Vratislav Blechta

Product operator formalism applied to large spin systems, with INEPT and DEPT examples

Abstract A modification of the product operator formalism for the description of NMR multipulse experiments is proposed. In the modified formalism the single spin operators Ipx and Ipy of particular spins p are replaced by raising and lowering operators Ip+ and Ip− in the product operator base. Use of a such mixed product operator base simplifies the formula for spin system evolution under scalar coupling interaction. Consequently, analysis of experiments on large weakly coupled spin ( I = 1 2 ) systems becomes feasible in the mixed base. The procedure is illustrated by the treatment of INEPT and DEPT experiments performed on spin systems with various hetero- and homonuclear couplings.

Structure determination of dihydroxamic acids and their trimethylsilyl derivatives by NMR spectroscopy

Homologous series of dihydroxamic acids [HONHCO(CH2)nCONHOH with n = 0, 1, 2, 3, 4 and 6] were prepared and trimethysilylated [1(n) and 2(n)]. The solution NMR spectra (1H, 13C, 15N) of 1(n) show that the hydroxamic end groups assume Z–Z and Z–E combinations of conformers. An exception is oxalodihydroxamic acid, which assumes only one combination. 13C cross‐polarization magic angle spinning reproduces the solution chemical shift in this compound and indicates the Z–Z combination as determined earlier by x‐ray diffraction. The trimethylsilylation produces compounds with a hydroximic structure on both ends, both groups being disilylated. Z–Z, Z–E and E–E isomer combinations are visible in the spectra and their ratio can be determined. Again, oxalodihydroximic acid derivatives are an exception: only one silylated product was found and its geometry could not be determined. Selective decoupling experiments (15N{1H} and 13C{1H}) are an inexpensive alternative to 15N enrichment used to identify E and Z conformers. To differentiate hydroxamic and hydroximic structures, the most reliable parameter is the 15N chemical shift, which differs in the two classes of compounds by about 120 ppm. To differentiate E and Z hydroxamic conformers 13C chemical shifts of C O groups are preferable to 15N chemical shifts but for distinguishing E and Z isomers of the hydroximic structure both 15N and 13C NMR of the C N group are useful. 17O NMR data are of no practical value in this respect. Copyright

STRUCTURE OF SILYLATED BENZOHYDROXAMIC ACIDS

NMR spectra (13C, 15N and 29Si) of the products of trimethylsilylation and tert‐butyldimethylsilylation of benzohydroxamic acid and model derivatives of benzohydroximic acid were studied in solutions. The products are shown to have the structure of (Z)(syn)‐O,O′‐bis(trimethylsilyl) and ‐O,O′‐di(tert‐butyldimethylsilyl) derivatives of benzohydroximic acid [i.e. (Z)‐trimethylsilyl ester of N‐(trimethylsiloxy)benzoimidic acid and (Z)‐tert‐butyldimethylsilyl ester of N‐(tert‐butyldimethylsilyl)benzoimidic acid, respectively]. Whereas 15N NMR chemical shifts are the most useful NMR parameter for differentiation between derivatives of hydroxamic and hydroximic tautomers, differentiation between Z and E stereoisomers of hydroximic acid derivatives is more reliable on the basis of 1J(13C,13C) coupling within the C—CN moiety. Copyright

Polymerization of nitrophenyl propargyl ethers with transition metal catalysts and characterization of polymers

o-, m- and p-Nitrophenyl propargyl ethers were prepared, characterized (n.m.r., i.r.) and polymerized with tungsten and molybdenum based metathesis catalyst systems. Soluble high-molecular-weight (weight-average molecular weight about 4 × 105) poly(p-nitrophenyl propargyl ether)s were prepared in good yields using catalyst systems MoOCl4/Me4Sn and MoCl5/EtAlCl2. Insoluble poly(m-nitrophenyl propargyl ether)s and poly(o-nitrophenyl propargyl ether)s were prepared in moderate yields with the same catalyst systems. I.r. and n.m.r. spectra of all polymers confirmed polyacetylene type polymer structure with nitrophenoxymethyl pendant groups.

Polymerization of p-nitrophenylacetylene with metathesis catalysts. Photoelectrical properties of phenylacetylene/p-nitrophenylacetylene copolymer

Preparation of substituted acetylene polymers carrying nitro group is reported for the first time. Homopolymers of p-nitrophenylacetylene (NPA) and its copolymer with phenylacetylene (PA) have been prepared by polymerizations induced by WOCl 4 /3Me 4 Sn as a catalyst in benzene and benzene/dioxane as solvents. WOCl 4 alone does not polymerize, oligomerize and/or cyclotrimerize NPA. The two-component catalysts WOCl 4 /2Ph4Sn and MoOCl 4 /3Me 4 Sn produce low amounts of oligomers only. The homopolymer of NPA is insoluble, whereas the copolymer containing one NPA unit per 4.38 units of PA is soluble in aromatic solvents and THF. The IR and NMR spectra of the homopolymer and the IR spectrum of the copolymer are reported. The photoconductive properties of the copolymer are also reported and compared with those of poly(PA). Introduction of NO 2 groups onto polymer chains was found to increase the quantum efficiency of charge carrier photogeneration but to deteriorate the charge carrier transport properties of the polymer.

