Trends In Shifts Research Articles

Group 4 organometallic compounds. Part 8. Preparation and Mössbauer spectra of five- and six-co-ordinate di- and tri-organotin compounds containing mixed phenyl and butyl groups on tin

Mossbauer spectroscopic data and preparative details are reported for a range of [SnIVBuPh], [SnIVBu2Ph], and [SnIVBuPh2] compounds. The stereochemical preferences of the butyl and phenyl groups in octahedral and trigonal-bipyramidal co-ordination in these mixed-ligand complexes, relative to their symmetrical analogues, have been examined on the additivity model, with comparison between theory and experiment based on our previously reported data on ligand partial quadrupole splittings. For the derivatives of the two triorganotins, trends in centre shift (c.s.) and quadrupole splitting (q.s.) parameters are reported, which are consistent with the additivity model. An approximate inverse correlation has been noted between q.s. and element–oxygen stretching frequency in comparing complexes of [SnBuxPh3–x](x= 0, 1, or 2) with PPh3O and AsPh3O.

Experimental support for a right‐handed vertical double helix

AbstractThe experimentally observed geometry of a miniature double helix of high anti (χCN fixed ≃ 120°) nucleic acid structure indicates substantial interstrand base stacking with little intrastrand base stacking. This geometry is consistent with the right‐handed vertical double helix in which the base planes are parallel to helical axis proposed by Olson [(1977) Proc. Natl. Acad. Sci. USA 74, 1775] from theoretical calculations. The experimental data do not agree with the left‐handed model in which the base planes are perpendicular to helical axis that has been proposed by Yathindra and Sundaralingam [(1976) Nucl. Acids Res. 3, 729]. The theoretically computed chemical shift changes for the various double‐helical configurations show that for the system examined, only the vertical model can explain the experimentally observed shift trends. The melting curve for the helix–coil transition for high anti dinucleoside monophosphate has been observed for the first time. Even though the experimental data support vertical double helices when χCN is fixed at 120°, data on naturally occurring nucleic acid structures indicate that they have no proclivity to enter into vertically stabilized double‐helical arrays.

CONFORMATIONAL ANALYSIS WITH LANTHANIDE SHIFT REAGENTS DIFFERENCES IN THE SOLUTION AND CRYSTAL STRUCTURE CONFORMATIONS OF A COMPOUND

Abstract The solid and LSR-determined solution conformations of 2-benzyl-3-oxo-6-exo-(2,6-diiodo-4-methylphenoxy)-2-azabicyclo[2.2.1]heptane are substantially different. The “best” solution conformation contains a close I···H contact of 2.65 Å, in agreement with trends in the 1H chemical shifts in several similar compounds. Crystal structure data may not provide an accurate picture of the major solution conformer in cases where interconversion barriers are low.

Beiträge zur Chemie des Silieiums und Germaniums, XXIX [1] 29Si-NMR-spektroskopische Untersuchungen von geradkettigen und verzweigten Silanen / Contributions to the Chemistry of Silicon and Germanium, XXIX [1] 29Si NMR Spectroscopic Investigations on Straight and Branched Silanes

, The structure of the straight silanes SinH2n+2 (1 ≤ n ≤ 7) and of the branched isomers isotetrasilane, iso- and neopentasilane, 2-silyl- and 3-silylpentasilane, 2,3-disilyltetrasilane as well as 2-silyl- and 3-silylhexasilane could be unambiguously elucidated by 29Si NMR spectroscopy. Systematic trends in the chemical shift and coupling parameters were observed and an empirical relationship between δ(29Si) of a 29Si nucleus and the number of the neighbouring silicon atoms in αβor γposition is given. In 1H decoupled spectra the nuclear Overhauser effect on the 29Si resonance was found to increase with increasing distance of the 29Si nucleus from the end of the chain. A complete analysis of the multi spin systems of disilane, trisilane and isotetrasilane was performed and the signs of the 29Si,H- and H,H-long range coupling constants were determined with regard to the negative sign of 1J(29SiH).

The constituents of Schizandra chinensis Baill. VI. 13C nuclear magnetic resonance spectroscopy of dibenzocyclooctadiene lignans.

The 13C NMR spectra of thirteen dibenzocyclooctadiene lignans, isolated from the iruits of Schizandra chinensis BAILL. (Schizandraceae), were analyzed and their carbon shifts assigned. Several important chemical shift trends are described.

Oxygen-17 nuclear magnetic resonancé spectra of transition metal carbonyl compounds

The natural abundance 17O NMR shielding values observed by FTNMR in the present study on a representative variety of terminal transition metal carbonyl complexes are shown to range 400–300 ppm downfield from 17OH 2. The carbonyl 17O chemical shift trends are generally opposite to those for the carbonyl 13C chemical shifts and this is explained by metal π-backbonding to the carbonyl π ★ orbitals. A metal triad effect is observed for 17O chemical shifts. There is an upfield shift in the carbonyl oxygen shielding values on descending a given group in the periodic table. This is the same effect as observed previously for the 13C shieldings of carbonyl groups.

