Thorvald Pedersen

Structure of fluorobenzene

Abstract The microwave spectra of four ring-substituted fluorobenzenes have been measured and the molecular structure of fluorobenzene has been determined by the substitution method from the data so obtained in combination with earlier published data for deuterated fluorobenzenes. The result shows a small shortening of the two C-C bonds nearest fluorine, while the rest of the molecule is practically like benzene. The C-F bond length is 1.35 A, as in vinyl fluoride.

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 .

A projector formulation for the Van Vleck transformation: I. Degenerate case

The Van Vleck transformation as previously described is generalized so that it can isolate several levels simultaneously. It is shown that the contact transformation involves projectors in an implicit way and that it can be considered as a special case of the generalized Van Vleck transformation. We discuss to what extent the recursion relations (for further transformation of the Nth transform) given in part I can be carried over to the generalized Van Vleck transformation and to the contact transformation. An effective hamiltonian for a particular ‘cluster’ of zeroth-order levels can be obtained in several ways. We compare the results of the usual Van Vleck transformation, Soliverezs transformation and the contact transformation.

Microwave spectra of nitromethane and D3-nitromethane

Abstract The microwave spectra of isotopic nitromethanes are presently being reinvestigated in order to increase our understanding of the interaction between the almost free internal rotation and the vibrations. The presence of relaxation terms depending on cos3p proves that the symmetry of H eff is D 3h and furthermore makes a determination of the structure of the methyl group possible.

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.

A projector formulation for the Van Vleck transformation

The Van Vleck transformation as previously described is generalized so that it can isolate several levels simultaneously. It is shown that the contact transformation involves projectors in an implicit way and that it can be considered as a special case of the generalized Van Vleck transformation. We discuss to what extent the recursion relations (for further transformation of the Nth transform) given in part I can be carried over to the generalized Van Vleck transformation and to the contact transformation. An effective hamiltonian for a particular ‘cluster’ of zeroth-order levels can be obtained in several ways. We compare the results of the usual Van Vleck transformation, Soliverezs transformation and the contact transformation.

Rate constants and activation energies for ozonolysis of isoprene methacrolein and methyl-vinyl-ketone in aqueous solution: Significance to the in-cloud ozonation of isoprene

Rate constants and activation energies for the reactions of ozone with isoprene, methacrolein, and methyl-vinyl-ketone in aqueous solution have been determined at temperatures from 5 to 30°C, using the stopped-flow-technique and monitoring ozone decay. The rate constants at 25°C and the activation energies have been found to be 4.1 (±0.2) × 105 M−1 s−1 and 19.9 (±0.5) kJ mol−1 for isoprene, 2.4 (±0.1) × 104 M−1 s−1 and 23.9 (±0.5) kJ mol−1 for methacrolein, and 4.4 (±0.2) × 104 M−1 s−1 and 18.0 (±0.5) kJ mol−1 for methyl-vinyl-ketone. A UV spectrum of a transient intermediate with a lifetime of about 15 s formed during the ozonation of isoprene was obtained in the range 220 to 300 nm. It rises steadily toward 220 nm. It is suggested that the spectrum can be attributed to the two unsaturated Criegee-intermediates (carbonyl oxides), which would conceivably be stabilized by resonance. Lifetime considerations indicate that the oxidation of isoprene and its first-generation reaction products, methacrolein and methyl-vinyl-ketone, by ozone and OH in the aqueous phase of a cloud environment play only a minor role compared to homogeneous gas-phase processing.

Activity coefficients in sea water using Monte Carlo simulations

A new approach to the estimation of activity coefficients in sea water has been established using statistical mechanics. The distribution of particles (ions and nonelectrolytes) are simulated with a Monte Carlo method combined with a particle insertion technique that allows calculation of the activity coefficients. The simulations are based on the primitive model of electrolytes and accounts for both short- and long-ranged forces, but does not suffer from the approximations used in the classical theory. Excellent agreement have been found with experimental data and calculations based on the Pitzer model.