Interaction of H2@C60 and Nitroxide through Conformationally Constrained Peptide Bridges

We synthesized two molecular systems, in which an endofullerene C₆₀, incarcerating one hydrogen molecule (H₂@C₆₀) and a nitroxide radical are connected by a folded 3₁₀‐helical peptide. The difference between the two molecules is the direction of the peptide orientation. The nuclear spin relaxation rates and the para → ortho conversion rate of the incarcerated hydrogen molecule were determined by ¹H NMR spectroscopy. The experimental results were analyzed using DFT‐optimized molecular models. The relaxation rates and the conversion rates of the two peptides fall in the expected distance range. One of the two peptides is particularly rigid and thus ideal to keep the H₂@C₆₀/nitroxide separation, r, as large and controlled as possible, which results in particularly low relaxation and conversion rates. Despite the very similar optimized distance, however, the rates measured with the other peptide are considerably higher and thus are compatible with a shorter effective distance. The results strengthen the outcome of previous investigations that while the para → ortho conversion rates satisfactorily obey the Wigner's theory, the nuclear spin relaxation rates are in excellent agreement with the Solomon–Bloembergen equation predicting a 1/r⁶ dependence.     

Collisional relaxation of metastable electronic states of Fe+

The overall rate constants for collisional relaxation of metastable excited states of Fe⁺ by He, Ar, Kr, H₂, ²H₂, CO, N₂, NO, CH₄, and CH₃OH have been studied by using charge-exchange ion-molecule reaction chemistry. The rate constants vary according to the nature of the quenching reagent as well as the energy level and electron configuration of the Fe⁺ ions. In general, NO, CH₄, and CH₃OH are the most efficient quenching reagents with rate constants that approach the Langevin collision rate, whereas the reaction rates for the rare gas atoms are slow and vary depending upon the specific electron configuration of the Fe⁺ ion. The mechanism of collisional relaxation is discussed with emphasis on a curve-crossing. mechanism for the rare gas atoms. An electron-transfer mechanism is described for the relaxation of high lying (Fe⁺)*.     

The effect of collisions on vibrational energy distribution in polyatomic molecules

We approach the problem of the effect of collisions on infrared absorption by an experimental and modeling study of a well-understood system, absorption of CO₂ laser radiation by cold CF₄. The spectroscopy, collisional energy transfer rates, and laser characteristics are all known. We conclude that the dominant collisional processes that influence the vibrational energy distribution during infrared laser absorption are rotational energy transfer-rotational relaxation and pressure broadening.     

Theory of relaxation due to spin-exchange collisions

A general method is developed for the evaluation of the relaxation rates due to spin-exchange collisions. For the case where nuclear spin I = $\text{1}\text{2}$, all the relaxation rates are found, account being taken of the hyperfine and Zeeman interactions. The present theory can be applied to the following systems: (1) H + H, (2) H + a different electron-spin-$\text{1}\text{2}$ atom, and (3) H + H₂.     

AB initio calculations of 2H and 14N quadrupolar coupling constants in hydrogen bonded dimers

SCF MO calculations at the 6-31G^** level of approximation are reported for ²H and ¹⁴N electric field gradients in HCN?HCN, HCN?HF, and CH₃CN?HF dimers, with emphasis on the configurational dependence of these quantities in (HCN)₂. In comparison with available experimental nuclear quadrupolar coupling constants, the calculated values for the monomers and dimers exhibit an accuracy of ≈ 10%, which is comparable to that of other spectroscopic parameters. The implications of hydrogen bonding for quadrupolar spin-lattice relaxation rates are briefly discussed.     

The collision complex in the exchange reaction Na + Na2. I. An ortho-para pumping experiment on Na2

The exchange reaction Na + Na₂ → Na₂ + Na is studied in the gas phase. The influence of this reaction on the nuclear spin states of the molecules is investigated by two different experiments. In an ortho-para pumping experiment the ortho-para conversion rate of Na₂ is determined with aid of laser induced molecular fluorescence. This rate is found to be twice the nuclear spin relaxation of the molecules, measured in a NMR experiment with nuclear spin polarized Na₂. Furthermore the influence of dissociation- and intramolecular relaxation processes on nuclear spin relaxation and ortho-para conversion is investigated.     

