CO Dimer Research Articles

Infrared spectroscopy of CO trapped in an argon matrix revisited

Abstract The infrared spectra of carbon monoxide in an argon matrix at cryogenic temperatures have been reinvestigated by FTIR spectroscopy over a wide range of experimental conditions, and new assignments are suggested for the peaks at 2136.7, 2138.5, and 2140.1 cm −1 . The former two peaks are assigned to the CO monomer, while the last peak is attributed to the CO dimer. A temperature dependence, observed for the absorption at 2138.5 and 2140.1 cm −1 , is explained by the coupling between librational modes and the lattice phonons. The increase in the CO dimer concentration upon annealing suggests the rearrangement of Ar toward the closely packed structure.

Palmitic acid and aquo-ruthenium complex monolayers at the liquid–air interface and electroactive Langmuir–Blodgett films

Liquid monolayers and Langmuir–Blodgett (LB) films of palmitic acid and Ru[L(4′-topty)(H2O)] (ClO4)2 (L=(C6H5)2PCH2CH2P(C6H5)2; 4-totpy= 4′-(4-tolyl-2,2′:6′,2″-terpyridine)) have been produced. Comparison of pressure–area and surface potential–area curves for pure palmitic acid monolayers and the mixed films of palmitic acid with ruthenium complex indicates that the minimum molecular area is determined only by the fatty acid. However, the metal complex bonding increases the monolayer compressibility, transforming the condensed state to expanded at higher area per molecule. The absence of any contribution of the metal complex to the minimum molecular area for the mixed films suggests that the complex may be ion paired with the hydrophilic head group of palmitic acid molecules at the condensed state. This observation is confirmed in the FTIR spectrum of the transferred films, by the absence of CO dimer stretching at 1700cm−1 suggesting the formation of the salt in the mixed films. The electronic spectra and cyclic voltammograms show the presence of the ruthenium complex in the films.

Unequivocal laboratory detection of CO dimer transitions in the millimeter wave region

CO dimer transitions were found for the first time in the millimeter wave region, i.e. in the frequency range from 142 to 172 GHz. An isotopic test was performed to authenticate the origin of the transition to the CO dimer. At present the millimeter wave data are not yet sufficient to secure a firm assignment. A brief overview of the history of this complex is given.

High-resolution electron ionization study of CO, (CO)2 and (CO)3: appearance energies and bond dissociation energies

Electron ionization cross-sections for CO, CO dimers and trimers have been investigated near the threshold with a newly constructed crossed beams apparatus using a hemispherical electron monochromator (HEM) to monochromatize the primary electron beam. The dissociative attachment cross-section curve for CO used to test the new set-up shows, in accordance with the high-resolution study of Stamatovic and Schulz, a vertical onset at the thermochemical threshold (9.63 eV) of the lowest possible DA channel O−(2P)+C(3P) in excellent agreement with the value of 9.63 eV derived from known thermochemical data, thereby indicating an energy scale accuracy of better than 10 meV. The appearance energies (AE) of some rare gases (Ar, Kr, Xe) and molecules (N2, O2, N2O) measured for calibration purposes agreed with the known ionization energies (IE) of these compounds within 10 meV. Using a novel data handling procedure, involving a simultaneous non-linear weighted least-squares fit of two functions, the following appearance energies were obtained from the measured ionization cross-section curves using the AE(CO+) as reference: AE((CO)2+)=13.19±0.10 eV and AE((CO)3+)=12.98±0.34 eV. These values are in fair agreement with ionization energies values obtained in a high-resolution photoionization experiment by Ng and co-workers yielding 13.05±0.04 and 12.91±0.04 eV, respectively.

Vibrational spectrum of (CO)2 on Cu(100): Quantum calculations with 18 coupled modes

We report calculations of the vibrational frequencies of CO dimer on Cu(100) using the recently developed vibrational self-consistent field code. Eighteen modes are treated explicitly within a three-mode representation of the potential. Nine symmetry-distinct doublets are observed and the corresponding frequencies are computed. The thermally broadened spectrum of the CO-stretch fundamental is calculated at various temperatures. Both the temperature and coverage dependence of both the average CO-stretch frequency and the corresponding linewidth are consistent with experimentally observed trends.

