D. B. Chesnut

Molecular‐Orbital Treatment of Isotropic Hyperfine Interactions in Simple Aliphatic Radicals

The problem of isotropic hyperfine interactions in the EPR spectra of aliphatic free radicals is approached from the molecular‐orbital picture of hyperconjugation. The ethyl, methylethyl, and 1,1‐dimethylethyl radicals are treated by this approximation; with a reasonable choice of parameters, the results can be correlated rather well with our present knowledge of aliphatic radicals. The calculated coupling constants of methyl group hydrogens are of the order of 15 to 25 gauss, do not decrease radically with the presence of additional methyl groups, and are very nearly proportional to the molecular‐orbital unpaired electron density at the central carbon atom.

Chemical shifts and bond modification effects for some small first‐row‐atom molecules

Chemical shifts and shift changes with bond modification have been calculated for some small first‐row‐atom molecules employing a (6311/311/1)=[4s, 3p, d] heavy atom, (31)=[2s] hydrogen basis in the GIAO approach. Shifts and shift anisotropies using this intermediate size basis agree as well with experiment as other, more extensive basis sets; heavy atom shifts are determined reasonably well on an absolute basis and hydrogen shifts are acceptable on a relative scale. Nearly all shift first derivatives are negative with respect to bond lengthening as are the second derivative terms. These results suggest that in most cases a negative temperature coefficient for the chemical shifts of the heavy atoms will be observed, an effect which in fact does obtain in most cases known experimentally.

ESR Study of Morpholinium TCNQ Complexes

Electron paramagnetic resonance studies have been carried out on three morpholinium–TCNQ charge resonance salts of varying stoichiometry. The three salts which have been studied include the previously known 1:1 and 2:3 complexes as well as a new, relatively unstable salt believed to be a 3:4 complex. All three materials exhibit resonance behavior similar to that found in other triplet‐exciton‐containing systems. A phase transition in the 1:1 salt has been discovered at 151°C which is accompanied by only minor changes in the zero‐field splitting tensor parameters and principal axes. The morpholinium salts are characterized by an anomolous “impurity” resonance which occurs along with the zero‐field doublet(s). The behavior of this “impurity” signal as a function of temperature suggests that it may be a characteristic property of the system.

Nuclear magnetic resonance chemical shifts using optimized geometries

Isotropic chemical shifts and shift anisotropies for carbon, nitrogen, oxygen, and fluorine in first‐row‐atom molecules have been calculated in the perturbed Hartree–Fock gauge including atomic orbital scheme for both experimental and optimized molecular structures using a 6‐311G* basis for heavy atoms and a scaled 4‐31G basis for hydrogen. Structure optimization leads to the expected shortening of bond lengths, which is accompanied by an increase in the isotropic chemical shifts. The increased shifts show much improved agreement with gas phase experimental values for nitrogen and oxygen, while the results for carbon are only mildly affected and remain good; shift anisotropies for all species tend to decrease in magnitude and also generally improve. Fluorine is anomalous, its increasing shifts upon structure optimization moving further away from experiment at this level of basis set. The trend in the optimized isotropic shifts is explained in terms of the general tendency for atoms in the right‐hand porti...

A basis set study of the NMR chemical shift in PH3

Chemical shift calculations varying the heavy atom valence shell basis functions have been carried out for PH3 as a prototype molecule containing second row atoms. A triple valence split with two sets of d polarization functions, (66211/6211/11)=[5s,4p,2d], appears to be adequate for second‐row atoms in the GIAO‐SCF approach. Chemical shift calculations on SiH4, H2S, and HCl as well as PH3 employing this basis yield results in good agreement with experiment and with calculations of others involving larger bases. Bond length and bond angle shift derivatives in these hydrides are negative, continuing the general trend observed before for molecules containing first‐row atoms.

Instability of a Linear Spin Array: Application to Würster's Blue Perchlorate

A linear array of coupled spins is shown under appropriate conditions to be unstable with respect to alternation of the lattice parameter much like the Peierls instability of a one‐dimensional metallic lattice. A second‐order phase transition occurs, below which transition temperature the magnetic susceptibility decreases to zero. The model is discussed with respect to the analogous physical system of solid Wursters blue perchlorate (N,N dimethyl paraphenylenediamine perchlorate) where qualitative agreement is observed.

Exciton‐Controlled Proton Relaxation Times in Some Ion Radical Salts of TCNQ

Proton spin‐lattice relaxation times in several ion–radical salts of TCNQ have been measured over a range of temperature. The temperature dependence of T1 in these materials (which contain thermally activated triplet excitons) suggests that ψc, the correlation time of the relaxation mechanism, may be written as ψc−1 = ψ0−1ρ, where ρ is the exciton density per lattice site and ψ0 a time characteristic of the system. This assertion is supported by a very close agreement of correlation‐time activation energies and triplet‐exciton activation energies.

Half‐field resonance study of triplet exciton exchange dynamics in (O/AsCH3)+(TCNQ)2−

The theory has been derived and experiments carried out on the effect of triplet exciton exchange on the half‐field resonance in (O/AsCH3)+(TCNQ)2−. The half‐field resonance is a single Lorentzian line which exhibits a line broadening with increasing temperature in agreement with the effect seen at high field; the thermal activation energies at low and high field are 0.127 and 0.116 eV, respectively, but can not be distinguished statistically. The intensity behavior, however, exhibits an activation energy (effective singlet–triplet splitting) of 0.077 eV at low field compared to that observed at high field of 0.090 eV. This difference cannot be atttributed to a thermal modification of the triplet dipole tensor.The theory has been derived and experiments carried out on the effect of triplet exciton exchange on the half‐field resonance in (O/AsCH3)+(TCNQ)2−. The half‐field resonance is a single Lorentzian line which exhibits a line broadening with increasing temperature in agreement with the effect seen at high field; the thermal activation energies at low and high field are 0.127 and 0.116 eV, respectively, but can not be distinguished statistically. The intensity behavior, however, exhibits an activation energy (effective singlet–triplet splitting) of 0.077 eV at low field compared to that observed at high field of 0.090 eV. This difference cannot be atttributed to a thermal modification of the triplet dipole tensor.

A one‐dimensional model for exciton motional correlation effects in dense triplet exciton systems

A model for motional correlation effects of random‐walking particles with spin in one dimension is developed. For a system of particle site density ρ, the hard sphere interactions between particles are modeled by considering a single particle moving on a line segment of length ρ−1 undergoing perfectly elastic reflective collisions at the end points of the lattice; the collision rate is used to determine the spin exchange rate. The model is applied to proton spin–lattice relaxation in the MTPA(TCNQ)2 triplet exciton ion radical salt. Using mean theoretical estimates for the anisotropy parameters, a jump rate νj is calculated indicative of an activated process with an activation energy of approximately 0.12 eV ith a preexponential factor of the order of 1.5×10+14 Hz. Limitations and extensions of the model are discussed.

Resolution of the exchange anomaly in triplet exciton ion radical salts

The temperature dependence of the D and E zero‐field splitting parameters in the triplet‐exciton‐containing ion radical salt (φ3AsCH+3)(TCNQ)−2 has been measured at low temperatures and used to correct observed splittings at higher temperatures where triplet–triplet exchange is present. Analysis of the exchange effects from the thermally corrected splittings leads to an exchange activation energy (0.110 eV) and frequency factor (1.4×1011 Hz) in good agreement with those obtained from previous width measurements.