Index of content:
Volume 95, Issue 5, 01 September 1991
95(1991); http://dx.doi.org/10.1063/1.460910View Description Hide Description
The C–H overtone absorption spectra of (CH3O)3B in liquid and gas phases are reported. The observed energies of the C–H stretching overtones corresponding to Δv=3, 4, 5, and 6 (liquid) are obtained by conventional spectroscopy. The C–H overtones Δv=5 and 6 (gas) are obtained by laser intracavity photoacoustic spectroscopy. Computer deconvolution of the gas‐phase absorptions shows two bands at each overtone which are assigned as the overtones of nonequivalent C–H bonds. In general, for molecules in which a methyl group is directly attached to an oxygen atom, nonequivalent C–H bonds are produced due to the ‘‘t r a n s effect.’’ A different mechanism seems to occur in the case of the methyl groups in trimethylborate even though they are adjacent to an oxygen atom. In order to interpret the experimental results, a b i n i t i o molecular‐orbital calculations were performed on (CH3O)BH2, (CH3O)2BH, and (CH3O)3B. Equilibrium geometries, vibrational frequencies, and infrared intensities were calculated at the Hartree–Fock level using the 3‐21G split valence basis set. In the three molecules studied, the equilibrium conformation is such that the methyl groups have one C–H bond (C–H a ) in the plane of the molecule and c i s to a B–O bond, and two equivalent C–H bonds located symmetrically above and below the plane of the molecule (C–H b ). In (CH3O)BH2, the in‐plane C–H bond C–H a is longer than the out‐of‐plane C–H bonds C–H b (1.082 Å vs 1.0795 Å). This trend is reversed in the case of (CH3O)3B, where the C–H a and C–H b bond lengths are 1.0792 and 1.0807 Å, respectively.
The situation is more complicated in the case of the molecule (CH3O)2BH, where in one methyl group the C–H a bond is shorter than the C–H b bond (1.0794 Å vs 1.0802 Å), while in the other methyl group the corresponding C–H a bond is longer than the C–H b bond (1.0829 Å vs 1.0802 Å). The nature of the difference in the C–H bond lengths is studied and discussed in terms of group orbital interactions in both, the σ and π systems. In addition, a correlation between the C–H bond length, the corresponding C–H stretching force constant, and the vibrational frequency is discussed based on vibrational frequency calculations performed on the deuterated species (CHD2O) n BH3−n for n=1, 2, 3 in cases where the isolated C–H was considered to be the in‐plane C–H a or the out‐of‐plane C–H b .
Overtone line narrowing and intramolecular vibrational energy redistribution in substituted toluenes95(1991); http://dx.doi.org/10.1063/1.460860View Description Hide Description
The vibrational overtone spectra of some substituted toluenes are presented. The aryl CH stretching overtone regions have a simple local mode interpretation. Distinct aryl CH stretching overtone progressions can be identified for each structurally nonequivalent type of aryl oscillator. The methyl CH stretching overtone regions are more complex. Both conformationally nonequivalent methyl CH bonds and unresolved methyl rotational structure may contribute to the methyl spectral profiles. An additional factor involves a strong alkyl CH stretch‐bend Fermi resonance. Two of the molecules studied, 2,3,5,6‐tetrafluorotoluene and 2,3,4,5,6‐pentafluorotoluene, stand out. The Δv CH=3 methyl overtone linewidths of these molecules are markedly reduced compared to the other toluenes studied and reveal underlying structure. The narrowing is interpreted as arising from a detuning of the alkyl CH stretch‐bend Fermi resonance due to the presence of multiple fluorine substituents.
95(1991); http://dx.doi.org/10.1063/1.460861View Description Hide Description
Circular dichroism and optical rotation by oriented molecules in the electric dipole approximation are studied for molecules of different symmetry groups. It is shown that these effects are not connected with each other by the Kramers–Kronig relation and are irreversible, like the Faraday effect in a magnetic field. The optical rotation by the nematic liquid crystal of p‐azoxyanisole, observed earlier by Williams [J. Chem. Phys. 5 0, 1342 (1969)], can be explained on the basis of this theory. Finally, the optical activity of an ensemble of molecules oriented in some direction and randomly oriented around this direction, is considered.
