Graphical Interactive Strategy for the Analysis of NMR Spectra in Liquid Crystalline Phases

Tertiary Phosphine and Arsine Complexes of Phosphorus Pentafluoride: Synthesis, Properties, and Electronic Structures

The reaction of PMe₃ or PPh₃ with PF₅ in anhydrous CH₂Cl₂ or hexane forms the white, moisture-sensitive complexes [PF₅(PR₃)] (R = Me, Ph). Similar reactions involving the diphosphines o-C₆H₄(PR₂)₂ afford the complexes [PF₄{o-C₆H₄(PR₂)₂}][PF₆]. The X-ray structures of [PF₅(PR₃)] and [PF₄{o-C₆H₄(PMe₂)₂}][PF₆] show pseudo-octahedral fluorophosphorus centers. Multinuclear NMR spectra (¹H, ¹⁹F{¹H}, ³¹P{¹H}) show that in solution in CH₂Cl₂/CD₂Cl₂ the structures determined crystallographically are the only species present for [PF₅(PMe₃)] and [PF₄{o-C₆H₄(PMe₂)₂}][PF₆] but that [PF₅(PPh₃)] and [PF₄{o-C₆H₄(PPh₂)₂}][PF₆] exhibit reversible dissociation of the phosphine at ambient temperatures, although exchange slows at low temperatures. The complex ¹⁹F{¹H} and ³¹P{¹H} NMR spectra have been analyzed, including those of the cation [PF₄{o-C₆H₄(PMe₂)₂}]⁺, which is a second-order AA'XX'B₂M spin system. The unstable [PF₅(AsMe₃)], which decomposes in a few hours at ambient temperatures, has also been isolated and spectroscopically characterized; neither AsPh₃ nor SbEt₃ forms similar complexes. The electronic structures of the PF₅ complexes have been explored by DFT calculations. The DFT optimized geometries for [PF₅(PMe₃)], [PF₅(PPh₃)], and [PF₄{o-C₆H₄(PMe₂)₂}]⁺ are in good agreement with their respective crystal structure geometries. DFT calculations on the PF₅-L complexes reveal the P-L bond strength falls with L in the order PMe₃ > PPh₃ > AsMe₃, consistent with the experimentally observed stabilities, and in the PF₅-L complexes, electron transfer from L to PF₅ on forming these complexes also follows the order PMe₃ > PPh₃ ≈ AsMe₃.

A combined LX-NMR and molecular dynamics investigation of the bulk and local structure of ionic liquid crystals

The unique power of NMR spectroscopy in anisotropic media (LX-NMR) as a tool to obtain local and bulk structural information, combined with the effectiveness of molecular dynamics simulations at the atomistic level, shows very attractive potentialities for the study of interesting, even though still poorly understood, materials such as Ionic Liquid Crystals (ILCs). In this work, we focused our attention, in particular, on the orientational ordering of two mesophases: 1-dodecyl-3-methylimidazolium chloride, [C12C1im]Cl, and 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C12C1im][BF4]. Both ILCs were studied by a 2H NMR direct investigation of the molecules forming the phases, suitably deuterated, and by 1H NMR spectroscopy, using the small rigid probe-solutes 1,4-dichlorobenzene (DCB), dissolved in [C12C1im][BF4] and [C12C1im]Cl, and 1,4-dibromobenzene (DBB) dissolved in [C12C1im][BF4], to probe the local, internal structure and organization of the mesophases. The experimental results were then compared with the predictions, by atomistic MD simulations, of the structure of the smectic phase of the two salts, at two selected temperatures, containing a single DCB molecule as a probe. The MD simulations show that the DCB solute is distributed only within the hydrophobic layers of the ILC. Orientational order parameters of the imidazolium cations and of the DCB molecule were obtained and compared with the experiments, showing a general good agreement and allowing a deeper understanding of the microscopic structure of the systems.

