Alvin L. Kwiram

E.S.R. spectra of matrix isolated alkali atom clusters

E.S.R. spectra assigned to Na3 molecules have been obtained by codepositing sodium atoms and diluent argon on a sapphire surface loosely coupled to a liquid helium cryostat and warmed by a heat leak, to allow aggregation before the alkali atoms are frozen into the argon matrix. The trimer spectra under these conditions were much more intense than the residual atom spectra. The trimer spectra indicate the unpaired electron in these molecules has predominantly s rather than p character. Approximately 95 per cent of the 3s spin density is equally distributed between two of the alkali atoms, with only about 7 per cent of the spin density on the third atom. The trimers thus are not merely van der Waals adducts but involve chemical bonding. Both a covalent molecular orbital model for linear or obtuse isosceles geometry and an ionic charge-transfer model giving M2 +M- or M+M2 - appear qualitatively consistent with the spectra.

Zero field splitting of the triplet state of porphyrins. II

The zero field splitting (zfs) for porphyrin is calculated for possible transitions b2u(π), a1u(π), a2u(π) → eg(π*). Calculations are done with pi−electron orbital coefficients determined from extended Huckel calculations on the deprotonated dianion, the free base, and the acid dication. The spin dipole−dipole couplings are evaluated exactly using a Gaussian−lobe expansion. The calculations are compared to the reported zfs of zinc and free base porphin and to the zfs reported herein of Zn etioporphyrin in various lattice sites in an octane matrix and of Zn tetraphenylporphin. The calculations fit experimental D and E values within 10% under the following assumptions: (i) For Zn etioporphyrin and Zn porphin the triplet state is 3[a1u(π) → eg(π*)], but for Zn tetraphenylporphin it is 3[a2u(π) → eg(π*)]; (ii) the larger value for D observed in free base porphin is due to configuration interaction between these two transitions; (iii) variable axis directions set up by the lattice crystal field cause the varia...

ESR of matrix isolated alkali superoxides

Abstract ESR spectra of NaO2, KO2, RbO2, and CsO2 in rare gas matrices are consistent with an ionic M+O2− model of isosceles symmetry, in accord with recent vibrational spectra. However, the ESR for CsO2 also shows evidence for an inversion of the uppermost occupied molecular orbitals which can be attributed to a slight covalent mixing of the oxygen valence orbitals with the inner-shell p-orbitals of the metal.

Fast computer calculation of ESR and nonlinear spin response spectra from the fast motion to the rigid lattice limits

Abstract Fast computer simulation of the electron spin resonance and adiabatic rapid passage spectra of spin labels characterized by rotational correlation times ranging from the fast motion to the rigid lattice limits is demonstrated. Calculations are based upon a modification of the stochastic Liouville equation for the density matrix which explicitly includes interaction of the spins with applied radiation and modulation fields. Several mathematical simplifications of previous calculations are demonstrated, permitting computation with core and CPU requirements compatible with small computers.

Triplet state properties of pyridine derivatives

The phosphorescence and zero‐field splittings have been systematically investigated for a number of methyl, amino, and cyano derivatives of pyridine in both the neutral and protonated forms. In pyridinium and the methylpyridines, decay from the triplet state appears to have mechanisms similar to those in benzene. Interaction of lowest ππ* triplets with low‐lying 3nπ* states appears to have little effect on zfs, but nonradiative decay rates are distinctly affected. Cyanopyridines and aminopyridines offer less of a comparison to pyridinium. An nπ*‐type configuration may contribute to large D′s for protonated cyanopyridines. Charge‐transfer effects are large in 4‐aminopyridine, although the 2‐ and 3‐aminopyridines apparently do not show significant charge‐transfer character.

Wavelength dependence of the zero-field splittings in the triplet state of tryptophan☆

Abstract It is shown that abrupt discontinuities occur in the zero-field splitting parameters of the isolated amino acid, tryptophan, and its derivatives as a function of the wavelength of the optically detected emission from the excited triplet state. These discontinuities are attributed to the convolution of the magnetic transitions associated with each of the elementary vibronic components assuming that the frequency of the magnetic transition is a linear function of the wavelength for each elementary vibronic component.

Molecular and applied modulation effects in electron—electron double resonance. III. Bloch equation analysis for inhomogeneous broadening

Abstract The dependence of electron—electron double resonance (ELDOR) reduction factors, obtained employing field or frequency swept modes of operation and dispersion or absorption modes detection, upon experimental condition such as Zeeman modulation frequency, Zeeman modulation phase, and pump microwave power and upon molecular conditions such as homogeneous and inhomogeneous broadening of the hyperfine lines are experimentally and theoretically investigated. Experimental studies were carried out on solutions of 2,2,6,6-tetramethyl-4-piperidenol-1-oxyl and perdeuterio-2,2,6,6-tetramethyl-4-piperidone-oxyl in sec-butylbenzene at −63 °C. For 1 × 10−3M tetramethylpiperidinol spin label, the methyl and methylene proton nuclear relaxation rates are significantly longer than nitrogen (14N) nuclear relaxation rates that the electron spin resonance (ESR) hyperfine lines demonstrate the characteristics of inhomogeneous broadening. Frequency swept dispersion and absorption ELDOR linewidths are observed to correspond to the linewidth (approximately one megahertz between points of maximum slope) of an ESR spin packet or superhyperfine line as compared to the ESR envelope or hyperfine linewidth (approximately four megahertz for the mI(14N) = +1 (low field) line when a peak to peak modulation field of 0.24 gauss is employed). Moreover, field dispersion ELDOR reduction factors are observed to depend strongly upon modulation frequency (in contrast to the behavior for homogeneous lines reported earlier), with reduction factors decreasing with modulation frequency. Theoretical calculations based upon a modified (interaction of the spins with Zeeman modulation and multiple radiation fields is introduced) decreasing equation Bloch treatment quantitatively reproduce the experimental results when interaction of the unpaired electron with nearby protons as well as with 14N is taken into account.

Triplet state properties of pyridinium

Abstract Pyridinium phosphorescence originates at 334 nm and has a lifetime of 3.5 s; the quantum yield is roughly 0.04. The ODMR spectrum gives a zero-field splitting of |D| = 0.134 cm −1 , |E| = 0.030 cm −1 . Implications for the lowest triplet state of pyridine are discussed.

Structural study of sulfamic acid at 4.2 K by ENDOR‐detected NMR

The electron–nuclear double resonance method (ENDOR) is used to obtain greatly enhanced NMR signals from the distant deuterium nuclei of perdeuterated sulfamic acid (ND3SO3) in single crystals at 4.2 K. The structural information is obtained by measuring the quadrupole coupling tensors of the deuterium atoms and their direction cosines. There is very good agreement between the N–D bond angles derived from these ENDOR‐detected NMR (EDNMR) results and bond angles from neutron diffraction data at 78 K. Relationships between the deuterium quadrupole constants and N–D and D⋅⋅⋅O bond lengths are derived and discussed.

Triplet state decay and spin-lattice relaxation rate constants in tyrosinate and tryptophan

Abstract The decay rates and spin-lattice relaxation rates are presented for the lowest excited triplet state in tryptophan and in tyrosinate in zero-field at 1.34 K.