R. A. Bernheim

Electron Paramagnetic Resonance of Triplet Alternant Methylenes. Propargylene and Homologs

The electron paramagnetic resonance of a number of alternant methylene compounds has been observed, and zero‐field splitting parameters have been obtained. The results are D=0.6276±0.0002 cm−1 and E=0.0000±0.0008 cm−1 for H–↑C↑–C≡C–H,D=0.6263±0.0002 cm−1 and E=0.0000±0.0008 cm−1 for H–↑C↑–C≡C–CH3,D=0.5413±0.0004 cm−1 and E=0.0035±0.0003 cm−1 for H–↑C↑–C≡C–C6H5,D=0.6087±0.0001 cm−1 and E=0.0000±0.0004 cm−1 for H–↑C↑–C≡C–C≡C–CH3,D=0.6055±0.0001 cm−1 and E=0.0000±0.0004 cm−1 for H–↑C↑–C≡C–C≡C–C(CH3)3,D=0.5530±0.0002 cm−1 and E=0.0000±0.0006 cm−1 for H–↑C↑–C≡C–C≡C–C6H5, and D=0.8629±0.0003 cm−1 and E=0.0000±0.0010 cm−1 for H–↑C↑–C≡N. The interpretation of the data is based upon the observed spin‐density distribution in the alternant free radicals having structures similar to the above triplet molecules. Thus, propargylene, H–↑C↑–C≡C–H, is discussed in terms of a superposition of two π‐electron systems having the same spin‐density distribution as in the allyl radical. Both positive and negative triplet spin de...

Rydberg states of 7Li2 by pulsed optical–optical double resonance spectroscopy: Molecular constants of 7Li2+

Three Rydberg series of electronic states of 7Li2 have been characterized by pulsed optical–optical double resonance spectroscopy. The observed Rydberg states, which include the previously reported E 1Σ+g and G 1Πg states, have been identified as 3–10sσ 1Σ+g, 3–10dσ 1Σ+g, and 3–15dπ 1Πg. The molecular constants for several of the upper members of each of the above series have been used to deduce the ionization potential of 7Li2 and molecular constants for the X 2Σg+ state of 7Li2+. The former was determined to be T0(∞)=41 496±4 cm−1. The latter were found to be in good agreement with recent ab initio calculations.

Spectroscopic study of the E 1Σ+g ‘‘shelf’’ state in 7Li2

The E 1Σ+g ‘‘shelf ’’ state of the 7Li2 molecule was investigated using a pulsed optical optical double resonance technique. The measurements cover the vibrational levels in the range 0≤v≤29 including the shelf region around 13≤v≤15. Molecular constants have been determined. Using an inverted perturbation approach, an effective potential energy curve has been generated within the adiabatic approximation whose quantum mechanical energy eigenvalues reproduce all the measured term values to within 0.113 cm−1 for the range 0≤v≤23 and 0≤J≤47.

Hyperfine splitting of the 1 3Δg Rydberg state of 7Li2

The hyperfine structure of the 1 3Δg Rydberg state of Na2 has been studied by sub‐Doppler CW perturbation facilitated optical–optical double resonance (PFOODR) spectroscopy via A 1Σu+∼b 3Πu mixed intermediate levels.

Anisotropies and Absolute Signs of the Indirect Spin–Spin Coupling Constants in 13CH3F

Proton and 19F magnetic resonance experiments on 13CH3F partially oriented in the liquid crystal solvent p,p′‐di‐n‐hexyloxyazoxybenzene show conclusively that JCH = + 148.8 ± 0.4 Hz, JCF = − 161.9 ± 0.7 Hz, and JHF = + 46.3 ± 0.3 Hz, with the signs absolutely determined. Assuming that there is no change of molecular geometry between the gas phase and the nematic solution, and neglecting solvent effects, isotope effects and corrections due to vibrational averaging, the experimental results suggest unexpectedly large anisotropies of the spin couplings, (J‖ − J⊥)CH = + 1890 Hz, (J‖ − J⊥)CF = + 700 Hz (measured with respect to the molecular symmetry axis), whereas JHF has negligible anisotropy. The anisotropies in Jij are examined with respect to their sensitivities upon the appropriate average molecular geometries used in their determination, and the effects of solvent are also discussed. The accuracy of the above anisotropies cannot be determined without a knowledge of the complete vibrational potential of ...

A spectroscopic study of the E1Σ+g and F1Σ+g states of 7Li2 by pulsed optical–optical double resonance

The results of pulsed optical–optical double resonance spectroscopic studies of the E 1S+g and F 1S+g states of 7Li2 are presented. Observations were carried out on the v = 0 through v = 30 levels of the F 1S+g state, representing about 60% of the dissociation energy. A set of Dunham molecular constants was derived and an RKR potential was generated. Franck–Condon factors were determined for the range of observed vibrational levels. The spectroscopic results on the E 1S+g state are preliminary and are included because of a probable homogeneous anticrossing between it and the F 1S+g state. It is suggested that the E 1S+g state correlates with the (2 2S)+(3 2S) atomic lithium states while the F 1S+g correlates with the (2 2P)+(2 2P) states.

Proton and 19F Magnetic Resonance of Partially Oriented Fluoromethanes

It is shown that the absolute sign of the indirect proton–fluorine nuclear spin–spin coupling constant JHF is positive in the fluoromethanes from their spectra in the nematic liquid crystal p,p′‐di‐n‐hexyloxyaz‐oxybenzene and the knowledge of the previously determined absolute signs of the 19F magnetic shielding anisotropies in CH3F and CHF3. Values of the 19F and proton magnetic shielding anisotropies are determined and the problems of shift referencing are discussed. If it is assumed that the contributions to magnetic shielding of the H and F nuclei are axially symmetric about the respective C–H and C–F bonds, the experimental values can be transformed into magnetic shielding anisotropies along bond directions.

A study of the E 1Σ+g state of 7Li2 by pulsed optical–optical double resonance spectroscopy

The results of pulsed optical–optical double resonance spectroscopic studies of the E 1Σ+g state of 7Li2 are presented. Observations were carried out on the v = 0 through v = 12 levels. A set of Dunham molecular constants was derived and an RKR potential was generated. Franck–Condon factors for transitions to the A 1Σ+u state were determined for the range of observed vibrational levels. An apparent deviation from the expected dissociation energy is observed consistent with the suggestion that a double minimum exists for this state.

A spectroscopic study of the G 1Πg state of 7Li2 by pulsed optical–optical double resonance

The results of a pulsed optical–optical double resonance spectroscopic study of the G 1Πg state of 7Li2 are presented. Observations were made on the v*=0 through v*=20 levels, representing about 60% of the dissociation energy. A set of Dunham molecular constants was derived. The RKR potential was generated and Franck–Condon factors were determined for the range of observed vibrational levels. A dissociation energy was estimated from a Birge–Sponer extrapolation of the Gv values which suggests a correlation with the (2 2P)+(2 2P) atomic lithium states at the dissociation limit.

Electron paramagnetic resonance of triplet alternant methylenes

Abstract In a previous study the alternant methylenes, propargylene, and some of its homologs, were investigated with electron paramagnetic resonance, and the zero-field splitting parameters of their triplet ground states determined assuming that the g -factors for all the molecules were equal to the free electron value. The same molecules have been reinvestigated at two microwave frequencies, 9.5 GHz and 15.7 GHz, for each sample preparation. With the possible exception of g zz for cyanomethylene and propargylene, the assumption of the free electron g -value is valid for a determination of the zero-field splitting parameters precise to about 1%.