Volume 96, Issue 5, 01 March 1992
Index of content:
96(1992); http://dx.doi.org/10.1063/1.461937View Description Hide Description
The vibrational band of H2 and the first four rotational bands of H2 dissolved in ice have been observed by Raman spectroscopy. The presence of ice is indicated by its Raman spectrum. The H2 vibrational band is shifted to higher frequency than in the gas phase. The rotational bands show small shifts in frequency and widths that are relatively constant with changing J.
96(1992); http://dx.doi.org/10.1063/1.461938View Description Hide Description
A differential equation for the polarizability of a small, continuum, spherically symmetric medium is developed in the local‐dielectric‐response approximation. The dynamic polarizability of a small metal sphere is then calculated through this equation using the Drude dielectric expression with the spatial dispersion of the free electron density given by the results of self‐consistent density functional calculations on jellium. This approach is used to examine the size dependence of the plasmon resonance absorption of small metal particles. It is able to account for the observed broadening and peak frequency shifts, both ‘‘red’’ and ‘‘blue,’’ as well as the additional absorption feature seen for small particles. To the extent that comparison with experimental data is possible, good agreement is found.
96(1992); http://dx.doi.org/10.1063/1.461939View Description Hide Description
Model calculations are performed on the amide‐I infrared (ir) bands of globular proteins by assigning one oscillator with a transition dipole to each peptide group. Coupling between these oscillators is introduced through the transition dipole coupling mechanism. As examples of application of the model, the ir spectra in the amide‐I region of eight representative proteins, viz., carbonmonoxy myoglobin, ribonuclease A, α‐lactalbumin, lysozyme, flavodoxin, carboxypeptidase A, concanavalin A, and β‐trypsin are calculated. Good agreement is obtained between the calculated and observed amide‐I band envelopes. Some structure‐spectrum correlations are discussed on the basis of the model calculations. The presence of bands with significant ir intensities for myoglobin in the region below 1640 cm−1 is consistent with its x‐ray structure having no β sheet. Analysis of the contributions of β sheets to the amide‐I band envelopes shows that parallel, antiparallel, and mixed parallel/antiparallel β sheets give rise to strong ir bands at a similar position in the wave number region below 1650 cm−1, and that no band in the region above 1650 cm−1 can be regarded as a reliable marker of antiparallel β sheets. The contributions of non‐α‐non‐β parts spread over a wide wave number region. The differences in the amide‐I band envelopes between α‐lactalbumin and lysozyme originate most probably from the structural differences between the α‐helical parts near the N termini of these proteins.
96(1992); http://dx.doi.org/10.1063/1.461940View Description Hide Description
Resonant two‐photon ionization (R2PI) time‐of‐flight mass spectroscopy is used to record S 0–S 1spectra of the neutral complexes C6H6–H2O, C6H6 –HDO, C6H6–D2O, C6H6–(H2O)2, and C6H6–(D2O)2. In C6H6–H2O, the lack of an S 0–S 1 origin transition and the presence of a splitting at 61 0 (which is absent in C6H6 –HDO) provide vibronic level evidence that the water molecule is on the sixfold axis undergoing internal rotation about that axis. Rotational band contour analysis of the 61 0 transitions of the isotopomers confirms this picture and also determines a ground state center‐of‐mass separation between C6H6 and D2O of 3.32±0.07 Å, very close to that predicted by a b i n i t i o calculations. R2PI scans of the van der Waals structure in the isotopic series C6H6–H2O, C6H6 –HDO, and C6H6–D2O provide tentative assignments for three of the six van der Waals modes in the complex. In C6H6–(H2O)2, rotational band contour analysis of the origin transition provides a best‐fit structure in which the two water molecules reside on the same side of the benzene ring at a H2O–H2O separation close to that in the free water dimer.
Qualitatively, the structure of the 1:2 cluster is thus one which maximizes the strength of the water–water hydrogen bond at the expense of a somewhat poorer interaction of the second water molecule with the benzene ring in an off‐axis geometry. Several intriguing features of the structure are suggested by our analysis, but are near the limit of our ability to distinguish from band contour fitting. Among these features are (i) the on‐axis water molecule is pulled slightly in toward the ring from that in the 1:1 complex; (ii) the water dimer prefers an orientation bisecting a C–C bond in the benzene ring; (iii) the water–water separation is ∼0.2 Å less than that in the free water dimer; and (iv) the water dimer axis is tilted by about 10° relative to the plane of the benzene ring. Finally, the van der Waals structure in C6H6–(H2O)2 and C6H6–(D2O)2 suggests the possibility of large amplitude motion in these complexes as well. We postulate that this motion involves a hindered rotation of the on‐axis water molecule.