The mystery of the CO dimer: assignments from variable-temperature jet-cooled infrared spectra

The infrared absorption spectrum of the CO dimer was studied in the vicinity of the CO fundamental band in both axisymmetric and planar supersonic jet expansions. By probing the jet at varying distances downstream, we observed (CO) 2 spectra over a range of effective rotational temperatures from about 15 down to 1 K. The temperature dependence of the transitions enabled us to identify one perpendicular and two parallel subbands arising from the lowest rotational levels of the dimer. The effective intermolecular separation for (CO) 2 was determined to be about 4.35 Å. The only previous assignment of CO dimer spectra is incompatible with our results.

An ab Initio and Diffusion Monte Carlo Study of the Potential Energy Surface of the CO Dimer

A potential energy surface has been constructed for the CO dimer using the methods of intermolecular perturbation theory. The electrostatic and induction terms are described using distributed multipoles and distributed polarizabilities, the dispersion using anisotropic dispersion coefficients calculated by Rijks & Wormer, and the repulsion−penetration by an anisotropic exponential site-site function fitted to points calculated by ab initio intermolecular perturbation theory. The dispersion and induction are damped using Tang-Toennies damping functions. The potential has been used in diffusion Monte Carlo calculations of the ground rovibrational state, and the vibrationally averaged rotational constants agree well with experiment. The virial coefficient is also in good agreement with experiment. In order to achieve this agreement it is necessary to include C9 and C10 dispersion terms. The potential energy surface has two symmetry-equivalent versions of each of two low-lying minima, both of which are planar...

Optimum geometry of CO dimer and FT-IR spectra of CO in solid argon

To gain insight into the optimum geometry of the CO dimer, conventional ab initio and density functional calculations have been carried out. The potential energy surface of the CO dimer appeared to be very flat. Nonetheless, the planar distorted T-shaped structure was computed to be the most stable structure. The slipped antiparallel structure was also computed to be a local minimum. Both the linear and the nonplanar crossed structures seemed unstable. The slipped parallel structure was calculated to correspond to a saddle point. In addition to computing the optimum geometry of (CO) 2, the matrix isolated infrared spectra were taken for CO dispersed in solid argon. Three peaks were identified at 2136.4, 2138.4, and 2139.9 cm −1. The most distinct peak at 2138.4 cm −1 could be assigned to monomeric CO. The other two peaks were attributed to distorted T-shaped dimeric CO species, the 2136.4 cm −1 peak due to (CO) 2 bound through the CO…OC interaction and the 2139.9 cm −1 peak due to (CO) 2 bound through the OC…CO interaction.

Multi-property predictions from recent He-CO potential energy surfaces and related comments on the nature of heteronuclear-rare gas interactions

Recently two potential energy surfaces, an empirical potential V (3,3,3) and a potential of the exchange-Coulomb (XC) form, have been determined by fitting their adjustable parameters to the observed line positions of the same high resolution infra-red spectra of He–CO. Both yield results for the IR spectra in excellent agreement with experiment and with each other but, interestingly, there are significant differences between the XC and V (3,3,3) potentials as a function of relative orientation and interspecies distance for all regions of configuration space, and not just for the repulsive walls. In this paper several properties of the He–CO dimer, second virial, binary diffusion and shear viscosity (both interaction and mixture) coefficients, are used to discriminate between XC, V (3,3,3) and several other literature potentials. The explicit calculations of the transport properties are carried out in the Mason–Monchik approximation and then scaled to give good estimates of close coupling classical trajec...

Pulsed molecular beam infrared absorption spectroscopy of HCCHCO

The rotationally resolved absorption spectrum of the HCCHCO dimer was observed in the region of the CO monomer vibration by a rapid-scan infrared diode laser and a pulsed molecular beam. The observed spectrum of HCCHCO supported a linear molecule. The vibrational wavenumber is determined to be 2149.3444(13) cm −1 and the shift is 6.073 cm −1 from the fundamental of the CO monomer.

The 28-Electron Tetraatomic Molecules: N4, CN2O, BFN2, C2O2, B2F2, CBFO, C2FN, and BNO2. Challenges for Computational and Experimental Chemistry

