Index of content:
Volume 78, Issue 10, 15 May 1983

Infrared photoacoustic spectroscopy of undoped polyacetylene
View Description Hide DescriptionPhotoacoustic infrared spectroscopy has been applied to undoped polyacetylene powders and films and the results are compared to transmission infrared spectra obtained on both types of samples. the photoacoustic technique has been found to be particularly suitable for the study of polyacetylene powders which are more sensitive to air exposure than films, due to their higher surface area. The method yields spectra whose major features are essentially identical to transmission spectra, but allows more flexibility with respect to sample thickness, size, and shape. A band near 1920 cm^{−} ^{1}, also observed but not discussed in previous studies, is assigned to the presence of cumulative carbon double bonds. Bands observed near 1375 and 890 cm^{−} ^{1} on films and, more prominently, on powders are assigned to the incorporation of small amounts of oxygen during preparation and handling, resulting in inadvertent doping. The large surface area of powders also results in more pronounced oxidation of the polymer backbone.

Conformational mapping of fatty acids in ordered and disordered phases: A spectroscopic study
View Description Hide DescriptionThe infrared active CD_{2} rocking vibration of selectively deuterated palmitic and stearic acids is used as a probe for the mapping of the local conformation of these molecules in the solid and liquid phases. The infrared spectra at different temperatures are analyzed and interpreted by means of normal coordinate calculations and conformational energy calculations. Evidence is collected that in the Csolid modification molecules are fully trans planar. The cooperatively distorted local conformation of the carboxyl group in the liquid phase is discussed.

Laser light scattering studies of the dynamics of molecular reorientation of a viscoelastic liquid: α‐phenyl o‐cresol
View Description Hide DescriptionThe reorientational motion of α‐phenyl o‐cresol molecules in the normal liquid and supercooled liquid state in the temperature range between 97 and −49.3 °C has been investigated using the dynamic light scattering technique. Above 5 °C, Fabry–Perot interferometry is used, but the photon correlation spectroscopic technique is employed below −28.9 °C. With the combined use of these two light scattering spectroscopic techniques, the orientational relaxation time over ten decades of change is measured. The shape of the VH time correlation function is nonexponential. The bimodal and the Williams–Watts distribution functions are used to fit the shape of the VH correlation functions at various temperatures. The Williams–Watts function gives the best fit to the observed shape. The correlation time and the shape of the VV time correlation function are found to be similar to that of the VH, suggesting that the reorientational process is coupled to the localized translational motion for molecules in the supercooled state. Angular dependent studies of the VH and VV time correlation functions indicate that in the supercooled state, the shear wave (a collective translational motion) influences the reorientational motion only negligibly. Various theoretical predictions about the profile of the central component of the VH spectrum based on the coupling of the reorientation to the shear wave are not supported by the experiment result. The temperature dependence of the orientational relaxation time does not follow the Arrhenius equation, but it follows the Antoine‐WLF equation reasonably well. The Debye–Stokes–Einstein equation is found to be adequate to describe the temperature and viscosity dependence of the orientational relaxation time over ten decades of change.

EPR study of the structure and spin distribution at the binding site in human nitrosylhemoglobin single crystals^{a)}
View Description Hide DescriptionThe anisotropicEPR spectrum was measured in single crystals of human nitrosylhemoglobin. The principal values and axis directions of the g factor and of the Nhyperfineinteraction of the NO ligand were determined for an alpha and a beta subunit. A model for the geometry, orientation and orbital spin distribution at NO bound to the Fe atom in the subunits is derived from the measured g values and hyperfineinteraction. The relationship between the NO bond, the Fe crystal field and the g and A principal axis directions are discussed.

Preliminary rovibrational analysis of the nν_{6}+ν_{1}−nν_{6} vibration in HCN⋅⋅⋅HF
View Description Hide DescriptionA preliminary rotation‐vibration analysis of the n=0 and n=1 subbands associated with the nν_{6}+ν_{1}−nν_{6} hydrogen‐bonded vibration in HCN⋅⋅⋅HF has been completed. The following excited staterotational constantsB′ and band origin frequencies ν_{0} have been determined for the complex. The results are consistent with a rotation‐vibration interaction constant α_{1}=−68.3±1 MHz which correlates with an excited stater(N⋅⋅⋅F) internuclear distance of 2.762 Å, a decrease of 0.034 Å relative to the ground state.Excited state lifetimes associated with assigned transitions are demonstrated to be ≥1.8×10^{−} ^{1} ^{0}s while the x _{1} _{6} anharmonic constant is evaluated to be 4.01±0.03 cm^{−} ^{1}.

ENDOR evidence for crystal imperfections: Case of N‐acetylglycine
View Description Hide DescriptionThe formation of several very similar radicals in irradiated N‐acetylglycine was shown to be associated with the crystal imperfections which were produced during growth, heating, and irradiation processes.

