I. Ozier

Rotational spectrum, structure, and chlorine nuclear quadrupole coupling constants of the van der Waals complex Ar–Cl2

The pure rotational spectrum of the van der Waals complex Ar–Cl2 has been observed between 5 and 14 GHz using a Balle–Flygare type pulsed molecular beam microwave Fourier transform spectrometer. Ten a‐type rotational transitions of Ar–35Cl2, as well as five a‐type rotational transitions of the mixed isotopomer Ar–35Cl37Cl, have been assigned. The rotational constants and quartic centrifugal distortion constants have been determined. Unlike its isovalent linear isomer Ar–ClF, Ar–Cl2 has been found to be a T‐shaped complex. This confirms the result from electronic spectroscopy and is in accord with the atom–atom additive model. The distance from the Ar nucleus to the center of mass of the Cl2 subunit has been calculated to be 3.7190 A for Ar–35Cl2 and 3.7184 A for Ar–35Cl37Cl, respectively. The van der Waals stretch and bend force constants, as well as their corresponding harmonic vibrational frequencies, have been derived and compared with the theoretically predicted values. The nuclear hyperfine splitting...

The torsional spectrum of CD3CD3

The torsional spectrum of gaseous CH3CD3 has been measured between 230 and 280 cm−1 under relatively large pressure‐path length conditions. The observations were made using a modified Bomem spectrometer at a resolution of 0.016 cm−1 and an absorption path of 20 m. The gas temperature and pressure were 190 K and 82 Torr, respectively. The prominent features of the spectrum are the P, Q, and R branches of the torsional fundamental (v6=1←0). The P and R branches consist of broad peaks from 0.3 to 0.5 cm−1 wide showing partially resolved rotational structure. A frequency analysis combining the present far‐infrared frequencies with molecular beam, microwave, and mm‐wave measurements from previously reported experiments yielded an improved set of torsion–rotation parameters. An intensity analysis was carried out to obtain the torsional dipole moments. The torsional dipole components μT⊥ and (μT∥−μT⊥) were determined to be 14.0(1.5) and −39.6(4.0) μD, respectively. The conditions under which there is agreement b...

The microwave rotational spectrum of the Ar–CO dimer

Pure rotational spectra of four different isotopomers of the dimer Ar–CO have been investigated between 8 and 18 GHz using a pulsed beam cavity Fourier transform spectrometer. The spectra confirm that the complex is a prolate near‐symmetric rotor with an essentially T‐shaped structure, and that it undergoes large amplitude zero‐point motion. It is shown that on the average the argon is closer to the oxygen than to the carbon. The transitions measured obey a‐type selection rules with ΔJ=+1, ΔKa=0, and ΔKc=+1. For 40Ar–12C16O, transitions have been observed for Ka=0 and 1 with lower state J values of 1, 2, and 3. For 40Ar–13C16O and 40Ar–13C18O, a similar series was measured, but only for Ka=0. For 40Ar–13C17O, the 17O quadrupole hyperfine pattern was resolved in the rotational transition JKaKc = 202–101. Determinations have been made for rotational and centrifugal distortion constants, as well as for the 17O quadrupole coupling constant χaa. Effective values have been obtained for the length of the line fr...

Avoided‐crossing molecular‐beam experiments on fluoroform (CF3H) and fluoroform‐d (CF3D)

The avoided‐crossing molecular‐beam method for studying normally forbidden (ΔK≠O) transitions in symmetric tops has been applied to fluoroform (CF3H) and fluoroform‐d (CF3D), thus marking the extension of the method to systems which are not near spherical rotors. In order to reach the high electric fields required while still retaining the necessarily narrow linewidth, the electric resonance spectrometer has been equipped with a new pair of Stark plates capable of providing electric fields up to about 20 kV/cm with a homogeneity of 1 part in 105 over a length of 3 cm. The anticrossing (J,K) = (1,0)↔(1,±1) has been studied for both CF3H and CF3D. In each case, the rotational constant C0 along the symmetry axis has been obtained to 0.002%. From anticrossing spectra observed in combined electric and magnetic fields, the signs of the rotational g factors g∥ and g⊥ have been shown to be negative. From a conventional molecular beam study for each isotopic species, a value of the permanent electric dipole moment...