95(1991); http://dx.doi.org/10.1063/1.460862View Description Hide Description
We have recorded the 351 nm photoelectron spectra of Bi− 2, Bi− 3, and Bi− 4. The spectrum of Bi− 2 shows transitions to at least seven electronic states of Bi2 neutral, four of which are observed with vibrational resolution. Term energies, bond lengths, and vibrational frequencies are obtained for the anion ground state and for the first three excited states of Bi2. These results are compared to previous spectroscopic measurements and to the a b i n i t i o calculations presented in the accompanying paper. The photoelectron spectrum of Bi− 3 reveals some of the electronic structure of Bi3 and the results are discussed in comparison to recent theoretical work. Adiabatic electron affinities are obtained for Bi2 [1.271(8) eV] and for Bi3 [1.60(3) eV]. The electron affinity of Bi4 is estimated from the onset of photodetachment to be 1.05(10) eV.
95(1991); http://dx.doi.org/10.1063/1.460863View Description Hide Description
We compute the spectroscopic constants of 26 electronic states of Bi2 and six electronic states of Bi− 2. In addition, the potential energy curves of electronic states of Bi2 dissociating into Bi(4 S)+Bi(4 S), Bi(4 S)+Bi(2 D), Bi(4 S)+Bi(2 P), Bi(2 D)+Bi(2 D), and Bi(4 S)+Bi(4 P) limits are computed. We use a complete active space multiconfiguration self‐consistent field (CAS‐MCSCF) followed by first‐order configuration interaction (FOCI) and second‐order configuration interaction (SOCI) methods. In addition, the spin–orbit effects are included through the relativistic configuration interaction (RCI) method. Our computed spectroscopicproperties facilitate the assignment of recently observed negative ion photodetachment spectra as well as the electronic spectra accumulated up to now. The observed lifetime and transition moment dependence on internuclear distance are also explained based on computed potential energy curves.
Methyl torsional fine structure in the high‐resolution S 0→T(nπ*) excitation spectrum of acetophenone and its three methyl‐deuterated isotopomers95(1991); http://dx.doi.org/10.1063/1.460864View Description Hide Description
The cold‐beam excitation spectrum of the S 0→T(nπ*) origin band of acetophenone shows two maxima separated by about 1.2 cm−1, interpreted as due to methyl tunneling. Consecutive deuteration of the methyl group leads to three maxima in acetophenone‐d 1, two maxima in acetophenone‐d 2 and a single broad maximum in acetophenone‐d 3, with maximum separations of about 7 cm− 1 in ‐d 1 and about 3.5 cm− 1 in ‐d 2. These separations are due to the occurrence of two rotamers for isotopomers without threefold permutation symmetry. The spectra are analyzed in terms of hindered rotor potentials. It is shown that S 0 and T(nπ*) share the same torsional equilibrium configuration, relative to which the S 1(nπ*) equilibrium configuration is rotated by 60°. However, both nπ* states have very shallow torsional potentials with barrier heights of 70 and 90 cm− 1 for T and S, respectively, compared with a barrier of about 800 cm− 1 in the ground state. Hence, relative to S 0 the two potentials are quite similar despite the shift. The potential terms entering upon partial deuteration are quite similar for S 0 and T(nπ*) and hence depend only weakly on the barrier height. They are ascribed to kinetic coupling of the asymmetric rotor of the CH2D or CHD2 group with other modes in the molecule.
