A computational approach to understanding the mechanism of aromatic bromination using quaternary ammonium tribromides

Quaternary ammonium tribromides (QATBs) have garnered interest for nearly a century now. Various types of tribromides have been synthesized over the years and their diverse applications have been extensively reported. However, despite the fact that these reagents are touted as safer alternatives to the very poisonous bromine, there is insufficient information on the structure of Br₃^? in QATB systems and there is still no clear explanation for how the tribromide ion (Br₃^?) participates in bromination reactions. This paper reports a through structural assessment of Br₃^?, followed by an attempt was made to fully understand the mechanistic behaviour of tribromide during bromination of aromatic compounds.     

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.     

Supramolecular fluoride and chloride anions

The Cambridge Structural Database has been searched for supramolecular anions of the form H_nX_n + 1⁻ where X is fluorine (n = 1 to 6) and chlorine (n = 1 to 5); no credible evidence was found for comparable bromine or iodine species. Several structural isomers have been identified, and an experimental measure has been developed which seems to distinguish between two-centre and three-centre bonded species. There are also examples of suprafluoride and suprachloride anions as ligands in organometallic complexes. Most of these anions have not been previously described.     

A Raman spectroscopic study of photochromic benzothiazolium dyes

Resonance Raman spectra using several excitation wavelengths have been obtained in the region 300–1750 cm⁻¹ for two photochromic benzothiazolium dyes containing a crown ether ring. Bands arising from both cis- and trans-isomers have been distinguished by comparing the spectra observed at different resonances and under different sample conditions. Preliminary assignments of the bands have been made, allowing insight into the changes in structure and bonding which occur on photoisomerization.     

Theoretical study of the polarized electronic absorption spectra of vanadium-doped zircon

The polarized electronic absorption spectra of a blue vanadium-doped zircon single crystal, grown by the flux method, has been studied by quantum chemical (CI) calculations in order to determine the position of the V⁴⁺ dopant in the zircon host structure. Particularly, the excitation energies and polarizations of V⁴⁺ occupying alternate positions, either the zirconium or silicon position or the interstitial site 16g, have been considered. It is concluded that the observed electronic absorption spectra and the color of zirconblue can only be explained if the V⁴⁺ chromophore is placed on the respective interstitial position.     

Pressure-tuning Raman and infrared spectra of N-thiocyanato[tris(o-tolyl)]tin(IV), (o-CH3C6H4)3SnNCS

The effect of high external pressures on the Raman and IR spectra of the title compound (I) has been examined at ambient temperature. A pressure-induced phase transition was observed at 13–16 kbar, which is most likely second-order, resulting from slight rotations of the phenyl rings and/or the CH₃ groups under the influence of pressure. No new peaks were observed in the spectra with increasing pressure indicating that no pressure-induced linkage isomerism or SnNCS⋯Sn bridging took place. The average pressure sensitivity (dν/dP) of the Raman-active vibrational modes is lower in the low-pressure region (0.23 cm⁻¹/kbar) than in the high-pressure one (0.47 cm⁻¹/kbar). In general, the IR-active modes are less sensitive to increasing pressure than are the Raman-active modes and the average dν/dP value for the IR-active modes in the low-pressure region is quite similar to that in the high-pressure region, i.e., about 0.23 cm⁻¹/kbar.     

Single-crystal raman study of dirhenium decacarbonyl

A single-crystal Raman study of Re₂(CO)₁₀ is reported from which 20 of the 21 Raman-active modes of this D_4d molecule have been assigned. All six lattice modes were identified below 70 cm^-1. In respect of the long-disputed assignment of v (Re--Re), for which two possibilities exist (107 and 129 dm^-1), the balance of the new evidence is in favour of the lower band. Comparisons are drawn with assignments for Mn₂(CO)₁₀ and Re(CO)₅X, (X = halogen), respectively and anomalies noted.     

