Comparative performance of CF3I,CD3I and CH3I in an atomic iodine photodissociation laser

We have compared the performance of CF₃I, CD₃I, and CH₃I in an atomic iodine photodissociation laser over the pressure range 1–200 torr. At pressures below 5 torr, CD₃I produces larger energy outputs, while above 5 torr CF₃I gives superior performance. The crossing of the laser energy output versus pressure curves is explained on the basis of collisional quenching of I(²P_{$\text{1}\text{2}$})(≡I^*) by undissociated alkyl iodide.     

Vibrational state analysis of unrelaxed BaI from the reactions Ba + CH3I and Ba + CH2I2

The reactions Ba + CH₃I → BaI + CH₃ and Ba + CH₂I₂ → BaI + CH₂I have been investigated by the method of laser-induced fluorescence. Excitation spectra are reported for BaI products formed under single-collision conditions in a “beam-gas” arrangement. The production of BaI for Ba + CH₂I₂ is found to be a major reaction pathway with a cross section about twice that for Ba + CH₃I. The relative vibrational populations show for both reactions bell-shaped distributions peaking close to υ = 21 for Ba + CH₃I and υ = 39 for Ba + CH₂I₂. The corresponding average fraction of the total reaction exoergicity that appears as BaI vibration is $\text{f}$_υ = 0.18 for Ba + CH₃I and $\text{f}$_υ = 0.29 for Ba + CH₂I₂. In the case of Ba + CH₃I, an estimate for the average relative translational energy of the products, obtained from the primitive angular distribution measurements of Lin, Mims and Herm, can be combined with the average vibrational excitation of BaI to provide evidence that the internal excitation of the methyl radical exceeds that of BaI. A model is discussed which postulates an electron jump in the exit valley of the Ba + CH₃I reaction to account for this feature of the reaction dynamics.     

Photoacoustic measurements of photofragmentation in CH3I

Photoacoustic measurements are described giving branching ratios for the I(²P_{$\text{1}\text{2}$}) and I(²P_{$\text{3}\text{2}$}) atom production following vapour-phase photolysis of CH₃I. A range of excitation wavelengths are used from the long wavelength tail up to 248 nm. The presence of three bands is shown within the σ^* ← n continuum; in the strong-coupling model these are E ← A₁(⊥), A^*₁ ← A₁(||) and E ← A₁(⊥) with only the A^*₁ ← A₁ transition giving excited iodine atoms.     

The enthalpies of formation of CH3Mn(CO)5 and of CH3Re(CO)5, and the strengths of the CH3Mn and CH3Re bonds

From measurements of the heats of iodination of CH₃Mn(CO)₅ and CH₃Re(CO)₅ at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔH^o_f[CH₃Mn(CO)₅, c] = ?189.0 ± 2 kcal mol^?1 (?790.8 ± 8 kJ mol^?1), ΔH^o_f[Ch₃Re(CO)₅,c] = ?198.0 ± kcal mol^?1 (?828.4 ± 8 kJ mo^?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn₂(CO)₁₀ and Re₂(CO)₁₀, values are derived for the dissociation energies: D(CH₃Mn(CO)₅) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol^?1 and D(CH₃Re(CO)₅) = 53.2 ± 2.5 kcal mol^?1. In general, irrespective of the value accepted for D(MM) in M₂(CO)₁₀, the present results require that, D(CH₃Mn) = $\text{1}\text{2}$D(MnMn) + 18.5 kcal mol^?1 and D(CH₃Re) = $\text{1}\text{2}$D(ReRe) + 30.8 kcal mol^?1.     

