N.W. Larsen

Microwave spectra of the six mono-13C-substituted phenols and of some monodeuterated species of phenol. Complete substitution structure and absolute dipole moment

Abstract The microwave spectra of the six mono- 13 C-phenol species have been investigated and a complete substitution structure ( r s ) is reported. The r s bond lengths (A) are: r CC 1.391–1.395, average 1.3933; r CH 1.080–1.086, average 1.083; r CO 1.374; r OH 0.957. The angles (degrees) are: ∠ CCC 119.2–120.9; ∠ CCH 119.2–121.6; ∠ COH 108.8. In the hydroxyl group, O and H are on either side of the line through C 1 and C 4 ; the angle between this line and the CO bond is 2.5°. The dipole moment has been measured for [3-D] -, [5-D] -, and [7-D] phenol, making it possible to determine its direction in the molecule, and allowing definite assignment of isotopic species to observed spectrum. The dipole moment in the parent species is: μ = 1.224 D, with μ a = −0.133 ± 0.003 D, and μ b = 1.217 ± 0.008 D. The changes in the dipole moment on deuteration are from 0.009 to 0.020 D.

Microwave spectra of the six monodeuteriophenols. Molecular structure, dipole moment, and barrier to internal rotation of phenol

Abstract The microwave spectra of the six monodeuteriated phenols have been measured and a partial substitution structure of phenol has been determined. The selection rules for Class III molecules 5 have been derived, and the Stark effect has been discussed. Stark coefficients for [2-D]- and [6-D]phenol have been measured and the dipole moment of phenol deduced, Splitting data for phenol, [ 18 O]phenol, [4-D]phenol, and [7-D]phenol were used to determine the barrier to internal rotation, V 2 = 1175 ± 20 cm −1 .

Far-infrared gas spectra of phenol, 4-fluorophenol, thiophenol and some deuterated species: barrier to internal rotation

Abstract The far-infrared gas spectra (400-50 cm−1) are reported for phenol, [7-D]phenol, [4-D]phenol, 4-fluorophenol, 4-fluoro[7-D]phenol and thiophenol. The spectra are assigned and the barrier (V2) to internal rotation is calculated from combined far infrared and microwave data. The results for V2 are (using the same sequence as above): 1213, 1243, 1213, 1007, 1034, 267 cm−1. The origin of the barrier differences is discussed.

Equilibrium configuration and barriers of four fluorine substituted nitrobenzenes, obtained by microwave spectroscopy

Abstract MW Spectra of 3- and 4-fluoronitrobenzene in several torsional states and of 2- and 2,4,6-trifluoronitrobenzene in the ground state have been obtained. The inertial defects have been used to show that the 3- and 4-substituted molecules are planar and to find their twofold barriers (1428 cm−1 and 1213 cm−1 respectively). The 2- and 2,4,6-substituted molecules have very large negative inertial defects indicating non-planarity. The out-of-plane angles have been calculated (32° and 57° respectively).

Kinetic study of the formation of isotopically substituted ozone in argon

The kinetics of the formation of ozone was studied by using pulse radiolysis coupled with time-resolved UV absorption at 275 nm and at T = 294.9±0.6 K. The rate constant for the formation of ozone 16O16O16O in argon was determined to be k3a = (3.38±0.04) × 10−34 cm6 molecule−2 s−1. The rate constants for the reactions 18O + 16O16O (k3b), 16O + 16O18O (k3c), 16O + 18O18O (k3d), 18O + 16O18O (k3e), and 18O + 18O18O (k3f) were studied, and the following parameters were determined: (k3b + k3d)/(2k3a) = (1.184±0.037), (k3c + k3e)/(2k3a) = (1.155±0.062), and k3f/k3a = (0.977±0.021). The values for (k3b + k3d)/(2k3a) and (k3c + k3e)/(2k3a) obtained here are equal to the values derived from the product studies and the recently reported relative rate study but higher than the reported values for (k3b + k3d)/(2k3a) and (k3c + k3e)/(2k3a) obtained by using CO2 as a third body. The parameter k3f/k3a = (0.977±0.021) is lower than the value of k3f/k3a obtained by using CO2 as a third body and the value derived from the product studies. These different values of k3f may be partly due to changes in third body efficiency or due to resonance interactions between the excited ozone molecules and the third body. The absolute measurements reported here together with literature data suggest that the nature of the third body is an important factor in controlling the enhancements of the rate constants for ozone formation and that asymmetry of neither ozone nor dioxygen ensure a fast ozone formation rate.

First direct kinetic study of isotopic enrichment of ozone

The formation kinetics of ozone has been studied using isotopes and pulse radiolysis combined with time-resolved UV absorption spectroscopy. An enhancement of (9.8±2.6)% was found for the rate constant for the reaction of 18O with 18O18O relative to that for the 16O+ 16O16O reaction. The average formation rate for unsymmetric 18O16O16O and 16O18O18O from O2 and 18O18O respectively was enhanced by (14.7±2.8)%. For the formation of a mixture of symmetric and unsymmetric ozone species from O18O the enhancement was (3.6±2.4)%. This leads to the conclusion that both mass and symmetry affect the rate constant for formation of isotopic ozone. The results are compared with recent enhancement studies from the literature, and an apparent conflict is discussed.

