Ring currents, local anisotropy, and the problem of the out-of-plane protons: a reinvestigation of the nuclear magnetic resonance spectrum of [10]-paracyclophane

1977 ◽  
Vol 55 (13) ◽  
pp. 2575-2581 ◽  
Author(s):  
Arvind Agarwal ◽  
John A. Barnes ◽  
John L. Fletcher ◽  
Michael J. McGlinchey ◽  
Brian G. Sayer
Keyword(s):  
Ring Current ◽  
Chemical Shifts ◽  
Current Model ◽  
Classical Model ◽  
Local Anisotropy ◽  
Nmr Spectrum ◽  
H Nmr ◽  
Out Of Plane ◽  
Mechanical Approach ◽  
Quantum Mechanical Approach

The1H nmr spectrum of [10]-paracyclophane has been recorded at 220 MHz and the methylene protons assigned on the basis of homonuclear decoupled spectra. Local anisotropic contributions to chemical shifts for protons sited above or near the aromatic ring of [10]-paracyclophane have been calculated using a classical model previously proposed by Grant. The residual incremental shift was shown to follow the Waugh–Fessenden–Johnson–Bovey classical ring current model, but the loop separation originally invoked was shown to be unnecessary. These corrected ring current contributions also correlate very well with the quantum-mechanical approach of Haigh and Mallion.

Part II. Conformational analysis of 4- and 5-substituted thiazolidine-2-thiones

1980 ◽  
Vol 58 (6) ◽  
pp. 604-616 ◽  
Author(s):  
F. Chanon ◽  
M. Rajzmann ◽  
M. Chanon ◽  
J. Metzger ◽  
G. Pouzard ◽  
...  
Keyword(s):  
Chemical Shifts ◽  
Alkyl Substituent ◽  
Chair Conformation ◽  
Equatorial Orientation ◽  
Anomeric Effect ◽  
H Nmr ◽  
Transannular Interaction ◽  
Conformational Properties ◽  
Out Of Plane ◽  
C Nmr

Variation of the conformational properties in a series of 4-R-thiazolidine-2-thiones (4-R = Me, Et, iPr, tBu, OH) and 5-R-thiazolidine-2-thiones (5-R = Me, Et, iPr, tBu) is deduced from 1H nmr (Karplus 3J, chemical shifts, 2J44′, 2J55′) 13C nmr data and CNDO-2 calculations. In the absence of any transannular interaction, vicinal constraints and van der Waals requirements of the substituents control the conformational equilibrium. The pseudo-axial arrangement is favoured when 4-R = Me, while the substituent presents no preferential orientation when 5-R = Me. As the size of the 5-alkyl and the 4-alkyl substituent increases, its pseudo-equatorial orientation and the predominance of the C4C5 out-of-plane half-chair conformation are favoured. This does not lead to conformational exclusivity even when 4-R or 5-R is tBu. When 4-R is hydroxy, there is a definite pseudo-axial conformation (anomeric effect) and the ring preference is of the C4 out-of-plane type.

Addition of the nitroxyl radical TEMPO to 1-naphthylketene: formation of an unusual adduct

1999 ◽  
Vol 77 (5-6) ◽  
pp. 806-809 ◽  
Author(s):  
Jennifer Carter ◽  
Michael H Fenwick ◽  
Wen-wei Huang ◽  
Vladimir V Popik ◽  
Thomas T Tidwell
Keyword(s):  
Nitroxyl Radical ◽  
Chemical Shifts ◽  
Wolff Rearrangement ◽  
Nmr Spectrum ◽  
X Ray ◽  
H Nmr ◽  
Naphthyl Ring ◽  
Nmr Signals ◽  
Very High

1-Naphthylketene (2), generated by thermal Wolff-rearrangement, is trapped in situ by 2,2,6,6-tetramethylpiridinyloxy radical (TEMPO, TO·) to form the adduct 1-naphthCH(OT)CO2T (4), whose structure is confirmed by an X-ray determination. The 1H NMR spectrum of 4 displays three CH3 groups with very high field chemical shifts (δ 0.10-0.47), and this is attributed to the location of these groups in the shielding region above the π system of the naphthyl ring. At -40°C, doubling of most of the 1H NMR signals occurs, and this is attributed to a freezing out of two conformations differing by rotation around the naphthyl—CH bond.Key words: ketene, TEMPO, restricted rotation, 1H NMR, conformational analysis, free radicals.

