Medium effects on the fluorine chemical shifts of non-polar molecules in liquids

The gas-to-infinite dilution fluorine chemical shifts of CF₄, SiF₄, SF₆, C₆F₆, p-fluorotoluene and p-difluorobenzene have been measured for a series of non-polar solvents. Downfield displacements ranging from 3–16 p.p.m. have been observed. Comparisons with proton solvent shifts make it evident that the London dispersion forces are the principal agent in causing the shifts. However, unlike proton resonance where the local diamagnetic shielding is mostly affected, it is probably through the local paramagnetic shielding that solvents alter fluorine chemical shifts.     

Conformational analysis of the chemical shifts for molecules containing diastereotopic methylene protons

Quantum chemistry SCF/GIAO calculations were carried out on a set of compounds containing diastereotopic protons. Five molecules, including recently synthesized 1,3-di(2,3-epoxypropoxy)benzene, containing the chiral or pro-chiral center and the neighboring methylene group, were chosen. The rotational averages (i.e. normalized averages with respect to the rotation about the torsional angle τ with the exponential energy weight at temperature T) calculated individually for each of the methylene protons in 1,3-di(2,3-epoxypropoxy)benzene differ by ca. 0.6 ppm, which is significantly less than the value calculated for the lowest energy conformer. This value turned out to be low enough to guarantee the proper ordering of theoretical chemical shifts, supporting the interpretation of the (1)H NMR spectrum of this important compound. The rotational averages of chemical shifts for methylene protons for a given type of conformer are shown to be essentially equal to the Boltzmann averages (here, the population-weighted averages for the individual conformers representing minima on the E(τ) cross-section). The calculated Boltzmann averages in the representative conformational space may exhibit completely different ordering as compared to the chemical shifts calculated for the lowest-energy conformer. This is especially true in the case of molecules, for which no significant steric effects are present. In this case, only Boltzmann averages account for the experimental pattern of proton signals. In addition, better overall agreement with experiment (lower value of the root-mean-square deviation between calculated and measured chemical shifts) is typically obtained when Boltzmann averages are used.     

Conformational dynamics of photoexcited conjugated molecules

Time-dependent photoexcitation and optical spectroscopy of pi-conjugated molecules is described using a new method for the simulation of excited state molecular dynamics in extended molecular systems with sizes up to hundreds of atoms. Applications are made to poly(p-phenylene vinylene) oligomers. Our analysis shows self-trapping of excitations on about six repeat units in the course of photoexcitation relaxation, identifies specific slow (torsion) and fast (bond-stretch) nuclear motions strongly coupled to the electronic degrees of freedom, and predicts spectroscopic signatures of molecular conformations.     

Fine-structure fluorescence of conjugated chain molecules

The fine-structure fluorescence spectra of conjugated chain compounds—distyrylbenzene (DSB), its fluorine-substituted derivative (FDSB), 1,4-bis-(5-phenyl-2-oxazolyl) benzene (POPOP), and 1,4-bis-(5-phenyl-2-oxadiazolyl) benzene (PDPDP)—are obtained at 4.2 K using the Shpolskii method. The multiplicity of the spectra is revealed and their vibrational analysis is performed involving Raman scattering data. It is shown that some vibrational frequencies of the fluorescence spectra are common for all the molecules under study, while other frequencies are specific for the spectra of the pairs DSB-FDSB and POPOP-PDPDP. The relation between the spectra of the molecules and their structure is considered.     

AB initio calculation of isotope shifts in BaII

Hartree-Fock and pseudo-relativistic Hartree-Fock calculations have been made for the 6s, 5d and 6p configurations of BaII and used to provide estimates of screening effects and specific mass isotope shifts for the 6p-5d transition.     

Third-order optical polarizability of conjugated organic molecules

We present a theoretical investigation of the π-electron contribution to linear and non-linear polarizabilities of conjugated chains. Using simple models the respective influence of chain length and bond alternation on optical non-linearities are analyzed. The results are applied to symmetrical cyanines and polyenes.     

On the magnetic properties of conjugated molecules

Recent criticisms of the gauge factors usually employed in the ‘test-dipole’ method for the calculation of ‘ring current’ chemical shifts in conjugated molecules, are discussed. It is shown that, in a simple semi-empirical scheme of the London-McWeeny type, insertion of a dipole contribution into the vector potential appearing in the gauge factor, whilst having no effect on the calculated ‘ring current’ intensities, is algebraically analogous (and, at large distances from ring centres, numerically equivalent) to estimating the secondary field at the origin due to a set of classical line currents, as discussed originally by Salem; these ‘line currents’ are of the same magnitude as the quantum-mechanical ‘bond currents’ implicit in the ‘ring currents’ calculated using the simpler gauge factors originally due to London, but their contributions to the secondary magnetic field experienced by the peripheral protons, are estimated classically. Extensive numerical comparison is made between experimentally-observed proton chemical shifts in some conjugated hydrocarbons, and secondary fields estimated by this semi-classical formalism, and its predictions are found to correlate as well with experiment as do those of the original McWeeny approach. It is concluded that any further illegitimacy involved in the procedure of inserting a dipole contribution into the gauge factor, is evidently quite simply compensated for, numerically, by an appropriate empirical parametrization. Such empirical parametrizations are also thought to absorb errors due to all the other various approximations of the ‘ring current’ theories (with apparently unwarranted efficiency), and they should, therefore, be treated with more scepticism than has previously been thought necessary.     

