David C. Moule

Vibration-rotation-inversion Hamiltonian of formaldehyde

Abstract An expression for the Hamiltonian of a vibrating-rotating-inverting formaldehyde molecule is derived. In this derivation, we have used one curvilinear coordinate corresponding to the angle between the CO bond and the bisector of the angle HĈH, and five rectilinear coordinates (linearized valence coordinates). Making use of the zeroth-order Hamiltonian, we have fitted to least-squares (i) the three observed Δ G(v 4 + 1 2 ) values for inversion of H2CO and (ii) five of D2CO, both belonging to the A 1 A 2 excited electronic states, in two separate calculations. For this, we have employed two model potential functions: one consisting of a sum of quadratic and Gaussian and the other a sum of quadratic and Lorentzian terms. In each case, the refined parameters, when transferred to the isotopic molecules (D2CO and HDCO in the one case; H2CO and HDCO in the other), could not account for their observed Δ G(v 4 + 1 2 ) values to the expected degree. We attribute the discrepancies to the inadequacy of the model chosen for the formaldehyde molecule which takes into account only one large amplitude bending motion and which neglects vibration-inversion interactions. We have also obtained a number of quadratic squared sum relations among the Coriolis coupling constants ζ kl α and the inertial constants a k αβ . These are applicable to any molecule undergoing a large amplitude bending motion provided the reference configuration is chosen as described in the text.

The ã3A2 ← X̃1A1 absorption spectrum of thioformaldehyde: A vibrational and rotational analysis

Abstract The results of a vibrational and rotational analysis of the banded a 3 A 2 ← X 1 A 1 transition in CH 2 S CD 2 S are presented. Only three of the six vibrational modes are active in the spectrum with ν′ 2 = 1320 1012 , ν′ 3 = 859 798 , and 2ν′ 4 = 711 516 cm −1 . The spin forbidden transition gains intensity primarily by a mixing of the 1 A 1 (π ∗ ,π) and 3 A 2 (π ∗ ,n) states. This is confirmed by a rotational analysis of the 000 band of both isotopes. The rotational analysis shows that the coupling in the a 3 A 2 state is near Hunds case b and that the spin constants are nearly 10 times greater than those observed for CH2O. A CNDO 2 calculation shows that this difference is due to the greater spin orbit coupling of S in CH2S and to the smaller energy differences between the B 1 A 1 (π ∗ ,π) , b 3 A 1 (π ∗ ,π) , X 1 A 1 , and the a 3 A 2 (π ∗ ,n) states. The r0 structure calculated from the rotational constants is r CS = 1.683 A , r CH = 1.082 A , βHCH = 119.6°, and α (out of plane) = 16.0°. A simultaneous fit of the vibrational levels in ν′4 of CH2S and CD2S to a double minimum potential function yielded a barrier to molecular inversion of 13 cm−1 and an equilibrium out-of-plane angle of 15°.

A vibronic analysis of the ã3A2 ← X̃1A1, ∗ ← n electronic transition in thiophosgene

Abstract The visible asborption spectrum of thiophosgene has been recorded under conditions of high resolution and long path length and has been assigned as an π∗ ← n, a 3 A 2 ← X 1 A 1 electronic transition. Progressions in ν1′, ν2′, ν3′, ν4′, ν1″, ν3″, and ν4″ have been identified in the spectrum and have been analyzed in terms of vibronic transitions between a planar ground state and a nonplanar excited state. A barrier height of 726 cm−1 and a nonplanar equilibrium angle of 32° were obtained for the upper state from a fit of the energy levels of a Lorentzian function to the observed levels in ν4. The origin of the system was assigned to a band at 17 492 cm−1. The oscillator strength of the singlet-triplet system relative to the corresponding singlet-singlet system was obtained by comparing the intensities of selected singlet and triplet bands in their region of spectral overlap. The high intensity, f = 2 × 10−5, observed for the transition results from a spin-orbit coupling between the a 3A2(π∗,n) state and the 1 A 1 (π ∗ ,π) state. This mechanism predicts that the major bands in the triplet system should be polarized in the direction of the thiocarbonyl bond.

The T1(nπ*)←S0 laser induced phosphorescence excitation spectrum of acetaldehyde in a supersonic free jet: Torsion and wagging potentials in the lowest triplet state

The laser induced T1(nπ*)←S0 phosphorescence excitation spectrum of jet‐cooled acetaldehyde has been observed for the first time with a rotating slit nozzle excitation system. The vibronic origins were fitted to a set of levels that were obtained from a Hamiltonian that employed flexible torsion‐wagging large amplitude coordinates. The potential surface extracted from the fitting procedure yielded barriers to torsion and inversion of 609.68 and 869.02 cm−1, respectively. Minima in the potential hypersurface at θ=61.7° and α=42.2° defined the equilibrium positions for the torsion and wagging coordinates. A comparison to the corresponding S1‐state parameters showed that the torsion barrier (in cm−1) does not greatly change, S1/T1=710.8/609.7, whereas the barrier height for the wagging‐inversion barrier increases dramatically, 574.4/869.0.

