Spectroscopic Signatures of Halogens in Clathrate Hydrate Cages. 1. Bromine

Interaction energy of Cl2 and Br2 with H2 O: Exchange, dispersion and density the crucial ingredients

Modern Density Functional Theory models are now suitable for many molecular and condensed phase studies. The study of noncovalent interactions, a well-known drawback, is no longer an insurmountable obstacle through design and empirical corrections. However, using empirical corrections as in the DFT-D methods might not be an all-in-one solution. This work uses a simple system, X₂ -H₂ O with X = Cl or Br, with two different interactions, halogen-bonded (XB) and hydrogen-halogen (HX), to investigate the capability of current density functional approximations (DFA) in predicting interaction energies with eight different exchange-correlation functionals. SAPT(DFT) provides, for all the studied cases, better predictions than the widely used supermolecular approach. In addition, the components of the interaction energy suggest where some of the shortcomings originate in each DFA. The analysis of the functionals used confirms that PBE0 and ω-B97X-D have a physically correct behavior. Using SAPT(DFT) and PBE0, and ω-B97X-D, we obtained the interaction energy of Cl₂ and Br₂ inside different clathrate cages and satisfactorily compared with wavefunction results; hence, the lower and upper limits of this value are defined: Cl₂ @5¹² , -5.3 ± 0.3 kcal/mol; Cl₂ @5¹² 6² , -5.5 ± 0.1 kcal/mol; Br₂ @5¹² 6² , -7.6 ± 1.0 kcal/mol; Br₂ @5¹² 6³ , -10.6 ± 1.0 kcal/mol; Br₂ @5¹² 6⁴ , -10.9 ± 0.8 kcal/mol.

Tandem Efficient Bromine Removal and Silver Recovery by Resorcinol-Formaldehyde Resin Nanoparticles

Halogen wastewater greatly threatens the health of human beings and aquatic organisms due to its severe toxicity, corrosiveness, and volatility. Efficient bromine removal is therefore urgently required, while existing Br₂-capture materials often face challenges from limited water stability and possible halogen leaking. We report a facile and efficient aqueous Br₂ removal method using submicron resorcinol-formaldehyde (RF) resin nanoparticles (NPs). The abundant aromatic groups dominate the Br₂ removal by substitution reactions. An excellent Br₂ conversion capacity of 7441 mg g_RF^-1 was achieved by RF NPs that outperform state-of-the-art materials by ∼2-fold, along with advantages including good water stability, low cost, and easy fabrication. Two recycling-coupled (electrochemical or H₂O₂-involved) Br₂ removal routes further reveal the feasibility of in-depth halogen removal by RF NPs. The brominated resin can be downstream upcycled for silver recovery, realizing the harvesting of precious metal, reducing of heavy-metal pollution, and resource utilization of brominated resin.

Regularized vibrational energies prediction and potential energy function study for some interhalogen diatomic molecules

Full vibrational energies and corresponding analytical potential energy functions (APEFs) for [Formula: see text] of ClF, [Formula: see text] of ICl, [Formula: see text] of IBr, and [Formula: see text]of IF are obtained by four-term variational algebraic energy-consistent method [VAECM(4)]. Three major methods are used to handle the regularization problems and make the results more reliable: (1) physical constraints like progressive dissociation behavior are used to overcome overfitting problems; (2) reasonable variational method is used to expand the experimental dataset and make the model optimization much faster; and (3) validation dataset is used to further enhance the reliability. After these treatments, our results agree well with experiment and have a good physical converged behavior in molecular dissociation and asymptotic region. This study provides important reference data for the related molecules.

The anomalous halogen bonding interactions between chlorine and bromine with water in clathrate hydrates

Clathrate hydrate phases of Cl₂ and Br₂ guest molecules have been known for about 200 years. The crystal structure of these phases was recently re-determined with high accuracy by single crystal X-ray diffraction. In these structures, the water oxygen–halogen atom distances are determined to be shorter than the sum of the van der Waals radii, which indicates the action of some type of non-covalent interaction between the dihalogens and water molecules. Given that in the hydrate phases both lone pairs of each water oxygen atom are engaged in hydrogen bonding with other water molecules of the lattice, the nature of the oxygen–halogen interactions may not be the standard halogen bonds characterized recently in the solid state materials and enzyme–substrate compounds. The nature of the halogen–water interactions for the Cl₂ and Br₂ molecules in two isolated clathrate hydrate cages has recently been studied with ab initio calculations and Natural Bond Order analysis (Ochoa-Resendiz et al. J. Chem. Phys. 2016, 145, 161104). Here we present the results of ab initio calculations and natural localized molecular orbital analysis for Cl₂ and Br₂ guests in all cage types observed in the cubic structure I and tetragonal structure I clathrate hydrates to characterize the orbital interactions between the dihalogen guests and water. Calculations with isolated cages and cages with one shell of coordinating molecules are considered. The computational analysis is used to understand the nature of the halogen bonding in these materials and to interpret the guest positions in the hydrate cages obtained from the X-ray crystal structures.

