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15 Dec 1981

Volume 75, Issue 12, pp. 5587-5977

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ESR and ENDOR studies of irradiated cysteine HCl: Decarboxylation

Kiyoshi Matsuki, William H. Nelson, and Joseph H. Hadley

J. Chem. Phys. 75, 5587 (1981); http://dx.doi.org/10.1063/1.441995 (7 pages) | Cited 3 times

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K‐band ESR and ENDOR spectra of irradiated single crystals of L‐cysteine hydrochloride monohydrate at 65 K provide evidence for an unstable carbon‐centered radical produced by decarboxylation. The unpaired electron is localized in a 2p orbital on the alpha carbon. An α‐proton hyperfine coupling with principal values −104.8, −65.4, and −33.1 MHz has been measured. Five β‐proton couplings have also been observed, of which three are exchangeable.
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82.50.-m Photochemistry
76.30.Rn Free radicals
76.70.Dx Electron-nuclear double resonance (ENDOR), electron double resonance (ELDOR)

Chemical shift scaling in NMR of rotating solids

J. A. DiVerdi and S. J. Opella

J. Chem. Phys. 75, 5594 (1981); http://dx.doi.org/10.1063/1.441996 (2 pages) | Cited 2 times

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An improved version of the chemical shift scaling multipulse cycle is demonstrated that eliminates the dependence of the direction of the axis of precession on the extent of scaling. This prevents the formation of a zero‐frequency artifact in the final spectrum. Results from liquid, as well as stationary and spinning solid samples, are described.
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76.60.Cq Chemical and Knight shifts

Superparamagnetic effects in the ferromagnetic resonance of silica supported nickel particles

V. K. Sharma and A. Baiker

J. Chem. Phys. 75, 5596 (1981); http://dx.doi.org/10.1063/1.441997 (6 pages) | Cited 31 times

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A silica supported Ni catalyst has been investigated by electron magnetic resonance between 293 and 4.2 K. A relatively narrow absorption signal with linewidth ΔHpp?400 Oe is observed at temperatures ?150 K as a result of motional averaging of the magnetocrystalline anistropy field Ha of the Ni particles resulting from superparamagnetism. Broadening of the resonance occurs at lower temperatures because of a reduction of the influence of thermal fluctuations. A line shape analysis of the observed resonance signals is carried out employing the theory of ferromagnetic resonance in polycrystalline material. The calculated anisotropy field is reduced in the particles as compared to bulk Ni. The Landau–Lifshitz relaxation frequency obtained from the calculated ’’single crystal’’ linewidths are found to be larger than those of bulk Ni single crystals. Concurrently with line broadening, the resonance shifts to lower magnetic fields. The resonance shifts are approximately Ha/2. The appearance of magnetoelastic effects in the resonance below 120 K indicates the presence of small but significant strain in the particles resulting from particle‐support interaction.
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76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance

Infrared diode laser spectrum of the ν1 band of CF2 (math 1A1)

P. B. Davies, W. Lewis‐Bevan, and D. K. Russell

J. Chem. Phys. 75, 5602 (1981); http://dx.doi.org/10.1063/1.441998 (7 pages) | Cited 29 times

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The ν1 fundamental band of difluorocarbene (CF2) in its ground electronic state has been investigated using diode laser absorption spectroscopy. Several RQ branches have been assigned and analyzed. The effects of asymmetry doubling become measurably large in Q branches with low K and in RQ3 provide a rigorous test of the assignment. Combined with microwave data for the ground state, the diode laser spectra yield v = 1 rotational constants and the first accurate measurement of the band center: ν1 = 1225.0793(40) cm−1.
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33.20.Ea Infrared spectra
82.80.Dx Analytical methods involving electronic spectroscopy
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
33.15.Mt Rotation, vibration, and vibration-rotation constants

Isotopically decoupled vibrational spectra and proton exchange rates for crystalline NH3 and ammonia hydrate

