Index of content:
Volume 63, Issue 8, 15 October 1975

A comparison of experimental determinations of electron affinities of pi charge transfer complex acceptors
View Description Hide DescriptionThe absolute electron affinities of pi charge transfer complex acceptors have been examined and the ’’best’’ values have been chosen. All of the results obtained by the magnetron method, including the estimates for hexafluorobenzene and tetracyanoethylene, were accepted. However, the magnetron results for anthraquinone and benzoquinone are not in agreement with charge transfer complex and half‐wave reduction potential data. The half‐wave reduction potential data and the charge transfer complex data for all of the other acceptors for which absolute electron affinities are available were found to be consistent with the usual correlation equations and their associated assumptions. The parameters from these correlations have been used to calculate the absolute electron affinities for about 150 acceptors.

Intermolecular vibration in the low‐frequency Raman spectra of l‐alanine crystal
View Description Hide DescriptionThe low‐frequency, ? 150 cm^{−1}, polarized Raman spectra of l‐alanine single crystals determined by Wang and Storms can be interpreted simply in terms of the intermolecular vibrations, i.e., librations and/or translations, of the four oriented alanine molecules in the unit cell of the orthorhombic crystal.

Potential surface distortion and orbital reorganization upon change of electronic state. Formaldehyde
View Description Hide DescriptionNear Hartree–Fock calculations on the ^{1} A _{1} ground, ^{2} B _{1}, ^{2} B _{2} cationic, ^{2} B _{1} anionic, and ^{1,3} A _{2} optical excited states of formaldehyde are reported. Reorganization effects are analyzed at the group symmetry and orbital levels. The results are as follows: E n e r g y—large reorganization effects are found on excitation to all states. Comparison of equivalent excitations to the isoelectronic molecule ethylene shows that the reorganization effects are significantly more severe in the polar H_{2}CO. O r b i t a l s—the b _{2} and π orbitals are strongly sensitive to the electronic state, leading to poor frozen orbital predictions of excited stateproperties (e.g., dipole moments). The frontier 2b _{2} orbital contains strongly bonding and antibonding regions depending on the electronic state; consequently it is better to label it as frontier sigma (σ_{ f }) rather than as nonbonding (n). D i p o l e m o m e n t s—the large dipole moments in the σ_{ f }π* states can be largely understood from the centroids of σ_{ f } and 1b _{1}(π) shifting towards the oxygen in σ_{ f }π*. P o t e n t i a l s u r f a c e s—only the B _{1} anionic and A _{2} optical states are predicted to have nonplanar potential minima. Fermi correlation alone cannot explain either the smaller out‐of‐plane angle or larger inversion barrier height of the ^{1} A _{2} vs ^{3} A _{2} states.

Fast metastable fragments produced by dissociative excitation of carbonyl sulfide
View Description Hide DescriptionDissociative excitation of OCS by electron impact has been studied using the method of translational spectroscopy. Time‐of‐flight distributions and excitation functions of the fast metastable fragments have been measured. The results are compared with similar measurements on CO_{2} and show that a variety of metastable fragments including CO(a ^{3}Π), S(^{5} S), O(^{5} S) as well as long lived high lying atomic and molecular Rydberg fragments can contribute to dissociation.

Angular distribution of N^{+} from dissociative ionization of N_{2} near threshold
View Description Hide DescriptionTranslational kinetic energy and angular distributions of N^{+} from dissociativeionization of N_{2} have been measured at electron energies close to threshold. For electron energies above 50 eV the energy distributions agree favorably with earlier measurements of Kieffer and Van Brunt. The ion energy distributions at electron energies below 50 eV are dominated by a feature peaked at a kinetic energy below 1 eV in agreement with recent measurements of Locht e t a l. For electron energies closest to threshold the angular distribution of N^{+} associated with this feature is consistent with the previously proposed mechanism involving excitation of the C ^{2}Σ^{+} _{ u } state followed by predissociation. At higher electron energies the degree of anisotropy increases and then rapidly decreases suggesting a significant contribution from direct excitation to the ^{4}Σ^{+} _{ g } repulsive state of N^{+} _{2} leading to fragments in their ground states.

Variational time‐dependent Hartree–Fock calculations. I. Applications to four‐electron atomic and molecular systems
View Description Hide DescriptionSolutions of the time‐dependent Hartree–Fock (TDHF) equations are obtained by a variational treatment that employs Slater basis sets. Application is made to several four‐electron atomic and molecular systems. For the beryllium isoelectronic sequence the variational approach is comparable in accuracy to numerical integrations of the TDHF equations. It provides also a description of the resonances that arise from excitation of the inner‐shell electrons. Similar calculations for the molecule LiH show that the TDHF method is capable of producing accurate oscillator strengths for molecular systems. Results are given for LiH and for the isoelectronic BeH^{+} molecule.

