ENDOR study of 1H couplings in single crystals of 2-aminoethyl hydrogen sulfate x-ray irradiated at 295 K. Radical geometry analysis using effective dipole center approximations and UHF-INDO calculations

Abstract

Single crystals of 2-aminoethyl hydrogen sulfate, H3N+CH2CH20SOs-, were X-irradiated at 295 K and studied using EPR, ENDOR, and FSE techniques. Among several radicals trapped at this temperature, two carboncentered radicals were identified. These are identical in structure but are formed in each of the two molecules constituting the asymmetric unit of the crystal, their common structure being H3N+CH2CHOSO3-. The aand j3-proton hyperfine coupling tensors of these two radicals show that one of them has a planar center, while the other exhibits a nonplanar site for the lone electron density. This is probably due to differences in the electrostatic interactions between each of the two radical centers and the lone pairs centered on their neighboring oxygen atoms. A revised model for obtaining the lone electron orbital spin density from the a-tensor in a nonplanar radical by considering a three-center approximation of the lone electron orbital is proposed. UHF-INDO calculations are carried out for a number of hypothetical radical configurations. The a-proton tensor is calculated for these geometries, and the three-center model is discussed in light of the results from the MO calculations.

EPR and ENDOR studies of single crystals of 2-oxazolidinone X-irradiated at 295 K

“…The lone electron orbital (LEO) will then acquire some 2s-orbital character that y An ENDOR line is superscripted with + if it belongs to the M s ¼ 1/ in turn will reduce the negative isotropic spin density at the a-proton. [24][25][26][27] This model is further explored using density functional theoretical (DFT) calculations below.…”

“…15 Refinements including a total of nine angles (y, f, and the starting angle for each plane) were made, using a non-linear refinement procedure converging to a minimum root mean square (rms) value for the complete data set. Providing conservative estimates for the measurement uncertainties, an error analysis was made using the method of propagation of errors 14,16 yielding standard errors for the principal values of the hfc tensors and for the components of the corresponding eigenvectors. The refined values of the measurement angles obtained from the fitting of the a-coupling were used throughout the analyses of the other couplings.…”

ESR and ENDOR Studies of Hippuric Acid Single Crystals X-Irradiated at 295 K: A Reinvestigation

“…Looking at the tensor data in Table , there clearly is a spread in the dipolar coupling of the two protons. The data in Table , however, suggest that this most probably is due to variations in the C−H bonding lengths and not indicative of bending at the α-carbon atom 3 Carbon 2p π Spin Densities Calculated from the Isotropic and Dipolar Components of the α-Proton Coupling Tensors of Radical 1, Measured at 77 K 6 and at 295 K (Table ) as Well as the Isotropic Spin Density Calculated Using the RHF/CI INDO MO Method a 77 K295 KRHF/CI INDO isotropic 0.713 0.717 0.708 dipolar 0.683 0.715 a All experimental values are mean values of the two tensors. …”

“…The data in Table 3, however, suggest that this most probably is due to variations in the C-H bonding lengths and not indicative of bending at the R-carbon atom. 18 3.1.2. Nitrogen Hyperfine Coupling.…”

EPR and ENDOR Studies of Single Crystals of α-Glycine X-ray Irradiated at 295 K

“…The discrepancy between these two values is again ascribed to a nonplanarity around the radical center so that the carbon lone electron orbital (LEO) exhibits a significant degree of 2s character. Sørnes et al showed that, under similar circumstances, the (orthogonal) hfc principal axis system is determined by the direction of the LEO and the external bisector of the N−C 2 −C 1 fragment. Thus, the eigenvector for the minimum principal value will not represent the true C−H bond direction, but the projection of this into the N−C 2 −C 1 plane.…”