Index of content:
Volume 68, Issue 3, 01 February 1978

Analytical potentials from a b i n i t i o computations for the interaction between biomolecules. IV. Water with glycine and serine zwitterions
View Description Hide DescriptionThe glycine zwitterion, in two different conformations, and the serine zwitterion, in three different conformations, have been analyzed in their interaction with a molecule of water placed at 281 different orientations and distances. For these orientations and distances SCF–LCAO–MO computations have been performed on the glycine–water and serine–water complexes, using a Gaussian basis set. The computed interaction energies have been fitted with a sum of pair potentials. Two simple forms of the pair potentials have been considered: the first is the one used in the previous paper and is of the form ‐A _{ i j }/r ^{6} _{ i j } +B _{ i j }/r ^{12} _{ i j } +C _{ i j } q _{ i } q _{ j }/r _{ i j }; the second contains a more flexible description of the electrostaticinteraction, and is of the form ‐A _{ i j }/r ^{6} _{ i j } +B _{ i j }/r ^{12} _{ i j } +C _{ i j } q _{ i } q _{ j }/r _{ i j } +D _{ i j } q _{ i } q _{ j }/r ^{2} _{ i j }, where A, B, C, and D are numerical constants, and q _{ i } and q _{ j } are the atomic charges of the ith atom of M and of the jth atom of H_{2}O at infinite separation. The pair potentials are discussed by computing potential curves and contour maps of isoenergy levels.

^{14}N PNQR study of the effect of pressure on the phase transition and molecular reorientation in s‐triazine
View Description Hide Description^{14}N pure nuclear quadrupole resonance frequencies and inverse linewidth in s‐triazine were measured as functions of hydrostatic pressure to 6.2 kbar along two isotherms at 23.3 and −0.4 °C. The second order phase transition, previously studied at atmospheric pressure, was observed at high pressures. Above the transition pressure p _{ c } each resonance line splits into two. The line splittings were fitted to the equation Δν=B (p−p _{ c })^{β}, and the critical exponent β was found to be close to 0.5. The pressure dependence of the transition temperature T _{ c } is described by the (linear) equationT _{ c }(p) =198.8+18.2p, [T _{ c }(°K);p (kbar)]. The present results along isotherms were compared with previous results along an isobar. In the low pressure region the inverse linewidthT _{2}* is dominated by hindered rotations of the s‐triazine molecule. The activation volume for this motion is ΔV*=26 cm^{3}/mole.

Absorption studies of Cs_{2} and Rb_{2} molecular bands in the visible and near visible
View Description Hide DescriptionBy analyzing the temperature dependence of the absorptionspectrum of Cs_{2} molecules in the visible region of the spectrum we have demonstrated that a group of unusually narrow bands near 7100 Å originate from a very shallow and possibly slightly repulsive part of a ground state potential curve. This may be part of the ^{3}Σ ground‐state potential curve. An analogous set of narrow bands occurs for Rb_{2} near 6000 Å. The remaining prominent visible absorption bands for Cs_{2} and Rb_{2} all originate from molecules near the bottom of the ^{1}Σ ground state.

Reactions of singly and doubly charged argon ions with N_{2} and O_{2} in a steady state hollow cathode discharge
View Description Hide DescriptionIon–molecule reactions between the ions Ar^{+}, Ar^{++}, and N_{2} molecules are studied in the negative glow region of a cylindrical steady‐state hollow cathodedischarge at pressures between 0.25 and 0.5 torr and at [N_{2}]/[Ar] density ratios of 0.001, 0.002, 0.005, and 0.01. The following rate constants are obtained: for Ar^{++}+N_{2}→N_{2} ^{+}+Ar^{+}, k=2×10^{−10} cm^{3} s^{−1}; Ar^{++}+N_{2}→N^{+}+N+Ar, k =1×10^{−10} cm^{3} s^{−1}; Ar^{+}+N_{2}→N_{2} ^{+}+Ar, k =0.2–1.5×10^{−11} cm^{3} s^{−1}. A value of k=4×10^{−13} cm^{3} s^{−1} for the partial charge exchange Ar^{++}+Ar→2 Ar^{+} is consistent with the measurements. For comparison the rates of Ar^{++}+O_{2} (k=1.7×10^{−9} cm^{3} s^{−1}) and of Ar^{+}+O_{2} (k=5.8×10^{−11} cm^{3} s^{−1}) are determined. The error limits of the k values are ±45%. Dissociative volume recombination of Ar_{2} ^{+} with slow plasma electrons (α=3.1±1.1×10^{−7} cm^{3} s^{−1} at an electron temperature of 1200 K) is the dominant loss process for these ions. Wherever possible, the results are compared with previous ones.