Cynthia Thornton, M. S. Khatkale, and J. Paul Devlin

J. Chem. Phys. 75, 5609 (1981); http://dx.doi.org/10.1063/1.441999 (6 pages) | Cited 6 times

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Codeposits of NH3 with ND3 or D2O have been prepared at liquid nitrogen temperatures in the absence of proton exchange. Vibrational data for the anhydrous cubic crystalline ammonia, containing isolated NH3 or ND3, confirm that, relative to water ice, intermolecular coupling in ammonia ice exerts a relatively minor influence on the infrared and Raman spectra. Nevertheless, sizeable decoupling shifts, particularly for ν1, have been observed and attributed to a combination of factors including correlation field and Fermi resonance effects. The Raman polarization data has also affirmed long standing assignments of ν1 and ν3 for ammonia ice. Warming of the ammonia thin films resulted in limited isotopic scrambling at 130 K, apparently possible only through the agency of trace concentrations of water. The vibrational coupling pattern for the resultant NHD2 and NH2D molecules suggest that proton (deuteron) migration away from the exchange centers is impossible at temperatures up to 150 K. By contrast, isotopic scrambling was rapid and complete at 140 K for amorphous ammonia hydrate films (∼35% NH3, ∼65% D2O) which were also prepared without exchange at ∼90 K. The proton (deuteron) exchange rate is much greater for the amorphous ammonia hydrate at 140 K than for pure water ice. Such exchange requires both ion‐pair defect formation and proton mobility. Since the NH3 suppresses the H3O+ concentration via formation of NH+4, a suppression the likes of which has been shown to stop proton exchange in water ice, the evidence strongly suggests that NH4+ in ammonia, like H3O+ in water, is an effective proton transfer agent, probably acting through a tunneling mechanism (i.e., H3N+–H⋅⋅⋅NH3→H3N⋅⋅⋅H–N+H3 etc.) to render the proton mobile in the ammonia hydrate. This mobility combined with the greater NH4+ concentration, relative to the H3O+ concentration in H2O ice Ic, results in isotopic scrambling at the reduced temperature.
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78.30.Hv Other nonmetallic inorganics

Resonance enhanced dynamic Rayleigh scattering

Susan G. Stanton, R. Pecora, and Bruce S. Hudson

J. Chem. Phys. 75, 5615 (1981); http://dx.doi.org/10.1063/1.442000 (12 pages) | Cited 40 times

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A semiclassical theory relating the molecular resonance Rayleigh scattering intensity to the extinction coefficient of an electronic transition is presented. For isotropic molecules, the theory is shown to be equivalent to classical fluctuation theory. The semiclassical theory is then extended to include anisotropic polarizabilities. Depolarized scattering intensities and depolarization ratios are calculated for a model system, and the general features of previous experimental data are explained. Inclusion of inhomogeneous broadening in the theory predicts increased scattering intensities and decreased vibrational resolution of the scattering profile. Depolarized Rayleigh linewidths and intensity profiles are presented for a series of diphenylpolyenes, several coumarin laser dyes, and 3,4‐dinitrophenol in its neutral and ionic forms. The ideal molecular properties and experimental conditions for the use of resonance enhanced dynamic Rayleigh scattering are deduced.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.20.-t Molecular spectra
33.70.Fd Absolute and relative line and band intensities
33.70.Jg Line and band widths, shapes, and shifts

On the ESR detection of the σ‐radical RNHĊO in irradiated crystals of 5,5‐dihydroxybarbituric acid (alloxan) trihydrate

Einar Sagstuen and Halvard Skjærvø

J. Chem. Phys. 75, 5627 (1981); http://dx.doi.org/10.1063/1.442001 (3 pages)

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In single crystals of 5,5‐dihydroxybarbituric acid (alloxan) trihydrate, an ESR absorption was observed after irradiation at 77 K which is ascribed to a σ‐radical RNHĊ = O. Its spectral parameters are described and compared to those from the structurally similar σ‐acyl radical in malonic and succinic acid. It is suggested that the radical is formed from the primary oxidation product RCO(OH) through electronic rearrangements leading to an opening of the pyrimidine ring.
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76.30.Rn Free radicals

Absorption and circular dichroism line shapes of a molecular dimer in the strong electronic coupling limit

