Fourier transform NMR studies of organometallic compounds. II—synthesis and NMR spectroscopy of propenyl and isopropenyl derivatives of silicon and lead

Isomeric mixtures of compounds Me_nM(CH?CHMe)_4?n (M=Si, Pb; n=0?3) have been prepared and studied, as well as pure Me₃M(CMe?CH₂) and mixtures containing propenyl isopropenyl residues bonded to silicon and lead. ¹H, ¹³C, ²⁹Si and ²⁰⁷Pb NMR data are presented; as previously observed for the corresponding tin compounds, the ²⁹Si and ²⁰⁷Pb shifts for the Me₃MC₃H₅ isomers can be used to calculate the shifts expected for the other isomers; while for lead the agreement is good, calculated and observed values for silicon diverge with decreasing n due, at least in part, to steric factors.     

NMR spectroscopy on heavy nuclei of transition metal complexes. The role of polarization effect

Chemical shifts (δ) and spin-spin coupling constants (J) in the NMR spectra on heavy nuclei M (⁵¹V, ⁵⁵Mn, ⁵⁷Fe, ⁹⁵Mo, ¹⁰³Rh, ¹⁸⁷Os, ¹⁹⁵Pt) for 27 series of transition metal complexes have been analyzed. In general case in the absence of steric factors the values of δ and J depend on three effects of substituents X: inductive, resonance, and polarization. The latter effect increases with the decrease in the distance between M and X and with growing charge on atom M. The contribution of the polarization effect varies from 0 to 80% depending on the type of the series.     

17O NMR spectroscopy of transition metal carbonyls

NMR spectra of ¹⁷O in natural abundance have been obtained for a range of metal carbonyls. Linewidths of less than 10 Hz for the derivatives of Cr, Mo, W and Fe have been observed, although they are slightly larger for the derivatives of metals having an electric quadrupole moment. Comparison with the corresponding ¹³C NMR data shows that: (i) the chemical shift ranges are comparable, (ii) the ordering of chemical shifts is not very different, (iii) the linewidths are more favourable for ¹⁷O than ¹³C for some Mn and Co derivatives and allow an extension of the range of temperature in which information on the fluxional dynamics are obtainable in the rapid exchange limit.     

Coordination chemistry of scandium. VIII[1] detection of complex formation in solution by 45Sc NMR spectroscopy.

⁴⁵Sc nmr spectra of aqueous solutions of ScCl₃, ScBr₃, ScI₃, Sc(ClO₄)₃, Sc(NO₃)₃ and Sc₂(SO₄)₃ are reported. The concentration dependence of the chemical shift and the line width have been studied and attempts made to relate these dependences to chemical species in solution. Raman shifts and ⁴⁵Sc nmr spectra have been obtained for aqueous solutions of ScCl₃ containing 0–12 M added HCl. ⁴⁵Sc nmr spectra of tetrahydrofuran solutions of ScCl₃, ScCl₃·4H₂O, ScBr₃, ScBr₃·4H₂O, Sc(NO₃)₃, Sc(NO₃)₃·3H₂O and Sc(ClO₄)₃·6H₂O are also reported. A study of ⁴⁵Sc chemical shifts in water-tetrahydrofuran mixtures has demonstrated the strong solvating power of water towards the scandium (III) ion. The complexation of scandium (III) with some macrocyclic polyether ligands in aqueous solution has been investigated.     

Characterization of noninnocent metal complexes using solid-state NMR spectroscopy: o-dioxolene vanadium complexes