GET-SERF, a new gradient encoded SERF experiment for the trivial edition of 1H-19F couplings

A new spatially encoded heteronuclear (1)H-(19)F selective refocusing NMR experiment (GET-SERF) is proposed. This sequence allows editing in one single 2D experiment all couplings between a selected fluorine site and all the proton nuclei of the molecule. Its efficiency is illustrated in the case of diflunisal, a difluorinated anti-inflammatory drug, in isotropic and anisotropic media.

Smectic order parameters via liquid crystal NMR spectroscopy: Application to a partial bilayer smectic A phase

Solute molecules were dissolved in the liquid crystal 4-cyano-4'-n-octyloxybiphenyl (8OCB), known to form a partial bilayer smectic-A phase. Through measurement of solutes' and solvent's orientational order parameters via nuclear magnetic resonance spectroscopy, and their analysis via a statistical thermodynamic density functional theory, values of the solvent's positional order parameters and solutes' positional-orientational distribution functions were obtained. Near to the transition to the nematic phase, the main positional order parameter of the smectic liquid crystal turned out to be comprised in the interval 0.4-0.6, though the quality of the fittings assuming the phase as nematic all across the temperature range investigated was only slightly worse. This may be ascribed to the looseness of the partial bilayer smectic structure. Solutes were found to preferentially lie in those regions where liquid crystal molecule terminal chains are located.

Rigid probe solutes in a smectic-A liquid crystal: an unconventional route to the latter's positional order parameters

Biphenylene and pyrene were dissolved in the nematic and smectic-A phases of the liquid crystal 4,4'-di-n-heptyl-azoxybenzene and the orientational order parameters of both solutes and solvent measured via proton and deuteron nuclear-magnetic-resonance spectroscopy. This new data set was then merged with the one previously obtained, formed by 4,4'-di-chloro-benzene and naphthalene as solutes in the same solvent, and the resulting overall data set analyzed with a statistical thermodynamic density-functional theory to provide positional-orientational distribution functions of the various solutes along with the smectic solvent's positional order parameters.

Experimental assessment of the vibration-reorientation contribution to liquid crystal NMR dipolar couplings: the case of tetramethylallene dissolved in a nematic mesophase

In the present paper, the peculiar orientational behavior, studied by liquid crystal NMR (LXNMR) spectroscopy, of the D(2d) symmetry quasi-spherical molecule of tetramethylallene (TMA) dissolved in the nematic solvent I52 is exploited to attempt a quantitative experimental assessment of the correlation between molecular vibrations and overall rotations in weakly oriented molecules. The analysis of the very small D(HH) and (1)D((13)C-H) dipolar couplings, available from the natural abundance LXNMR spectra of TMA at different temperatures, allows for a derivation leading (by making a few approximations) to the quantification of the vibration-reorientation (also called nonrigid) contribution affecting the observed direct (1)D((13)C-H) dipolar coupling. The obtained results show that, under the particular conditions of the studied system (very weak orientational ordering of a highly symmetric molecule), this contribution is particularly important, in order to reproduce the whole value of the "observed" dipolar coupling. This issue is discussed and commented on at length in the work, also, by making reference to the analogy with perfectly symmetric molecules (such as methane and analogues) dissolved in liquid crystalline phases.

A general strategy for obtaining 19F-19F and 13C-19F residual dipolar couplings in perfluorocarbons from the NMR spectroscopy of liquid crystalline samples

A two-dimensional Fluorine Detected Local Field (FDLF) NMR experiment is demonstrated on a sample of perfluoropropyl iodide dissolved in the nematic solvent ZLI1132. In analogy to the proton detected local field (PDLF) technique, for each resolved site of the carbon spectrum, a simple map of the heteronuclear coupling network is obtained in the indirect dimension. A full analysis of the FDLF spectrum was achieved with the aid of two-dimensional (19)F-(13)C HETCOR and (13)C, D-resolved spectra (with D representing the anisotropic spin-spin coupling). A one-dimensional (19)F spectrum was recorded on the same sample at intermediate resolution, and values of the residual spin-spin couplings T(CF) and T(FF) obtained from both experiments were combined and used to provide starting parameters for the analysis of a very high resolution (19)F spectrum, including the weak satellite lines from single-(13)C isotopomers. The high-precision, residual, anisotropic couplings were used to explore whether they have an appreciable contribution from the anisotropic electron-mediated spin-spin couplings.

