Alkyl Halide Molecules Research Articles

Photocatalytic Degradation of Alkyl Halide Molecules Promoted by CsPbBr3 Perovskite Quantum Dots

Photocatalytic Degradation of Alkyl Halide Molecules Promoted by CsPbBr₃ Perovskite Quantum Dots

Partial hydration of n-alkyl halides at the water–vapor interface: a molecular simulation study with atmospheric implications

Classical molecular dynamics simulations with a polarizable force field were used to study adsorption of gas-phase alkyl halides to the surface of liquid water and their hydration properties in the interfacial environment. A systematic investigation has been performed for a set of monosubstituted alkyl chlorides, bromides and iodides of the alkyl chain length from one to five carbon atoms (C n H2n+1X, n = 1–5, X = Cl, Br, or I). All alkyl halides readily adsorb to the water surface and exhibit a strong preference for interfacial (partial) hydration. When adsorbed, the alkyl halide molecules reside primarily in the outermost region of the water–vapor interface. The (incomplete) hydration shell of the surface-adsorbed methyl halide species is centered on the methyl end of the molecule, with the halogen atom largely exposed and facing away from water into the gas phase. The maximum hydration of the longer-chain alkyl halides is localized around the α-CH2 group next to the halogen. With an increasing chain length, the alkyl halide molecules align more parallel to the surface. However, ethyl and propyl halides still have the halogen atom rather exposed, pointing almost freely into the gas phase. The behavior of butyl and pentyl halides on the water surface resembles that of alcohols, with the polar region of the CH2X group interacting with water and the rest of the increasingly nonpolar hydrocarbon chain pointing on average away from water. Consequently, the halogen atom becomes more, albeit not fully, hydrated. The propensity of alkyl halides for the water–vapor interface along with the specific character of the partial hydration of the surface-adsorbed alkyl halides and their preferred interfacial orientation is likely to be of importance for heterogeneous chemical processes, involving alkyl halides adsorbed on the surface of aqueous aerosol droplets or ice particles in the atmosphere.

Students' understanding of alkyl halide reactions in undergraduate organic chemistry

Organic chemistry is an essential subject for many undergraduate students completing degrees in science, engineering, and pre-professional programs. However, students often struggle with the concepts and skills required to successfully solve organic chemistry exercises. Since alkyl halides are traditionally the first functional group that is studied in undergraduate organic chemistry courses, establishing a robust understanding of the concepts and reactions related to them can be beneficial in assuring students' success in organic chemistry courses. Therefore, the purpose of this study was to elucidate and describe students' understanding of alkyl halide reactions in an undergraduate organic chemistry course. Participants were interviewed using a think-aloud protocol in which they were given a set of questions dealing with reactions and mechanisms of alkyl halide molecules in order to shed light on the students' understanding of these reactions and elucidate any gaps in understanding and incorrect warrants that may be present. These interviews were transcribed and analyzed using a qualitative inquiry approach and a modified Toulmin scheme. In general, the findings from this study show that the students exhibited gaps in understanding and incorrect warrants dealing with: (1) classifying substances as bases and/or nucleophiles, (2) assessing the basic or nucleophilic strength of substances, and (3) accurately describing the steps that take place and reactive intermediates that form during alkyl halide reaction mechanisms. In addition, implications for teaching and future research are discussed.

Incorporation and Exclusion of Long Chain Alkyl Halides in Fatty Acid Monolayers at the Air−Water Interface

Mixed monolayers of deuterated palmitic acid C(15)D(31)COOH (dPA) and deuterated stearic acid C(17)D(35)COOH (dSA) with 1-bromoalkanes of different alkyl chain length (C(4) to C(16)) at the air-water interface were investigated. Alkanes and 1-chlorohexadecane ClC(16)H(33) (ClHex) were also studied to compare the effects of the halogen on the mixed monolayers. Surface pressure-area isotherms and Brewster angle microscopy (BAM) were used to obtain the organization and phase behavior, providing a macroscopic view of the mixed monolayers. A molecular-level understanding of the interfacial molecular organization and intermolecular interactions was obtained by using vibrational sum frequency generation (SFG) spectroscopy and infrared reflection-absorption spectroscopy (IRRAS). It was found that from the alkyl halide molecules investigated 1-bromopentadecane, BrC(15)H(31) (BrPent), 1-bromohexadecane, BrC(16)H(33) (BrHex), and ClHex incorporate into the fatty acid monolayers. Alkanes of 15- and 16-carbon chain length do not incorporate into the fatty acid monolayer, which suggests that the halogen is needed for incorporation. Isotherms and spectra suggest that BrHex molecules are squeezed out, or excluded, from the fatty acid monolayer as the surface pressure is increased, while BAM images confirm this. Additionally, SFG spectra reveal that the alkyl chains of both fatty acids (dPA and dSA) retain an all-trans conformation after the incorporation of alkyl halide molecules. BAM images show that at low surface pressures BrHex does not affect the two-dimensional morphology of the dPA and dSA domains and that BrHex is miscible with dPA and dSA. We also present for the first time BAM images of BrHex deposited on a water surface, which reveal the formation of aggregates while the surface pressure remains unchanged from that of neat water.