Beyond the Halogen Bond: Examining the Limits of Extended Polybromide Networks through Quantum‐Chemical Investigations

The bonding environments of some polybromide monoanions and networks were examined by quantum‐chemical methods to investigate electronic interactions between dibromine–dibromine contacts. Examination of thermodynamic parameters and a bond critical point analysis give strong evidence for such bonding modes, which have been previously treated disparately in the literature. The thermodynamic stability of large polybromides up to [Br₃₇]^? was also predicted by these methods.     

Theoretical study of the electronic spectrum of diimide by AB initio methods

A series of ab initio calculations is reported for the ground and low-lying valence and Rydberg states of diimide N₂H₂. Symmetric bending potential curves for both the cis and trans forms of this system have been obtained at the SCF level of treatment. In addition Cl calculations have been carried out for the trans-diimide ground state equilibrium nuclear conformation, using a configuration selection procedure described elsewhere; an associated energy extrapolation scheme is also employed which enables the effective solution of secular equations with orders of up to 40000. The ensuing Cl wavefunctions are interpreted in the discussion and the corresponding calculated energy differences between the various electronic states are compared with experimental transition energy results for both diimide and for related systems such as trans-azomethane. A more detailed analysis of the observed absorption bands in the ¹B_g-X¹A_g transition in N₂H₂ is also given, making use of calculated potential curve data as well as the pertinent Cl vertical energy difference. The dipole-forbiddenness of the excitation process is thereupon concluded to result in a distinct non-verticality for this electronic band system, causing its absorption maximum to occur at a position some 0.6 eV to the blue of the so-called vertical transition, i.e., that for which maximum vibrational overlap is obtained.     

Polarised single crystal Raman spectroscopy of sinhalite,MgAlBO4

Polarised single-crystal Raman spectra for sinhalite have been collected at 80 K over the range 1300-10 cm⁻¹. All 36 of the first-order Raman bands predicted by the factor group analysis (FGA) of the olivine structure type have been observed and assigned. Sinhalite has the smallest unit cell volume of any crystal with the olivine structure and is significantly more distorted than are the silicate olivines. This leads to distortion-related consequences in the vibrational spectra, such as greater wavenumber ranges for each type of mode and reduced Raman intensity for symmetric BO₄ modes. Intense Raman bands in the region 796-468 cm⁻¹ are assigned to BO₄ rotations/translations which deform the AlO bond without displacement of the aluminium atoms which lie on the M1 sites. Raman-active modes involving M1 cation displacements are incompatible with the FGA.     

Raman scattering study of the geometrically frustrated pyrochlore Y2Ru2O7

Geometrically frustrated pyrochlore Y₂Ru₂O₇, which shows a spin-glass-like transition at T_G ∼ 80 K, were investigated by temperature-dependent Raman scattering. Three discernable phonons appear around 315, 410, and 510 cm⁻¹ without any abrupt change in the number of Raman active modes within the temperature range of 10–300 K. Fitting each phonon with Lorentz oscillators, we analyzed the effects of temperature on the phonon frequencies and the linewidths. The temperature-dependence of the mode near 510 cm⁻¹ shows abnormal behavior below T_G, while the other two phonons follow the usual thermal effect of lattice vibration. This behavior can be understood in terms of spin–phonon coupling. Considering the atomic modulations of each phonon mode, it is conjectured that the 510 cm⁻¹ phonon mode is isotropically coupled to the spin degree of freedom, while the other modes are not.     

{[K.18-Crown-6]Br3}n: a unique tribromide-type and columnar nanotube-like structure for the oxidative coupling of thiols and bromination of some aromatic compounds

Reaction of 18-crown-6 with KBr and then with bromine (Br₂) afforded a unique tribromide salt with a nanotube-like structure ({[K.18-crown-6]Br₃}_n). Oxidative coupling of thiols and bromination of some aromatic compounds is the first report on the chemistry of this reagent.     

