Order-disorder effects of small guests in clathrates studied by nuclear quadrupole resonance and crystallography. Part I. Carbon tetrachloride and related molecules in Ni(SCN)2(3-methylpyridine)4

NQR spectroscopy and X-ray crystallography have been jointly applied to the study of the small guest molecules CH₂Cl₂, CHCl₃, CCl₄, CBr₂Cl₂, C(CH₃)2Cl₂ etc. in the Werner host complex Ni(SCN)₂(3-methylpyridine)₄. Specific host-guest dipole-dipole interactions play an important role in order-disorder effects for guest molecules in the cavities of orthorhombic system,Fddd. Thus a weakly polar molecule such as CBr₂Cl₂ exhibits a disordered alternation, both the bromine and the chlorine atoms occupying the two possible sites in theC ₂ cavity, whereas for the polar molecule C(CH₃)₂Cl₂, an ordered structure is observed. In addition, two new types of crystal structures,C2/c andP , are reported here for dichloromethane and chloroform clathrates.     

Accurate equilibrium structures of fluoro- and chloroderivatives of methane

This work is a systematic study of molecular structure of fluoro-, chloro-, and fluorochloromethanes. For the first time, the accurate ab initio structure is computed for 10 molecules (CF₄, CClF₃, CCl₂F₂, CCl₃F, CHClF₂, CHCl₂F, CH₂F₂, CH₂ClF, CH₂Cl₂, and CCl₄) at the coupled cluster level of electronic structure theory including single and double excitations augmented by a perturbational estimate of the effects of connected triple excitations [CCSD(T)] with all electrons being correlated and Gaussian basis sets of at least quadruple-ζ quality. Furthermore, when possible, namely for the molecules CH₂F₂, CH₂Cl₂, CH₂ClF, CHClF₂, and CCl₂F₂, accurate semi-experimental equilibrium (r^SE_e) structure has also been determined. This is achieved through a least-squares structural refinement procedure based on the equilibrium rotational constants of all available isotopomers, determined by correcting the experimental ground-state rotational constants with computed ab initio vibration–rotation interaction constants and electronic g-factors. The computed and semi-experimental equilibrium structures are in excellent agreement with each other, but the r^SE_e structure is generally more accurate, in particular for the CF and CCl bond lengths. The carbon–halogen bond length is discussed within the framework of the ligand close-packing model as a function of the atomic charges. For this purpose, the accurate equilibrium structures of some other molecules with alternative ligands, such as CH₃Li, CF₃CCH, and CF₃CN, are also computed.     

β-carboline as a probe for the sonolysis of alcohols and chloromethanes

Sonolysis at 20 and 500 kHz of alcohols, chloromethanes and dilute mixtures of the latter in the former was studied by ultraviolet spectrophotometry in the presence of β-carboline. Acidic degradation products were detected at low frequency sonication, especially for solutions of carbon tetrachloride in alcohols.     

Decomposition of Chloromethanes in Gliding Discharges

Gliding discharge plasma was used for decomposition of tetrachloromethane and trichloromethane. Air containing 8000 ppm or 20 ppm of water vapor was the carrier gas. The course and yield of the process were studied as a function of initial concentration of tetrachloromethane and trichloromethane, gas flow rate, and water vapor content. The conversion was high in all cases—it reached 90% for tetrachloromethane and 100% for trichloromethane. The conversion rate of the chloromethane compounds increased with increasing initial concentration of these compounds in the reaction mixture. Changing the water vapor content in the reaction mixture from 20 ppm to 8000 ppm increased the conversion rate of chloromethanes.     

Fluorescence Quenching of 2-Amino-9-Hydroxy Fluorene by Chloromethanes

Fluorescence quenching of 2-amino-9-hydroxyfluorene by CCl₄, CHCl₃, and CH₂Cl₂ was investigated in solvents of different polarity. The results reveal that the mechanism of quenching is purely dynamic (complex formed in excited state). The bimolecular quenching-rate constants (k_q) correlate well with electron affinity of the quenchers. They are also dependent on the polarity of the solvents, confirming the formation of charge-transfer nonemissive exciplex.