Free radicals in pyrimidines: E.S.R. of a γ-irradiated single crystal of 5-nitrouracil

Free radicals are observed in γ-irradiated single crystals of 5-nitrouracil with the unpaired electron showing hyperfine interaction with one nitrogen atom. The principal values of hyperfine coupling are A_x = 22·5 g, A_y = 25·2 g, and A_z = 40·0 g, and the principal values of the spectroscopic splitting factor are g_u = 2·0117, g_v = 2·0064 and g_w = 2·0027. The relationship of the directions of the corresponding principal axes to the molecular orientations show that the unpaired electron must be located in an sp ² orbital on either N₍₁₎ or N₍₅₎. Considerations of the mechanism of radical formation and comparison to radiation damage in other molecules make the N₍₁₎ location seem more probable. The π interaction of the nitro group on C₍₅₎ evidently prevents the formulation of free radicals with the unpaired electron on C₍₅₎. That carbon atom is the most common location of unpaired electron density in other pyrimidine free radicals.     

Interferometric measurements of the dipole polarizability α of molecules between 300 K and 1100 K

The temperature dependence of the dipole polarizability α(λ, T) of free atoms and molecules is determined by precise measurements of the refractive index n of gases in the extended temperature range between 300 K and 1100 K for wavelength λ = 632·99 nm, using a specially constructed Michelson twin interferometer. α of the noble gases is observed to be independent of T. α of the molecular gases H₂, N₂, O₂, and CH₄ increases with increasing temperature by an amount of approximately 1 per cent per 1000 K. These results are in excellent agreement with theoretical predictions. They will be compared to previously measured temperature dependent polarizabilities.     

Mittlere Energien und Driftgeschwindigkeiten von Elektronen in Stickstoff,Argon und Xenon,ermittelt aus Bildverstärkeraufnahmen von Elektronenlawinen

Local and temporal development of electron avalanches in a pulsed discharge gap (d=3,00 cm) are investigated in N₂, Ar, Xe and mixtures of N₂ and CH₄ by simultaneously applying high gain image intensifier- and photomultiplier techniques. Electron drift velocities are obtained from time-of-flight and way-of-flight measurements in these gases. The mean energy of agitation of the electrons is derived both from electron mobility and avalanche image trace profile (diffusion broadening). The results obtained (for 20°C), being in fair agreement with one another, read N₂: (4·6...5·0) eV forE/p=50...200 V/cm Torr; Ar: (9·0...9·5) eV forE/p=24... 45 V/cm Torr; Xe: (4·8...5·0) eV forE/p= 40... 90 V/cm Torr; CH₄(10% N₂): 6·3 eV forE/p= 89 V/cm Torr. The mean energy of agitation does not change very much withE/p in the ranges investigated. Some results concerning the radiation properties of these gases are included such as lifetime of the excited states, quenching pressure etc.     

Tunable diode laser measurements of the band strength and collision halfwidths of nitric oxide

The strength of the fundamental absorption band of nitric oxide at 5.3 μm and collision halfwidths of nitric oxide lines broadened by nitrogen, argon, and combustion gases were measured in absorption cell, flat flame and shock tube experiments using a tunable diode laser. Room temperature absorption measurements were made in an absorption cell filled with NO/N₂ or NO/Ar mixtures or with probe-extracted combustion gases. High temperature (to 2500 K) absorption measurements were performed for NO in N₂ and NO in Ar using a shock tube, and for NO in combustion gases using a flat flame burner.Absorption measurements were made on lines from 1860–1925 cm^?1, ($\text{Ω=}\text{1}\text{2}$ and $\text{3}\text{2}$,P($\text{5}\text{2}$-R ($\text{29}\text{2}$)) resulting in a band strength of 123±8 cm^-2 atm^?1 at 273.2 K. Collision halfwidth dependencies for each broadening species were examined as a function of rotational quantum number and temperature.     

Kinetic mechanisms for the initiation of supersonic combustion of a hydrogen-air mixture behind a shock wave under the excitation of molecular vibrations in initial reagents

Mechanisms for the initiation of autoignition in hydrogen-air mixtures in a supersonic flow behind a shock at temperatures ≤700 K when the H₂ and N₂ molecule vibration is selectively excited are considered. By exciting molecular vibration in the gases, one can initiate detonation combustion behind the shock front even at weak shocks at gas temperatures ≤500 K. It is established that even a small (<0.15%) amount of vibrationally excited ozone present in the reacting mixture may considerably shrink the induction zone.     