13C nuclear magnetic resonance spectra of some C19-diterpenoid alkaloids and their derivatives

The natural abundance carbon-13 nuclear magnetic resonance spectra of some C19-diterpenoid alkaloids and their alkamines (lappaconitine, lappaconine, lapaconidine, ranaconine, 14-dehydrobrowniine, aconine, pseudoaconine, deoxyaconine, and hypaconine) have been determined at 15.03 MHz. With the aid of proton decoupling techniques, additivity relationships, and comparison with spectra of related alkaloids, self-consistent and unambiguous assignments of nearly all carbon resonances for these alkaloids have been made. Some important chemical shift trends have been observed, which are useful for identifying the basic C19-diterpenoid alkaloid skeleton and the hydroxy and methoxy group substitution patterns in these alkaloids. On the basis of 13C nmr spectra of lappaconitine and lappaconine, the anthranoyl ester moiety is assigned to the C-4 position in lappaconitine. The 13C nmr spectra of lapaconidine, aconine, and pseudoaconine taken in pyridine and chloroform have been compared to determine the conformational changes of the ring A hydroxy groups in these alkaloids.

Stimulus dependence of ocular dominance of complex cells in area 17 of the feline visual cortex

Stimulus dependence of ocular dominance of 31 deep-layer complex cells was assessed from detailed monocular directional tuning curves for motion of bar stimuli or fields of static visual noise, in area 17 of normal adult cats, lightly anaesthetised with N2O/O2 supplemented with pentobarbitone. Virtually all cells were binocularly driven, with the anticipated ocular dominance distribution. Interocular differences in directional bias and sharpness of directional tuning for noise were observed in eleven cells, whereas preferred direction and sharpness of tuning for bar stimuli were similar for each eye. In the majority of cells (20/31), any differences between noise and bar tuning in one eye were replicated in the other. Ocular dominance of about half the cells (17/31) for noise and for bar motion was similar, or marginally shifted by up to one ocular dominance group. Substantial shifts in ocular dominance were seen in 14 cells — by up to two ocular dominance groups in 12 cells and by up to three ocular dominance groups in two cells. In three cases these shifts involved a reversal of eye dominance. Notwithstanding these changes, there were no obvious trends in shifts of ocular dominance in favour of the ipsilateral or contralateral eye, nor was there any tendency towards increased binocularity for noise.

A decoupled rigid bender model for the anharmonic force field of octahedral molecules

The molecular vibrational Hamiltonian for an octahedral molecule is derived to fourth order, subject to the constraint that the bond lengths are held constant. By assuming that the force field for the bending motions is a points-on-a-sphere potential, the location of the first difference bands for the pure bending motions is calculated; mode couplings are neglected. With these results, systematic trends in the anharmonic shifts can be predicted for the complete family of metal-hexafluorides. The effects of mode couplings and the importance of these calculations for understanding the initial process in the multiphoton dissociation of SF 6 are briefly discussed.

Proton magnetic resonance investigation of hydrogen bonding in aqueous polyalcohols

The proton NMR spectra of aqueous solutions of 1,2-ethanediol, 1,2-propanediol, and glycerol have been investigated as a function of mole fraction at 0 and 25°C. It was observed that for the latter two polyols (at the lower temperature), the resonances due to the water and the alcohol's hydroxyl protons were resolved at all concentrations and exhibited chemical shift trends similar to those observed in aqueous solutions of monohydric alcohols. However, in contrast to the simple alcohols, the alkyl group chemical shifts were found to be temperature dependent in all three polyols studied.

Carbon‐13 NMR spectra of some chlorinated polycyclodiene pesticides

AbstractCarbon‐13 NMR spectra have been obtained for aldrin, dieldrin, isodrin, endrin, heptachlor, heptachlor oxide and 24 of their degradation/conversion products. Chemical shifts were assigned on the basis of (1) comparison among the spectra, (2) single frequency, off‐resonance decoupling experiments, (3) substituent effects derived for norbornanes and (4) spectra obtained with an added lanthanide shift reagent. The substituent effects previously reported for norbornanes were found to be applicable for establishing trends in the chemical shifts for many of the compounds studied.

Vibrational bandshapes of monohydric saturated alcohols

Calculations are presented for three models of alcohols dissolved in carbon tetrachloride in the slow modulation limit. A number of liquid-state models is considered. The calculated bands differ markedly in some cases from the often assumed gaussian distribution. The Raman spectra of the OH-stretching vibration of methanol, t-butanol and adamantanol-1 in dilute CCl4 solution are presented. It is concluded that the second moments of these bands compare well with the calculated values in two of the solvent models. The experimental bands are narrower than the computed bands, indicating some motional narrowing. Trends in the vapour-solution frequency shift are also well reproduced in the calculations.

