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The microwave spectra of three isotopologues of the gas-phase heterodimer formed between cis-1,2-difluoroethylene and hydrogen chloride are obtained in the 5–21 GHz region using Fourier transform microwave spectroscopy. The molecular structure, determined from the analysis of the spectra and supported by quantum chemistry calculations, has the hydrogen atom of the hydrogen chloride molecule interacting with both fluorine atoms of the fluoroethylene and no interaction between the chlorine atom and the olefin. Although the equilibrium structure has two inequivalent H⋯F interactions, zero-point motion averages over the two equivalent choices for these interactions, rendering the pairs of like atoms (C, H, and F) of the fluoroethylene equivalent, retaining the C2v symmetry of the olefin. This results in only one unique singly substituted 13C isotopologue and in the observed effects on transition intensities due to nuclear spin statistics. The heterodimer structure allows for a strong, linear hydrogen bond between the HCl donor and the fluoroethylene acceptor that is more important here than in the analogous acetylene containing complex, where the interaction between the π electrons of acetylene and an electrophilic hydrogen atom on the olefin compensates for the loss of linearity required for binding to a geminal F/H pair. [ABSTRACT FROM AUTHOR]

The microwave rotational spectrum of the gas-phase bimolecular heterodimer formed between cis-1,2-difluoroethylene and acetylene is obtained using Fourier transform microwave spectroscopy from 5.9 to 21.2 GHz. Rotational constants derived from the analysis of the spectra for the normal isotopologue and singly substituted 13C isotopologues, obtained in natural abundance, allow the determination of the structure of the complex, which, in the absence of a fluorine-hydrogen atom pair located cis to each other, adopts a sterically disfavored geometry ("side-binding") in which the acetylene interacts with a geminal fluorine-hydrogen atom pair. Structural details are found to be similar to those of previously studied heterodimers with side-binding of acetylene to fluorine while reflecting the degree of halosubstitution. A detailed comparison with the (Z)-1-chloro-2-fluoroethylene-acetylene complex reveals information regarding the relaxed steric requirements for hydrogen bonding to chlorine as opposed to hydrogen bonding to fluorine. [ABSTRACT FROM AUTHOR]

Gas phase homodimers of 3,3,3-trifluoro-1,2-epoxypropane (TFO), a molecule which has shown promise as an effective chiral tag for determining the absolute stereochemistry and the enantiomeric composition of chiral analytes, are explored using a variety of quantum chemistry models and rotational spectroscopy. The potential surface governing the interaction of the two molecules is rapidly explored using the artificial bee colony algorithm for homodimer candidates that are subsequently optimized by quantum chemistry methods. Although all model chemistries employed agree that the lowest energy form of the heterochiral homodimer of TFO (RS or SR) is lower in energy than that of the homochiral dimer (RR or SS), the energy spacings among the lower energy isomers of each and indeed the absolute energy ordering of the isomers of each are very model dependent. The experimental results suggest that the B3LYP-D3BJ/def2-TZVP model chemistry is the most reliable and provides excellent estimates of spectroscopic constants. In accord with theoretical predictions the non-polar lowest energy form of the heterochiral homodimer is not observed, while two isomers of the homochiral dimer are observed and spectroscopically characterized. Observation and assignment of the spectra for all three unique singly-substituted 13C isotopologues, in addition to that of the most abundant isotopologue for the lowest energy isomer of the homochiral homodimer of TFO, provide structural information that compares very favorably with theoretical predictions, most notably that the presence of three fluorine atoms on the trifluoromethyl group removes their direct participation in the intermolecular interactions, which instead comprise two equivalent pairs of CH⋯O hydrogen bonds between the two epoxide rings augmented by favorable dispersion interactions between the rings themselves. [ABSTRACT FROM AUTHOR]

