Br Stretch Research Articles

Status of organochlorine pesticides in Ganga river basin: anthropogenic or glacial?

Abstract. This study reports the occurrences of organochlorine pesticides (OCPs) in the Ganga river basin covering 3 states, i.e., Uttarakhand, Uttar Pradesh and Bihar comprising 72% of total river stretch consisting of 82 sampling points covered through 3 sampling campaigns. Samples were monitored for 16 major OCPs, including hexachlorocyclohexanes (HCHs), Endosulfan group, Aldrin group, DDTs and Heptachlor group pesticides. The results showed the ng L−1 levels contamination of OCPs in all the stretches sampled during these campaigns. The results also revealed that different types of OCPs were dominating in different stretches in accordance with the land use practices and agricultural runoff generated from those stretches. HCHs were most frequently detected (detection rate = 75%) in mountainous stretch; Endosulfans were prominent in UP (detection rate = 75%) stretch while in BR stretch Aldrin group pesticides were paramount (detection rate = 34%). Source apportionment of the OCP's revealed that in the upper reaches of the Ganges i.e., in the state of Uttarakhand, the glacial melt may be responsible for the presence of OCP's. In the lower reaches, intensive agriculture and industrial activities may be significantly contributing these pesticides. The samples from tributaries of Ganga river were found to contain higher numbers of pesticides as well as higher concentrations. The maximum total pesticide concentration in an individual sample during these sampling campaigns was found in the Son river sample (0.17 μg L−1, Location: Koilwar, Bhojpur, Bihar).

The structure, properties, and nature of C–Br⋯F halogen bond involving HArF: Substitution, hybridization, and nonadditivity

Quantum chemical calculations have been performed to investigate the halogen bond of HArF with some brominated hydrocarbons at the MP2/aug-cc-pVTZ level. The C–Br bond in F3CBr–FArH complex is contracted, while it is elongated in other halogen-bonded complexes. However, the C–Br stretch vibration has a small red shift in all complexes. The strength of halogen bond becomes stronger in order of Csp3–Br<Csp2–Br<Csp–Br. The substitution position in CH2CHBr has a prominent effect on the strength of halogen bond. With the number of F atom in CH2CHBr, the halogen bond is stronger and a negative nonadditivity is present for F substitution. The average contribution of each F atom to the interaction energy of halogen bond is estimated to be −4kJ/mol. Additionally, two hydrogen-bonded complexes of BrCCH–FArH and HCCBr-pi-HArF have also been studied. These complexes have been analyzed with the electrostatic potentials and NBO theory.

Photoionization of two substituted methyl radicals: Cyanomethyl and bromomethyl

The photoionization of two substituted methyl radicals is presented. Cyanomethyl (CH 2CN) and bromomethyl (CH 2Br) were generated by flash pyrolysis from chloroacetonitrile and bromonitromethane. Mass-selective threshold photoelectron spectra of both radicals were recorded. The adiabatic ionization energies of CH 2CN and CH 2Br have been determined to be 10.28 eV ± 20 meV and 8.62 eV ± 10 meV. CH 2Br shows a vibrational progression upon ionization that is assigned to the C–Br stretch mode. The photoionization of the precursors as well as the dissociative photoionization of CH 2Br were examined. A 0 K heat of formation of 174.5 kJ mol −1 is derived for CH 2Br and a C–Br bond dissociation energy of 334.1 kJ mol −1.

Low-energy electron collisions with CH3Br: the dependence of elastic scattering, vibrational excitation, and dissociative attachment on the initial vibrational energy

Using the laser photoelectron attachment (LPA) method at an energy width of 1–2 meV, the Br− yield due to dissociative electron attachment to the molecule CH3Br has been measured over the energy range 1–180 meV at a gas temperature of 600 K. The data clearly exhibit the vibrational Feshbach resonance predicted by Wilde et al (2000 J. Phys. B: At. Mol. Opt. Phys. 33 5479) and associated with the v3 = 4 vibrational level of the C–Br stretch mode in the neutral molecule. We also report the dependence of the rate coefficient for Rydberg electron attachment (measured at high principal quantum numbers n ≈ 150) on gas temperature and find an Arrhenius-type behaviour compatible with the results of R-matrix calculations and with earlier findings of electron swarm experiments. With the aim to stimulate experiments, we report absolute cross sections for elastic scattering and vibrationally inelastic scattering involving the C–Br stretch mode v3 of CH3Br for several initial vibrational levels in the range v3 = 0–4; they were obtained with R-matrix calculations based on the same potential input as that involved in the computations of the cross sections for dissociative attachment.

