Assignment of the lowest triplet state of benzene using phosphorescence-microwave double resonance (PMDR) techniques

Phosphorescence-microwave double resonance (PMDR) techniques are used and the zero-field (zf) origin of the 0, 0 band of the phosphorescence of benzene-h₆ in benzene-d₆ at 1.6°K is determined to be the τ_y level. This is consistent with a ³B_1u assignment for a distorted D_2h benzene in its triplet state and contradicts a recently suggested ³B_2u assignment.     

Simulation of vibrational spectra of isotopic benzenes by an extended molecular mechanics method

Vibrational spectra of benzene and benzene-d₆ in the gas and liquid phase have been simulated by a molecular mechanics method including the calculation of equilibrium structures, thermodynamic quantities, normal frequencies and vibrational transition probabilities. The potential parameters have been estimated by referring to the observed frequencies of benzene, benzene-d₆ and 1,3,5-benzene-d₃ and also to results of ab initio calculations. Four and ten independent parameters are required, respectively, for elucidating the infrared absorption and Raman intensities of these compounds in the gas and liquid phase. The infrared absorption spectrum of benzene-d₁ is reproduced well by using the potential and the intensity parameters estimated for benzene and benzene-d₆. The change of relative band intensities on the phase change has been elucidated in terms of the change of various intensity parameters.     

Vibronic coupling activity in benzene

The absorption spectra of benzene-h₆, -d₃ and -d₆ have been remeasured in the region of the hot band transitions 1⁰₁6⁰₁ and 8⁰₁. Whereas for the first an intense doublet is found, for the latter two only a single intense peak is observed. This is taken as evidence of the inactivity of ν″₈ in inducing intensity in contradiction to some recent calculations. The strong doublet for benzene-h₆ has been attributed to Fermi resonance.     

Two photon excitation spectrum of benzene and benzene-d6 in the gas phase: Assignment of inducing modes by hot band analysis

The two-photon absorption spectrum for benzene-h₆ and -d₆ is reported and assigned, leading to the frequency assignment for hitherto unknown vibrations in the electronically excited state. In addition absorption has been measured from hot bands in the ground state. This latter technique has allowed us to make an unequivocal assignment of the vibrational modes responsible for inducing the otherwise forbidden two-photon process. The result is in disagreement with current theory, for this prototype two-photon spectrum in the gas phase.     

1H NMR study of the reversible cis / trans isomerization of {(μ-CH2)(μ-CO)[η5-C5H5Fe(CO)]2}

The cis / trans isomerization of the bridging methylene complex {(μ-CH₂)(μ-CO)[η⁵-C₅H₅-Fe(CO)]₂} was studied in solution by ¹H NMR spectrometry, using solvents with different polarities (acetone-d₆, chloroform-d₁ and benzene-d₆). Equilibrium constants and rate constants for the forward and reverse steps were measured between 278 and 323 k. Both reactions show first-order kinetics. A possible mechanism for the isomerization is proposed, involving the breaking of a FeFe bond in the rate-determining step.     

A precise determination of the first ionization potential of benzene

The first ionization potentials of benzene and benzene-d₆ have been precisely determined by the extrapolation of three-photon resonant Rydberg states in the four-photon ionization spectrum of the jet-cooled molecule. The convergence of resolved transitions in two Rydberg series for principal quantum numbers as high as 14 (-h₆) and 15 (-d₆) establish adiabatic thtesholds of 74573.0 ± 2.0 cm^?1, and 74592.5 = ± 1.2 cm^?1, respectively. These results are crucial for the understanding of the many excited states of benzene in terms of quantum defect theory. Precise quantum defects have been obtained for several Rydberg series and their variation with principal quantum number is reported. The results strongly suggest that the R? series of Wilkinson is derived from aπ(e_1g)→ nf±1 Rydberg excitation.     

