Pronounced stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the selenophenyl oximes as shown by NMR spectroscopy and GIAO calculations

In the ¹H and ¹³C NMR spectra of 1‐(2‐selenophenyl)‐1‐alkanone oximes, the ¹H, the ¹³C‐3 and ¹³C‐5 signals of the selenophene ring are shifted by 0.1–0.4, 2.5–3.0 and 5.5–6.0 ppm, respectively, to higher frequencies, whereas those of the ¹³C‐1, ¹³C‐2 and ¹³C‐4 carbons are shifted by 4–5, ～11 and ～1.7 ppm to lower frequencies on going from the E to Z isomer. The ¹⁵N chemical shift of the oximic nitrogen is larger by 13–16 ppm in the E isomer relative to the Z isomer. An extraordinarily large difference (above 90 ppm) between the ⁷⁷Se resonance positions is revealed in the studied oxime isomers, the ⁷⁷Se peak being shifted to higher frequencies in the Z isomer. The trends in the changes of the measured chemical shifts are well reproduced by the GIAO calculations of the ¹H, ¹³C, ¹⁵N and ⁷⁷Se shielding constants in the energy‐favorable conformation with the syn orientation of the? C?N? O? H group relative to the selenophene ring. Copyright © 2009 John Wiley & Sons, Ltd.     

Solid‐state 13C, 15N and 29Si NMR characterization of block copolymers with CO2 capture properties

Natural abundance solid‐state multinuclear (¹³C, ¹⁵N and ²⁹Si) cross‐polarization magic‐angle‐spinning NMR was used to study structures of three block copolymers based on polyamide and dimethylsiloxane and two polyamides, one of which including ferrocene in its structure. Assignment of most of the resonance lines in ¹³C, ¹⁵N and ²⁹Si cross‐polarization magic‐angle‐spinning NMR spectra were suggested. A comparative analysis of ¹³C isotropic chemical shifts of polyamides with and without ferrocene has revealed a systematic shift towards higher δ ‐values (de‐shielding) explained as the incorporation of paramagnetic ferrocene into the polyamide backbone. In addition, the ¹³C NMR resonance lines for ferrocene‐based polyamide were significantly broadened, because of paramagnetic effects from ferrocene incorporated in the structure of this polyamide polymer. Single resonance lines with chemical shifts ranging from 88.1 to 91.5 ppm were observed for ¹⁵N sites in all of studied polyamide samples. ²⁹Si chemical shifts were found to be around ?22.4 ppm in polydimethylsiloxane samples that falls in the range of chemical shifts for alkylsiloxane compounds. The CO₂ capture performance of polyamide‐dimethylsiloxane‐based block copolymers was measured as a function of temperature and pressure. The data revealed that these polymeric materials have potential to uptake CO₂ (up to 9.6 cm³ g^?1) at ambient pressures and in the temperature interval 30–40 °C. Copyright © 2016 John Wiley & Sons, Ltd.     

Stereospecificity of 1H, 13C and 15N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on 1H chemical shifts

In the ¹³C NMR spectra of methylglyoxal bisdimethylhydrazone, the ¹³C‐5 signal is shifted to higher frequencies, while the ¹³C‐6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the ¹H‐6 chemical shift and ¹J(C‐6,H‐6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the –CH═N– bond does not change. This paradox can be rationalized by the C–H?N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum‐chemical calculations including Bader's quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ(¹H‐6) and ¹J(C‐6,H‐6) parameters. The effect of the C–H?N hydrogen bond on the ¹H shielding and one‐bond ¹³C–¹H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The ¹H, ¹³C and ¹⁵N chemical shifts of the 2‐ and 8‐(CH₃)₂N groups attached to the –C(CH₃)═N– and –CH═N– moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8‐(CH₃)₂N group conjugate effectively with the π‐framework, and the 2‐(CH₃)₂N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N‐2– and N‐8– nitrogen lone pairs to the π‐framework varies, which affects the ¹H, ¹³C and ¹⁵N shieldings. Copyright © 2012 John Wiley & Sons, Ltd.     