Proton tunneling in the benzoic acid dimer studied by high resolution ultraviolet spectroscopy

High resolution ultraviolet spectroscopy has been used to investigate the rotationally resolved excitation spectrum of the first singlet–singlet transition in the benzoic acid dimer. The measured spectrum consists of two overlapping components. The corresponding lines in the two components are shown to originate in different levels of the ground state potential separated by a tunneling splitting produced by concerted proton exchange between the two subunits forming the dimer. The frequency separation between the two components is equal to the difference between the tunneling splittings in the ground and the excited electronic state. This frequency separation is found to be 1107±7 MHz. From the analysis, it is estimated that the barrier for proton tunneling changes by about 20% upon electronic excitation. The structure of the dimer in the ground state is determined to be linear, while in the excited S1 state it is slightly bent (3.4°±1.7°).

Study of the torsion-rotation Hamiltonian for symmetric tops using the millimeter wave spectrum of methyl silane

Abstract The torsion-rotation Hamiltonian for symmetric tops has been tested in methyl silane by combining recent anticrossing molecular beam measurements in the ground torsional state (v = 0) with pure rotational spectra taken for v as high as 4. The earlier microwave data set which consisted of J = 1 ← 0 and 2 ← 1 has been greatly extended by studying millimeter transitions for J = 4 ← 3, 5 ← 4, and 13 ← 12. An analysis of the 72 rotational frequencies for v ≤ 2 and the 15 anticrossing data for v = 0 yielded an excellent fit using 14 rotational, torsional, and distortion constants including the effective values for the A rotational constant and the barrier height V3. No satisfactory fit could be obtained when the data set was extended to include measurements for (v = 3) or (v = 4). For each of these higher torsional levels, the difference between the observed frequencies and the predictions based on the best (v ≤ 2) constants can be expressed in terms of a shift δBv in the B rotational constant, where δBv is a smooth function of the torsional energy. This disagreement is of particular interest because it may result from the fact that the molecule passes from hindered to free rotation as v is increased from 2 to 4. The possibility of perturbation by a low-lying vibrational level is considered briefly. The information contained in the different types of spectra is discussed; the redundancy relations are treated and a Fourier expansion of the diagonal torsional matrix elements is introduced. For 12CH329SiH3, 12CH330SiH3, and 13CH328SiH3 pure rotational spectra for v = 0 were studied briefly in natural abundance. The results were combined with existing data for two deuterated symmetric rotors to obtain a structure based only on symmetric top rotational constants.

Pure rotational spectra of the van der Waals complexes Ne–CO, Kr–CO, and Xe–CO

The pure rotational spectra of the van der Waals dimers of Ne, Kr, and Xe with CO have been measured using a pulsed jet, cavity microwave Fourier transform spectrometer. All transitions measured were a-type R-branches, obeying selection rules ΔJ=+1, ΔKa=0, and ΔKc=+1. Spectra with Ka=0 were measured for 7 isotopomers of Ne–CO, 13 of Kr–CO, and 17 of Xe–CO. Transitions with Ka=1 were measured for 20Ne–12C16O and 84Kr–12C16O. Rotational constants and centrifugal distortion constants have been determined for all species, as well as the 17O quadrupole coupling constants χaa for 84Kr–13C17O and 20Ne–13C17O. Effective structural parameters have been calculated from the rotational constants. Results derived from the 17O quadrupole coupling constants and centrifugal distortion constants indicate that Ne–CO is considerably more flexible than Ar–CO, Kr–CO, or Xe–CO. Failure to observe hyperfine structure due to the 21Ne, 83Kr, and 131Xe nuclei is discussed in terms of the weak rare gas–CO bonding. Comparisons have ...

