Synthesis and nmr analysis of cyclo-[85% 13C-asp]-pro: 13C-13C vicinal coupling constants and conformation

The dipeptide cyclo(Asp-Pro) where the aspartic acid residue was 85% ¹³C enriched, was synthesized with the aim of analyzing its conformation in solution by using ¹C-¹H, ¹³H-¹H and ¹³C-¹³C coupling constant parameters. The values of these couplings agree well with each other and show that the side chain of the aspartic acid residue adopts privileged conformations the proportions of which vary somewhat with pH, and more weakly, with a change in solvent. The ¹³C-¹³C interresidue coupling constants ³J_Cl'–_C2^β and ³J_Cl'–_C2^γ obtained after long accumulation of the signals of unenriched carbons, have different values; they show puckering in the pyrrolidine ring similar to that found in cyclo(Leu-Pro) in the solid state. It was concluded that ¹³C-¹³C coupling constants represent an excellent means of determining the side chain conformation (whenever the incorporation of an enriched amino acid into the peptide is possible) that will find applications particularly in the case of peptides with complicated proton spectra.     

29Si and 13C NMR Studies of organofunctional di- and trisilanes

A series of di- and trisilanes of general structure Ph₃SiSiMe₂R and (Ph₃Si)₂SiR′R″ were synthesized, and the ²⁹Si and ¹³C chemical shifts and one-bond siliconsilicon coupling constants (¹J_SiSi) were measured. The coupling constants of the disilanes were found to be primarily dependent upon the inductive effect of the alkyl group, R, as measured by the Taft o^★ constant. In both series of compounds, increasing alkyl substitution at silicon led to a decrease in ¹J_SiSi.     

Comparison of the effects of symmetric versus asymmetric H bonding on 2H and 17O nuclear quadrupole coupling constants: Application to formic acid and the hydrogen diformate anion

In this paper, we analyse the effects on the nuclear quadrupole coupling behaviour of ²H and ¹⁷O nuclei of a shift in H-bond character from asymmetric to symmetric. Using ab initio methods, the coupling amplitudes (nuclear quadrupole coupling constant e²_qQ/h) coupling anisotropies (asymmetry parameter η), and orientations of the electric field gradient (EFG) principal axis (PA) system of the H-bonded deuterium and oxygen nuclei in the formic acid dimer and related monomers, and the deuterium diformate anion are calculated. In addition, the relative contributions to the ²H and ¹⁷O EFGs of nuclear and electronic terms, and also the convergence of the EFG as a function of contributions from increasingly distant nuclei in the molecule are investigated. The trends in the calculated ¹⁷O EFGs on going from an asymmetric to a symmetric H-bonded environment are correlated with experimental nqr data in order to establish the hitherto unknown e²qQ/h sign, EFG assignments and PA orientation of symmetrically H-bonded oxygen. The possibility that the e²qQ/h value of deuterium is negative for symmetric H bonds is discussed, and difficulties in the computation of ²H EFGs for symmetric - as distinct from asymmetric - H-bonded systems are pointed out. In strong disagreement with assumptions in the literature, it is found that nearest neighbour terms do not dominate the ²H EFG in symmetrically H-bonded systems.     

Calculation of the 1J(PP) angular dependence in P2H4

Using the results obtained from an MO SCF ab initio calculation on P₂H₄ in four different conformations, the ¹J(PP) NMR coupling constants have been calculated. The ¹J(PP) values are highly dependent upon the rotational angle φ (?238.0 Hz in the eclipsed conformation, 10.9 Hz in the staggered one). The shape of the theoretical ¹J(PP) plot seems to be in good agreement with experimentally measured ¹J(PP) values.     

Nuclear hyperfine splitting in the BX electronic band system of 127I2

The spacings between seven hyperfine components in the R(127) line of the 11-6 band of the B-X electronic transition of ¹²⁷I₂ are fitted, with a standard deviation of 17.3 kHz, by varying the nuclear quadrupole coupling constants eqQ′ and egQ′', the magnetic spin—rotation constant C_I and the tensor and scalar nuclear spin—spin coupling constants d′ and a′ in the hyperfine hamiltonian. The P(13) line of the 430 band is also analysed using an identical hamiltonian and a standard deviation of 6.25 kHz is obtained. No evidence for a magnetic octupole coupling is found to the precision of the data although this effect was invoked by Hackel et al. for the P(13) line.     

