Experimental,semi-experimental and ab initio equilibrium structures

The determination of equilibrium structures of molecules by spectroscopic methods or by quantum mechanical calculations is reviewed. The following structures are described in detail: experimental equilibrium structures, empirical structures, semi-experimental structures and ab initio structures. The approximations made by the different methods are discussed and their accuracies are compared.     

Microwave spectrum of dimethylallene excited states and strong Coriolis coupling

The rotational spectra of six excited vibrational states of dimethylallene were measured and assigned to the corresponding vibrational levels, and for three more excited state spectra at least the rotational constants could be determined. Between the two lowest excited levels of symmetry species b₂ and b₁ of group C_2v a strong a-type Coriolis coupling was found to exist. The evaluation of the resulting perturbation by a diagonalization of the energy matrix yielded ζ^(a) = 0.36 and a precise value for the vibrational energy difference 48.761 GHz (1.6 cm^?1). The state b₂ is believed to be the first excited torsional substate (01, 10)₁ of methyl internal rotation, and the rotational transitions of this state as well as those of the strongly coupled state b₁ presented very irregular multiplet splittings. On the other hand, the splittings of the next-higher excited state of species a₂ which could be identified as the partner torsional substate (01, 10)₂, followed the regular pattern, yielding an internal rotation barrier V₃ (2079 cal/mole) not unlike that derived earlier from ground state splittings.     

Microwave spectrum of dimethylketene: Centrifugal distortion and Coriolis interaction

For the prolate asymmetric (κ = ?0.58) top molecule dimethylketene the centrifugal constants have been determined in terms of the Δ constants by means of ΔK_? ≠ 0 rotational transitions. Two earlier assigned torsional excited states (01, 10)₁ and (01, 10)₂ of symmetry B₂ and A₂ of C_2v could be confirmed. A third fundamental vibration v_r(b₁), probably the inplane skeletal rock, has been found and analyzed in the v_r = 1 and v_r = 2 states. The variation of the centrifugal constants and of the inertial defects with the state of excitation could be explained by Coriolis coupling between the v_r(b₁) = 1 vibrational and v_t(b₂) = 1 torsional excited states. The Coriolis constant $\text{ζ}{}_{\mathrm{rt}}{}^{\mathrm{a}}$ and the torsional frequency ω_t(b₂) have been estimated to be $\text{ζ}{}_{\mathrm{rt}}{}^{\mathrm{a}}\text{≈ 0.24}$ and ω_r(b₁) - ω_t(b₂) ≈ 5.8 cm^?1, respectively.     

The chiral molecule CHClFI: first determination of its molecular parameters by Fourier transform microwave and millimeter-wave spectroscopies supplemented by ab initio calculations

The rotational spectrum of chlorofluoroiodomethane (CHClFI) has been investigated. Because its rotational spectrum is extremely crowded, extensive ab initio calculations were first performed in order to predict the molecular parameters. The low J transitions were measured using a pulsed-molecular-beam Fourier transform spectrometer, and the millimeter-wave spectrum was measured to determine accurate centrifugal distortion constants. Because of the high resolution of the experimental techniques, the analysis yielded accurate rotational constants, centrifugal distortion corrections, and the complete quadrupole coupling tensors for the iodine and chlorine nuclei, as well as the contribution of iodine to the spin-rotation interaction. These molecular parameters were determined for the two isotopologs CH35ClFI and CH37ClFI. They reproduce the observed transitions within the experimental accuracy. Moreover, the ab initio calculations have provided a precise equilibrium molecular structure. Furthermore, the ab initio molecular parameters are found in good agreement with the corresponding experimental values.     

Microwave and high resolution infrared spectra of vinyl chloride, ab initio anharmonic force field and equilibrium structure

The quadratic, cubic, and semi-diagonal quartic force field of vinyl chloride has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality. The spectroscopic constants derived from this force field are compared with the experimental values. To make this comparison more complete, the rotational constants of the lowest excited state, v₉ = 1 at 395 cm⁻¹ have been determined by microwave spectroscopy and the ν₁₂ band (around 618 cm⁻¹) has been investigated by high-resolution infrared Fourier transform spectroscopy. The equilibrium structure has been derived from experimental ground state rotational constants and ab initio rovibrational interaction parameters. This semi-experimental structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-ζ quality and a core correlation correction. The experimental mass-dependent r_m structures are also determined and their accuracy is discussed. The recommended equilibrium geometry is: r (CC) = 1.3262(10), r (CCl) = 1.7263(10), r (CH_g) = 1.0784(10), r (CH_c) = 1.0795(10), r (CH_t) = 1.0797(10), ∠(CCCl) = 122.77(10)°, ∠(CCH_g) = 123.86(10)°, ∠(CCH_c) = 121.80(10)°, ∠(CCH_t) = 119.29(10)°.     

Rotational spectrum and structure of asymmetric dinitrogen trioxide, N2O3

The rotational spectra of the ground vibrational state and the ν₉ = 1 torsional state have been reinvestigated and accurate spectroscopic constants have been determined. The torsional frequency, ν₉ = 70(15) cm⁻¹, has been determined by relative intensity measurements. The assignment of the infrared spectrum has been slightly revised and an accurate harmonic force field has been calculated. The equilibrium structure has been determined using different, complementary methods: experimental, semi-experimental and ab initio, leading to r(NN) = 1.870(2) Å, in particular.     

The 14-22 μm absorption spectrum of nitrous acid studied by high-resolution Fourier-transform spectroscopy: New analysis of the ν5 and ν6 interacting bands of trans-HONO and first analysis of the ν6 band of cis-HONO

High-resolution Fourier-transform absorption spectra of nitrous acid (HONO) were recorded in the 400-700 cm⁻¹ spectral region. For the trans-HONO isomer a very extensive analysis of the ν₅ (ONO bend) and ν₆ (torsion) interacting fundamental bands of trans-HONO located near 595.620 and 543.879 cm⁻¹, respectively, was performed starting from the results of a previous study [C.M. Deeley, I.M. Mills, L.O. Halonen, J. Kauppinen, Can. J. Phys. 63 (1985) 962-965]. In addition, for the less abundant cis-HONO isomer, the first high-resolution study of the ν₆ band located at 639.7432 cm⁻¹ was achieved, the ν₅ band is presumed to be located near 609.2 cm⁻¹. For both isomers the strong A-type and B-type Coriolis interactions linking the {5¹, 6¹} interacting energy levels were accounted for during the energy level calculations. Finally, using these results a list of positions and of realistic relative line intensities has been generated for both isomers, and a comparison with some ab initio predictions was performed.     

Rotational Spectrum, Hyperfine Structure, and Internal Rotation of Methyl Carbamate

Methyl carbamate, an isomer of glycine, is a possible candidate for interstellar detection. It might be more abundant than glycine and its dipole moment is much larger. Furthermore, using high-level quantum chemical calculations, it is shown that syn methyl carbamate has a lower energy than glycine. The quadrupole hyperfine structure due to ¹⁴N has been measured using microwave Fourier transform spectroscopy. The rotational spectrum of the ground vibrational state has been measured from 8 to 240 GHz and accurate spectroscopic constants have been determined for the A internal rotation components of the rotational lines. Finally, the internal rotation splittings have been analyzed.