Yannis G. Lazarou

Atmospheric chemistry of CF3CF═CH2 and (Z)-CF3CF═CHF: Cl and NO3 rate coefficients, Cl reaction product yields, and thermochemical calculations.

Rate coefficients, k, for the gas-phase reactions of Cl atoms and NO(3) radicals with 2,3,3,3-tetrafluoropropene, CF(3)CF═CH(2) (HFO-1234yf), and 1,2,3,3,3-pentafluoropropene, (Z)-CF(3)CF═CHF (HFO-1225ye), are reported. Cl-atom rate coefficients were measured in the fall-off region as a function of temperature (220-380 K) and pressure (50-630 Torr; N(2), O(2), and synthetic air) using a relative rate method. The measured rate coefficients are well represented by the fall-off parameters k(0)(T) = 6.5 × 10(-28) (T/300)(-6.9) cm(6) molecule(-2) s(-1) and k(∞)(T) = 7.7 × 10(-11) (T/300)(-0.65) cm(3) molecule(-1) s(-1) for CF(3)CF═CH(2) and k(0)(T) = 3 × 10(-27) (T/300)(-6.5) cm(6) molecule(-2) s(-1) and k(∞)(T) = 4.15 × 10(-11) (T/300)(-0.5) cm(3) molecule(-1) s(-1) for (Z)-CF(3)C═CHF with F(c) = 0.6. Reaction product yields were measured in the presence of O(2) to be (98 ± 7)% for CF(3)C(O)F and (61 ± 4)% for HC(O)Cl in the CF(3)CF═CH(2) reaction and (108 ± 8)% for CF(3)C(O)F and (112 ± 8)% for HC(O)F in the (Z)-CF(3)CF═CHF reaction, where the quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. NO(3) reaction rate coefficients were determined using absolute and relative rate methods. Absolute measurements yielded upper limits for both reactions between 233 and 353 K, while the relative rate measurements yielded k(3)(295 K) = (2.6 ± 0.25) × 10(-17) cm(3) molecule(-1) s(-1) and k(4)(295 K) = (4.2 ± 0.5) × 10(-18) cm(3) molecule(-1) s(-1) for CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF, respectively. The Cl-atom reaction with CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF leads to decreases in their atmospheric lifetimes and global warming potentials and formation of a chlorine-containing product, HC(O)Cl, for CF(3)CF═CH(2). The NO(3) reaction has been shown to have a negligible impact on the atmospheric lifetimes of CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF. The energetics for the reaction of Cl, NO(3), and OH with CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF in the presence of O(2) were investigated using density functional theory (DFT).

THEORETICAL INVESTIGATION OF THE THERMOCHEMISTRY OF HYDROFLUOROETHERS

Abstract The enthalpies of formation and the C–H bond strengths of fluorinated dimethylethers were derived at 298.15 K by ab-initio calculations at the MP2/3-21++G(2d,2p), MP2/6-31++G(2d,2p) and MP2/6-311++G(2d,2p) levels of theory, and are compared with the results of recent calculations by the G2(MP2) method. Theoretical calculations were also performed for the corresponding fluorinated ethanes in order to compare the results with those derived by the BAC–MP4 method and with the available experimental data. The calculations at the MP2/6-31++G(2d,2p) level are considered more reliable on the basis of the closer agreement between theory and experiment. The results suggest that the introduction of the ether linkage –O– leads to lower C–H bond strengths only in mildly F-substituted dimethylethers. Heavily F-substituted dimethylethers possess stronger C–H bonds than their fluoroethane counterparts and therefore their tropospheric reactivity is expected to be lower.

Absolute reaction rate of chlorine atoms with iodomethane

Abstract The reaction of atomic chlotine with iodomethane was studied in the gas phase with the very low pressure reactor technique over the temperature range 273–363 K. The absolute rate constant was given by the expression k = (1.33 ± 0.49) × 10−11 exp [ −(573 ± 1.00 kJ mol −1 RT ] cm 3 molecule −1 s −1 . The reaction proceeds through an intermediate weakly bound adduct CH3ICl and results in the formation of HCl and iodomethyl radical CH2I. The kinetic isotope effect k H k D of the reaction was independent of temperature and found to be 1.09 ± 0.4.

Absolute rate coefficient determination and reaction mechanism investigation for the reaction of Cl atoms with CH2I2 and the oxidation mechanism of CH2I radicals.