NMR study (1H, 13C and 29Si) of the methylallylsilanes

¹H, ¹³C and ²⁹Si NMR data for the compounds (CH₃)_xSi(CH₂CHCH₂)_4-x are reported. The ¹H resonances from the π system are indicative of the electron-supplying inductive effect (+I) of the (CH₃SiCH₂, moiety but the corresponding ¹³C_π chemical shifts seem to be influenced by a sterically induced polarization of the C-H bonds. The ¹³C_All, ¹³C_Me and ²⁹Si chemical shift data reveal an important neighbour anisotropy contribution originating from the π system. Ultraviolet study of the compounds mentioned above gives indication of a σ—π conjugation in accordance with PES and ab initio results [1—5]. The trend observed in the various coupling constants is too small to be Interpreted.     

Symmetry constraints in energy transfer between state-selected Ar*(3P2, 3P0) metastable atoms and ground state H atoms

The excitation-transfer reaction in thermal energy collisions of state-selected metastable Ar*(³P₂) and Ar*(³P₀) atoms with ground state H atoms, giving excited H*(n = 2) atoms, has been studied with the stationary afterglow technique. The rate constant for the excitation of H atoms by Ar*(³P₂) has been found to be more than one order of magnitude larger than in excitation by Ar*(³P₀). This difference in the reactivity of two metastable species is explained to be a consequence of the attractive nature of the D(²II) and E(²Σ⁺) potentials that develop from the Ar*(³P₂)+H entrance channel and which give curve crossing with the B(²II) and C(²Σ⁺ potentials, respectively, leading to the Ar+H*(n=2) exit channel, whereas only a repulsive ⁴II (Ω=$\text{1}\text{2}$) potential develops from the Ar*(³P₀+H entrance channel.     

1H and 199Hg NMR spectra of methylmercury(II) complexes. Effects of basicity and ortho substitution in pyridines(L) in complexes [MeHgL]NO3

Linear complexes [MeHgL]NO₃ (L = substituted pyridine) have been prepared and their ¹H and ¹⁹⁹Hg NMR spectra measured and compared with other complexes of this series reported previously. The coupling constant J(¹H?¹⁹⁹Hg) correlates directly with pKa and with the gas phase enthalpy of ionization [ΔG_i(g)] of LH⁺; with J(¹H?¹⁹⁹Hg) decreasing with increasing pKa or ΔG_i(g). The chemical shift, δ, for ¹⁹⁹Hg does not correlate with either pKa or ΔG_i(g). Complexes without substituents in the 2 position of pyridine have δ¹⁹⁹Hg ca. 80–100 ppm downfield from MeHgNO₃, those with one methyl group in the 2 position ca. 125–150 ppm, and those with methyl groups in the 2 and 6 position (or benzyl or 3′-methylpyridyl groups in the 2 position) ca. 160–200 ppm downfield from MeHgNO₃. The coupling constant J(¹H?¹⁹⁹Hg) is found to be more useful than δ¹⁹⁹Hg in determination of solution structures of MeHg(II) complexes of this type of ligand.     

Vibrational relaxation of CO2(0001) in pure CO2

The rate constant for collisional deactivation of CO₂(00⁰1) in pure CO₂ has been measured at room temperature using the laser fluorescence technique. The relaxation rate has been found to be (1.0 ± 0.2) × 10^?14 cm³ s^?1 which is in favorable agreement with previously published values.     

Energy transfer in collisions of Ar(3P0,2 metastable atoms with H(2S) atoms. I. Total rate constant and mechanism

The Ar(4s,³P₂) + H(1S,²S) reaction, which gives excited H(n=2) atoms, has been studied. The room temperature rate constant for Lyman-α (2p-1s) excitation was measured as 2.4 × 10^?10 cm³ mol^?1 s^?1. The method was based upon comparison of the Lyman-α emission intensity with the Kr resonance emission intensity produced from Ar(³P₂) + Kr, which has a known rate constant. The H atom excitation, which has a large energy defect of 1.3 eV, is discussed in terms of a curve crossing mechanism.     

