A Time-Resolved Electron Paramagnetic Resonance Investigation of the Spin Exchange and Chemical Interactions of Reactive Free Radicals with Isotopically Symmetric (¹⁴N−X−¹⁴N) and Isotopically Asymmetric (¹⁴N−X−¹⁵N) Nitroxyl Biradicals

Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl radicals (N−X−N) were studied by time-resolved electron paramagnetic resonance (TR-EPR). Reactive spin-polarized free radicals (r#), with non-Boltzmann population of spin states were produced by laser flash photolysis of benzil dimethyl monoketal or of (2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (the superscript # symbol indicates electron spin polarization). Both isotopically symmetric nitroxyl biradicals (14N−X−14N) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing 15N and the other nitroxyl bearing 14N (14N−X−15N), were employed as probes of the spin exchange and chemical interactions between r and the nitroxyl biradicals. The interaction of r# with the asymmetric ortho-nitroxyl biradical (14N−O−15N), which exists in a condition of strong spin exchange, proved to be particularly informative. In this case, spin polarized (14N−O−15N)# (product of spin exchange with r#) and two polarized monoradicals (r14N−O−15N)# and (14N−O−15Nr)# (products of chemical reaction with r#) were observed. The latter three species possess three distinct TR-EPR spectra with different line splittings. The relative cross sections for spin exchange (Rex) and chemical reaction (Rrxn) were achieved through computer simulation of the TR-EPR spectra. The cross section for spin exchange, Rex, between r# and (N−X−N) biradical is estimated to be 4−6 times larger than the cross section of chemical reaction, Rrxn, between r# and (N−X−N). The para-nitroxyl biradical (14N−P−15N) exists in weak spin exchange, and behaves as an equimolar mixture of 14N and 15N mononitroxyls

Characterization of the External Surface of Silicalites Employing Electron Paramagnetic Resonance^†^,^‖

Sensitive and structurally specific methods for investigating silicalite external surface have been developed using electron paramagnetic resonance (EPR). The absorption of an EPR silent probe ortho-methyldibenzyl ketone (oMeDBK) (4 in Scheme ) on the external surface of a series of monodisperse silicalite crystals was studied using an initially coadsorbed EPR active nitroxide probe. The displacement of the initially adsorbed nitroxide probe by coadsorbed oMeDBK shows that the adsorbate molecules first adsorb on stronger binding sites characterized by slow rotational motion of the probe (broad EPR lines) and after the stronger sites are saturated, the displaced EPR probe molecules adsorb on weaker binding sites characterized by fast rotational motion of the probe (narrow EPR lines). The transition point from slow to fast rotational motion provides a quantitative measurement of the stronger binding sites on the silicalite external surface area and the external surface area of silicalite crystals. The adsorption strength is molecular structure-dependent, and polar functional groups provide significant contribution to the binding strength. Sequential adsorption of 14N and 15N spin-labeled nitroxides shows the presence of the dynamic exchange between the adsorbates on the strong binding sites and those in solution or on the weak binding sites, while concurrent coadsorption of 14N and 15N spin-labeled nitroxides provides another sensitive means of studying the molecular structural dependence of the binding strength

Photoactive Additives for Cross-Linking Polymer Films: Inhibition of Dewetting in Thin Polymer Films

In this report, we describe a versatile photochemical method for cross-linking polymer films and demonstrate that this method can be used to inhibit thin polymer films from dewetting. A bifunctional photoactive molecule featuring two benzophenone chromophores capable of abstracting hydrogen atoms from various donors, including C−H groups, is mixed into PS films. Upon exposure to UV light, the bis-benzophenone molecule cross-links the chains presumably by hydrogen abstraction followed by radical recombination. Photoinduced cross-linking is characterized by infrared spectroscopy and gel permeation chromatography. Optical and atomic force microscopy images show that photocrosslinked polystyrene (PS) thin films resist dewetting when heated above the glass transition temperature or exposed to solvent vapor. PS films are inhibited from dewetting on both solid and liquid substrates. The effectiveness of the method to inhibit dewetting is studied as a function of the ratio of cross-linker to macromolecule, duration of exposure to UV light, film thickness, the driving force for dewetting, and the thermodynamic nature of the substrate

Distance-Dependent Paramagnet-Enhanced Nuclear Spin Relaxation of H₂@C₆₀ Derivatives Covalently Linked to a Nitroxide Radical

A series of H₂@C₆₀ derivatives covalently linked to a nitroxide radical has been synthesized. We report distance-dependent nuclear spin relaxivity of H₂ in these derivatives. The results clearly indicate that the relaxivity of H₂ is distance-dependent and in good agreement with the Solomon−Bloembergen equation, which predicts a 1/<i>r</i>⁶ dependence