Photofragmentation of CH3Br in the A band

CH₃Br is photodissociated in the first continuum. Dissociation takes place into ground state CH₃ and Br [ = Br(²P_{$\text{3}\text{2}$}] or Br* [ = Br(*P_{$\text{1}\text{2}$})]. Time of flight and angular distributions of the CH₃ fragments are measured. The Br*/Br ratios upon excitation at 222 and 193 nm are found to be 1.00 and 0.20 respectively. The anisotropy parameters at these wavelengths are β = 0.28±0.04 and β = ?0.23±0.02, respectively. The total absorption cross section is decomposed into partial absorption cross sections of the ¹Q, ³Q₀ and ³Q₁ states. It appears that excitation at 222 nm takes place to the ³Q₀ and ³Q ₁ states whereas at 193 nm the ¹Q and ³Q₀ states are excited. Contrary to CH₃I, the adiabatic curve crossing between the ³Q₀ and the ¹Q states in Ch₃Br is not important. The dissociation energy of the CBr bond is determined to be D₀(CH₃Br) = 2.87±0.02 eV.     

Crossed Molecular Beam Study of H+CH4 and H+CD4 Reactions: Vibrationally Excited CH3/CD3 Product Channels

We study the H+CH₄/CD₄→H₂/HD+CH₃/CD₃ reactions using the time sliced velocity map ion imaging technique. Ion images of the CH₃/CD₃ products were measured by the (2+1) resonance enhanced multi-photon ionization (REMPI) detection method. Besides the CH₃/CD₃ products in the ground state, ion images of the vibrationally excited CH₃/CD₃ products were also observed at two collision energies of 0.72 and 1.06 eV. It is shown that the angular distribution of the products CH₃/CD₃ in vibrationally excited states gradually vary from backward scattering to sideways scattering as the collision energy increases. Compared to the CH₃/CD₃ products in the ground state, the CH₃/CD₃ products in vibrationally excited states tend to be more sideways scattered, indicating that larger impact parameters play a more important role in the vibrationally excited product channels.     

Vibrational relaxation of N2(A3Σu+,υ = 1, 2,3) by CH4 and CF4

N₂(A, υ = 0-3) produced by the Ar(³P_0,2) + N₂ reaction and detected by laser-induced fluorescence undergoes rapid, stepwise vibrational relaxation but slow electronic quenching with added CH₄ or CF₄. Rate constants, k_Q^υ, of 1.5, 3.1, and 5.0 × 10^?12 cm³ s^?1 are measured for Q = CH₄, υ = 1-3, and 0.47, 1.8, and 5.5 × 10^?12 cm³ s^?1 for Q = CF₄, υ = 1-3, with ≈±20% accuracy (1σ). Information is also obtained for the unrelaxed, relative υ populations.     

Observation of nonlinear molecular hyperfine level crossings in CD3I

Level crossings between hyperfine levels were observed in the nonlinear absorption of CD₃I ν₂^QP(4,1) transition by using the P(16) laser line of the CO₂ 10.6 μm band. From the electric fields of crossings, the dipole moments of this molecule were determined. These are μ₀ = 1.6514 ± 0.0025 and μ₁ = 1.6477 ± 0.0025 debye, in the ground state and in the ν₂ state, respectively.     

Vibrational energy transfer in CH3I

Previous works have reported vibration—vibration and vibration—translation transfer rates in the methyl halides. Using the technique of laser induced infrared fluorescence we have studied energy transfer in the concluding member of this series, CH₃I. Following excitation by resonant lines of a Q-switch CO₂ laser, infrared fluorescence has been observed from the v₂, v₅ as well as the 2v₅, v₁, v₄ vibrational energy levels of CH₃I. All the observed states exhibit a single exponential decay rate of 23 ± 2 ms^?1 torr^?1. Measurements have also been made on deactivation of the various modes by rare gases. The risetime of the v₂, v₅ levels was found to be approximately 101 ± 20 ms^?1 torr^?1, while that of the 2v₅, v₁, v₄ levels was approximately 225 ± 45 ms^?1 torr^?1. Fluorescence was not detected from the v₃ level. These results are discussed in terms of SSH type theoretical calculations, and comparison is made with the results obtained for other members of the methyl halide series, namely CH₃F, CH₃Cl and CH₃Br.     