A New Approach for the Determination of Organobromine in Harmful Waste

Determination and estimation of organic sulphur, nitrogen, phosphor and halides in harmful chemical waste and estimation of emission of SO₂, NO_X, HCI, HBr or HI during incineration are the primary task of risk assessment. In our paper we present a new analytical approach for determination of bromine in a heterogeneous sample. The method applied is based on combustion of chemical waste under oxygen atmosphere (pressure 20 bars) and transformation to corresponding anions. These anions can be measured by ion chromatography. Using this method all hetero atoms can be transformed to corresponding anions (except for oxygen) and can be measured by ion chromatography. In this paper we report of determination of bromine content of chemical waste. During the combustion of organobromine compounds different types of inorganic bromine are formed. All types of bromine must be converted to Br^–. To fulfil it the above, ascorbic acid solution was used for absorbing and transforming all types bromine compounds to Br^–. We will discuss all the details about this new approach and give all the parameters to get efficient conversion from organic bromine to bromide anions.This revised version was published online in November 2005 with corrections to the Cover Date.     

Surface enhanced Raman scattering of ammonia

A surface enhanced Raman scattering (SERS) spectrum of 0.5 M NH₃ in 4.0 M KCl has been observed on a silver electrode. An approximate enhancement factor of 3 × 10⁵ is calculated, and additional evidence for the enhanced nature of the spectrum is provided by the observation that totally symmetric vibrations are depolarized and by the strong potential dependence of the intensity of surface lines. Assignments have been given to the SERS lines with the low-frequency lines assigned to a AgCl and AgN stretch. The positive shift of the maximum of the intensity versus voltage curve with a lower laser excitation frequency is taken as evidence for the occurrence of a charge transfer process from ammonia to the silver electrode. The fact that the SERS spectrum of NH₃ on Ag can only be observed at large electrolyte concentrations is attributed to the breaking of hydrogen bonding at the electrode-solution interface.     

Raman scattering study of CaB6 and YbB6

Phonon spectra of CaB₆ and RB₆ (R=Yb, Ce, and Pr) have been investigated by Raman scattering. We found clear spectral difference between divalent cation hexaboride and trivalent one. E_g mode shows the doublet spectra for only the divalent crystals of CaB₆ and YbB₆. The doublet spectra are understood by the two-dimensional charge distribution on B₆ without lattice distortion. In addition, the scattered intensities of the phonons change at around the ferromagnetic Curie temperature for YbB₆ and at T?600 K for CaB₆. These are the characteristic temperatures due to the change of the electronic system.     

Exchange reactions in selectively excited Br2(B3Π)

Laser-controlled bimolecular isotopic exchange reactions in bromine leading to alterations in natural isotopic abundance ratios have been directly observed using laser-modulated molecular beams and mass analysis. Several instances of selective excitation and subsequent rapid isotope exchange are observed within the 7 GHz gain curve of the exciting 514.5 nm line of an argon-ion laser. These exchange processes are attributed to the direct photochemical participation of excited molecular states in the B³Π_ou⁺ system of Br₂.     

Regioselective Bromination of Aromatic Amines and Phenols Using N-Benzyl-DABCO Tribromide

N-Benzyl-DABCO tribromide, a stable, solid organic ammonium tribromide, has been used as a bromine source for the regioselective and high-yielding bromination of aromatic amines and phenols. Mono-bromination proceeds well in the presence of a stoichiometric amount of bromine source at room temperature.     

The vibrational Raman spectra of selenium trioxide ag

The vibrational Raman spectra of selenium trioxide have been observed in the solid and vapour phases for the same sample. The spectrum of the vapour phase, which essentially arises from the monomeric species SeO₃, has provided for the first time values of the Raman-active ν₁, ν₃ and ν₄ wavenumbers of SeO₃. The spectrum of the solid phase represents an improvement on previous work and has enabled a partial assignment of the tetrameric species (SeO₃)₄ to be made.     

Photochemical reactions of disulphide radical anions in Frozen water-salt matrix

Photochemical reactions of disulphide radical anions CH₃SSCH ₃ ^– induced by light with =436, 405 and 365 nm in a watersalt matrix of 6M LiCl at 77 K have been studied. It has been shown that upon excitation disulphide radical anions may enter into electron photodetachment reaction and the S–S bond is cleft. Kinetical nonequivalence of disulphide radical anions in photochemical reactions has been shown to manifest itself in different mechanisms of their transformations.