Internal rotation in methyl silane by avoided-crossing molecular-beam spectroscopy

Abstract The avoided-crossing molecular-beam electric-resonance technique was applied to methyl silane in the ground torsional state. A new type of anticrossing is introduced which breaks the torsional symmetry and obeys the selection rules Δ J = 0, K = +1 / a 3 −1. For these “barrier” anticrossings, the values of the crossing fields E c yield directly the internal rotation splittings; the E c are independent of the difference ( A - B ) in the rotational constants. Such anticrossings were observed for J from 1 to 6. Studies were also conducted of several “rotational” anticrossings ( J , K ) = (1, ±1) / a 3 (2, 0) for which E c does depend on ( A - B ). The normal rotational transition ( J , K ) = (1, 0) ← (0, 0) was observed in the ground torsional state using the molecular beam spectrometer. The present data on CH 3 28 SiH 3 were combined with Hirotas microwave spectra and analyzed with the torsion-rotation Hamiltonian including all quartic centrifugal distortion terms. In addition to evaluating B and several distortion constants, determinations were made of the moment of inertia of the methyl top I α = 3.165(5) amu-A 2 , the effective rotational constant A eff = 56 189.449(32) MHz, and the effective height of the threefold barrier to internal rotation V 3 eff = 592.3359(73) cm −1 . The correlations leading to these two effective constants are discussed and the true values of A and V 3 are determined within certain approximations. For the isotopic species CH 3 30 SiH 3 , barrier and rotational anticrossings were observed. The isotopic changes in A and V 3 were determined, as well as an upper limit to the corresponding change in I α .

Torsion-rotation-vibration effects in the degenerate vibrational fundamental (v12 = 1 ← 0) of CH3SiH3

Abstract The lowest-lying degenerate fundamental of a polar symmetric rotor undergoing hindered internal rotation has been studied. The spectrum arising from the excitation ( v 12 = 1 ← 0) of the silyl rock in CH 3 SiH 3 has been measured between 485 and 565 cm −1 with Fourier transform spectroscopy at an effective resolution of 0.0045 cm −1 . Almost 1500 lines originating in the ground torsional state have been identified. Two interesting torsional effects are observed in the spectra. First, resonant or near-resonant perturbations are seen between ( v 12 = 1, v 6 = 0) and ( v 12 = 0, v 6 = 3) for many different ( K , σ) series, where v 6 is the principal torsional quantum number and σ labels the torsional sublevels. The perturbation is shown to result from the xy -Coriolis interaction and a higher-order Coriolis-like term that arises from the off-diagonal part of the operator responsible for the l -doubling. Second, for given lower state J and K , the unperturbed σ-splittings in ( v 12 = 1, l = −1) and in ( v 12 = 1, l = +1) are almost identical and, except for a scale factor, approximate the σ-splittings for the lower state. This effect is shown to result from a z -Coriolis operator linear in the torsional and vibrational angular momenta. The form of the effective Hamiltonian is severely constrained by the large number of precision data obtained earlier on different torsional levels in the ground vibrational state. The global data set of 2011 frequencies is fitted to within experimental error by a model which includes 12 molecular parameters in addition to the 21 constants introduced earlier for ( v 12 = 0). The effective Hamiltonian for a doubly degenerate vibrational fundamental of E 1 symmetry and the leading interactions between this state and the ground vibrational state are discussed in detail.

The microwave centrifugal distortion moment spectrum of 16OPF3

Abstract The microwave centrifugal distortion moment spectrum of the symmetric top 16 O 31 P 19 P 3 in its ground vibronic state has been studied between 9 and 18 GHz with a Stark modulated spectrometer of high sensitivity. The frequencies of 152 transitions obeying the selection rules Δ J = 0, Δ K = ±3 have been measured. A unique identification of the lines has been made by using the current results in a reanalysis of the allowed R -branch spectrum reported recently. From the Q -branch data, values were obtained for the rotational constant difference ( A 0 - B 0 ), two quartic distortion constants and three sextic distortion constants. In kHz, these are: ( A 0 - B 0 ) = 217,495 ± 2; D JK = (1.2971 ± 0.0007); D K = −(1.114 ± 0.012); H JK = (9.10 ± 0.27) × 10 −6 ; H KJ = −(6.8 ± 0.8) × 10 −6 ; H K = −(73 ± 30) × 10 −6 . The value of ( A 0 - B 0 ) has been combined with the R -branch values of B 0 for 16 OPF 3 and 18 OPF 3 to give improved structural parameters. By comparing the intensities of two phosphoryl fluoride lines with the intensities of two silane lines of matching frequencies, the distortion dipole moment of 16 OPF 3 has been determined to be (4.0 ± 1.4) × 10 −6 D.