Refinement of β-LiIO3 crystal structure by use of zeeman effect on the NQR line and ir-reflection spectra

The NQR Zeeman effect of ¹²⁷I quadrupole resonance (±$\text{1}\text{2}$ ↓ ±$\text{3}\text{2}$ transition) and the polarized IR spectra of a single crystal of β-LiIO₃ have been studied. The z-axes of the EFG orientation in the unit cell were determined; the EFG asymmetry parameter for the ¹²⁷I nuclei was refined giving η = 0.027 ± 0.003 at 300 K for β-LiIO₃. The dichroic ratio in the (100) crystal plane over the I-O stretching vibration region has been measured from polarized spectra and calculated on the basis of an “oriented gas” model by the use of refined structural data. Comparison of some spectral, structural and quantum-chemical characteristics of both β- and α-modifications of LiIO₃ crystal has been made.     

Λ-doublet populations in CH(A2Δ) produced in the 193 nm multiphoton dissociation of (CH3)2CO, (CD3)2CO, (CH3)2S and CH3NO2

Unequal intensities of the Λ-doublet components were observed in the CH(A²Δ-X²Π) emission following the multiphoton dissociation of (CH₃)₂CO, (CH₃)₂S and CH₃NO₂ by an ArF laser (193 nm). The power dependence of the emission intensity was estimated to be cubic (3.1±0.2) when the laser power was below ≈ 8×10¹⁷ photons cm^?2 pulse^?1. The Λ-doublet populations depended on the rotational quantum number N′ and the preferred level changed at N′ = 20. A similar behavior was observed for the CD(A²Δ) from (CD₃)₂CO. Rotational distributions show bimodal behavior, having a hump around N′ = 13 in CH(A²Δ) and N′ = 11 in CD(A²Δ). These trends indicate that the CH(A²Δ) is produced through multiple processes where stepwise mechanisms are operative via either CH₂ or CH₃, or both radicals as intermediates.     

Yield of I(2Psol:12) in the photodissociation of CH2I2

The production of I(²P_{$\text{1}\text{2}$}) in the photolysis of CH₂I₂ has been studied optoacoustically at excitation wavelengths between 365.5 and 247.5 nm. Bands found at 32200 and 47000 cm^?1 correlate with I(²P_{$\text{3}\text{2}$}) whilst those at 34700 and 40100 cm^?1, which correlate with I(²P_{$\text{1}\text{2}$}), give final ²P_{$\text{3}\text{2}$}/²P_{$\text{1}\text{2}$} ratios of 1.75 and 1.1, respectively, after curve crossing.     

Microwave transitions of CD2=NCN

In continuation of our studies of the pyrolysis (1000°C) of (CH₃)₂NCN to alleged CH₂=NCN (I), N-cyanoformimine, (CD₃)₂NCN has been prepared and pyrolyzed (1000°C) to CD₂=NCN (II) as seen by its observed rotational transitions (18.6–40.0 GHz). The published rigid structural model of I and a corresponding structure of II are practically identical.     

Isotope effects on intensities in the K-shell excitation spectra of CH4 and CD4 studied by 2.5 keV electron impact

The isotope effect on the intensity of the 1a₁(K) » 3a₁(3s) transition in methane has been examined by inner shell electron energy loss spectroscopy. The intensity ratio, I_K»3s(CD₄)/I_K»3s(CH₄) is 0.81 ± 0.08 which is at variance with a recent theoretical prediction by Bagus et al. It is shown that the measured ratio is compatible with a Herzberg-Teller vibronic coupling mechanism for this transition.     

The crystal structure of N-sodiohexamethyldisilazane,Na[N{Si(CH3)3}2]

The crystal structure of Na[N{Si(CH₃)₃}₂] has been determined from single-crystal x-ray diffraction data collected by counter methods. N-sodiohexamethyldisilazane crystallizes in the mono-clinic space group p21/n with unit cell parameters $\text{a}$ = 9.426(3), $\text{b}$ = 6.921(3), c = 17.974(5)Å, β = 93.83(2)°, and ?_calc = 1.04 g cm^?3 for z = 4 formula units. Least-squares refinement gave a final conventional R value of 0.034 for 1244 independent observed reflections. In the solid state the compound exists in a polymeric arrangement with an average Na-N distance of 2.355(4)Å. The methyl group are configured such that the angle of rotation of the trimethylsilyl moiety about the Si-N bond is 30° from the eclipsed position. The Si-N bond length is 1.690(5)Å, and the Si-N-Si bond angle is 125.6(1)°.     