We report the first experimental gas-phase observation of an asymmetric, trans-N2O4 formed by the dimerization of NO2. In additional to the dominant 14N2 16O4 species, rotational transitions have been observed for all species with single 15N and 18O substitutions as well as several multiply substituted isotopologues. These transitions were used to determine a complete substitution structure as well as an r0 structure from the fitted zero-point averaged rotational constants. The determined structure is found to be that of an ON-O-NO2 linkage with the shared oxygen atom closer to the NO2 than the NO (1.42 vs 1.61 Å). The structure is found to be nearly planar with a trans O-N-O-N linkage. From the spectra of the 14N15NO4 species, we were able to determine the nuclear quadrupole coupling constants for each specific nitrogen atom. The equilibrium structure determined by ab initio quantum chemistry calculations is in excellent agreement with the experimentally determined structure. No spectral evidence of the predicted asymmetric, cis-N2O4 was found in the spectra. [ABSTRACT FROM AUTHOR]

The T-shaped OH-C2H2 complex is formed in helium droplets via the sequential pick-up and solvation of the monomer fragments. Rovibrational spectra of the a-type OH stretch and b-type antisymmetric CH stretch vibrations contain resolved parity splitting that reveals the extent to which electronic angular momentum of the OH moiety is quenched upon complex formation. The energy difference between the spin-orbit coupled ²B1 (A'') and ²B2 (A') electronic states is determined spectroscopically to be 216 cm-1 in helium droplets, which is 13 cm-1 larger than in the gas phase [Marshall et al., J. Chem. Phys. 121, 5845 (2004)]. The effect of the helium is rationalized as a difference in the solvation free energies of the two electronic states. This interpretation is motivated by the separation between the Q(3/2) and R(3/2) transitions in the infrared spectrum of the helium-solvated 2 3/2 OH radical. Despite the expectation of a reduced rotational constant, the observed Q(3/2) to R(3/2) splitting is larger than in the gas phase by 0.3 cm-1. This observation can be accounted for quantitatively by assuming the energetic separation between ²π3/2 and ²π1/2 manifolds is increased by 40 cm-1 upon helium solvation. [ABSTRACT FROM AUTHOR]

Guided by ab initio calculations, Fourier transform microwave spectra in the 6-21 GHz region are obtained for seven isotopomers of the complex formed between 1-chloro-1-fluoroethylene and acetylene. These include the four possible combinations of 35Cl- and 37Cl-containing CH2CClF with the most abundant acetylene isotopic modification, HCCH, and its H13C13CH analogue, as well as three singly substituted deuterated isotopomers. Analysis of the spectra determines the rotational constants and additionally, the complete chlorine quadrupole hyperfine coupling tensors in both the inertial and principal electric field gradient axis systems, and where appropriate, the diagonal components of the deuterium quadrupole coupling tensors. The inertial information contained in the rotational constants provides the structure for CH2CClF-HCCH: a primary, hydrogen bonding interaction existing between the HCCH donor and the F atom acceptor on the 1-chloro-1-fluoroethylene moiety, while a secondary interaction occurs between the acetylenic bond on the HCCH molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene, which causes the hydrogen bond to deviate from linearity. This is similar to the structure obtained for 1,1-difluoroethylene-HCCH [H. O. Leung and M. D. Marshall, J. Chem. Phys. 126, 154301 (2006)], and indeed, to within experimental uncertainty, the intermolecular interactions in CH2CClF-HCCH and its 1,1-difluoroethylene counterpart are practically indistinguishable, even though ab initio calculations at the MP2/6-311G++(2d, 2p) level suggest that the former complex is more strongly bound. [ABSTRACT FROM AUTHOR]

Fourier transform microwave rotation spectra in the 7–21 GHz region are obtained for the complex formed between 1,1-difluoroethylene and hydrogen fluoride, including the normal isotopomer and two singly substituted 13C species obtained in natural abundance. Spectra are also obtained for the analogous three species formed using deuterium fluoride. Analysis of the spectra provides rotational and hyperfine constants that are used, in combination with information from the analogous complex, 1,1-difluoroethylene-acetylene, to determine a structure for CH2CF2–HF. This structure is similar to that obtained for vinyl fluoride-HF [G. C. Cole and A. C. Legon, Chem. Phys. Lett. 400, 419 (2004)] in that a primary, hydrogen bonding interaction exists between the HF donor and a F atom acceptor on the 1,1-difluoroethylene moiety, while a secondary interaction occurs between the F atom on the HF molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene and causes the hydrogen bond to deviate from linearity. A comparison of the structures of 1,1-difluoroethylene complexes with the protic acids HF, HCl, and HCCH demonstrates that the hydrogen bond length increases with decreasing gas-phase acid strength, whereas a comparison of HF complexes with vinyl fluoride, 1,1-difluoroethylene, and 1,1,2-trifluoroethylene indicates that the nucleophilicity of the F atoms decreases with increasing fluorine substitution, but that the secondary interaction length is remarkably similar in all three complexes. [ABSTRACT FROM AUTHOR]