The rotational levels of the ground vibrational state of formaldehyde

A variational procedure for rovibrational energy levels and wavefunctions of centrally connected tetra-atomic molecules is extended to include high rotational states, and in particular, J ? 10 levels for the vibrational ground state of formaldehyde. It is very important to do this because it has made possible the calculation of the usual rotational spectroscopic constants which correspond to the forcefield and geometry. A direct comparison with the ‘observed’ spectroscopic constants is therefore possible. The geometry and forcefield are refined against 65 J = 0 levels of H₂CO, 6 J = 0 levels of D₂CO, 42 J = 1, 70 J = 2 and 98 J = 3 levels of the ground and fundamentals of H₂CO and D₂CO, using an iterative scheme. The mean absolute error of the J = 0 levels is 1·10 cm^?1 and that for J ≠ 0 is 0·005 cm^?1, and the predicted geometry is CH = 1·10064 Å, CO = 1·20296 Å and HCO = 121·648°. Finally, the rotational constants A, B, and C for the ground state are 281956, 38846 and 34003 MHz, compared with the observed values 281971, 38836, and 34002 MHz. The centrifugal distortion constants Δ_J , Δ_JK , Δ_K and δ_J , are 77, 1275, 18113 and 11 kHz compared with 75, 1291, 19422 and 10 kHz. These results underline the accuracy of the new quartic forcefield.     

Molekularstrahlmessungen von Stoßquerschnitten für Übergänge zwischen definierten Rotationszuständen zwei-atomiger Moleküle

Inelastic collision cross sections for transitions between specified rotational states designated by (J, M) have been measured in a molecular beam apparatus. With an electrostatic four pole field molecules in a specified rotational state are separated out of a molecular beam and focussed into a gas filled scattering chamber. Molecules which have been scattered by less than 1/2° are then collected in a second four pole field, located directly behind the scattering chamber, and are analyzed for their rotational state. From a comparison of the measured pressure dependence with calculated curves a determination of inelastic collision cross sections for specified quantum jumps is possible. Measured inelastic scattering cross sections for the transitions (2,0→3,0) are reported for the gases He, Ne, Ar, Kr, CH₄, SF₆, H₂, O₂, Air, N₂O, H₂O, CF₂Cl₂. The values range between about 5 and 100 Ų in the order indicated. The scattering gases NH₃ und ND₃ yielded larger cross sections of about 600 Ų and, in addition, the transitions (3,0)→(2,0),(1,0)→(2,0), (2,0)→(1,0) and (3,0)→(1,0) were observed. Total cross sections for the same gases were also measured with the apparatus.     

Der Einfluß der Eigenraumladung auf die Trägervermehrung einer Lawine in N2, CH4 und N2+CH4

In gases with highγ, e.g. N₂, in which the carrier amplification is max. 10⁴ at the static breakdown, electron avalanches with a carrier amplification up to 10⁹ are produced by overvoltage. These avalanches allow to investigate the influence of the own space charge on the carrier amplification above 10⁶. In the homogeneous field of a pulsed 3 cm discharge gap avalanches were produced in N₂, CH₄ and N₂+CH₄. The investigation of avalanches shows an exponential growth according to\(e^{\alpha _0 \nu _ - \cdot t} \) up to 10⁶. Between 10⁶ and ?5·10⁸ the electron avalanches increase underexponentially owing to the own space charge. These results are in accordance with former measurements in vapours. A comparison of the lowering ofα/α ₀ by the space charge with the theory allows the calculation of avalanche diameters and of the thermal energies of the electrons. The calculated values of the electron temperature vary from 2·0 to 2·5 eV for N₂, and from 1·0 to 2·0 eV for CH₄ and N₂+CH₄ atn<10⁷. Above ?·10⁷ the avalanche diameter increases considerably.     