13C-NMR Spectroscopy of Tropane Alkaloids

The natural abundance 13C-NMR spectra of tropine, atropine, scopolamine, cocaine, atropine methonitrate, and dl-tropic acid were determined at 22.63 MHz. With the aid of proton decoupling techniques and by comparison with analogous simpler compounds, it was possible to make self-consistent and unambiguous assignments of all carbon resonances for these alkaloids. Some important chemical shift trends were observed and should be useful in the identification of similar systems.

Studies on Si-O bonding in silicate and aluminosilicate glasses based on SiK? emission X-rays

The SiKβ X-ray emission spectra of silicate and aluminosilicate glasses and crystals have been measured with an X-ray fluorescence spectrometer and the chemical shifts of the SiKβ peak relative to that of SiO2 glass have been determined. Trends in the chemical shift of the SiKβ peak for Na2O-SiO2, Li2O-SiO2 and Li2O-Al2O3-SiO2 glasses were similar to those of their crystalline counterparts, indicating that the strength of the Si-O bond varies in the same way with composition in both glass and crystal. The chemical shift in alkali aluminosilicate glasses containing equal molar proportions of aluminium oxide and alkali oxide increased with decreasing SiO2 content, indicating that the Si-O bond also weakened in aluminosilicate glasses. The same effect was also observed when zirconia was added. The SiKβ bandwidths for Na2O-Al2O3-SiO2 and Li2O-Al2O3-SiO2 glasses with the Al2O3: M2O ratio equal to one, remained almost the same as for SiO2 glass, indicating that the addition of alkali does not result in the formation of non-bridging oxygens in the framework if accompanied by Al2O3.

Carbon‐13 NMR studies of thyroid hormones and model compounds. The conformational analysis of diphenyl ethers

AbstractThe 13C spectra of a series of thyroid hormones and derivatives including thyroxine (T4) sodium salt, T4‐N‐acetyl methyl ester, triiodothyronine (T3), T3‐sodium salt, T3‐methyl ester hydrochloride, sodium triiodothyroacetate and ethyl triiodothyropropionate were assigned using chemical shift trends from model compounds, coupling constants and relaxation times. The 13C spectra of a series of diphenyl ethers were assigned and the chemical shift trends observed were analyzed on the basis of conformational changes engendered by mesomeric or steric effects.

Spatial configuration of mRNA 5'-terminus.

Nuclear magnetic resonance investigation has revealed that the 5'-terminus m7G5'ppp5'Am of mRNA displays a spatial configuration in which the bases form stacked arrays. Details of the conformation as derived from coupling constants, shift trends and ring current considerations are discussed.

Nuclear magnetic resonance study of the impact of ribose 2'-O-methylation on the aqueous solution conformation of cytidylyl-(3' leads to 5')-cytidine.

AbstractIn order to obtain information about the conformational characteristics at the nearestneighbor level in the 2′‐O‐methylated region of t‐RNA, as well as in the bizarre 5′‐terminus of eucaryotic mRNA, a detailed nuclear magnetic resonance study of 2′‐O‐methyl‐cytidylyl‐(3′ → 5′)‐cytidine (CmpC) was conducted. Proton spectra were recorded at 270 MHz in the Fourier mode in D2O solutions, 0.01M, pD 7.3 in the temperature range 5–80°C. Complete accurate sets of nmr parameters were derived for each of the nucleotidyl units by a combination of homo‐nuclear decouplings and simulation iteration methods. The data were translated into conformational parameters using procedures developed in earlier studies from these laboratories.It is shown that the ribofuranose ring exists at a 2E ⇄ 3E equilibrium with clear preference [(75–80)%] for the 3E mode. The C(4′)‐C(5′) and C(5′)‐O(5′) bonds form a stable conformational network with outspoken preference for conformers in which Ψ1, Ψ2 ≃ 60° and ϕ2 ≃ 180°. The orientation of the 3′‐phosphate and 2′‐O‐methyl groups is such that ϕ1′ ≃ 210° and ϕ″ ≃ 60°. The phosphodiester bonds are flexible and shift trends for base, H(1′), and H(5″) suggest the existence of a conformational blend of right‐handed stack (g−g−), left‐handed stack (g+g+), and unstacked arrays (tg− and tg+).Elevation of temperature perturbs the 2E ⇄ 3E equilibrium accompanied with modest depopulation of ψ1, ψ2 ≃ 60° and ϕ2 ≃ 180° conformers. The major effect of elevation of temperature is in the increase of unstacked arrays at the expense of g−g− and g+g+ conformers. The shift trend of Cmp‐H(3′) with temperature shows that torsional variation about O(3′)‐P is facilitated by increase in temperature and the preferred rotamer about O(3′)‐P in the unstacked form is t (ω1′ = 180°).A detailed comparison of the aqueous solution conformations of CpC and CmpC reveals that 2′‐O‐methylation causes: (i) a reduction in the magnitude of χ1; (ii) an increase in the population of 3E pucker at the 3′‐nucleotidyl unit; and (iii) modest perturbations in the O(3′)‐P and P‐O(5′) bond conformations. Comparison of the aqueous solution conformations of AmpA and CmpC makes clear that the conformational properties of pyrimidine‐pyrimidine and purine‐purine dimers which carry a 2′‐O‐methylated 3′‐nucleotidyl unit are significantly different.