Excess Gibbs energies and volumes of the ternary system chloroform + tetrahydrofuran + cyclohexane at 298.15 K

Vapour–liquid equilibria and densities for the ternary system chloroform + tetrahydrofuran + cyclohexane and for the binary mixtures containing chloroform have been determined at 298.15 K. Vapour–liquid equilibrium data have been collected by head-space gas-chromatographic analysis of the vapour phase directly withdrawn from an equilibration apparatus. Density measurements have been carried out by means of a vibrating tube densimeter. Molar excess Gibbs energies G^E and volumes V^E, as well as activity coefficients and apparent molar volumes of the components, have been obtained from the measured quantities and discussed. The binary chloroform + tetrahydrofuran displays negative deviations from ideality, while chloroform + cyclohexane positive deviations, for both volume and Gibbs energy. The G^E's and V^E's for the ternary system are positive in the region rich in cyclohexane while negative in the region rich in chloroform + tetrahydrofuran. This indicates that hydrogen bonding between chloroform and tetrahydrofuran molecules produces negative values of G^E and V^E and strongly influences the behaviour of the ternary system.     

Proton-transfer effect in hydrogen-bonded complexes of phospho-diester groups with amines in non-aqueous solvents

The proton transfer in 1 : 1 mixtures of phosphoric acid-di-n-butylester with various aromatic amines has been studied in deutero-chloroform and DMSO-d₆ by ¹H NMR spectroscopy. The 50% equilibrium was found for both solvents at ΔpK_a(50%)-values from 4.4 to 5.2, i.e. the strength of the base is 4.4 to 5.2 higher than that of the acid. The titration curves are steeper in DMSO than in chloroform, indicating a higher potential barrier in POH … N ? PO^? … H⁺N hydrogen bonds in the more polar solvent.     

Raman study of reorientational motion of liquid benzene

Raman spectra of the 943 cm^?1 band of liquid benzene-d₆ were measured at temperatures up to 120°C and at pressures up to 4000 bar. Rotational diffusion constants for tumbling and spinning motions were obtained from Raman band widths and NMR T₁ data. Experimental results were compared with theoretical diffusion constants computed on the basis of various models. It is found that the Hynes—Kapral—Weinberg theory gives the best prediction for the temperature and pressure dependence of the diffusion constants and for the rotational anisotropy in liquid benzene.     

On the tautomerism of pyrazolones: the geminal J[pyrazole C-4,H-3(5)] spin coupling constant as a diagnostic tool

The tautomerism of pyrazolones unsubstituted at position 3(5) has been investigated by ¹³C- and ¹H NMR spectroscopic methods. Apart from chemical shift considerations and NOE effects the magnitude of the geminal ²J[pyrazole C-4,H3(5)] spin coupling constant permits the unambiguous differentiation between 1H-pyrazol-5-ol (OH) and 1,2-dihydro-3H-pyrazol-3-one (NH) forms. Whereas 1H-pyrazol-5-ols and 2,4-dihydro-3H-pyrazol-3-ones (CH-form) exhibit ²J values of approximately 9-11 Hz, in 1,2-dihydro-3H-pyrazol-3-ones this coupling constant is considerably reduced to 4-5 Hz. This can be mainly attributed to the removal of the lone-pair at pyrazole N−1 in the latter due to protonation or alkylation. According to the data obtained, 2-substituted 4-acyl-1,2-dihydro-3H-pyrazol-3-ones exist predominantly as pyrazol-5-ols in CDCl₃ or benzene-d₆ solution, whereas in DMSO-d₆ also minor amounts of NH tautomer may contribute to the tautomeric composition. 2,4-Dihydro-2-phenyl-3H-pyrazol-3-one (1-phenyl-2-pyrazolin-5-one) exists in benzene-d₆ solely in the CH-form, in CDCl₃ as a mixture of CH and OH-form, whereas in DMSO-d₆ a fast equilibrium between OH and NH isomer (with the former far predominating) is probable. For 11 compounds, including neutral and protonated molecules, we have calculated at the B3LYP/6-311++G** level, the ²J(¹H,¹³C) coupling constants which are in good agreement with those measured experimentally.     