Singlet methylene kinetics: Direct measurements of removal rates of ã1A1 and b̃1B1 CH2 and CD2

Rate constants for collisional removal of ã¹A₁ and b?¹B₁ CH₂ and CD₂ have been directly measured, using IR laser induced multiple photon dissociation to prepare the radicals, and time resolved laser induced fluorescence to observe them. For CH₂(ã¹A₁) removal by He, Ne, Ar, Kr, Xe, N₂, H₂, O₂, CO and CH₄, rate constants of 3.1, 4.2, 6.0, 7.0, 16, 8.8, 130, 30, 56 and 73 × 10^?12 cm³ molecule^?1 s^?1 were found respectively. These represent significant increases over the previously accepted values. Essentially no isotope effect is observed in the removal of CD₂(ã¹A₁) by the rare gases. The rate determining step in removal by the rare gases and N₂ is thought to be singlet—triplet intersystem crossing controlled by long range attractive forces, and the results are discussed in terms of both isolated and mixed state theoretical models of these processes. For the other molecular collision partners, bimolecular chemical removal channels are possible, and may account for the relatively fast rates observed. Radiative lifetimes of five Σ vibronic levels of CH₂(b?¹B₁) and three Σ vibronic levels of CD₂(b?¹B₁) have been measured and found to lie in the range 2.5–6.0 μs, and collisional quenching rates for CH₂(b?¹B₁) are found to be of the order of the gas kinetic collisional frequency.     

Vibration-vibration energy transfer iln CH3F

Previous works have reported vibration-vibration and vibration-translation transfer rates in CH₃F and CH₃FX mixtures. In this letter we report the study of the fast VV transfer rate populating the 3ν₃, ν₁ and ν₄ states of CH₃F. Gaseous CH₃F was initially excited to the ν₃ state by a TEA CO₂ laser operating on the P(20 9.6 μ line and collisional pumping to the 3ν₃, ν₁ and ν₄ states was measured by monitoring the rise time of the fluorescence at 300 cm⁻¹. The rate constant was found to be 2.2 × 10⁵ sec⁻¹ torr⁻¹.     

Collisional depolarization of excited114Cd 5s5p 3 P 1 atoms by collisions with molecular gases

Depolarization of excited¹¹⁴Cd 5s5p ³ P ₁ atoms induced by collisions with various molecular gases (N₂, H₂, D₂, CO, CO₂, CH₄, C₂H₆, C₂H₄) has been investigated using polarized fluorescence spectroscopy. After pulsed optical excitation of the Cd 5³ P ₁ level with appropriately polarized light the temporal behaviour of Zeeman quantum beats has been observed showing the influence of collisional destruction of orientation and alignment. By analyzing the signal curves at different molecular gas pressures the corresponding depolarization cross sections for¹¹⁴Cd atoms in the 5³ P ₁ state have been obtained. With regard to a test of a nuclear spin decoupling model for the collisions the cross sections were compared with previously measured hyperfine structure transfer cross sections of¹¹³Cd 5s5p ³ P ₁ atoms.     

Vibrational relaxation induced population inversions in laser pumped polyatomic molecules

Conditions for population inversion in laser pumped polyatomic molecules are described. For systems which exhibit metastable vibrational population distributions [slow vibration—translation/rotation (V—T/R) relaxation], large, long lived inversions are possible even when the vibrational modes are strongly coupled by rapid collisional vibration—vibration (V—V) energy transfer. Overtone states of a hot mode are found to invert with respect to fundamental levels of a cold mode even at V—V steady state. Inversion persists for a V—T/R relaxation time. A gain of 4 m^?1 for the 2v₃ → v₂ transition in CH₃F (λ ≈ 15.9 μ) was found assuming a spontaneous emission lifetime of 10 s for this transition. General equations are derived which can be used to determine the magnitude of population inversion in any laser pumped, vibrationally metastable, polyatomic molecule. A discussion of factors controlling the population maxima of different vibrational states in optically pumped, V—V equilibrated metastable polyatomics is also given.     

