Advantages of deuterium modification of nitroxide spin labels for biological epr studies

The spin label, perdeuterio-N-(1-oxy 1-2,2,6,6-tetramenthyl-4-piperidinyl)maleimide (DMSL) was synthesized and its EPR and saturation transfer EPR spectra were compared to those of the hydrogen analogue, HMSL- The labels were studied as freely tumbling entities and also bound to bovine serum albumin (BSA). Significant gains in spectral resolution and detectability were observed for DMSL relative to HMSL.     

Effects of geometric distortions and substituents on rates of intersystem crossing; diazomethane and pyrazoline

We report a qualititive perturbation-theoretic analysis of the effects of substituents on the spin-orbit coupling and on the density of states in diazomethane and pyrazoline systems. The importance of the form of the distortion which accompanies approach to the singlet-triplet crossing zone is also discussed with the aim of determining when reactions involving a spin multiplicity change can compete in rate with spin-conserving reactions.     

A theoretical approach to substituent effects: Stabilities and conformational preferences in β-substituted ethyl radicals

Ab initio molecular orbital theory is used to study substituent effects in a series of β-substituted Et radicals XCH₂CH₂. For X = BH₂ (plan.), CH₃, NH₂, OH and F, only slight conformational preferences and weak stabilizations are indicated. Such behaviour may be rationalized, using a PMO model, in terms of opposing changes, accompanying variation in X, in positive and negative hyperconjugation between the XCH₂ group and the CH₂ centre. On the other hand, for groups containing an appropriately oriented, low-lying vacant orbital, viz. X = Li, BeH and BH₂ (perp.), there is a pronounced preference for the perpendicular conformation of the radical. This is attributed to 1,3-interaction between the singly-occupied 2p(C) orbital and the vacant 2p(X) orbital.     

A theoretical approach to substituent effects: Interaction between directly bonded groups in the isoelectronic series XNH3+, XCH3, and XBH3−

Ab initio molecular orbital theory including full geometry optimization at the 4-31G level is used to examine the interactions between substitutents X(X = Li, BeH, BH₂, CH₃, NH₂, OH and F) and substrates Y(Y = NH₃⁺, CH₃, BH₃^?) in the isoelectronic series XNH₃⁺, XCH₃ and XBH₃^?. The results indicate that the interaction energies are dominated by σ-effects. NH₃⁺ is found to interact favorably with the σ-donors (e.g. Li, BeH and BH₂) and unfavorably with the σ-acceptors (e.g. F, OH, NH₂). The reverse pattern a observed for XBH₃^?. The range of interaction energies for XCH₃ is considerably smaller than for XNH₃⁺ and XBH₃^?.     

Die rotationsisomerie des bicyclopropyls.: II. Die gauche/trans-isomerisierungsenthalpie und-entropie von bicyclopropyl aus ir-intensitäts-messungen. Ein experimenteller test

The determination of the enthalpy ΔH and entropy ΔS of the isomerization bicyclopropy l(trans)? bicyclopropyl(gauche) in the liquid phase by the IR intensity method is described. It is assumed that the ratio of the integral absorption coefficients of the two reference bands at 1351 cm^?1 (gauche) and 1291 cm^?1 (trans), which both belong to the same type of vibration, is temperature independent. The two values ΔH = ?160 ± 40 cal Mol^?1 and ΔS = ?0.4 ± 0.5 cal (Mol · Grad)^?1 respectively. ΔS_U = ?1.8 ± 0.5 cal (Mol · Grad)^?1 thus determined agree well with the corresponding results obtained from NMR and electron diffraction measurements. However, from the pair of reference bands at 695 cm^?1 (gauche) and 1291 cm^?1 (trans), which do not belong to the same type of vibration, strongly differing values for ΔH and ΔSresult under the same assumption as above, which apparently is not applicable in this case.It is shown through these data that the “Fateley-Test” does not provide a suitable tool to decide whether the absorption coefficients of the reference bands are temperature independent or not. The reason for this insignificance is the relatively poor accuracy and reproducibility of measured IR band intensities obtainable up to now.The relative density of bicyclopropyl between ?60°C and + 50°C was determined.     

Production of the n2 herman infrared system by the energy pooling reaction of N2(Δ3Σ+u) metastable nitrogen molecules

Molecular N₂ emission, observed from an Ar(³Po, 2) and Xe(³P₂) + N₂ flowing afterglow apparatus, indicates that the energy pooling reaction by 2N₂(A ³Σ⁺_u) generates the emission from the Herman infrared system, which is an unassigned nitrogen band system. A lower limit to the formation rate constant for the upper state of the Herman infrared system was found to be 2.5 × 10^-11 cm³ molecule^?1 s^?1. The information presented here may help in the identification of the upper and lower states of the emission system. The 2N₂(A) energy pooling reaction also forms N₂(B³ Π_g, v? 8) but a rate constant cannot be assigned from the present data.     