The assignment of the Rydberg transitions in the electronic absorption spectrum of formaldehyde

The electronic spectra of H2CO, D2CO, HDCO, and D213CO have been recorded between 1250 and 2000 A and the vibrational fine structure associated with the n→3s, 3px, 3py, 3pz, 3d transitions assigned. The electronic origin bands in the Rydberg transitions show quite unusual isotope effects, with the ν (D2CO) –ν (H2CO) shift for the n→3d transition being somewhat smaller than the limiting value of the ion while the lower energy n→3s and 3py transitions have anomalously large values. As these effects cannot be accommodated by the changes in the totally symmetric in‐plane frequencies on excitation, they are attributed to large differences in ν5 and ν6 in the upper and lower electronic states. Direct information concerning the reduction in ν6 on excitation comes from the identification of a series of low frequency bands in the n→3py system of D2CO which were assigned to quanta in ν6. The potential function which was derived from this data was found to be very anharmonic and could contain a double minimum. The r...

An ab initio determination of the bending-torsion-torsion spectrum of dimethyl ether, CH3OCH3 and CD3OCD3

We have calculated the potential energy hypersurface of dimethyl ether with respect to the COC bending coordinate α and the torsional angles of the two methyl groups, θ1 and θ2. Two sets of ab initio calculations were carried out. The first was made at the level MP2/6‐31G(d,p) in which the structural coordinates were fully relaxed except for the grid points on the hypersurface. More extensive calculation were carried out with MP4 corrections for electron correlation with the same molecular structure. The torsional bending Hamiltonian matrix was symmetrized by the operations of the G36 nonrigid group and was solved variationally. The effect of explicitly considering the bending mode in the three‐dimensional treatment was determined by a comparison to the two‐dimensional model in which the flexibility of the frame was absorbed into the calculation by the fully relaxed method. It was found that the three‐dimensional calculation gave a much better account of the sin(3θ1)sin(θ2) intermode coupling than the two...

An analysis of the methyl rotation dynamics in the S0 (X̃ 1A1) and T1 (ã 3A2) states of thioacetone, (CH3)2CS and (CD3)2CS from pyrolysis jet spectra

Jet‐cooled, laser‐induced phosphorescence excitation spectra (LIP) of thioacetone (CH3)2CS/(CD3)2CS have been recorded over the region 16 800–18 500 cm−1 using the pyrolysis jet spectroscopic technique. The responsible electronic transition, T1←S0, a 3A‘←X 1A1, results from an n→π* electron promotion and gives rise to a pattern of vibronic bands that were attributed to activity of the methyl torsion and the sulphur out‐of‐plane wagging modes. The intensities of the torsional and wagging progressions in the excitation spectra were interpreted in terms of a C2v–Cs molecular distortion of the triplet molecule from its singlet ground state equilibrium structure. A complete unrestricted Hartree–Fock (UHF) ab initio molecular orbital (MO) structural optimization of the T1 state predicted that the sulphur was displaced by 27.36° from the molecular plane and the methyl groups were rotated by 10.93° in clockwise–counterclockwise directions. Restricted Hartree–Fock (RHF) calculations were used to generate the V(θ...

Analysis of the 2973‐Å Absorption System of Phosgene

The near‐ultraviolet absorption of phosgene has been assigned to a π* ← n, 1A2 ← 1A1, electronic transition from vapor‐phase spectra recorded under conditions of high resolution and low temperature. Progressions in ν1′, ν2′, ν3′, ν4′, and ν4″ have been identified in the spectrum and have been analyzed in terms of vibronic transitions between a planar ground and a nonplanar excited state. A barrier height of 3170 cm−1 and a nonplanar equilibrium angle of 32.5° were calculated for the upper state from a fit of the energy levels of a Lorentzian–quadratic potential function to the observed levels of ν4. The false origin, 401, of the spectrum has been assigned to the band at 33 631 cm−1. An oscillator strength of f = 1.04 × 10−3 has been obtained for the 1A2 ← 1A1 transition.

The far ultraviolet spectrum of thioformaldehyde

Abstract The electronic absorption spectrum of thioformaldehyde has been recorded from 2200 to 1800 A. Four electronic transitions have been identified in the spectrum and have been assigned to the π → π*, n → 4s, n → 4p y and n → 4p z electron promotions.

Thioketone spectroscopy: An analysis of the lower electronic transitions in thioacetone and thioacetaldehyde

Abstract Visible and ultraviolet spectra of the unstable species, thioacetaldehyde, CH 3 CHS, thioacetone, (CH 3 ) 2 CS, and thioacetone- d 6 have been studied in the gas phase. The valence n → π* excitations, A ← X and a ← X, have been identified. Rydberg n → 4s, 4p y and 4p z and π → π* valence excitations have been found in thioacetone. In thioacetaldehyde a Rydberg-valence interaction mixes the n ,4s and π,π* states which leads to a broad absorption of mixed character between 200 and 220 nm.