The vibrational spectrum of the normal and perdeuterated tetramethylammonium ion

Lewis Acidic Aluminosilicates: Synthesis, 27Al MQ/MAS NMR, and DFT-Calculated 27Al NMR Parameters

Porous aluminosilicates are functional materials of paramount importance as Lewis acid catalysts in the synthetic industry, yet the participating aluminum species remain poorly studied. Herein, a series of model aluminosilicate networks containing [L-AlO3] (L = THF, Et3N, pyridine, triethylphosphine oxide (TEPO)) and [AlO4]- centers were prepared through nonhydrolytic sol-gel condensation reactions of the spherosilicate building block (Me3Sn)8Si8O20 with L-AlX3 (X = Cl, Me, Et) and [Me4N] [AlCl4] compounds in THF or toluene. The substoichiometric dosage of the Al precursors ensured complete condensation and uniform incorporation, with the bulky spherosilicate forcing a separation between neighboring aluminum centers. The materials were characterized by 1H, 13C, 27Al, 29Si, and 31P MAS NMR and FTIR spectroscopies, ICP-OES, gravimetry, and N2 adsorption porosimetry. The resulting aluminum centers were resolved by 27Al TQ/MAS NMR techniques and assigned based on their spectroscopic parameters obtained by peak fitting (δiso, CQ, η) and their correspondence to the values calculated on model structures by DFT methods. A clear correlation between the decrease in the symmetry of the Al centers and the increase of the observed CQ was established with values spanning from 4.4 MHz for distorted [AlO4]- to 15.1 MHz for [THF-AlO3]. Products containing exclusively [TEPO-AlO3] or [AlO4]- centers could be obtained (single-site materials). For L = THF, Et3N, and pyridine, the [AlO4]- centers were formed together with the expected [L-AlO3] species, and a viable mechanism for the unexpected emergence of [AlO4]- was proposed.

Surface enhanced spectroscopic investigations of adsorption of cations on electrochemical interfaces

The adsorption of alkali and tetraalkylammonium cations on Pt is investigated using surface enhanced infrared absorption spectroscopy and carbon monoxide as a probe molecule. Alkali cations exhibit a stronger adsorption than organic cations, with potassium showing the strongest effect, followed by sodium and lithium.

Synthesis and characterization of salts containing the BrO(3)F(2)(-) anion; a rare example of a bromine(VII) species

The BrO(3)F(2)(-) anion has been prepared by reaction of BrO(3)F with the fluoride ion donors KF, RbF, CsF, [N(CH(3))(4)][F], and NOF. The BrO(3)F(2)(-) anion is only the fourth Br(VII) species to have been isolated in macroscopic quantities, and it is one of only three oxide fluorides that possess D(3)(h)() symmetry, the others being XeO(3)F(2) and OsO(3)F(2). The fluoride ion acceptor properties of BrO(3)F contrast with those of ClO(3)F, which does not react with the strong fluoride ion donor [N(CH(3))(4)][F] to form the analogous ClO(3)F(2)(-) salt. The single-crystal X-ray structures of [NO](2)[BrO(3)F(2)][F] and [N(CH(3))(4)][BrO(3)F(2)] confirm the D(3)(h)() symmetry of the BrO(3)F(2)(-) anion and provide accurate Br-O (1.593(3)-1.610(6) A) and Br-F (1.849(5)-1.827(4) A) bond lengths. The salt, [NO](2)[BrO(3)F(2)][F], is fully ordered, crystallizing in the monoclinic space group, C2/c, with a = 9.892(3) A, b = 12.862(4) A, c = 10.141(4) A, beta = 90.75(2) degrees , V = 12460(7) A(3), Z = 4, and R(1) = 0.0671 at -173 degrees C, whereas [N(CH(3))(4))][BrO(3)F(2)] exhibits a 2-fold disorder of the anion, crystallizing in the tetragonal space group, P4/nmm, with a = 8.5718(7) A, c = 5.8117(6) A, V = 427.02(7) A(3), Z = 2, and R(1) = 0.0314 at -173 degrees C. The (19)F chemical shift of [N(CH(3))(4))][BrO(3)F(2)] in CH(3)CN is 237.0 ppm and is more deshielded than those of the previously investigated Br(VII) species, BrO(3)F and BrF(6)(+). The vibrational frequencies of the BrO(3)F(2)(-) anion were determined by use of Raman and infrared spectroscopy and were assigned with the aid of electronic structure calculations and by analogy with the vibrational assignments reported for XeO(3)F(2) and OsO(3)F(2). The internal and symmetry force constants of BrO(3)F(2)(-) were determined by use of general valence force field and B-matrix methods, respectively, and are compared with those of XeO(3)F(2), OsO(3)F(2), and the unknown ClO(3)F(2)(-) anion. The instability of ClO(3)F(2)(-) relative to BrO(3)F(2)(-) has been investigated by electronic structure calculations and rationalized in terms of atomic charges, Mayer bond orders, and Mayer valencies, and the enthalpies of fluoride ion attachment to BrO(3)F and ClO(3)F.