Solvent smectic order parameters from solute nematic order parameters

In liquid crystals, while the second and fourth rank orientational order parameters characterizing a nematic phase can be experimentally determined via several techniques, there is no straightforward experiment rendering the positional order parameters characterizing a smectic A phase. This work illustrates a novel method to estimate the positional order parameters of a smectogenic liquid crystal solvent from knowledge of the orientational order parameters of a number of solutes dissolved therein. The latter order parameters can be experimentally determined via liquid crystal NMR spectroscopy. These data can be then analyzed with a statistical-thermodynamic density functional theory, whose basic ingredient is a model for solute-solvent intermolecular interactions. Its parametrization and the subsequent fitting procedure eventually permit one to obtain the positional order parameters of the solvent besides the positional-orientational distribution function of the solutes. The method is applied to the smectogen 4,4(')-di-n-heptyl-azoxybenzene, in which the solutes 1,4-dichlorobenzene and naphthalene have been dissolved. With the help of this exploratory practical example, pros and cons of the method are pointed out and further developments prospected.

Intrinsic Information Content of NMR Dipolar Couplings: A Conformational Investigation of 1,3-Butadiene in a Nematic Phase

The conformational equilibrium of 1,3-butadiene in a condensed fluid phase is investigated by liquid-crystal NMR spectroscopy. The full set of D(HH) and D(CH) dipolar couplings is determined from the analysis of the (1)H spectra of the three 1,3-butadiene most-abundant isotopomers (i.e. the all (12)C and the two single-labeled (13)C isotopomers) for a total of 21 independent dipolar couplings. A very good starting set of spectral parameters for the analysis of the (1)H spectrum is determined in a semiautomated way by the analysis of the (N-1) (specifically, N=6, the number of 1/2 spin nuclei in the spin system) quantum refocused (5QR), and not (5Q), spectra. As an alternative approach, a Monte Carlo (MC) numerical simulation, capable of predicting the solute ordering, is tested to simulate the 5QR spectrum. The set of D(ij) couplings is very good, proving that the MC method can represent a novel, valid alternative to the existing spectral simplification procedures. The experimentally determined dipolar-coupling data set is fully compatible with the 1,3-butadiene conformational distribution reported in the literature for isolated molecules, indicating the presence of about 99 % of s-trans conformer. With regards to the remaining 1 %, in spite of the direct and very strong dependence of the observables on the molecular structure, it was not possible to discriminate between the planar s-cis and s-gauche forms, both of which produce a very good fit of the dipolar couplings. Vibrational corrections, up to the anharmonic term, were applied; the calculated geometrical parameters are in good- although not exact-agreement with those reported in the literature from experimental and theoretical investigations. This result can be considered as supporting the methodology used for obtaining the structure and conformational distribution of a flexible molecule in a liquid phase.

Molecular Ordering and Structure of Quasi-spherical Solutes by Liquid Crystal NMR and Monte Carlo Simulations: The Case of Norbornadiene

Norbornadiene (a C2v symmetry bicyclic rigid hydrocarbon) dissolved in three different nematic mesophases has been studied by liquid crystal NMR, to contribute to a better understanding of the influence of solvents on the solute's ordering and structure. The main results achieved by this work can be summarized as follows: (i) the order parameters obtained by the analysis of the 1H NMR spectra (at different temperatures) were successfully reproduced by a recently proposed model of solute/liquid crystal interactions, by using Monte Carlo numerical simulations; (ii) the theoretical (B3LYP/6-31++G**) "equilibrium" geometry of norbornadiene, vibrationally corrected by using the force field calculated at the same level, is compatible (within, at most, a 5% error) with experimental LXNMR data. This leads to the conclusion that the structure is not significantly distorted by the tested solvents.