(51)V solid-state NMR (SSNMR) studies of a series of noninnocent vanadium(V) catechol complexes have been conducted to evaluate the possibility that (51)V NMR observables, quadrupolar and chemical shift anisotropies, and electronic structures of such compounds can be used to characterize these compounds. The vanadium(V) catechol complexes described in these studies have relatively small quadrupolar coupling constants, which cover a surprisingly small range from 3.4 to 4.2 MHz. On the other hand, isotropic (51)V NMR chemical shifts cover a wide range from -200 to 400 ppm in solution and from -219 to 530 ppm in the solid state. A linear correlation of (51)V NMR isotropic solution and solid-state chemical shifts of complexes containing noninnocent ligands is observed. These experimental results provide the information needed for the application of (51)V SSNMR spectroscopy in characterizing the electronic properties of a wide variety of vanadium-containing systems and, in particular, those containing noninnocent ligands and that have chemical shifts outside the populated range of -300 to -700 ppm. The studies presented in this report demonstrate that the small quadrupolar couplings covering a narrow range of values reflect the symmetric electronic charge distribution, which is also similar across these complexes. These quadrupolar interaction parameters alone are not sufficient to capture the rich electronic structure of these complexes. In contrast, the chemical shift anisotropy tensor elements accessible from (51)V SSNMR experiments are a highly sensitive probe of subtle differences in electronic distribution and orbital occupancy in these compounds. Quantum chemical (density functional theory) calculations of NMR parameters for [VO(hshed)(Cat)] yield a (51)V chemical shift anisotropy tensor in reasonable agreement with the experimental results, but surprisingly the calculated quadrupolar coupling constant is significantly greater than the experimental value. The studies demonstrate that substitution of the catechol ligand with electron-donating groups results in an increase in the HOMO-LUMO gap and can be directly followed by an upfield shift for the vanadium catechol complex. In contrast, substitution of the catechol ligand with electron-withdrawing groups results in a decrease in the HOMO-LUMO gap and can directly be followed by a downfield shift for the complex. The vanadium catechol complexes were used in this work because (51)V is a half-integer quadrupolar nucleus whose NMR observables are highly sensitive to the local environment. However, the results are general and could be extended to other redox-active complexes that exhibit coordination chemistry similar to that of the vanadium catechol complexes.     

NMR observations of drawn polymers. VIII. Doubly oriented nylon 66

A wide-line NMR study of chain segmental motion in nylon 66 has been made on a rolled sheet having “double orientation.” In this sheet the crystallite c axis, i.e., the molecular chain axis, is oriented preferentially along the roll direction, and the crystallographic (010) plane lies predominantly parallel to the roll plane, or the plane of the sheet. The direction of the applied magnetic field with respect to the sheet is characterized by two angles, the polar angle γ subtended by the roll direction and the magnetic field, and an azimuthal angle ?. NMR spectra were taken at various values of the angles γ and ? and at three temperatures ?196°C, 20°C, and 180°C. The second moments of the absorption spectra taken at 180°C were compared with theoretical predictions of second moments based on two models for the high-temperature segmental motion (called the α_c process) in crystalline regions of nylon 66. One model consists of rotational oscillation with amplitudes δ of segments around their axies. The second model is denoted 60° flip-flop motion and consists of rotational 60°C jumps of the segments around their axes between two equilibrium sites with the possibility that the segments also oscillate with a general amplitudes δ around each site. The experimental results are consistent with fairly large amplitudes δ, in which case both models approach the limiting case of full segment rotation. For this reason the experiments do not allow a distinction between the two models. From the second moments at ?196°C and 20°C the decrease in second moment due to the low temperature segmental motion, the γ process, is obtained. This motion occurs in noncrystalline regions of nylon 66 and is found to cause a decrease in second moment which is strongly dependent on the two angles γ and ?, implying double orientation of the noncrystalline segments. It is suggested that at low temperatures the noncrystalline segments become immobilized in sites dictated by the crystallite orientation through the extensive hydrogen bonding known to exist in nylon 66.     

Quenching echo modulations in NMR spectroscopy.

In NMR spectroscopy, homonuclear scalar couplings normally lead to modulations of spin echoes that tend to interfere with the accurate determination of transverse relaxation rates by Carr-Purcell-Meiboom-Gill (CPMG) multiple refocusing experiments. Surprisingly, the echo modulations are largely cancelled when the refocusing pulses applied to the coupling partner deviate slightly from ideal pi rotations due to tilted effective radio-frequency (RF) fields, even at offsets that are much smaller than the radio-frequency amplitude. Experiments and simulations illustrate these effects for two-spin IS systems containing donor and acceptor (15)N nuclei I=N (D) and S=N(A) in RNA Watson-Crick base pairs with homonuclear scalar couplings J(IS)=(2h)J(N(D), N(A)) across the hydrogen bonds.     

Quenching and recoupling of echo modulations in NMR spectroscopy.