Far‐infrared spectra of hydrogen dimers: Comparisons of experiment and theory for (H2)2 and (D2)2 at 20 K95(1991); http://dx.doi.org/10.1063/1.460865View Description Hide Description
Far‐infrared spectra of weakly bound complexes of molecular hydrogen have been studied using an infrared Fourier transformspectrometer and a long absorption path (98 m) through equilibrium gas at low temperature. The dimer transitions accompany pure rotational transitions of H2 or D2monomers. Para‐H2 was studied in the S 0(0) region (350 cm−1 ), normal H2 in the S 0(1) region (590 cm−1 ), and ortho‐D2 in the S 0(0) region (180 cm−1 ). The extensive and well resolved (13 sharp lines) spectrum observed for (D2)2 was of special interest. A new empirical (rigid rotor) fit potential of the H2–H2 system has been used for calculating eigenvalues and numerical eigenfunctions of the dimers in the close coupled formalism. Dipole moment transition probabilities were calculated by using the previously tested induced dipole moment surface of Meyer. In order to compare with experiment, bound–bound transition frequencies have been calculated for the three cases, along with the full collision‐induced spectrum for the para‐H2 S 0(0) case. It was found that the dimer binding energies give quite good agreement with experiment, and that the line and continuum intensities were also in satisfying agreement with the measurements. However, there is evidence that the rigid rotor potential function is not completely adequate, especially for dimers containing rotationally excited monomers. The next step will be to utilize a nonrigid rotor potential including the effects of the individual hydrogen monomer bond lengths.
95(1991); http://dx.doi.org/10.1063/1.460866View Description Hide Description
A computational method for the prediction of spin trajectories in systems of two or more spins evolving under the influence of the static field, an applied radio frequency field, chemical shifts, spin–spin couplings, and magnetic dipolar relaxation has been described. The program has been tested to establish its correct operation by calculation of limiting cases with known solutions. It is shown that it is necessary to retain the nonsecular elements of the relaxation matrix in order to obtain correct results, primarily elements controlling cross relaxation between transverse components of magnetization of different spin states belonging to the same quantum number of the total magnetization along the axis of the static field. As a result of the retention of these terms, two unexpected features are predicted: frequency and phase shifts in spectra of two non‐spin‐coupled nuclei with relaxation rate comparable to frequency separation and unusually intense magnetizations in the locked state of two spins with the radio frequency applied at the center of the spectrum.
95(1991); http://dx.doi.org/10.1063/1.460867View Description Hide Description
It is shown that the sum of the intensities for magnetic dipole transitions between crystal‐field components of two free‐ion levels in lanthanide spectra is almost independent of the symmetry of the environment. A mean theoretical sum value of 18×10−7D2, 94×10−7D2, and 9×10−7D2 for, respectively, the 5 D 1←7 F 0, 5 D 0←7 F 1, and 5 D 2←7 F 1 dipole strengths has been found. Experimental values of the dipole strength for Eu3+ in different lattices support within reasonable limits the theoretically derived sum rule. We therefore propose to use these magnetic dipole transitions in the Eu3+ spectrum as standards for further Judd–Ofelt parametrization.
Feshbach–Fano formalism in Hilbert space: Application to shape resonances in molecular photoionization95(1991); http://dx.doi.org/10.1063/1.460868View Description Hide Description
An explicit Hilbert‐space formulation of Feshbach–Fano theory is described which is particularly well suited for treating the problem of shape resonances in molecular photoionization. The prepared states of Fano and Cooper are employed to resolve the irreducibly infinite degeneracy of molecular electronic continua that arises from the noncentral nature of molecular (body‐frame) potentials. Previously defined L 2 Stieltjes states generated employing appropriate test functions are shown to converge to these prepared states, avoiding prior construction of the associated degenerate channel functions. A novel choice of zeroth‐order state particularly suitable for applications of the L 2 Feshbach–Fano method to molecular shape resonances is introduced and shown to avoid many of the difficulties associated with more conventional treatments employing resonancescattering theory. Specifically, the energy of this zeroth‐order state is seen to be a weighted average over the correct photoionization cross section, the associated background cross section is shown to vanish for all energies, and calculations of the associated ρ(E) and q(E) parameters are avoided entirely. Computational applications in the static‐exchange approximation to selected valence‐shell channels in N2, O2, and N2O illustrate the attributes of the method and provide accurate cross sections in each case. The L 2 Feshbach–Fano formalism is seen to provide improved results from precisely the same information—pseudostate energies and oscillator strengths—as is required in the conventional L 2 Stieltjes development, with no significant increase in computational effort required.
