Hyperfine Interactions In Molecules Research Articles

Hyperfine interactions in molecules through laser spectroscopy

The infrared and millimeter wave spectroscopy, laser Stark spectroscopy, and beam maser spectroscopy of CH3CN and its isotopic species will be discussed. The beam maser spectroscopy and hyperfine structure of molecules like NCCCD, ClCCD and CH3CCH are reviewed. The laser magnetic resonance and hyperfine structure in CF, CH and CH2 free radicals will be discussed. The Lamb dip spectroscopy and laser-induced fluorescence in I2 involving theB 3Π(0 u + ) state are reviewed with special reference to its hyperfine structure. The splitting of the rotational levels of N 2 + in itsX 2Σ g + andB 2Σ u + states due to hyperfine interactions (along with the hyperfine structure) in laser-induced fluorescence in theB−X transition is discussed. Recent results obtained in the laser photo-acoustic spectrum of ICl in the transitionX 1Σ+ −A 3Π1 will be presented and the possibility of the use of this technique in studying the hyperfine structure will be discussed.

Study of hyperfine interactions by Mössbauer spectroscopy in mixed crystals

A mixed crystal technique, appropriate for the Mössbauer study of hyperfine interactions in molecules with D > 0, is described. It is based on the magnetic behavior of a |± 1 2 〉 Kramers doublet. The method has been applied with success to the study of hyperfine structure of Fe(methdtc) 2Br.

Nuclear self coupling via the hyperfine interaction in molecules

Nuclear self coupling via hyperfine interaction occurs in molecules as well as in metals. To obtain a finite result for this coupling, the effect of vibrational modes will have to be taken into account in molecules.

On the Theory of Spin-Orbit and Hyperfine Interactions in Molecules. Application to the Hydrogen Molecule—Ion

Previously, calculations on molecular systems have employed spin-orbit and hyperfine operators which possess strong singularities at the nuclei of the molecule. The matrix elements of the latter operators may, depending on the symmetries of the wave functions, involve divergent integrals. The spin-orbit and hyperfine interactions are re-examined. The correct operators are derived from a reduction of the Dirac equation. It is shown that the correct forms of the spin-orbit and hyperfine operators have finite and unique matrix elements. The theory is specifically applied to the hydrogen molecule—ion. It is found that the anisotropic part of the spectroscopic splitting factor is very small. The dipole—dipole coupling parameter D is calculated for the ground state of H2+. At the equilibrium internuclear distance, D=−0.1067. The latter result does not agree with the result of other authors who, by using the strongly singular operators, found a positive sign for D.

Theory of Isotropic Hyperfine Interactions in π-Electron Radicals

Indirect proton hyperfine interactions in π-electron radicals are first discussed in terms of a hypothetical CH fragment which holds one unpaired π electron and two σ-CH bonding electrons. Molecular orbital theory and valence bond theory yield almost identical results for the unpaired electron density at the proton due to exchange coupling between the π electron and the σ electrons. The unrestricted Hartree-Fock approximation leads to qualitatively similar results. The unpaired electron spin density at the proton tends to be antiparallel to the average spin of the π electron, and this leads to a negative proton hyperfine coupling constant. The theory of indirect proton hyperfine interaction in the CH fragment is generalized to the case of polyatomic π-electron radical systems; e.g., large planar aromatic radicals. In making this generalization there is introduced an unpaired π-electron spin density operator, ρN, where N refers to carbon atom N. Expectation values of the spin density operator ρN are called ``spin densities,'' ρN, and can be positive or negative. In the simple one-electron molecular orbital approximation a π-electron radical always has a positive or zero spin density at carbon atom N, 0≤ρN≤1. In certain π-electron radical systems; e.g., odd-alternate hydrocarbon radicals, the spin densities at certain (unstarred) carbon atoms are negative when the effects of π—π configuration interaction are included in the π-electron wave function. The previously proposed linear relation between the hyperfine splitting due to proton N, aN, and the unpaired spin density on carbon atom N, ρN,aN=QρNis derived under very general conditions. Two basic approximations are necessary in the derivation of this linear relation. First, it is necessary that σ—π exchange interaction can be treated as a first-order perturbation in π-electron systems. Second, it is necessary that the energy of the triplet antibonding state of the C–H bond be much larger than the excitation energies of certain doublet and quartet states of the π electrons. This derivation of the above linear relation makes no restrictive assumptions regarding the degree of π—π or σ—σ configuration interaction. The validity of the above approximations is discussed and illustrated by highly simplified calculations of the proton hyperfine splittings in the allyl radical, assuming the π—π configuration interaction—and hence the negative spin density on the central carbon atom—to be small. Isotropic hyperfine interactions in molecules in liquid solution can also arise from spin-orbital interaction effects, and it is shown that these effects are negligible for proton hyperfine interactions in aromatic radicals.