The energies and structures of the 28-electron tetraatomic molecules, composed of the first row nonmetallic elements: N4 (1), CN2O (2), BFN2 (3), C2O2 (4), B2F2 (5), CBFO (6), C2FN (7), and BNO2 (8) have been studied uniformly by ab initio methods including coupled-cluster theory. New estimates of the stability of the N4 isomers, tetraazatetrahedrane and tetraazacyclobutadiene, are presented, and a new triplet NNNN open-chain isomer has been established computationally. Potential energy surfaces of the nonpolar 1 and the polar 2 are compared. Three-membered cyclic C 2v fluorodiazaboririne has been found to be the most stable isomer similar to diazirinone (Chem. Phys. Lett. 1994, 227, 312). Linear triplet CO and BF dimers, OCCO, FBBF, and OCBF, are the most stable forms of 4, 5, and 6, respectively. The singlet cyanofluoromethylene, NCCF, the global energy minimum of 7, is 7 kcal/mol more stable than isomeric CNCF and 10 kcal/mol lower in energy than 3-fluoroazacycloprop-2-ylidene. The singlet and triplet forms of nitrosoboroxide, OBNO, the most stable isomers of 8, were found to have similar energies, within 1 kcal/mol, and the isomeric triplet OBON lies only 4 kcal/mol above OBNO. Singlet−triplet energy separations and dissociation energies to diatomic fragments are compared for the series of linear 14- and 14-electron (NNNN, NNCO, OCCO, FBBF, and OCBF) and open-chain 13- and 15-electron (NCNO, OBCF, NCCF, and OBNO) dimers. Trends in the chemical bonding in the series of 28-electron tetraatomic molecules, 1−8, stability, and possible synthetic routes, are discussed.

The investigation of the (CO)+2 ion by dissociative ionization of argon/carbon monoxide clusters

The formation of (CO)+2 ion has been investigated by the dissociative ionization of Ar(CO)2 clusters in a threshold photoelectron photoion coincidence experiment. The kinetic energy released (KER) in the reaction Ar(CO)+2→(CO)+2+Ar has been measured as a function of the internal energy of the Ar(CO)+2 ion. The comparison between the experimental KER and the statistically expected KER allows one to extrapolate back to zero KER which corresponds to the thermochemical threshold for the formation of (CO)+2. The ionization potential (IP) of (CO)2 is determined to be 12.24±0.15 eV. This leads to a binding energy of the (CO)+2 of 1.80 eV one of the strongest known for any ionized van der Waals dimer. Our data are in line with a trans planar structure of the CO dimer ion as suggested by our ab initio calculations at the CI level.

Ab initio studies of the complexes of benzene with carbon monoxide and formaldehyde

Benzene...carbon monoxide and benzene...formaldehyde complexes are studied using ab initio methods with the highest calculations at the MP4SDTQ/6–31+G**//MP2/6–31+G** level. The benzene...carbon monoxide dimer forms a π complex of Cs symmetry where the CO top is nearly parallel with the benzene plane. In the benzene...formaldehyde π complex the dimer is without any symmetry. In this arrangement a weak hydrogen bond is expected between the elements where benzene acts as the acceptor, while in a linear benzene...formaldehyde alignment benzene has been identified as a weak hydrogen bond donor to the carbonyl oxygen. Changes in the intramolecular geometric parameters upon dimerization are small. Interaction energy of the benzene...CO dimer seems to be underestimated compared to the experimental value. No experimental value has been found for the benzene...formaldehyde binding energy. The calculated value is more negative by 0.7 kcal/mol with reference to the benzene...CO dimer. Calculated intermolecular vibrational frequencies are in partial agreement with the experiment. The stretching frequency of the benzene...CO dimer is well reproduced, intermolecular bending and torsional frequencies are overestimated. The intramolecular vibrational frequencies for the monomers show over and underestimation in the high and low frequency ranges, respectively. Experimental results in the literature suggest an almost free internal rotation of the CO top above benzene. The calculated barrier to internal rotation is 0.01 kcal/mol in good agreement with the experimental value. Based on this theoretical value the model with the almost free internal rotation was supported. Analysis for the benzene...formaldehyde dimer suggests more hindered rotation, if at all, with a H2CO top.

The Microwave Spectrum and Structure of the CH 3OHCO Dimer

The rotational spectrum of CH 3OHCO has been observed in the region 7-18 GHz with a pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometer. In order to obtain detailed structural information the spectra of CH 3OH, CH 3OD, CD 3OH, and CD 3OD combined with CO and 13CO were examined. Each of the isotopic species studied exhibits two states, which are interpreted as A and E symmetry states arising from internal rotation of the methyl group. The E-state assignments were verified by observing their first-order Stark effect. The structure of the complex is a bent hydrogen bond of the carbon atom of CO at a distance of 2.41 Å from the hydroxyl hydrogen of methanol and planar heavy atoms. The effective barrier to internal rotation for CH 3OHCO, V 3 = 183.0 cm −1, is one-half of the value for the methanol monomer.

