Volume 103, Issue 10, 08 September 1995
Index of content:

Pulsed‐nozzle Fourier‐transform microwave investigation of the large‐amplitude motions in HBr–CO_{2}
View Description Hide DescriptionMicrowave spectra of H^{79}Br–CO_{2} and H^{81}Br–CO_{2} and their D and ^{18}O isotopomers have been measured using a pulsed‐nozzle Fourier‐transform microwave spectrometer. The spectra are consistent with a T‐shaped Br–CO_{2} geometry, as concluded previously by Zeng et al. [Y. P. Zeng, S. W. Sharpe, S. K. Shin, C. Wittig, and R. A. Beaudet, J. Chem. Phys. 97, 5392 (1992)] from an investigation of the rotationally resolved infrared spectrum of the asymmetric C=O stretching vibration of the complex. Only b‐type K _{ a }=1←0 transitions are observed, with the symmetry‐allowed a‐type ΔK _{ a }=0 transitions being too weak to be detected. The absence of a strong a‐type spectrum implies that the HBr axis is nearly parallel to the b‐inertial axis of the complex, which itself is parallel to the C_{∞} axis of the CO_{2}. The K _{ a }=1←0 energy level spacing is approximately 1.2 GHz larger than that predicted from the infrared rotational constants due to an additional contribution to the splitting arising from the hindered‐rotation tunneling of the HBr through a C_{ s } or C_{2v } transition state. Because the Bose–Einstein statistics of the spin‐zero oxygen nuclei allow only symmetric tunneling states for K _{ a } even and antisymmetric tunneling states for K _{ a } odd, no doubling of the lines is observed. No evidence was obtained for this tunneling motion in the infrared spectrum of Zeng et al., since the tunneling state selection rules are symmetric↔symmetric and antisymmetric↔antisymmetric for the band studied. A dynamical modeling of the ^{79}Br and ^{81}Br nuclear quadrupole coupling constants gives an equilibrium ∠CBrH angle of ∼103° and an HBr zero‐point bending amplitude of ∼24°. The implication of this study on the interpretation of experiments on the photoinitiated reaction of H atoms with CO_{2} using an HBr–CO_{2} precursor are discussed.

Ab initio studies of the nuclear magnetic resonance chemical shifts of a rare gas atom in a zeolite
View Description Hide DescriptionThe intermolecular chemical shift of a rare gas atom inside a zeolite cavity is calculated by ab initio analytical derivative theory using gauge‐including atomic orbitals (GIAO) at the Ar atom and the atoms of selected neutral clusters each of which is a 4‐, 6‐, or 8‐ring fragment of the zeolite cage. The Si, Al, O atoms and the charge‐balancing counterions (Na^{+}, K^{+}, Ca^{2+}) of the clusters (from 24 to 52 atoms) are at coordinates taken from the refined single crystal x‐ray structure of the NaA, KA, and CaAzeolites. Terminating OH groups place the H atom at an appropriate O–H distance along the bond to the next Si or Al atom in the crystal. The chemical shift of the Ar atom located at various positions relative to the cluster is calculated using Boys–Bernardi counterpoise correction at each position. The dependence of the rare gas atom chemical shift on the Al/Si ratio of the clusters is investigated. The resulting shielding values are fitted to a pairwise additive form to elicit effective individual Ar–O, Ar–Na, Ar–K, Ar–Ca intermolecular shielding functions of the form σ(^{39}Ar, Ar...O_{zeol})= a _{6} r ^{−6}+a _{8} r ^{−8}+a _{10} r ^{−10}+a _{12} r ^{−12}, where r is the distance between the Ar and the O atom. A similar form is used for the counterions. The dependence of the Ar shielding on the Al/Si ratio is established (the greater the Al content, the higher the Ar chemical shift), which is in agreement with the few experimental cases where the dependence of the ^{129}Xe chemical shift on the Al/Si ratio of the zeolite has been observed.