The ejection anisotropy in the Coulomb explosion of some alkyl halide molecules under strong ps laser fields

The ejection anisotropy of Coulomb explosion fragments released from alkyl halide molecules (C 2H 5X, 1-C 3H 7X and 1-C 4H 9X, where X = Br, Cl) under strong 35 ps laser irradiation is studied by means of time-of-flight mass spectrometry. By comparing the angular distributions widths of the halogen fragment ions originating from multiply charged molecular ions it is concluded that the ejection anisotropy presents a weak dependence on the mass of the halogen atom, observed especially in the case of ethyl halide molecules, while a stronger dependence is found on the size of the alkyl chain.

Chemomechanical Nanolithography: Nanografting on Silicon and Factors Impacting Linewidth

We present a two-fold extension of previous work on Atomic Force Microscope-based chemomechanical functionalization: (1) chemomechanical nanografting, which extends chemomechanical functionalization to a more stable initial surface, and (2) linewidth studies that show the impact of force and Atomic Force Microscope probe tip wear on patterning resolution. Alkene, alcohol, and alkyl halide molecules were nanografted to silicon and imaged with in situ atomic force microscopy, time-of-flight secondary ion mass spectrometry with Automated eXpert Spectrum Image Analysis, and scanning electron microscopy. Chemomechanical nanografting demonstrated linewidths down to 50 nm. Lines written on hydrogen-terminated silicon were used to explore the impact of tip radius and tip wear on linewidth when using Si3N4 coated tips.

Isotope, molecular and surface effects on hyperthermal surface induced dissociated ionization

Organic molecules acquired with hyperthermal (1–20 eV) kinetic energy undergo efficient surface ionization. This hyperthermal surface ionization (HSI) may produce both positive and negative ions. The dissociative ionization of alkyl halides results in the production of negative ions of the functional group having high electron affinity, and positive ions of the alkyl radical, which can further dissociate into smaller fragments. The effect of the alkyl chain lenght and the bromine isotope effect on the obtained HSI mass spectra were studied in several alkyl halide molecules. While the negative ion formation yield is found to be independent of the size of the alkyl radical, the positive ion formation yield strongly increases with the size of the C n H 2n + i radical for n = 1–4 and then a quasi saturation is observed. The observed radical fragmentation increases with the incident molecular kinetic energy and was affected by the molecular structure and the surface temperature and cleanliness. A considerable (up to 24%) heavy brominem isotope increased ionization is observed at intermediate molecular kinetic energies. Piperidine HSI on a rhenium filament exhibits a single (M - 1) ion while its HSI from an oxidized rhenium filament having a much higher work function is characterized by a much richer fragmentation pattern. The dissociative ionization mechanism is rationalized in terms of a surface—molecule electron transfer followed by an immediate dissociation into a negative halogen ion and an alkyl radical. This radical, which usually has a low ionization potential, can transfer an electron to the surface and scatter as a positive ion. At high kinetic energies, the radicals or positive ions can further dissociate near the surface, and then scatter away as lower mass ions with ion yield which depends on their surface reneutralization probabilities. Thus, the observed fragmentation pattern is governed by surface chemicl and electron transfer processe and not by gas phase unimolecular ion dissociation, as found with large polyatomic molecules.

Ion—molecule chemistry of pyrene in an ion‐trap mass spectrometer

AbstractThe quadrupole ion‐trap mass spectrometer is used to investigate aspects of the ion‐molecule chemistry of the pyretic molecular radical cation (M+), the [M  H]+ ion and the [M  H2]+˙ ion. Cycloaddition and alkyl halide addition reactions with the reagents pyrene, methyl iodide, ethyl iodide, isopreue and 2‐iodopropane are investigated. The pyrene molecular radical cation is remarkably unreactive while the closed shell [M  H]+ ion is highly reactive and the [M  H2]+˙ ion shows intermediate reactivity. Analogies are found between the behaviour of the pyrene molecular radical cation and the corresponding [M  H]+ ion on the one hand, and the benzene radical cation and the phenylium ion on the other. The total lack of reactivity of the M+˙ ion is attributed to π‐delocalization of the radical and the cation over the large aromatic system. Addition of isoprene to the [M  H]+ and [M  H2]+˙ ions of pyrene occurs via cycloaddition reactions. A characteristic fragmentation of each adduct ion is methyl radical loss and evidence obtained using pyrene‐d10 indicates that the product in the case of [M  H]+ is an ionized indene. Addition of methyl iodide, ethyl iodide and 2‐iodopropane to the [M  H]+ ion of pyrene occurs to give an iodonium product (C16H9–I+–R) which fragments by loss of I˙ or CH3˙ to give the methylpyrene or iodopyreue product ions. Reactions of ethyl iodide and 2‐iodopropane with the dehydropyrene ion, [M  H2]+˙, give primarily an HI addition product, together with small contributions from the intact adducts. Evidence is provided suggesting that in this case the HI and alkyl halide molecules add across a CC bond rather than forming an iodine‐bound adduct. An experiment in which nine consecutive isolation and activation steps (MS10) are used to couvert the [2M  H]+ adduct, C32H, to C32H˙ exemplifies the intriguing ion chemistry and the stability of the carbon skeleton of the adduct ions.