The gas-phase reaction of atomic chlorine with diiodomethane was studied over the temperature range 273-363 K with the very low-pressure reactor (VLPR) technique. The reaction takes place in a Knudsen reactor at pressures below 3 mTorr, where the steady-state concentration of both reactants and stable products is continuously measured by electron-impact mass spectrometry. The absolute rate coefficient as a function of temperature was given by k = (4.70 +/- 0.65) x 10-11 exp[-(241 +/- 33)/T] cm3molecule-1s-1, in the low-pressure regime. The quoted uncertainties are given at a 95% level of confidence (2sigma) and include systematic errors. The reaction occurs via two pathways: the abstraction of a hydrogen atom leading to HCl and the abstraction of an iodine atom leading to ICl. The HCl yield was measured to be ca. 55 +/- 10%. The results suggest that the reaction proceeds via the intermediate CH2I2-Cl adduct formation, with a I-Cl bond strength of 51.9 +/- 15 kJ mol-1, calculated at the B3P86/aug-cc-pVTZ-PP level of theory. Furthermore, the oxidation reactions of CHI2 and CH2I radicals were studied by introducing an excess of molecular oxygen in the Knudsen reactor. HCHO and HCOOH were the primary oxidation products indicating that the reactions with O2 proceed via the intermediate peroxy radical formation and the subsequent elimination of either IO radical or I atom. HCHO and HCOOH were also detected by FT-IR, as the reaction products of photolytically generated CH2I radicals with O2 in a static cell, which supports the proposed oxidation mechanism. Since the photolysis of CH2I2 is about 3 orders of magnitude faster than its reactive loss by Cl atoms, the title reaction does not constitute an important tropospheric sink for CH2I2.

Novel polycarboxylated EDTA-type cyclodextrins as ligands for lanthanide binding: study of their luminescence, relaxivity properties of Gd(III) complexes, and PM3 theoretical calculations

Novel -type cyclodextrin (CD) derivatives, , and , bearing 6, 7 and 8 bis(carboxymethyl)amino (iminodiacetic acid) groups, respectively, were prepared, and their complexation with Eu(iii), Tb(iii) and Gd(iii) ions was studied. Luminescence titrations and mass spectrometry showed formation of multimetal complexes ( 2 to 3, mainly 3 and exactly 4 metal ions), whereas luminescence lifetime measurements revealed the presence of exchangeable water molecules. Semiempirical quantum mechanical calculations, performed by the PM3 method and assessed by DFT calculations on model ligands, indicated efficient multi-metal complexation, in agreement with the experiment. The structures showed coordination of the metal ions in the outer primary side of the CDs via 4 carboxylate O atoms, 2 N atoms and a glucopyranose O atom per metal ion. Coordination of water molecules was also predicted, in accordance with experimental results. Calculated bond lengths and angles were in agreement with literature experimental values of lanthanide complexes. Calculated energies showed that complex stability decreases in the order > > . (1)H NMR molecular relaxivity measurements for the Gd(iii) complexes of , or in water afforded values 4 to 10 times higher than the relaxivity of a commercial contrast agent at 12 MHz, and 6 to 20 times higher at 100 MHz. Solutions of and Gd(iii) complexes in human blood plasma displayed relaxivity values at 100 MHz 7 and 12 times, respectively, higher than the commercial agent. MTT tests of the Gd(iii) complexes using human skin fibroblasts did not show toxicity. Attempts to supramolecularly sensitize the luminescence of the lanthanide complexes using various aromatic CD guests were ineffective, evidently due to large guest-metal distances and inefficient inclusion. The described lanthanide complexes, could be useful as contrast agents in MRI.

S‐Nitroso‐β‐Cyclodextrins as New Bimodal Carriers: Preparation, Detailed Characterization, Nitric‐Oxide Release, and Molecular Encapsulation

