David O. Harris

Ring Puckering in Five‐Membered Rings. II. The Microwave Spectrum, Dipole Moment, and Barrier to Pseudorotation in Tetrahydrofuran

The microwave spectrum of tetrahydrofuran has been studied. Nine complete rotational spectra have been observed. These arise from the ground and eight excited states. All of these states are less than 200 cm−1 from the ground state. The rotational constants and dipole moments exhibit a strong nonlinear dependence on the quantum number of the excited state. Vibration–rotation interaction is strong and the spectra of the first four states deviate from that of rigid rotor spectra. These deviations permit the determination of two energy separations: Δ01 = 0.67 cm−1 and Δ23 = 1.5 cm−1. All of the results are interpreted in terms of a model of restricted pseudorotation with a potential function of [30(1‐cos2φ) / 2] + [40(1‐cos4φ) / 2] cm−1, where φ is the angle of pseudorotation. The dipole moment varies from 1.52 to 1.76 D depending upon the pseudorotation state. The details of this variation indicate that the twisted configuration is at lower energy than the bent configuration.

Ring Puckering in Five‐Membered Rings. I. General Theory

As a general introduction to a subsequent series of papers on pseudorotation and five‐membered rings, the two‐ring puckering modes of a five‐membered ring are discussed in general terms. Five cases are separated depending upon the nature of the potential function for each mode. Correlation diagrams connecting the various cases are given. Consistent with our basic policy, we have gone directly from fundamental theory to an arithmetical development using large digital computers. The classical section of this development is outlined. The quantum‐mechanical section is developed in the following paper.

Barriers to Internal Rotation in Asymmetric Molecules: 3‐Fluoropropene

The potential function hindering internal rotation in 3‐fluoropropene has been determined using microwave and far‐infrared spectroscopic data. It is given by the six potential constants in cm−1 (calories/mole): V1 = − 247 ± 30(− 707), V2 = 185 ± 25(530), V3 = 857 ± 15(2449), V4 = 188 ± 25(538), V5 = 7 ± 5(20), V6 = − 93 ± 10(− 265). From these the cis–gauche and gauche–trans–gauche barrier heights were determined to be 1090 + 75 − 25 cm−1 and 520 ± 40 cm−1, respectively. The spectra were interpreted using a one‐dimensional model in which the only internal motion is the rotation about the bond connecting the top (–CH2F) to the frame (H2C=CH–); the various parameters in the Hamiltonian which was derived using the theory of internal rotation in asymmetric molecules developed by Quade and Lin were determined numerically and expressed as Fourier expansions in the internal coordinate α. In addition, the general theory was rederived and extended to include the case where neither the top nor the frame has a plane...

Microwave optical double resonance spectroscopy with a cw dye laser: BaO X1Σ and A1Σ

A tunable, single frequency, continuous wave, dye laser has been used to optically pump various lines of the BaO A 1Σ‐X 1Σ electronic transition. Microwave optical double resonance (MODR) spectra are recorded as changes in the intensity of dye laser induced photoluminescence. Fourteen microwave rotational transitions in the X 1Σ (ν = 0,1) and A 1Σ (ν = 0–5) states of 138Ba16O and one transition in the A 1Σ (ν = 1) state of 137Ba16O have been observed. Partially deperturbed rotational constants obtained for BaO A 1Σ are B(ν) = 0.25832(2) − 0.001070(5) (ν + 1/2) cm−1. Two physical models are described which account for microwave optical double resonance effects in the strong (nonlinear) and weak (linear) optical pumping limits. Observed changes in photoluminescence polarization caused by excited state microwave transitions are predicted by a semiclassical transition dipole model. A three level steady state kinetic treatment of microwave optical double resonance indicates that the BaO MODR transitions report...

Laser spectroscopy of the A-X transitions of CaOH and CaOD☆

Abstract The A - X transitions of gas phase CaOH and CaOD, produced in a low-pressure flow system, were studied by low- and high-resolution laser spectroscopy. The vibrational structure of the low-lying vibrational levels of both the A and X states were determined. A detailed rotational analysis of the (0, 0, 0)-(0, 0, 0) bands of the A - X system is presented. The spectra are well described by a 2 Π- 2 Σ model, where the molecule is linear in both states. A preliminary analysis of the (0, 1, 0)-(0, 1, 0) bands indicates that the Renner parameter ϵ of the A 2 Π electronic state is equal to 0.073(1).

