Quasi-symmetric Top Molecules Research Articles

Monodromy in the spectrum of a rigid symmetric top molecule in an electric field

We show that for rigid symmetric top molecules in electric fields the phenomenon of monodromy arises naturally as a “defect” in the lattice of quantum states in the energy-momentum diagram. This makes it impossible to use either the total angular momentum or a pendular quantum number to label the states globally. The monodromy is created or destroyed by classical Hamiltonian Hopf bifurcations from relative equilibria. These phenomena are robust and should be observable in quasi-symmetric top molecules with field strengths ℰ satisfying μE/b>4.5, where μ is the dipole moment and b the rotational constant perpendicular to the symmetry axis of the molecule.

Ab initio potential energy surfaces for the large-amplitude motions of quasi-symmetric top molecules: CH 3NCS and SiH 3NCO

The potential energy surfaces of methyl isothiocyanate, CH 3NCS, and silyl isocyanate, SiH 3NCO, have been investigated in quantum mechanical ab initio calculations at the SCF, MP2-MP4 and CCSD (T) levels of theory. Total energies of the two molecules were calculated as functions of coordinates of the two large-amplitude motions: the CNC/SiNC bending motion and internal rotation of the methyl/silyl group. At the most advanced level employed, CCSD(T) with the cc-pVTZ basis set, the theoretical results are in good agreement with experimental data and consistent with the quasi-symmetric top model of both molecules.

Ab initio potential energy surfaces of large-amplitude motions for quasi-symmetric top molecules: CH 3NCO

The potential energy surface of methyl isocyanate, CH 3NCO, has been investigated in quantum mechanical ab initio calculations at the SCF, MP2–MP4, and CCSD(T) levels of theory. The total energy of the molecule was calculated as a function of the coordinates of the two large-amplitude motions: the CNC bending motion and internal rotation of the methyl group. At the most advanced level employed, CCSD(T) with cc-pVTZ basis set, the barriers to linearity of the CNCO skeleton and to internal rotation of the methyl group were determined to be 920 and 22 cm −1, respectively. The theoretical results are in good agreement with experimental data and consistent with the quasi-symmetric top model of the molecule.

ChemInform Abstract: The Microwave Spectrum of Methyl Isoselenocyanate: CH3NCSe as a Quasi‐Symmetric Top.

Abstract The microwave spectrum of methyl isoselenocyanate, CH3NCSe, has been measured in the region 18–40 GHz. The a-type J + 1 ← J rotational transitions (J = 5 to 10) in the ground vibrational state of the molecule have been assigned for the three isotopic species with 82Se, 80Se, and 78Se. For the most abundant isotopic species, with 80Se, the rotational transitions in the excited states of the CNC bending mode v10 = 1 and 2 have also been assigned. Contrary to the previous investigation [T. Sakaizumi et al., Bull. Chem. Soc. Japan 59, 1614–1616 (1986)], it has been shown that the observed spectrum is consistent with that of a quasi-symmetric top molecule. The rotation-vibration constants have been determined, showing an unusual variation with excitation of the CNC bending mode. The CNC bending potential function has been determined in preliminary calculations using a quasi-symmetric top model. The potential function is calculated to be very anharmonic, with an equilibrium CNC angle of about 162° and a barrier to linearity of the CNCSe skeleton of about 25 cm−1. From accidental resonances observed for the ground state transitions the barrier to internal rotation of the methyl group was estimated to be about 3 cm−1.

The microwave spectrum of methyl isoselenocyanate: CH 3NCSe as a quasi-symmetric top

The microwave spectrum of methyl isoselenocyanate, CH 3NCSe, has been measured in the region 18–40 GHz. The a-type J + 1 ← J rotational transitions ( J = 5 to 10) in the ground vibrational state of the molecule have been assigned for the three isotopic species with 82Se, 80Se, and 78Se. For the most abundant isotopic species, with 80Se, the rotational transitions in the excited states of the CNC bending mode v 10 = 1 and 2 have also been assigned. Contrary to the previous investigation [T. Sakaizumi et al., Bull. Chem. Soc. Japan 59, 1614–1616 (1986)], it has been shown that the observed spectrum is consistent with that of a quasi-symmetric top molecule. The rotation-vibration constants have been determined, showing an unusual variation with excitation of the CNC bending mode. The CNC bending potential function has been determined in preliminary calculations using a quasi-symmetric top model. The potential function is calculated to be very anharmonic, with an equilibrium CNC angle of about 162° and a barrier to linearity of the CNCSe skeleton of about 25 cm −1. From accidental resonances observed for the ground state transitions the barrier to internal rotation of the methyl group was estimated to be about 3 cm −1.