Richard Friesner and Robert Silbey

J. Chem. Phys. 75, 5630 (1981); http://dx.doi.org/10.1063/1.442002 (10 pages) | Cited 16 times

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We have derived approximate analytical expressions for the absorption and circular dichroism line shapes of a molecular dimer composed of two identical molecules (each with a single excited electronic state and single vibrational mode) in the strong electronic coupling limit. These are compared with numerical results and yield excellent agreement. We compare our approach to other theories and discuss experimental applications and possible extensions of this work.
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33.20.Ea Infrared spectra
33.55.+b Optical activity and dichroism
33.57.+c Magneto-optical and electro-optical spectra and effects
33.70.Jg Line and band widths, shapes, and shifts
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.20.-t Molecular spectra

Polarized two‐photon fluorescence excitation studies of pyrimidine

Patrik R. Callis, Thomas W. Scott, and A. C. Albrecht

J. Chem. Phys. 75, 5640 (1981); http://dx.doi.org/10.1063/1.442003 (7 pages) | Cited 23 times

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Two‐photon fluorescence excitation studies are reported for pyrimidine (1, 3‐diazine) dissolved in a hexane solution at room temperature. Quantitative two‐photon absorption strengths for the three lowest electronic absorption bands are measured relative to the benzene B2u ← A1g two‐photon transition. The B1(nπ∗) ← A1 excitation profile matches the one‐photon absorption band closely, indicating an allowed two‐photon transition. However, the B2(ππ∗) two‐photon intensity is vibronically induced by a 1600 cm−1 b2 vibration, even though transitions into this state are formally two‐photon allowed. In addition, polarization analysis is used to uncover a new transition in pyrimidine in a region of overlapping absorptions. This new transition is assigned to the lowest A2(nπ∗) state. If this assignment is correct, the A2(nπ∗), B1(nπ∗) splitting is 1.4 eV. Spectroscopic CNDO and INDO calculations of the two‐photon absorption strengths are also reported.
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33.20.Kf Visible spectra
33.50.Dq Fluorescence and phosphorescence spectra
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Wz Other multiphoton processes

Relaxation parameters for power‐broadened microwave line shapes. I. Anderson theory

Keith L. Peterson

J. Chem. Phys. 75, 5647 (1981); http://dx.doi.org/10.1063/1.441992 (8 pages) | Cited 2 times

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A model is developed for the calculation of collisional cross sections for all spherical tensor orders. The model is based on the Anderson low‐power linewidth theory. General expressions are derived for the cross sections and then evaluated for the case of multipole–multipole interaction potentials. Motivated by application to power‐broadened microwave line shapes, the dependence of the cross sections on tensor order is discussed. It is shown that for the typically dominant case of dipole–dipole potentials, many of the cross sections are independent of tensor order. The order dependence for other potentials is discussed.
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33.20.Bx Radio-frequency and microwave spectra
34.50.Ez Rotational and vibrational energy transfer
33.70.Jg Line and band widths, shapes, and shifts

Relaxation parameters for power‐broadened microwave line shapes. II. Sudden approximation

Keith L. Peterson

J. Chem. Phys. 75, 5655 (1981); http://dx.doi.org/10.1063/1.441993 (7 pages) | Cited 4 times

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A sudden approximation, utilizing linear trajectories and a dipole–dipole potential as developed by Cross [J. Chem. Phys. 55, 510 (1971)], is extended to enable calculation of collisional relaxation parameters of arbitrary tensor order. Expressions are given for linear rotors and symmetric tops. The relaxation parameters factorize into a dynamical part and an angular momentum part and are expressible as linear combinations of collisional transition probabilities out of the j = 0 rotational level. This is in agreement with previous discussions of the infinite order sudden approximation to the coupled states equations. An experiment is proposed which would enable the determination of all polarization relaxation times of a given transition. The results of such an experiment would help to define the validity of the sudden approximation in calculating relaxation cross sections. Finally, application of the sudden approximation to four‐level double resonance experiments is discussed.
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33.20.Bx Radio-frequency and microwave spectra
34.50.Ez Rotational and vibrational energy transfer
33.70.Jg Line and band widths, shapes, and shifts