Guided by ab initio calculations, Fourier transform microwave rotation spectra in the 6.5–22 GHz region are obtained for the complex formed between trans-1,2-difluoroethylene and hydrogen fluoride, including the normal isotopomer and two singly substituted 13C species in natural abundance. Spectra are also obtained for the analogous three species formed using deuterium fluoride. Analysis of the spectra provides rotational and hyperfine constants that are used to determine a structure for trans-CHFCHF–HF. This structure is similar to that obtained for 1,1-difluoroethylene-HF [H. O. Leung et al., J. Chem. Phys. 131, 204301 (2009)] in that a primary, hydrogen bonding interaction exists between the HF donor and a F atom acceptor on the 1,2-difluoroethylene moiety, while a secondary interaction occurs between the F atom on the HF molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene and causes the hydrogen bond to deviate from linearity. Because the two F atoms and the two H atoms in trans-1,2-difluoroethylene form electrostatically equivalent pairs, the structure of the complex with HF provides insight into the contribution of steric effects to the observed geometries of fluoroethylene-protic acid complexes. A comparison of the observed hydrogen bond lengths and deviations from linearity in 1,1-difluoroethylene-HF and trans-1,2-difluoroethylene-HF suggests that the F atoms in trans-1,2-difluoroethylene are more nucleophilic than those in 1,1-difluoroethylene and that the H atoms are similarly more acidic. Ab initio calculations of electrostatic potentials mapped onto total electron density surfaces for these two molecules support these conclusions. [ABSTRACT FROM AUTHOR]

Rotational spectra have been observed for 16OH–16OH2, 16OH–18OH2, 18OH–16OH2, and 18OH–18OH2 with complete resolution of the nuclear magnetic hyperfine structure from the OH and water protons. Transition frequencies have been analyzed for each isotopic form using the model of Marshall and Lester [J. Chem. Phys. 121, 3019 (2004)], which accounts for partial quenching of the OH orbital angular momentum and the decoupling of the electronic spin from the OH molecular axis. The analysis accounts for both the ground (2A′) and first electronically excited (2A″) states of the system, which correspond roughly to occupancy by the odd electron in the py and px orbitals, respectively (where py is in the mirror plane of the complex and px is perpendicular to py and the OH bond axis). The spectroscopic measurements yield a parameter, ρ, which is equal to the vibrationally averaged 2A′-2A″ energy separation that would be obtained if spin-orbit coupling and rotation were absent. For the parent species, ρ=-146.560 27(9) cm-1. 18O substitution on the water increases |ρ| by 0.105 29(10) cm-1, while substitution on the OH decreases |ρ| by 0.068 64(11) cm-1. In the OH–OH2 complex, the observed value of ρ implies an energy spacing between the rotationless levels of the 2A′ and 2A″ states of 203.76 cm-1. Ab initio calculations have been performed with quadratic configuration interaction with single and double excitations (QCISD), as well as multireference configuration interaction (MRCI), both with and without the inclusion of spin-orbit coupling. The MRCI calculations with spin-orbit coupling perform the best, giving a value of 171 cm-1 for the 2A′-2A″ energy spacing at the equilibrium geometry. Calculations along the large-amplitude bending coordinates of the OH and OH2 moieties within the complex are presented and are shown to be consistent with a vibrational averaging effect as the main cause of the observed isotopic sensitivity of ρ. [ABSTRACT FROM AUTHOR]