Relaxations Studies of 2IZSZ Spin Order with Signal Enhancement by Coherence Transfer

The pulse sequence for generating coherence transfer (or polarization transfer) is invoked to enhance the signal of heteronuclear two spin order (e.g. 2I_ZS_Z) in a spin system with CH moiety. This allows the observation of selective conversion of Zeeman order, in a sample with natural abundant ¹³C nuclei, into two spin order for measuring cross-correlation of chemical shift anisotropy and dipole-dipole interactions. The molecular reorientational correlation time and the orientation of the C–H axis with respect to the principal axes of carboxyl CSA tensor may be determined simultaneously in the relaxation profile of two spin order.     

Photo-oxidation of diglycine in confined media Relaxation of longitudinal magnetization in spin correlated radical pairs

Time-resolved electron paramagnetic resonance spectra (X-band) of correlated radical pairs created in AOT reverse micelles are presented and simulated using the microreactor model. They are discussed in terms of the two-site model with a particular emphasis on longitudinal relaxation mechanisms. The geminate radical pair is created by photo-oxidation of dyglicine by the excited triplet states of an anthraquinone salt. The strong chemically induced electron spin polarization observed is due to three mechanisms: TM, RPM, and SCRPM. Relative contributions from these mechanisms depend on the water pool volume and the time of observation. There are three types of longitudinal relaxation in radical pairs. The first is relaxation of the RPM induced longitudinal magnetization in spin correlated radical pairs. The second is the longitudinal relaxation in radical pairs which are not correlated (with a zero value of the double quantum coherence). In such pairs, the generation of longitudinal magnetization due to RPM is impossible, and the spin-selective recombination of the pairs is ineffective. Under all experimental conditions, the first type of relaxation is slower than the second type. For both, the physical mechanism leading to relaxation is modulation of the Heisenberg electron spin exchange interaction. This is an internal relaxation process. The third relaxation type occurs in radical pairs due to ordinary longitudinal relaxation in non-interacting radicals. Normally, relaxation of the third type is the slowest of the three. This explains time and micelle size dependence of the relative contribution of RPM into TREPR spectra. It seems reasonable to suggest that the creation of the initial spin state populations is partially adiabatic.     

Nuclear spin–spin coupling and nuclear motion

The vibration and rotation of molecules affects nuclear spin–spin coupling constants. This manifests itself as a temperature dependence of the coupling and also as an isotope effect (after allowing, where necessary, for differing magnetogyric ratios of the two nuclei involved in the isotopic substitution). Within the Born–Oppenheimer approximation, a nuclear spin–spin coupling surface can be defined for each pair of coupled nuclei. This surface is sampled by the nuclei as they undergo the excursions about equilibrium geometry that are governed by the force field. An accurate ab initio carbon–proton spin–spin coupling surface for the methane molecule has been calculated. This was obtained by summing the surfaces for each of the four contributions—Fermi contact, spin–dipolar, orbital paramagnetic, and orbital diamagnetic—expressed as power series in terms of symmetry coordinates. Preliminary calculations for ¹³CH₄ and ¹³CD₄ give a difference of only 6% between the calculated and observed nuclear motion contributions. The observed temperature dependence is also accounted for by the calculations. For these isotopomers, bond stretching plays the dominant role. © 1994 John Wiley & Sons, Inc.     

