Microwave Spectroscopy

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

This review is the first of a series dealing with developments in the field of microwave spectroscopy and covers papers included in Chemical Titles for 1971. Although this has the result that some papers published towards the end of 1971 have not been included, continuity will be preserved since such papers will be included in Chemical Titles for 1972 and therefore covered in the next review. Because of the detailed treatment which can now be made for diatomic and triatomic molecules, these are discussed as separate topics and are followed by sections on inorganic and organic molecules. Developments in treating vibration–rotation interaction are conveniently taken together, and lastly a number of areas of growing interest are reviewed.

1 Diatomic Molecules

Equilibrium internuclear distances for diatomic molecules, or at least their relative values in the absence of a more accurate value of Planck's constant, are among the most accurately determined physically significant properties of molecules. This year has seen the establishment of the same value of re for four isotopic species of one molecule within very close limits.

Equilibrium internuclear distances have been evaluated for carbon monoxide (re = 1.12823 [+ or -] 0.00005 Å) and for hydrogen chloride (re = 1.27460 [+ or -] 0.00005 Å). A sub-millimetre-wave investigation has been made of a number of isotopic species of hydrogen halide. Transitions in the region 0.38 — 1.0 mm wavelength have been measured using a spectrometer which employs a klystron-driven crystal harmonic generator and a 1.6 K InSb photo-conducting detector. As a result of this work, re values have now been obtained for all of the hydrogen halides using microwave measurements (Table 1). The re values are calculated using the relationship BIb = 5.05376 x 105 a.m.u. Å2 MHz. For hydrogen chloride, four isotopic species have been examined and all the re values lie within 10-6 Å. Further investigations have been made of silver halides using a specially designed high temperature Stark cell. Improved quadrupole coupling constants have been obtained from observations on low transitions for AgCl (J = 1 <- 0) and AgBr (J = 3 <- 2). Transitions in the microwave spectrum of silver iodide have been obtained for the first time. Nuclear quadrupole coupling coefficients reported for the three molecules are given in Table 2. Equilibrium internuclear distances have been obtained for the bromide and iodide:

re(Ag-Br) = 2.393100 [+ or -] 20.000029 Å and re(Ag-I) = 2.544611 [+ or -] 0.000031 Å.

2 Triatomic Molecules

Linear triatomic molecules for which new information has been obtained include HCN, HCP, OCS, and SCSe. New microwave measurements have been made of rotational transitions within vibrationally excited states of four isotopic species of [MATHEMATICAL EXPRESSION OMITTED] and have led to improved values of the relevant Bv constants. Values for r0 internuclear distances have been obtained for all possible pairs of B0 values and compared with r8, and re values. Conclusions from this work are summarized in the section on vibrational influence on structural determination. Further investigation of the spectrum of the phosphorus analogue of HCN, methylidene phosphine (HCP), has been reported. A millimetre-wave investigation has been made of transitions in the ground state and in the v2 = 1 state. Previous studies have given enough information to determine Be for HCP, but not for DCP, where α2 and α3 remained undetermined. The new investigation has led to a value for α2 but α3 still remains to be determined. A value of q1 has also been obtained for DCP. The microwave spectrum of thiocarbonyl selenide (SCSe) has been observed in the (0, 1[+ or -], 0) and (0, 2[+ or -], 0) vibrational states. Ground-state rotational constants have been obtained by extrapolation from the observations on the two vibrationally excited states. The rotational constants which are given in Table 3 differ considerably from those suggested previously. A lack of isotopic information for sulphur species prevents the determination of an r8,-structure, but the following internuclear distances have been obtained from the ground-state rotational constants: r(C — S) = 1.553 A and r(C — Se) = 1.695 Å. Of these, the latter is an r8-value; it is significantly smaller than the corresponding bond distance for OCSe (1.708 Å). By using the value found for q[??] and the three vibrational frequencies, the molecular force field has been calculated : fC-Se = 5.72, fC-S = 7.97, frr = 0.59, and [MATHEMATICAL EXPRESSION OMITTED] mdyn Å-1. The dipole moment was measured in the (0, 2[+ or -]2, 0) state, transitions for the (0, 1[+ or -], 0) state not being fully modulated, and found to be 0.03 1 D.

For OCS the absolute signs of dipole moment derivatives have been obtained. The determination is based on the use of four pieces of information: the change in dipole moment on excitation to the state with one quantum of the bending mode, the intensities of the two stretching fundamentals, and the newly measured intensity of the bending overtone. These four serve to overdetermine three quantities: the first derivative of the dipole moment with respect to each of the two stretching co-ordinates and the second derivative with respect to bending. For the carbonyl bond, the oxygen becomes more negative as the bond is stretched, and the dipole is estimated to change at a rate of 7 D Å-1. For the C = S bond a similar result is found and the gradient is estimated as 4 D Å-1.

Among bent symmetric XY2 molecules studied were GeF2, HDO, HDS, D2S, and O3. A detailed study has been made of the GeF2, molecule by Takeo, Curl, and Wilson in which rotational constants have been obtained for 76GeF2, 74GeF2, 72GeF2, and 70GeF2. Changes in each of the rotational constants (α, β, and γ) on vibrational excitation have been obtained for each of the three fundamentals and the equilibrium parameters were found to be: re(Ge-F) = 1.7321 Å, and αe = 97°10'; the dipole moment was found to be 2.61 D. Changes in inertial defect on vibrational excitation have been used to...