Synthesis, characterization, and computational study of the trans-IO2F5(2-) anion

The combination of CH(3)CN solutions of [N(CH(3))(4)][F] and a mixture of cis- and trans-[N(CH(3))(4)][IO(2)F(4)] produces the novel trans-IO(2)F(5)(2)(-) anion. Under the given conditions, only the trans-IO(2)F(4)(-) anion acts as a fluoride ion acceptor, thus allowing the separation of isomerically pure, soluble cis-IO(2)F(4)(-) from insoluble trans-IO(2)F(5)(2)(-). The trans-IO(2)F(5)(2)(-) and cis-IO(2)F(4)(-) anions were characterized by infrared and Raman spectroscopy and theoretical calculations at the LDFT and HF levels of theory. The trans-IO(2)F(5)(2)(-) anion has a pentagonal-bipyramidal geometry with the two oxygen atoms occupying the axial positions. It represents the first example of a heptacoordinated main group AO(2)X(5) species and completes the series of pentagonal-bipyramidal iodine fluoride and oxide fluoride species. The geometries of the pentagonal-bipyramidal series IO(2)F(5)(2)(-), IOF(5)(2)(-), IF(5)(2)(-), IOF(6)(-), IF(6)(-), and IF(7) and the corresponding octahedral series IO(2)F(4)(-), IOF(4)(-), IF(4)(-), IOF(5), IF(5), and IF(6)(+) were calculated by identical methods. It is shown how the ionic charge, the oxidation state of the iodine atom, the coordination number, and the replacement of fluorine ligands by either an oxygen ligand or a free valence electron pair influence the stuctures and bonding of these species.

Structure and vibrational study of the trimethylammonium hexafluorosilicate [(CH3)3NH]2SiF6 compound

The X-ray powder diffraction pattern of [(CH(3))(3)NH](2)SiF(6) was obtained and indexed on the basis of a centred cubic unit cell with the P4(1)32 as the likely space group. The Infrared and Raman spectra of this compound have been recorded at room temperature and discussed in relation to the crystal structure. In this salt, the bands corresponding to the cation vibrational modes show that the symmetry of these cations is distorted from the free C(3v) one and that they are strongly hydrogen-bonded to the respective anions. However, the spectra of the anions can be interpreted in term of ordered groups as indicated by the splitting of the bands corresponding to some degenerate vibrational modes. The harmonic frequencies, corresponding to the (CH(3))(3)NH-SiF(6)-NH(CH(3))(3) optimised geometry, were calculated using the SCF semi-empirical MNDO-PM3 method.

Vibrational study of (CH3)4NSbCl6 and [(CH3)4N]2SiF6

(CH3)4NSbCl6 and [(CH3)4N]2SiF6 are face-centred cubic compounds at ambient temperature with a = 11.548 and 11.172 A, respectively. The vibrational spectra of these two compounds are discussed in relation to their crystal structure and other compounds containing (CH3)4N+ ions. The assignment of the observed bands is discussed. The Raman and infrared spectra of [(CH3)4N]2SiF6 show that the cations and anions are not distorted inside the crystals and are weakly hydrogen-bonded to each other. The infrared spectrum of (CH3)4NSbCl6 confirms a cubic structure for this compound at ambient temperature, in which cations are in octahedral environments and can be interpreted as being disordered groups. No evidence could be found for the existence of hydrogen bonding between cations and anions in this compound. The phase transition of (CH3)4NSbCI6 is studied by means of Raman spectroscopy. It is believed to be governed by the reorientational motions of tetramethylammonium cations and may be classified as an 'order-disorder' type.