Photochemical and photophysical dynamics of I_{2} isolated in a rare gas cage
View Description Hide DescriptionThe photochemical and photophysical properties of I_{2} in Ar, Kr, and Xe matrices have been investigated and are reported here. Following laser excitation high into the B ^{3}π(0^{+} _{ u }) and ^{1}π_{ lu } dissociative states, prompt emission is observed from the B ^{3}π(0^{+} _{ u }), A ^{3}π_{1u }, and A′ ^{3}π_{2u } electronic states depending on the matrix host. In all matrices the emission attributed to the B electronic state lies 200 nm to the red of the excitation wavelength and is generally featureless. The time resolved emission for this state has a detector limited rise and fall. The emission attributed to the A ^{3}π_{ lu } electronic state was resolved with a monochromator and the source determined to be the v′=0 level of the A state. The lifetime for this emission is 260 μs and was found to be roughly the same in all matrices. Emission from the A′ ^{3}π_{2u } state was also detected but only in Xe matrices. The measured lifetime for this state was 6.3 ms. Data from a number of previous experiments on matrix isolated halogens in Ar, Kr, and Xe are reviewed and a consistent model for the relaxation pathways for intramolecular electronic energy transfer is presented. The implications of this model for studies of the solvent cage effect on I_{2}photodissociation and the development of the I_{2}chemical laser are also discussed.

Pressure broadening of CO infrared lines perturbed by H_{2} and He
View Description Hide DescriptionWe present results of infrared linewidthmeasurements at low and room temperatures for CO diluted in H_{2} and He using a tunable diode laser in the 4.7 μm spectral region (CO fundamental band). Information on the intermolecular potential is obtained by comparison with results of dynamical calculations. In the case of CO–H_{2} pressure broadening (PB), data are compared to direct measurements of inelastic cross sections by infrared–infrared double resonance and to ab initioquantum calculations. In the case of CO–He, experimental PB coefficients are compared to theoretical cross sections derived from two ab initiopotential energy surfaces.

Line interference effects in the rotational Raman spectrum of C_{2}H_{4}
View Description Hide DescriptionLine interference effects have been observed in the rotational Raman spectrum of the nearly prolate, asymmetric top C_{2}H_{4} at pressures ∼1 atm. An interferometric detection scheme is used which enhances the contribution of quasiperiodic, symmetric‐top‐allowed lines to the spectrum. A simple, cluster model provides an adequate description of interference effects in these lines. Optimum values of the cluster spectral window and width parameters are determined from a least‐χ^{2} fitting between experimental and theoretical Raman interferogram spectra. The value obtained for the cluster linewidth is in good agreement with the results of previous experiments on C_{2}H_{4}, and provides a physically meaningful alternative to the a d h o cline shape introduced to account for the presence of non‐Lorentzian line shapes in this latter spectra. The cluster model is also compared with the results of a model calculation of interference effects in a simple spectrum.

Fluorescence depolarization by anisotropic rotational diffusion of a luminophore and its carrier molecule
View Description Hide DescriptionThe analytical expression for the polarizationanisotropy is derived for a luminophore undergoing rotational diffusion about a single axis while attached to a nonluminescing, rotationally diffusing, symmetrical carrier molecule. In contrast to previous related calculations, the rotation axis of the luminophore is assumed to have an arbitrary orientation relative to the carrier. Additionally, the polarizationanisotropy is measured for bovine serum albumin (BSA) labeled with dansyl, NBD, rhodamine, or eosin that is: (a) surface adsorbed to a glass/buffer interface, using a variation of the technique of total internal reflectionfluorescence spectroscopy (TIRFS), or (b) bulk dissolved, using conventional transmitted illuminationfluorescence spectroscopy. With this theory, using previously published values for the rotational diffusion constants of BSA and the fluorescence lifetimes of the fluorophores, the rotational diffusion constant of the covalently bound probes is estimated from the measuredanisotropy values. The results indicate a wide variability in the rotational diffusion constant of the probes (from ∼10^{7} s^{−} ^{1} for dansyl to ∼10^{9} s^{−} ^{1} for eosin) attached to both the surface adsorbed and bulk dissolved forms of BSA. Contrasting the rotational diffusion constant for each probe for surface adsorbed BSA vs bulk dissolved BSA indicates surface adsorption of the BSA molecule inhibits the rotational motion of the probe. These results have important implications in the application of other fluorescence techniques, such as singlet–singlet energy transfer, where the rotational mobility of the probe is important.