Guided by ab initio calculations, Fourier transform microwave rotational spectra in the 6–22 GHz region are obtained for the complex formed between 1,1,2-trifluoroethylene and acetylene, including the normal isotopomer, three of four singly substituted 13C species obtained in natural abundance, and using commercially available isotopic varieties of acetylene, species containing HCCD and H13C13CH. Although the ab initio calculations suggest two possible low energy planar arrangements for the molecules in the complex, only a single, unique structure is obtained from a combined analysis of the rotational constants derived from the spectra and atomic positions determined using Kraitchman [Am. J. Phys. 21, 17 (1953)] substitution coordinates. This structure is similar to that obtained for the CF2CHF[Single_Bond]HF complex [H. O. Leung and M. D. Marshall, J. Chem. Phys. 126, 114310 (2007)] in which both the primary and secondary interactions occur between the HCCH molecule and a F atom and a H atom bonded to the same carbon of CF2CHF. The 2.748(15) Å hydrogen bond has acetylene as the donor and 1,1,2-trifluoroethylene as the acceptor and forms a 104.49(15)° C[Single_Bond]F...H angle. The 2.8694(9) Å secondary interaction between the π bond of acetylene and the H atom geminal to the acceptor F atom causes the hydrogen bond to deviate 69.24(67)° from linearity. This large deviation from linearity and the similarity of the two intermolecular bond lengths suggest that the two interactions are becoming comparable in importance. [ABSTRACT FROM AUTHOR]

Fourier transform microwave rotational spectra in the 6–22 GHz region are obtained for the complex formed between 1,1,2-trifluoroethylene and hydrogen fluoride, including the normal isotopomer, the two singly substituted 13C species, and the complex obtained with DF. A unique planar structure for the complex is determined from a combined analysis of the rotational constants derived from the spectra and atomic positions obtained using Kraitchman [Am. J. Phys. 21, 17 (1953)] substitution coordinates. Consistent with this structure, no hyperfine splitting of rotational lines due to the nuclear quadrupole coupling interaction is observed for the D-containing species. Although the primary interaction in the complex is a hydrogen-fluorine hydrogen bond, as is the case for all previously studied Lewis acid-fluoroethylene complexes, the CF2CHF–HF complex adopts a distinctly different geometry in which both the primary and secondary interactions occur between the HF molecule and a F atom and a H atom, respectively, bonded to the same carbon of CF2CHF. The 2.020(41) Å hydrogen bond has hydrogen fluoride as the donor and 1,1,2-trifluoroethylene as the acceptor and forms a 109.0(13)° C–F...H angle. The secondary interaction between the hydrogen fluoride F atom and the H atom geminal to the acceptor F atom causes the hydrogen bond to deviate 41.6(51)° from linearity. Structural comparisons with analogous complexes formed with mono- and difluorinated ethylenes suggest that the primary hydrogen bond strength and the fluoroethylene fluorine atom basicity both decrease with increasing fluorine substitution. In the course of this work, it was necessary to obtain additional rotational spectra for the 1,1,2-trifluroethylene monomer and to improve the precision of the values of the structural parameters for this molecule. [ABSTRACT FROM AUTHOR]

Fourier transform microwave, rotational spectra in the 6–21 GHz region are obtained for the complex formed between 1,1-difluoroethylene and acetylene, including the normal isotopomer and each singly substituted 13C species along with complexes derived from commercially available isotopic varieties of acetylene (HCCD, DCCD, and H13C13CH). Although two possible planar structures are consistent with the rotational constants derived from analysis of the spectra, ab initio calculations, as well as chemical intuition, support only one of the two as the structure of the complex. Nuclear quadrupole coupling constants for D-containing species show no evidence of electric field gradient perturbation and are consistent with the structures obtained from inertial data. The primary interaction between the two molecules is a 2.646(11) Å hydrogen bond with acetylene as the donor and a 1,1-difluoroethylene fluorine as the acceptor that forms a 122.41(79)° C–F...H angle. A secondary interaction between the acetylenic bond and the difluoroethylene hydrogen atom cis to the acceptor fluorine atom causes the hydrogen bond to deviate 53.25(24)° from linearity. Structural comparisons with the related complex, 1,1-difluoroethylene-hydrogen chloride [Z. Kisiel et al., J. Chem. Soc., Faraday Trans. 88, 3385 (1992)], suggest that the hydrogen bond in the acetylene complex is weaker, whereas comparisons with vinyl fluoride-acetylene [G. C. Cole and A. C. Legon, Chem. Phys. Lett. 369, 31 (2003)] indicate that the fluorine atoms in 1,1-difluoroethylene are less basic than the one in vinyl fluoride. [ABSTRACT FROM AUTHOR]