Hydrodynamics of pulsed supersonic underexpanded jets: Spatiotemporal characteristics

The gasdynamic parameters of nonsteady expansion of He, Ar, N₂, and SiH₄ from a sonic nozzle into a space with reduced background gas pressure were experimentally studied for moderate values of n (10³–10⁶) and the Reynolds number (Re_L∼10⁰–10²). The jet set times necessary for the formation of pulsed jets of a given finite duration are determined. The results are generalized in terms of dimensionless similarity parameters. The laws of motion of the leading and trailing fronts in pulsed jets of various gases are established. The leading front of a pulsed jet propagates at a velocity significantly smaller than the limiting steady value. The jet expansion dynamics is determined by the ratio of the momentum of the expanding gas to that of the background gas displaced from the flow region. The length of the steady flow region in a pulsed jet monotonically decreases downstream from the source and drops with increasing background gas pressure because of the loss of jet particles in the trailing rarefaction wave; this length increases with the initial momentum because the background gas is more intensively displaced from the flow region.     

Rotational diffusion of carbon monoxide in various simple liquids

Infra-red spectra of CO dissolved in liquid N₂, O₂, Ar, Xe and CH₄ have been recorded both in the fundamental and the first overtone. From the band profiles, rotational correlation functions, band moments and intermolecular mean-square torques have been calculated. With the help of Gordon's rotational diffusion model [1] we computed the theoretical correlation functions in the ‘M-diffusion’ (including or not a distribution of angular momentum correlation times) and ‘J-diffusion’ limits. The validity of the Gordon and Debye diffusion models in the various dense gases is discussed.     

Elektronennachlieferung in Niederdruckentladungen durch metastabile Stickstoffmoleküle und deren Stoßlöschung durch Fremdgase

Oscilloscopic studies of pre-breakdown discharge currents in pure nitrogen and mixtures of nitrogen with other gases show that metastable N₂ molecules influence the discharge by releasing secondary electrons when they collide with the electrodes. By this slow process an after current is produced, which lasts a time of milliseconds. It was found that the diffusion coefficient of the metastable N₂ molecule is 167 cm² s^?1 ±15% at 1 Torr and 20°C, its radiative life time is larger than 40 ms, and its cross section for deexciting collisions with ground state N₂ molecules is smaller than 7 · 10^?22 cm². There is strong evidence that the metastables are in theA ³Σ _u ⁺ state. From measurements in gas mixtures cross sections for deexciting collisions with other molecules are derived. The results are for O₂ 1.9 · 10^?17 cm², for CO 1.5 · 10^?17 cm², for H₂ 2.5 · 10^?20 cm², and for CO₂ smaller than 5 · 10^?20 cm².     

An anharmonic force field for carbonyl sulphide

A 16-parameter force field for OCS based on curvilinear coordinates is presented in table 2 and the agreement between observed and calculated transitions and rotational constants is indicated in tables 4, 5 and 6. The computational approach differs from that of Morino and Nakagawa [2] and similar work on other molecules by other authors, in that perturbation theory is eschewed and the calculated vibrational transitions and rotational constants are derived via the numerical solution of a large matrix. The usual parameters x_ij , y_ijk , α _i ^B and γ _ij ^B are thereby rendered inappropriate intermediates. The approach also leads to estimates of B _e - B ₀ for each isotopic species and thence suggests equilibrium distances r _CO = 1·155386(21) Å and r _CS = 1·562021(17) Å.     

Narrow band model parameters for the CO2 4·3 μm band

The narrow band model parameters of the carbon dioxide 4·3 μm band have been obtained from absorption experiments with four broadening gases, namely CO, N₂, O₂, and Ar. The temperature and wave number dependence of the parameters are discussed. The experiments were performed at temperatures of about 303, 598 and 1185°K, with CO₂ partial pressures between 0·01 and 1·0 atm, at a total pressure of 1·0 atm. The Elsasser and the statistical models were applied to thebands in order to obtain applicable parameters and to determine a suitable model for the band at each temperature. It has been found that the Elsasser model is an adequate representation of the band at low temperatures whereas the statistical model is preferred at elevated temperatures. The effect on the line half-widths of the broadening gases is discussed. Good agreement is obtained between absorptance calculated with the present parameters and experimental results obtained by other authors.     