Continued fraction formalism and its application to lattice dynamics
View Description Hide DescriptionThe continued fraction formalism of Mori is derived systematically and its application to nonequilibrium statistical mechanics is carefully studied. We choose applications from lattice dynamics and give general solutions for semi‐infinite and infinite harmonic oscillator chains in terms of finite continued fractions. Special cases of these general solutions are considered in detail.

Time lag in diffusion‐controlled nucleation
View Description Hide DescriptionNucleation occurring in the presence of a dense solvent is accompanied by spatial correlations among the reactant clusters and monomers which affect the aggregation rates. The classical theory of nucleation, which does not take such correlations into account, overestimates the observed nucleation rate and underestimates the time lag in condensed systems. An explicit expression for the time lag in diffusion‐controlled nucleation is derived. Values associated with miscibility gap experiments differ from the classical predictions by factors of 5 to 10.

Ion pair and partially hydrated Li^{+}NO_{3} ^{−} ion pair structures: Correlation of molecular orbital results with matrix isolation data
View Description Hide DescriptionThe results of a molecular orbital study for the ion pair Li^{+}NO_{3} ^{−}, alone and hydrated, have been related to the vibrational data for the same species matrix isolated in argon. Initially it was established that a plot of the N–O bond overlap population vs empirical F _{NO} force constants, for fourteen molecules, is impressively linear. Subsequently this plot has been used to convert the Gaussian 70‐derived N–O overlap populations for the N–O bonds of Li^{+}NO_{3} ^{−}, alone or hydrated, to F _{NO} values for the particular species. In the simple Li^{+}NO^{−} _{3} pair, for example, the minimum energy bidentate structure has a significantly smaller overlap population in the two metal‐coordinated N–O bonds. These overlap populations indicate an N–O stretching force constant difference of 3.94×10^{2} N m^{−1} which can be converted to a 245 cm^{−1} splitting of the otherwise degenerate ν_{3} antisymmetric stretching mode. The latter value compares well with the experimental value of 265 cm^{−1} and, together with equally encouraging comparisons for the hydrated ion pair, suggests that the Gaussian 70 results for these systems, even using the STO‐3G minimum basis set, realistically depict the bonding for Li^{+}NO_{3} ^{−} as well as the influence of hydration on that bonding.

Calculations of proton magnetic shielding constants in polyatomic molecules
View Description Hide DescriptionProtonmagnetic shielding constants in H_{2}O, NH_{3}, and CH_{4} molecules have been evaluated within the framework of coupled and uncoupled Hartree–Fock perturbation theories for the Fock–Dirac density matrix, employing several basis sets of gaugeless Gaussian functions. The results yielded by the largest basis sets are in good agreement with experiment, as well as other a b i n i t i o calculations. Some quantities, useful in testing the degree of gauge independence of computed screening constants and in evaluating this property for a gauge whatsoever, are also presented. Numerical tests demonstrate the basic soundness of the uncoupled approach to magnetic shielding proposed here, because of its higher accuracy with respect to other uncoupled schemes and its practicality for larger molecules.

Molecular dynamics study of clustering. I
View Description Hide DescriptionA study of the cluster formation in a two‐dimensional dilute gas of hard sphere square well discs is presented. The temperature range is chosen close to the gas‐liquid phase transition. The evolution of the system is discussed. Abundances of clusters with different topologies are estimated.

Experimental determination of relative signs of dipole moment derivatives: HCN and DCN
View Description Hide DescriptionAn alternative numerical analysis for the experimental determination of the relative signs of the dipole moment derivatives with respect to the normal coordinates is presented. Use is made of the polar tensor and effective charge equations and the G–intensity sum rule of Crawford. The relative signs are determined by sufficiently accurate individual infrared fundamental intensities of one molecule and only the sum of these intensities for an isotopically related molecule. The intensity data of HCN and DCN are used to illustrate the procedure. The as yet unreported values of the atomic polar tensors and effective charges of these molecules are also reported.

An analysis of the NMR line shapes of the ammonium ion undergoing composite tunneling and reorientational motions at low temperatures
View Description Hide DescriptionAn analysis is made of the NMR line shapes of the rotationally tunneling ammonium (NH_{4} ^{+}) ion which also undergoes rapid reorientataional jumps around its C _{3} or C _{2} symmetry axis. The proposed composite motion, believed to be brought about by the coupling of rotor states of the ion with lattice phonons, leads to line narrowing and offers an interpretation of experimental line shape and second moment data not explained previously. In particular, the proton resonance line shapes of (NH_{4})_{2}SnCl_{6}, NH_{4}BF_{4}, NH_{4}ClO_{4}, (NH_{4})_{2}GeF_{6}, (NH_{4})_{2}PbCl_{6}, and (NH_{4})_{2}PtCl_{6} at liquid helium temperatures are discussed. The analysis suggests that protonmagnetic resonance line shape measurements on single crystals may provide an assessment on the state of ordering of the tunneling and reorienting NH_{4} ^{+} ions.