The effect of solvent accessible surface on Hammett‐type dependencies of infinite dilution 29Si and 13C NMR shifts in ring substituted silylated phenols dissolved in chloroform and acetone

Infinite dilution ²⁹Si and ¹³C NMR chemical shifts were determined from concentration dependencies of the shifts in dilute chloroform and acetone solutions of para substituted O‐silylated phenols, 4‐R‐C₆H₄‐O‐SiR′₂R″ (R = Me, MeO, H, F, Cl, NMe₂, NH₂, and CF₃), where the silyl part included groups of different sizes: dimethylsilyl (R′ = Me, R″ = H), trimethylsilyl (R′ = R″ = Me), tert‐butyldimethylsilyl (R′ = Me, R″ = CMe₃), and tert‐butyldiphenylsilyl (R′ = C₆H₅, R″ = CMe₃). Dependencies of silicon and C‐1 carbon chemical shifts on Hammett substituent constants are discussed. It is shown that the substituent sensitivity of these chemical shifts is reduced by association with chloroform, the reduction being proportional to the solvent accessible surface of the oxygen atom in the Si‐O‐C link. Copyright © 2012 John Wiley & Sons, Ltd.     

15N chemical shifts of a series of isatin oxime ethers and their corresponding nitrone isomers

In this article, we describe the characteristic ¹⁵N chemical shifts of isatin oxime ethers and their isomer nitrone. These oxime ethers and nitrones are the alkylation reaction products of isatin oximes. In our study, the ¹⁵N chemical shifts observed in these oxime ethers were in the 402–408 (or 22–28) ppm range, although those for their corresponding nitrone series were in the 280–320 (or ?100 to ?60) ppm range. This remarkable difference in ¹⁵N NMR chemical shift values could potentially be used to determine the O‐ versus N‐alkylation of oximes, even when only one isomer is available. In this paper, the differences in ¹⁵N NMR chemical shifts serve as the basis for a discussion about how to distinguish both regioisomers derived from the oximes alkylation. Copyright © 2010 John Wiley & Sons, Ltd.     

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 © 2011 John Wiley & Sons, Ltd.     

Configurational assignment of the geometric isomers of α,β-disubstituted vinyl methyl ethers from 13C nmr shift data

¹³C chemical shifts of the C atoms in the geometric isomers of some α,β-disubstituted vinyl methyl ethers have been measured. Configurational assignment is readily accomplished from the relative ¹³C chemical shift values of the β-C atoms, the signal of the Z isomer (with the β substituent in a cis position with respect of the MeO group) lying 11–15 ppm downfield from that of the E isomer. The higher chemical shifts of the β-C of the Z form are ascribed to reduced conjugation in the vinyloxy system, due to the nonplanar gauche configuration of the MeO group about the O-C(sp²) bond. Structural effects on the other ¹³C shift values are also discussed.     

29Si and 13C NMR spectra of permethylpolysilanes

²⁹Si, ¹³C and ¹H NMR spectra are reported for the series of linear permethylpolysilanes Me(SiMe₂)_nMe where n = 1 to 6, for the cyclic permethylpolysilanes (Me₂Si)_n where n = 5 to 8, and for a few related compounds. For linear polysilanes the ²⁹Si and ¹³C chemical shifts can be accurately calculated from simple additivity relationships based on the number of silicon atoms in α, β, γ and δ positions. Adjacent (α) silicon atoms lead to upfield shifts in the ²⁹Si and ¹³C resonances, whereas more remote silicon atoms lead to downfield shifts. The ²⁹Si chemical shifts of the polysilane chains are linearly related to the ¹³C shifts of the carbon atoms attached to the silicon. The ²⁹Si and ¹³C resonances of the cyclic silanes deviate from this relationship. Ring current effects arising from σ delocalization are suggested as an explanation for the deviations. Proton-coupled ²⁹Si NMR spectra are reported for Me₃SiSiMe₃ and for (Me₂Si)_n, n = 5 to 7.     

Improved pulse sequences for measurement of small long‐range couplings between heteronuclei (29Si?13C) at natural abundance

The gradient pulse sequences for measurement of small long‐range couplings between heteronuclei (²⁹Si? ¹³C) in natural abundance reported to date (INEPT‐(Si,C)gCOSY and INEPT‐(Si,C,Si)HMQC) suffer from significant signal loss when these nuclei (²⁹Si, ¹³C) are coupled through one‐bond couplings to protons. This negative effect can be completely eliminated by using non‐gradient versions (INEPT‐(Si,C)COSY) or by switching proton decoupling off during gradient pulses (modified INEPT‐(Si,C,Si)gHMQC pulse sequence). The beneficial effects of these two approaches on the quality of the spectra are demonstrated here. Copyright © 2009 John Wiley & Sons, Ltd.     