Electronic spectra of jet‐cooled tropolone–M_{ n } (n=1,2) clusters. Microscopic solvent effects on proton tunneling in the S _{1} state
View Description Hide DescriptionThe S _{1}←S _{0}fluorescence excitation spectra of jet‐cooled tropolone (TRN)–M_{ n } (M=Ar, Kr, Xe, N_{2}, CH_{4}/CD_{4}, C_{2}H_{6}, C_{3}H_{8}, CCl_{4}; n=1,2) clusters have been measured in the wavelength region near the electronic origin to investigate the effects of van der Waals interactions on protontunneling in the electronic excited S _{1} state. The solvation of TRN with the rare gas atom(s) has small effect on the 0^{0} _{0}tunneling splitting, while the solvation with the molecule(s) considerably decreases the tunneling splitting. The decrease in the tunneling splittings of the TRN clusters has been explained by strong coupling of intermolecular vibration with intramolecular vibration of TRN, increasing the effective potential barrier height and/or tunneling distance. The anisotropy in the intermolecular interactions, and the configuration and number of solvent molecules are suggested to be important factors for the changes in the tunneling splitting.

High‐pressure study of the far‐infrared collision‐induced absorption of nondipolar liquids
View Description Hide DescriptionFar‐infrared collision‐induced absorption of liquid benzene, hexafluorobenzene, and 1,3,5‐trifluoro‐ benzene were measured at pressures and room temperature with a synchrotron radiationsource. The zeroth and second spectral moments of the absorption bands were obtained as a function of pressure. The collision‐induced dipole moments and their time derivatives were obtained from the spectral moments for these liquids and previously measuredcarbon disulfide and carbon tetrachloride. The induced‐dipole moments decrease with increasing pressure for the liquids studied except carbon disulfide for which the moment is nearly independent of pressure. This decrease in the moments is probably caused by increase in the cancellation effect of three‐molecule correlation with an increase in pressure. The time derivatives of the collision‐induced moments, on the other hand, increase with increasing pressure. This is explained by increase in collision frequencies with pressure.

Zero kinetic energy proton and deuteron production from photoionization of H_{2} and D_{2}
View Description Hide DescriptionA zero ion kinetic energy spectrometer has been developed to study the production of near zero energy protons and deuterons from dissociativephotoionization of H_{2} and D_{2}. Both H^{+} and D^{+} spectra show four peaks on top of a continuum. The continuum was found to be in excellent agreement with the single center Coulomb calculation for the direct dissociation through the X ^{2}Σ^{+} _{ g } state of H^{+} _{2}. The observed structures were shown to originate from autoionization of the doubly excited Q _{1} ^{1}Σ^{+} _{ g }(1), Q _{1} ^{1}Σ^{+} _{ u }(1), Q _{1} ^{1}Σ^{+} _{ u }(2), and Q _{2} ^{1}Σ^{+} _{ u }(1) states, of which the Q _{1} ^{1}Σ^{+} _{ g }(1) state is dipole forbidden.

Rotational spectra and structures of the C_{6}H_{6}–HCN dimer and Ar_{3}–HCN tetramer
View Description Hide DescriptionA comparative study has been made of the rotational properties of C_{6}H_{6}–HCN and Ar_{3}–HCN, observed with the Balle/Flygare pulsed beam, Fourier transformmicrowave spectrometer. C_{6}H_{6}–HCN is found to be a prolate symmetric top and Ar_{3}–HCN an oblate one, both with the H in the middle. The rotational constantsB _{0}, D _{ J }, and D _{ JK } of the parent species are 1219.9108(4) MHz, 1.12(3) kHz, and 18.32(8) kHz for C_{6}H_{6}–HCN, and 886.4878(1) MHz, 10.374(2) kHz, and 173.16(1) kHz for Ar_{3}–HCN. Rotational constants are reported for the isotopic species C_{6}H_{6}–H^{13}CN, ‐HC^{15}N, and ^{13}CC_{5}H_{6}–HC^{15}N, and for Ar_{3}–HC^{15}N and ‐DCN. Analysis of the ^{14}N hyperfineinteraction χ finds its projection on the figure axis to be −4.223(4) MHz in C_{6}H_{6}–HCN and −1.143(2) in Ar_{3}–HCN. They correspond to average projection angles θ between the HCN and figure axes of 15.2° and 45.3°, respectively. A pseudodiatomic analysis of the rotational constants gives the c.m. to c.m. distance to be 3.96 Å in C_{6}H_{6}–HCN and 3.47 Å in Ar_{3}–HCN. While the rotational properties of C_{6}H_{6}–HCN are ‘‘normal,’’ those of Ar_{3}–HCN display a long list of ‘‘abnormalities.’’
They include a J‐dependent χ(^{14}N) similar to that of Ar–HCN; a very large projection angle θ; large centrifugal distortion including higher‐order terms in H_{ J } and H_{ JK }; splitting of the K=3 transitions into J‐dependent doublets; and the ready observation of an excited vibrational state. These behavioral differences are related qualitatively to the interactionsurfaces for the two clusters, calculated with the molecular mechanics for clusters (MMC) model, and discussed. The potential minimum for C_{6}H_{6}–HCN is smooth, circular, steep except for a flat bottom, and deep (1762 cm^{−1}). That for Ar_{3}–HCN is tricuspid, with large gullies, and shallow (507 cm^{−1}). In addition to the dispersion forces, the dominant interaction forming C_{6}H_{6}–HCN is between the benzene quadrupole moment and the HCN dipole moment, a strong 4‐2 potential. That in Ar_{3}–HCN is polarization of the spherical Ar by the HCN dipole and quadrupole moments, a weak 0–2,4 potential.