Temperature dependence of rotational correlation times in tribromobenzene

The ¹³C relaxation times and nuclear Overhauser enhancements of the protonated carbons in 1,3,5-tribromobenzene were measured as a function of temperature in the solvent benzene-d₆. Rotational correlation times, τ_C(CH), calculated by the Microviscosity/Free Rotor and Hu-Zwanzig “slip” models are substantially below the measured values. In contrast, correlation times predicted by the Hynes—Kapral—Weinberg model are in near quantitative agreement with experiment at all temperatures studied.     

Surface-enhanced Raman spectra of benzene and benzene-d6 on vapor-deposited sodium

Enhanced Raman spectra have been observed from small amounts of benzene and benzene-d₆ adsorbed on vapor-deposited sodium surfaces held at 15 K. Many lines were observed which can be assigned to normally Raman-active benzene fundamentals as well as some that are normally forbidden in D_6h symmetry.     

Charge separation reactions of doubly charged benzene ions

Photoion-photoion coincidence spectra of benzene and benzene-d₆ photoionized by He(II) light and synchrotron radiation show the existence of six major and eight minor charge-separation reactions of the [C₆H₆]²⁺ ion. Three main groups of ion pairs are related to [C₃H₃]⁺ + [C₃H₃]⁺, [C₂H₃]⁺ + [C₄H₃]⁺ and [CH₃]⁺ + [C₅H₃]⁺, with appearance energies of 32.2 ± 0.5 eV, 31.3 ± 0.5 eV and 28.4 ± 0.3 eV. The kinetic energy release is the same for all pairs within a group, irrespective of hydrogen number, but differs from group to group. Results are interpreted in terms of fast, direct charge separation of [C₆H₆]²⁺, and subsequent hydrogen loss by the singly charged fragments.     

Structures of 18-crown-6, 15-crown-5 and their metal complexes in methanol solution as studied by Raman spectroscopy

Raman spectra of 18-crown-6, 15-crown-5, and their complexes with Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺ and Ba²⁺ have been investigated in methanol solution. Normal coordinate calculations are presented for the D_3d⁻ and C_i-symmetric structures of 18-crown-6. Analysis of the Raman spectra gives information on the ring conformations of the crown ethers and the stoichiometry of complexation. The uncomplexed ethers adopt diverse conformational states in methanol at room temperature, the D_3d state being the most stable in 18-crown-6 and various states equally stable in 15-crown-5. Most of the cations form 1:1 or 2:1 crown—metal complexes depending on the cation size relative to the hole size of crown. Exceptionally, Cs⁺ forms both 1:1 and 2:1 complexes with 18-crown-6. The ring structures in complexes of 18-crown-6 are not much distorted from the D_3d one, though the distortion is rather large in the 2:1 Cs²⁺ and 1:1 Ca⁺ complexes. Complexes of 15-crown-5 exhibit for types of ring structure depending on the size and charge of the cation. These structures are likely to involve distortion from the gauche or gauche′ conformation about the CC bonds and various conformation about the CO bonds.     

Photogeneration of singlet molecular oxygen (1O2, 1Δ g ) using monomeric and aggregated forms of copper tetra-4-(morpholine-4-yl)-tetra-5-(2-naphthoxy)phthalocyanine in organic solvents

Luminescence and chemical acceptors methods were used to study the photosensitized formation of singlet molecular oxygen (¹O₂, ¹?? _g ) by monomer and aggregated forms of copper tetra-4-(morpholine-4-yl)-tetra-5-(2-naphthoxy)phthalocyanine in benzene, benzene-d ₆, acetone, DMF, and pyridine. The values of ¹O₂ quantum yield were determined. The effectiveness of the ¹O₂ photoproduction was shown to be reduced due mainly to the aggregation of the copper phthalocyanine molecules.     