C?H···N and C?H···O intramolecular hydrogen bonding effects in the 1H, 13C and 15 N NMR spectra of the configurational isomers of 1‐vinylpyrrole‐2‐carbaldehyde oxime substantiated by DFT calculations

According to the ¹H, ¹³C and ¹⁵N NMR spectroscopic data and DFT calculations, the E‐isomer of 1‐vinylpyrrole‐2‐carbaldehyde adopts preferable conformation with the anti‐orientation of the vinyl group relative to the carbaldehyde oxime group and with the syn‐arrangement of the carbaldehyde oxime group with reference to the pyrrole ring. This conformation is stabilized by the C? H···N intramolecular hydrogen bond between the α‐hydrogen of the vinyl group and the oxime group nitrogen, which causes a pronounced high‐frequency shift of the α‐hydrogen signal in ¹H NMR (～0.5 ppm) and an increase in the corresponding one‐bond ¹³C–¹H coupling constant (ca 4 Hz). In the Z‐isomer, the carbaldehyde oxime group turns to the anti‐position with respect to the pyrrole ring. The C? H···O intramolecular hydrogen bond between the H‐3 hydrogen of the pyrrole ring and the oxime group oxygen is realized in this case. Due to such hydrogen bonding, the H‐3 hydrogen resonance is shifted to a higher frequency by about 1 ppm and the one‐bond ¹³C–¹H coupling constant for this proton increases by ～5 Hz. Copyright © 2008 John Wiley & Sons, Ltd.     

1,1‐allylboration of bis(silyl)ethynes: electron‐deficient Si?H?B bridges and novel heterocycles via intramolecular hydrosilylation

The reaction of bis(silyl)ethynes 2 – 4 , bearing one, two and three hydrides at one of the silicon atoms, with triallylborane 1 leads primarily to alkenes 5 , 8 and 11 respectively by 1,1‐allylboration. In these alkenes, the diallylboryl and the silyl group bearing one or more Si? H functions are in cis‐positions at the C?C bond, giving rise to the formation of an electron‐deficient Si? H? B bridge. This follows unambiguously from the consistent set of NMR data, in particular from the observation of isotope‐induced chemical shifts ²Δ^10/11B(²⁹Si). The activation of the Si? H bond in 5 , 8 and 11 induces intramolecular hydrosilylation under very mild reaction conditions to give 1,4‐silabora‐cyclo‐2‐heptenes 7 , 10 and 13 respectively. Upon heating, these seven‐membered heterocycles undergo ring contraction by 1,1‐deallylboration to give the 1‐sila‐cyclo‐2‐hexenes 14 – 16 , and bear an exocyclic diallylboryl group in 3‐position. All proposed structures are based on consistent ¹H, ¹¹B, ¹³C and ²⁹SiNMR data. Copyright © 2004 John Wiley & Sons, Ltd.     

Stereospecificity of shielding constants of carbon-13 nuclei in 13C NMR spectra of oximes of hetarenecarbaldehydes and alkyl heteryl ketones

It has been discovered that the chemical shifts of carbon atoms in ¹³C NMR spectra of oximes having pyrrolyl, furyl, benzofuryl, thienyl, and pyridyl rings as substituents are changed systematically on going from the E- to the Z-isomer. This makes it possible to use the indicated chemical shifts for establishing the configuration of oximes with heterocyclic substituents and studying the special features of their electronic structure. Dedicated to Academician of the Russian Academy of Sciences B. A. Trofimov on his 70th jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1523–1531, October, 2008.     