Relaxation parameters for power‐broadened microwave transitions. III. Numerical calculations

Keith L. Peterson and R. H. Schwendeman

J. Chem. Phys. 75, 5662 (1981); http://dx.doi.org/10.1063/1.441994 (10 pages) | Cited 3 times

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The collisional relaxation parameters needed for the calculation of power‐broadened line shapes of the j = 2←1 transition in OCS and the (j, k) = (3, 3) transition in NH3 have been calculated by the modified Anderson theory and the sudden approximation described in papers I and II of this series. Relaxation parameters of all tensor orders have been obtained, enabling a comparison of the Karplus–Schwinger–Townes line shape and the recently‐derived continued‐fraction line shape for power‐broadened microwave transitions. It is shown, for the two transitions studied and the relaxation parameters calculated, that the Karplus–Schwinger–Townes equation is a very good representation of the continued‐fraction line shape. However, the (T1/T2)0 obtained is 5–10% larger than the true T1/T2.
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33.20.Bx Radio-frequency and microwave spectra
34.50.Ez Rotational and vibrational energy transfer
33.70.Jg Line and band widths, shapes, and shifts

Intersystem crossing in cold isolated molecules of naphthalene

Fred M. Behlen and S. A. Rice

J. Chem. Phys. 75, 5672 (1981); http://dx.doi.org/10.1063/1.442004 (13 pages) | Cited 43 times

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We report the results of an extensive study of the rate of radiationless decay in very cold, isolated, naphthalene molecules. The experimental method employed, namely, pulsed narrow band laser excitation of a pulsed seeded supersonic free jet, avoids the complications imposed by the sequence congestion generated by the thermal excitation characteristic of ordinary vapor samples. Our analysis of the fluorescence decay includes a method for transforming room temperature quantum yield data to quantum yields characteristic of the molecules in the supersonic free jet. It is concluded that (a) the out of plane vibration 2(b2g) is very likely a promoting mode for intersystem crossing; (b) there is no need to invoke near resonant coupling of levels in the prepared singlet and final triplet manifolds to explain features of the radiationless decay; and (c) in general, the broad features of radiationless decay in naphthalene are very much like those of the corresponding decay in benzene, and can be similarly explained.
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33.50.Dq Fluorescence and phosphorescence spectra
33.50.Hv Radiationless transitions, quenching

Fluorescence spectroscopy of cold and warm naphthalene molecules: Some new vibrational assignments

Fred M. Behlen, Daniel B. McDonald, V. Sethuraman, and Stuart A. Rice

J. Chem. Phys. 75, 5685 (1981); http://dx.doi.org/10.1063/1.442005 (9 pages) | Cited 45 times

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The fluorescence excitation spectrum of naphthalene seeded in a supersonic free jet and the single vibronic level fluorescence spectrum of room temperature naphthalene vapor have been used to determine assignments for a number of combination and sequence transitions that were previously unassigned. Our data, when combined with data in the literature, require some changes in previous assignments. The spectroscopic literature on naphthalene and naphthalene‐d8 is reviewed and up‐to‐date tables of vibrational frequencies are presented.
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33.50.Dq Fluorescence and phosphorescence spectra
33.20.Tp Vibrational analysis

On the separation of resonance Raman scattering into orders in the time correlator theory