Benzhydroximic acids — NMR study of trimethylsilyl derivatives

Abstract NMR spectra ( 1 H 13 C, 15 N, and 29 Si) of trimethylsilylated para and meta substituted benzhydroxamic acids were studied in chloroform solutions. The silylation products have the structure of Z-O,O ′-bis(trimethylsilyl) derivatives of benzhydroximic acid, independently of the ring substituent. According to aromatic proton chemical shifts, the geometry of the hydroximic group and its torsion angle with the ring plane are not affected by the para substituent. The chemical shifts of the nuclei in the hydroximic part of the molecule show surprisingly strong dependence on the remote ring substituent. The two 29 Si chemical shifts exhibit essentially the same sensitivity to substitution despite the fact that the Si(O 1 ) is one bond closer to the substituent than the Si(O 4 ) silicon. It is suggested that while electron donor substituents increase the shielding of the silicon atoms they also increase the basicity of the oxygen in the SiO moiety, thus leading to the stronger hydrogen bonding with the solvent. Association with chloroform partially compensates the direct substituent effect on the shielding in the case of Si(O 1 ) silicon. The influence of other factors not covered by substituent constants is demonstrated by excellent correlations with the chemical shifts in analogous tert -butyldimethylsilyl derivatives.

Geminal 2J(29Si‐O‐29Si) couplings in oligosiloxanes and their relation to direct 1J(29Si‐13C) couplings

Absolute values of (79) geminal 2J(29Si‐O‐29Si) couplings were measured in an extensive series of (55) unstrained siloxanes dissolved in chloroform‐d. Signs of 2J(29Si‐O‐29Si) in some (9) silicon hydrides were determined relative to 1J(29Si‐1H) which are known to be negative. It is supposed that positive sign of the 2J(29Si‐O‐29Si) coupling found in all studied hydrides is common to all siloxanes. Theoretical calculations for simple model compounds failed to reproduce this sign and so their predictions of bond length and angle dependences cannot be taken as reliable. Useful empirical correlations were found between the 2J(29Si‐O‐29Si) couplings on one side and the total number m of oxygen atoms bonded to the silicon atoms, sum of 29Si chemical shifts or product of 1J(29Si‐13C) couplings on the other side. The significance of these correlations is briefly discussed. Copyright

Relative signs of 29Si13C couplings

Two pulse sequences applicable to the determination of relative signs of coupling constants, gHSQC‐RELAY(P) and gHSQC‐RELAY(D), were developed and tested. These sequences are suitable for determination of relative signs of long‐range coupling constants (<2 Hz) between two heteronuclei of low abundance (such as 29Si and 13C), and are applicable even to cases in which one of the heteronuclei (29Si) does not exhibit coupling with some of the detected protons (1H). The two sequences differ in the manner in which they suppress undesirable homonuclear coherence transfers. Each of the sequences can be combined with an isotope filter for better suppression of the centerlines arising from more abundant NMR‐inactive isotopes. The sequences were tested on ethoxytrimethylsilane and (E)‐(buta‐1,3‐dienyloxy)trimethylsilane, and we conclude that 2J(29Si‐O‐13C) is positive while 3J(29Si‐O‐C‐13C) is negative in both compounds. Copyright

Cationic nickel(II) complexes with azine diphosphines—structural and electrochemical study

Abstract Nickel(II) complexes of azine diphosphine ligands, PR2CH2C(But)NNC(But)CH2PR2 (R=Ph, But, Pri, C6H11), were prepared for the first time by reactions of anhydrous NiX2 (X=Cl, Br, I) with ligands. Complexes are cationic, the counterion being either simple halide anion or, in two cases, 1/2[NiCl4]2−. The azine diphosphines are coordinated terdentately in (E,Z) configuration forming thereby a bicyclic ligand frame (five and six-membered ring) with diphosphine bite angles 160–165°. A balance of stereoelectronic factors governing square planar versus tetrahedral coordination of the ligands is suggested. Structures of four of the complexes with bulky ligands (R=But, X−=Cl−, Br−, I−; R=C6H11, X−=1/2[NiCl]2−) were determined by X-ray diffraction and qualitative Huckel MO calculations on model undistorted square planar [trans-NiX(NH2)(PH3)2]+ were carried out to elucidate the contributions of steric and electronic factors. Two reduction and two oxidation processes depending on the substituent R and the halide were identified from cyclic voltammetry data on the complexes.

DOSY OF SILYLATED SACCHARIDES

With steadily improving NMR hardware and software, diffusion ordered spectroscopy (DOSY) is becoming increasingly popular for analysis of mixtures (for a review and leading references, see ref. 1). In principle, DOSY enables separation of NMR signals from molecules that differ in their diffusion rates (or diffusion coefficients, D). In contrast to LC-NMR, DOSY achieves signal separation without physical separation of compounds by a suitably chosen pulse sequence that employs pulsed (magnetic) field gradients (PFG). The gradients attenuate NMR signals from slowly moving molecules less than the signals from fast moving molecules. Using a series of different gradient strengths, the signal attenuation is measured and analysis yields a diffusion coefficient for each resolved line in the spectrum. Theoretically, signals coming from the same molecule should have the same diffusion coefficient; hence on an appropriate 2D plot the NMR signals from the same molecule should appear on a line parallel to the NMR chemical shift axis. DOSY is an attractive option for analysis of mixtures of oligosaccharides differing in molecular size. This straightforward analysis is based on the generally accepted idea that, the larger the molecule, the slower it diffuses. It has recently been reported that hydrogen bonding can substantially affect diffusion rates. Although this effect was observed in a study of phenol and cyclohexanol, variation in the number of hydroxyl groups present in different carbohydrate molecules will complicate the interpretation of DOSY spectra in terms of molecular size. An obvious remedy would be to block the hydroxyl groups by suitable substituents. In this communication we want to demonstrate that trimethylsilylation offers such a possibility and, in addition, that DOSY methodology can be applied to Si J. CARBOHYDRATE CHEMISTRY, 20(1), 87–91 (2001)