Perturbations of 15N chemical shifts following N‐oxidation of the alkaloid cryptolepinone determined by 1H–15N GHSQC and GHMBC at natural abundanceMartin, Gary E.; Hadden, Chad E.; Blinn, James R.; Sharaf, Maged H. M.; Tackie, Albert N.; Schiff, Paul L.
doi: 10.1002/(SICI)1097-458X(199901)37:1<1::AID-MRC391>3.0.CO;2-Hpmid: N/A
Direct and long‐range
1H–15N correlations for cryptolepinone
and its N5‐oxide as a mixture in
DMSO‐d6 were determined using a combination
of 1H–15N GHSQC and GHMBC experiments.
The 15N chemical shifts of cryptolepinone in the mixture
were comparable to those reported previously for a pure sample of the
alkaloid. Following N‐oxidation, the N‐5
resonance shifted from 103.4 ppm in cryptolepinone to 188.1 ppm
(+84.7 ppm) in the N5‐oxide.
The N‐10 resonance is also shifted from 111.4 ppm downfield by
+23.9 ppm to 135.3 ppm in the N‐oxide. The
downfield shift observed for N‐10 is attributed to a transfer
of electronic effects via the intervening
C‐5a—C‐10a carbon–carbon double bond in a
manner comparable to that observed for fully aromatic diazines and
their corresponding N‐oxides, although the shift of
N‐10 is in the opposite direction of that observed for diazine
systems and their N‐oxides. The downfield shift of
N‐10 is consistent with the calculated partial charges
at N‐10 before and after oxidation, which are identical in
sense with the partial charges at N‐5, the actual site of
oxidation, although much smaller in magnitude. © 1999 John Wiley
& Sons, Ltd.
Ring inversion of fluorocyclohexane in its solid thiourea inclusion compound †Harris, Robin K.; Nordon, Alison; Harris, Kenneth D. M.
doi: 10.1002/(SICI)1097-458X(199901)37:1<15::AID-MRC403>3.0.CO;2-Jpmid: N/A
The rate of ring inversion of fluorocyclohexane in the
solid inclusion compound that it forms with thiourea was studied
using a variety of NMR techniques. Special use was made of 19
F NMR spectra acquired with high‐power proton decoupling.
Bandshape analysis was used in the range 237–300 K. The
occurrence of ring inversion at lower temperatures was established by
the two‐dimensional EXSY method and the rates thereof were
determined by selective polarization inversion. Measurements of
spin–lattice relaxation in the rotating frame showed a minimum
at 300 K, yielding an additional value for the ring inversion rate.
Variable‐temperature triple‐channel 13
C–{1H, 19F} spectra were
studied. It is concluded that the average activation parameters
(from the 19F work) for the forward and backward
ring inversion processes are ΔH‡
=39.4±2.6 kJ mol−1 and Δ
S‡=−12±11 J mol
−1 K−1. The near equality of the
axial and equatorial populations makes accurate determination of
separate barriers for the equatorial→axial and
axial→equatorial processes problematic. The values are discussed
in relation to those for other cyclohexane derivatives in their
thiourea inclusion compounds and for fluorocyclohexane in other
media. The superiority of the 19F measurements over the
use of 13C spectra is emphasized. © 1999 John Wiley
& Sons, Ltd.
Methyl β‐substituent effect on NMR 17O chemical shifts in two‐coordinated oxygen atoms: DFT GIAO and NBO and experimental studiesPeralta, J. E.; Contreras, R. H.; Taurian, O. E.; Ortiz, F. S.; de Kowalewski, D. G.; Kowalewski, V. J.
doi: 10.1002/(SICI)1097-458X(199901)37:1<31::AID-MRC396>3.0.CO;2-Epmid: N/A
Methyl β‐substituent effects on 17
O chemical shifts in dicoordinated oxygen atoms in compounds of
the type CH3OX are found to be insensitive
to the nature of the X substrate, even for strongly conjugating
groups. Such behavior is rationalized in terms of intramolecular
electron delocalizing using the natural bond orbitals approach to
calculate NBO E(2) charge‐transfer
energies. The level of theory employed in this analysis was chosen in
such a way that DFT GIAO at the same level could describe the
experimental trends adequately. In this way, it was found that charge
transfers to (OC)* antibonds render important
deshielding effects on the 17O chemical shifts. In the
particular case of the methyl β‐substituent effect quoted
above, such charge transfer originates in the preferential
orientation that the CαH2—C
βH3 group adopts in these compounds CH
3OX. Analogies with 19F chemical
shifts are discussed. © 1999 John Wiley & Sons, Ltd.