Properties of cationic pnicogen-bonded complexes F4-nHnP+:N-base with H–P···N linear and n = 1–4

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes F_4-nH_nP⁺:N-base, for n = 1–4, each with a linear or nearly linear H_ax–P···N alignment. The sp³-hybridised nitrogen bases include NH₃, NClH₂, NFH₂, NCl₂H, NCl₃, NFCl₂, NF₂H, NF₂Cl, and NF_3, and the sp bases are NCNH₂, NCCH₃, NP, NCOH, NCCl, NCH, NCF, NCCN, and N₂. Binding energies increase as the P–N distance decreases, with an exponential curve showing this relationship when complexes with sp³ and sp hybridised bases are treated separately. However, the correlations are not as good as they are for the complexes F_4-nH_nP⁺:N-base for n = 0–3 with F–P···N linear. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid–base pair is a factor in determining the binding energies of these complexes.

Three different charge-transfer interactions stabilise these complexes, namely N_lp→σ*P–H_ax, N_lp→σ*P–F_eq, and N_lp→σ*P–H_eq. Unlike the corresponding complexes with F–P···N linear, N_lp→σ*P–H_ax is not always the dominant charge-transfer interaction, since N_lp→σ*P–F_eq is greater in some complexes. N_lp→σ*P–H_eq makes the smallest contribution to the total charge-transfer energy. The total charge-transfer energies of all complexes increase exponentially as the P–N distance decreases in a manner very similar to that observed for the series of complexes with F–P···N linear.

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin–spin coupling constants ^1pJ(P–N) across the pnicogen bond vary with the P–N distance, but different patterns are observed which depend on the nature of the acid, and for some acids, on the hybridisation of the nitrogen base. ^1pJ(P–N) values for complexes of F₃HP⁺ initially increase as the P–N distance decreases, reach a maximum, and then decrease with decreasing P–N distance as the P···N bond acquires increased covalent character. ^1pJ(P–N) for complexes with H–P···N linear and those with F–P···N linear exhibit similar distance dependencies depending on the number of F atoms in equatorial positions and the hybridisation of the base. Complexation may increase, decrease, or leave the P–H_ax distance unchanged, but ¹J(P–H_ax) always decreases relative to the corresponding isolated ion. Decreasing ¹J(P–H_ax) can be related to decreasing intermolecular P–N distance.     

Geometry,strength of binding and Br2 charge redistribution in the complex OC ··· Br2 determined by rotational spectroscopy

The ground-state rotational spectra of the six isotopomers ¹⁶O¹²C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁸¹Br, ¹⁶O¹²C ··· ⁷⁹Br⁸¹Br, ¹⁶O¹³C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹³C ··· ⁸¹Br⁷⁹Br, were observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectroscopic constants B _O, D _J, χ _aa (Br_i), χ _aa (Br_o), M_bb (Br_i) and M _bb (Br_o), where i = inner and o = outer, were determined for each isotopomer. The complex is linear, with the weak bond between the C atom of CO and Br_i. The rotational constants were used to determine the distance r(C ··· Br_i) = 3.1058Å and to show that the Br—Br bond lengthens by ～0.005–0.01Å on complex formation. The intermolecular stretching force constant kσ = 5.0 Nm^?1 was obtained from D_J and the Br nuclear quadrupole coupling constants were interpreted to reveal that a fraction δ = 0.02 of an electronic charge is transferred from Br_i to Br_o when Br₂ is subsumed into the complex. Properties of the two series OC ··· XY and H₃N ··· XY, where XY = C1₂, Br₂ and BrC1, are compared.     

Observation of collisionless plasma heating by strong shock waves

The heating of a plasma by collisionless shock waves is investigated by measuring the variation of magnetic field (with magnetic probes), density and electron temperature (from Thomson scattering of laser light) in the shock waves. The compression waves are produced in a tube of 14 cm diameter by the fast rising magnetic field (12 kG in 0.5Μsec) of a theta pinch. For shocks with Mach numbers between 2 and 3 propagating into a hydrogen or deuterium plasma with a localΒ of about 1 (Β=ratio of particle pressure to magnetic pressure) the measured jump in density and magnetic field across the front is 2 to 4, and the electron temperature increases in the front from 3 to 50 eV with a further rise to between 100 and 250 eV in the piston region. Only about 20% of the measured electron heating can be explained by adiabatic heating and resistive heating based on binary collisions, indicating a high turbulent plasma resistance. Both the observed electron heating and the width of the shock front, which is about 0.6 ·c/Ω _p, can be accounted for using an effective collision frequency close to the ion plasma frequencyΩ _p. The ion heating in the almost stationary shock fronts can be inferred indirectly from the steady state conservation relations. For shock waves with Mach numbersM<M _crit it seems to be consistent with an adiabatic heating process, whereas forM>M _crit the calculated ion temperatures exceed those one would except for a merely adiabatic heating.     

