. N-Demethylation of this substrate by cell-free extracts of Cunninghamella bainieri in an n.m.r, tube gave norcodeine and ¹³C-labelled formaldehyde. Fourier-transform ¹³C-n.m.r. spectroscopy was used to observe the N-demethylation process at selected temperatures. The labelled formaldehyde liberated was trapped with sodium sulphite, and the sulphite adduct, as well as intermediates, were located in the n.m.r, spectrum at each temperature. Intermediate resonances assignable to codeine-N-oxide were not detected during these enzyme-transformation studies. These data suggest that the observed ¹³C-n.m.r. signals correspond to the chemically labile carbinolamine intermediate formed during N-demethylation. A methine ¹³C signal was not observed. Thus, N-demethylation of codeine by Cunninghamella bainieri occurs by direct C-oxidation and not via an N-oxide intermediate.     

Assignment of chemical shifts in benzohydroxamic acid and structure of its silylated derivatives by 15N enrichment

¹⁵N isotopic enrichment was necessary for the unequivocal assignment of the ¹H NMR lines to the protons in the NH–OH fragment of benzohydroxamic acid, BHXA, C₆H₅CONHOH, in dry dimethyl sulfoxide solutions. The assignment [δ(NH) = 11.21, δ(OH) = 9.01, ¹J(¹⁵N,¹H) = 102.2 Hz, ²J(¹⁵N,¹H) <1.5 Hz], which is opposite to that used by other authors, confirms the assignment extended to BHXA by Brown and co‐workers from the spectra of acetohydroxamic acid. The enrichment allowed also assignment of the ²⁹Si lines in the spectra of disilylated benzohydroxamic acid, (Z)‐tert‐butyldimethylsilyl N‐tert‐butyldimethylsilyloxybenzoimidate (2) and (Z)‐tert‐butyldiphenylsilyl N‐tert‐butyldiphenylsilyloxybenzoimidate (3), and confirmed structure of the monosilylated products, N‐tert‐butyldiphenylsilyloxybenzamide (4) and N‐tert‐butyldiphenylsilyloxy benzoimidic acid (5). Copyright © 2003 John Wiley & Sons, Ltd.     

Effect of a controlled dietary change on carbon and nitrogen stable isotope ratios of human hair

The carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) stable isotope ratios of human hair can be used for the interpretation of dietary habits and nutritional status in contemporary or past populations. Although the results of bulk or segmental isotope ratio analysis of human hair have been used for the reconstruction of an individual's diet for years, only limited data of controlled dietary changes on the carbon and nitrogen isotopic composition of human hair are available. Hair of four individuals, two males and two females, who participated in a dietary change experiment for 28 days was segmentally analysed for δ¹³C and δ¹⁵N. The dietary change included a change from C3 to C4 plant enriched diets and a simultaneous replacement of terrestrial animal products by marine products. This resulted in an increase in δ¹³C_diet of +8.5 to +9.9‰ and in δ¹⁵N_diet of +1.5 to +2.2‰. All subjects showed significant increases in δ¹³C_hair and δ¹⁵N_hair during the dietary change period, although no subject reached a new steady state for either carbon or nitrogen. The change in δ¹⁵N_hair was faster than the change in δ¹³C_hair for all individuals. The magnitude of change of the isotopic composition during the dietary change period could be attributed to the degree of physical activity of the individuals, with a higher physical activity resulting in a faster change. Copyright © 2009 John Wiley & Sons, Ltd.     

The use of MM/QM calculations of 13C and 15N chemical shifts in the conformational analysis of alkyl substituted anilines

The calculation of the ¹³C and ¹⁵N NMR chemical shifts by a combined molecular mechanics (Pcmodel 9.1/MMFF94) and ab initio (GIAO (B3LYP/DFT, 6-31 + G(d)) procedure is used to investigate the conformations of a variety of alkyl substituted anilines. The ¹³C shifts are obtained from the GIAO isotropic shielding (Ciso) with separate references for sp³ and sp² carbons (δc = δref − Ciso). The ¹⁵N shifts are obtained similarly from the GIAO isotropic shielding (Niso) with reference to the ¹⁵N chemical shift of aniline. Comparison of the observed and calculated shifts provides information on the molecular conformations. Aniline and the 2,6-dialkylanilines exist with a rapidly inverting symmetric pyramidal nitrogen atom. The 2-alkylanilines have similar conformations with the NH₂ group tilted away from the 2-alkyl substituent. The N,N-dialkylanilines show more varied conformations. N,N-dimethylaniline has a similar structure to aniline, but N-ethyl, N-methylaniline, N,N-diethylaniline, and N,N-diisopropylaniline are conformationally mobile with two rapidly interconverting conformers. In contrast, the anilines substituted at C₂ and the nitrogen atom exist as one conformer where the steric interaction between the C₂ substituent and the N substituent determines the conformation. In 2-methyl-N-methylaniline, the nitrogen atom is pyramidal as usual with the N-methyl opposite to the 2-methyl, but in 2-methyl-N,N-dimethyl aniline, the NMe₂ group is now almost orthogonal to the phenyl plane. This is also the case with 2-methyl-N,N-diethylaniline and 2,6-diisopropyl-N,N-dimethylaniline. The comparison of the observed and calculated ¹⁵N chemical shifts confirms the above findings, in particular the pyramidal conformation of aniline and the above observations with respect to the conformations of the N,N-dialkylanilines.     