The preparation and rearrangements of some 8,8-dimethylhomotropylium cations and their iron tricarbonyl complexes

1985 ◽  
Vol 63 (2) ◽  
pp. 418-425 ◽  
Author(s):  
Ronald F. Childs ◽  
Aravamuthan Varadarajan
Keyword(s):  
Low Temperatures ◽  
Chemical Shifts ◽  
Major Product ◽  
Membered Ring ◽  
Tricarbonyl Complex ◽  
Nmr Spectrum ◽  
H Nmr ◽  
Free Energy Of Activation ◽  
Iron Tricarbonyl ◽  
Proton Chemical Shifts

The circumambulatory rearrangements of some 8,8-dimethylhomotropylium cations and their corresponding iron tricarbonyl complexes have been examined. 8,8-Dimethylhomotropylium cation, 4, has been prepared by protonation of 8,8-dimethyl-bicyclo[5.1.0]octa-3,5-dien-2-ol in FSO3H/SO2ClF. Evidence for homoaromatic delocalization in 4 was found in its characteristic 1H nmr spectrum, particularly when this was compared to that of the non-cyclically delocalized 8,8-dimethyl-bicyclo[5.1.0]octadienyliron tricarbonyl complex 7. Cation 4 was found to be stable at low temperatures but to isomerize to isopropyltropylium ion, 8, at −50 °C, [Formula: see text] The specifically deuterated cation 4D was prepared and a slow circumambulation of C8 around the basal 7-membered ring of this cation was found to occur at −90 °C. The free energy of activation for this circumambulation is 14.5 kcal/mol. Homotropylium cation 4 was also prepared by protonation of 8,8-dimethylbicyclo[3.2.1]octa-3,6-dien-2-ol, 14, at −120 °C. No evidence for the presence of the corresponding bicyclo[3.2.1]octadienyl ion 12 was obtained. The 2,8,8-trimethylhomotropylium cation, 17, was prepared and shown to undergo a circumambulatory rearrangement to give the 4,8,8-trimethyl substituted cation 20 as the major product. In this trimethyl series it also proved possible to generate the 2,8,8-trimethylbicyclo[3.2.1]octadienyl cation, 18, and observe its 1H nmr spectrum at very low temperatures (−116 °C). The proton chemical shifts of 18 suggest that the charge is predominantly located on the 3-carbon bridge. Cation 18 isomerized to 17, 20, and other products at temperatures above −100 °C. The 2,8,8-trimethylbicyclo[5.1.0]octadienyliron tricarbonyl complex 22 was also shown to undergo a circumambulatory rearrangement in which both C8 and the Fe(CO)3 grouping migrate.

Benzene isomers. II. 3,4-Dimethylenecyclobutene

1968 ◽  
Vol 21 (7) ◽  
pp. 1807 ◽  
Author(s):  
BAW Coller ◽  
ML Heffernan ◽  
AJ Jones
Keyword(s):  
Dipole Moment ◽  
Chemical Shifts ◽  
Current Model ◽  
Chemical Properties ◽  
Membered Ring ◽  
Thermal Rearrangement ◽  
Local Anisotropy ◽  
Satellite Spectrum ◽  
Anisotropy Model ◽  
Physical And Chemical

Theoretical considerations indicate that the electron distribution in 3,4,-dimethylenecyclobutene is non-uniform and a dipole moment of 0.71 D has been predicted by inclusion of non-neighbour core resonance integrals. Preliminary studies of the microwave spectrum of this compound provide a dipole moment of 0.618 � 0.011 D. The term "pseudo-alternant" is proposed to describe this phenomenon. In the present paper additional physical and chemical properties of 3,4,-dimethylene- cyclobutene are described. The observed low-field proton chemical shifts for 3,4-dimethylenecyclobutene are not accounted for using a ring current model and a small calculated paramagnetic current indicates properties usually associated with H�ckel 4n-hydrocarbons. A local anisotropy model provides a better account of the observed shift. The carbon-13- proton satellite spectrum of 3,4-dimethylenecyclobutene indicates similarities in the geometry of the four-membered ring to that in cyclobutene. In addition, infrared and ultraviolet spectra support the conclusion that this hydrocarbon be regarded as a cross-conjugated diene. A detailed discussion of the products arising from the bromination of 3,4-dimethylenecyclobutene under a variety of conditions indicates consecutive 1,4-addition of bromine across the four-membered ring, a property which has been associated with cross-conjugation. Hydrogenolysis, chlorination, iodination, and attempted Diels-Alder addition reactions are also described. 3,4-Dimethylenecyclobutene was prepared by the thermal rearrangement of hexa-1,5-diyne. The mechanisms of thermal and photolytic rearrangement of hexa-1,5-diyne are discussed using Woodward-Hoffmann postulates. 1,2-Dideutero-,1-ethyl-, and 1-n-propyl-3,4-dimethylenecyclobutene were also prepared by the thermal rearrangement of the corresponding diyne. In addition, the thermal rearrangement of hexa-1,5-diyne over the temperature range 290-700� to give variable quantities of 3,4-dimethylenecyclobutene, fulvene, and benzene is described. 3,4- Dimethylenecyclobutene is thermally rearranged to benzene at 680�.