ChemInform Abstract: Electric Dipole Moment Functions of CH3NCO and CH3NCS.

Abstract The Stark effect of some rotational transitions of two quasi-symmetric top molecules, namely methyl isocyanate (CH3NCO) and methyl isothiocyanate (CH3NCS), has been investigated and analyzed in terms of a five-dimensional quasi-symmetric top model. The electric dipole moment is expressed as a function of coordinates of the large-amplitude motions: the CNC bending and internal rotation. The function parameters have been determined using the experimental data combined with the results of ab initio SCF calculations. The effective electric dipole moments of both molecules have been estimated for the ground and excited CNC bending states.

Electric dipole moment functions of CH 3NCO and CH 3NCS

The Stark effect of some rotational transitions of two quasi-symmetric top molecules, namely methyl isocyanate (CH 3NCO) and methyl isothiocyanate (CH 3NCS), has been investigated and analyzed in terms of a five-dimensional quasi-symmetric top model. The electric dipole moment is expressed as a function of coordinates of the large-amplitude motions: the CNC bending and internal rotation. The function parameters have been determined using the experimental data combined with the results of ab initio SCF calculations. The effective electric dipole moments of both molecules have been estimated for the ground and excited CNC bending states.

Characteristic patterns in microwave spectra of quasi-symmetric top molecules of the WH 3XYZ type

Characteristic patterns in microwave spectra of quasi-symmetric top molecules of the WH 3 XYZ type are discussed in terms of the quasi-symmetric top model accounting for the large-amplitude WXY bending motion, and internal and overall rotation. The molecules SiH 3NCS, SiH 3NCO, CH 3NCS, CH 3NCO, and CH 3SCN, exemplifying the symmetric, quasi-symmetric, and asymmetric top molecules, are considered. The correlation parameter γ v is defined to quantify the position of a WH 3 XYZ molecule between the two symmetric and asymmetric top limiting cases. The quasi-symmetric top model is compared to the usual approach to the rotational-vibrational problem of symmetric top molecules.

The microwave spectrum of methyl isothiocyanate

Abstract The microwave spectrum of methyl isothiocyanate, CH 3 NCS, has been measured in the range from 5 to 26 GHz. Almost 300 lines have been assigned to the a -type J + 1 ← J rotational transitions ( J = 0 to 4) arising from molecules in the ground state and excited torsional and CNC bending states. The observed spectrum has been analyzed and successfully interpreted in terms of the five-dimensional quasi-symmetric top molecule model accounting explicitly for the large-amplitude CNC bending motion, and internal and overall rotation. In the least-squares fitting to the observed transition energies the values of the parameters of the CNC bending-torsional potential function have been determined.

The microwave spectrum of methyl isocyanate

The microwave spectrum of methyl isocyanate, CH 3NCO, has been measured in the range from 8 to 40 GHz. More than 200 lines have been assigned to the a-type J + 1 ← J rotational transitions ( J = 0 to 3) arising from molecules in the ground state and excited torsional and CNC bending states. The observed spectrum has been analyzed and successfully interpreted in terms of the five-dimensional quasi-symmetric top molecule model accounting explicitly for the large-amplitude CNC bending motion, and internal and overall rotation. In the least-squares fitting to the observed transition energies the values of the parameters of the CNC bending-torsional potential function have been determined.

Quasi-symmetric top molecule approach to the rotational-vibrational problem of CH 3XY molecules: Application to CH 3OD

The quasi-symmetric top molecule approach proposed previously [J. Koput, J. Mol. Spectrosc. 104, 12–24 (1984)] to calculate vibration-rotation energy levels of a CH 3XY molecule is used to study the COD bending-torsion-rotation energy levels of monodeuterated methanol, CH 3OD. The available 150 transition frequencies (microwave, millimeter wave, and infrared data) have been fitted and the barrier to linearity of the COD skeleton has been found to be about 7000 cm −1. The effective barrier to internal rotation in the ground state has been determined to be 365.79 cm −1 and that in the first excited state of the COD bending mode has been predicted to be 380.62 cm −1.