Four-dimensional quantum study on exothermic complex-forming reactions: Cl−+CH3Br→ClCH3+Br−

The exothermic gas-phase bimolecular nucleophilic substitution (S(N)2) reaction Cl(-)+CH(3)Br (upsilon1',upsilon2',upsilon3')-->ClCH(3) (upsilon1,upsilon2,upsilon3)+Br- and the corresponding endothermic reverse reaction have been studied by time-independent quantum scattering calculations in hyperspherical coordinates on a coupled-cluster potential-energy surface. The dimensionality-reduced model takes four degrees of freedom into account [Cl-C and C-Br stretching modes (quantum numbers upsilon3' and upsilon3); totally symmetric modes of the methyl group, i.e., C-H stretching (upsilon1' and upsilon1) and umbrella bending vibrations (upsilon2' and upsilon2)]. Diagonalization of the Hamiltonian was performed employing the Lanczos algorithm with a variation of partial reorthogonalization. A narrow grid in the total energy was employed so that long-living resonance states could be resolved and extracted. While excitation of the reactant umbrella bending mode already leads to a considerable enhancement of the reaction probability, its combination with vibrational excitation of the broken C-Br bond, (0, 1, 1), results in a strong synergic effect that can be rationalized by the similarity with the classical transitional normal mode. Exciting the C-H stretch has a non-negligible effect on the reaction probability, while for larger translational energies this mode follows the expected spectatorlike behavior. Combination of C-Br stretch and symmetric C-H, (1,0,1), stretch does not show a cooperative effect. Contrary to the spectator mode concept, energy originally stored in the C-H stretching mode is by no means conserved, but almost completely released in other modes of the reaction products. Products are most likely formed in states with a high degree of excitation in the new C-Cl bond, while the internal modes of the methyl group are less important. Reactants with combined umbrella/C-Br stretch excitation, (0, 1, 1), may yield products with two quanta in the umbrella mode.

Formation and reactivity of high quality halogen terminated Si(1 1 1) surfaces

High quality halogenated silicon surfaces have been produced using gas phase reactions of H-terminated Si(1 1 1) with molecular chlorine or bromine at room temperature. STM images show that the resulting surfaces maintain the low defect densities of H/Si(1 1 1). High resolution electron energy loss spectra confirm halogenation of the surface through the observation of the Si–Cl or Si–Br stretch modes and also indicate the absence of significant oxidation or contamination of the surface even after brief air exposure. While chlorine terminated surfaces are found to be surprisingly stable to exposure to water vapour, dipping in water destroys the surface by insertion of oxygen into silicon backbonds and formation of terminal H and O–H groups. In lab air, oxidation is observed to proceed rather slowly and in a patch-like fashion with substantial chlorine still present after >10 h exposure.

Vibrational frequencies and structural determination of N-bromodifluoromethylenimine

The vibrational frequencies and corresponding normal mode assignments of N-bromodifluoromethylenimine (CF2NBr) are examined theoretically using the gaussian98 set of quantum chemistry codes. All normal modes were successfully assigned to one of seven types of motion predicted by a group theoretical analysis (C–F stretch, CN stretch, N–Br stretch, CN–Br bend, CF2 scissors, CF2 wag, and CF2 rock). Theoretical infrared and Raman intensities are reported. The molecular orbitals and bonding of N-bromodifluoromethylenimine are examined.