Study of three-dimensional structures of benzoylpyridine oximes and their ethers by 1H and 13C NMR spectroscopy

The ¹H and ¹³C NMR spectra of the E and Z isomers of 2-, 3-, and 4-benzoylpyridine oximes and their ethers were analyzed thoroughly, and the ¹H-¹³C spin-spin coupling constants (SSCC) were determined. It was established that the magnitude of the effect for the quaternary carbon atoms in the E and Z isomers depends on the site of substitution in the pyridine ring. It was assumed that the intermolecular hydrogen bond is stronger in the E form than in the Z form. The existence of the Z isomer of 2-benzoylpyridine oxime in deuterochloroform with an intramolecular hydrogen bond was proved.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 203–208, February, 1990.     

Saturated amine oxides: part 10. hydroacridines: part 32. linear 17O/13C and 13C/13C chemical shift correlations between saturated azaheterocyclic N‐methylamine N‐oxides,and the methiodides of their parent amines

Two kinds of good linear correlations were found between the chemical shifts of saturated six‐membered azaheterocyclic N‐methylamine N‐oxides and the chemical shifts of the methiodides of their parent amines. One of the correlations occurs between the ¹⁷O chemical shift of the N⁺―O^– oxygen in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9778). This correlation enables unambiguous configuration assignment of the N⁺―O^– bond, even if the experimentally observed ¹⁷O chemical shift of only one N‐epimer is available, provided the ¹³C chemical shifts of both N⁺―CH₃ groups in the methiodide are known and assigned; furthermore, it can be used also for the estimation of ¹⁷O chemical shifts of the N⁺―O^– oxygens in N‐epimeric pairs of N‐oxides, for which observed ¹⁷O data hardly become available. The second correlation is observed between the ¹³C chemical shift of the N⁺―CH₃ methyl group in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9785). It can be used for safe configuration assignment of the N⁺―CH₃ group and, indirectly, also of the N⁺―O^– bond in an amine N‐oxide, even if no ¹⁷O NMR data, and the ¹³C chemical shift of only one N‐epimer is available. Copyright © 2015 John Wiley & Sons, Ltd.     

The 1H and 13C NMR chemical shifts of Strychnos alkaloids revisited at the DFT level

The density functional theory calculation of ¹H and ¹³C NMR chemical shifts in a series of ten 10 classically known Strychnos alkaloids with a strychnine skeleton was performed at the PBE0/pcSseg-2//pcseg-2 level. It was found that calculated ¹H and ¹³C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.67 ppm in the range of 150 ppm for carbons, so that a mean absolute percentage error was as small as ~1% in both cases.     

Study of conformations and hydrogen bonds in the configurational isomers of pyrrole‐2‐carbaldehyde oxime by 1H, 13C and 15N NMR spectroscopy combined with MP2 and DFT calculations and NBO analysis

The ¹H, ¹³C and ¹⁵N NMR studies have shown that the E and Z isomers of pyrrole‐2‐carbaldehyde oxime adopt preferable conformation with the syn orientation of the oxime group with respect to the pyrrole ring. The syn conformation of E and Z isomers of pyrrole‐2‐carbaldehyde oxime is stabilized by the N? H···N and N? H···O intramolecular hydrogen bonds, respectively. The N? H···N hydrogen bond in the E isomer causes the high‐frequency shift of the bridge proton signal by about 1 ppm and increase the ¹J(N, H) coupling by ～3 Hz. The bridge proton shows further deshielding and higher increase of the ¹J(N, H) coupling constant due to the strengthening of the N? H···O hydrogen bond in the Z isomer. The MP2 calculations indicate that the syn conformation of E and Z isomers is by ～3.5 kcal/mol energetically less favorable than the anti conformation. The calculations of ¹H shielding and ¹J(N, H) coupling in the syn and anti conformations allow the contribution to these constants from the N? H···N and N? H···O hydrogen bondings to be estimated. The NBO analysis suggests that the N? H···N hydrogen bond in the E isomer is a pure electrostatic interaction while the charge transfer from the oxygen lone pair to the antibonding orbital of the N? H bond through the N? H···O hydrogen bond occurs in the Z isomer. Copyright © 2010 John Wiley & Sons, Ltd.     