Electrochemically catalyzed synthesis of cyclic carbonates from CO2 and propargyl alcohols

A convenient and efficient electrochemical method has been developed for the synthesis of the α-alkylidene cyclic carbonates from carbon dioxide (CO₂) and propargyl alcohols at room temperature. The electrosynthesis was successfully carried out with a copper anode and a nickel cathode in an undivided cell containing n-Bu₄NBr-MeCN electrolyte with a constant current under 3 MPa pressure of CO₂, and the α-alkylidene cyclic carbonates were obtained in good to excellent isolated yields in the secondary and tertiary terminal propargylic alcohols cases. The experimental results show that the electrogenerated Cu⁺ ions and strong bases in situ could efficiently catalyze or promote the coupling reaction under the cooperation of electrolytic medium MeCN and supporting electrolyte n-Bu₄NBr. The plausible mechanism of the coupling reaction was also discussed briefly.     

Conformational analysis of methylsuccinic acid by 1H NMR spectroscopy and circular dichroism

The pH-dependence of the ¹H NMR and Circular Dichroism (CD) spectra of 2-methylsuccinic acid was investigated. Both spectra undergo dramatic changes between pH 4 and 6, where both carboxylic groups become ionized. From the coupling constants of the tertiary proton with the assigned¹ diastereotopic methylene protons, it is concluded that below pH 4 the syn-clinal (2) and above pH 6 the anti-periplanar (1) conformation of methylsuccinic acid prevail. The diesters of methylsuccinic acid also assume mainly the syn-clinal conformation (2). The pH-dependence of the CD spectra is discussed in terms of conformation and/or ionization effects.     

Synthesis of 13c-labelled [s]-proline and its conformational analysis by nuclear magnetic resonance

In order to analyze completely the ring conformation of [S]-Proline (l-thiazolidine-4-carboxylic acid) this compound has been synthesized with its C_δ atom enriched at 90% in ¹³C. The ¹H-¹H, ¹³C-¹H, ¹³C-¹³C vicinal coupling constants measured at several pH values describe well the geometry of the thiazolidine ring. The results suggest that the ring has an average planar conformation only in a narrow range of pH close to the pK of the amine group. Above and below this pH the ring tends to adopt preferentially the S_γendo and S_γexo puckered conformations respectively. It is concluded that a good correlation exists between the value of the dihedral angle ø and the character S_γendo or S_γexo of the ring.     

Rotational isomerism. A gas-phase 1H nuclear magnetic resonance study of 1-bromo-2-chloroethane

The average vicinal coupling constants of 1-bromo-2-chloroethane at different temperatures have been obtained in a gas-phase ¹H NMR study of 1,2-disubstituted ethanes. Analysis of the experimental data, assuming a ”static” model with constant values for the vicinal coupling constants of the individual rotamers, yielded unacceptable results. A ”dynamic” model, which takes into account the torsional vibrations, has therefore been developed and used to analyse the gas-phase results. The values at 305 K for the vicinal coupling constants of the individual rotamers are: trans rotamer J_T = 13.6 Hz, J'_t = 4.9 Hz; gauche rotamer $\text{1}\text{2}$(J_G1 + J_G2) = 0.9 Hz, $\text{1}\text{2}$(J'_G1 + J'_G2) = 7.8 Hz. These couplings are temperature-dependent with J_T changing about four times as much as the other coupling constants. Analysis of the average vicinal coupling constants, measured in different solvents, with the “dynamic” model gave values for the vicinal coupling constants of the individual rotamers in excellent agreement with those from the gas-phase data. The value of $\text{1}\text{2}$(J_G1+ J_G2) decreases noticeably with increasing dielectric constant of the solution. This can be explained by changes in the average dihedral angle between the coupling protons, in the gauche rotamers, which leads to changes of J_G1 and J_G2 in the same direction.     

The rα-structure of thieno[2,3-b] thiophene. A 1H-NMR study in a nematic phase including 13C-satellites in natural abundance

The ¹H-NMR spectrum of thieno[2,3-b]thiophene in a nematic phase has been recorded and analysed. The ¹H,¹³C dipole—dipole couplings were obtained from ¹³C satellites in natural abundance. The indirect ¹H,¹H and ¹H,¹³C spin—spin coupling constants were determined from an isotropic sample. The dipole—dipole couplings were corrected for harmonic vibrational contribution. The corrected dipole—dipole couplings were used to evaluate the r_α-structure of the molecule.     