Infrared emission spectra of C_{3}: the Renner effect in the ã^{3}Π_{ u } and b̃^{3}Π_{ g } electronic states
View Description Hide DescriptionFive new triplet bands of ^{12}C_{3}, and three corresponding bands of ^{13}C_{3}, have been observed in emission between 6000 and 6600 cm^{−1} with a Fourier transformspectrometer. Rotational analysis shows that these bands arise from excited bending vibrations of the b̃^{3}Π_{ g }→ã^{3}Π_{ u } electronic transition, and that all components of the (010) vibrational level of the b̃^{3}Π_{ g } state exhibit unusual perturbations. Renner parameters for both electronic states have been extracted from an analysis of the spin‐orbit parameters of the (000) ^{3}Π, (010) ^{3}Δ, (020) ^{3}Φ, and (030) ^{3}Γ levels. The Renner effect is shown to be large in both electronic states (ε′ = +0.447, ε″ = +0.566). Approximate bending vibrational frequencies are also obtained (ω_{2} ^{′}∼345 cm^{−1}, ω_{2} ^{″}∼505 cm^{−1}).

Ab initio calculation of rovibronic transition spectra of CaH
View Description Hide DescriptionThe ground (1 ^{2}Σ^{+},X) and excited electronic states, 1 ^{2}Π (A), 2 ^{2}Σ^{+} (B,B′), 1 ^{2}Δ, 2 ^{2}Π (E), 3 ^{2}Σ^{+} (D), 4 ^{2}Σ^{+} (C), 5 ^{2}Σ^{+} (K), 3 ^{2}Π (L), and 2 ^{2}Δ, of CaH are studied by ab initio calculation, with extensive configuration interactions. Calculated spectroscopic constants, in particular, the excitation energies, are more accurate than in previous calculations and agree well with the currently available experimental data. This work confirms the existence of a single vibrational level, v′=4, bound to the second well of the B state calculated from the experimental data by Martin [J. Chem. Phys. 88, 1797 (1988)]. Calculated oscillator strengths for the B–X and D–X show irregular band structures because of multiple avoided crossings of the potential energy curves. The avoided crossing between the D and C states should be taken into account to explain the apparent perturbations observed in the experimental spectra. The wave functions,dipole moments, and transition dipole moments of these states are discussed in detail.

Pressure‐induced d–π charge transfer in one‐dimensional phthalocyanine conductors, NiPc(AsF_{6})_{0.5} and CoPc(AsF_{6})_{0.5}
View Description Hide DescriptionPressure dependence of optical absorptionspectra were measured for (phthalocyaninato)‐nickel hexafluoroarsenate, NiPc(AsF_{6})_{0.5} and (phthalocyaninato)‐cobalt hexafluororarsenate, CoPc(AsF_{6})_{0.5}. We found a charge transfer from the central metal ion to the macrocycle at 0.5 GPa in NiPc(AsF_{6})_{0.5} and 1.1 GPa in CoPc(AsF_{6})_{0.5}. According to the electron‐filling process in the π‐conduction band, a metal–insulator transition and an evolution of the energy gap was observed in NiPc(AsF_{6})_{0.5}. On the other hand, nonmetallic CoPc(AsF_{6})_{0.5} showed a continuous change.