Multiphoton ionization studies of clusters of immiscible liquids. II. C6H6–(H2O) n , n=3–8 and (C6H6)2–(H2O)1,296(1992); http://dx.doi.org/10.1063/1.461941View Description Hide Description
Resonant two‐photon ionization (R2PI) time‐of‐flight mass spectroscopy is used to record S 0–S 1spectra of the neutral complexes C6H6–(H2O) n with n=3–8 and (C6H6)2–(H2O)1,2. Due to limitations imposed by the size of these clusters, a number of vibronic level arguments are used to constrain the gross features of the geometries of these clusters. Among the spectral clues provided by the data are the frequency shifts of the transitions, their van der Waals structure, the fragmentation of the photoionized clusters, and the complexation‐induced origin intensity and 61 0 splitting. In the 1:3 cluster, simple arguments are made based on the known structures of the 1:1 and 1:2 clusters which lead to the conclusion that all three water molecules reside on the same side of the benzene ring.
Three structures for the 1:3 cluster are proposed which are consistent with the available data. Of these, only one is also consistent with the remarkable similarity of the 1:4 and 1:5 spectra to those of the 1:3 cluster. This structure involves a cyclic water trimer in which one of the water molecules is near the sixfold axis in a π hydrogen‐bonded configuration. This structure is then expanded in the 1:4 and 1:5 clusters to incorporate the fourth and fifth water molecules in cyclic structures which place the additional water molecules far from the benzene ring without disturbing the interaction of the other water molecules with the benzene ring. For 1:n clusters with n≥6, subtle and then significant changes are observed in the spectra which indicate changes in the way the water cluster interacts with the benzene ring. This development occurs at precisely the water cluster size which calculations predict that cagelike water cluster structures will begin to compete and eventually be favored over large cyclic structures. Finally, cursory scans of the 2:1 cluster show that this cluster also fragments efficiently upon photoionization by loss of a single water molecule and that it possesses a distinctly asymmetric structure with a sizable 61 0 splitting.
Two potential structures are proposed for the 2:1 cluster which can both reasonably explain the observations—a BBW structure in which the water molecule (W) is π hydrogen bonded to a single benzene molecule (B) and a BWB structure in which the water molecule bridges the two benzenes via π hydrogen bonds to both.
96(1992); http://dx.doi.org/10.1063/1.461942View Description Hide Description
The gas‐phase infrared spectrum of the BH3 ν3 band was observed in the 2600 cm−1 region by a Fourier transforminfrared spectrometer. The borane molecule was produced with a discharge in a B2H6 and He mixture. The spectral pattern was consistent with the D 3h structure of borane. The observed spectrum between 2450–2730 cm−1 was analyzed to determine band origins, rotational constants, and centrifugal distortion constants. The ν3 band origins of 11BH3 and 10BH3 were 2601.5743(15) and 2615.7935(21) cm−1, respectively, with one standard deviation in parentheses. The B–H bond length was determined for the first time experimentally to be r 0=1.190 01(1) Å from the ground state rotational constantB 0.
An experimental study of collision broadening of rotational spectral lines of 13CH3 13C15N in the ground and nν8, n=1 and 2 vibrationally excited states at microwave frequencies96(1992); http://dx.doi.org/10.1063/1.461943View Description Hide Description
A double modulation microwave spectrometer was used to determine the linewidth parameter for some rotational components in the nν8 vibrations of methyl cyanide under 13C and 15N substitution. The linewidth parameters for self‐broadening of the ΔJ=2←1 rotational components for the ground, ν8, and 2ν8 vibrations were determined over a pressure range of 1–13 mTorr and at a temperature of 300 K. An experimental method is presented to correct for modulation broadening when high derivatives are used to extract the absorption profile information from the signals. The eighth derivative profile was explored to determine if the spectralline shape remains Lorentzian over a range of modulation levels from 10% of Δν to more than 100% of Δν. These tests showed that the ratio of inner to next inner peak separations, designated in this paper as Δ, of the eighth derivative was the same as that for an assumed Lorentzian line shape. Thus, line shapes were assumed to be Lorentzian for theoretical analysis of the derivative profiles and comparisons made between experiment and theory on that basis. Dipole moments for vibrationally excited energy levels for the ν8 and 2ν8 vibrations were calculated from the linewidth parameter data after all corrections had been made for modulation, Doppler, and other nonpressure broadening effects.