A new water‐soluble and lipid‐insoluble spin probe: application to the study of aqueous sucrose solutionsBelton, Peter S.; Sutcliffe, Leslie H.; Gillies, Duncan G.; Wu, Xiaoping; Smirnov, Alex I.
doi: 10.1002/(SICI)1097-458X(199901)37:1<36::AID-MRC405>3.0.CO;2-8pmid: N/A
A new ESR spin probe, sodium
1,1,3,3‐tetrakis(trideuteromethyl)isoindolin‐2‐
yloxyl‐4‐sulphonate, which is related to the extensively studied
generic free radical 1,1,3,3‐tetramethylisoindolin‐2‐yloxyl
(TMIO), was synthesized. The latter has greater chemical and biochemical
stability than any other nitroxyl radical and in low‐viscosity media it exhibits
near isotropic tumbling motion. It is therefore a very useful spin probe for ESR investigations
of (i) microviscosity, (ii) measurements of oxygen concentrations and
rates of oxygen diffusion and (iii) ESR imaging experiments. Substituents can be
introduced into the basic TMIO radical: here the synthesis of the sulphonated form is described;
it has a very high solubility in water, which is important for in vivo biological
applications where a low‐volume, high‐concentration spin probe may need to be
injected. In this paper, its use is demonstrated in a study of the microviscosities of aqueous
sucrose solutions at concentrations up to 65% and over a range of temperatures. General
assumptions were tested concerning the effect of inhomogeneous broadening from unresolved proton
hyperfine coupling constants on the calculations of rotational correlation times. For this purpose,
the well known spin probes Fremy' salt (peroxylamine disulphonate) and
di‐tert‐butyl nitroxide were used. The ESR results are compared with those from
previous NMR relaxation studies of aqueous sucrose solutions. © 1999 John Wiley & Sons, Ltd.
Electronic structure and phosphorus shielding: transition between PN double and triple bonding in iminophosphines ROPNMes*Gudat, D.; Niecke, E.; Grossmann, G.; Krüger, K.
doi: 10.1002/(SICI)1097-458X(199901)37:1<43::AID-MRC407>3.0.CO;2-Apmid: N/A
The 31P nuclear magnetic shielding tensors of
five iminophosphines of the type ROPNMes*
(R=organyl, silyl and organosulfonyloxy;
Mes*=2,4,6‐tBu3C6H
2) were characterized by means of solid‐state
31P cross‐polarization magic angle spinning NMR
studies and IGLO quantum chemical model calculations. The results
revealed a systematic variation in the principal values σ
ii (i=1, 2, 3) of the shielding
tensors which can be correlated with the transition from a formal PN
double to a triple bond. According to the IGLO calculations, the
principal axis 2 is in all compounds nearly orthogonal on the
local OPN plane, while the principal axis 1 deviates
5–15° from the PO bond and the axis 3 deviates
11–35° from the PN bond. © 1999 John Wiley & Sons,
Ltd.
Complete assignment of the 1H and 13C NMR spectra of the Coccinellidae‐defensive alkaloids myrrhine, precoccinelline and hippodamine, their N ‐oxides and the corresponding hydrochloridesLebrun, B.; Braekman, J. C.; Daloze, D.
doi: 10.1002/(SICI)1097-458X(199901)37:1<60::AID-MRC417>3.0.CO;2-Gpmid: N/A
The 1H and 13C NMR spectra of the
2‐methylperhydro‐9b‐azaphenalene alkaloids
characteristic of coccinellid beetles, myrrhine, precoccinelline,
hippodamine, their N‐oxides and the corresponding
hydrochlorides were completely assigned for the first time by a
one‐ and two‐dimensional homo‐ and heteronuclear
study (1H, 13C, 1H–1
H COSY, HMQC, HMBC) at 600 and 150.87 MHz. The influence of
the ring junction stereochemistry and of the N‐oxide
function on the proton and carbon chemical shifts in this series of
compounds is discussed. Protonation shifts are also considered ©
1999 John Wiley & Sons, Ltd.