Paramagnetic NMR shifts of alkyl halide solvents induced by pyridine complexes of Co(II) and Ni(II)

1. A delocalization of the spin density from paramagnetic complexes of Co2+ and Ni2+ to alkyl halide molecules has been detected. 2. The transfer of spin density takes place most affectively on the interaction of the C-X fragments of the alkyl halides with halogen ligands of the paramagnetic centers.

Photodissociation of aryl and aryl–alkyl halides at 193 nm: Fragment translational energy distributions

Molecular beams of a number of aryl and aryl–alkyl halide molecules were photodissociated using an excimer laser at 193 nm and fragment translational energy distributions measured. Previous work had shown that dissociation of aryl halide molecules takes place by a spin–orbit dependent crossing from an intermediate (π, π*) delocalized state to a triplet (σ, σ*) state localized on the C–X bond (X=Cl,Br,I). The vibrational energy of the intermediate state remains in the aromatic framework; thus the fragment translational energy distribution reflects only the electronic but not the vibrational energy of the intermediate state. This fact is exploited in identifying the electronic nature of this state. Present work indicates that both phenyl and naphthyl halide molecules dissociate at 193 nm to produce vibrationally hot aryl radicals. The actual dissociation pathway depends on the competition between intersystem crossing and internal conversion which are functions of both ring size and halogen substituents. Iodobenzene dissociates from S3, S2, and S1 whereas chlorobenzene internally converts to S1 entirely before dissociating. Bromobenzene also dissociates from all three singlet states whereas 2-bromonaphthalene dissociates from S1 even though it is excited to S4 at 193 nm. Fluorination (C6F5I and C6F5Br) drastically reduced the kinetic energy of the fragments which seems to indicate dissociation from a highly distorted aryl ring configuration which may be related to the well-known benzene structural isomers. At lower photon energies it had been found that the flux of fragments of aryl iodides was anisotropic with respect to the E vector of the dissociating light but the flux from aryl bromides was isotropic. The same results are found at 193 nm with a somewhat different interpretation. In all cases, C6H5CH2X and C6H5C2H4X produced translational energy distributions peaking at zero energy which implies that intersystem crossing is so slow that the molecule internally converts from S3 to S0 with subsequent statistical unimolecular decomposition.

Potential energy surface for the M2X+ion and the ionization potential of M2X

Potential energy surfaces for the M2X+ ions (M denotes alkali atom, X halogen atom) have been calculated by a classical ionic model based upon the Rittner model. In all cases M2X+ has substantial binding energy (∼1·5–2 eV) with respect of M+ + MX. The most stable configuration varies from linear for heavy M light X, to strongly bent for light M heavy X. The adiabatic ionization potential of the M2X molecule I ad (M2X) is found to be surprisingly low; less than that of the alkali atom I(M). The vertical ionization potential I v(M2X) is only slightly greater than the adiabatic value. This facilitates a second electron jump in the reactions of alkali dimers with polyhalide and alkyl halide molecules. The main implications of the M2X+ potential surface for the dynamics of the reaction M+ + M′X → MX + M′+ are briefly noted.

Dielectric Behavior of Some Binary Mixtures of Polar Liquids, One Being an Alcohol

Dielectric constants and losses in the 50 Hz—5 MHz range are reported for some binary mixtures of an alkyl halide and an alcohol at low temperatures. The dielectric spectrum presents two absorption ranges: the low frequency region seems of the Debye type, like for the structural relaxation of alcohols; the high frequency region has two contributions, attributed to the alkyl halide molecules and the alcohol free molecules. This last region cannot be experimentally separated into two dispersion regions and in this respect is like the dielectric spectrum of mixtures between two alkyl halides in a nonpolar solvent or not. It is concluded that the high frequency region in the alkyl halide—alcohol mixture has to be linked to a cooperative process between two species of molecules.