96(1992); http://dx.doi.org/10.1063/1.461944View Description Hide Description
The vibrational analysis of the X 2(3/2)–X 1(1/2) system of BiO in the near infrared region, 6400–7700 cm−1, has been extended. Transitions in the Δv=−1, 0, and +1 sequences have been observed and assigned up to v=6 in the upper state and no transitions have been observed with v’>6. The Δv=−1 sequence is shown to form a head of heads. The upper state appears to be strongly perturbed, especially at v=6.
The optimization of single mode basis functions for polyatomic vibrational problems with application to the water molecule96(1992); http://dx.doi.org/10.1063/1.461945View Description Hide Description
We consider the optimization of the wave functions for coupled vibrations represented by linear combinations of products of functions depending only on a single vibrational coordinate. The functions themselves are optimized as well as the configuration list. For the H2O molecule, highly accurate results are obtained for the lowest 15 levels using significantly shorter expansions than would otherwise be possible.
The NH stretch in pyrrole: A study of the fundamental (Δv=1) and third overtone (Δv=4) bands in the bulk gas and in a molecular beam96(1992); http://dx.doi.org/10.1063/1.461946View Description Hide Description
We have examined the NH stretching vibration in pyrrole both in the infrared and visible regions of the spectrum. Three techniques were used—direct absorption spectroscopy, intracavity photoacoustic spectroscopy, and optothermal molecular beam spectroscopy. In the bulk gas, the Δv=1 transition is composed of a well‐resolved manifold of subbands. Molecular beamspectra of a number of those subbands reveal detailed structure due to single state‐to‐state transitions. An asymmetric rotor model was found to provide an adequate description of the spectra and a complete set of rotational parameters has been determined. Little evidence of rotational perturbations was observed in the Δv=1 spectrum. The bulk gas Δv=4 overtone band is composed of three vibrational transitions, whose rotational contours overlap. These transitions involve Fermi resonance among assignable NH and CH stretching excited states. Attempts to detect Δv=4 overtone transitions on the molecular beam failed, in spite of the large anticipated signal‐to‐noise ratio calculated by extrapolation from the Δv=1 data. This negative result is a likely consequence of extensive level mixing in the Δv=4 region.
96(1992); http://dx.doi.org/10.1063/1.461947View Description Hide Description
Rotational spectra of several isotopic species of the SiF4–NH3 dimer were obtained with the Mark II Flygare–Balle FT‐MW spectrometer. This is the first determination of the gas phase structure of a penta‐coordinated silicon. The spectra indicate a symmetric top, trigonal bipyramid SiF3N structure with the lone electron pair of the NH3 pointed at one face of the SiF4 and the three equatorial F’s splayed significantly away from the N. The ∠Fax–Si–Feq is about 12° less than tetrahedral. The Si–N distance is determined to be 2.090 Å. The experimental results are in excellent agreement with the SiF4 deformation predicted by the a b i n i t i o calculations of Rossi and Jasinski [Chem. Phys. Lett. 1 6 9, 399 (1990)]. The measured dipole moment is 5.61 D, an enormous enhancement compared to the sum of the monomer moments, 1.47 D. The increase of 4.14 D is due largely to the geometric distortion of the tetrahedral SiF4 molecule upon dimer formation, demonstrating that the Si–F bond is much more ionic than covalent. If the charge is simply partitioned between Si and F so as to obtain a moment of 4.14 D, the Si has a charge of +3.52 and each F, −0.88. This conclusion is consistent with recent applications of the atoms‐in‐molecules charge partition methodology developed by Bader and co‐workers.