Optical–optical double resonance laser spectroscopy of BaO
View Description Hide DescriptionBaO molecules were sequentially excited by collinearly propagating radiation from two cw lasers, an argon ion laser (488.0 and 496.5‐nm lines) and a tunable (560–630 nm) rhodamine 6G dye laser. This sequential excitation technique, called optical–optical double resonance (OODR), is a special case of two‐photon spectroscopy in which two visible or uv wavelength photons of different frequencies resonantly excite a molecule from an initial level (v ^{″},J ^{″}) to a final level (v*,J*) by way of a real intermediate level (v′,J′). Two types of optical double resonance experiments were performed on BaO. The first, excitation spectroscopy, revealed 19 vibronic levels of ^{1}Σ^{+} electronic symmetry between 36 490 and 38 620 cm^{−1} above E (X ^{1}Σ,r _{ e }). These 19 ^{1}Σ^{+} levels belong to two or more perturbed electronic states. No ^{1}Π levels were found, although levels belonging to the upper state of the BaO B (^{1}Π) –X ^{1}Σ Parkinson bands occur in this energy region. The second type of experiment, photoluminescencespectroscopy, enabled observation of extended rotationally resolved photoluminescence progressions out of various two‐photon‐excited levels (v*,J*) into BaO X ^{1}Σ (v ^{″}=0–34). Vibrational energies and rotational constants obtained from these photoluminescence progressions were used to construct an RKR potential energy curve for BaO X ^{1}Σ^{+} through v ^{″}=40, from which it was shown that the X ^{1}Σ^{+} and a ^{3}Π curves cross at r=0.276 nm and E=22 750 cm^{−1} [above E (X ^{1}Σ,r _{ e })]. Laser frequency jitter of less than 10^{−3} cm^{−1} resulted in large intensity fluctuations of the OODR signal, implying that the double resonance linewidth was narrower than the Doppler width of the (v′,J′) → (v*,J*) transition. This sub‐Doppler width effect resulted from excitation of (v′,J′) molecules that were velocity selected relative to the laser propagation direction.

Reactions of O^{−} with N_{2}, N_{2}O, SO_{2}, NH_{3}, CH_{4}, and C_{2}H_{4} and C_{2}H_{2} ^{−} with O_{2} from 300 °K to relative kinetic energies of ∼2 eV
View Description Hide DescriptionThe energy dependences of the reaction rate constants of O^{−} ions with the neutral species N_{2}, N_{2}O, SO_{2}, NH_{3}, CH_{4}, and C_{2}H_{4} have been measured from thermal energy to approximately 2 eV relative kinetic energy. The energy dependences of the over‐all reaction rate constants do not fit available theories for collision rate constants. The reaction of O^{−} with C_{2}H_{4} produces four different negative‐ion product channels, in addition to the associative‐detachment channel. The branching ratios are found to change drastically with reactant energy. The reactions establish the following upper limits on electron affinities: EA(:C=CH_{2}) <0.43 eV and EA(C_{2}H_{3}) ≳0.4 eV. The reactions also establish an upper limit for the heat of formation of the ion C_{2}H_{3}O^{−} (ΔH _{ f }°<−0.62 eV).

Electronic symmetries, dipole moments, and polarizabilities of molecules in Rydberg states determined from electric‐field effects on absorption spectra
View Description Hide DescriptionThe effects of intense electric fields (∼ 2×10^{5} 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 Å band of acetone assigned to the 3s←n (^{1} B _{2}←^{1} A _{1}) Rydberg transition and the 1959 and 2284 Å bands of dimethylsulfide assigned to the 4p←n (^{1} A _{1}←^{1} A _{1}) and 4s←n (^{1} B _{1}←^{1} A _{1}) 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} cm^{3}, 11.7±1.3×10^{−24} cm^{3}, and 9.3±2.0×10^{−24} cm^{3}, respectively. The transition polarizations were all found to be in agreement with their particular Rydberg assignments. Derivations of the mathematical equations necessary for the interpretation of the experimental data are presented.