6-Monoand 6-multi-S-nitroso-b-cyclodextrins (SNObCDs) were prepared from their corresponding thiols (SHbCDs) and characterized in detail for the first time in terms of their conformational preferences and SNO content, thermal and photochemical stability, their ability to encapsulate guest molecules, and their cell toxicity and permeation. The prevalence of gauche–trans (gt) over gauche–gauche (gg) conformations (with respect to rotation about the C5 C6SH bond and, hence, to the bCD cavity) and the presence of syn-to-anti equilibria (with respect to the C6S-NO configuration) in SNO-CDs and in a reference compound, S-nitrosoglutathione (GSNO), were suggested by H and N NMR spectroscopy. Quantum mechanical calculations indicated that the gt conformations indeed prevail in mono-SH-bCD and mono-SNO-bCD, whereas a blend of gt/gg conformations prevail in per-SH-bCD and per-SNO-bCD. This reflected the presence of two potentially dissimilar SNO groups with diverse stabilities toward NO release and propensities for forming interglucose S S bonds. Moreover, syn conformers were energetically preferred in all cases. MonoSNO-bCD is water soluble, thermally more stable than GSNO, and undergoes photocontrolled NO release. Furthermore, the CD cavity is available for guest encapsulation without noticeable perturbation of the SNO functionality whilst hosting, for example, the chemotherapeutic tamoxifen. Nitrosation of per-SH-bCD to form per-SNO-bCD was found to compete with SNO decomposition and disulfidebridge formation, thereby resulting in an average of 5.2 SNO groups instead of 7. Upon isolation, SNO-CDs, as well as GSNO, suffer a small additional loss of SNO groups, mostly to afford disulfides. Multi-SNO-bCD is soluble in DMSO and displays better thermal stability than GSNO and cell permeability. Both SNO-CDs were found to be chemically non-toxic to cells at high incubation concentrations (>200 mm); thus, they represent a potentially new family of bimodal drug-delivery systems.

Ab initio computational study of the interaction of Cl atoms with HI, CH3I and CH3OCH2I

Abstract Ab initio calculations at the MP2/3–21 + + G(2d,2p) level of theory suggest that the interaction of chlorine atoms with RI (R = H, CH 3 and CH 3 OCH 2 ) molecules leads to weakly bound adducts with potential energy wells of −31.839, −57.749 and −59.760 kJ mol −1 , respectively. The corresponding standard enthalpies of the adduct formation reactions at 298 K were calculated to be −31.056, −52.409 and −51.337 kJ mol −1 . The structure of the parent RI molecules were only slightly perturbed during the chlorine atom attachment to the iodine atom. The ICl bond length was always found to be ca. 2.8 A and the RICl angle was close to 80°.

Kinetic studies of the reaction of Cl atoms with SiH4

The reaction kinetics of chlorine atoms with silane has been studied over the temperature range 273–363 K using the very low pressure reactor technique. The reaction proceeds via a hydrogen-atom abstraction and leads to SiH3 and HCl products. The absolute rate constant was found to be independent of temperature with a value k1=(3.25 ± 0.40)× 10–10 cm3 molecule–1 s–1. Ab initio calculations at the MP2/6–311G(d,p) level of theory did not show any evidence for the formation of an intermediate SiH4–Cl adduct. The thermochemical version of conventional transition-state theory indicated that the transition-state geometry is almost linear, with a Si—H—Cl angle of ca. 170°.

IR Laser Chemistry of Dimethylnitramine and Diethylnitramine

The infrared multiphoton decomposition of dimethylnitramine and diethylnitramine in the gas phase under almost collisionless conditions has been studied with a tunable CO2 laser. The steady state rate coefficient for their unimolecular decomposition were found to be kDENA(st) = 105.5 ± 0.1(I/MW cm–2) s–1, and kDENA(st) = 105.2 ± 0.1(I/MW cm–2) s–1, for laser intensities in the range 3–15 MW cm –2. The dependence of rate constants and product yields on laser fluence have indicated that the unimolecular dissociation of both nitramines occurs via scission of the N-NO2 bond, and the main secondary reaction leading to the final photolysis products (nitrosamine and nitroxide) is the oxidation reaction of nascent dialkylamino radical with parent nitramine or NO2 product.

Infrared multiphoton dissociation of tetramethylsilane: formation of electronically excited trimethylsilyl radical

C.M. Michael, Y. Lazarou and P. Papagiannakopoulos Department of Chemistry, University of Crete, 71409 Heraklion, Crete, Greece Received 13 January 1992 The infrared multiphoton excitation of tetramethylsilane and chloromethyltrimethylsilane molecules, under collision-free and collisional conditions, with a tunable CO2 laser resulted in an intense ultraviolet-visible luminescence. The spectral analysis and the time profile of this luminescence has been studied, in order to provide a reasonable mechanism of its formation. Lifetime measurements of this emission were performed at various alkylsilane pressures, and its radiative lifetime was found to be 365 + 10 ns. The quenching rates of this luminescence by several colliders were also determined from lifetime measurements. The proposed mechanism leading to the production of this chemiluminescence involves the formation of electronically excited trimethylsilyl radicals ( 1 2E state) via an inverse electronic relaxation process. 1. Introduction The chemical reactivity of many intermediate sil- icon compounds is very important to the prepara- tion of silicon films with the method of chemical va- por deposition (CVD). In particular alkylsilanes such as tetramethylsilane (CH