Complete thermodynamic analysis of the ionization of ortho-methoxybenzoic acid. Influence of the medium on the ortho effects

The ionization and solution enthalpies of o-methoxybenzoic acid have been measured in H₂O-DMSO mixtures. In the same solutions, the ΔG⁰ values for the ionization process have been determined by potentiometric measurements. A study of the ortho effects by means of a linear combination of the ordinary polar, proximity polar, and steric effects was also performed. The CH₃O group seems to undergo a partial steric inhibition of resonance. Finally, the proton transfer process from the ortho derivative to benzoic acid, compared with the same process in the gaseous phase, is presented.     

Substituent effects in donor-acceptor complexes with D→A coordination bonds (D = N, O, S; A = B, Al, Ga, Sn, Sb) and related systems

The effect of substituent on the enthalpy ΔH ⁰ and free energy ΔG ⁰ of complexation, on the dipole moments of complexes μ_C and coordination bonds μ_DA, and on the degree of charge transfer Δq was analyzed for 20 series of complexes with D→A coordination bonds (D = N, O, S; A = B, Al, Ga, Sn, Sb), hydrogen bonds, and charge transfer. It was found that ΔH ⁰, ΔG ⁰, μ_C, μ_DA, and Δq depend not only on the inductive and resonance effects, but also on the polarization effect of substituents; its contribution varies in a wide range and can exceed 50%.     

Mode-to-mode vibrational energy transfer in D2CO: Evidence of coriolis-enhanced rotational selectivity

Infrared—ultraviolet double resonance spectroscopy is used to demonstrate rapid collision-induced V-V transfer between the v₆ and v₄ vibrational manifolds of D₂CO. The rate of transfer is at least gas-kinetic and is explained in terms of Coriolis coupling and rotationally specific, quasi-resonant relaxation channels     

Ultraviolet and nuclear magnetic resonance studies of the fumaric nitrile-benzene charge transfer complex

The fumaric nitrile-benzene charge transfer complex has been investigated. The equilibrium constant K has been determined by both ultraviolet and nuclear magnetic resonance spectroscopy, following the method of Benesi-Hildebrand. The experimental result k_NMR > K_UV is associated with the structure of the complex.     

The yields of1 P and1 D resonances in the He(e, 2e)He+ reaction

In the first Born approximation the dependence of the yields of the¹ P and¹ D resonances in the He(e, 2e)He⁺ reaction on the momentum transfer in the recoil peak region at incident energiesE ₀=1000 eV is studied. It is shown that in a certain range of the ejection angle and for the large momentum transfer the yield of the¹ D resonance dominates over the¹ P resonance one.     

Spin-chemical approach to photochemistry: reaction control by spin quantum operation

In this review, the contribution of spin chemistry (in particular, magnetic resonance-related chemistry) to the photochemical field is briefly introduced. First, the development of a time-resolved EPR method and its significant application to radical-related physical phenomena and chemical reactions are presented. Second, a reaction-control method by means of electron spin operations is introduced, and several reaction yield-detected magnetic resonance (RYDMR) methods are presented as applications of this concept. One of the most important physical conclusions is the introduction of the concept of “spin phase relaxation” termed singlet–triplet (S–T) and triplet–triplet (T–T) dephasing, instead of the traditional concepts of longitudinal (T₁) and transversal relaxations (T₂). The effects of strong microwave power on the RYDMR spectrum and time-domain data are analyzed according to this concept. Furthermore, a new detection method is introduced, termed “photoconductivity detected magnetic resonance” (PCDMR), which is applicable exclusively to the system of charge transfer reactions.     

The collision cross sections for the fine-structure mixing of caesium 6P levels induced by collisions with potassium atoms

The cross section for the fine-structure excitation transfer Cs(6P _1/2) → Cs(6P _3/2), induced by collisions with the ground state potassium atoms, has been measured by resonant Doppler-free two-photon spectroscopy. The population densities of caesium 6P _J (J=1/2, 3/2) levels were probed by thermionic detection of the collisionally ionized caesium atoms from the Cs(6P _J ) → Cs(10S _1/2) excitation channel. The cross section for the transfer process at the temperatureT=503 K has been found to be σ(1/2 → 3/2)=45 Ų ± 20%. The result is compared with previously published experimental cross sections for fine-structure transfer in resonance states of other alkali elements perturbed by potassium and a thoeretical value of the Li(2P _J )-K system calculated in a simple approach.     

Electronic spectra of 1- and 9-fluorenecarboxylic acids

The absorption and fluorescence spectra of 1-fluorenecarboxylic acid and its anion have indicated that charge transfer interaction between the COOH group and the fluorene ring is more than that of the COO⁻ group with the ring, both in the S₀ and S₁ states. This has been explained by the greater possible resonance interaction between the aromatic ring and the COOH group than with the COO⁻ group. The respective interactions between the COOH or the COO⁻ group and the fluorene ring in the case of 9-fluorenecarboxylic acid is virtually negligible, both in the S₀ and S₁ states. The COO⁻ group becomes coplanar with the ring when deprotonation takes place at position 9. Various proton transfer reactions have been studied and pK_a values in the S₀ and S₁ states have been determined and discussed. The behaviour of the acids is confirmed by undertaking a similar study with their respective esters.     