John B. Page and D. L. Tonks

J. Chem. Phys. 75, 5694 (1981); http://dx.doi.org/10.1063/1.442006 (15 pages) | Cited 84 times

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A detailed study is carried out of the separation of resonance Raman scattering into orders within the time‐correlator formulation of Hizhnyakov and Tehver (HT). This formulation is exact for a multimode system at all temperatures and for all electron–phonon coupling strengths within well‐defined ’’standard assumptions.’’ As in any Raman theory, the full m‐phonon Raman scattering involves the electron–phonon coupling to infinite order, owing to virtual phonon transitions accompanying the m real final state phonon transitions. The HT separation into orders is seen to correspond to a particularly convenient grouping of the contributions, such that the full m‐phonon scattering is expressed as a power series in explicit orders of the electron–phonon coupling, but with each term of the series also containing factors involving the electron–phonon coupling implicitly to infinite order. Each term of this series except the first vanishes for T→ 0, with the result that the (explicitly) mth order scattering and the full m‐phonon scattering are identical at T = 0. Most importantly, at any temperature the contributions from each order to the resonance Raman excitation profile line shapes are obtainable from the optical absorption in a direct and simple way. We have previously stressed the usefulness of the first‐order version of this feature for the analysis of experimental data, and here that work is contrasted with recent work of Hassing and Mortensen, who expressed the full one‐phonon series for a single‐mode system in a different form. For completeness, a detailed derivation of the entire theory is given, including proofs via phonon many‐body techniques of the two fundamental identities upon which the HT approach rests and which were not proven in HT’s rather condensed presentation of their time‐correlator formulation.
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31.10.+z Theory of electronic structure, electronic transitions, and chemical binding
33.70.Jg Line and band widths, shapes, and shifts

Classical, diatomic molecule, kinetic theory cross sections. II. Collision dynamics

C. F. Curtiss

J. Chem. Phys. 75, 5709 (1981); http://dx.doi.org/10.1063/1.442007 (5 pages) | Cited 4 times

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The dynamics of collisions between nonvibrating diatomic molecules is discussed in terms of a system of collision coordinates introduced earlier in the development of classical expressions for the generalized or kinetic theory cross sections. The expressions developed earlier are then rewritten in terms of precollision values of functions introduced in the discussion of the dynamics, which may be evaluated either through a perturbation solution of the equations or a stepwise numerical solution.
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34.10.+x General theories and models of atomic and molecular collisions and interactions (including statistical theories, transition state, stochastic and trajectory models, etc.)
51.10.+y Kinetic and transport theory of gases

Picosecond kinetics of p‐dimethylaminobenzonitrile

D. Huppert, S. D. Rand, P. M. Rentzepis, P. F. Barbara, W. S. Struve, and Z. R. Grabowski

J. Chem. Phys. 75, 5714 (1981); http://dx.doi.org/10.1063/1.442008 (6 pages) | Cited 62 times

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The nanosecond and picosecond resolved dual fluorescences of p‐dimethylaminobenzonitrile (DAB), in various solvents and glasses excited by 266 nm 20 ps FWHM laser pulses, have been investigated. Pulse‐limited rise times are exhibited by the b∗‐state emission whose decay in turn feeds directly the risetime of a∗‐state emission at 440–600 nm in most solvents studied. The a∗‐state emission was monitored at 520–600 nm in order to eliminate contribution from the b∗‐state. Within the experimental resolution, the b∗‐state fluorescence decay times vary approximately linearly with solvent viscosity. The a∗‐state fluorescence decay times vary with both solvent and temperature, and may reflect either thermally assisted intersystem crossing from the solvated singlet a∗‐state (presumably of twisted internal charge transfer character) to a corresponding solvated triplet of slightly higher energy, or a thermally activated internal conversion of the 1TICT to the ground state.
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78.55.Bq Liquids
78.55.Kz Solid organic materials

Decay kinetics of NCl(b 1Σ+, v′)

A. T. Pritt, D. Patel, and R. D. Coombe

J. Chem. Phys. 75, 5720 (1981); http://dx.doi.org/10.1063/1.442009 (8 pages) | Cited 10 times

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Measurements have been made of the time decay of NCl(b 1Σ+,v′) generated by the laser photolysis (193 nm) of ClN3 diluted in helium. Rate constants for collisional quenching of the v′ = 0 level of the b 1Σ+ state are 47.7±1.3(ClN3), 0.54±0.25 (Cl2), 7.1±0.6 (F2), 38±3 (ClF), 7.4±0.3 (H2), 59±3 (HF), 33±3 (HCl), and 1.71±0.06 (CO2) in units of 103 s−1 Torr−1. These results are interpreted in terms of possible reactive and energy transfer mechanisms for electronic quenching. With the exception of ClN3, quenching within the vibrational manifold of the b 1Σ+ state is faster than electronic quenching for each of the chaperone gases tested. Analysis of the b 1Σ+ decay rate for quenching by ClN3 indicates that 13±2% of the vibrational energy contained in NCl(b 1Σ+) is utilized in enhancing the rate of the quenching process.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Pm Rate constants, reaction cross sections, and activation energies

