M. B. Robin

Optical and Photoelectron Spectra of Small Rings. III. The Saturated Three‐Membered Rings

The electronic states of cyclopropane, ethylene oxide, ethylenimine, and diaziridine were investigated using gas‐phase and condensed‐phase vacuum ultraviolet spectroscopy, photoelectron spectroscopy, and Gaussian orbital self‐consistent field calculations. Correlation of the gas‐phase and condensed‐phase optical spectra of cyclopropane with the first band in its photoelectron spectrum confirms the presence of Rydberg transitions in the optical spectrum involving excitation of an electron from the 3e′ sigma level. The valence shell spectrum of cyclopropane is complex but is dominated by two very strong 1A1′ → 1E′, σ → σ*, excitations. The Rydberg spectrum of ethylene oxide is reassigned to include the two absorptions in the 55 000–65 000‐cm−1 region, all bands originating from the 2b2π orbital, while its valence shell spectrum is closely related to that of cyclopropane. Similar but less conclusive results were obtained for ethylenimine. Virtual orbital calculations of σ → σ* excitation energies gave result...

Electronic States of the Amide Group

The presence of a moderately strong band falling between the n→π* and π→π* (N→V1) transitions of the amide group has been observed for the first time. All‐electron SCF calculations in Gaussian bases aimed at assigning the bands of formamide suggest that the new band is an n→σ* excitation in which the σ* orbital is Rydberg‐like. A second π→σ* big‐orbit transition follows the π→π* (N→V1) excitation. In a basis containing expanded orbitals, the usual virtual orbital—configuration interaction calculations predict that every upper state will be of Rydberg‐orbital dimensions. However, this obvious error can be avoided by indirect SCF calculations on the upper states, which show that in formamide, all states below 13 eV terminating at π* are valence‐shell transitions, whereas all states terminating at σ* are big‐orbit Rydberg states.

Electronic Spectra of Isoelectronic Amides, Acids, and Acyl Fluorides

The first four bands in the gas‐phase spectra of amides, carboxylic acids, and acyl fluorides are thought to be n → π*, n → 3s Rydberg, π → π*, and n → 3p Rydberg excitations. That the second and fourth bands are Rydberg, whereas the first and third are valence shell is demonstrated in a comparison of gas‐phase and condensed‐phase absorption and circular dichroism spectra. All‐electron, SCF Gaussian orbital calculations are also presented which qualitatively explain the trends in the spectra of HCOX molecules, and predict several quantities of interest, such as upper‐state dipole moments and magnetic transition moments, which have not been measured as yet.

Photoelectron spectroscopy of simple amides and carboxylic acids

Abstract The accidental near-degeneracies of the no (oxygen nonbonding) and π2 (antisymmetric pi) MOs of formamide and formic acid have been investigated using photoelectron spectroscopy and SCF calculations of the successive ionization potentials. It is concluded that ionization from no preceeds that from π2 in both of these compounds, but that the reverse order obtains in M-methylformamide and N,N-dimethylformamide. The close correspondence between the profiles of the no and π2 photoelectron bands and the Rydberg band profiles observed optically is used in making a more detailed assignment of the optical bands. All ionization potentials below 20 eV are reported for each of the four compounds.

Assignment of the Ultraviolet Mystery Band of Olefins

The weak mystery band found in the mono‐olefins at 5–6 eV is here reassigned as a symmetry‐allowed π→CH*(1Ag→1B2u) transition in which the CH* sigma upper orbital has considerable ns Rydberg character. Evidence supporting this assignment and militating against the recent CH→π*(1Ag→1B3g) assignment of Berry [J. Chem. Phys. 38, 1934 (1963)], the only other plausible assignment, is drawn from (a) ab initio calculations of the electronic spectrum and orbital energies of ethylene in large Gaussian bases, (b) a study of the intensities and vibronic band shapes of the mystery bands of cis—trans pairs of dialkylethylenes, (c) the observed effect of ring strain on the mystery‐band intensity, (d) a measurement of the Cotton effect and rotational strength of the mystery‐band transition, and (e) the spectra of olefins in condensed phases which demonstrate the Rydberg nature of the mystery‐band upper state. Evidence is also presented that shows that the first electron affinity of ethylene places an electron in the CH*...

Assignments in the Ultraviolet Spectra of Olefins

Indirect SCF calculations in a Gaussian basis of the lower π→σ* and σ→π* states of ethylene demonstrate that the lower π→σ* excitations are Rydberg and involve 3s and 3p orbitals on carbon, whereas the lower σ→π* excitations are strictly valence shell. Calculations in an identical basis using the virtual‐orbital approximation instead lead to excited states which are mixtures of Rydberg and valence‐shell configurations. The calculations also show that the oscillator strengths of π↔σ transitions should not exceed 0.1, and that in twisted olefins, the rotational strengths of π→σ* excitations, in general, are smaller than those for σ→π* excitation. The e.ectronic spectrum of tricyclo[3.3.0.02,6]oct‐3‐ene(TCO) in the gas phase shows that an olefin can have up to four π↔σ transitions preceding the π→π* absorption. Comparison of the spectra of ethylene in high‐pressure nitrogen and of TCO in rare‐gas matrices show that the lowest transition in the latter is a valence‐shell transition, but that the second and thi...