Synthesis, Isomer Count, and Nuclear Spin Relaxation of H₂O@Open-C₆₀ Nitroxide Derivatives

H₂O@C₆₀ derivatives covalently linked to a nitroxide radical were synthesized. The ¹H NMR of the guest H₂O revealed the formation of many isomers with broad signals. Reduction to the diamagnetic hydroxylamines sharpened the ¹H NMR signals considerably and allowed for an “isomer count” based on the number of observed distinct signals. For H₂O@K-8, 17 positional isomeric nitroxides are predicted, not including additional numbers of regioisomers; indeed, 17 signals are observed in the ¹H NMR spectrum

Observations of Interfacial Population and Organization of Surfactants with Sum Frequency Generation and Surface Tension

The complementary interface-sensitive methods of sum frequency generation (SFG) and surface tension have been used to investigate surfactants at the air/water interface. SFG yields information on the density and orientation of surfactants at the interface, whereas surface tension yields information on the surface excess of surfactants, which, however, is not their interfacial density. The rapid rise in the SFG signal to its maximum value at 1 mM, which remained essentially constant thereafter, was attributed to the formation of large surfactant domains. Above 1 mM, the domains, all of which have the same density, dominated the SFG signal. As a consequence, the SFG signal remained essentially constant. The surface excess had an equally rapid rise to its maximum value at 1 mM, which remained constant until reaching a concentration of 3 mM, where it dropped to a zero value, indicating that a full surfactant monolayer had been formed. This coincides with the formation of bulk micelles, which is commonly referred to as the critical micelle concentration. The orientation of the surfactant carbonyl chromophore was obtained from polarization measurements of the SFG signal and showed a small change at and above a 1 mM concentration. The SFG results and the surface tension results, though inherently different, were found to be consistent with each other

Paramagnet Enhanced Nuclear Spin Relaxation in H₂O@Open‑C₆₀ and H₂@Open‑C₆₀

Relaxation rates of <i>endo</i>-H₂O in H₂O@Open-C₆₀ in the presence of a nitroxide radical and of their nitroxide derivatives have been measured and are compared with effects for <i>endo</i>-H₂ in similar cages. <i>T</i>₁ relaxation enhancement of the <i>endo</i>-H₂O and H₂ induced by either intra- or intermolecular interaction is relatively insensitive to the presence of a cage opening. Enhancement of intermolecular relaxation is observed, however, when the cage opening has an OH group

Fundamental Optical Properties of Linear and Cyclic Alkanes: VUV Absorbance and Index of Refraction

VUV absorbance and index of refraction data for a series of linear and cyclic alkanes have been collected in order to understand the relationship between the electronic excitation wavelength (or absorbance edge), index of refraction, and molecular structure. The absorbance edge and index for a homologous series of both linear and cyclic alkanes increase with increasing carbon number. The optical properties of complex cycloalkanes do not vary predictably with increasing carbon number but instead depend on variations in the hydrocarbon structure in addition to hydrocarbon size. An understanding of the fundamental optical properties of this class of compounds is directly applicable to the identification of a high index and low-absorbance fluid for 193 nm immersion lithography

ENDOR Evidence of Electron–H₂ Interaction in a Fulleride Embedding H₂

An endofulleropyrrolidine, with H₂ as a guest, has been reduced to a paramagnetic endofulleride radical anion. The magnetic interaction between the electron delocalized on the fullerene cage and the guest H₂ has been probed by pulsed ENDOR. The experimental hyperfine couplings between the electron and the H₂ guest were measured, and their values agree very well with DFT calculations. This agreement provides clear evidence of magnetic communication between the electron density of the fullerene host cage and H₂ guest. The <i>ortho-H</i>_<i>2</i>/<i>para-H</i>_<i>2</i> interconversion is revealed by temperature-dependent ENDOR measurements at low temperature. The conversion of the paramagnetic <i>ortho-H</i>_<i>2</i> to the diamagnetic <i>para-H</i>_<i>2</i> causes the ENDOR signal to decrease as the temperature is lowered due to the spin catalysis by the paramagnetic fullerene cage of the radical anion fulleride

Stereochemical Features of the Physical and Chemical Interactions of Singlet Oxygen with Enecarbamates

Oxazolidinone-substituted enecarbamates represent a mechanistically rich system for the study of stereoelectronic, steric, and conformational effects on stereoselectivity and mode selectivity in 1O2 [2 + 2] and ene reactions. Photooxygenation of these enecarbamates with 1O2 leads to diastereomerically pure dioxetanes that decompose to yield an oxazolidinone carbaldehyde and one of the two enantiomers of methyldesoxybenzoin in enantiomeric excess. Stereoselectivity originates at the allylic stereocenter, a result supported by quenching studies, computational analysis, and deuterium solvent isotope effects