Experimental appraisal of the maximal‐entropy theory of multiphoton ionization‐fragmentation: The alternative ionization pathway test

D. A. Lichtin, R. B. Bernstein, and K. R. Newton

J. Chem. Phys. 75, 5728 (1981); http://dx.doi.org/10.1063/1.442010 (7 pages) | Cited 16 times

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An experimental test of the Silberstein–Levine (SL) maximal‐entropy theory of multiphoton ionization‐fragmentation has been devised and applied to a diverse group of polyatomic molecules. The MPI fragmentation pattern of a given compound is strongly dependent upon the laser wavelength (which governs the ’’pathway’’ to the continuum) and the laser power density (and pulse energy). Nevertheless, it is found possible to obtain similar fragmentation patterns at several wavelengths (i.e., via different pathways) by adjusting the laser power so as to excite the molecule/ion system to the same average value of internal energy 〈E〉. Data are presented for benzene, toluene, t‐butylbenzene, triethylenediamine, quinuclidine, and iodomethane, all of which tend to confirm the major premise of the SL theory that the branching fraction pattern is governed by the single parameter 〈E〉.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Wz Other multiphoton processes
33.15.Ta Mass spectra

Statistical fragmentation patterns in multiphoton ionization: A comparison with experiment

J. Silberstein and R. D. Levine

J. Chem. Phys. 75, 5735 (1981); http://dx.doi.org/10.1063/1.442011 (9 pages) | Cited 28 times

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Breakdown curves computed in the statistical limit are presented for benzene, toluene, t‐butylbenzene, triethylenediamine, and iodomethane. The statistical limit is that of the fragmentation pattern of maximal entrophy (subject only to conserved quantities). In this limit the fragmentation pattern is governed by the mean energy 〈E〉 absorbed per molecule. By varying 〈E〉 it is found possible to span the range from low to extensive fragmentation. For the five molecules tested, the statistical limit reproduces the experimental multiphoton ionization‐fragmentation patterns reported by Lichtin, Bernstein, and Newton as is expected on the basis of their experimental, ’’alternative doorway’’ appraisal. Another prediction, not yet tested by experiment, is that in the statistical limit it should prove possible to obtain similar fragmentation patterns for different isomers (e.g., naphthalene and azulene). Internal state distributions offer a third way in which the statistical limit can be compared with experiment.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Wz Other multiphoton processes
33.15.Ta Mass spectra

Quasidecay of harmonic oscillator coherent states in nonharmonic potentials

Juergen Brickmann and Peter Russegger

J. Chem. Phys. 75, 5744 (1981); http://dx.doi.org/10.1063/1.442012 (15 pages) | Cited 35 times

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The time evolution of harmonic oscillator coherent states (HOCS’s) (displaced ground state wave functions) in symmetrical and nonsymmetrical nonharmonic potentials is studied numerically. The potentials were modeled with an ansatz V(ϵ, q) = (1−ϵ) VHO+ϵ VNH, where VHO is the harmonic oscillator potential and VNH is chosen as a convenient Morse oscillator (nonsymmetrical) or as a negative Gaussian (symmetrical) so that the potential near the minimum is not distorted. The initially well located HOCS’s decay within a medium time scale of 5–100 classical oscillations to a wave packet which is delocalized in the position area of the potential well between the classical turning points. The time period up to this delocalization is designed as quasidecay time τQ, and it is demonstrated with some examples that in the symmetrical potentials the HOCS is refocused after a period of τR = 8τQ, while, as a rule, in the nonsymmetrical potential the recurrences need much longer time and could not be observed in the time scale under investigation. It is shown that the absolute values of the quasidecay times cannot be related either to the energy moments 〈H〉 and ΔH nor to the contribution of a quasicontinuum to the initial HOCS but that there is a strong correlation of ΔH and τQ for the nonsymmetrical potential considering only initial wave packets of the same energy.
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31.70.Hq Time-dependent phenomena: excitation and relaxation processes, and reaction rates
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
03.65.Db Functional analytical methods