Performance of local correlation methods for halogen bonding: The case of Br2-(H2O)n,n = 4,5 clusters and Br2@5(12)6(2) clathrate cage

The performance of local correlation methods is examined for the interactions present in clusters of bromine with water where the combined effect of hydrogen bonding (HB), halogen bonding (XB), and hydrogen-halogen (HX) interactions lead to many interesting properties. Local methods reproduce all the subtleties involved such as many-body effects and dispersion contributions provided that specific methodological steps are followed. Additionally, they predict optimized geometries that are nearly free of basis set superposition error that lead to improved estimates of spectroscopic properties. Taking advantage of the local correlation energy partitioning scheme, we compare the different interaction environments present in small clusters and those inside the 5(12)6(2) clathrate cage. This analysis allows a clear identification of the reasons supporting the use of local methods for large systems where non-covalent interactions play a key role.

Hydrate Shell Growth Measured Using NMR

Benchtop nuclear magnetic resonance (NMR) pulsed field gradient (PFG) and relaxation measurements were used to monitor the clathrate hydrate shell growth occurring in water droplets dispersed in a continuous cyclopentane phase. These techniques allowed the growth of hydrate inside the opaque exterior shell to be monitored and, hence, information about the evolution of the shell's morphology to be deduced. NMR relaxation measurements were primarily used to monitor the hydrate shell growth kinetics, while PFG NMR diffusion experiments were used to determine the nominal droplet size distribution (DSD) of the unconverted water inside the shell core. A comparison of mean droplet sizes obtained directly via PFG NMR and independently deduced from relaxation measurements showed that the assumption of the shell model-a perfect spherical core of unconverted water-for these hydrate droplet systems is correct, but only after approximately 24 h of shell growth. Initially, hydrate growth is faster and heat-transfer-limited, leading to porous shells with surface areas larger than that of spheres with equivalent volumes. Subsequently, the hydrate growth rate becomes mass-transfer-limited, and the shells become thicker, spherical, and less porous.

Molecular dynamics simulation of halogen bonding in Cl2, BrCl, and mixed Cl2/Br2 clathrate hydrates

Clathrate hydrate phases of dihalogen molecules have properties that differ from those of other guest molecules of similar size. The water oxygen–chlorine distances in the structure I (sI) Cl2 hydrate are smaller than the sum of the van der Waals radii of oxygen and chlorine. Bromine hydrate forms a unique clathrate hydrate structure that is not seen in other guest substances. In mixed Cl2/Br2 structure I hydrate, the water oxygen–bromine distances are also smaller than the sum of the oxygen and bromine van der Waals radii. We previously studied the structure of three dihalogen clathrate hydrates using single crystal X-ray diffraction and described these structural features in terms of halogen bonding between the dihalogen and water molecules. In this work, we perform molecular dynamics simulations of cubic sI Cl2, mixed Cl2/Br2, and BrCl clathrate hydrate phases. We perform quantum chemical computations on the dihalogen molecules to determine the nature of σ-hole near the halogen atoms. We fit the electrostatic potential of the molecules to point charge models including dummy atoms that represent σ-holes adjacent to the halogen molecules. Molecular dynamics simulations are used to determine the lattice constants, radial distribution functions, and guest dynamics in these phases. We determine the effect of guest size and difference in halogen bonding on the properties of the clathrate hydrate phase. Simulations for the Cl2, BrCl, and mixed Cl2/Br2 hydrates are performed with small cages of the sI clathrate hydrate phases completely full or filled with experimental occupancies with Cl2 guests.

Role of bromide in hydrogen peroxide oxidation of CTAB-stabilized gold nanorods in aqueous solutions

In recent years hydrogen peroxide has often been used as the oxidizing agent to tune the resonance wavelength of gold nanorods (AuNRs) through anisotropic shortening in the presence of cetyltrimethylammonium bromide (CTAB). However, a complete picture of the reaction mechanism remains elusive. In this work, we present a systematic study on the mechanism of the AuNR oxidation by revealing the important role of bromide. Hydrogen peroxide slowly oxidizes bromide into elemental bromine. The latter two form tribromide, which exhibits a characteristic 272 nm absorption peak. The peak intensity, representing the concentration of tribromide, is found to have a linear correlation with the oxidation rate of AuNRs. Tribromide approaches AuNRs through conjugating strongly with CTA cationic micelles, which leads to the oxidation occurring on the surface of AuNRs. In contrast, the CTA micelles protect AuNRs from the direct oxidation by hydrogen peroxide. Our findings are believed to provide new insights into the reaction mechanism occurring in the relevant CTAB-AuNR systems, which can be important for understanding the principles governing the reaction dynamics.

