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 evaluate Coriolis constants, and then by combining ζ13(c) with the three vibrational frequencies, harmonic force constants have been obtained : [MATHEMATICAL EXPRESSION OMITTED]. The vibration-rotation interaction constants α, β, and ζ for each of the vibrational states have further been used to obtain cubic force constants.

Millimetre and submillimetre studies have been made for HDS and HDO, and in both cases improved centrifugal distortion analyses have been made. For HDS it was found necessary to include both P4 and P6 distortion effects and for HDO a partial set of P8 and P10 terms were needed as well. For HDO, 21 new transitions have been measured and a total of 53 were used in the calculation of 15 rotation and distortion constants. In a related study, Bellet and Steenbeckeliers have made distortion treatments for D2O and HDO using 24 transitions for the former and 30 for the latter to fit to 20 parameters. For H2O, far-i.r. transitions were included and a 16-parameter model was employed. Two other papers have also appeared on isotopic species of water. Using a millimetre beam maser the nuclear hyperfine structure of the 110 -> 101 transition of D2S at 91.4 GHz has been resolved. By combining the data with that previously obtained for H2S and HDS, the deuterium quadrupole coupling tensor has been obtained. The components in its principal axis system are Xzz = 149.0 Xyy = 59.8, and Xzz = -89.2 kHz. The z-axis is perpendicular to the plane of the molecule and the x-axis is rotated 1º35' from the D — S bond in the direction away from the D — S — D obtuse angle. The rotation is similar to that of 1°20' found for D20. A millimetre-wave study of ozone has led to a recalculation of the rotational constants. From Stark-effect measurements on six transitions, an r.m.s. value of 0.5324 [+ or -] 0.0024 D is found for the dipole moment.

Further investigations of the microwave spectrum of hypochlorous acid have extended previous studies, which had obtained B+C values. Through the observation of the a-type transitions, the separate rotational constants B and C have been evaluated for four isotopic species (HO35Cl, HO37Cl, DO35Cl, and DO37Cl). The following rs-structural parameters were obtainecl: r(O — H) = 0.959 [+ or -] 0.005 Å, r(O — Cl) = 1.6895 Å, and HOCl = 102°29'[+ or -] 27'. Nuclear quadrupole coupling coefficients have also been evaluated. Stark-effect measurements have been made on the 101<- 000 transitions for HO35Cl and DO35Cl and values of μa for the two species are found to be 0.367 [+ or -] 0.008 and 0.412 [+ or -] 0.15 D. The difference is attributed largely to the different orientation of the principal axes systems in the two molecules. On this basis the dipole moment of the hypochlorous acid molecule is calculated to be 1.3 [+ or -] 0.3 D and to be inclined at approximately 73° to the O — Cl bond direction. For the structurally related molecule NHCl2, μa has been found to have a value of 1.22 [+ or -] 0.03 D. Rotational constants have been obtained for four isotopic species of NSCl (14N32S35Cl, 14N32S37Cl, 14N34S35Cl, and 15N32S35Cl). From these constants an rs structure has been obtained: r(N — S) = 1.450 Å, r(S — Cl) = 2.161 Å, and NSCl = 117°42'. The S — Cl bond distance is rather longer than those observed for S2Cl2, SOCl2, or SOCl2. Nuclear quadrupole coupling coefficients were also obtained: χaa = -38.51, χbb = 23.51, and χcc = 15.00 MHz. These lead to χbond = -43.36 MHz. In another investigation of the NSCl molecule, the Stark effect for 111<- 000, 211<- 202, and 312<-303 transitions has been used to evaluate the dipole moment and leads to a value of μ = 1.87 [+ or -] 0.02 D.

3 Inorganic Molecules

An earlier study of trans-nitrous acid has now been extended and a detailed study has also been made of the cis-isomer. For the trans-isomer isotopic substitution has been made for all four atoms and an rs-structure has been evaluated. For the cis-isomer the a and b co-ordinates of the hydrogen and nitrogen atoms have been obtained from Kraitchman's equations, and the remaining co-ordinates from Ib and the usual moment conditions. It is seen from the two structures (1) and (2) shown below that there are significant differences in the corresponding nitrogen–oxygen bond lengths. The central and N — O bond cis-isomer is ca. 0.04 Å shorter than the trans-isomer and this is attributed to increased double-bond character, which is in accord with the higher OH torsional frequency for cis-nitrous acid. The N=O bond lengths for both isomers are noticeably longer than those for the nitrosyl halides, and this is regarded as being consequent on some double-bond character in the central bond. The dipole moments are found to be: cis-isomer, 1.423 [+ or -] 0.005; trans-isomer, 1.855 [+ or -] 0.016 D. Nitrogen nuclear quadrupole coupling coefficients are also reported.

The C4v symmetry of BrF5, is confirmed from the statistical weighting of the different K-levels arising from nuclear spin degeneracy. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants have been obtained for both 79Br and 81Br species. An analogous study has confirmed the C4v symmetry of the IF5 molecule and again the rotational and quadrupole coupling constants have been obtained. Another molecule of C4v symmetry, SF535Cl, has been further studied. Transitions for four singly excited vibrational states have been assigned, and Bv, DJJ, and av evaluated for each of these (v4, v6, v10, and v11).

Structure and bonding in molecules with a central sulphur atom continues to attract interest, and microwave studies have been made on sulphuryl chloride and three derivatives of the sulphuryl halides, namely sulphuryl fluoride chloride (SO2FCl), dimethyl sulphone [(CH3)2SO2] and methanesulphonyl fluoride (CH3SO2F). Rotational constants for two isotopic species of sulphuryl chloride fluoride, 32S16O235Cl19F and 32S16O237Cl19F, have been obtained and chlorine nuclear quadrupole coupling coefficients have been evaluated. Although a complete structural determination has not been made, some structural information has been obtained. By making assumptions about the SO bond length, from comparison with SO2Cl2, and SO2F2, two values have been obtained: r(S — F) = 1.55 and r(S — Cl) = 1.985 Å. The former is a little longer than r(S — F) in SO2F2 and the latter a little longer than r(S — Cl) in SO2Cl2. The three rotational constants for dimethyl sulphone have been used to obtain structural information. The structure of the methyl groups may reasonably be assumed by comparison with other molecules. One further assumption is necessary, and the authors examine in turn the assumption of either the r(S — O) or the r(S — C) values obtained from an electron-diffraction study of dimethyl sulphone. The first assumption leads to an OSO angle of 122.02° and the second to 120.82°. The angle is sufficiently insensitive to the value taken for r(S — O), so the authors argue that the mean microwave value, 121.4°, is to be preferred to the electron-diffraction value. For the CH3SO2F the barrier to methyl rotation is determined from relative-intensity measurements for ground- and excited-state lines to be 2.52 [+ or -] 0.35 kcal mol-1. The molecular dipole moments are found to be: (CH3)2SO2, μ = 4.50 [+ or -] 0.10 D; CH3SO2F, μ = 3.88 [+ or -] 0.04 D. The dramatic difference between these and that for SO2F2 (μ = 1.11 D) is rationalized on the basis of comparatively large bond moments for S — O and S — F bonds and a much smaller bond moment for H3C — S. A further microwave study of sulphuryl chloride has also been made, and centrifugal distortion constants have been obtained for the species SO235Cl2. First-order Watson theory was first applied to transitions below J = 40 and ΔJK, ΔK, ζJ, and ζK were evaluated. After higher J transitions had been assigned using these values, first- and second-order theory was used for all transitions between J = 10 and J = 70 to obtain first- and second-order distortion constants.