Kr Complexes Research Articles

Activation of the C-H Bonds in Neopentane and Neopentane-d12 by (.eta.5-C5(CH3)5)Rh(CO)2: Spectroscopic and Temporal Resolution of Rhodium-Krypton and Rhodium-Alkane Complex Intermediates

The photoinitiated reaction of Cp*Rh(CO)[sub 2] with neopentane and neopentane-d[sub 12] in liquid krypton has been studied with low temperature IR flash kinetic spectroscopy. Photolysis of Cp*Rh(CO)[sub 2] generates a single transient absorption at 1946 cm[sup [minus]1] which is assigned to the Cp*Rh(CO)(Kr) complex. This complex reacts with (CH[sub 3])[sub 4]C to form the C-H activated neopentyl hydride product observed at 2008 cm[sup [minus]1]. Confirming earlier flash kinetic results with cyclohexane, the results are consistent with a pre-equilibrium mechanism in which an initially formed transient krypton complex Cp*Rh(CO)(Kr) is in rapid equilibrium with a transient (uninserted) alkane complex Cp*Rh(CO)(CH[sub 3][sub 4]C) which then proceeds to form the neopentyl hydride in a unimolecular step. Under most conditions our mechanism requires that both Cp*Rh(CO)(Kr) and Cp*Rh(CO)(alkane) exhibit unresolved carbonyl stretching absorptions at 1946 cm[sup [minus]1]. 44 refs., 7 figs., 1 tab.

Molecular Modeling: An Experimental Tool

Molecular Modeling: An Experimental Tool

Electric Dipole Moments of Rare Gas-Carbonyl Sulfide van der Waals Complexes

The μ a and μ b electric dipole moment components of the Ne-OCS, Ar-OCS, and Kr-OCS van der Waals complexes were determined using Fourier transform microwave spectroscopy. The accuracy of these results was evaluated by comparing the Ar-OCS moments with molecular beam electric resonance measurements. The experimental moments are compared with values calculated from projections of the dipole moment of the OCS monomer, corrected for effects from harmonic bending and induced dipole moments in the rare gas atoms. The μ a and μ b components were reproduced within 0.01 D for the Ar and Kr complexes and within 0.02-0.025 D for the Ne complex.

Metal carbonyl complexes with xenon and krypton: IR spectra, carbon monoxide substitution kinetics, and bond energies

IR spectra for complexes between Xe and Kr and M(CO) 5 (M=Cr, W) have been obtained using laser photolysis of M(CO) 6 and rapid-scan FTIR spectroscopy of liquid rare-gas solutions. The Kr complexes have lifetimes of ∼0.1 s at 150 K in liquid Kr while W(CO) 5 Xe has a lifetime of ∼1.5 min at 170.0 K in liquid Xe. W(CO) 5 Xe reacts with CO to form W(CO) 6 and the CO substitution kinetics were investigated in liquid Xe over a range of temperatures (173.0-198.0 K) and CO concentrations.

Half-collision studies: Action spectroscopy of electronic energy transfer within the Cd⋅CH4 van der Waals complex

