Microwave Spectroscopy

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

1 Introduction

This chapter preserves continuity with the Report in Volume 2. It covers papers included in Chemical Titles for the period from April 1973 to March 1974 inclusive, and follows the general pattern used in previous years. During the year under review a new structural parameter rm has been introduced (see ref. 274). The new parameter can be calculated solely from ground-state data, making use of 'mass-dependence' of rotational constants of isotopically substituted molecules, and leads to values close to re values. An advance in understanding electrical properties of molecules has been made by the combined use of i.r. intensities and the dependence of Stark effects on both vibrational state and isotopic substitution to evaluate a molecular dipole moment function (see ref. 20). For molecules with internal rotation, where V3 terms in the barrier hindering rotation have now been evaluated for many molecules, the significance of values of V6 terms which are beginning to be obtained has been called into question (see ref. 129–133). For some large molecules it has been shown how low-resolution microwave spectroscopy can now be applied to conformational problems (see ref. 275).

A further attempt has been made to settle the difficult question of the equilibrium structure of formamide (see ref. 108). It is concluded from a careful analysis of microwave and i.r. spectra that the lowest-lying vibrational mode, the NH2 wag, is governed by a single minimum potential function and that therefore the atoms are coplanar at equilibrium. Rotational spectra of two formally non-polar molecules, allene and methane, have been reported. For the weakly polar species H2C[??]C[??]CD2 ground-state transitions have been detected by conventional techniques (see ref. 119), and ΔJ = 0 transitions in the vibronic ground state of CH4 (allowed by the centrifugal distortion dipole moment) have been observed both through double resonance (see refs. 302, 303) and conventionally (see ref. 304). In addition, Mizushima-Venkateswarlu type ΔJ = 0 transitions within the v3 = 1 state of CH4 have succumbed to the double resonance technique (see refs. 333, 334). Double resonance experiments of the microwave-optical variety (see ref. 350), on the other hand, have allowed the accurate measurement of rotational transitions in an electronically excited state of BaO. The first observation of the gas-phase electron magnetic resonance spectrum (see ref. 265) of a non-linear free radical, namely HCO, has been followed by the observation of the pure microwave spectrum (see refs. 266, 268) of the same molecule. This may indicate that the study of triatomic free radicals could follow the pattern already seen in the study of diatomic free radicals where resonance studies have often been followed by microwave studies. For the most part, the parameters evaluated from microwave spectroscopy are molecular parameters, but it has recently been shown that thermodynamic functions can be obtained through the measurement of absolute intensities of transitions (see ref. 273). A new development in molecular beam maser spectroscopy has been the measurement of scattering cross-section for NH3 for two different cases, the first with beam molecules in the upper inversion state, and the second for a coherent superposition state with equal amplitudes of upper and lower inversion states (see ref. 31 1). A significant instrumental development is the spectrometer described by Krupnov et al. (see ref. 396). This has both high sensitivity and the facility for broad-banded operation in the millimetre regions by virtue of an acoustic detector.

2 Diatomic Molecules

A number of new developments have occurred in the study of diatomic molecules. The microwave spectrum of IF has been obtained for the first time and the sign of the dipole moment of ClF reported in Volume 2 has been called into question. Zeeman effect studies have been made for several diatomic molecules, and new observations have been made on rotational spectra of a number of alkali halides.

Nuclear quadrupole hyperfine structure for 39K127I has been measured on the J = 2 <- 1 transition at 7.2 GHz. Observations on vibrational states up to v = 3 lead to the following quadrupole coupling constants: eq0Q(39K)= -4.12 [plus or minus] 0.10 MHz and eqvQ(1271)= [-85.32 - 2.93(v + 1/2) [+ or -] 0.12] MHz. Using a specially designed high-temperature spectrometer for Zeeman effect measurements, Honerjager and Tischer have obtained gJ-factors for CsF, CsCl, CsBr, and CsI. Magnetic susceptibility anisotropies were also obtained for CsF and CsCl. A partial resolution of the J = 1 -> 0 transition of 6Li19F has been achieved by using electric resonance spectroscopy in the millimetre region ; magnetic spin rotation and lithium nuclear quadrupole coupling constants have been obtained. Microwave spectra of AlBr and AlI which were reported for the first time in 1972, have been further examined and nuclear quadrupole coupling constants have been evaluated from measured hyperfine structures of J = 1 <- 0 and J = 2 <- 1 transitions, respectively. The values are collected in Table 1 which also includes values for AlCl obtained by calculation on a reassignment of the spectrum.

For All, the vibrational dependence of the iodine coupling constant was also obtained as was the magnetic spin-rotation coupling for this case. Similar measurements have also been reported for TlF.

Diatomic oxides and sulphides of some Group IV elements have been further investigated. Wollrab and Rasmussen have investigated the lifetime of gas-phase carbon monosulphide by monitoring the J = 1 <- 0 transition of CS produced by a discharge through carbon disulphide. Half-lifetimes have been obtained as a function of the initial pressure of carbon disulphide and the mechanism of decay of carbon monosulphide has been considered. Measurements of the Zeeman effect have been made for Ge0, SiS, Sn0, and Pb0. The gJ-factors and magnetic susceptibility anisotropies [??]T have been obtained and are collected in Tables 2 and 3.

Molecular quadrupole moments and asymmetries of electronic charge distributions in these two molecules have also been evaluated. Two papers on the oxygen molecule have appeared. One clears up a discrepancy concerning the optical and microwave values of B0 and D0 for 16O2. The other reports the observation and analysis of the millimetre and sub-millimetre wave spectrum of 18O2. Among the data...