Free jet infrared spectroscopy of SiF_{4}‐rare gas complexes
View Description Hide DescriptionThe rotation‐vibration spectra of ^{28}SiF_{4}‐Ar and ^{28}SiF_{4}‐^{84}Kr complexes have been studied in the 10 μm region. The triply degenerate ν_{3} (F _{2}) band of SiF_{4} reduces to nearly degenerate parallel (A) and perpendicular (E) band of the complex with the C _{3v } structure. The two bands coupled by an A−ECoriolis interaction have been analyzed simultaneously. No splitting due to internal motion has been observed. Band shifts by the formation of the complex are discussed in conjunction with the magnitudes of A−E splitting.

Coherent isotropic averaging in zero‐field nuclear magnetic resonance. I. General theory and icosahedral sequences
View Description Hide DescriptionWe present a general theory of coherent isotropic averaging in nuclear magnetic resonance(NMR). In a zero external field, magnetic‐field pulses can selectively average the internal spin Hamiltonians, while preserving the intrinsic invariance of the spectrum with respect to the sample orientation. The theory predicts the limits of the scaling factors for tensor interactions of different ranks. Time reversal is found to be possible for first‐ and second‐rank tensors with scaling factors of −1/3 and −1/4, respectively. Explicit sequences, based on icosahedral symmetry, are given for a number of optimal scaling factors. To illustrate the theory, an experiment is also presented in the special case of rank‐selective decoupling. As in high‐field NMR, applications can be expected from the introduction of coherent averaging schemes for zero‐field techniques: for example, decouplings (by rank or nuclear species), time reversal, and multipolar experiments (zero‐field analog of multiple‐quantum NMR).

Coherent isotropic averaging in zero‐field nuclear magnetic resonance. II. Cubic sequences and time‐reversal of spin couplings
View Description Hide DescriptionWe present a special case of the theory of coherent isotropic averaging in zero‐field NMR, given in part I of this work. In a zero external field, combinations of the magnetic‐field pulses restricted to π/2 rotations along the three coordinate axes can selectively average internal spin Hamiltonians while preserving the intrinsic invariance of the spectrum with respect to the sample orientation. Compared with the general case, the limits of the allowed scaling factors of first‐ and second‐rank interactions are slightly reduced. For instance, time reversal is possible for second‐rank tensors with a −1/5 scaling factor, instead of −1/4 in general. Finite pulse compensations are analyzed and experimental illustrations are given for two optimum time‐reversal sequences. The cubic sequences, though less efficient than the icosahedral sequences, are technically more feasible and may be used in zero‐field experiments such as decoupling (by rank or nuclear species), time reversal or multipolar experiments (the zero‐field equivalent of multiple‐quantum NMR).

Nonperturbative approach to femtosecond spectroscopy: General theory and application to multidimensional nonadiabatic photoisomerization processes
View Description Hide DescriptionA general nonperturbative approach to calculate femtosecond pump‐probe (PP) signals is proposed, which treats both the intramolecular couplings and the field‐matter interaction (numerically) exactly. Experimentally as well as in a perturbative calculation it is straightforward to distinguish between different spectroscopic processes through the direction of the wave vector of the emitted radiation. A nonperturbative calculation, on the other hand, yields the overallpolarization of the system, which is the sum of all these contributions. We present a general and practical method that allows to extract the individual spectroscopic signals, which are resolved in time, frequency, and direction of the emission, from the overall polarization. We briefly derive the basic expressions for the time‐ and frequency‐resolved PP signals under consideration, and discuss in detail the simplifications that arise when the usual assumptions (i.e., weak laser fields, nonoverlapping pulses, slowly‐varying envelope assumption and rotating‐wave approximation) are invoked. The computational procedure is illustrated by nonperturbative calculations of the polarizations and PP signals for a one‐dimensional shifted harmonic oscillator. To demonstrate the capability of the approach we have evaluated the polarization as well as PP signals for a three‐dimensional model system with vibronically coupled potential‐energy surfaces, which describes ultrafast nonadiabatic isomerization dynamics triggered by the twisting of a double bond. We consider various wavelengths and pulse durations of the laser fields and study integral and dispersed PP spectra as well as coherent photon‐echo signals. It is shown that the time‐ and frequency‐resolved PP signals reflect in real time the disappearance of the reactants and the delayed appearance of the products.