Polarized emission from doublet state organic free radicals
View Description Hide DescriptionThe m‐ or p‐fluorobenzyl radicals were prepared by x‐irradiation of the precursor fluorotoluenes in argon at 10 K. The samples in an external magnetic field were excited with circularly polarized light obtained by passing the output of a He–Cd laser through a λ/4 plate. The right and left circular polarization components of the fluorescence output were alternately sampled by means of a photoelastic modulator and a digital synchronous computer. The results showed that there is net circularly polarized emission from the excited electronic states of doublet organic free radicals, thus demonstrating that the populations of the excited state magnetic sublevels are related to the population distribution in the ground state. The intensity of the effect is linearly dependent upon external magnetic strength because of the increase in the Zeeman splitting.

Density functional theory of nonlinear optical response
View Description Hide DescriptionThis work is concerned with the formulation and testing of a self‐consistent nonlinear response theory of optical processes based upon the density functional formalism. An expression is developed for the third order susceptibility of a spherically symmetric system using a generalization of a time‐dependent local density approximation (TDLDA) previously developed for linear response. Explicit calculations of the coefficient of third harmonic generation for the rare gases are in good agreement with experiment. Applications to other nonlinear phenomena are pointed out.

Electron paramagnetic resonance study of Mn^{2} ^{+} doped ammonium iodide single crystals
View Description Hide DescriptionERP of Mn^{2} ^{+}doped in ammonium iodide single crystals has been studied at room temperature (∼300 K) and at X band (∼9.3 GHz). An axial complex with the following spin‐Hamiltonian parameters has been observed: g _{∥}=2.0014±0.0005, g _{∥}=2.0028±0.0005, b ^{0} _{2}=−1611±2 G, b ^{0} _{4}=6±1 G, ‖A‖=‖B‖=90±1 G. Some unusual features observed in the spectra at X band due to the large value of b ^{0} _{2} have been discussed.

Experimental evidence of collective vibrations in DNA double helix (Raman spectroscopy)
View Description Hide DescriptionThe collective vibrational motions in the solid DNA fiber have been found from the wave vector dependence of the low frequency Raman spectra. The main part of the low frequency modes have been assigned to the α_{ xz }(α_{ yz }) component of the Raman tensor from the polarized Raman spectroscopy.

ESR investigations of H_{2}O^{+}, HDO^{+}, D_{2}O^{+}, and H_{2} ^{1} ^{7}O^{+} isolated in neon matrices at 4 K
View Description Hide DescriptionThe H_{2} ^{1} ^{6}O^{+}, HD ^{1} ^{6}O^{+}, D_{2} ^{1} ^{6}O^{+}, and H_{2} ^{1} ^{7}O^{+} ion radicals have been generated by photoionization, trapped in neon matrices at 4 K, and investigated by ESR(electron spin resonance)spectroscopy. The magnetic parameters are g _{iso}=2.0093(3), A _{iso}(H)=73.7(6) MHz, and A _{iso}(^{1} ^{7}O)=83.5(6) MHz. A weak electronic absorption feature at 6036(3) Å was observed which is tentatively assigned to a known gas phase band. The H_{2}O^{+} radical appears to be rotating in neon and exhibits an unusual intensity increase as the matrix temperature is lowered from 9 to 4 K. Comparisons between H_{2}O^{+} and the isoelectronic radicals NH_{2}, NH^{+} _{3}, and CH_{3} are made.

Matrix ESR spectra of polyatomic alkali metal clusters
View Description Hide DescriptionESRspectra have been obtained for polyatomic sodium and potassium clusters in argon and nitrogen matrices. The spectra, tentatively assigned to alkali septemer molecules, show a large isotropic hf (a _{1}) from two equivalent alkali nuclei plus a much smaller splitting (a _{2}) from an additional (at least) five equivalent nuclei. For an argon matrix, the measured hf constants and g values are a _{1}=118.16(4) G, a _{2}=6.66(12), g _{0}=1.9994 (1) and a _{1}=30.55(6) G, a _{2}=1.79(2) G, g _{0}=2.0017(1) for Na_{7} and K_{7}, respectively. The ESR spin populations, ρ_{ i }=a _{ i }/a(atom), are compared with the orbital symmetries predicted by the simple Huckel approximation for a range of M _{7}, M _{9}, M _{11}, and M _{13} structures. Aside from the relatively unstable ‘‘T‐shaped’’ arrangement, only a few three‐dimensional structures give a satisfactory agreement with the ESRspectra. For M _{7}, a likely geometry is the pentagonal bipyramid. The ESR spin populations for Na_{7} are almost identical in magnitude to those of K _{7}. If a _{2}<0, the total isotropic spin population is Σ_{ i }ρ_{ i }≂0.64, which implies an ∼ 36% p character for the unpaired electron.

The generalized log‐derivative method for inelastic and reactive collisions^{a)}
View Description Hide DescriptionA generalization of the log‐derivative method is presented which is useful for both reactive and nonreactive scattering problems. In the coupled system of radial equations for this problem a first derivative term is included for complete generality. Thus, this method may be used when, as is often the case in reactive or curve crossing problems, the equations contain a first derivative term. When no first derivative term is present and no reactive channels are present, the method reduces to the standard log‐derivative method. A reactive scattering problem is solved as an example.