95(1991); http://dx.doi.org/10.1063/1.460869View Description Hide Description
The excited singlet state intermolecular proton transferreaction in jet‐cooled clusters of 1‐naphthol/ammonia and water is investigated employing mass resolved excitation, threshold photoionization, and emission spectroscopy. The complete data set indicates that no proton transfer occurs for 1‐naphthol(NH3)1,2 and (H2O) n , n=1,...,20 clusters. Proton transfer occurs for (at least) one configuration of the 1‐naphthol(NH3)3 cluster, as well as all 1‐naphthol(NH3) n , n≥4, clusters. The (at least) two configurations of 1‐naphthol(NH3)3 clusters are distinguished by threshold photoionization studies. The 1‐naphthol(NH3)3 cluster for which proton transfer is indicated has a threshold photoionizationenergy ∼2000 cm−1 below the other 1‐naphthol(NH3)3 cluster configurations. These results are employed to explain the previous discrepancy between static spectroscopic experiments and picosecond time resolved dynamic experiments concerning proton transfer in the 1‐naphthol(NH3)3 cluster. Calculations of cluster geometry in 1‐naphthol/ammonia and water cluster systems suggest some qualitative explanations of these observations.
95(1991); http://dx.doi.org/10.1063/1.460870View Description Hide Description
The electron‐paramagnetic‐resonance spectrum of C4 in a neon matrix at ∼4 K formed by the vacuum‐ultraviolet photolysis of 1,3‐butadiene has been investigated to determine if there is any evidence of the bending of the molecule, which had previously been indicated by the results of EPR and Fourier‐transform infrared measurements made on C4 trapped in argon at 10–12 K. The observed spectra, which include 13C hyperfine splitting and zero‐field splitting data, confirm the splitting of the perpendicular x y 1 and x y 2 lines of triplet C4 into separate x and y components, which is indicative of the inequivalent axes of a bent molecule.
95(1991); http://dx.doi.org/10.1063/1.461780View Description Hide Description
We compare the spectral diffusion broadening of mesoporphyrin substituted horseradish peroxidase with the chromophore doped host glass. Whereas the broadening in the glass is smooth and follows a power law with an exponent close to (3)/(2), there is a a steplike broadening in the protein which occurs around 14 K. The observed phenomena are interpreted on the basis of a two domain interaction of the chromophore with its environment.
An analysis of the methyl rotation dynamics in the S 0 (X̃ 1 A 1) and T 1 (ã 3 A 2) states of thioacetone, (CH3)2CS and (CD3)2CS from pyrolysis jet spectra95(1991); http://dx.doi.org/10.1063/1.460871View Description Hide Description
Jet‐cooled, laser‐induced phosphorescence excitation spectra (LIP) of thioacetone (CH3)2CS/(CD3)2CS have been recorded over the region 16 800–18 500 cm−1 using the pyrolysis jet spectroscopic technique. The responsible electronic transition, T 1←S 0, ã 3 A‘←X̃ 1 A 1, results from an n→π* electron promotion and gives rise to a pattern of vibronic bands that were attributed to activity of the methyl torsion and the sulphur out‐of‐plane wagging modes. The intensities of the torsional and wagging progressions in the excitation spectra were interpreted in terms of a C 2v –C s molecular distortion of the triplet molecule from its singlet ground state equilibrium structure. A complete unrestricted Hartree–Fock (UHF) a b i n i t i o molecular orbital (MO) structural optimization of the T 1 state predicted that the sulphur was displaced by 27.36° from the molecular plane and the methyl groups were rotated by 10.93° in clockwise–counterclockwise directions. Restricted Hartree–Fock (RHF) calculations were used to generate the V(θ1,θ2) potential surface governing methyl rotation for the S 0 state. This was incorporated into a two‐dimensional Hamiltonian, symmetrized for the G 36 point group and solved variationally for the torsional frequencies. The calculated frequencies of 159.97/118.94 for the ν17(b 1) mode of S 0 (CH3)2CS/(CD3)2CS were found to agree with the experimental values, 153.2/114.7 cm−1.