Complex formation between benzene and [Cr(CO)3(η6-arene)]

1/1 complex formation between benzene-d₆ and [Cr(CO)₃(η⁶-arene)] (arene = C₆H₅R (R = H, Me, OMe, Cl), C₁₀H₈) has been observed by PMR spectroscopy. The equilibrium constant in the case of arene =C₆H₆ confirms earlier chemical evidence that the [Cr(CO)₃] unit exerts an electron-with-drawing effect upon aromatic rings which is approximately equal to that of the nitro group.     

Nuclear magnetic resonance studies of isoprenyllithium derived from 1,1-diphenyl-n-butyllithium-3,4-d5, and isoprene

The equimolar reactions of 1,1-diphenyl-n-butyllithium-3,4-d₅ (RLi) with isoprene (I) and isoprene-1,4-d₄ (Id) were carried out in benzene-d₆ quantitatively to give isoprenyllithiums, RILi and RIdLi, respectively. From the NMR spectrum of the RILi it was proposed that the isoprene unit had cis-1,4, cis-4,1 and some unknown structures in benzene-d₆. When RILi was prepared in the presence of about one equivalent of THF to RILi, the anion was considered to include an isoprene unit in cis-1,4, trans-1,4, cis-4,1, and probably 3,4 structures. The same anion was obtained even if an equimolar THF was added afterward to the RILi prepared in benzene-d₆ The RLi was reproduced by the reverse reaction from RILi, when a large excess of THF was added or the temperature of the solution was elevated. The results obtained were correlated with those of anionic polymerizations of isoprene by lithium initiators.     

Low molecular weight organogelators derived from threefold symmetric tricarbamates

A group of new low molecular weight organogelators based on threefold symmetric tricarbamate were synthesized and characterized. The tricarbamates with long alkyl chains were able to gelate a wide range of polar and nonpolar organic solvents such as acetonitrile and cyclohexane, generally at concentrations lower than 20 g/L. The best organogel formation was obtained using a threefold symmetric tricarbamate in n-dodecane, in which a sufficiently transparent gel was formed at the critical gelation concentration 1 g/L. Intermolecular hydrogen bonding by the tricarbamate in a nonpolar solvent benzene-d₆ was indicated by ¹H NMR spectra. Its maximum UV absorption was 11 nm higher in chloroform than in n-dodecane, and this red shift indicated increased conjugation between the benzene core and the carbamate substituents, which confirmed a change in its conformation from nonpolar to polar solvent. The self-assembling behavior of the tricarbamate in dilute solutions was investigated by TEM. Fiber-like networks were observed in a large range of solution concentrations.     

Angular distribution for the major photoelectron bands of nitrogen below 45 eV binding energy

High-resolution optically detected magnetic resonance (ODMR) and optically detected electron—electron double resonance (ODEEDOR) have been performed on the lowest excited triplet states of pyrazine-h₄ and pyrazine-d₄, as dilute guests in polycrystalline benzene and cyclohexane host matrices. Computer simulated spectra have been fitted to those experimentally observed, using a spin hamiltonian which takes into account the ¹⁴N and ¹H (or ²D) nuclei of the pyrazine guest molecule. Good agreement is obtained when one ¹⁴N nuclear quadrupole energy difference, |ε_y^N - ε_z^N| ? 1.3 MHz, and the remaining component, |ε_x^N| = 0.6 ± 0.2 MHz for pyrazine-d₄ in benzene-d₆.     

Pre-resonance Raman effect and vibronic coupling of benzene and benzene-d6

The pre-resonance Raman effect was observed for liquid benzene and benzene-d₆. Great intensity enhancement was found for ν₁₀(e_1g), 2ν₈ and 2ν₁₄. The effect was expained in terms of vibronic couplings between the electronic states of the molecules.