On the behavior of CO, CO 2 and their complexes with HCl in liquefied argon

Solutions in liquefied argon of CO, CO 2 and of mixtures of CO and CO 2, CO and HCl and CO 2 and HCl have been investigated in the temperature range from 80 to 120 K, using infrared spectroscopy. No evidence for the formation of the van der Waals dimers of CO or CO 2, or of van der Waals complexes of CO with CO 2, and CO 2 with HCl, was found. The enthalpy of formation of the van der Waals complex between CO and HCl was determined to be 5.4 ± 0.3 kJ mol −1.

The microwave rotational spectrum of the Ar–CO dimer

Pure rotational spectra of four different isotopomers of the dimer Ar–CO have been investigated between 8 and 18 GHz using a pulsed beam cavity Fourier transform spectrometer. The spectra confirm that the complex is a prolate near-symmetric rotor with an essentially T-shaped structure, and that it undergoes large amplitude zero-point motion. It is shown that on the average the argon is closer to the oxygen than to the carbon. The transitions measured obey a-type selection rules with ΔJ=+1, ΔKa=0, and ΔKc=+1. For 40Ar–12C16O, transitions have been observed for Ka=0 and 1 with lower state J values of 1, 2, and 3. For 40Ar–13C16O and 40Ar–13C18O, a similar series was measured, but only for Ka=0. For 40Ar–13C17O, the 17O quadrupole hyperfine pattern was resolved in the rotational transition JKaKc = 202–101. Determinations have been made for rotational and centrifugal distortion constants, as well as for the 17O quadrupole coupling constant χaa. Effective values have been obtained for the length of the line from the center of mass of the CO subunit to the argon nucleus, and for the angle between this line and the CO bond.

An ab lnitio Calculation of the Low Rotation-Vibration Energies of the CO Dimer

On the basis of a previously published ab initio potential surface for the CO dimer [A. van der Pol, A. van der Avoird and P. E. S. Wormer, J. Chem. Phys.92, 7498-7504 (1990); the potential assumes fixed CO bondlengths] we have calculated the J < 3 rotation-vibration energies below 25 cm−1. This should be suitable for interpreting the absorption spectrum of CO dimer cooled to about 10 K. We have made uncoupled one- and three-dimensional calculations of the energies, as well as two different four-dimensional calculations. The first four-dimensional calculation uses an adiabatic separation of the intermolecular coordinate R, and the second uses the full close-coupling method. It is shown that because of the strong couplings on this surface there are differences between the results obtained from these calculations. However, even using the close-coupling results we cannot assign the five published experimental frequencies of CO dimer at 10K [P. A. Vanden Bout, J. M. Steed, L. S. Bernstein, and W. Klemperer, Astrophys. J.234, 503-505 (1979)].

Microwave spectrum, dipole moment, and dynamic structure of the ‘‘linear’’ HCCH–CO dimer

Rotational spectra have been observed for several 13C/18O isotopic species of the pseudolinear acetylene–carbon monoxide dimer using the pulsed, Fourier transform Balle/Flygare microwave spectrometer. A quadrature detection system has been incorporated in the data processing system to avoid folding of the spectrum onto itself in taking the Fourier transform. Dipole moments of 0.3112(12) and 3.29(3) D were determined for the parent HCCH–CO species and for HCN–HCCH, respectively. Structural properties were derived from the various measurements, including a well depth of 300 cm−1 from DJ, and the results are compared with those reported earlier for HCN–HCCH. A substitution analysis is developed in which the rotational properties of a pseudolinear dimer are treated as a symmetric top. A simple method is described for determining effects of the off-axis bending of the monomers.

Dynamics of reactive intermediates as probed by flash photolysis: the rhodium(I) complexes RhCl(CO)L2 [L = PPh3, P(p-tolyl)3, or PMe3

Abstract

The reaction of (SO 2) 2 and (CO 2) 2 with Ba

The reactions of (CO 2) 2 and (SO 2) 2 with Ba have been investigated using a crossed beam arrangement and the laser-induced fluorescence technique. Internal energy in the BaO product was probed in order to study differences between monomeric and dimeric reactions. The reaction cross section for the dimers of CO 2 was found to be between four and eightfold larger than that of the monomers. This can be explained by the change in the reaction mechanism due to the positive electron affinity of the dimers versus the negative electron affinity of the monomers. The product BaO from the dimeric reactions is much colder rotationally than in the monomeric case. This phenomenon can be explained based on the kinematics.