Obtaining the structure and bond rotational potential of a substituted ethane by NMR spectroscopy of solutions in nematic liquid-crystalline solvents

Partially averaged dipolar couplings (also referred to as residual dipolar couplings) D(ij) can be obtained from the analysis of the NMR spectra of molecules dissolved in liquid-crystalline solvents. Their values for a nonrigid molecule depend upon the bond lengths and angles, the rotational potentials, and the orientational order of the molecules. The molecule studied, 1-chloro-2-bromoethane, is one of the simplest example of a substituted alkane in which the rotational potential has three minimum-energy positions, trans and gauche+/-conformations, and the present investigation explores the problems inherent in deriving the form of the potential and the molecular geometry from the set of partially averaged couplings between the protons, and between protons and (13)C nuclei. The geometrical parameters and the rotational potential obtained are compared with the results from a density-functional theory method.

Conformational Analysis of 2,2‘-Bithiophene: A 1H Liquid Crystal NMR Study Using the 13C Satellite Spectra

We have obtained a very large data set of spectral parameters from the analysis of (1)H NMR and (13)C satellite spectra of 2,2'-bithiophene dissolved in anisotropic, partially orienting mesophases. In particular, this parameter set includes 33 dipolar couplings, which are directly related to the interatomic distances, the dihedral angle phi between the two thiophenic rings, and the anisotropic solute-solvent interaction potential. This allows an exhaustive investigation of the conformational equilibrium of 2,2'-bithiophene in a liquidlike phase. Comparison with the predictions of high-level theoretical calculations for the isolated molecule provides evidence of a strong flattening as well as the sharpening effect of the medium on the conformer population. The approximations needed to apply vibrational corrections to flexible molecules are discussed in detail and some general conclusions concerning their effect on structure and conformational equilibria are proposed.

NMR Spectroscopy Investigation of the Cooperative Nature of the Internal Rotational Motions in Acetophenone

The proton NMR spectrum of the doubly enriched acetophenone-carbonyl,methyl-13C2 isotopomer dissolved in a liquid-crystalline solvent (LXNMR) was analyzed to yield a data set of 19 dipolar couplings. The presence of so many couplings, and in particular the dependence of some of them on the acetyl carbons enabled the investigation of the structure of the acetyl moiety and of possible cooperative motions about the aryl-carbonyl and carbonyl-methyl bonds. Methodological aspects, and approximations relating to the application of the vibrational correction procedure in the presence of large-amplitude torsional motions, are discussed. Results show that it is possible to discriminate between a continuous and a discrete conformer distribution about the angle phi(1) but not among a few proposed continuous shapes of U(iso)({phi}). In this study, the use of dipolar couplings with a non-negligible contribution from the indirect spin-spin coupling tensor J, (D(C8C9) in our case), for structural determination is extended from rigid to flexible molecules. The 1/2J(aniso)(C8C9) contribution was derived theoretically using the density functional theory linear response (DFT-LR) first-principles calculation of the J(C8C9) spin-spin coupling tensor.

The Structure and Conformations of 2-Thiophenecarboxaldehyde Obtained from Partially Averaged Dipolar Couplings

The proton NMR spectra of samples of 2-thiophenecarboxaldehyde dissolved in a nematic liquid crystalline solvent, including those from all five singly labelled 13C isotopomers, have been obtained. These have been analysed to yield sets of partially averaged dipolar couplings which have been used to determine the structure and the relative amounts of the cis and trans forms, which are the two minimum-energy structures generated by rotation about the ring-aldehyde bond. A procedure for applying vibrational corrections to the dipolar couplings in the presence of large amplitude motions is discussed.