Vibrational frequencies and structural determination of N-bromodifluoromethylenimine

The vibrational frequencies and corresponding normal mode assignments of N-bromodifluoromethylenimine (CF 2NBr) are examined theoretically using the gaussian98 set of quantum chemistry codes. All normal modes were successfully assigned to one of seven types of motion predicted by a group theoretical analysis (C–F stretch, CN stretch, N–Br stretch, CN–Br bend, CF 2 scissors, CF 2 wag, and CF 2 rock). Theoretical infrared and Raman intensities are reported. The molecular orbitals and bonding of N-bromodifluoromethylenimine are examined.

Transient resonance Raman investigation of CHBr 2 and CHBrCl radicals

Transient resonance Raman spectra of CHBr 2 and CHBrCl radicals following ultraviolet photolysis of CHBr 3 and CHBr 2Cl, respectively, in methanol solution are reported. Most of the Raman intensity appears in the C–Br stretch and Br–C–Br bend or Cl–C–Br bend fundamentals accompanied by moderate intensity in their overtones and/or combination bands. The CH 2Br radical was previously observed to have only one predominant Franck–Condon active mode (the C–Br stretch) with a strong overtone progression. The Franck–Condon region photodissociation dynamics of the CHBr 2 and CHBrCl radicals appear to have more multidimensional character compared to halomethyl radicals like CH 2Br.

Investigation of quantum effects on vibrational excitation of CF3Br scattering from graphite

Mixed quantum–classical molecular dynamics simulations of vibrational excitation of CF3Br in collisions with graphite have been carried out. The quantum problem is made numerically manageable by a reduced dimensionality treatment of the internal degrees of freedom including only the C–Br stretch and the umbrella motion. These two coordinates are treated quantum mechanically as a 2D wavepacket whereas the translational and rotational degrees of freedom and the surface atoms are treated classically. A 3D wavepacket scheme where the distance to the surface is included among the quantum degrees of freedom is also considered as well as purely classical trajectories. It is found that the total vibrational excitation is quite low. Excitation of the C–Br stretch dominates with only a very small fraction of the excitation energy going into the umbrella motion. The 2D wavepacket calculations and the classical trajectories (initiated without zero point energy) give nearly identical results for the total excitation but the 2D wavepacket model excites the umbrella motion to a lesser extent. The 3D wavepacket method shows a considerable dependence on the initial width of the wavepacket due to the underlying mean-field approximation. All methods show a linear increase in vibrational excitation with increasing surface temperature, in agreement with previous experimental results.

Vibrational frequencies and structural determinations of 3,5-dibromo-1,2,4-trithia-3,5-diborolane

The vibrational frequencies and corresponding normal mode assignments of 3,5-dibromo-1,2,4-trithia-3,5-diborolane (B 2S 3Br 2) are examined theoretically using the Gaussian98 set of quantum chemistry codes. All normal modes were successfully assigned to one of six types of motion predicted by a group theoretical analysis (BS stretch, BBr stretch, SS stretch, SBS bend, BBr wag, B(SSBr) umbrella motion) utilizing the C 2 v symmetry of the molecule. The vibrational modes of the naturally isotopically substituted (1- 10B and 2- 10B) forms of B 2S 3Br 2 were also calculated and compared against experimental data. The molecular orbitals of B 2S 3Br 2 are examined. The calculations suggest that a considerable amount of π bonding occurs in B 2S 2Br 2.

Visible Wavelength Electronic Spectroscopy of the DCBr ÖX̃ Vibronic Transition

The laser excitation and dispersed fluorescence spectra of the DCBr ÖX̃ vibronic bands between 570 and 600 nm were successfully obtained in a direct current (DC) discharge supersonic free jet expansion. The analysis of the dispersed fluorescence spectra results in information on the vibrational structure of the DCBr X̃1A′ state for the first time. Perturbations due to the background levels of the ã3A″ state can be recognized. The data are consistent with the previously reported triplet-singlet energy gap (the zero-point level energy gap) of 5.73±0.02 kcal/mol for HCBr. We have determined the vibrational frequencies to be 832 cm−1 (bend) and 663 cm−1 (C–Br stretch) for the X̃1A′ state. In addition to the X̃ state vibrational structure, the Ã(0,6,2) and Ã(0,5,0) levels were also observed for the first time.