High-resolution laser excitation spectra of linear triatomic molecules: Analysis of the B2Σ+-X2Σ+ system of SrOH and SrOD

Abstract Single-mode cw dye laser excitation spectra of the (0, 00, 0)-(0, 00, 0), (1, 00, 0)-(1, 00, 0), (0, 11, 0)-(0, 11, 0) bands of the 611- to 607-nm system of SrOH and SrOD were observed and assigned. The spectrum is consistent with a 2Σ-2Σ system where the molecule is linear in both electronic states. The rotational analysis is obtained by measuring P-R separations in the excitation spectrum. A technique which greatly aided in the spectral assignment that utilizes a monochromator as a bandpass filter in obtaining excitation spectra is described. A new spin-rotation constant, γ(+) - γ(−), was found necessary to fit the l-type doubling of the (0, 11, 0) mode in B2Σ+ of SrOH. This new constant is assumed to arise from two 2 Σ 1 2 vibronic states, split by Renner-Teller interaction, of the (0, 11, 0) mode of A2Π. From a perturbation treatment of this electronic interaction, ϵω2 of the (0, 11, 0) mode of A2Π was found to be 14 cm−1. The linear structure of SrOH and SrOD in both electronic states is consistent with the model, in the one-electron approximation, in that the transition involves the promotion of the unpaired 5s to the 5p orbital localized on the strontium.

On the Determination of Barriers to Internal Rotation

A method is presented for determining barriers to internal rotation directly from the spectrum of torsional energy levels without an a priori knowledge of either molecular geometry or the dynamics of the motion. The method is applied to the H2O2 and D2O2 molecules. It is also shown that the neglect of the variation of the effective inverse moment of inertia of threefold rotors can lead to an apparent V6 contribution to the potential function.

Microwave optical double resonance and continuous wave dye laser excitation spectroscopy of NO2: Rotational assignment of the K = 0−4 subbands of the 593 nm band☆

Abstract A rotational assignment of approximately 80 lines with Ka′ = 0, 1, 2, 3, and 4 has been made of the 593 nm 2 A 1 → 2 B 2 band of NO2 using cw dye laser excitation and microwave optical double-resonance spectroscopy. Rotational constants for the 2B2 state were obtained as A = 8.52 cm−1, B = 0.458 cm−1, and C = 0.388 cm−1. Spin splittings for the Ka′ = 0 excited state levels fit a simple symmetric top formula and give (ϵ bb + ϵ cc ) 2 = −0.0483 cm −1 . Spin splittings for Ka′ = 1 (N′ even) are irregular and are shown to change sign between N′ = 6 and 8. Assuming that the large inertial defect of 4.66 amu A2 arises solely from A, a structure for the 2B2 state is obtained which gives r (NO) = 1.35 A and an ONO angle of 105°. Alternatively, weighting the three rotational constants equally gives r = 1.29 A and θ = 118°.

Comparison of three numerical techniques for calculating eigenvalues of an unsymmetrical double minimum oscillator

Disagreement between calculated vibrational eigenvalues for the E, F 1Σ+g state of H2 obtained by three different numerical techniques has been interpreted in the literature in terms of inherent inadequacies of these techniques. We have tested these three numerical procedures by using each to obtain the eigenvalues of an unsymmetrical double minimum potential. An analytical potential was used to eliminate uncertainties introduced by interpolation. All three techniques are shown to give accurate results when properly applied.

Microwave optical double resonance and reanalysis of the CaF A2Πr-X2Σ band system

Abstract A laser excitation spectrum of the (0,0) and (1,1) bands of the CaF A 2 Π- X 2 Σ system has been recorded and all 12 rotational branches assigned on the basis of laser induced fluorescence measurements. Microwave optical double resonance (MODR) measurements were also made on several low- J transitions to obtain precise spectroscopic constants of the X 2 Σ state. The present set of molecular constants for both the A 2 Π and X 2 Σ states represent a vast improvement in precision over previously reported values because of the larger number of line position measurements and the use of a more reasonable Hamiltonian. The excellent agreement between the rotational constants derived from the optical and microwave measurements fully validates the assignments. Important derived constants are listed below (in cm −1 ) with 95% confidence limits in parentheses.