95(1991); http://dx.doi.org/10.1063/1.460872View Description Hide Description
In this paper, a semiempiricaltheory for the spectral shifts of the electronic origin of the S 0→S 1 transition of (aromatic molecule)⋅(rare‐gas) n heteroclusters is advanced and applied. Neglecting the modifications of intermolecular overlap and exchange interactions upon electronic excitation, the dispersive contributions to the spectral shift are evaluated to second order, accounting for finite‐size structural features of the large molecule by the utilization of the multicenter monopole representation of the intermolecular interactions. The spectral shifts for nonpolar aromatic hydrocarbons in or on rare‐gas heteroclusters are represented in terms of differences between electrostatic interactions involving an electrostatic field (due to the molecular transition monopoles charge distribution) and an induced dipole (originating from the rare‐gas polarizability). The transition monopoles incorporated all the one‐ and two‐electron ππ* excitations of the aromatic molecule, which were represented by Hückel or self‐consistent molecular orbitals (MO). The dispersive spectral shifts were semiempirically scaled to correct the systematic overestimate of the transition monopoles within these simple MO schemes. The red spectral shift was recast in terms of a sum of two‐atom (carbon atom–rare‐gas) and three‐atom (carbon atom–rare‐gas–carbon atom) contributions, which are subsequently summed over the contributions of all the rare‐gas atoms, with each term involving products of an electronic factor and a geometric factor.
The electronic factors exhibit a linear dependence of the spectral shift on the rare‐gas polarizability, while the geometric factors incorporate the structural effects of the contributions of the individual rare‐gas atoms to the spectral shift, predicting the nonadditivity of the spectral shift per added rare‐gas atom and the exhibition of distinct spectral shifts for different structural isomers. The semiempiricaltheory in conjunction with structural information emerging from (pairwise) potential optimization for small and medium‐sized (n=1–8) heteroclusters and from molecular dynamics simulation for medium‐sized and large heteroclusters (n=5–34) was applied. The theory accounts for the spectral shifts of small and medium‐sized rare‐gas heteroclusters of pentacene and tetracene as well as for small, medium‐sized, and large heteroclusters of 9,10 dichloroanthracene, but not for the general pattern of the spectral shifts for anthracene⋅A n (A=Ar, Kr; n=1–8) and for perylene⋅Ar n (n=2). The confrontation between theory and experiment for the spectral shifts of pentacene⋅Ar n (n=1–8), tetracene⋅Ar n (n=1–8), tetracene⋅Kr n (n=1–8), 9,10 dichloroanthracene⋅Ar n (n=1–34) and 9,10 dichloroanthracene⋅Kr n (n=1–20), allows for the elucidation of the size dependence of spectral shifts, for the identification of structural isomers in small and medium‐sized heteroclusters, and for the exploration of spectral inhomogeneous broadening in large heteroclusters.