CNDO/2 calculations of rotational barriers in N.N-dimethylamino-azines: Lineshape study of the barrier in 3-dimethylamino-1,2,4-triazine

3-Dimethylamino-1,2,4-triazine has been prepared and the hindered rotation around the exocyclic C-N bond has been studied by a complete lineshape method. CNDO/2 calculations on dimethylamino derivatives of pyridine, pyrimidine, 1,2,4-triazine and 1,3,5-triazine have been performed, and it is found that the calculations satisfactorily reproduce the experimental data. The barrier heights are also discussed in terms of total and π-charge densities obtained from CNDO/2 calculations.     

Vibrational spectroscopic investigations of snse8-n molecules

The fundamental vibrations of 13 cyclic S_nSe_8-n (n = 7—2) molecules have been calculated using a modified Urey—Bradley force field with 9–14 independent force constants whose values have been adapted from those of Se₈ and S₈. Calculated wavenumbers are compared to those obtained by Raman spectroscopy for sulfur—selenium phases prepared by recrystallizing quenched molten mixtures of the elements as previously described. Agreement between the observed spectra and calculated wavenumbers is closest by assuming the existence of selenium—selenium bonds and the absence of isolated selenium atoms in S_nSe_8-n molecules. It is assumed that sulfur—selenium phases are mixed crystals with the following components in varying concentrations: 1,2-S₆Se₂, 1,2,3-S₅Se₃, 1,2,3,4-S₄Se₄, 1,2,5,6-S₄Se₄, 1,2,3-S₃Se₅ and 1,2-S₂Se₆. The presence of S₈ and Se₈ in some of the phases is indicated by the Raman spectra.     

Effect of quadratic interactions on the resonance raman scattering of T × t2 jahn-teller systems

Raman excitation profiles and depolarization dispersion curves are calculated for T × t₂ Jahn-Teller systems that are subject to quadratic vibronic (Renner-Teller) coupling as well as frequency changes of the active vibrational mode.     

Crystalline hydrogen bonded adducts of oxygen bases with peroxybenzoic acids

The preparation and isolation of several crystalline 1 : 1 adducts of substituted peroxybenzoic acids with triphenylphosphine oxide, triphenylarsine oxide, and pyridine oxides, is reported. Infrared spectra indicate the presence of strong intermolecular hydrogen bonds in these adducts.     

On the planarity and resonance effect of tetraphenylporphyren and its metallocomplexes

Based on Lennard-Jones potential and resonance effect calculations it has been demonstrated that in solution tetraphenylporphyrin assumes a structure closer to coplanar by 17° from that in the solid state. In the case of metalloporphyrin the tendency towards coplanarity is even greater, about 40°. The minimum in the energy curve is of the same order of magnitude as reported values of free energy of activation for the phenyl rotation.     

High-pressure (2+2)cycloaddition of toluene-4-sulphonyl isocyanate to glycals

High-pressure (2+2)cycloaddition of toluene-4-sulphonyl isocyanate to glycals is examined. Reactions proceed regiospecifically to afford single products in case of all 3-substituted glycals. Upon heating or even after standing at room temperature adducts undergo retro-addition to give starting glycals. Various aspects of the cycloaddition are discussed, especially ret-ro-reaction and rearrangement of β-lactams to α,β-unsaturated amides.     

A new approach to the measurement of the momentum densities in solids using an electron microscope

A new approach to the direct determination of electron momentum distributions in solids, using a combination of electron microscopy and electron spectroscopy, is described. The technique is several orders of magnitude more sensitive than currently available ones. The results of measurements on amorphous carbon are presented and used to demonstrate the potential of this new technique.     

Calculation of the spectral distribution of absorption coefficients in vibrational spectra of complicated molecules,polymers and molecular crystals

The interpretation of vibrational spectra of complicated systems is necessary for the subsequent use of these spectra in the analysis of the structure and physical properties of complicated molecules, polymers and molecular crystals.This interpretation is accomplished with the aid of various methods including calculations of frequencies and in certain cases use of the intensities of absorption bands.     

The structure of naphthalene

The structure of naphthalene has been determined by ab initio gradient computation at the Hartree—Fock level using the 4–21 basis set. Special attention is paid to the small differences between the nearly equal C-C distances and to evaluation of the ability of electron diffraction experiments to distinguish among them. The computed orbital energies are used to assign the photoelectron spectrum.