The technique of action spectroscopy has been used to characterize the ground state and excited states of the Cd–CH4 van der Waals complex and to study ‘‘half-collision’’ electronic energy transfer processes within the complex. By tuning one laser pulse in the spectral region near an atomic transition of the free Cd atom and monitoring a predissociation product with a second, delayed laser pulse, it has been possible to obtain ‘‘action’’ spectra which provide information about the excited Cd–CH4 potential surfaces from which electronic energy transfer processes occur. Excitation of the Cd⋅CH4 complex to the red of the Cd(5s5p 3P1 ← 5s5s 1S0) transition while monitoring Cd(5s5p 3P0) produces an anharmonic series of blue-shaded bands which have been assigned to the Cd⋅CH4(A 30+ ← X 10+) transition. The spectra are similar to the A 30+ ← X 10+ laser-induced fluorescence (LIF) spectra of the Cd⋅Ar and Cd⋅Kr complexes, except that there are weaker subbands ∼9 cm−1 to the blue of each main band. Computer simulations of the rotational structure of several of the bands were successful, and spectroscopic parameters for both the X 10+ ground state and the A 30+ upper state are reported. It is postulated that Cd⋅CH4 is a hindered rotor with C3v (facial) geometry in both ground and upper states. Detailed considerations of the possible nuclear-spin isomers of symmetries A, F, and E, using the CH4 hindered-rotor calculations of Endo and Ohshima, lead to an assignment of the main bands as A 30+(E,K′ = 1) ← X 10+(E,K″ = 1) transitions and the subbands as A 30+(2F,K′ = 1) ← X 10+(2F,K″ = 1) transitions. It is suggested that the expected A 30+(F,K′ = 0) ← X 10+(F,K″ = 0) and A 30+(A,K′ = 0) ← X 10+(A,K″ = 0) transitions were not observed because the K′=0 states lack the rotational motion about the C3v axis which can couple the A 30+ (A1) state with the a 30− (A2) state [which correlates with Cd(5s5p 3P0)]. Action spectra of Cd⋅CH4 were also obtained to the red of the Cd(5s5p 1P1 ← 5s5s 1S0) transition when the probe laser was monitoring Cd(5s5p 3P2) products. The severely broadened spectra are consistent with extremely rapid predissociation of the C 1Π1 (C 11) van der Waals state by the repulsive c 3Σ1 (c 31) state, which correlates with Cd(5s5p 3P2) + CH4.

Electronic spectroscopy and vibrational predissociation dynamics of OH–Kr and OD–Kr

OH/D–Kr complexes have been characterized via laser excitation of the A–X system. Progressions of the OH/D–Kr stretch were seen in conjunction with the OH/D 0–0 and 1–0 transitions. Rotational analyses of 17 bands provided ground state rotational constants of B″=0.0835 (OH–Kr) and 0.0806 cm−1 (OD–Kr). These constants correspond to an intermolecular separation of 3.78±0.01 Å. Assignment of the A state vibrational levels was accomplished by means of the 84Kr/86Kr isotope effect. A lower limit for the dissociation energy of OH(A,v=0)–Kr of De ≳ 1840 cm−1 was derived from the vibrational constants. Extrapolation of the rotational constants yielded an equilibrium intermolecular separation of 2.67±0.12 Å. The properties defined by the spectroscopic analysis indicated weak, van der Waals bonding in the ground state, and incipient chemical bonding in the excited state. Vibrational predissociation of OH/D–Kr was observed through homogeneous broadening of the rotational lines. The predissociation rates ranged from <1×1010 to 4×1011 s−1, with a general trend of decreasing decay rate with increasing excitation of the intermolecular stretch. Wavelength resolved fluorescence spectra showed that predissociation of OH–Kr partitioned ∼40% of the available energy into OH(A,v=0) rotation. Predissociation of OD–Kr released ≊25% of the energy to OD(A,v=0) rotation.

Electronic spectroscopy of silver dimer rare gas complexes

Vibrationally resolved electronic spectra are reported for the metal dimer-rare gas complexes Ag2–Ar and Ag2–Kr. These spectra are obtained using resonant two-photon photoionization in the energy region near the Ag2 B←X electronic transition (280–285 nm). Both complexes exhibit extensive activity in three vibrational modes, making it possible to determine vibrational constants, anharmonicities, and cross-mode couplings. An unusual cancellation of factors results in the Kr complex (ω′e =72.6 cm−1) having nearly the same metal-rare gas stretching frequency as the Ar complex (ωe=73.9 cm−1). Progressions extending over a significant range of the excited state potential surfaces make it possible to derive the excited state dissociation energies (D′0=755 and 1205 cm−1 for Ar and Kr, respectively). Combination with the red-shifted electronic state origins yields the corresponding ground state dissociation energies (D■0=275 and 394 cm−1 for Ar and Kr, respectively). Potential energy surfaces are investigated for excited and ground states of both complexes.

Van der Waals bond lengths and electronic spectral shifts of the benzeneKr and benzeneXe complexes

Rotationally resolved UV-spectra are presented for the 6 1 0 bands of benzeneKr and benzeneXe complexes yielding precise rotational constants and van der Waals bond lengths for the ground and excited vibronic state, and electronic band shifts. These value complement the previously published data for the other rare gases and the various quantities have now been determined for all the benzene—rare gas complexes. Measured values of the bond length were used to calculate the band shifts from recent theoretical predictions. They are compared with the experimental values of this work.