Dual fluorescence lifetimes of pyrimidine: Intermediate radiationless decay

We have observed a dual fluorescence decay from the lowest n → π* excited singlet state of pyrimidine. The vibronic states 0-0, 6a¹, 12¹, 6a¹12¹, 12² and 6a¹12² have two exponential decays with lifetimes ranging from 2.7-0.7 nsec and from 410-234 ns at 0.02 torr. The ratio of pre-exponentials is pressure independent but the long decay is very sensitive to collisions. The four lower energy states have effective impact diameters of 16 A and the highest energy state is quenched by gas kinetic collision diameters (≈ 5.5 Å). The dual fluorescence decay and collisional fluorescence quenching by rotational relaxation is consistent with the available models of singlet-triplet mixed state decay. Using these models we have computed the rates for singlet-triplet crossing, the number of coupled triplet levels, and the decay rates for internal conversion. The model used our measured fluorescence decay parameters and our estimate of a triplet loss rate. The estimated triplet loss varies from 0.2 to 2.0 × 10⁶ s^?1 and the singlet internal conversion rate varies from ≈ 0.4 to 56 × 10⁷ s^?1. The singlet-triplet radiationless rate suggests that 50–100 times more triplet levels are effective in the state mixing than can be expected from the triplet vibronic density. Such an enhanced coupling of ro-vibronic triplet levels is 5–10 times larger than previously observed for the dicarbonyls. The observation of reduced collisional quenching of higher energy vibronic levels is quantitatively interpreted by a different model than used previously for the dicarbonyls.     

Toward a Complete Understanding of the Vinyl Fluoride Spectrum in the Atmospheric Region

A deep and comprehensive investigation of the vinyl fluoride (CH₂CHF) spectrum in the atmospheric window around 8.7 μm is presented. At first, the ro‐vibrational patterns are modelled to an effective Hamiltonian, which also takes into account the coupling of the C? F stretching vibration, ν₇, with the neighbouring vibrational combination ν₉+ν₁₂. The obtained Hamiltonian gives very accurate simulations and predictions of the ro‐vibrational quantum energies. Then, in the main part of the work, an experimental and theoretical study of vinyl fluoride self‐broadening collisions is carried out for the first time. The broadening coefficients obtained experimentally are compared with those calculated by a semiclassical theory, demonstrating a significant contribution of collisional coupling effects between lines connecting pairs of degenerate (or nearly degenerate) rotational levels. Finally, the experimentally retrieved integrated absorption coefficients are used to calculate the absorption cross‐section of the ν₇ normal mode, from which dipole transition moments are derived. The obtained results provide a deep insight into the spectral behaviour of vinyl fluoride, in a spectral region of primary relevance for atmospheric and environmental determinations. Indeed, the data presented constitute an accurate model for the remote sensing of vinyl fluoride—a molecule of proved industrial importance which can lead to hazardous effects in the atmosphere and affects human′s health.     

Nuclear Magnetic Resonance Relaxivities: Investigations of Ultrahigh‐Spin Lanthanide Clusters from 10 MHz to 1.4 GHz

Paramagnetic relaxation enhancement is often explored in magnetic resonance imaging in terms of contrast agents and in biomolecular nuclear magnetic resonance (NMR) spectroscopy for structure determination. New ultrahigh‐spin clusters are investigated with respect to their NMR relaxation properties. As their molecular size and therefore motional correlation times as well as their electronic properties differ significantly from those of conventional contrast agents, questions about a comprehensive characterization arise. The relaxivity was studied by field‐dependent longitudinal and transverse NMR relaxometry of aqueous solutions containing Fe^III₁₀Dy^III₁₀ ultrahigh‐spin clusters (spin ground state 100/2). The high‐field limit was extended to 32.9 T by using a 24 MW resistive magnet and an ultrahigh‐frequency NMR setup. Interesting relaxation dispersions were observed; the relaxivities increase up to the highest available fields, which indicates a complex interplay of electronic and molecular correlation times.     

