Gideon Fraenkel

Theoretical studies of heavy‐atom magnetic shielding in some small polyatomic molecules

Perturbed Hartree‐Fock calculations are performed on the molecules CH4, C2H2, C2H4, C2H6, CH3F, CH3OH, H2O, NH3, and C6H6 to obtain the heavy‐atom NMR shielding constants and magnetic susceptibilities. Slater‐type atomic orbitals (STOs) are used as the basis set. Various aspects of this model, including the dependence on basis‐set size and on the origin of the vector potential, are examined. The results are compared with the available experimental data and with other ab initio and semiempirical treatments.

NMR spectroscopy of benzocyclobutene and biphenylene

Abstract NMR parameters were determined for benzocyclobutene, six substituted benzocyclobutenes and biphenylene. Values calculated for the dihedral angles between the benzylic CH bonds of benzocyclobutene indicate that the four-membered ring is skewed. Analysis of the NMR spectrum of biphenylene shows the presence of two (and not three) ring currents.

On a Perturbed Hartree‐Fock Study of the Magnetic Susceptibility and the 13C and 17O NMR Shielding Constants in Formaldehyde with Slater Basis Sets

The theoretical study of Flygare et al. on one‐electron properties of the formaldehyde molecule is extended to include the paramagnetic susceptibility and the 13C and 17O NMR shielding constants and to investigate their sensitivity to Slater basis‐set quality. The Gaussian transform method is used to evaluate the necessary integrals. Using the fully coupled perturbed Hartree‐Fock formalism, we find that improvements in the wavefunction produce significant changes in most components of the second‐order properties. The best shielding calculations, obtained with a double‐zeta Slater set, are within the rather large experimental uncertainties, whereas the agreement between the theoretical and experimental susceptibility results is less satisfactory. The complete paramagnetic tensors are analyzed in terms of MO and AO contributions. Comparisons are made with other ab initio calculations and with simplified forms of the theory.

Modulation of Spin Echoes in Multi‐Half‐Spin Systems. I. Closed Formulas of Carr–Purcell Spin Echoes in Several AaBXx Systems

A density‐matrix approach to derive the effects of chemical shift and both homo‐ and heteronuclear spin coupling in a molecule on the Carr–Purcell spin‐echo (CPSE) train is developed. Closed formulas are derived for several multi‐half‐spin systems of the “AaBXx” type where a and x are arbitrary numbers of spins, and X is either the same nuclear species as A and B or different. As a special case of this type, the formulas are also obtained for both homo‐ and heteronuclear AA′XX′ systems. In heteronuclear multi‐half‐spin systems, the general equation is derived as a function of all the coupling constants. Features of the modulation patterns in “AaBXx” systems are discussed and the following relation is obtained: By introducing “effective internal chemical shifts” the modulation pattern of AaB resonance in a heteronuclear AaBXx system can be related to that in the homonuclear AaB system. Modulation patterns in homonuclear “AaBXx” systems are not summations of those in simpler spin systems and these facts ari...

Effects of ligands on ion-pairing behavior of benzylic lithium compounds

The 11 adduct of t-butyllithium and α-methylstyrene (II) has been generated in cyclopentane in the presence of a variety of ether and t-amine ligands as well as unsolvated, giving stable solutions in every case. NMR spectra of the solvated species are the same for all ligands but differ from that of the unsolvated compound. The results are consistent with a salt which contains a conjugated t-benzylic anion and exists as a loose ion-pair in the presence of ligands and as a tight ion-pair in cyclopentane alone. In contrast, benzyllithium behaves like a tight ion-pair in the presence of all ligands tried. Steric hindrance to tight ion-pairing at Cα of II is concluded to be responsible for the results observed. A covalently-bonded dilithium compound, 4,4-dimethyl-2-lithio-2-(p-lithiophenyl)pentane has been generated.

Condensed cyclobutane aromatic compounds—XXIII : Some transformation products of 1,2,5,6-dibenzocyclo-octatetraene

Abstract The reaction of bromine with 1,2,5,6-dibenzocyclo-octatetraene involves normal halogen addition to the olefinic linkages, rather than transannular addition as previously assumed. Both a very unreactive tetrabromide and a highly reactive dibromide can be obtained. Solvolysis of the dibromide gives derivatives of 1,2,5,6-dibenzocycloheptatriene. The structures of several transformation products of this dibromide which appear in the literature are revised. The usual geometry of the tetrabromide of dibenzocyclo-octatetraene is correlated with the observed NMR spectrum.

Some observations on the relationship between charge and 13C chemical shift in cyclohexadienylic anions and cations

Abstract Carbon-13 chemical shifts δ± in a variety of cyclohexadienylic cations and anions have been separated semi-empirically into contributions from (a) the hypothetical neutral framework, (b) charge effects and (c) interactions between ions and between ions and solvent molecules. Carbon-13 shifts for sites in neutral model compounds, δ0, approximate contribution (a) the shift differences for the model carbons δ±3,5-δ0 3,5 are assumed to reflect electric field effects (b). Thus what remains, δ±s-δ0s- (δ±3,5-δ03,5) comes from the charge. The summation of these terms for each of the ions comes to ca. 165 ppm. This shows that a changein charge of ±1 of the neutral model, with correction for the electric field, brings about a change in shift of 165 ppm, thus supporting the linearity of charge with shift.

Reaction of t-butyllithium with 1-phenylcycloalkenes

Abstract The course of reaction of 1-phenylcycloalkenes, Ph CCH(C H2)n with t-butyllithium in the presence of THF or TMEDA is found to vary with ring size as follows: n  2, polymerization; n  3, allylic deprotonation and addition to the double bond; n  4, allylic deprotonation only; n  5, metalation of the aromatic ring. These results are in accord with the stereochemistries of starting olefins.

Structure and Dynamic Behavior of Organolithium Compounds

The study of carbanions necessitates investigating organometallic compounds. Except for the highly conjugated ones, most carbanions are not free but exist as the partially negative part of some organometallic compound. Ideally, in order to learn about carbanions one should study organocesium or organorubidium compounds since these materials are most likely to contain carbanions. Unfortunately, such compounds do not form stable solutions; they either react violently with potential solvents or are inert and insoluble, as with alkanes.

Preparation of N,N,N′,N′-2,3-Hexamethyl-2,3-butanediamine

Abstract The title compound has been prepared by aminolysis (dimethylamine) of the corresponding hexamethyl aziridinium iodide.