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 within a range very similar to the range found for HCN. The mean rs values are r(B — H) = 1.1692 and r(S — B) = 1.5994 Å. Both HCN and HCP have been the subject of molecular Zeeman studies and molecular quadrupole moments have been evaluated.

A similar study has been made for some of the halogen cyanides. The molecular quadrupole moments (in units of 10-26 e.s.u. cm2) for the various cyanides and for methylidenephosphine are as follows:

35ClN 79BrCN 127ICN HCN
HCP
-3.87 [+ or -] 1.0 6.02 [+ or -] 1.1 -7.33 [+ or -] 1.1 3.1 [+ or -] 0.6
4.4 [+ or -] 1.2

The isotopic dependence of the molecular g-values has also been used to show that for chlorine cyanide the sign of the dipole moment is +ClCN-. For iodine cyanide a detailed re-examination has been made of the microwave spectrum. Rotational constants for 127I12C14N and 127I12C15N have been obtained for nine vibrational states and l-doubling constants for three states. This amount of information has made possible the calculation of a number of rotational parameters, and at the same time the change of the iodine nuclear quadrupole coupling coefficient with vibrational state has been examined.

Among bent triatomic molecules, H2O and H2S have continued to be further studied. Microwave spectra of the radioactive species HTO and T20 have been observed for the first time. Twenty-six lines have been measured for HTO and five for T20, and the rotational constants given in Table 3 evaluated. Revised rotational and distortion constants have been evaluated for H216O as a result of a submillimetre microwave study. New transitions have also been reported in the submillimetre region for H218O, and the distortion constants obtained for this species have been compared with those calculated on the basis of H216O data. Excellent agreement is obtained, which is gratifying since the evaluations made for H216O and H218O include very different types of i.r. data as well as different higher-order terms in the Hamiltonian. A very detailed study of centrifugal distortion for D216O has been made by Steenbeckeliers and Bellet. Twenty-two constants have been determined for the ground state and nine for the v2 state. The discussion includes an examination of the contribution of the more important distortion terms to the energies of a number of rotational states. Dipole moment measurements have been made for D2O and HDO. The dipole moment of D2O has been determined from the Stark effect on microwave transitions and confirmed by high-resolution Stark spectroscopy using the HCN 337 pm laser. The dipole moment of HDO has also been determined from microwave measurements. The work indicates that the rotationless dipole of water increases slightly on deuteriation, and this is accounted for in terms of anharmonicity of the bending mode, analogously with the interpretation of a similar effect found for ammonia. Stark coefficients of millimetre-wave transitions for H2O, D2O, and HDO have also been reported. Anharmonic force constants for water have been evaluated using data on H2O, D2O, and HDO. Centrifugal distortion has also been investigated for H2S and D2S. For H2S the distortion terms of order P4 have been obtained and a partial set of order P6 and P10 was found to be necessary in the analysis.3a For D2S a millimetre and submillimetre study has led to the measurement of sixty-six transitions, and these have been analysed with a twenty-two parameter distortion treatment. However, it is found that only a partial set of terms of order P6 and P10 is necessary. Hyperfine structure for deuteriated hydrogen selenide has been investigated by beam maser spectroscopy and quadrupole constants and spin–rotation and spin–spin coupling constants involving deuterium nuclei have been evaluated.

The microwave spectrum has been reported for the first time of two isotopic species of hypofluorous acid, HOF and DOF. The rs structural parameters are found to be:

r(O-H) r(O-F) HOF

0.964 [+ or -] 0.01 Å 1.442 [+ or -] 0.001 Å 97.2 [+ or -] 0.6

Interestingly the HOF angle is smaller than the angles in either H2O or F2O, which parallels the finding for HOCl, by comparison with H2O and Cl2O. In a further study, more than forty lines each for both HOF and DOF have been measured and a centrifugal distortion treatment has been applied, leading to the quartic distortion constants.