96(1992); http://dx.doi.org/10.1063/1.461948View Description Hide Description
The dimerization of SiF4 and NH3 is accompanied by a substantial 1.63 a.u. increase in dipole moment. Theoretical analysis of the charge distribution of the SiF4–NH3 system using the theory of atoms in molecules shows that the origin of this increase is primarily the large geometrical change which occurs within the SiF4 molecule when the dimer is formed, rather than the changes in atomic polarization or interatomic or intermolecular electron transfer. The relative displacement of the highly charged fluorine atoms of SiF4 away from NH3 upon dimer formation accounts for nearly 90% of the dipole moment enhancement. This result is in agreement with predictions made recently based on experimental results.
Application of Floquet theory to the nuclear magnetic resonance spectra of homonuclear two‐spin systems in rotating solids96(1992); http://dx.doi.org/10.1063/1.461949View Description Hide Description
The theory for the nuclear magnetic resonance(NMR) spectra of homonuclear two‐spin systems under sample spinning has been developed. The propagator describing the time‐evolution of the systems, which is driven by the homogeneous Hamiltonian composed of the chemical‐shift difference and the flip–flop parts of the dipolar and J couplings, is treated using Floquet theory; the Floquet eigenvalues and eigenvector components determine the resonance frequencies and intensities, respectively. Nondegenerate Rayleigh–Schrödinger perturbation theory is used to solve the Floquet secular equation. Recurrence equations for the perturbation corrections have been derived, allowing us to evaluate efficiently very high‐order terms, such as 20th‐order terms. Analytical expressions for the resonance frequency obtained in the low‐order approximations provide an intuitive understanding of the main spectral features; the second‐order equations can describe the conspicuous solid‐state effects on the magic‐angle spinning (MAS) spectra, such as the unique line shapes and the additional shifts, while the zeroth and first‐order approximations yield spectra similar to those in solution‐state NMR. The spectra should, however, be calculated up to the perturbation order where line shape function converges, to reproduce precisely the experimental results, especially the change of the additional shifts with the spinning frequency which were obtained in the MAS spectra for 13C doubly‐labeled sodium acetate. The theoretical treatment shows that off‐magic angle spinning (OMAS) spectra of two‐spin systems should exhibit line shapes dependent on the mutual orientations of the dipolar and the chemical‐shift tensors of the two nuclei. Also, the nondegenerate perturbation treatment yields criteria for the conditions under which the normal and n=0 rotational resonance occur.
Population transfer by stimulated Raman scattering with delayed pulses: Analytical results for multilevel systems96(1992); http://dx.doi.org/10.1063/1.461900View Description Hide Description
Stimulated Raman scattering involving adiabatic passage (STIRAP) is a proven technique for population transfer in three‐level systems with strong oscillator strengths. We show that the STIRAP process should also yield high transfer efficiencies when the densities of states near the intermediate and final levels are high, provided certain criteria for the experimentally controllable parameters are met. We also show that high transfer efficiencies may even be possible when the pump and Stokes lasers can access levels outside of the three‐level system. The criteria are derived with an approach that emphasizes the adiabatic nature of the transfer process in an eigenvector of the interaction Hamiltonian that resembles a trapped state. The results are compared to density matrix calculations.
96(1992); http://dx.doi.org/10.1063/1.461901View Description Hide Description
The high resolution electronic spectrum of Ne⋅OH has been recorded in a supersonic free jet expansion using the laser‐induced fluorescence technique. From an analysis of the spectrum which yields rotational constants, we are able to obtain Ne⋅OH bond lengths in several vibrational (hindered rotor) levels of the excited state and the vibrationless level of the ground state. We also measure the Fermi contact constant in the Ã state which is, unlike Ar⋅OH, insignificantly perturbed from the value in the OH monomer. However, we now measure a parity doubling of the X̃ state rotational levels which is tenfold larger than the upper limit we established for such an interaction in Ar⋅OH. We interpret these latter measurements to imply weaker and more isotropic bonding in Ne⋅OH compared to Ar⋅OH in both electronic states.