The Structure of Acrolein in a Liquid Crystal Phase

The (1)H NMR spectrum of a sample of acrolein dissolved in the nematic liquid crystal phase I52 has been analysed to yield 18 dipolar couplings between all the magnetic nuclei in the molecule; moreover, the (13)C and (13)C{(1)H} NMR spectra of a sample of acrolein in CDCl(3) were recorded and analysed to determine the indirect J(ij) couplings. The data were used to obtain the relative positions of the carbon and hydrogen atoms, assuming that these are independent of the conformations generated by rotation around the C--C bond through an angle phi, and to obtain a probability distribution P(phi). It has been found that in the liquid phase, the distribution is a maximum at the trans form whereas the abundance of the cis form is significantly smaller compared with that found by microwave spectroscopy or high level quantum mechanical calculations. Such calculations produced also a suitable force field needed to develop suitable strategies for vibrational correction procedure in the case of flexible molecules.

Is styrene planar in liquid phases?

The proton NMR spectra of two (13)C-labeled isotopomers of styrene dissolved in two liquid crystalline solvents have been obtained and analyzed to yield four sets each of 24 dipolar couplings. These couplings were then used to investigate the structure of the ring and the ene fragments of the molecule, and the position of the maximum, phi(0), in the ring-ene bond rotational probability distribution. To do this, the effect on the dipolar couplings of small-amplitude vibrational motion was taken into account using vibrational wave functions calculated by molecular orbital and density functional methods. It is concluded that the NMR data are consistent with the ring fragment, averaged over the ring-ene rotation, planar, while the ene fragment is not. The value of phi(0) is found to be 18.0 degrees +/-0.2 degrees for the two solutions, compared with a value of 27 degrees calculated by the molecular method MP2/6-31G(*).

The NMR spectra of samples dissolved in liquid-crystalline phases: automatic analysis with the aid of multiple quantum spectra--the case of flexible molecules

Homonuclear N(S) = 0 and heteronuclear N(S) not equal 0 multiple quantum spectra, involving changes in the magnetic number m(I) by (N(I)-1), (N(I) - 2), and (N(I)-3), with N(I) and N(S) the number of interacting nuclei of magnetogyric ratio gamma(I) and gamma(S), are used for the automatic analysis of (1)H NMR spectra of flexible molecules dissolved in liquid-crystalline phases. The automatic procedure has been applied to study molecules of general formula Ph-CH(2)-X starting from a parameter set having all the spectral parameters set to zero. The results of such an analysis are then used as starting parameters for analysis of the single quantum spectrum. The method was first tested when X = Br and X = H in order to compare strategies differing for the types of parameters used and was then applied to the analysis of 3-phenylprop-1-yne.

Symmetry and phase-selected NMR spectra of liquid crystalline samples

It is demonstrated that the NMR spectra of liquid crystalline samples can be simplified by using multiple quantum filtering. In a system of N spin-12 nuclei, the N or (N-1)-multiple quantum filtered spectra (NQF or (N-1)QF) contain lines which originate only from transitions among the eigenstates belonging to the highest symmetry class of the spin permutation group. In addition the NQF spectra are divided further into two sets of lines which differ in phase by 180 degrees. A method for simulating and analysing multiple quantum filtered spectra is described, with examples from molecules with up to eight interacting spins.

Measurement of interproton, nuclear spin dipolar couplings in liquid crystalline samples by combining variable angle sample spinning, isotope dilution, and deuterium decoupling

The proton NMR spectra of liquid crystalline samples can be simplified by selective, partial deuteration followed by deuterium decoupling. This leaves strong peaks from the protons at the undeuterated sites, but also weak resonances from the coupling of these protons to those at low abundance (2% in the present case) at the deuterated sites. The weak resonances can be separated from the strong by recording spectra while spinning the sample at a series of angles to the magnetic field. Copyright 1997 Academic Press. Copyright 1997Academic Press