The entrance channel leading to the addition reaction between the hydroxyl radical and acetylene has been examined by spectroscopic characterization of the asymmetric CH stretching band of the π-hydrogen bonded OH-acetylene reactant complex. The infrared action spectrum observed at 3278.6 cm-1 (origin) consists of seven peaks of various intensities and widths, and is very different from those previously reported for closed-shell HF/HCl-acetylene complexes. The unusual spectrum arises from a partial quenching of the OH orbital angular momentum in the complex, which in turn is caused by a significant splitting of the OH monomer orbital degeneracy into 2A′ and 2A″ electronic states. The magnitude of the 2A′-2A″ splitting as well as the A rotational constant for the OH-acetylene complex are determined from the analysis of this b-type infrared band. The most populated OH product rotational state, jOH=9/2, is consistent with intramolecular vibrational energy transfer to the ν2 C≡C stretching mode of the departing acetylene fragment. The lifting of the OH orbital degeneracy and partial quenching of its electronic orbital angular momentum indicate that the electronic changes accompanying the evolution of reactants into products have begun to occur in the reactant complex. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A model is developed for the rotational energy levels and electric dipole transition intensities of nonlinear OH-containing complexes in which the OH is hydrogen bonded to its partner. Both the 2A′ and 2A″ electronic states arising from the lifting of the OH monomer electronic orbital degeneracy are explicitly included. Consequently, the model smoothly spans the entire range of the difference potential associated with the separation between these two states, and the model accounts for the partial quenching of the OH monomer electronic angular momentum in such complexes. The more familiar cases of completely unquenched and completely quenched electronic angular momentum are recovered in the limits of zero and very large difference potential, respectively. The sensitivity of rovibrational spectra to the value of the difference potential is investigated, and it is shown that spectra of reactant complexes reveal the extent of quenching, which must occur along the reaction coordinate as the system evolves from weakly interacting partners to addition product. The model is successfully applied to the analysis of the OH overtone spectrum of the OH-acetylene complex. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A hydrogen-bonded complex between the hydroxyl radical and acetylene has been stabilized in the reactant channel well leading to the addition reaction and characterized by infrared action spectroscopy in the OH overtone region. Analysis of the rotational band structure associated with the a-type transition observed at 6885.53(1) cm-1 (origin) reveals a T-shaped structure with a 3.327(5) Å separation between the centers of mass of the monomer constituents. The OH (v=1) product states populated following vibrational predissociation show that dissociation proceeds by two mechanisms: intramolecular vibrational to rotational energy transfer and intermolecular vibrational energy transfer. The highest observed OH product state establishes an upper limit of 956 cm-1 for the stability of the π-type hydrogen-bonded complex. The experimental results are in good accord with the intermolecular distance and well depth at the T-shaped minimum energy configuration obtained from complementary ab initio calculations, which were carried out at the restricted coupled cluster singles, doubles, noniterative triples level of theory with extrapolation to the complete basis set limit. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Intermolecular vibrations of the linear OH-CO reactant complex have been observed as combination bands in the OH overtone region using infrared action spectroscopy. Rotational analyses and simulations of the band structures have been carried out for transitions to geared bend, excited spin-orbit, and H-atom bend states with 50-250 cm[sup -1] of intermolecular excitation. The projection quantum number associated with each of these upper states is identified through the intensity profile of the band contour, missing rotational lines, and/or parity splitting of individual rotational lines. Intermolecular states with projection quantum numbers P = ½ and 5/2 are observed for each of the two bending modes, arising from coupling of the unquenched angular momentum of OH with the vibrational angular momentum associated with the bending motion of the complex. An additional P = ½ state is attributed to spin-orbit excitation, which shifts to higher energy than in free OH and gains infrared transition strength through the spin-decoupling interaction. The intermolecular energy level pattern is also examined in the context of the Renner-Teller interaction and spin-orbit coupling. The intermolecular bends of the OH-CO complex are of special interest because they probe portions of the reaction path leading to trans-HOCO formation. [ABSTRACT FROM AUTHOR]