Lifetimes of IBr(B ^{3}π^{+} _{0}) by laser‐excited time‐resolved fluorescence
View Description Hide DescriptionFluorescence lifetimes for the v′=2,3, and 4 levels of the B ^{3}π^{+} _{0} state of IBr have been measured by means of laser‐excited time‐resolved fluorescence. A technique is described which reduces the presence of Br_{2} and I_{2} to an acceptable level, allowing an unambiguous determination of the IBr lifetime. The observed lifetime for the three vibrational levels studied is essentially the same, 0.54±0.05 μs.

Time resolved measurements in high pressure mass spectrometry: An analysis of assumptions
View Description Hide DescriptionThe assumptions underlying the use of time resolvedmeasurements for the determination of ion–molecule reaction rate constants in high pressure chemical ionization sources have been examined and problems introduced by diffusion and drift are presented; these require conversion from ion currents to ion concentrations in the presence of applied fields when ion current ratios of different ions are used. Moreover, the existence of an irresolvable difference in the significance of arrival times of product and reactant ions is demonstrated, but it is shown how this error can be minimized. A method for the measurement of total disappearance rate constants for association reactions is demonstrated and applied to the reaction CO^{+} _{2}+2CO_{2}→ (CO_{2})^{+} _{2}+CO_{2}; the third order rate constant for this process is 1.5±0.1×10^{−28} cm^{6} sec^{−1} at 409° K.

Infrared‐to‐visible conversion in (Y, Yb, Er)F_{3} and (Y, Yb, Ho)F_{3}
View Description Hide DescriptionVisible emission spectra from (Y_{0.80}Yb_{0.19}Er_{0.01})F_{3} and (Y_{0.80}Yb_{0.19}Ho_{0.01})F_{3} were obtained upon excitation at 970 nm. Emission bands at 656, 548, 520, 409, and 380 nm are observed in the Er^{3+}phosphor, and at 746, 645, 544, and 489 nm in the Ho^{3+}phosphor. The origin of each of these bands is discussed. Data presented for the 656 nm emission in the Er^{3+}phosphor show that it can be the result of only one of three previously proposed mechanisms. The 746 nm band from the Ho^{3+} system has not been previously discussed in the literature even though it is the most intense emission band. Data presented for the temperature dependence of the upconversion process in these two phosphors show that generalizatons made about the mechanism of energy transfer in the Ho^{3+} system based on the more widely studied Er^{3+} system are not valid.

Mean dipole moment derivatives, atomic anisotropies, and effective charges of diatomic hydrides
View Description Hide DescriptionMean dipole moment derivatives, atomic anisotropies, and effective charges for the first and second row diatomic hydride molecules have been calculated from the theoretical CEPA values of the equilibrium dipole moment,dipole moment derivative, and internuclear distance reported by Meyer and Rosmus. The behavior of these values as a function of atomic number, both across and down the periodic table, is extensively analyzed. The theoretical values are compared with those obtained from experiment. The ratio of the equilibrium dipole moment to the internuclear distance and the dipole moment derivative show linear dependencies on the atomic number for the Group III through Group VII first and second row diatomic hydrides. This information is used to estimate these quantities for the metal diatomic hydrides of the third and fourth row.

Energy transfer cross sections in relation to generalized oscillator strengths for the donor–acceptor pair benzene–acetone
View Description Hide DescriptionGeneralized oscillator strengths have been determined for the ? ^{1} B _{2u }←? ^{1} A _{1g } transition in benzene and the ? ^{1} A _{2}←? ^{1} A _{1} transition in acetone. From the data, and the spectroscopic data and analysis of other investigators, the generalized oscillator strengths for the reverse transition in benzene have been calculated approximately. Huo’s theory of energy transfer cross sections [J. Chem. Phys. 66, 3572 (1977)] and the generalized oscillator strengths have been used to calculate the cross section for the resonant energy transfer process benzene (? ^{1} B _{2u }) + acetone (? ^{1} A _{1}) → benzene (? ^{1} A _{1g })+acetone (? ^{1} A _{2}). The energy transfer cross section calculated in this way is found to be 39 Å^{2} with an estimated uncertainty of ±5 Å^{2}. The experimental value is 33±2 Å^{2} [K. Janda and F. S. Wettack, J. Am. Chem. Soc. 94, 305 (1972)]. Huo’s theory is based on the Born approximation, neglecting electron exchange. The agreement between calculated and observed cross sections shows that the Born approximation (without exchange) is holding even though the molecules are colliding with relative velocities in the thermal range.