Differential cross section from an operator viewpoint
View Description Hide DescriptionIt is shown that a differential cross section can be identified as a matrix element of a transition superoperator between an observable (density operator) describing unit planar flux and an observable operator for unit spherical flux per steradian. The analysis exactly parallels the usual treatment of scattering of wavefunctions, but is done entirely in terms of density operators. The large distance (far from the scattering center) behavior of the scattered density operator is conveniently described in terms of the equivalent Wigner distribution function. For simplicity of presentation, rearrangement collisions are excluded from discussion, but the treatment may easily be extended to include such effects.

CO_{2}–N_{2} intermolecular V–T and V–V potentials via CNDO
View Description Hide DescriptionV–T and V–Vintermolecular potentials have been calculated within the CNDO approximation for the CO_{2} (asymmetric mode) –N_{2} system. The procedure followed was to compute the total energy of the CO_{2}–N_{2} system for a variety of intermolecular separations, orientations, and normal mode displacements, and subtract the relevant isolated CO_{2} and isolated N_{2} total energies. The V–T and V–Vintermolecular potentials were then obtained by fitting a second‐order polynomial expansion in the normal mode displacements of both molecules to the total intermolecular energy for each intermolecular orientation and displacement. The resulting elastic potentials are much too deep. The inelastic potentials are highly orientation dependent, generally not well represented by a P _{2}(ϑ) expansion in the relative angular orientation, and different for each specific V–T or V–V process.

Radiative and nonradiative transitions in the first excited singlet state of simple linear aldehydes
View Description Hide DescriptionRadiative and nonradiative rates of asymmetric linear aldehydes (C _{2}∼C _{6}) have been measured in the gas phase. Observe radiative lifetimes are compared to calculated radiative lifetimes obtained from the Strickler–Berg (SB) expression. The calculated values are about the same as the observed values in large aldehydes, but the former is about six times greater than the latter in acetaldehyde. The radiative rates are about four orders of magnitude smaller than the nonradiative rates. The two most important nonradiative processes are S _{1}→T _{1} intersystem crossing and type II processes, the latter playing an increasingly important role at high energy in the aldehydes with γ‐hydrogens. The contrasting radiative behavior of the asymmetric aldehydes from that of the symmetrical ketones is intriguing and suggests important differences in their excited state equilibrium geometries and transition moments.

Electron paramagnetic resonance study of hydrogen and fluorine atom addition to SO_{2}
View Description Hide DescriptionThe electron paramagnetic resonancespectrum of the species formed in the near uv photolysis of mixtures of argon or krypton containing 1% HI and SO_{2}, 1% CF_{3}OF and SO_{2}, or F_{2}SO_{2} has been investigated at 4.2 K. The species formed when an HI/SO_{2} mixture is photolyzed has been identified as the symmetric σ radical HSO_{2}. The assignment of the principal components of the g and hyperfine tensors was aided by our observation of preferential orientation of the trapped radical. The species formed when a CF_{3}OF/SO_{2} mixture is photolyzed in the far uv has been identified as the symmetric σ radical FSO_{2}. The g and A tensors have been determined from an exact solution of the spin Hamiltonian assuming a noncollinear tensor axis system.

Electronic state of iron in the oxygen and carbon monoxide adducts of heme proteins
View Description Hide DescriptionThe Fe Kβ fluorescenceemission spectrum was used to study the coordination of iron in myoglobin adducts. The electronic structure of oxymyoglobin can be described as an equilibrium between a singlet state with a coordinated oxygen molecule and a triplet state with a coordinated superoxide anion. Carbon monoxide myoglobin contains divalent, zero spin iron.

Pure‐state molecular beams: Production of rotationally, vibrationally, and translationally selected CsF beams
View Description Hide DescriptionBeams of CsF in selected vibrational (v=0,1,2,3,4) and rotational (J,M=1,0) states, with narrow velocity distributions, have been produced via an improved microwave molecular beam electric resonance (MBER) method utilizing ’’displacement focusing.’’ The apparatus consists of an effusive graphite oven source, a slotted‐disk velocity selector, and two electrostaticquadrupole fields. The first field focuses the (J,M) = (2,0) state through an off‐axis collimating orifice into a microwave cavity. Here microwave radiation stimulates the J=2→1 transition for a specified v state. Then a second (’’refocusing’’) quadrupole delivers the specified (v,J,M) = (v,1,0) state to an image point (a collimated surface ionization mass filter detector). The final focused and state‐selected beam, say (0,1,0), has a purity ≳95% with respect to J,M. However, the best over‐all (v,J,M) state purity achieved was 83% (0,1,0), with 17% of ’’background’’ [of which 4% is actually (0,1,0), 4% is (1,1,0), 4% (v,1,0) with v?2, and the remaining 5% unselected (scattered) CsF]. Using the present effusive oven source, the best intensity achieved for a state‐selected (0,1,0) beam (of velocity 3.65×10^{4} cm sec^{−1}) was 1×10^{5} molecule sec^{−1}, corresponding to a flux density of 1.3×10^{9} molecule cm^{−2} sec^{−1}.