Laser excitation and dispersed fluorescence spectra of the HCBr ÖX̃ vibronic transition

We recorded the laser-induced fluorescence (LIF) excitation spectra of several HCBr ÖX̃ vibronic transitions between 580 nm and 630 nm in a direct current (DC) discharge supersonic free jet expansion. Dispersed fluorescence spectra, following excitation of these HCBr ÖX̃ vibronic transitions were successfully obtained for the first time. The analysis of the dispersed fluorescence spectra reveals the details of the vibrational structure of the X̃ 1A′ state as well as that of the ã 3A″ state. The vibrational frequencies were determined as 1119 cm−1 (bend), 676 cm−1 (C–Br stretch) for the X̃ 1A′ state and 994 cm−1 (bend), 733 cm−1 (C–Br stretch) for the ã 3A″ state. In addition, based upon the observation of the triplet state levels, the value of the triplet–singlet energy gap (the zero-point level energy gap) could be extracted from the dispersed fluorescence spectra and is to be approximately 2006±8 cm−1 (5.73±0.02 kcal/mol).

Unusual temperature dependence in dissociative electron attachment to 1,4-chlorobromobenzene

Electron attachment to 1,4-chlorobromobenzene yields the ionic fragments Cl − and Br − appearing in the energy range ≈0–1 eV with Br − by about a factor of 70 more intense at room temperature. Increasing the temperature of the target molecule up to 540 K results in an unexpected temperature dependence of the cross-section in that it first increases and after passing a maximum (400 K for Br − and 460 K for Cl −) decreases. This behaviour is qualitatively interpreted by the temperature dependent population of the relevant normal modes containing the C–Cl and C–Br stretch vibration.

Solvation Effects on the A-Band Photodissociation of Dibromomethane: Turning a Photodissociation into a Photoisomerization

We have obtained A-band resonance Raman spectra of dibromomethane in the gas and solution phase and nanosecond time-resolved resonance Raman spectra of dibromomethane photoproducts. The A-band resonance Raman spectra suggest the short-time dynamics are similar to other dihalomethanes that are known to have direct photodissociation reactions in the gas phase. Several power dependent A-band resonance Raman bands were tentatively assigned to the C−Br stretch overtone progression of the CH2Br radical which has a strong absorption band that is coincident with the A-band resonance Raman excitation wavelength. Two-color nanosecond time-resolved resonance Raman spectra (266.0 nm pump/341.5 nm probe) were obtained and comparison of the vibrational frequencies to the results for density functional theory calculations indicate that the iso-CH2Br−Br species is mainly responsible for the transient photoproduct absorption band ∼360 nm. Our preliminary results for A-band photoexcitation of dibromomethane in conjunction ...

Transient resonance Raman and density functional theory investigation of bromomethyl radical

We have obtained transient resonance Raman spectra of CH 2Br produced following A-band excitation of dibromomethane and performed density functional theory computations to estimate the vibrational frequencies and electronic transition energies of the CH 2Br radical. Our results indicate the CH 2Br radical has a C–Br bond longer than in the parent CH 2Br 2 molecule but shorter than in a typical bromoalkane. Both the experimental and calculated resonance Raman spectra which display a strong overtone progression in the nominal C–Br stretch mode suggest that the first excited electronic state of the CH 2Br radical experiences motion mostly along the C–Br bond in the Franck–Condon region.

Pressure and temperature-dependent gauche-trans isomerization of 1-bromopropane: Raman measurement and statistical thermodynamic analysis

Raman measurements of the isomerization equilibrium in liquid 1-bromopropane are compared with perturbed hard-body fluid predictions. The integrated areas of the Raman bands arising from the C–Br stretch of the gauche and trans conformations are used to track the isomerization equilibrium as a function of pressure and temperature. Repulsive solvent–solute interactions are treated using the recently developed excluded-volume-anisotropy model (based on realistic molecular structures for the two isomers and the equation of state of liquid 1-bromopropane), and cohesive interactions are treated using the van der Waals mean field approximation. The results illustrate the delicate balance of attractive and repulsive solute–solvent interactions which underlie the effects of solvation on chemical equilibria. Comparison of the measured and predicted changes in ΔH with pressure, and ΔV with temperature, are used to determine parameters describing the attractive mean field and cavity formation energies of the two isomers, as well as the isomerization enthalpy and entropy of the isolated molecule.