Direct energy minimization of excited singlet states

Luc Bouscasse, Danielle Bouin‐Roubaud, Thierry Avignon, and H. H. Jaffé

J. Chem. Phys. 75, 5759 (1981); http://dx.doi.org/10.1063/1.442013 (5 pages) | Cited 2 times

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A general method is developed which permits the minimization of the energy of an excited singlet or triplet state. The conditions for energy minimization are obtained without introduction of Lagrange multipliers by a highly concise matrix formulation. The method is applicable to excited states expressed as linear combinations of an unlimited number of singly excited configurations. The method reduces to an iterative system of matrix diagonalizations and is thus equivalent to a pseudoeigenvalue problem. Calculations for the lowest n→π∗ and π→π∗ states of thiazole and 2‐methylthioimidazole are reported.
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31.50.Df Potential energy surfaces for excited electronic states
31.15.-p Calculations and mathematical techniques in atomic and molecular physics

A study of the arsenic, black phosphorus, and other structures derived from rock salt by bond‐breaking processes. I. Structural enumeration

Jeremy K. Burdett and Timothy J. McLarnan

J. Chem. Phys. 75, 5764 (1981); http://dx.doi.org/10.1063/1.442014 (10 pages) | Cited 28 times

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The structures of arsenic and black phosphorus can be regarded as distortions of the simple cubic or NaCl structure. The presence of more than four electrons per site results in the breaking of three of the six linkages around each atom in the NaCl structure. A generalization of Pólya’s enumeration theorem is proven which allows counting of the 36 possible ways of breaking bonds to produce structures containing trigonal pyramidal coordination at each site and having the same size unit cell as NaCl. Larger cells are considered briefly.
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61.66.Bi Elemental solids
61.50.Lt Crystal binding; cohesive energy

A study of the arsenic, black phosphorus, and other structures derived from rock salt by bond‐breaking processes. II. Band structure calculations and the importance of the gauche effect

Jeremy K. Burdett, Peter Haaland, and Timothy J. McLarnan

J. Chem. Phys. 75, 5774 (1981); http://dx.doi.org/10.1063/1.442015 (8 pages) | Cited 16 times

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Band structure calculations based on the extended Hückel method order energetically the 36 arsenic‐like structures having the NaCl size unit cell in a way which is compatible with the observed crystal structures. Black phosphorus is the most stable of the 36, and the arsenic layer type is number seven. Energy differences among these structures can be explained by different energetic contributions from syn‐, gauche, and anti‐arrangements, of both bonded and nonbonded pairs of atoms, and by a destabilization accompanying small bond angles. The order of stability is gauche ≳ anti ≳syn. Lone pair repulsions are important in determining the total energy, but play only a minor role in the relative energies of these structures. The energies ascribed to all these local geometries are compatible with molecular orbital calculations on small molecules and with observed molecular conformations described by the ’’gauche effect.’’
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61.66.Bi Elemental solids
61.50.Lt Crystal binding; cohesive energy
71.20.Mq Elemental semiconductors

Wave functions in momentum space. I. Iterative computation for the helium atom in Hartree–Fock approximation

Hendrik J. Monkhorst and Krzysztof Szalewicz

J. Chem. Phys. 75, 5782 (1981); http://dx.doi.org/10.1063/1.442016 (3 pages) | Cited 14 times

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The possibilities of obtaining wave functions for atoms and molecules by iteration of equations in momentum space have been examined. Using a first‐iterate orbital some approximations to the Hartree–Fock ground state energy have been calculated. The accuracy of these results is strongly dependent on a variation of the parameters that characterize the orbital. The second‐iterate orbital contains logarithmic and arctangent functions and there appear unfamiliar special functions in the third‐ and higher‐iterate orbitals.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
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