Microwave spectra and internal rotation of 4-fluorophenol, 4-chlorophenol and 4-bromophenol

Abstract The microwave spectra of 4-fluorophenol, 4-chlorophenol and 4-bromophenol are analysed in the frequency region 18–40 GHz. The vibrational ground state with two torsional sublevels are assigned for the following molecular species: 4-F-C 6 H 4 OH(I), 4-F-C 6 H 4 OD(II), 4- 35 Cl-C 6 H 4 OH(III), 4- 37 Cl-C 6 H 4 OH(IV), 4- 35 -Cl-C 6 H 4 OD(V), 4- 79 Br-C 6 H 4 OH(IV) and 4- 81 Br-C 6 H 4 OH(VII). The spectra consist of μ a - and μ b -lines. The μ b -lines appear as doublets with approximately constant splittings of 354 MHz(I), 2.25 MHz(II), 159 MHz (III and IV), 0.75 MHz(V) and 139 MHz(VI and VII). The μ a -lines are unsplit except for I in which a few μ a -lines appear as closely spaced doublets. Coriolis type interaction between the torsional substates was observed for I and was proved to be responsible for the splitting of the μ a -lines. Assuming a semirigid model the effective twofold barriers to internal rotation of the hydroxyl group are calculated to be 1006 cm −1 (I), 1035 cm −1 (II), 1148 cm −1 (III and IV), 1179 cm −1 (V) and 1172 cm −1 (VI and VII). All the molecules are shown to be planar and structural information about the OH group has been obtained for I and III.

Model study of polar stratospheric clouds and their effect on stratospheric ozone: 2. Model results

We use the detailed microphysical/chemical/dynamical two-dimensional model described in part I [De Rudder et al., this issue] to study the effect of heterogeneous reactions occurring on the surface of polar stratospheric clouds (PSCs) on stratospheric ozone. The calculations show that the heterogeneous reactions occurring on the surface of PSCs are the likely causes of the ozone decrease observed from 1980 to 1990 in both Antarctica and the Arctic. The calculation shows that the dense sulfate aerosol cloud produced by the eruption of Mount Pinatubo (in the Philippines, 1991) has enhanced the formation rate of type I PSCs in the Arctic and the Antarctic. In the calculation, the concentration and surface area of type I PSCs are enhanced. The effect on the ozone depletion in the Antarctic is, however, limited due to the fact that the conversion from ClONO2 to ClO on PSCs is almost “saturated” under no-volcanic conditions. For the potential ozone depletion, the enlargement in the area covered by PSCs may therefore be more important than the increase in PSC density. The calculation also shows that, in the future, the density of PSCs in the Arctic could be enhanced owing to the potential emission of water vapor and nitrogen species by high altitude aircraft. The increase in PSC density could lead to a maximum of ozone depletion of 10% at the northern high latitudes in winter.

Conformation, barrier to internal rotation, and structure of the PH2-group in phenylphosphine, studied by microwave spectroscopy

Abstract The microwave spectra of phenylphosphine (C 6 H 5 PH 2 ) and of mono- and dideuterated phenylphosphine (C 6 H 5 PHD and C 6 H 5 PD 2 ) were investigated. The two torsional sublevels of the vibrational ground state were assigned for each molecular species. Only μ a -and μ b -lines were observed, the μ b -lines being split by approximately twice the splitting of the torsional sublevels. By means of internal rotation calculations with a program that determines the effective Hamiltonian for one top molecules without symmetry restrictions, the twofold barriers to internal rotation were determined to be 94.2 cm −1 (C 6 H 5 PH 2 ), 91.1 cm −1 (C 6 H 5 PHD), and 85.5 cm −1 (C 6 H 5 PD 2 ). No effects due to inversion of the PH 2 group were observed in the spectra. For C 6 H 5 PH 2 and C 6 H 5 PD 2 the internal rotation potential is of the cos 2γ type with the minimum position corresponding to the configuration with the hydrogen atoms of the phosphine group on opposite sides of the phenyl ring plane, which is thus a plane of symmetry for the molecule. The C 6 H 5 PHD potential, however, contains an additional sin 2γ term with the effect of rotating the minimum 8.5° in the direction from D toward H. The direct monosubstitution method for determination of the hydrogen atoms of the PH 2 group gave inconsistent results, but the disubstitution method yielded a result comparable to the internal rotation calculation. The structural parameters determined for the PH 2 group are: PH = 1.414 A, HPH = 94.27°, CPH = 97.29°, and tilt angle (CP to internal rotation axis) = 2.96° (P and H being on opposite sides of the internal rotation axis).

Structure and electric dipole moment of 1,6-dioxa-2,5-diaza-6aλ4-thiapentalene from microwave spectra

Abstract The microwave spectrum of dioxadiazathiapentalene has been analysed in the ground state and in a few vibrationally excited states. The ground-state rotational constants (in MHz) were found to be A = 4452.8265(12); B = 1697.56541(45); C = 1229.04579(48). The spectrum is perfectly normal of a rigid, C2v, asymmetric rotor, as was the case with 1,6-dioxa-6aλ4-thiapentalene. It is therefore concluded that there is no evidence of a double-minimum electronic ground state. The electric dipole moment is |μb| = 2.883(20) D.