1H and 13C NMR spectra of Strychnos alkaloids: Selected NMR updates

The PBE0/pcSseg-2//pcseg-2 calculations of ¹H and ¹³C NMR chemical shifts were performed for a classical series of 12 Strychnos alkaloids (except for the earlier studied parent strychnine), namely akuammicine, isostrychnine, rosibiline, tsilanine, spermostrychnine, diaboline, cyclostrychnine, henningsamide, strychnosilidine, strychnobrasiline, holstiine, and icajine. It was found that the calculated ¹H and ¹³C NMR chemical shifts show markedly good correlations with available experimental data, as characterized by a mean absolute error of 0.22 ppm for the range of 8 ppm for protons and 1.97 ppm for the range of 180 ppm for carbons. Complementarily, the present results provide essential NMR update and fill a gap in the NMR data of this distinguished group of vitally important natural products.     

Silyl modification of biologically active compounds. 11. Synthesis,physico‐chemical and biological evaluation of N‐(trialkoxysilylalkyl)tetrahydro(iso,silaiso) quinoline derivatives

N‐(trialkoxysilylalkyl) derivatives of 1,2,3,4‐tetrahydroquinoline, 1,2,3,4‐tetrahydroisoquinoline and 4,4‐dimethyl‐4‐sila‐1,2,3,4‐tetrahydroisoquinoline were prepared and characterized by elemental analysis, ¹H, ¹³C and ²⁹Si NMR spectroscopy. In vivo psychotropic properties and in vitro cytotoxic effects of 3‐[N‐(1,2,3,4‐tetrahydroisoquinolyl)]propyltriethoxysilane methiodide and 3‐[N‐(1,2,3,4‐tetrahydroisoquinolyl)]propylsilatrane are reported. Comparative study of ²⁹Si shifts in newly synthesized compounds suggested donor–acceptor interaction between nitrogen and silicon atom, which increased electron density at Si nuclei, revealing a stronger increment of N → Si transannular bond in comparison with N → Si α‐effect. The molecular structure of 3‐[N‐(1,2,3,4‐tetrahydroisoquinolyl)]propylsilatrane features a penta‐coordinate silicon atom having CSiO₃ pattern and Si…N intramolecular interaction. Copyright © 2005 John Wiley & Sons, Ltd.     

Empirical and DFT GIAO quantum‐mechanical methods of 13C chemical shifts prediction: competitors or collaborators?

The accuracy of ¹³C chemical shift prediction by both DFT GIAO quantum‐mechanical (QM) and empirical methods was compared using 205 structures for which experimental and QM‐calculated chemical shifts were published in the literature. For these structures, ¹³C chemical shifts were calculated using HOSE code and neural network (NN) algorithms developed within our laboratory. In total, 2531 chemical shifts were analyzed and statistically processed. It has been shown that, in general, QM methods are capable of providing similar but inferior accuracy to the empirical approaches, but quite frequently they give larger mean average error values. For the structural set examined in this work, the following mean absolute errors (MAEs) were found: MAE(HOSE) = 1.58 ppm, MAE(NN) = 1.91 ppm and MAE(QM) = 3.29 ppm. A strategy of combined application of both the empirical and DFT GIAO approaches is suggested. The strategy could provide a synergistic effect if the advantages intrinsic to each method are exploited. Copyright © 2010 John Wiley & Sons, Ltd.     

13C and 15N NMR spectra of high‐energy polyazidocyanopyridines

¹³C and ¹⁵N NMR spectra of high‐energy 2,4,6‐triazidopyridine‐3,5‐dicarbonitrile, 2,3,5,6‐tetraazidopyridine‐4‐carbonitrile and 3,4,5,6‐tetraazidopyridine‐2‐carbonitrile are reported. The assignment of signals in the spectra was performed on the basis of density functional theory calculations. The molecular geometries were optimized using the M06‐2X functional with the 6‐311+G(d,p) basis set. The magnetic shielding tensors were calculated by the gauge‐independent atomic orbital method with the Tao–Perdew–Staroverov–Scuseria hybrid functional known as TPSSh. In all the calculations, a polarizable continuum model was used to simulate solvent effects. This approach provided accurate predictions of the ¹³C and ¹⁵N chemical shifts for all the three compounds despite complications arising due to non‐coplanar arrangement of the azido groups in the molecules. It was found that the ¹⁵N chemical shifts of the N_α atoms in the azido groups of 2,4,6‐triazidopyridines correlate with the ¹³C chemical shifts of the carbon atoms attached to these azido groups. Copyright © 2016 John Wiley & Sons, Ltd.