Conformational properties of purine-pyrimidine and pyrimidine-purine dinucleoside monophosphates.

The detailed conformational features and dynamics of heterodinucleoside monophosphates ApU, ApC, GpU, GpC, UpA, CpA, UpG, and CpG have been studied in aqueous solution by high field nuclear magnetic resonance (NMR) spectroscopy. Analysis of the resultant NMR parameters leads to a number of discernible trends throughout the series. Thus the ribose rings of the dimers exist as equilibrium mixtures of C(2')-endo(2E) in equilibrium C(3')-endo(3E) conformers with a proclivity for the 3E pucker in most cases; the C(4')-C(5') bonds of both nucleotidyl units show significant preference (74-96%) for a gg conformation and the dominant conformer (85-89%) about C(5')-O(5') is g'g'. Orientation about the C(3')-O(3') bond is coupled to the ribose conformational equilibrium and the system exists with a bias for the 3Eg- coupled conformation in which the H(3')-C(3')-O(3')-P dihedral angle occupies the narrow range of 33-35 degrees. Dimerization, on the average, causes about 10% increase in gg and g'g' populations and the g-domain becomes increasingly populated about the C(3')-O(3') bond. The ribose equilibrium 2E in equilibrium 3E shifts in favor of 3E upon dimerization, the effect being very conspicuous for the pu-py series (similar to 40 yields 60%) and less noticeable for the py-pu systems (similar to 47 yields 58%), clearly suggesting a correlation between sequence and ribose conformational equilibrium. The temperature and dimerization data for the heterodinucleoside monophosphates show that the transition 2E yields 3E is directly related to XCN changes induced by dimerization and stacking. Analysis of the ribose coupling data shows that the percentage populations of stacked species vary from dimer to dimer with GpC displaying a maximum of 45% stacked population and UpG about 10%. However, in general, the pu-py dimers show a higher preference (27-45%) for stacked conformations than py-pu dimers (10-25%). It is proposed that the pronounced deshielding of H(5') of the 5'-nucleotidyl units upon dimerization is associated with the presence of right-handed stacks (g-g-), whereas the chemical shift trends of H(5') and H(5') of 3'-nucleotidyl units are due to the presence of left-handed stacks (g+g+) in all the dimers. In pu-py dimers, the population of the g-g- species is found to be greater than that of g+g+. Also the population of g-g- stacks in pu-py dimers is generally greater than in their corresponding matched py-pu dimers. Thus the base sequence has not only an explicit effect on the overall populations of stacked species, but also on the handedness of the stacks. The present results further confirm the interdependence of conformational bonds throughout the nucleotidyl framework.

ChemInform Abstract: VIBRATIONAL AND NUCLEAR MAGNETIC RESONANCE SPECTRA OF SOME COMPLEXES OF TRIHALOGENO(TRIHALOGENOPHOSPHINE)PLATINATE(II), AND THEIR METHANOLYSIS REACTIONS

AbstractDie Schwingungs‐ und NMR‐Spektren der Trihalogenophosphinplatinate(II) (I)‐(III), die überwiegend als Tetrabutylammoniumsalze isoliert sind, werden mitgeteilt und diskutiert.

A theoretical investigation of electronic reorganizations accompanying core and valence ionization in simple hydrogen bonded systems

Non-empirical LCAO MO SCF calculations have been carried out on the ground state and core ionized states of some hydrogen bonded dimers, and in the particular case of H2O the trimer has also been investigated. Comparison of absolute and relative binding energies and relaxation energies with respect to the corresponding monomers reveals that substantial changes occur in going to the associated species. The relaxation energies for a given core hole are shown to increase on going from monomer to dimer indicating that intermolecular contributions to relaxation energies are of the same sign irrespective of the sign for the shift in core binding energy. Creation of a core hole in the dimer species is shown to give rise to substantial changes in hydrogen bond energies compared with the neutral species. In the particular case of valence holes dominantly of 2s and 2p character it is shown that trends in shifts and relaxation energies parallel those for the core hole states.