Vibronic-level-specific heavy-atom effect on the fluorescence decay of the rare gas complexes of 9-methoxyanthracene

Laser-induced fluorescence (LIF) spectroscopy is applied to study the dynamics of electronically excited van der Waals (vdW) complexes of 9-methoxyanthracene (MEOA) with Ar, Kr and Xe. The fluorescence decay features of these rare gas complexes vary widely depending on the number of coordinated atoms as well as on the vibronic levels of MEOA. The heavy-atom effect observed for the Kr and Xe complexes in the electronic origin is manifested more strongly in the vibronically excited 12 0 1 and 12 0 2 levels. Excitation to levels higher than 12 0 1 leads to vibrational predissociation of the complexes, yielding in many cases non-exponential decays with long-lived components (21–25 ns) which reflect the production of fragments in vibrationless states.

Optothermal-infrared and pulsed-nozzle Fourier-transform microwave spectroscopy of rare gas–CO2 complexes

Sub-Doppler infrared spectra of Ne–CO2, Ar–CO2, and Kr–CO2 have been recorded near 3613 and 3715 cm−1, in the region of the 2ν02+ν3/ν1+ν3 Fermi diad of CO2, using an optothermal molecular-beam color-center laser spectrometer. In addition, pulsed-nozzle Fourier-transform microwave spectra are reported for the ground vibrational states of the complexes. The infrared and microwave spectra are consistent with T-shaped complexes as shown originally by Steed, Dixon, and Klemperer for Ar–CO2.1 The infrared band origins for the Ar and Kr complexes are red shifted, from that of free CO2, by 1.09 and 0.95 cm−1 for Ar–CO2 and by 1.97 and 1.76 cm−1 for 84Kr–CO2. For Ne–CO2, blue shifts of 0.15 and 0.19 cm−1 are observed. The lower Fermi components are free of perturbations, whereas the upper components of Ar–CO2 and Kr–CO2 are perturbed. For Ar–CO2 the perturbation is strong, shifting the positions of the observed Q-branch lines of the Ka =1←0 subband by as much as 500 MHz.

Van der Waals potentials from the infrared spectra of rare gas–HF complexes

High-resolution infrared spectra of the Ar–HF, Kr–HF, and Xe–HF van der Waals molecules have been recorded in the vicinity of the H–F stretching fundamentals, ν1, under thermal equilibrium conditions at T≂211 K with a tunable difference-frequency laser. Rotational structure has been observed up to or approaching rotational predissociation, permitting us to model the effective radial van der Waals potentials for these complexes. These potentials provide good estimates for the binding energies, D0, and the van der Waals stretching frequencies, ν3, in the ground (v1=0) and excited (v1=1) states of the molecules. For v1=0 in Ar–HF, Kr–HF, and Xe–HF, we find D0=102, 133, and 181 cm−1 and ν3=39.2, 41.1, and 43.4 cm−1, respectively. The ν3 modes characterized by the model potentials aid in the assignment of the ν1+ν3−ν3 hot bands observed in our spectra. The band centers for the ν1 fundamentals are all down shifted in frequency from the isolated HF monomer by Δν=−9.654, −17.518, and −29.185 cm−1 for the Ar, Kr, and Xe complexes, respectively, indicating that the van der Waals bonds are some 10% to 15% stronger in the excited vibrational state. This increased vibrational attraction also results in a contraction of the van der Waals radial coordinate manifest in the larger rotational constants observed for ν1; ΔB/B0=+0.35%, +1.00%, and +1.75% for Ar–, Kr–, and Xe–HF. We have also observed the Q branch of the ν1+ν2 stretch–bend combination band in Ar–HF some 70.2 cm−1 above the ν1 fundamental with a large negative ΔB/B0=−2.00% implying a strong anisotropy in the potential.