Theoretical prediction of the structure and infrared spectrum of the molecule–ion complexes NH_{3}–H^{−}, NH_{3}–D^{−}, and ND_{3}–H^{−}
View Description Hide DescriptionThe potential energy and dipole moment functions of the molecule–ion complex NH_{3}–H^{−}, and its isotopomers, NH_{3}–D^{−} and ND_{3}–H^{−}, have been calculated by the CEPA‐2 method. From these functions we have computed the vibration–rotation–inversion states for J=0 and J=1, and the rovibrational transition probabilities between them. The complexes are found to have a nearly rigid structure in the ground state, with the H^{−} or D^{−} ion localized near a hydrogen (or deuterium) atom of the ammonia, and a small probability of tunneling between the three equivalent equilibrium positions. For the vibrationally excited states, however, the probability of this threefold tunneling increases considerably. The umbrella inversion of the ammonia is nearly quenched by the presence of the ion. The character of the rovibrational excitations is determined, and is found to be affected by the isotope substitutions. In order to investigate whether it is possible to evaluate the rovibrational transition probabilities for other total J transitions from the present calculations, an approximate expression for the vibrational transition probabilities has been derived. The formula appears to be valid for the ortho species, for the para species it is found to be rather crude.

Infrared absorption of cis‐ and trans‐alkali‐metal peroxynitrites (MOONO, M=Li, Na, and K) in solid argon
View Description Hide DescriptionPotassium nitrate (KNO_{3}) isolated in solid argon at 13 K was irradiated with emission from an ArF excimer laser at 193 nm. Recombination of the photofragments led to formation of both cis‐ and trans‐potassium peroxynitrites (KOONO). The cyclic conformer, cis‐KOONO, absorbs at 1444.5, 952.3, 831.6, and 749.1 cm^{−1}, whereas trans‐KOONO absorbs at 1528.4, 987.4, and 602.2 cm^{−1}. The assignments are based on observed ^{18}O‐ and ^{15}N‐isotopic shifts and comparison with similar compounds, cis–cis and trans–perp HOONO. Ab initio calculations using density functional theory at a Becke3LYP level predicted similar line positions and isotopic shifts for both conformers. Photoconversion among these three isomers was achieved at various wavelengths and periods of irradiation; cis‐KOONO was photolyzed readily at 308 nm, whereas trans‐KOONO increased slightly in intensity initially and was eventually transformed to KNO_{3} on prolonged irradiation. Similar results were obtained for LiNO_{3} and NaNO_{3}; cis‐LiOONO and cis‐NaOONO absorb at (1423.4, 1422.0), 966.2, 874.2, 792.3 cm^{−1} and (1437.4, 1434.6), 961.4, 840.7, (770.9, 768.7) cm^{−1}, respectively, whereas trans‐LiOONO and trans‐NaOONO absorb at (1581.6, 1580.4), (998.3, 995.6), 600.4 cm^{−1} and (1549.3, 1540.6), (996.3, 994.1), (609.4, 607.4) cm^{−1}, respectively; the numbers in parentheses are due to line splitting.

van der Waals binding energies and intermolecular vibrations of carbazole⋅R (R=Ne, Ar, Kr, Xe)
View Description Hide DescriptionMass‐selective ground‐state vibrational spectra of jet‐cooled carbazole⋅R (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating ground‐state intra‐ and intermolecular levels via stimulated emissionpumping, followed by time delayed resonant two‐photon ionization of the vibrationally hot complex. By tuning the dump laser frequency, S _{0} state vibrational modes were accessed from ≊200 cm^{−1} up to the dissociation energyD _{0}. Upon dumping to ground‐state levels above D _{0}, efficient vibrational predissociation of the complexes occurred, allowing us to determine the S _{0} state van der Waals binding energies very accurately. The D _{0}(S _{0}) values are <214.5±0.5 cm^{−1} (R=Ne), 530.4±1.5 cm^{−1} (R=Ar), 687.9±4.0 cm^{−1} (R=Kr), and 890.8±1.6 cm^{−1} (R=Xe). In the S _{1} state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cm^{−1}, 576.3±1.6 cm^{−1}, 756.4±4.5 cm^{−1}, and 995.8±2.5 cm^{−1}, respectively.