Accurate Backbone 13C and 15N Chemical Shift Tensors in Galectin-3 Determined by MAS NMR and QM/MM: Details of Structure and Environment Matter

Chemical shift tensors obtained from solid-state NMR spectroscopy are very sensitive reporters of structure and dynamics in proteins. While accurate ¹³C and ¹⁵N chemical shift tensors are accessible by magic angle spinning (MAS) NMR, their quantum mechanical calculations remain challenging, particularly for ¹⁵N atoms. Here we compare experimentally determined backbone ¹³C^α and ¹⁵N^H chemical shift tensors by MAS NMR with hybrid quantum mechanics/molecular mechanics/molecular dynamics (MD-QM/MM) calculations for the carbohydrate-binding domain of galectin-3. Excellent agreement between experimental and computed ¹⁵N^H chemical shift anisotropy values was obtained using the Amber ff15ipq force field when solvent dynamics was taken into account in the calculation. Our results establish important benchmark conditions for improving the accuracy of chemical shift calculations in proteins and may aid in the validation of protein structure models derived by MAS NMR.     

Two-stage formation of chloro (N-o-chlorobenzamido-meso-tetraphenylporphyrinato) zinc(II) methylene chloride solvate: Zn(N-NHCO(o-Cl)C6H4-tpp)Cl · CH2Cl2

The coordinating properties of N-o-chlorobenzamido-meso-tetraphenylporphyrin (N-NHCO(o-Cl)C₆H₄-Htpp; 11) have been investigated for the Zn²⁺ ion. Insertion of Zn results in the formation of the zinc complex Zn(N-NCO(o-Cl)C₆H₄-tpp)(MeOH) · MeOH (12 · MeOH). The diamagnetic 12 · MeOH can be transformed into the diamagnetic Zn(N-NHCO(o-Cl)C₆H₄-tpp)Cl · CH₂Cl₂ (13 · CH₂Cl₂) in a reaction with aqueous hydrogen chloride (2%). X-ray structures for 12 · MeOH and 13 · CH₂Cl₂ have been determined. The coordination sphere around the Zn²⁺ ion in 12 · MeOH is a distorted trigonal bipyramid with N(2), N(4) and O(2) lying in the equatorial plane, whereas for the Zn²⁺ ion in 13 · CH₂Cl₂, it is a square-based pyramid in which the apical site is occupied by the Cl(1) atom.     

Interaction of amines with rhodium(II) tetracarboxylates in solution: formation of nitrogenous stereogenic center

The ¹H, ¹³C, and ¹⁵N NMR spectra of amines: N,N-dimethylisopropylamine, N-ethyl-N-methylbenzylamine, N,N-dimethyl-1-phenylethylamine and N-methyl-1-phenylethylamine in the presence of dirhodium(II) tetratrifluoroacetylate and a dirhodium(II) Mosher’s acid derivative were measured in CDCl₃ as the solvent. Dirhodium(II) salts with amines form 1:1- and 1:2-adducts, respectively, depending on the amine and dirhodium salt molar ratio. The formation of the Rh–N bond slows down the nitrogen atom inversion process and causes either non-equivalency of the two methyl groups in N(CH₃)₂ or the formation of a nitrogenous stereogenic center in the molecule having an –NR′R″ group. The latter causes the formation of additional diastereoisomers in solution. Application of NMR spectroscopy at low temperature (253 K) allows us to observe separately the signals of all compounds in solution, despite ligand chemical exchange between species.     

Diorgano-gallium and -indium complexes with N-heterocyclic carboxylic acids: Synthesis, characterization and structures of [Me2M(O2C-C5H4N)]2, M = Ga or In

The reaction of triorgano-gallium and -indium etherate with heterocyclic carboxylic acids in benzene at room temperature yields complexes of the type [R₂M(L)]_n(M = Ga or In; R = Me or Et; L = 2-(C₅H₄N)CO₂, 2-(C₄H₃N₂)CO₂ or 2-(C₉H₆N)CO₂). These complexes have been characterized by elemental analysis, IR, UV-vis, NMR (¹H and ¹³C{¹H}) and mass spectral data. Complexes with L = (C₅H₄N)CO₂- and (C₉H₆N)CO₂- showed photoluminescence on excitation with ∼250 or ∼310 nm radiation, respectively. Single crystal X-ray structural analysis of [Me₂M(O₂C-C₅H₄N-2)]₂ (M = Ga or In), revealed a dimeric structure with five-coordinate metal atoms arising from the presence of two tridentate bridging picolinate ligands.     

1H, 13C and 15N NMR study of vinyltetrazolium salts and processes of their synthesis

The ¹H, ¹³C and ¹⁵N NMR spectra of several N- and C-vinyltetrazolium salts have been recorded and the observed chemical shifts together with the data of quantum-chemical calculations have been used for evaluation of electronic structure of the investigated substances and selectivity of the exhaustive alkylation of 1,5- and 2,5-substituted tetrazoles.