Absorption and pitch relationships in dichroic guest–host liquid crystal systems
View Description Hide DescriptionThe absorption of a dichroic dye dissolved in chiral nematic hosts in the Grandjean texture increased with decreasing pitch of the host. Absorption was higher for hosts of low birefringence and high order parameter. Boundary condition, homeotropic or parallel (homogeneous), had no appreciable effect on absorption, but threshold voltage was higher for the parallel than homeotropic boundary. The results agreed with theoretical plots relating absorption to pitch.

Threshold behavior of endoergic bimolecular reactions: A statistical phase space approach
View Description Hide DescriptionA statistical phase space theory is developed for threshold and near threshold behavior of endoergic bimolecular reactions. A simple, exact threshold law for the cross section is derived for collisions between atomic, diatomic, or polyatomic species in all combinations. An approximate expression is derived for the cross section that is valid between threshold and about 0.5 eV above threshold. Analytical expressions are obtained in both cases that allow cross section versus energy curves to be generated using only hand‐held electronic calculators. The utility of the theory is demonstrated by comparing the theoretical predictions with experiment for the reactions C^{+}⋅ (D_{2},D⋅)CD^{+}, H_{2}O^{+}⋅ (H_{2},OH⋅)H^{+} _{3}, and D_{3}O^{+}(D_{2},D_{2}O)D^{+} _{3}. The first reaction behaves statistically in the threshold region while the second and third do not. A correlation is made between the statistical behavior of the reaction and the stability of the collision complex: The intermediate CD^{+} _{2}⋅ is very stable (98 kcal/mole) while the ions H_{4}O^{+}⋅ and D_{5}O^{+} are unstable. The utility of the approximate threshold theory for exploring the molecular origins for the cross‐sectional changes with energy is emphasized.

Kinetics of the reactions of O(^{3} P) atoms with the amines CH_{3}NH_{2}, C_{2}H_{5}NH_{2}, (CH_{3})_{2}NH, and (CH_{3})_{3}N over the temperature range 298–440 °K
View Description Hide DescriptionAbsolute rate constants for the reaction of O(^{3} P) atoms with CH_{3}NH_{2}, C_{2}H_{5}NH_{2}, (CH_{3})_{2}NH, and (CH_{3})_{3}N have been determined over the temperature range 298–440 °K using a flash photolysis–NO_{2}chemiluminescence technique. The Arrhenius expressions obtained were k _{2}[CH_{3}NH_{2}] =9.02×10^{−12} exp[−(1650±200)/R T] cm^{3} molecule^{−1} sec^{−1}, k _{2}[C_{2}H_{5}NH_{2}]=1.13×10^{−11} exp[−(1275 ±200)/R T] cm^{3} molecule^{−1} sec^{−1}, k _{2}[(CH_{3})_{2}NH]=1.52×10^{−11} exp[−(550±200)/R T] cm^{3} molecule^{−1} sec^{−1}, k _{2}[(CH_{3})_{3}N] =1.08×10^{−11} exp[(415±200)/R T] cm^{3} molecule^{−1} sec^{−1}, with room temperature rate constants of (5.65±0.57) ×10^{−13} cm^{3} molecule^{−1} sec^{−1}, (1.33±0.14) ×10^{−12} cm^{3} molecule^{−1} sec^{−1}, (6.12±0.62) ×10^{−12} cm^{3} molecule^{−1} sec^{−1}, and (2.20±0.22) ×10^{−11} cm^{3} molecule^{−1} sec^{−1}, for CH_{3}NH_{2}, C_{2}H_{5}NH_{2}, (CH_{3})_{2}NH, and (CH_{3})_{3}N, respectively. Possible reaction mechanisms are discussed.

Ionizing collisions of cesium and potassium atoms with water
View Description Hide DescriptionCrossed beam studies of K and Cs collisions with H_{2}O and D_{2}O have been made for laboratory alkali beam energies from 10 to 200 eV. The only ionic products observed were K^{+}, Cs^{+}, OH^{−}, and OD^{−}. The positive and negative ion thresholds are equal to within ∼0.1 eV and correspond to the H(^{2} S)+OH^{−}(^{1}Σ) limit of H_{2}O^{−}. The positive and negative ion energy and angular distributions are discussed including the observation of an OD^{−} ’’pickup’’ reaction.