Intermolecular nuclear relaxation in paramagnetic solutions: from free radicals to rare earths

The principles of the intermolecular relaxation of a nuclear spin by its fluctuating magnetic dipolar interactions with the electronic spins of the paramagnetic surrounding species in solution are briefly recalled. It is shown that a very high dynamic nuclear polarization (DNP) of solvent protons is obtained by saturating allowed transitions of free radicals with a hyperfine structure, and that this effect can be used in efficient Earth field magnetometers. Recent work on trivalent lanthanide Ln³⁺ aqua complexes in heavy water solutions is discussed, including paramagnetic shift and relaxation rate measurements of the ¹H NMR lines of probe solutes. This allows a determination of the effective electronic magnetic moments of the various Ln³⁺ ions in these complexes, and an estimation of their longitudinal and transverse electronic relaxation times T_1e and T_2e. Particular attention is given to Gd(III) hydrated chelates which can serve as contrast agents in magnetic resonance imaging (MRI). The full experimental electronic paramagnetic resonance (EPR) spectra of these complexes can be interpreted within the Redfield relaxation theory. Monte-Carlo simulations are used to explore situations beyond the validity of the Redfield approximation. For each Gd(III) complex, the EPR study leads to an accurate prediction of T_1e, which can be also derived from an independent relaxation dispersion study of the protons of the probe solutes.     

Nuclear spin relaxation dynamics of 13CO2 sorbed in polyisobutene rubber

Recently we presented the dynamics of ¹³CO₂ molecules sorbed in silicone rubber (PDMS) ascertained from spin relaxation experiments. Results of a similar investigation for ¹³CO₂ sorbed in polyisobutene (PIB) are presented in this report. The spin-lattice and spin-spin relaxation times as well as nuclear Overhauser enhancements (NOE) were determined as a function of temperature and Larmor frequency. The relaxation mechanisms found to be important for ¹³CO₂/PIB system are intermolecular dipole-dipole relaxation and chemical shift anisotropy with a minor contribution from spin rotation relaxation. We have determined the parameters which characterize correlation times for ¹³CO₂ collisional motion, rotational motion, and translational motions in the PIB. The self-diffusion coefficient of 5.15 × 10^?8 cm²/s obtained from the nuclear magnetic resonance (NMR) data is close to the literature value of the mutual diffusion coefficient of CO₂ in PIB at 300 K obtained from permeability measurements. In contrast to the case of CO₂/PDMS in which a broad distribution (characterized by a fractional exponential correlation function of the Williams-Watts type with α = 0.58) is observed, a sharp distribution with a fractional exponent, α, of 0.99 is found for the CO₂/PIB system. Instead of assuming an Arrhenius type temperature dependence, we used a Williams-Landel-Ferry type temperature dependence and found it to be better suited to describe the behavior of this system. PIB is a densely packed “strong” chain polymer which responds gradually to the temperature variation and gas sorption. In contrast PDMS is a relatively loosely packed “fragile” polymer with a propensity to exhibit rapid dynamic responses to the temperature change and gas sorption. © 1993 John Wiley & Sons, Inc.     

Fragmentation of the enolate ion of 2,3-butanedione. Characterization of the acetyl anion by charge inversion mass spectrometry

The unimolecular and collision-induced fragmentation reactions of the enolate ion of 2,3-butanedione, [CH₃COCOCH₂]^?, have been studied, Unimolecular fragmentation on the metastable ion time-scale forms [HCCO]^?, [C₂H₃O]^?, [C₃H₅O]^? and [CH₃CO₂]^?. Charge inversion mass spectrometry shows that the [C₂H₃O]^? ion is the acetyl anion while the [C₃H₅O]^? product is the acetone enolate ion; formation of the latter product involves a large release of kinetic energy (T _1/2 = 0.99 eV). The fragmentation reactions occurring following collisional activation have been determined for 8 keV collisions and over the range 1.5–30 eV center-of-mass collision energy. Formation of [HCCO]^? and [CH₃CO]^? are of the most important reactions following collisional activation and it is concluded that the two reactions have similar critical reaction energies even though formation of [HCCO]^? is favored thermochemically.