Ringstrom bei cyclischen Polyenen mit alternierenden und ausgeglichenen Bindungsabständen

1967 ◽  
Vol 22 (1) ◽  
pp. 103-112 ◽  
Author(s):  
F. Baer ◽  
H. Kuhn ◽  
W. Regel
Keyword(s):  
Ring Current ◽  
Chemical Shifts ◽  
Electron Systems ◽  
Dimensional Electron ◽  
Nmr Spectrum ◽  
One Dimensional ◽  
Bond Alternation ◽  
Dimensional Electron Gas ◽  
The One ◽  
Proton Chemical Shifts

The ring current effect in NMR has generally been used to distinguish between equal and alternant bonds in cyclic π-electron systems, assuming a strong ring current in equal-bonds-hydrocarbons and no ring current in alternant-bonds-hydrocarbons. A calculation of the ring current is presented based on the one dimensional electron gas model. The effect of bond alternation is considered by a sine curve potential.Proceeding from a model with equal bonds to a model with alternant bonds the ring current is strongly reduced in rings with more than 10 members; it is reduced by 24% and 51% only in 6 and 10 membered rings, respectively. In non-HücKEL type rings with 4 to 16 carbon atoms the ring current is directed opposite to the classical current and is strongest in cyclobutadiene. The contribution of the π-electrons to the susceptibility of these compounds is therefore paramagnetic and ring current shifts opposite to those in HÜCKEL rings are to be expected. These results are confirmed by the proton chemical shifts in the NMR spectrum of [16]-Annulene.

Proton magnetic resonance spectrum of cellulose triacetate. Temperature effects, the rotational conformation of the 6-acetoxymethyl group, and computer modelling of methyl 4,6-di-O-acetyl-β-glucopyranoside

1985 ◽  
Vol 63 (9) ◽  
pp. 2507-2510 ◽  
Author(s):  
Vanga S. Rao ◽  
Françoise Sauriol ◽  
Arthur S. Perlin ◽  
M. T. Phan Viet
Keyword(s):  
Resonance Spectrum ◽  
Computer Modelling ◽  
Chemical Shifts ◽  
Cellulose Triacetate ◽  
Proton Magnetic Resonance Spectrum ◽  
Spin Coupling ◽  
Supporting Evidence ◽  
Nmr Spectrum ◽  
H Nmr ◽  
Spin Spin Coupling

Pronounced narrowing of the resonance signals in the 400-MHz 1H nmr spectrum of cellulose triacetate in CDCl3 between 25 and 50 °C, as well as shielding and deshielding changes in 1H chemical shifts, suggest that thermal disruption of intermolecular aggregates is accompanied by a conformational modification. Analysis of the spin–spin coupling patterns clearly evident at 50 °C indicates that although there is a reversal in the chemical shifts of H-6R and H-6S relative to those for acetylated D-glucopyranose derivatives, the rotational conformations of the exocyclic 6-acetoxymethyl groups of the polymer and the model compounds all favor RHS and RCS rotameric forms. Supporting evidence for this conclusion is obtained from the 2-dimensional 1H spectrum of a trisaccharide, O-β-D-glucopyranosyl-(1 → 3)-O-β-D-glucopyranosyl-(1→ 4)-O-β-D-glucopyranose undecaacetate. Computer-generated models of methyl 4,6-di-O-acetyl-β-glucopyranoside are examined in relation to the stereochemistry of 6-acetoxymethyl groups.

1H nuclear magnetic resonance spectral parameters of toluene. Implications for conformational analysis and isotope shifts

1985 ◽  
Vol 63 (10) ◽  
pp. 2597-2600 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Glenn H. Penner
Keyword(s):  
Chemical Shifts ◽  
Low Frequency ◽  
Spectral Parameters ◽  
Nmr Spectrum ◽  
H Nmr ◽  
Conformational Preferences ◽  
Precise Analysis ◽  
1H Nuclear Magnetic Resonance ◽  
Direct Measures ◽  
Proton Chemical Shifts

A precise analysis of the 1H nmr spectrum of toluene as a dilute solution in carbon disulfide yields a revised set of spectral parameters. The chemical shift of the para proton lies 12.6 ppb to low frequency of that of the ortho protons at 300 K. The ring proton chemical shifts are discussed and compared with 1H and 3H shifts observed in carbon tetrachloride. The long-range couplings between methyl and ring protons can be said to be quantitatively understood in terms of σ and σ–π electron transmitted mechanisms. The changes observed in these three couplings in phenylacetaldehyde can be quantitatively reproduced in terms of these mechanisms and also illustrate how these changes are direct measures of the conformational preferences in this molecule.

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