Motion of Br2 molecules in clathrate cages. A computational study of the dynamic effects on its spectroscopic behavior

This work looks into the spectroscopic behavior of bromine molecules trapped in clathrate cages combining different methodologies. We developed a semiempirical quantum mechanical model to incorporate through molecular dynamics trajectories, the effect movement of bromine molecules in clathrate cages has on its absorption spectra. A simple electrostatic model simulating the cage environment around bromine predicts a blue shift in the spectra, in good agreement with the experimental evidence.

Relativistic four-component potential energy curves for the lowest 23 covalent states of molecular bromine (Br2)

The covalent excited states and ground state of the Br2 molecule has been investigated by using four-component relativistic COSCI and MRCISD methods. These methods were performed for all covalent states in the representation Ω((±)). Calculated potential energy curves (PECs) were obtained at the four-component COSCI level, and spectroscopic constants (R(e), D(e), D0, ω(e), ω(e)x(e), ω(e)y(e), B(e), α(e), γ(e), Te, Dv) for bounded states are reported. The vertical excitations for all covalent states are reported at COSCI, MRCISD, and MRCISD+Q levels. We also present spectroscopic constants for two weakly bounded states (A':(1)2u and B':(1)0(-)u) not yet reported in the literature, as well as accurate analytical curves for all five relativistic molecular bounded sates [the ground state X:0 g(+) and the excited states A:(1)1(u), B:(1)0(u)(+), C:(2)1(u), and B':(1)0(u)(-)] found in this work.

Analysis of the visible absorption spectrum of I2 in inert solvents using a physical model

Absorption spectra of I(2) dissolved in n-heptane and CCl(4) are analyzed with a quantum gas-phase model, in which spectra at four temperatures between 15° and 50 °C are least-squares fitted by bound-free spectral simulations to obtain estimates of the excited-state potential energy curves and transition moment functions for the three component bands--A ← X, B ← X, and C ← X. Compared with a phenomenological band-fitting model used previously on these spectra, the physical model (1) is better statistically, and (2) yields component bands with less variability. The results support the earlier tentative conclusion that most of the ~20% gain in intensity in solution is attributable to the C ← X transition. The T-dependent changes in the spectrum are accounted for by potential energy shifts that are linear in T and negative (giving red shifts in the spectra) and about twice as large for CCl(4) as for heptane. The derived upper potentials resemble those in the gas phase, with one major exception: In the statistically best convergence mode, the A potential is much lower and steeper, with a strongly varying transition moment function. This observation leads to the realization that two markedly different potential curves can give nearly identical absorption spectra.

Spectroscopic methods in gas hydrate research

Gas hydrates are crystalline structures comprising a guest molecule surrounded by a water cage, and are particularly relevant due to their natural occurrence in the deep sea and in permafrost areas. Low molecular weight molecules such as methane and carbon dioxide can be sequestered into that cage at suitable temperatures and pressures, facilitating the transition to the solid phase. While the composition and structure of gas hydrates appear to be well understood, their formation and dissociation mechanisms, along with the dynamics and kinetics associated with those processes, remain ambiguous. In order to take advantage of gas hydrates as an energy resource (e.g., methane hydrate), as a sequestration matrix in (for example) CO(2) storage, or for chemical energy conservation/storage, a more detailed molecular level understanding of their formation and dissociation processes, as well as the chemical, physical, and biological parameters that affect these processes, is required. Spectroscopic techniques appear to be most suitable for analyzing the structures of gas hydrates (sometimes in situ), thus providing access to such information across the electromagnetic spectrum. A variety of spectroscopic methods are currently used in gas hydrate research to determine the composition, structure, cage occupancy, guest molecule position, and binding/formation/dissociation mechanisms of the hydrate. To date, the most commonly applied techniques are Raman spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy. Diffraction methods such as neutron and X-ray diffraction are used to determine gas hydrate structures, and to study lattice expansions. Furthermore, UV-vis spectroscopic techniques and scanning electron microscopy (SEM) have assisted in structural studies of gas hydrates. Most recently, waveguide-coupled mid-infrared spectroscopy in the 3-20 μm spectral range has demonstrated its value for in situ studies on the formation and dissociation of gas hydrates. This comprehensive review summarizes the importance of spectroscopic analytical techniques to our understanding of the structure and dynamics of gas hydrate systems, and highlights selected examples that illustrate the utility of these individual methods.

