Lawrence C. Snyder

Spectra of porphyrins: Part II. Four orbital model

Abstract Porphyrin molecules are treated by combining an LCAO-MO (Huckel) calculation with a simplified treatment of configuration interaction. The Huckel calculation requires special parameters α N = α C + 2 β CC and β CN = 0.5 β CC for certain internal porphyrin nitrogen atoms. The configuration interaction parameters are determined on Zn tetraphenylporphin. Calculations are carried out for reduced porphyrins, azaporphins, and benzporphins. Spectra are predicted, and chemical and magnetic properties are discussed.

ESR of the Triplet States of Randomly Oriented Molecules

The observation of the Δm = 1 transitions of the triplet states of randomly oriented molecules by electron spin resonance (ESR) is discussed. The shape of the absorption curve permits the ready determination of the zero‐field parameters from the spectrum. In a derivative presentation, the widths of the lines observed give direct information on environmental and hyperfine broadening. Computer simulations of triplet spectra are presented. Good agreement is found with the observed spectra of phosphorescent and ground‐state triplets.

Analysis of Nuclear Magnetic Resonance Spectra of Molecules in Liquid‐Crystal Solvents

A convenient method to analyze the high‐resolution nuclear magnetic resonance spectra of molecules dissolved in liquid‐crystal solvents is described and related to basic theory of magnetic resonance. The method is applied to analyze the proton and fluorine NMR spectra of monofluorobenzene in a nematic liquid‐crystal solvent. Parameters deduced from this analysis and describing the anisotropic molecular motion, the direct magnetic dipole—dipole interactions Dij, the indirect spin—spin couplings Jij, and the anisotropy of the diamagnetic shielding constant of F in the C–F bond are summarized and compared with the values for benzene and hexafluorobenzene in the same solvent. It is shown that agreement of experimental and computer‐simulated theoretical spectra is probably limited by the accuracy of relative bond lengths in the assumed geometry for fluorobenzene. It is concluded that for many molecules having a large number (N≥3) of spin‐½ nuclei, it is possible in the analysis of these spectra to reject incor...

Unrestricted Hartree—Fock Calculations. I. An Improved Method of Computing Spin Properties

Spin properties calculated using a spin‐polarized single‐determinant wavefunction in the unrestricted Hartree—Fock method can be unreliable because the wavefunction is not a spin eigenstate and contains unwanted spin components. New expressions are derived for the spin properties after the most important of these has been removed by an annihilator. Expressions are given for 〈S2〉 and for the spin‐ and charge‐density functions, all in terms of the unrestricted bond‐order matrices for electrons of α spin and of β spin. The cases of orthogonal and nonorthogonal atomic orbitals are included. The role of corresponding orbitals in the derivations is outlined. The results are illustrated by calculations on the pi‐electron radicals, allyl, pentadienyl, naphthalene positive and negative ions, and the lowest‐energy naphthalene triplet configuration. An estimate is made of the error due to the unwanted components which still remain in the wavefunction, and this will usually be small. A simple, approximate expression ...

Unrestricted Hartree—Fock Calculations. II. Spin Properties of Pi‐Electron Radicals

Unrestricted Hartree—Fock wavefunctions have been computed for a large number of conjugated‐hydrocarbon pi‐electron radicals. Pi‐electron spin‐density and charge‐density functions have been computed with and without annihilation of the major contaminating spin multiplet in the wavefunction.Three different empirical relations have been used to relate our results to proton isotropic hyperfine splittings measured in ESR experiments. These are the simple McConnell expression; the Colpa and Bolton relation which includes a term depending on the excess charge density at the carbon atom; and that due to Giacometti, Nordia, and Pavan which introduces a contribution from the spin densities in the bonds between the carbon and its nearest neighbors. When the Pariser—Parr method with semiempirical electron‐repulsion integrals is employed in the unrestricted Hartree—Fock calculation, then the experimental splittings are almost always bounded by those computed with spin densities before and after the annihilation of th...

Molecular Structure of Cyclobutane from Its Proton NMR in a Nematic Solvent

The protonnuclear magnetic resonancespectrum of cyclopropane has been measured in nematic p,p′‐di‐n‐hexyloxyazoxybenzene. The spectrum has been analyzed by determining the parameters of a spin Hamiltonian which permit its computer simulation. Several ways to determine the structure of cyclopropane from the elements of the spin Hamiltonian are employed, with all giving very similar geometric parameters. With the assumption of D 3h symmetry for cyclopropane, and a C–C bond length of 1.510 A determined by electron diffraction, we are able to fit the NMRspectrum with a structure having a C–H bond length of 1.123 A and an HCH angle of 114.4°, which are to be compared with the corresponding gas phase electron diffraction values of 1.089±0.003 A and 115.1±1°. The anisotropic motion corresponds to the molecular plane tending to be parallel to the applied magnetic field. For indirect spin—spin couplings we find J HH(cis)=+9.5±1 Hz, J HH(trans)=+5.5±1 Hz, and a positive sign for J CH, the indirect coupling between 13C and a directly bonded proton. The effects on the observed spectrum of J HH(gem) and J CH′, the indirect coupling of carbon 13 to a proton on another carbon, are too small to permit their determination. The shortness we find of the C–C bond relative to the C–H bond length is interpreted as a result of molecular vibrations. It appears that the analysis of NMR in nematic media provides an important method of precise molecular structure determination.

ESCA: chemical shifts of K-shell electron binding energies for first-row atoms in molecules

Abstract Chemical shifts of k-shell electron binding energies for first-row atoms in molecules due their different chemical environments have been obtained from double-zeta basis SCF-MO calculated ground state 1s orbital energies using Koopmans theorem. This computational procedure is valid because it is known that the energy quantities neglected thereby are either approximately invariant for a given atom, or small. The calculated chemical shifts are characteristics of an atom in a functional group, show a linear relationship with net charge, and agree with experimental numbers in the few cases where comparison is possible.

Anisotropy of Nuclear Spin‐Spin Coupling in Methyl Fluoride

Equations for indirect nuclear spin coupling tensors are developed within the framework of Hartree‐Fock perturbation theory. Calculated values for CF, CH, HF, and HH spin‐spin coupling anisotropies (pseudodipolar couplings) in methyl fluoride are compared with the results suggested by recent experimental work. An estimate of the importance of including pseudodipolar coupling in the determination of molecular geometry by NMR in liquid crystals is made.

Nuclear Magnetic Resonance Spectroscopy of Bicyclobutane

The proton and carbon‐13 nuclear magnetic resonance spectra of bicyclobutane have been analyzed in detail. The use of information from NMR spectra of bicyclobutane in a nematic solvent permits an unambiguous assignment of all chemical shifts and spin–spin interactions to specific nuclei. Some of the assignments differ from previously published ones. A long‐range indirect spin–spin coupling of 5.9 Hz is found between the two exo protons, and an indirect coupling of 16.0 Hz is observed between the exo proton and the carbon‐13 to which the other exo proton is bonded.

Electronic Structure of Thymine

The electronic structures of the ground and low‐lying excited states of thymine and the anions 1‐HT− and 3‐HT−, derived by removing a proton from nitrogen atom N3 and N1, respectively, have been described by all‐electron all‐integral LCAO–SCF calculations using Gaussian basis functions. In accord with experiment, we find the carbonyl bonds weakened in the anions relative to thymine because of a decrease in their overlap charge densities. On the other hand, our calculations and interpretation of the observed optical and ESR spectra suggest that both for T and 1‐HT− the lowest singlet and triplet π → π* states are very similar and localized in the region of the ethylenic bond of thymine.