Microwave Spectroscopy

BY A. C. LEGON AND D. J. MILLEN

This Report preserves continuity with the Report in Volume 1, by covering papers included in Chemical Titles for 1972. In an attempt to reduce the interval between publication of papers and their inclusion in the Report, the present Report also covers papers included in Chemical Titles up to March 1973, that is up to and including No. 6 for 1973. The Report is structured in the same way as previously. After reviews of diatomic and triatomic molecules there are sections on inorganic and organic molecules, and then sections on a number of special topics.

1 Diatomic Molecules

A number of studies have been made on diatomic molecules leading to Dunham coefficients and rather precise values of re. For a number of molecules, hyperfine structures have been measured for vibrationally excited states and the dependence of nuclear quadrupole coupling on vibrational state has been explored. Some further dipole moments have been measured and for some molecules the sign of the dipole moment has been determined, leading to a result of general interest in at least one case.

Microwave spectra are reported for the first time for AlBr and AlI, and improved measurements have been obtained for AlCl. Dunham coefficients have been evaluated and re/A values obtained as follows:

AlCl AlBr AlI

2.130 11(3) 2.294 80(3) 2.537 09(3)

The nuclear quadrupole coupling coefficient is also reported for the iodine nucleus in AlI. Measurements have been made of the J = 0 -> 1 transitions of four isotopic species of GaCl, including the hyperfine structure. Transitions for vibrational states up to v = 3 have been measured and Dunham coefficients as well as rotational constants have been obtained. A value of 2.201 681(3) Å has been obtained for re. The dependence of nuclear quadrupole coupling coefficients on vibrational state is given in Table 1. Hyperfine structure for InF has been measured, with a high-resolution-beam electric resonance spectrometer, for the J = 1 and J = 2 rotational states several vibrational levels, and eqQ values are reported. Thallium halides have been the subject of a number of studies. The Stark effect for TlF has been studied with a molecular-beam electric resonance spectrometer and the dipole moment has been obtained, among other parameters. Deviations from the Stark effect for a rigid-rotor model are discussed and additional terms arising from anharmonicity, centrifugal distortion, vibration – rotation interaction, and electronic polarizability are examined. The dipole moments/D of TlBr and TI1 have also been determined giving:

TlF TlBr TlI

4.2282(8) 4.493 [+ or -] 0.050 4.607 [+ or -] 0.070

Molecular-beam studies on TlCl include an examination of the Stark effect of different isotopic species in various vibrational states.

The microwave spectrum of CS has been used via the Zeeman effect to determine the sign of the electric dipole moment as —CS+, in the same direction as found for CO. The microwave spectrum of CSe has been reported for the first time and molecular g-values and the electric quadrupole moment have been evaluated from the observed Zeeman effect. For silicon monosulphide the sign of the dipole moment has also been shown to be +SiS-. The equilibrium internuclear separation is found to be re = 1.929 254(3) Å. A further study has been made of GeTe and hyperfine structures have been measured for the J = 2 -> 3 transition, leading to eqQ = 153.1 MHZ. Two independent studies have been made for phosphorus mononitride, PN. The J = 0 -> 1 transition has been measured for a number of vibrational states and Dunham coefficients have been evaluated, giving re = 1.490 85 A. In another study, rotational transitions from J = 1 -> 2 to J = 7 -> 8 have been observed in the millimetre and submillimetre regions. The analysis of the results leads to re = 1.490 80(2) A, in close agreement with value from the study of the J = 0 -> 1 transitions. A new analysis has been made of all frequency data at present available for the oxygen molecule, from microwave, submillimetre, and infrared spectroscopy, including some recent measurements. A least-squares analysis was used to fit 25 microwave frequencies and three submillimetre and i.r. frequencies and a new set of parameters for the oxygen molecule has been obtained in this way. A gas-phase electron paramagnetic resonance study of oxygen is mentioned in a later section of the report.

Chlorine and bromine monofluorides have been further examined. Molecular g-values and molecular quadrupole moments have been evaluated from the observed Zeeman effect. The isotopic dependence of the g-values gives the sign of the electric dipole moment for chlorine monofluoride as —CIF+, which is of interest in view of simple arguments based on the idea of electronegativity and also for comparison with conclusions drawn from the chlorine nuclear quadrupole coupling coefficient. The dipole moment itself has been remeasured [μ = 0.888 12(2) D] and the hyperfine structure examined using a molecular-beam electric resonance spectrometer. The chlorine nuclear quadrupole coupling coefficient [eqQ = -145.78211(9) MHz) and spin-spin interaction constants have been reported.

Hyperfine structures have been observed for 133Cs35Cl, 133Cs79 Br, and 133Cs127I. The dependence on vibrational state is summarized in Table 2. CsF has been studied using a molecular-beam electric resonance spectrometer. The analysis of the results includes the following for the nuclear quadrupole coupling for caesium:

[MATHEMATICAL EXPRESSION OMITTED]

Other alkali-metal halides investigated by molecular-beam spectrometry include LiC1, KF, and RbF.

2 Triatomic Molecules

A new molecule has been added during the year to the list of linear triatomic molecules investigated by microwave spectroscopy. Thioborine, HBS, an unstable molecule, which is isoelectronic with methylidenephosphine, HCP, shows a rotational spectrum typical of a linear rotor. Rotational and centrifugal distortion constants have been reported for eight isotopic species and l-doubling constants for four species. rs Structures have been calculated taking each isotopic species in turn as the parent molecule and all these calculations give structures falling...