Laser induced fluorescence and absorption measurements of NO in NH_{3}/O_{2} and CH_{4}/air flames
View Description Hide DescriptionLaser diagnostics have been used to probe NO in atmospheric pressureflames.Laser induced fluorescence techniques (LIF) were used to measure relative concentration profiles of NO at fuel equivalence ratios φ=1.28, 1.50, and 1.81 in NH_{3}/O_{2}/N_{2}flames and φ=1.7 and 1.8 in CH_{4}/air/O_{2}flames. Laser absorption measurements were made to derive an absolute concentration of NO in a lean NH_{3}/O_{2}/N_{2}flame. This measured NO concentration agreed well with the calculated equilibrium concentration. The fluorescence signals from rich flames were then calibrated by comparing the fluorescence signals to that of the lean flame where absolute concentrations were derived. In rich NH_{3}/O_{2}/N_{2}flames NO concentrations decay more rapidly throughout the burnt gases than one would expect from the conventional mechanism of ammonia oxidation. This suggests that new reactions such as NH_{2}+NH_{2} and NH+NH_{2} to ultimately yield N_{2} are important in these rich flames.LIF measurements on the CH_{4}/air/O_{2}flames were able to resolve the growth and decay of ‘‘prompt NO’’ within the flame front. The LIF technique is estimated to have a sensitivity of better than 1 ppm for NO in these atmospheric pressureflames.

Vibrational energy transfer in CO_{2}–C_{2}H_{2n+2} (n=0, 1, 2) mixtures from 220 to 300 K
View Description Hide DescriptionTime resolved laser induced infrared fluorescence studies have been performed to obtain the rate constants of energy transfer of CO_{2} (ν_{3}) in mixtures with C_{2}H_{2}, C_{2}H_{4}, and C_{2}H_{6} at temperatures between 220 and 300 K. A pulsed 10.6 μm CO_{2} laser light has been used to directly excite the CO_{2} molecules at low temperatures. The CO_{2} self‐relaxation was measured and the results are in agreement with previous measurements. The rate constants for the transfer of energy in the CO_{2}–C_{2}H_{2} system show normal temperature behavior, (rate constant increases as the temperature increases) associated with large energy defect and caused by short‐range repulsive forces. The rate constants for the systems CO_{2}–C_{2}H_{4} and CO_{2}–C_{2}H_{6} show an inverse temperature effect, (rate constant increases as the temperature decreases) associated with near resonant energy exchange and long‐range attractive forces between the molecules. The observed behavior with temperature for the three systems is explained using a Morse potential which includes repulsive and attractive terms to describe the interaction between the molecules and a probability expression formulated by H. K. Shin, to calculate the energy transfer probabilities of some important reactions considered for each system.

Test of variational transition state theory with a large‐curvature tunneling approximation against accurate quantal reaction probabilities and rate coefficients for three collinear reactions with large reaction‐path curvature: Cl+HCl, Cl+DCl, and Cl+MuCl
View Description Hide DescriptionThe large‐curvature ground‐state model for the transmission coefficient of generalized transition state theory [presented in a previous paper by B. C. Garrett, D. G. Truhlar, A. F. Wagner, and T. H. Dunning, J. Chem. Phys. 78, 4400(1983)] is tested against accurate quantal calculations of the rate coefficients for collinear reactions with very large inertial effects, namely Cl+HCl→ClH+Cl, Cl+DCl→ClD+Cl, and Cl+MuCl→ClMu+Cl. The tests cover the temperature range 200–2400 K. The accurate rate calculations are based on reaction probabilities obtained by a new numerical method for solving Schrödinger’s equation in Delves’ coordinates. Improved canonical variational transition state theory predicts rate coefficients 5.0–18 times smaller than those predicted by conventional transition state theory for the H transfer; for the D transfer, the ratio is 2.0–3.4; and for Mu it is 22–2.8×10^{7}. The large‐curvature model predicts transmission coefficients as large as 41, 8, and 206 for the H, D, and Mu‐transfer cases at 200 K, decreasing to 1.2, 1.1, and 1.4 at 2400 K. Despite the large effect of variationally optimizing the transition state location and the large size of the tunneling effect and the wide variation of both effects with temperature, improved canonical variational transition state theory with large‐curvature ground‐state transmission coefficients (ICVT/LCG) is accurate within a factor of 1.7 over a temperature range of a factor of 8, 300–2400 K, for all three reactions. At 200 K, the ICVT/LCG model underestimates the rate coefficients, by factors of 2.3, 1.9, and 1.5 for H, D, and Mu, respectively.