Vibrational spectra and rotational barriers of methyl titanium halides CH3TiX3 and CD3TiX3 (X = Cl,Br, I)

The IR and Raman spectra of gaseous and solid CH₃TiX₃ and CD₃TiX₃ species (X = Cl, Br, I) are reported. The gas phase spectra have been recorded between 4000 and 20 cm^?1 at pressures of 1 atm and 4 atm at 350 K and the Raman spectra of the solid phase recorded at 4.2 K. Internal rotation barriers and thermodynamic functions have been calculated.     

Phase equilibrium relations in the binary systems LiPO3CeP3O9 and NaPO3CeP3O9

The LiPO₃CeP₃O₉ and NaPO₃CeP₃O₉ systems have been investigated for the first time by DTA, X-ray diffraction, and infrared spectroscopy. Each system forms a single 1:1 compound. LiCe(PO₃)₄ melts in a peritectic reaction at 980°C. NaCe(PO₃)₄ melts incongruently, too, at 865°C. These compounds have a monoclinic unit cell with the parameters: a = 16.415(6), b = 7,042(6), c = 9.772(7)Å; β = 126.03(5)°; Z = 4; space group $\text{C}{}_2\text{c}$ for LiCe (PO₃)₄; and a = 9.981(4), b = 13.129(6), c = 7.226(5) Å, β = 89.93(4)°, Z = 4, space group $\text{P2}{}_1\text{n}$ for NaCe(PO₃)₄. It is established that both compounds are mixed polyphosphates with chain structure of the type |M^I_IM^III_II (PO₃)₄|_∞M^I_I: alkali metal, M^III_II: rare earth.     

Laser induced photodissociation of C2F5I. Radiative lifetime of the metastable iodine atom (2P12)

Excited iodine atoms I(²P_{$\text{1}\text{2}$}) are formed by laser irradiation of C₂F₅I at 2950 Å. The mean radiative lifetime τ of these metastable atoms and their bimolecular rate constant k₂ for deactivation in collissions with C₂F₅I were measured to be: τ = 108 ± 10 ms; k₂ = (1.8 ± 0.1) × 10^?17 cm³/molec s.     

Molecular beam chemiluminescence XI: kinetic and internal energy dependence of the NO + O3 → NO2*,→ NO23 reaction

Seeded supersonic NO beams were used to study the kinetic energy dependence of both the electronic (NO₂^*) and vibrational (NO₂³) chemiluminescence of the NO + O₃ reaction. In addition the electronic CL is found to be enhanced by raising the NO internal temperature. This is shown to be due to enhanced reactivity of the NO(²Π,_{$\text{3}\text{2}$}) fine structure component. By difference NO(²Π_{$\text{1}\text{2}$}) is concluded to yield predominantly groundstate NO₂³. The excitation function for NO₂^* formation from NO(²Π_{$\text{3}\text{2}$}) is of the form σ_{$\text{3}\text{2}$}(E) = C(E/E₀ - 1)ⁿ over the 3–6 kcal energy range where n = 2.4 ± 0.15, C = 0.163 Ų and E₀ = 3.2 ± 0.3 kcal/mole. Vibrational IR emission from NO₂³ has an energy dependence different from electronic NO₂^* emission, confirming that emitters are formed predominantly in distinct reaction channels rather than via a common precursor (either NO₂^* or NO₂³). The short wavelength cutoff of the CL spectra recorded at elevated collision energies E ? 15 kcal/mole corresponds to the total available energy. These and literature results are discussed in the light of general properties of the (generally unknown) ONO₃ potential energy surfaces. The formation of electronically excited NO₂^* rather than energetically preferred O₂ (¹ Δg) (Gauthier and Snelling) can be rationalized in terms of surface hopping near a known intersection of potential energy surfaces more easily than by vibronic interaction in the asymptotic NO₂ product.