Statistical mechanical calculations of the surface tension of fluids
View Description Hide DescriptionSurface tensions predicted from the Kirkwood–Buff formula for three different models of the interfacial doublet distribution function are compared. For liquid argon, the best agreement between theory and experiment is found for the model in which the interfacial pair correlation function is taken to have the value of a one‐phase fluid at the density halfway between the molecular pair. This model, in fact, predicts surface tensions whose magnitudes and temperature dependence are in excellent agreement with experiment for argon, nitrogen and methane. Some preliminary calculations for carbon dioxide and methanol indicate that the nonspherical and polar character of the interactions of the molecules of these fluids must be accounted for in attempting to apply any of the models to them.

Nonequilibrium velocity distributions and recombination rates in electron–ion and ion–ion recombinations in weakly ionized gases
View Description Hide DescriptionThe nonequilibrium velocity distributions and recombination rates in electron–ion and ion–ion recombinations in a heat bath of neutral molecules are investigated by solving the Boltzmann equation with the Monte Carlo simulation. The explicit time‐dependent velocity distributions and recombination rates from the initial equilibrium to quasisteady states are obtained for the typical electron–ion dissociative recombination e+N^{+} _{2}→N+N in the heat bath of N_{2} and for the typical ion–ion recombination H^{+}+H^{−} →H+H in the heat bath of H_{2}. The velocity distributions and the recombination rates indicate significant deviations from equilibrium. The decreases in the recombination rates from the equilibrium rates are 30% for e+N^{+} _{2}→N+N and 15% for H^{+}+H^{−}→H+H at quasisteady states for the heat bath temperature 300 °K and the charge concentrations X∼10^{−4} and 10^{−2}, respectively. The Monte Carlo result is compared with the Chapman–Enskog solution by Shizgal and Karplus for H^{+}+H^{−}→H+H. The Chapman–Enskog solution indicates a too large nonequilibrium for high charge concentration X≳10^{−4}.

The thermomagnetic force in the near‐Waldmann regime for CO, N_{2}, HD, and CH_{4}
View Description Hide DescriptionThe thermomagnetic force exerted on a disk with surface perpendicular to the heat flux has been measured in the gases CO, N_{2}, HD, and CH_{4} for conditions approximating those assumed by recent theory. Comparison is made with theory and previous experimental work, and the thermomagnetic force is shown to depend upon the surface composition of the disk.

Microwave spectra of e n d o‐ and e x o‐2‐methylbicyclo[2.1.0]pentane. Methyl group polarity and the sign of the dipole moment in bicyclo[2.1.0]pentane
View Description Hide DescriptionThe microwave spectra of the e n d o and e x o isomers of 2‐methylbicyclo[2.1.0]pentane have been identified and assigned. Rotational constants for the former isomer are A=6257.36±0.01, B=3412.19±0.01, and C=2843.34±0.01, and for the latter, A=7418.75±0.01, B=3101.66±0.01, C=2652.57±0.01, all values in megahertz. Stark effect measurements have provided dipole moment data, the magnitudes of the total moments being 0.295±0.010 D and 0.176±0.001 D, respectively. A b i n i t i o molecular orbital calculations give results in good agreement with experiment and show that the negative end of the dipole moment vector for the ring system is in the e n d o direction. The experimental data have been used to obtain a quantitative assessment of the polarity changes upon methyl substitution.

NMR study of the pressure effects on the molecular dynamics in the disordered, crystalline phase of neopentane
View Description Hide DescriptionThe NMRproton spin–lattice relaxation timesT _{1} have been measured in the disordered crystalline phase of neopentane as a function of pressure and temperature. The molecular reorientation and self‐diffusion contributions to the observed relaxation time are separated and the activation parameters are calculated for the two processes, respectively. The over‐all molecular reorientation has a very weak pressure dependence whereas the self‐diffusion exhibits a strong pressure dependence. The interesting result that the activation volume for self‐diffusion is close to the molecular volume in neopentane is discussed in terms of the diffusion mechanism. The data enable us to estimate the modulated portion of the second moment attributable to the over‐all molecular reorientation.