96(1992); http://dx.doi.org/10.1063/1.461902View Description Hide Description
High‐resolution, fast optical hole‐burning results are reported for the amorphous system cresyl violet in ethanolglass at 1.3 K. Holes are burned and detected using a novel technique which allows precise detection of narrow (∼0.03 cm−1 ), shallow (∼1%) holes 10 μs to 50 ms after their generation. The technique is described in detail along with careful tests demonstrating the validity of its results. The hole width is observed to increase linearly with time when plotted against log time. Using the four time correlation function description of optical hole burning, the time‐dependent increase in hole width (spectral diffusion) is shown to arise from a broad distribution of fluctuation rates in the glass with the probability of having a fluctuation at rate R proportional to 1/R. The 10 μs to 50 ms data is combined with hole‐width data spanning the range 100 ms to 10 000 s and with two‐pulse picosecond photon echo data. The two‐pulse photon echolinewidth is calculated by extrapolating the fluctuation rate distribution obtained from the hole‐width data to short times. The results are in excellent agreement with experimental echo results. The combined data from the two sets of hole‐burning experiments provides a detailed description of the glass dynamics over nine decades of time, 10 000 s to 10 μs. Together with the two‐pulse photon echo results, the data provide information on the glass dynamical behavior over seven decades faster in time as well. The net result is a description of the dynamics in low‐temperature ethanolglass on time scales spanning 16 decades.
96(1992); http://dx.doi.org/10.1063/1.461903View Description Hide Description
We use a simple model to study the color change taking place when sodium atoms are absorbed in the zeolitesodalite. The Hamiltonian is that of an electron moving in the electrostatic field created by the ions in the zeolite framework and by the alkali ion core. We examine the sensitivity of the absorptionspectrum on the magnitude of framework charges, the orientation of the Na4 cluster in the sodalite cells, the localization of the electron, the nature of the alkali impurity (Li, Na, K), and the laser polarization. Comparisons with the experiment help decide which framework charge models are consistent with the absorptionspectrum.
96(1992); http://dx.doi.org/10.1063/1.461904View Description Hide Description
Diagonal as well as off‐diagonal spin–orbit matrix elements were calculated for the ground state and a large number of excited states of CuO. They are based on previously calculated CI wave functions and energies. The spin–orbit splitting of the ground stateX 2Π is analyzed in detail. The excitation energies for a large number of excited states are predicted, for Ω quantum numbers 1/2, 3/2, 5/2, and 7/2. They are compared with experimentally observed states. The results suggest a new designation of the ‘‘δ 2Σ− ’’ state as 4Π contaminated by spin–orbit effects. Also ‘‘k 4Σ− ’’ is most likely the Ω=1/2 component of 2 4Π.
96(1992); http://dx.doi.org/10.1063/1.461905View Description Hide Description
Values and signs of Landè factors (g) have been measured for four rovibronic levels of the A 1Σ+ u state of the Na2 molecule. Because of a very small product gτ∼10−12 s, the level crossing signal of a dispersion shape was employed using circular light polarization and mutually orthogonal excitation, observation of laser induced fluorescence, and external magnetic field directions. The effects of the b 3Π u , B 1Π u , and a 3Σ+ u states on the g factors of the Na2(A 1Σ+ u ), as well as their dependence on vibrational and rotational quantum numbers, have been theoretically analyzed. An analytical connection has been found between the constants of Λ doubling (q) and Landè factors of the interacting singlet states. Simple expressions have been found for estimating the effects of distant electronic states on q and the g factors without summing over bounded and integrating over continuum levels of the perturbing state.
96(1992); http://dx.doi.org/10.1063/1.461906View Description Hide Description
The effects of features in the potential energy surface on the collinear H+H2reaction rate coefficient are investigated by the method of quantum functional sensitivity analysis (QFSA). The calculations use QFSA to connect features in the microscopic realm, with their response upon macroscopic quantities of chemical interest, via the intermediary sensitivities of the reactive transition probabilities. While the sensitivities of the individual transition probabilities show considerable structure, there is an attendant loss of structure in the rate coefficient sensitivities because of the thermal averaging. For the range of temperatures used in our study (200–2400 K), the most important region of the potential energy surface is found to be not at the top of the barrier, but rather at the lower energy shoulders of the barrier. There are also regions near the barrier where an increase in the potential surface actually increases the reaction rate! The effects of using different underlying potentials [the Porter–Karplus (PK2), Liu–Siegbahn–Truhlar–Horowitz (LSTH), and double many‐body expansion (DMBE) surfaces] on the nature of the results were also compared. The absolute sensitivity magnitudes on the PK2 surface vary considerably from the other two, but the relative change in the rate coefficient is about the same on all three surfaces. Furthermore, the identified regions of importance on the potential surfaces remain essentially the same. The reactive scattering calculations were performed with the log‐derivative version of the Kohn variational principle.