Methyl isocyanate: An analysis of the low- J rotational spectrum using the quasi-symmetric top molecule approach

The low- J rotational spectrum of methyl isocyanate (CH 3NCO) has been analyzed in terms of the quasi-symmetric top molecule model, accounting explicitly for the large-amplitude CNC bending motion, and internal and overall rotation. An assignment of 25 J = 1 ← 0 and 2 ← 1 rotational transitions arising from the various CNC bending and torsional states is proposed. The molecule is found to be a nearly freely internally rotating quasi-symmetric top, with a barrier to linearity of the CNCO skeleton of 1049 cm −1 and an equilibrium CNC valence angle of 140.2°.

Correlation diagrams for quasi-symmetric top molecules with a single rotor

The rotation-bending-internal rotation Hamiltonian for quasi-symmetric top molecules with a single rotor [A. Wierzbicki, J. Koput, and M. Krȩglewski, J. Mol. Spectrosc. 99, 102–115 (1983)] is used to calculate the correlation diagrams between the energy levels of the SiH 3NCO-type molecules in the symmetric and asymmetric top limits, and to predict various features of the microwave spectrum through trial calculations. The height of the barrier to linearity was varied from 0 to 6500 cm −1. For the barriers 0, 31, and 139 cm −1, the effects of hindered internal rotation are studied and several (Δ l = 3, Δ k = 0) resonances are found. In some cases almost all the microwave transitions from a given vibrational state can be perturbed.

Quasi-symmetric top molecule approach to the rotational-vibrational problem of CH 3XY molecules: Application to CH 3C 15N

A new approach to the rotational-vibrational problem of molecules of the CH 3XY type is proposed. The approach can be applied equally well to either a symmetric top or an asymmetric top CH 3XY molecule. The molecule is considered as vibrating, internally rotating about the CX bond of the effectively bent CXY skeleton, and rotating in space. The zeroth-order approximation, in which the CXY bending motion and internal and overall rotations are explicitly taken into account, is discussed. The approach is applied to the analysis of the rotational spectrum of [ 15N]methyl cyanide in the ground and first and second excited states of the CCN bending mode.

The large-amplitude motions in quasi-symmetric top molecules with internal C3 v rotors : The vibration-torsion-rotation Hamiltonian of a two-top molecule

An expression for the vibration-torsion-rotation Hamiltonian of a molecule with triatomic nonrigid frame and two internal C 3 v rotors has been derived. Three large-amplitude motions, namely skeletal bending and two torsions, are removed from the vibrational problem and are considered together with the rotational problem. The Hamiltonian obtained is applicable to any two- C 3v -top molecule with triatomic linear or bent frame. The zeroth-order skeletal bending-torsion-rotation Hamiltonian is derived and the method of solving the corresponding Schrödinger equation is discussed. The Hamiltonian obtained with only slight modifications is shown to be applicable to any single- C 3v -top molecule with a quasi-linear tetratomic nonrigid frame or to the problem of the large-amplitude bending motion in a pentatomic quasi-linear molecule.

The large-amplitude motions in quasi-symmetric top molecules with internal C3 v rotors : Interpretation of the low frequency Raman spectrum of disiloxane

The vibration-torsion-rotation Hamiltonian developed previously has been used to study the skeletal bending-torsion-rotation energy levels of disiloxane. The zeroth-order skeletal bending-torsion-rotation Hamiltonian for a molecule of this type is presented. The symmetry classification of the molecular wavefunctions and energy levels as well as infrared and Raman selection rules are considered. The energies of the skeletal bending-torsional-rotational states have been calculated and these results are used to interpret the low frequency Raman spectra of gaseous disiloxane- d 0 and disiloxane- d 6. It is shown that the treatment presented enables one to properly interpret these spectra. The results are compared to those obtained with other more approximate Hamiltonians.