On the Kirkwood theory of natural optical activity

Vibration-rotation intensities have been calculated for transitions between states of arbitrary vibrational symmetries, for tetrahedral molecules. For this purpose eigenfunctions of first order Coriolis interactions, which are assumed to be much smaller than anharmonic splittings, have been used. While some bands, among which fundamentals and overtones, follow the ΔR = 0 selection rule, for others the most intense vibration-rotation lines are those with ΔR maximum, in agreement with our double-resonance investigations of stretching levels of methane. One such investigation is presented here, in which the lower (3v₃, F ₂) level of CH₄ has been found at 8906·78 cm^-1, in close agreement with local-mode predictions. Several I.R. lines in this region have been assigned, the effective rotational constants being 5·214 and 5·24 cm^-1 for R = J and R ≠ J Coriolis sublevels respectively.     

A spectroscopic study of the Scorpio-Centaurus association

Rotational velocities as well as hydrogen and helium line intensities of one hundred and twelve members of the Scorpio-Centaurus association have been derived. For stars withM _v<0·0, the distribution of rotational velocities of both the upper Scorpius subgroup and the upper Centaurus -Lupus subgroup are similar and closely resemble those of the field stars. Stars withM _v>0·0, all of which are found in the dense upper Scorpius region, rotate much faster than their counter-parts amongst field stars, the Pleiades and Alpha-Persei cluster members. The measured equivalent width ofH _γ for 77 stars provide a distance modulus of 6·0 ± 0·09 magnitudes for the association. Evolutionary effects in the derived hydrogen line intensities are found between the two subgroups. The hydrogen-line intensities at all spectral types in the upper Centaurus-Lupus subgroup are systematically smaller than those of members in the upper Scorpius subgroup. Analysis of high dispersion spectra of five members of the association yield a helium abundance ofN _He/N _H=0·096 ± 0·004. Along with data available in the literature, the mean helium abundance of thirteen stars of this association is found to be 0·098 ± 0·004 by number. For the two main subgroups of this association, we derive a value of 0·105 ± 0·001 for the upper Centaurus-Lupus group from three stars and 0·096 ± 0·005 for the upper Scorpius group from ten stars. Within the capabilities of our present methods, there seems to be no difference between the two subgroups as far as rotational velocities and initial helium abundance are concerned, a fact that must be borne in mind in the interpretation of the difference found between the two subgroups from photoelectric photometry.     

A partial analysis of rotational structure in the 2710 Å system of p-fluorotoluene

The region of the 0-0 band of the 2710 Å system of p-fluorotoluene has been photographed at high resolution.

The rotational contour of the 0-0 band has been interpreted using two approximations in computed contours: (i) treating the molecule as a symmetric top with free internal rotation, and (ii) treating it as a rigid asymmetric top. The first approximation has enabled an estimate of AT' to be made where AT is the rotational constant of the methyl group about the top axis: the second approximation resulted in valves of AF', B' and C', where AF is the A rotational constant of the ring, although to a rather lower accuracy than has been the case in other substituted benzenes.

Interpretation of the rotational constants shows that in the excited state the methyl group has closed up a little: two extreme interpretations of the constant AT' result in either a decrease in the HCH angles of 1·2±0·7° or a decrease in the C-H bond-lengths of 0·009 ± 0·005 Å. The values of AF', B' and C' indicate that the C-CH₃ bond contracts by about 0·03 Å. This suggests a considerable increase in hyperconjugation in the excited state of p-fluorotoluene compared to the ground state.

The 0-0 band was found to be a type B band implying a polarization of the electronic transition moment along the short in-plane axis. The origin of the band was found to be at 36 859·6±0·3 cm^-1.