B. R. Russell

Vacuum ultraviolet absorption spectra of dimethylsulfide, dimethylselenide, and dimethyltelluride

The vapor phase absorption spectra of dimethylsulfide, dimethylselenide, and dimethyltelluride are reported for the 40 000–80 000 cm−1 vacuum ultraviolet spectral region. Three Rydberg series for each compound were assigned which converge on the ionization potentials of 8.706 ± 0.010, 8.400 ± 0.010, and 7.926 ± 0.010 eV for dimethylsulfide, dimethylselenide, and dimethyltelluride, respectively. These ionization potentials and corresponding Rydberg series members are assigned as originating from the central atom valence p orbital, which is perpendicular to the plane of the molecule. A correlation of the ionization potentials calculated for these molecules to ionization potentials of corresponding rare gases is presented. The term values of Rydberg members of the dimethyl chalcogens are also correlated to the term values of allowed atomic Rydberg series in the rare gases, thus atomic nomenclature is used in the assignment of the molecular Rydberg series. Higher energy absorptions in the spectra of dimethyls...

Vacuum ultraviolet absorption spectra of the bromomethanes

The vapor phase vacuum ultraviolet absorption spectra of methyl bromide, methylene bromide, bromoform, and carbon tetrabromide are reported for the region 40 000−90 000 cm−1 (250−110 nm). Many of the Rydberg and intravalent absorptions of the bromine electrons in the nonbonding p orbitals, which are perpendicular to the C−Br bonding axes, and the σ*←σ (C−Br) absorptions of the compounds have been assigned. A correlation of the molecular Rydberg series of methyl bromide to series in krypton, which has made possible the assignment of the 4d and 4f Rydberg absorptions, is presented. Expanded vibrational progressions of the first ns and np Rydberg transitions of methylbromide are presented. The analysis of these progressions indicates that transitions to the 5s and 5p Rydberg orbitals bring about a Jahn−Teller distortion of the molecule. Three ns Rydberg series of methylene bromide were assigned which converge on the ionization potentials of 10.629±0.010, 10.839±0.010, and 11.261±0.010 eV. The states formed u...

Electronic symmetries, dipole moments, and polarizabilities of molecules in Rydberg states determined from electric‐field effects on absorption spectra

The effects of intense electric fields (∼ 2×105 V cm−1) on the gas‐phase, ultraviolet absorption spectra of acetone and dimethylsulfide are investigated for the purpose of determining the changes in the electric dipole moments and polarizabilities and the transition polarizations corresponding to three bands assigned to Rydberg transitions. The bands investigated are the 1954 A band of acetone assigned to the 3s←n (1B2←1A1) Rydberg transition and the 1959 and 2284 A bands of dimethylsulfide assigned to the 4p←n (1A1←1A1) and 4s←n (1B1←1A1) Rydberg transitions, respectively. Interpretation of the results revealed that, for the transition in acetone and the 4p←n and 4s←n transitions in dimethylsulfide, the dipole moments are reduced from the ground‐state values by 0.48±0.02 D, 0.47±0.03 D, and 0.42±0.04 D, respectively, and the mean polarizabilities are increased by 17.3±1.5×10−24 cm3, 11.7±1.3×10−24 cm3, and 9.3±2.0×10−24 cm3, respectively. The transition polarizations were all found to be in agreement wit...

Electric dichroism spectroscopy in the vacuum ultraviolet. I. Cyclobutanone, cyclopentanone, cyclohexanone, and cycloheptanone

Electrochromism and electric linear dichroism have been used in a study of transitions to the second excited singlet of a series of simple cyclic ketones. The results of the work show that the transition under investigation is polarized perpendicular to the direction of the dipole moment in the ground state of cyclobutanone, cyclopentanone, cyclohexanone, and cycloheptanone, where the dipole moment changes for the transition to the second excited singlet state from ground state are found to be 1.44, 1.63, 4.30, and 4.92 D, respectively. The magnitude of the changes in dipole moment and mean polarizability support the perpendicular B2←A1,3sa1←nb2 molecular Rydberg assignment. The larger dipole moment changes for the larger compounds are attributed to ring delocalization of the positive core charges in this extravalent state, a situation that appears unfavorable in the small ring compounds.

Electric dipole moment and polarizability of the 1749 Å, 1B2 excited state of formaldehyde

The medium resolution modulated electric field (electrochromism) spectrum of the 1749 A band of formaldehyde in the gas phase has been obtained. This differential absorption results from changes in transition probability and transition energy in the presence of a strong electric field and is related to, among other excited state properties, the square of the dipole moment change. The data, after analysis, yield the dipole moment change upon excitation to be 2.66±0.14 D. This change is thought to reduce the dipole moment to near zero in this excited state. In addition, the excited state polarizability is estimated to be 6±3×10−23 cm3 in the state under consideration. These changes coupled with previously reported experimental and theoretical results support the assignment of this transition as the 3s (a1) ←n (b2), B2←A1 molecular Rydberg. Finally, the data suggest that in this energy region there is a state of the same symmetry lying nearby.

Gas phase electrochromism studies in the vacuum ultraviolet

An instrument and technique have been developed for the measurement of medium‐resolution electrochromism spectra of gas phase samples in the ultraviolet spectral region from 300.0 to 165.0 nm. The primary result of studies utilizing this method is the difference in dipole moment between the ground state and an excited state of a molecule. The electrochromism spectrum of aniline is presented for the B2←A1 (π*←π) transition at 293.84 nm. The absolute value of the change in dipole moment for this transition is ‖1.25±0.30‖ D, which compares favorably with the value of ‖0.92±0.10‖ D reported from Stark field studies of a high‐resolution spectrum. The primary advantage of of the technique is that it allows the determination of dipole moment changes associated with excited states, even when resolved rotational structure cannot be obtained.

Correlation of Molecular and Rare Gas Term Values

The assignment of absorption bands for polyatomic molecules in the vacuum ultraviolet spectral region is often quite difficult; however, the observation of trends in the spectra of a series of compounds can be helpful in aiding the spectral assignments. The specific details of many of the complexities of the absorption bands require the use of high resolution data; however, low resolution spectral studies can provide insight into the basic electronic features. Therefore, the energetics of the interactions of substituents with a specific chromophore of interest can be inferred from this data. It should be noted that the chromophore can be “perturbed” either by varying the substituents adjacent to it, or by maintaining the substituents constant and replacing the atom(s) of the chromophoric unit with electronically similar species. The later technique was utilized in a study herein presented.