Investigation of the short-time photodissociation dynamics of trans-1-bromo-2-iodoethane in the A -band absorption

We have obtained resonance Raman spectra and absolute Raman cross section measurements at five excitation wavelengths within the A-band absorption for 1-bromo-2-iodoethane in cyclohexane solution. The resonance Raman spectra have most of their intensity in the fundamentals, overtones, and combination bands of six Franck–Condon active vibrational modes; the nominal CCI bend, C–I stretch, C–Br stretch, C–C stretch, CH2 wag with the Br atom attached to the CH2 group, and CH2 wag with the I atom attached to the CH2 group. The resonance Raman intensities and A-band absorption spectrum were simulated using a simple model and time-dependent wave packet calculations. The simulation results and normal mode descriptions were used to find the short-time photodissociation dynamics in terms of internal coordinate displacements. The A-band short-time photodissociation dynamics for trans-1-bromo-2-iodoethane show that the C–I, C–Br, and C–C bonds as well as the CCI, CCBr, HCC, ICH, and BrCH angles have significant changes during the initial stages of the photodissociation reaction. This indicates the photodissociation reaction has a large degree of multidimensional character and suggests that the bromoethyl photofragment receives substantial internal excitation in so far as the short-time photodissociation dynamics determines the energy partitioning. Comparison of our results for 1-bromo-2-iodoethane with the A-band short-time dynamics of iodoethane, 1-chloro-2-iodoethane, and 1,2-diiodoethane and the trends observed for their A-band absorption spectra suggest that both the C–I and C–Br bonds experience a noticeable amount of photoexcitation.

Nanosecond time-resolved resonance Raman investigation of the T 1→T n transition of 2-bromonaphthalene

We have obtained nanosecond time-resolved resonance Raman spectra of the T 1→T n transition of 2-bromonaphthalene in methanol solvent. The transient resonance Raman spectra indicate that there are mostly structural changes in the C–C stretch ν 5a vibrational coordinate along with a smaller amount of change in the C–Br stretch and C–Br in plane deformation coordinates. This suggests that the T 1→T n transition of 2-bromonaphthalene does not initially excite a directly dissociative triplet state and a predissociative mechanism is probably responsible for the efficient T n state C–Br bond cleavage found in previous photochemistry studies.

COMPLEXES OF DI- AND TERVALENT COBALT, MONO- AND DIVALENT COPPER WITH 2-(2-ARYLTELLUROETHYL) PYRIDINES: SYNTHESIS AND SPECTRA

Abstract Complexes of 2-(2-phenyltelluroethyl)pyridine (L1) and 2-{2-(4-methoxyphenyl}telluro) ethylpyridine (L2) with Co(II and III) and Cu(I and II) having compositions [Co(L)Br2] (1, 2), [Co(L)2]BPh4 (3, 4), [Cu(L)Br]2 (5) and [Cu(L)2Br]2(BPh4)2 (6) have been synthesized and characterized by molecular weight, molar conductance, ESR, IR, 1H and 125Te{1H} NMR and electronic spectra in conjunction with magnetic susceptibility measurements and ESCA (in case of Cu (II)). The species 6 is diamagnetic and ESR silent because the odd electron of each copper is paired as two basal coordination planes are coplanar. The μeff values of 1 and 2 (3.5–3.7 BM) indicate the existence of the equilibrium which is supported by molecular weight and ΛM values. The v(Cu[sbnd]Br) stretch at 181 cm−1 in the IR spectrum of 5 indicates that it is a bromine bridged dimer. The 125Te {1H} NMR signal of L2 undergoes an upfield shift (141 ppm) on coordination with Cu(I). Deshielding of up to 1 ppm of the CH2Te signal in 1H NMR spectra...