Multiphoton ionization mass spectroscopy of acetaldehyde

Details of the positive ion fragmentation patterns of acetaldehyde produced by collisionless multiphoton ionization are presented. Fragments are generated in the ion collection region of a quadrupole mass spectrometer and analyzed with unit mass resolution. Daughter ions are found corresponding to the absorption of up to five photons. All fragment ions show the same excitation spectrum, which closely resembles that obtained with total ion collection in both the mass spectrometer and a conventional parallel plate ionization cell. Individual fragment ion production depends strongly on laser power, going approximately as Ip, where p=1.17±0.07 for CH3CHO+, up to p=3.54±0.09 for CH+3. Total ion production varies as p=2.19±0.08. Considerable deviation from a simple power law dependence can be found for some fragments, however. A rate equation approach is used to explain the experimental results.

Fluorination effects on the inner-shell spectra of unsaturated molecules

The applicability of the perfluoro effect to the X-ray spectra (300–800 eV) of unsaturated organic molecules is explored. The C1s and F1s (and Ols where appropriate) oscillator strength spectra of five fluoroethylenes, octafluorocyclopentene, formyi fluoride, carbonyl fluoride, hexa-fluorobutadiene, trifluoroacetic acid, hexafluorobutyne-2, hexafluoroacetone, and octafluorona-phthalene were derived from electron impact energy loss spectra recorded under electric-dipole scattering conditions. These spectra are analyzed and compared with those of their perhydro analogs, several of which (naphthalene, acetic acid, butyne-2) are reported for the first time. In unsaturated systems in which all the atoms lie in the molecular plane, such as ethylene, formaldehyde, benzene, etc., perfluorination results in approximately 10 eV shifts of the inner-shell energy loss spectra to higher energies, yet the term values for the C1s→1π∗ excitations are shifted by only 1 eV and often less. In direct contrast, the term values for the equivalent C1s→1π∗ excitations in unsaturated systems having atoms out of the molecular plane, such as butene-2 and acetone, are shifted upward by up to 3 eV upon perfluorination. These different spectral behaviors of planar and nonplanar systems on fluorination quantitatively parallel those which were observed earlier for valence-level ionization potentials (10–20 eV) and attributed to the perfluoro effect. It is observed for the first time that the C1s→1π∗ excitation energies in planar hydrocarbons are only very weakly dependent on the spatial extent of the π-electron system. An explanation involving a localized C1s hole is proposed to rationalize this behavior. The perfluoro effect also predicts that excitations to σ∗ MOs will become relatively low-lying in highly fluorinated planar systems. Such low-lying inner-shell excitations induced by fluorination are observed in the fluoroethylene series and in the fluorocarbonyls. When the negative-ion spectra of the fluoroethylenes are assigned in a self-consistent manner, a σ∗ MO is found to drop into the vicinity of 1π∗ upon fluorination. A similar intrusion of the lowest σ★ MO among the π∗ MOs is also observed upon fluorinating benzene, while evidence for this in the case of naphthalene is less clear, on account of the complex pattern of multiple C1s→nπ∗ transitions in this molecule. Inner-shell oscillator strength distributions are reported for all the spectra considered herein. In general, perfluorination increases the oscillator strengths of C1s→1π∗ transitions by up to a factor of two. Variation of the C1s→1π∗ and O1s→17π∗ oscillator strengths in the series H2CO, HFCO, F2CO shows clearly how the 17π∗ MO becomes more polarized toward C as fluorination proceeds. In some cases, C1s→σ∗ (C-F) oscillator strengths exceed those for C1s→1π∗ transitions.

The methyl iodide multiphoton ionization spectrum with intermediate resonance in the A‐band region

The 5pπ→6p Rydberg excitations of methyl iodide‐h3 and ‐d3 are observed using multiphoton ionization spectroscopy as two‐photon resonances when excited in the 28 000–32 000 cm−1 region. The two‐photon methyl iodide resonances abruptly cease at 32 000 cm−1, and at higher frequencies are replaced by resonances originating at the 2P1/2 and 2P3/2 levels of the iodine atom. A tentative assignment is also made for a two‐photon resonance in the methyl radical in this region. The replacement of methyl iodide resonances with those of iodine and methyl fragments beginning at 32 000 cm−1 correlates directly with the onset of a one‐photon intermediate resonance in the A band of methyl iodide. When pumping with light of frequency less than 16 000 cm−1, the 5pπ→6p excitations of methyl iodide are observed as four‐photon resonances. However, on entering the A‐band region via two‐photon resonance beyond 32 000 cm−1, no fragment transitions appear. Instead, we observe the three‐photon resonance to the (5pπ, 6s) state of t...

Excited electronic states of the simple alcohols

Abstract A Gaussian orbital calculation of the orbital structure of methanol is in excellent agreement with the photoelectron values determined here. Other alco