Vibrational predissociation in van der Waals complexes of glyoxal with Ar and Kr

Vibrational predissociation (VP) of glyoxal–Ar and glyoxal–Kr complexes formed in a supersonic expansion has been studied in several vibrational levels of the first excited singlet electronic state S1 (1Au) of glyoxal. Analysis of the fluorescence spectrum, together with lifetime measurements, has shown that VP is in competition with other intramolecular vibrational energy redistribution processes in these systems, as well as intersystem crossing (ISC). Glyoxal–Ar and glyoxal–Kr behave very similarly. In particular, two isomers exist with different dissociation schemes (but similar for the same isomer of Ar or Kr complexes), and Kr does not seem to enhance ISC. A comparison with collisional results is presented. Another interesting result of this work is that 710, a forbidden transition of free glyoxal, was found in the excitation spectrum of complexed glyoxal. The corresponding 7̄01 emission line of the complex was also observed, as well as combination bands. This suggests a strong interaction between mode 7 and one of the van der Waals modes. This interaction is the basis for a simple theoretical model which reproduces the experimental lifetime of the excited complex (7̄2).

Resonant two-photon ionization of fluorene rare-gas van der Waals complexes

Resonant two-photon ionization combined with time-of-flight mass spectrometry was applied for the interrogation of the S0 → S1 electronic-vibrational excitations of van der Waals complexes of fluorene (FL) with rare-gas atoms and N2 in supersonic jets. Energy-resolved and mass-resolved spectra of FL ⋅ Ne, FL ⋅ Arn (n=1–3), FL ⋅ Kr, FL ⋅ Xe, and FL ⋅ N2 were recorded over the energy range 0–800 cm−1 above the electronic origin of S1. The red microscopic spectral shifts of the electronic origins of FL ⋅ R (R=Ar, Kr, and Xe) complexes are dominated by dispersive interactions, being proportional to the polarizability of R. The vibrational level structure of FL ⋅ Rn (R=Ar, Kr, and Xe) complexes exhibits intramolecular vibrational excitations of FL, as well as intermolecular vibrations, which involve the relative motion of FL and R in the complex. The spectra of FL ⋅ Ne and FL ⋅ N2 reveal a rich vibrational structure in the vicinity of the electronic origin, indicating a substantial change of the nuclear configuration upon electronic excitation. Upper and lower bounds on the dissociation energies of FL ⋅ R (R=Ne, Kr, and Xe) and FL ⋅ Ar2 were inferred from the vibrational level structure in the mass-resolved spectra, where the disappearance of the signal of the parent van der Waals ion and the appearance of the ion signal of the fragments mark the onset of the vibrational predissociation process.

Bound atom–diatomic molecule complexes. Anisotropic intermolecular potentials for the hydrogen–rare gas systems

Spectra of the van der Waals complexes H2–Ar, Kr, Xe, and D2–Ne have been analyzed to obtain anisotropic intermolecular potentials and transition dipole moments. The Morse-spline–van der Waals functional form is used to describe the potentials. Eigenvalues were computed by solving the close coupling equations, and calculated transition frequencies were fit to the experimental Q1(0) and S1(0) peak frequencies by a least squares technique. For this purpose experimental peaks were chosen from the P and R as well as the N and T branches of the spectra. The absorption was computed for each transition. The total absorption contour for a spectrum was obtained by assuming a Lorentzian line shape for the individual transitions and summing contributions from overlapping lines. The agreement between calculated and experimental absorption contours was excellent. For the Ar, Kr, and Xe complexes it was necessary to include three terms in the angular expansion of the transition dipole moment rather than the usual two terms. These three terms interfere to produce the complex absorption contours observed in some spectra. The anisotropy of the intermolecular potential was not determined completely. Rather, a linear relationship between the repulsive and attractive anisotropy coefficients is found.

Spectra of H2–Ne and D2–Ne Van der Waals Complexes in the Collision-Induced Fundamental Bands of Hydrogen and Deuterium

Spectra of H2–Ne and D2–Ne Van der Waals complexes accompanying transitions in the collision-induced fundamental bands of hydrogen and deuterium have been obtained with an absorption path of 110 m at temperatures around 27 K in a specially designed multiple-traversal cell. The observed structure is similar to that of H2– and D2–Ar, Kr, and Xe complexes studied earlier, but the number of lines observed for the Ne complexes is fewer because of the shallower intermolecular potential. Well-resolved R and P branches (Δl= ±1, where l is the rotational quantum number of the complex) accompanying the overlap-induced Q1 (0) transitions are analyzed directly to give rotational (B) and centrifugal stretching (D) constants for the complexes. Unlike the other H2– rare gas complexes the spectra accompanying quadrupole-induced transitions show no direct evidence of anisotropy of the intermolecular forces.