Trains of spin echoes are normally modulated by homonuclear scalar couplings. It has long been known that echo modulations are quenched when the pulse-repetition rates are much larger than the offsets of the coupling partners, because the spin systems behave as if they consisted of magnetically equivalent spins when the offsets are suppressed. This type of quenching of the echo modulations can occur when the radio-frequency (RF) pulses are ideal, that is, when they are perfectly homogeneous, properly calibrated to induce rotations through an angle, pi, and have an RF amplitude, omega(1)=-gammaB(1), that is strong compared to the largest offset, Omega(S)=omega(0S)-omega(RF), with respect to the carrier frequency. Recently, it was discovered that echo modulations can also be quenched when the RF pulses are nonideal, that is, when they are too weak to bring about an ideal rotation of the magnetization of the coupling partners, so that the effective fields associated with the RF pulses are tilted in the rotating frame. This phenomenon typically occurs when the pulse-repetition rates are much slower than the offset of the coupling partner. Under such conditions, it turns out, however, that for certain offsets, when the phase, Phi(S) (which arises from a free precession of the magnetization of the coupling partner, S, in the pulse interval, 2tau, and the pulse length, tau(pi)), approaches a multiple of 2pi, the echo modulations are restored. However, the frequencies of these echo modulations are not simply determined by the homonuclear scalar coupling, J(IS). The Fourier transforms of the echo trains (the so-called "J spectra") reveal surprising multiplet patterns, and the amplitudes of the echo modulations depend on the offsets of the coupling partners. Herein, we present a unified theory, based on an average-Hamiltonian approach, to describe these effects for two-spin systems. Experimental evidence of echo modulations in a system of two spins is presented. Experiments with three and more spins, backed up by extensive numerical simulations, will be presented elsewhere.     

Functional genomics and NMR spectroscopy

The recent success of the human genome project and the continued accomplishment in obtaining DNA sequences for a vast array of organisms is providing an unprecedented wealth of information. Nevertheless, an abundance of the proteome contains hypothetical proteins or proteins of unknown function, where high throughput approaches for genome-wide functional annotation (functional genomics) has evolved as the necessary next step. Nuclear magnetic resonance spectroscopy is playing an important role in functional genomics by providing information on the structure of protein and protein-ligand complexes, from metabolite fingerprinting and profiling, from the analysis of the metabolome, and from ligand affinity screens to identify chemical probes.     

High-pressure x-ray diffraction and Raman spectroscopy of ice VIII

In situ high-pressure/low-temperature synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of ice VIII. The x-ray measurements show that the pressure-volume relations remain smooth up to 23 GPa at 80 K. Although there is no evidence for structural changes to at least 14 GPa, the unit-cell axial ratio ca undergoes changes at 10-14 GPa. Raman measurements carried out at 80 K show that the nu(Tz)A(1g)+nuT(x,y)E(g) lattice modes for the Raman spectra of ice VIII in the lower-frequency regions (50-800 cm(-1)) disappear at around 10 GPa, and then a new peak of approximately 150 cm(-1) appears at 14 GPa. The combined data provide evidence for a transition beginning near 10 GPa. The results are consistent with recent synchrotron far-IR measurements and theoretical calculations. The decompressed phase recovered at ambient pressure transforms to low-density amorphous ice when heated to approximately 125 K.     

Simultaneously cycled NMR spectroscopy

Simultaneously cycled (SC) NMR was introduced and exemplified by implementing a set of 2-D [1H,1H] SC exclusive COSY (E.COSY) NMR experiments, that is, rf pulse flip-angle cycled (SFC), rf pulse phase cycled (SPC), and pulsed field gradient (PFG) strength cycled (SGC) E.COSY. Spatially selective 1H rf pulses were applied as composite pulses such that all steps of the respective cycles were affected simultaneously in different slices of the sample. This increased the data acquisition speed for an n-step cycle n-fold. A high intrinsic sensitivity was achieved by defining the cycles in a manner that the receiver phase remains constant for all steps of the cycle. Then, the signal resulting from applying the cycle corresponded to the sum of the signals from all steps of the cycle. Hence, the detected free induction decay did not have to be separated into the contributions arising from different slices, and read-out PFGs, which not only greatly reduce sensitivity but also negatively impact lineshapes in the direct dimension, were avoided. The current implementation of SFC E.COSY reached approximately 65% of the intrinsic sensitivity of the conventional phase cycled congener, making this experiment highly attractive whenever conventional data acquisition is sampling limited. Highly resolved SC E.COSY yielding accurate 3J-coupling values was recorded for the 416 Da plant alkaloid tomatidine within 80 min, that is, 12 times faster than with conventional phase cycled E.COSY. SC NMR is applicable for a large variety of NMR experiments and thus promises to be a valuable addition to the arsenal of approaches for tackling the NMR sampling problem to avoid sampling limited data acquisition.     

NMR spectroscopy of cyclopentane derivatives: I. Cyclopentane and cyclopentanone

The deuterium-decoupled 100 MHz p.m.r. ¹³C-satellite spectra for cyclopentanone-2, 2,5,5-d₄ (2)) and cyclopentane-1,1,2,2,3,3-d₆ (5) have been determined at several temperatures and the LAOCOON 3 derived spectral parameters related to molecular conformation. For 2, the data were found to be consistent with the half-chair conformation; for 5, the data were fitted to an unrestricted pseudorotation model.