Solvation dynamics through Raman spectroscopy: hydration of Br2 and Br3(-), and solvation of Br2 in liquid bromine

Raman spectroscopy of bromine in the liquid phase and in water illustrates uncommon principles and yields insights regarding hydration. In liquid Br(2), resonant excitation over the B((3)Π(0u)(+)) ← X((1)Σ(g)(+)) valence transition at 532 nm produces a weak resonant Raman (RR) progression accompanied by a five-fold stronger non-resonant (NR) scattering. The latter is assigned to pre-resonance with the C-state, which in turn must be strongly mixed with inter-molecular charge transfer states. Despite the electronic resonance, RR of Br(2) in water is quenched. At 532 nm, the homogeneously broadened fundamental is observed, as in the NR case at 785 nm. The implications of the quenching of RR scattering are analyzed in a simple, semi-quantitative model, to conclude that the inertial evolution of the Raman packet in aqueous Br(2) occurs along multiple equivalent water-Br(2) coordinates. In distinct contrast with hydrophilic hydration in small clusters and hydrophobic hydration in clathrates, it is concluded that the hydration shell of bromine in water consists of dynamically equivalent fluxional water molecules. At 405 nm, the RR progression of Br(3)(-) is observed, accompanied by difference transitions between the breathing of the hydration shell and the symmetric stretch of the ion. The RR scattering process in this case can be regarded as the coherent photo-induced electron transfer to the solvent and its radiative back-transfer.

An ab initio calculation of the valence excitation spectrum of H2O...Cl2: comparison to condensed phase spectra

Valence electronic excitation spectra are calculated for the H(2)O...Cl(2) dimer using state-of-the art ab initio potentials for both the ground and the valence excited states, a basis set calculation of the ground state nuclear wave function, and a wave packet analysis to simulate the dynamics on the excited state surface. The peak of the H(2)O...Cl(2) dimer spectrum is blue-shifted by 1250 cm(-1) from that of the free Cl(2) molecule. This is less than the value previously estimated from vertical excitation energies but still significantly more than the blue shift in aqueous solution and clathrate-hydrate solid. Seventy percent of the blue shift is attributed to ground state stabilization, the rest to excited state repulsion. Spin-orbit effects are found to be small for this dimer. Homogeneous broadening is found to be slightly smaller for the dimer than for the free Cl(2). The reflection principle and spectator model approximations were tested and found to be quite satisfactory. This is promising for an eventual simulation of the condensed phase spectra.

Vibronic dynamics of I(2) trapped in amorphous ice: coherent following of cage relaxation

Four-wave mixing measurements are carried out on I(2)-doped ice, prepared by quench condensing the premixed vapor at 128 K. Coherent vibrational dynamics is observed in two distinct ensembles. The first is ascribed to trapping in asymmetric polar cages in which, as in water, the valence absorption of the molecule is blueshifted by 3500 cm(-1), predissociation of the B state is complete upon the first extension of the molecular bond, and the vibrational frequency in the ground state (observed through coherent anti-Stokes Raman scattering) is reduced by 6.5%. The effect is ascribed to polarization of the molecule. The implied local field and the ionicity of the molecule are extracted, to conclude that the molecule is oxygen bonded to one water molecule on one side and hydrogen bonded on the other side. The second ensemble is characterized by the transient grating signal, which shows coherent vibrational dynamics on the B state. The small predissociation rate in this site suggests a symmetric cage in which the local electric field undergoes effective cancellation; and consistent with this, the extracted blueshift of the valence transition in this site (approximately 1500 cm(-1)) coincides with that observed in clathrate hydrates of iodine. Remarkably, in this site, the vibrational period of the B state packet coherently stretches from an initial value of 245 fs to 325 fs in the course of five oscillations (1.3 ps), indicative of vibrationally adiabatic following of the cage expansion. The dynamics is characteristic of a molecule trapped in a tight symmetric cage, with a soft cage coordinate that relaxes without eliciting elastic response. Enclathration in low-density amorphous ice is concluded.

Theoretical Investigation of the Electronically Excited States of Chlorine Hydrate

As a step toward a first principles characterization of the optical properties of chlorine hydrate, we have calculated the electronic absorption spectrum of a chlorine molecule trapped in dodecahedral (H2O)20 and hexakaidodecahedral (H2O)24 cages. For comparison, spectra were also calculated for an isolated Cl2 molecule as well as for selected Cl2(H2O)n, n < or =8, clusters cut out of the Cl2(H2O)20 cluster, allowing us to follow the evolution of the low-lying excited states with increasing number of surrounding water molecules. Although encapsulation of a chlorine molecule within the water cages has relatively little effect on its low-lying valence transitions, it does result in a large number of solvent-to-solute charge-transfer transitions at energies starting near 48,000 cm(-1).