Wavelength-dependent triplet state populating mechanisms of naphthalene in heavy-atom hosts

We have made zero-field optically-detected magnetic resonance measurements on the triplet state of naphthalene-d₈ incorporated in the heavy-atom host crystals p-dichlorobenzene, p-dibromobenzene, and s-tetrachlorobenzene. The dependence of the relative triplet sublevel populating rates on the exciting wavelength is interpreted in terms of the electronic excitation transfer mechanisms. We find that upon excitation of the host S₁ ← S₀ transition in p-dichlorobenzene, energy transfer to the guest T₁ takes place via the guest S₁, whereas in the other host crystals the intermediate is mainly the host T₁ band. Upon host T₁ ← S₀ excitaton, trapping takes place from a spin-aligned excitor band in s-tetrachlorobenzene, whereas the initial alignment is lost prior to trapping in the dihalobenzene hosts.     

Effect of the resonance of molecular vibrations on the ir spectra of hydrogen bonds

The effects are considered of the vibrations occurring with the group of atoms peripheral towards the hydrogen bond on the IR spectrum of the model hydrogen bond. It. is shown that the resonance of the v_OHv_CH stretching vibrations in a hydrogen bonded molecule result in the division of the v_OH band into two other bands having different polarizations. The resonance interaction leads to redistribution of the intensity and perturbs the regular Franck—Condon type envelope of the spectrum.     

Quasi-resonant energy transfer in collisions: Na2(A1Σ u + )+K(4S)

Cross sections for electron energy transfer from the initial rotational stateJ′of the two lowest vibrational levelsv′=0 andv′=1 of excited dimers Na₂(A) to potassium atoms as described by Na₂(A¹Σ _u ⁺ ,v′J′)+K(4S)→Na₂ (X¹Σ _g ⁺ ,v″J″)+K(4P)+ΔE have been examined by laser-induced fluorescence. A strong increase of the cross section by as much as an order of magnitude has been observed for those dimerv′J′-levels for which the dipole transitions are close to resonance of the 4S-4P transitions in the atom (ΔE<4 cm^?1). The absolute cross sections for energy transfer have been calculated by the Rabitz approximation of first-order perturbation theory. In the case of closest energy resonance (ΔE=0.9 cm^?1) the cross section is Q=7.8×10^?13 cm².     

Molecular hydrogen emission in the vacuum ultraviolet from an inductively coupled plasma

Vacuum ultraviolet radiation is observed through a sampling orifice in a metal cone inserted directly into an Ar inductively coupled plasma (ICP). The many-lined spectrum of the H₂ Lyman bands is observed from an argon ICP with either H₂ gas or nebulized water added. The most prominent bands are from the ν = 3, 4, 5 or 6 vibrational levels of the B ¹Σ⁺_u electronic state of H₂. The energies of the ν = 3, 4 and 5 levels are close to those of the Ar metastable levels and of photons from the Ar resonance lines (11.5 to 11.8 eV). These H₂ levels are probably populated at least in part by selective energy transfer reactions between Ar metastable atoms and H₂ and/or by absorption of Ar resonance photons by H₂. The H₂ emission emanates only from the upstream reaches of the axial channel, i.e. the part of the axial channel that is inside the load coil and induction region.     

Time spectroscopy in theA 1∑ u + state of Li2

We have determined effective cross-sections for resonant transfer of excitation energy from state-selected Li₂ molecules to ground state Li atoms $$Li_2 A\left( {\upsilon ',J'} \right) + Li2S \to Li_2 X\left( {\upsilon '',J''} \right) + Li2P$$ with two different experimental techniques. Absolute cross-sections were derived from lifetime measurements of 114 rotational levels of theA-state subject to resonant collisions which belong to vibrational statesv′=4, 5, 6, 7 and 11 of⁶Li₂ and⁷Li₂. Cross-sections amount to several thousand Ų dropping off sharply with a resonance half-width of about 16 cm^?1. The resonance behaviour of the collision cross-section inferred from lifetime data could be confirmed by a double resonance technique probing the number of excited atoms with a second laser while the first laser was scanned over the Li₂ X?A absorption band. Due to their high reliability our data prove a good basis for the test of theoretical models. In contrast to earlier investigations significant deviations were found to exist between the measured cross-sections and those predicted by the first-order dipole-dipole scattering theory.     

The atomic form factor for vanadium: a treatment including correlation andLS-dependent relativistic effects

The atomic form factor for the ground state of vanadium is evaluated using non-relativistic and “relativistic-corrected” configuration interaction wavefunctions in the |LSM _L M _s 〉 representation. Relaxing the constraints imposed by the Hartree-Fock model results in a very small reduction of the atomic factor at small momentum transfer with a negligible change at higher momentum transfer. Better agreement with the relativistic Hartree-Fock atomic form factor at small momentum transfer is obtained when theLS-dependent relativistic effects are included in the Breit-Pauli approximation. The sensitivity of the atomic form factor to small changes in the magnitude of the expansion coefficients of the configurational functions is also discussed.     