Kinetics and mechanisms of photocatalytic degradation of (CH3)nNH(4-n)+ (0 < or = n < or = 4) in TiO2 suspension: the role of OH radicals

The photocatalytic degradation of a series of (CH3)nNH(4-n)+ (0 < or = n < or = 4) was systematically studied in the UV-illuminated TiO2 aqueous suspensions at pH ranges of 3-11. By investigating the pH-dependent kinetics and analyzing intermediates and products, we elucidated the mechanistic pathways and the role of OH radicals in the photocatalytic oxidation. The deprotonated neutral species more rapidly degraded than their protonated counterparts for these homologous compounds because the OH radicals favorably reacted with the lone-pair electron on the nitrogen atom. Therefore, the photocatalytic degradation was highly enhanced at alkaline solutions for all substances except (CH3)4N+. The H-atom abstraction (from (CH3)4N+) by OH radicals initiated successive demethylation processes to generate tri-, di-, and monomethylammonium/amine as an intermediate and NH3/NH4+ as a final product. On the other hand, the OH-addition to the N-atom with the lone-pair electron led to NO2-/NO3- whose production was highly favored at alkaline conditions. The photocatalytic degradation rates of (CH3)4N+ were comparable at both acidic and alkaline conditions, which could not be explained by a simple electrostatic surface charge model. By using OH-scavenging tert-butyl alcohol as a diagnostic probe into the mechanism, it is suggested that the photocatalytic oxidation of (CH3)4N+ at acidic conditions proceeds through free OH radicals in the solution bulk, not on the surface of TiO2.

The OsO4F-, OsO4F2(2)-, and OsO3F3- anions, their study by vibrational and NMR spectroscopy and density functional theory calculations, and the X-ray crystal structures of [N(CH3)4][OsO4F] and [N(CH3)4][OsO3F3]

The fluoride ion acceptor properties of OsO4 and OsO3F2 were investigated. The salts [N(CH3)4][OsO4F] and [N(CH3)4]2[OsO4F2] were prepared by the reactions of OsO4 with stoichiometric amounts of [N(CH3)4][F] in CH3CN solvent. The salts [N(CH3)4][OsO3F3] and [NO][OsO3F3] were prepared by the reactions of OsO3F2 with a stoichiometric amount of [N(CH3)4][F] in CH3CN solvent and with excess NOF, respectively. The OsO4F- anion was fully structurally characterized in the solid state by vibrational spectroscopy and by a single-crystal X-ray diffraction study of [N(CH3)4][OsO4F]: Abm2, a = 7.017(1) A, b = 11.401(2) A, c = 10.925(2) A, V = 874.1(3) A3, Z = 4, and R = 0.0282 at -50 degrees C. The cis-OsO4F2(2-) anion was characterized in the solid state by vibrational spectroscopy, and previous claims regarding the cis-OsO4F2(2-) anion are shown to be erroneous. The fac-OsO3F3- anion was fully structurally characterized in CH3CN solution by 19F NMR spectroscopy and in the solid state by vibrational spectroscopy of its N(CH3)4+ and NO+ salts and by a single-crystal X-ray diffraction study of [N(CH3)4][OsO3F3]: C2/c, a = 16.347(4) A, b = 13.475(3) A, c = 11.436(3) A, beta = 134.128(4) degrees, V = 1808.1(7) A3, Z = 8, and R = 0.0614 at -117 degrees C. The geometrical parameters and vibrational frequencies of OsO4F-, cis-OsO4F2(2-), monomeric OsO3F2, and fac-OsO3F3- and the fluoride affinities of OsO4 and monomeric OsO3F2 were calculated using density functional theory methods.

O-atom scrambling in the aqueous isomerization of pernitrous acid

Raman spectroscopy has been used to determine the isotopic distribution of oxygen during the isomerization of pernitrous acid to nitrate in 18OH2. Decomposition of unlabeled pernitrous acid in carbon-dioxide-free phosphate-buffered solutions of 18OH2 at pH 6.8 results in the incorporation of 13 +/- 1% 18O into the nitrate product with no detectable double-isotopic incorporation and 83% of the product having complete retention of its oxygen atoms. The mechanistic implications of this are contrasted for three frequently considered pathways for peroxide bond cleavage and it is concluded that the results are most consistent with the formation of reactive intermediate which can either rearrange or undergo hydrolytic O-atom exchange with water.