The calculation of potential energy curves of diatomic molecules: Application to halogen molecules

Methods for computing the vibrational turning points of diatomic molecules are summarized. An extension of the technique described by Vanderslice et al. is used to derive simple analytical expressions for calculation of the turning points from spectroscopic data. For most molecular states, the method gives results in agreement with those from sophisticated numerical procedures. Recently available spectroscopic data have been used to determine the potential energy curves for the¹∑⁺_g and ³Π(0⁺_u) states of Cl₂ and Br₂.     

On conjugated molecules with identical topological spectra

Properties of molecular graphs having identical topological spectra are investigated in some detail. It has been established that whole families of isospectral graphs can be derived from a molecular skeleton of vinyl benzene. A pair of isospectral graphs are obtained by an exchange of two non-equivalent residuals attached to particular sites of the skeleton. Additional pairs of isospectral graphs can be derived from related skeletons in which ortho positions of vinyl benzene are occupied by substituents. No constraints have to be imposed on the nature and form of the residuals or the substituting groups. This allows construction of larger isospectral systems including polymeric species. The theoretical foundation for the construction of isospectral graphs described in this work rests on the factorization of the eigenvalue problem of the adjacency matrix associated with molecular graphs which is analogous to a fragmentation of a secular determinant of a Hückel MO problem as outlined some time ago by Heilbronner. A formal proof of the procedure, expressed in a symbolic form, is presented in the Appendix. A discussion of some interesting features of isospectral systems reflected in special properties of topological eigenfunctions is presented.     

NMR chemical shifts in cuprous salts: The magnetic moment of Cu

An extensive study has been made of chemical shifts of the ⁶³Cu resonance in a number of cuprous salts. The observed trend in chemical shifts for the cuprous halides is in disagreement with theoretical ionicity estimates of either Pauling or Phillips. This trend has been explained by taking into account the relative importance of π bonding for the different halides. On the basis of the present data, CuBr is the most suitable Knight shift reference compound. The measurements indicate μ = 2.2206 nm for ⁶³Cu and μ = 2.3791 nm for ⁶³Cu for reference moment, without diamagnetic correction.     

Geometry-dependent electronic properties of highly fluorescent conjugated molecules

We present a combined experimental/theoretical study of the electronic properties of conjugated para-phenylene type molecules under high pressure up to 80 kbar. Pressure is used as a tool to vary the molecular geometry and intermolecular interaction. The influence of the latter two on singlet and triplet excitons as well as polarons is monitored via optical spectroscopy. We have performed band structure calculations for the planar poly(para-phenylene) and calculated the dielectric function. By varying the intermolecular distances and the length of the polymer repeat unit the observed pressure effects can be explained.     

Translation invariant calculation of nonperturbative quarkonium mass shifts

The mass shifts of $\text{q}\text{q}$ bound states in the nonperturbative QCD vacuum are calculated by applying field theoretic perturbation theory as derived within the formalism of the Bethe-Salpeter equation. We obtain corrections to heavy quarkonium masses of the order of, but somewhat smaller than those of previous authors, who employed ordinary perturbation theory of nonrelativistic quantum mechanics.     

13C contact shifts of diamagnetic solvent molecules

In order to study the hyperfine effects induced by molecular vibration, we expand the expression given by Moss [5] for the molecular hamiltonian, in order to include all relevant interaction terms involving nuclear magnetic moments. The transformation from space-fixed coordinates to molecular coordinates is done directly.     

Charge transfer and chemical shifts in zincblende compounds

Charge transfer and chemical shifts for some zincblende compounds are calculated using the electrostatic model of Shevchik et al.^3,4. A comparison of the results obtained from Phillips ionicities, the bond orbital model of Harrison and ionicities, recently proposed⁹, is performed. The predictions from the latter scale are in sufficient agreement with the experimental data. Additionally chemical shifts for some zincblende compounds are predicted, for which experimental data are not available.     

Properties of the self-consistent field treatment of conjugated molecules

The polarizability properties of π electron systems are treated by the self-consistent field method, as modified by Pople for application to conjugated molecules. An analytical treatment of the π electron configuration of substituted hydrocarbons deduces certain universal symmetry properties of this method, and contains the Hückel treatment of inductive substituents as a simplified special case. Whereas the usual Hückel theory derives results by approximate perturbation methods, the treatment given here is formally exact. In a later section s.c.f. polarizability coefficients, as calculated on an Elliott 402 digital computer, are given for benzene, naphthalene, anthracene and phenanthrene, and the predictions of chemical properties are shown to be in better agreement with experiment than predictions derived from Hückel theory.