Transient Raman detection of excited state electron,transfer to methyl viologen from photoexcited molecular reagents in fluid solution and anchored to SiO2

The lowest electronic excited state of the complexes [Ru(2,2′-bipyridine)₃]²⁺, fac-[ClRe (CO)₃(2,2′-bipyridine)], and fac-[(pyridine) Re (CO)₃(2,2′-bipyridine)]⁺ can be quenched by methyl viologen, MV²⁺, N,N′-dimethyl-4,4′-bipyridinium, in fluid solutions. The quenching obeys Stern—Volmer kinetics as deduced from plots of relative luminescence quantum yield vs [MV²⁺], and the data are consistent with a quenching process that is essentially diffusion controlled. Pulsed laser excitation (18 ns, 354.7 nm frequency tripled Nd: YAG) of the metal complexes in the presence of MV²⁺ shows that a detectable fraction of the quenching results in net electron transfer to form MV⁺. The MV⁺ is detectable by resonance Raman scattering from the trailing portion of the excitation pulse. Excited state electron transfer to MV²⁺ from a photo-excited complex anchored to SiO₂ has also been detected by transient Raman spectroscopy. High surface area SiO₂ was functionalized by reaction with 4-[2-(trimethoxysilyl)ethyl]pyridine to give [SiO₂]-SiEtpyr. Reaction of [SiO₂]-SiEtpyr with [(CH₃CN)Re(CO)₃(2,2′-bipyridine)]⁺ then yields [SiO₂]-[(SiEtpyr) Re (CO)₃ (2,2′-bipyridine)]⁺. Electron transfer quenching of the photo-excited immobilized Re complex occurs when suspended in CH₃CN solutions of MV²⁺ to yield MV⁺ as detected by resonance Raman scattering and by lifetime attenuation in the presence of MV²⁺.     

Classical model for electronically non-adiabatic collision processes resonance effects in electronic-vibrational energy transfer

A classical model for electronically non-adiabatic collision processes is applied to E → V energy transfer in a collinear system, A + BC (v = 1) → A¹ + BC (v = 0), resembling Br-H₂.The model, which treats electronic as well as translational, rotational, and vibrational degrees of freedom by classical mechanics, describes the resonance features in this process reasonably well.     

Reactions of indium phosphide cluster cations with ammonia and trimethylamine

Chemistry of indium phosphide clusters is studied using the powerful trapped ion cell techniques of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in conjunction with an external cluster source and ion guide. The external source is capable of generating a wide range of cluster ions which the ion guide loads with high efficiency into the FTICR cell. The differential pumping of the ion guide allows for operation of the FTICR at requisite low pressure conditions while extracting clusters generated in a high pressure environment. Highly selective reactions of indium phosphide clusters are observed with ammonia and trimethylamine. Of all the In_xP⁺_y cluster sizes and stoichiometries studied, only the indium dimer ion reacts exothermically with ammonia. Thermalized In⁺₂ reacts by indium ion transfer to ammonia. Owing to its much higher basicity, trimethylamine is much more reactive. The smaller indium phosphide clusters react by indium ion transfer to trimethylamine. As the clusters become larger, however, the reaction probability decreases to zero.     

Spin density distribution in oxamato-type transition metal complexes

The spin density distribution of the paramagnetic [ⁿBu₄N]₂[Cu(dana)] dana = N,N′-(naphthalene-2,3-diyl)-bis(oxamato) has been derived from angular dependent electron paramagnetic resonance measurements at room temperature. The results indicate a noticeable spin density transfer from the central metal to the coordinated N and O atoms. Quantum chemical studies using density functional theory reinforce the results.     

Study of the photophysical behavior of tetrasulfonated metallophthalocyanines in the presence of CdTe quantum dots

Förster resonance energy transfer (FRET) studies were carried out with CdTe quantum dots (QDs) synthesized in aqueous phase and various tetrasulfonated metallophthalocyanines (MPcS₄, M = aluminum ((OH)AlPcS₄), zinc (ZnPcS₄), silicon ((OH)₂SiPcS₄) and germanium ((OH)₂GePcS₄) in a H₂O:MeOH (1:1) solvent mixture. The QDs studied were capped with thioglycolic acid (TGA) or mercaptopropionic acid (MPA) with sizes ranging from 2.3 to 3.7 nm. Non-radiative energy transfer from QDs emission to MPcS₄ complexes was observed. Study of the photophysics of the MPcs in the presence of the QDs revealed high triplet state quantum yields (Φ_T, ranging from 0.41 to 0.85 in the presence of QDs), with corresponding long triplet state lifetimes (τ_T, which ranged from 140 to 610 μs in the presence of QDs) to allow for photosensitized reactions to occur. The efficiency of energy transfer and the donor–acceptor distance between the MPcs and the QDs were also evaluated.