Physical and chemical forms of radionuclides of 38S, 38Cl, 39Cl, and 82Br, produced at a high-energy proton accelerator facility

High-energy protons and neutrons produce various radionuclides in the air, mainly through the nuclear spallation of atmospheric elements. Air samples were collected from the EP-2 tunnel of the KEK 12 GeV proton synchrotron facility with a filter pack, which consisted of a membrane filter for aerosols, a Na₂CO₃-impregnated filter for acidic gases, and an activated carbon fiber filter for non-acidic gases. Sulfur-38 was found on the membrane and Na₂CO₃-impregnated filters, ³⁸Cl and ³⁹Cl were on the membrane, Na₂CO₃-impregnated, and ACF filters, and ⁸²Br was only on the ACF filter. The results on the relative abundances of aerosol and gaseous acidic components have indicated that ³⁸Cl produced by thermal neutron capture, which is the main reaction for ³⁸Cl production in the EP-2 tunnel air, are likely not to be present as aerosol or acidic gas. This was similar to the case of ⁸²Br produced by thermal neutron capture.     

Aufkl?rung der 1H- und 13C-NMR-spektren sowie der kristallstruktur unges?ttigter glycosylphosphonate und konformationsberechnungen an modellen1

By application of a modified LAOCN-3 program a complete elucidation of the ¹H NMR spectra of 2,3-unsaturated glycosyl phosphonates 1–5 could be achieved. The exactly determined ⁿJ(H, H) and novel ⁿJ(C, P) coupling constants are discussed, and permit a conclusive deduction of the conformations. The ¹³C NMR spectra of 1 to 6 can be fully interpreted. By a close inspection of chemical shifts as well as ¹J(C-H) and ⁿJ(C, P) coupling constants the conformational assignment is supported. For all the derivatives the correlations ¹J(C-1,H_e)>¹J(C-1, H_a) and ¹J(C-1, P_e) >¹J(C-1, P_a) can be used for the assignment of anomers. The crystal structure of 1 is in accordance with this assignment of anomers and displays an unexpected conformation. Calculations by MINDO-3 in resembling model compounds serve to elucidate such conformative effects.     

Microwave—optical double resonance in the B 3Π0+ excited state of ICl

Pure rotational transitions were observed in the electronically excited state B ³Π₀⁺ of ICl. This state was populated by optical pumping with a single-mode dye laser. From the observed spectrum precise values of the quadrupole hyperfine coupling constant cqQ of iodine and the effective rotational constant were derived for both isotopes I ³⁵Cl and I ³⁷Cl.     

The rα stucture of 1,6,6aλ4 trithiapentalene and 1,6-dioxa-6aλ4-thiapentalene. A 1H-NMR study in the nematic phase including 13C- satellites in natural abundance

The ¹H-NMR spectra of 1,6,6aλ⁴-trithiapentalene and l,6-dioxa-6aλ⁴-thiapentalene in nematic phases have been recorded and analyzed. The ¹³C-¹H dipole-dipole couplings were obtained from the ¹³C satellites in natural abundance. Contributions from the harmonic vibrations to the dipole-dipole couplings have been calculated. The vibration-corrected dipole-dipole couplings yielded the r_α structure in accordance with a rigid C_ZV symmetry of the molecules. No evidence for a rapid valence tautonierization in the molecules was found.     

Isolierung und struktur des dreikernigen hexamethylbenzolzirkonium-komplexes [(Me6C6)3Zr3Cl6]2+ [(Al2Cl7)−]2

Crystals of the trinuclear complex [(Me₆C₆)₃Zr₃Cl₆][Al₂Cl₇]₂ have been obtained from the reaction of ZrCl₄, hexamethylbenzene, AlCl₃, and Al in benzene. They are monoclinic, space group C2/2, with Z  4 and lattice parameters a 14.167(3), b 27.779(7), c 15.721(3) Å and β 94.27(4)°. The Zr atoms form a regular triangle. Each pair of Zr atoms is bridged by two Cl atoms. The fifth coordination site of each Zr atom is occupied by a h⁶-Me₆C₆ group. The cation is almost isostructural with the known trinuclear cation [(Me₆C₆)₃Nb₃Cl₆]²⁺. Important distances are: ZrZr 3.35, ZrCl 2.56, and Zrcenter of C₆ ring 2.17 Å. One of the two independent [Al₂Cl₇]^? anions occurs in a staggered conformation and one occurs in an eclipsed conformation.