The infrared spectrum of the linear OD-N[sub 2] complex has been recorded in the OD overtone region near 1.9 μm using an infrared-ultraviolet double resonance technique. The pure overtone band of OD-N[sub 2](2ν[sub OD]) was observed at 5173.99(1) cm[sup -1], and combination bands involving the simultaneous excitation of OD stretch and geared bend were identified at 5209.02(1) cm[sup -1] and 5214.59(2) cm[sup -1]. Assignments and spectroscopic constants have been derived from the rotationally resolved structure of each band, which are in good accord with model calculations based on an electrostatic interaction potential. Direct time-domain measurements yielded a vibrational predissociation lifetime of 150±16 ns for OD-N[sub 2] (2ν[sub OD]) and a three fold decrease in lifetime upon intermolecular excitation of the lower-energy geared bending state. The OD (v = 1) fragments of vibrational predissociation were found to be highly rotationally excited, indicating that predissociation proceeds by vibrational to rotational/translational energy transfer. The results obtained for OD-N[sub 2] are compared with an analogous study of OH-N[sub 2] [Marshall et al., J. Chem. Phys. 114, 7001 (2001)], revealing insights on the potential anisotropy along the geared bending coordinate and a change in the vibrational predissociation mechanism. [ABSTRACT FROM AUTHOR]

A two-experiment sequence is developed focusing on the noncovalent complex between the egg white protein avidin or the similar protein d, and the essential cofactor biotin. The first of the two experiments is a computational chemistry 'dry-lab' focusing on noncovalent interactions, and the second involves the measurement of the equilibrium constant for the binding of the dye-[(4-hydroxyphenyl)azo]benzoic acid to avidin.

A model is developed to treat the rotational energy levels of the OH-water complex with effective C (sub 2nu) symmetry that accounts for the difference potential (rho) which is responsible for quenching of the OH orbital angular momentum, and the spin-orbit interaction of the OH monomer. The changes in the rotational energy level structure are manifested in the microwave and infrared spectra of the OH-water complex.

Describes infrared-microwave double resonance studies of dipole moments in the v[sub 1] and v[sub 3] states of ammonia (NH[sub 3]). Stark effect in NH[sub 3]; Measurement of excited-state moments for a wide range of J,K states.

Infrared laser–radio frequency double resonance spectroscopy has been used to observe transitions across J=1 l-type doublets in vibrational states of monodeuterated acetylene, HCCD, that have both the C-H stretch and one degenerate bending mode excited. Electric dipole moments and l doublings have been measured for the ν1+ν4 and ν1+ν5 states, giving: μ14=-0.046 44(3) D, ql(14)=132.289(6) MHz, μ15=+0.031 26(2) D, and ql(15)=107.263(4) MHz. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vibrational predissociation of Ar–CO2 has been studied at the state-to-state level with the emphasis of the present work being on the determination of the rotational propensity rules governing this process and their dependence on the initial state of the parent complex. We find that for the low J, K=1 initial states associated with the (101)/(0201) Fermi diad of the parent complex, there is a strong propensity for the production of the CO2 fragments in odd j states. This propensity rule breaks down as J’ of the complex increases and preliminary results show that the final state distributions are also strongly dependent on K. The dependence of the photofragment angular distributions on the laser polarization direction is also reported, which provides additional dynamical information (vector correlations) on the nature of the photodissociation process. [ABSTRACT FROM AUTHOR]

A modified version of the optothermal technique has been used to measure photofragment angular distributions resulting from the vibrational predissociation of Ar–CO2 from both members of the (1001)/(0201) Fermi diad. These angular distributions show resolvable structure which can be assigned to individual final states of the resulting fragments. The emphasis of the present paper is on the role of the vibrational degrees of freedom of the CO2 fragment as depositories for the excess energy. The dissociation energy (D0) of the complex has been determined to be 166±1 cm-1. This result is used in conjunction with the spectroscopic data already available in the literature to refine two previously reported two-dimensional Ar–CO2 potential energy surfaces. [ABSTRACT FROM AUTHOR]