Electronic spectroscopy of jet‐cooled iron monocarbide. The ^{3}Δ_{ i }←^{3}Δ_{ i } transition near 493 nm
View Description Hide DescriptionWe report the first gas phase spectroscopic study of iron monocarbide. FeC molecules were generated in a laser vaporization molecular beamsource and detected by laser induced fluorescence. Twenty‐six vibronic bands have been recorded in the 430–500 nm region. Rotational analyses have been carried out for 22 of the bands. There are three lower states involved, two with Ω″=2 and one with Ω″=3. Based on our experimental observations and the ab initio calculations on RuC [Shim et al. J. Phys. Chem. 91, 3171 (1987)] the observed bands are interpreted as ^{3}Δ_{ i }←^{3}Δ_{ i } and ^{1}Δ←^{1}Δ transitions. The ground state electronic configuration is assigned to 1δ^{3}9σ^{1}. Evidence for three excited electronic states (one ^{3}Δ_{ i } state, one ^{1}Δ state and another state with Ω′=3) and for perturbations in several of their vibrational levels is presented.

Determination of multiple diabatic potentials by the inversion of atom–atom scattering data
View Description Hide DescriptionAn inversion algorithm based on first‐order functional sensitivity analysis and Tikhonov regularization is extended for the determination of multiple (diabatic) potentials from elastic and/or inelastic scattering data. Two methods of inversion are presented. In order to illustrate the methods, two‐state and three‐state models of the He^{+}+Ne system are employed in a simulated inversion, the former for the recovery of an entire potential matrix, and the latter to obtain multiple coupling elements, as well as to compare and contrast the two methods against each other. In the case of the two‐state model, good agreement between the recovered and the model potentials is achieved if initial guesses for V _{11}(r) and [V _{22}(r)−V _{22}(∞)] are within ±5% of the model, and those for the coupling element V _{12}(r) are within ±10%, thus indicating that the method may be useful for the simultaneous refining of ab initio calculations and the determination of coupling potentials. For the three‐state case, initial guesses differing from the model coupling potentials by as much as ±50% yield successful inversions using either method, therefore indicating that the procedure may be even more useful for the recovery of multiple coupling potentials.

Three‐dimensional study of predissociation resonances by the complex scaled discrete variable representation method: HCO/DCO
View Description Hide DescriptionPredissociationresonances of the radicals HCO and DCO were calculated using a three‐dimensional (J=0) complex scaled discrete variable representation (DVR) method that was applied previously to a study of the weakly bound van der Waals complex NeICl [Lipkin, Moiseyev, and Leforestier, J. Chem. Phys. 98, 1888 (1993)]. This study represents a first application of the complex scaling method to a full dimensional chemical reactive system described by a fitted ab initiopotential energy surface [Bowman, Bittman, and Harding, J. Chem. Phys. 85, 911 (1986)]. It is shown that the calculation method, being applied to a strongly coupled three‐dimensional system, provides a unique criteria that makes it possible to identify all resonances in a given energy range as stationary solutions with respect to a complex variational parameter, independently of the resonance widths and their mutual overlapping. About 50 resonances were found for the radical HCO in the energy range between the ground and the second vibrational state of the product diatomic CO, whereas only half of them were located in recent calculations [Dixon, J. Chem. Soc. Faraday Trans. 88, 2575 (1992); Wang and Bowman, J. Chem. Phys. 100, 1021 (1994)]. It was found that the labeling procedure based on a spectroscopic Hamiltonian fit of the bound states and resonance positions agrees completely with the assignment of HCO resonances given in previous calculations, and provides an assignment for the whole set of calculated resonances. Eighty‐three resonances of the radical DCO were found in the same energy range.

Buoyant convection in the Belousov–Zhabotinsky reaction. I. Thermally driven convection and distortion of chemical waves
View Description Hide DescriptionA numerical study of the complete system of reaction‐diffusion‐convection equations that govern the dynamics of Belousov–Zhabotinsky (BZ) reactive fluid has been carried out in order to better understand the interaction of chemical waves with thermally driven buoyant convection in the BZ system. It was shown by direct numerical simulation that such convection may be responsible for inducing the abrupt and continuous changes of chemical wave velocity which have been routinely observed in experiments using magnetic resonance imaging techniques. Distortions of chemical waves propagating in upright cylindrical tubes, by the axisymmetric mode of convection, were predicted by the numerical simulation that strongly resemble those observed experimentally.