The microwave spectrum of the DNO molecule, which has been previously studied by electronic spectroscopy, has been obtained by using a flow technique. The rotational constants resulting from the analysis are in good agreement with those found for the ground state from the electronic spectrum. The nitrogen nuclear quadrupole coupling coefficients are found to be similar to those for nitrosyl fluoride and nitrous acid, and the dipole moment of deuteriated nitroxyl is found to have a value of 1.70 [+ or -] 0.05 D. Following the work on DNO the spectrum of HNO has also been studied (see ref. 383). Two independent measurements have been made of the dipole moment of NSCl. A value of 1.83(6) [+ or -] 0.036 D has been obtained for 15N32S35Cl and 1.87 [+ or -] 0.02 for 14N32S35Cl. Centrifugal distortion coefficients have been obtained from an analysis of twenty-eight transitions for 14N32S35Cl. By combining these with vibrational frequencies and inertial defects force constants have been obtained:

F11 = 10.032 mdyn Å-1F22 = 1.44(4) mdyn A-1

F33 = 0.74(9) mdyn Å and F23 = -0.08(6) mdyn

The microwave spectrum of GeF2 has been further investigated. Rotational constants have been obtained for vibrationally excited states for 70GeF2, 72aGeF2, and 74GeF2 and a revised re structure has been obtained:

r(Ge-F) = 1.7320(9) Å FEeF = 97.148°

A centrifugal distortion analysis has been made and the question of the best use of centrifugal distortion constants and i.r. data in the calculation of the harmonic force field is discussed. Nuclear quadrupole coupling coefficients have been obtained for 73GeF2. A revised calculation has bcen made of the cubic potential constants for SeO2 using vibration-rotation interaction constants and new values of vibrational frequencies reported for matrix-isolated SeO2. The dipole moment has also been measured: μ = 2.62 [+ or -] 0.05. The millimetre-wave spectra of KOH and KOD have been examined, four ground-state frequencies being measured for the former and three for the latter. These are consistent with a set of rotational constants given in Table 4, but are not consistent with the frequency reported earlier for the J = 1 -> 2 transition. The pattern of satellites is qualitatively similar to those found previously for CsOH, RbOH and LiOH and is interpreted in the same way. A similar interpretation for NaOH leads to values included in Table 4 (see ref. 385). Other papers on linear triatomic molecules deal with vibration-rotation interaction and with centrifugal distortion of l-doublets.

3 Inorganic Molecules

There has been a growth in the number of papers on inorganic compounds, which reflects an increasing interest in applications in this area. A considerable number of phosphorus compounds have been studied, several papers deal with silicon compounds, and a number deal with germanium derivatives. As well as papers on compounds of the more familiar elements, nitrogen, sulphur, and the halogens, papers have also appeared on compounds of vanadium and tellurium.

Some further boron compounds have been studied, including the microwave identification of the molecule BF2OH. The analysis of the spectrum shows the molecule has a planar structure with the parameters given in Table 5.

The dipole moment μ = 1.86 [+ or -] 0.02 D. As shown in Figure 1, the BF2 group is tilted away from the OH group in the same way as the NO2 group is in the nitric acid molecule. For BF3 itself, l-type resonance has been reported and some cubic potential constants have been evaluated. Two further borane adducts have been investigated. Trimethylphosphine-borane, (CH3)3P,BH3, shows a symmetric-rotor spectrum. By using deuterium substitution and 10B and 11B isotopic species, the molecule is found to have the parameters collected in Table 6.

The structure of the closely related methylphosphine-borane, CH3PH3,BH3, has also been determined. Substitution co-ordinates were obtained for all the atoms except those of phosphorus which were calculated from firstmoment conditions. The resulting structure, which has a plane of symmetry, is shown in Figure 2. The methyl group is tilted towards the borane group and the origin of this tilt is considered in terms of electrostatic interactions. The dipole moments are found to be 4.99 [+ or -] 0.2 D for (CH3)3P,BH3 And 4.66 [+ or -] 0.05 for CH3PH2,BH3.

A number of papers have appeared on isocyanates, and a comparison of structures of isocyanates has been made. Chlorine isocyanate, ClNCO, has been shown to have a planar structure with the parameters listed in Table 7, having C1 and O trans to one An interesting feature of the structure is the bend in the NCO chain of about 8° away from the chlorine atom. A preliminary report has also appeared on the spectrum of the recently prepared cyanogen isocyanate, NCNCO.