The influence of laser polarization direction on the photofragment angular distributions resulting from vibrational predissociation of HF dimer has been studied in an effort to establish the correlation between the direction of the transition moment in the parent molecule (μ) and the recoil velocity vector of the fragments (ν). This is possible, even for the long-lived predissociative states considered here, since the dimer is initially excited to a single rotational level, with a well-defined time-independent, anisotropic rotational wave function. The mathematical framework appropriate for treating the photodissociation of this asymmetric rotor is presented and used to extract anisotropy parameters for the various initial states. The results clearly show that the recoil velocity vector of the fragments is preferentially aligned along the a axis of the dimer. The implications of this for the associated dynamics are discussed. Data of this type is directly related to the center-of-mass differential photodissociation cross section and thus can be used to establish further points of comparison with the theoretical calculations that are becoming possible for systems of this type. [ABSTRACT FROM AUTHOR]

The optothermal detection method has been used to obtain near-infrared spectra of the H2O–CO2 and H2O –HCCH van der Waals molecules in the 3 μm region. For H2O–CO2, the observed vibrational band correlates with the asymmetric O–H stretching vibration of the water monomer and gives a direct measurement of the internal rotation tunneling splitting, from which the height of the associated barrier is estimated. This experimental estimate is compared with results obtained from both a distributed multipole analysis and an ab initio calculation. In the case of H2O –HCCH, three different vibrational bands are observed. Although these spectra are not directly sensitive to the wide amplitude motion in this molecule, they are consistent with the quasiplanar picture suggested by the ab initio calculations. These calculations show the molecule to have a bent equilibrium structure, with a small barrier located at the planar geometry. [ABSTRACT FROM AUTHOR]

The state-to-state vibrational predissociation dynamics of the hydrogen fluoride dimer has been investigated in detail using a newly developed instrument which gives both initial state selection and photofragment state determination. Results are reported for a wide variety of initial states associated with the ν1 and ν2 vibrations. The final state distributions universally indicate that the preferred dissociation channels correspond to the production of one HF fragment that is highly rotationally excited and another that is not. This is explained in terms of an impulsive dissociation mechanism which proceeds from a geometry close to that of the equilibrium structure of the dimer. We find that nearly degenerate initial states can have rather different final state distributions. In particular, there is a distinct difference between the upper and lower members of the tunneling doublet, which is most likely attributable to their related symmetries. The dissociation energy (D0) is determined to be 1062±1 cm-1. [ABSTRACT FROM AUTHOR]

Infrared optothermal laser spectroscopy has been used to study the linear SCO–HCN complex. The observed spectrum corresponds to excitation of the C–H stretching vibration out of either the ground state or v=1 of the lowest frequency intermolecular bending state. Ab initio calculations have also been carried out for this system which give, at least for some properties, results which are in good agreement with experiment. A comparison between the SCO–HCN results and those of the linear CO2–HCN complex studied previously, reveals that the shifts in the C–H vibrational frequency upon complex formation are rather similar. This is despite the fact that OCS and CO2 have very different point multipoles, suggesting that the electrostatic part of the interactions are likewise different for these two partners. Nevertheless, a distributed multipole calculation shows that, at the intermolecular separations characteristic of these complexes, the electrostatic interactions are in fact quite similar at the oxygen ends of OCS and CO2, providing an explanation for the similarity between the frequency shifts of these two complexes. [ABSTRACT FROM AUTHOR]

The infrared absorption spectrum of the acetylene–carbon monoxide van der Waals molecule has been obtained in the 3 μ region. A color center laser was used to excite the vibration of the complex which corresponds to the antisymmetric hydrogen stretching mode in the monomer in a pulsed molecular jet. The form of the spectrum indicates that the molecule is a linear, hydrogen bonded complex, although excited state perturbations are present. The vibrational origin is at 3279.898(2) cm-1 and the ground state B value is 1397.4(6) MHz, corresponding to a separation of 5.011 Å for the monomer centers of mass. The spectrum cannot directly distinguish between the HCCH–CO and the HCCH–OC configurations, but the combined evidence of other hydrogen bonded complexes of carbon monoxide, the expectation that the observed hydrogen bond should be slightly shorter than the sum of van der Waals radii, and a distributed multipole analysis strongly favors the HCCH–CO configuration. [ABSTRACT FROM AUTHOR]