95(1991); http://dx.doi.org/10.1063/1.460873View Description Hide Description
We have performed molecular orbital calculations for SF6 and H2S, using the discrete–variational Xα method. Energies and cross sections of virtual states are obtained for theoretical spectra of sulfur K, L II,III and fluorine K x‐ray absorptions for the SF6 molecule. They are in good agreement with the experimental results. Through the same calculation procedures, the theoretical spectra of sulfur K and L II,III absorptions for H2S are derived. The obtained molecular orbitals represent the experimental spectra below the ionization energy very well. SF6 has distinct shape resonances above the ionization energy, in contrast to H2S which has pre‐edge peaks as a main structure. For the SF6 molecule, shape resonances come from the scattering at the steep change of attractive potential of surrounding fluorine atoms. It is demonstrated that phase shift due to the scattering causes the resonances without a potential barrier. Various spectral differences among the sulfur K, L II,III and the fluorine K absorptions in peak width and in base line are related to the character of wave functions. Other resonances among the fluorine wave functions contribute to the characteristics of the base line which was explained as the continuum state alone previously. For the H2S molecule, the reason for the absence of resonances is attributable to the weak scattering power of hydrogen atoms.
Continuous‐wave supersonic jet diode laser spectroscopy and dynamics of Ar–DCl: Rovibrational analysis of ν1 and ν1+ν1 2 and the effect of Coriolis coupling in the spectrum of ν1+2ν0 295(1991); http://dx.doi.org/10.1063/1.460874View Description Hide Description
The ν1 fundamental and ν1+ν1 2 intermolecular combination bands of the van der Waals dimer Ar–DCl have been observed using a sensitive tunable diode laser continuous‐wave (cw) supersonic jet spectrometer. The experimental results are compared with frequency and intensity predictions based on variational calculations using the H6(3) potential of Hutson. A possible assignment of the ν1+2ν0 2 combination band is proposed on the basis of anomalous intensity predictions from these calculations. The calculations indicate that the R branch of the ν1+2ν0 2 combination band is much stronger than the P branch due to a strong Coriolis mixing of the ν1+2ν0 2 and ν1+ν1 2 states.
An investigation of three‐body effects in intermolecular forces. II. Far‐infrared vibration–rotation–tunneling laser spectroscopy of Ar2HCl95(1991); http://dx.doi.org/10.1063/1.461799View Description Hide Description
A second Ar2HCl intermolecular vibration–rotation band centered at 37.2 cm−1 has been measured and assigned as a b‐type transition originating from the ground state. Nuclear hyperfine splittings were resolved for both chlorine isotopes. The rotational constants determined from the data indicate coupling between an Ar–Ar stretching or bending coordinate and the Ar2 –HCl vibrational coordinates. As a result of this particular vibrational motion, Ar2H 35Cl undergoes an axis‐switching transition while the Ar2H 37Cl isotope does not. In addition, the measured hyperfine projections indicate the possibility of coupling between the Ar2 –HCl stretching and bending modes, preventing an absolute vibrational assignment. These results indicate that the ‘‘reversed adiabatic’’ approximation employed by Hutson, Beswick, and Halberstadt in their theoretical study of Ar2HCl [J. Chem. Phys. 9 0, 1337 (1989)] is not appropriate for the complicated intramolecular dynamics presently observed in this system.
95(1991); http://dx.doi.org/10.1063/1.460875View Description Hide Description
This paper treats the vibrational and UV/visible spectra of oligomers of polyparaphe‐ nylenevinylene (PPV) and lays the background knowledge for the treatment of the spectra and structure of PPV which is treated in the paper that immediately follows. The problem of the molecular conformation of these systems is faced both with MNDO (modified neglect or differential overlap) calculations and with infrared‐Raman experiments. Oligomers are certainly twisted in noncrystalline phases while may approach coplanarity in the solid. Fourier transformRaman spectra are recorded and are interpreted in terms of the recently developed theory of the effective conjugation coordinate.
95(1991); http://dx.doi.org/10.1063/1.460876View Description Hide Description
The infrared and Raman spectra of oligomeric model molecules of polyparaphenylenevinylene (PPV) and of the polymer in the pristine state, and the infrared spectra of doped or photoexcited PPV are analyzed in terms of the theory of the effective conjugation coordinate (ECC). It is shown that all the spectroscopic observations (frequencies and intensities) are accounted for by ECC theory which shows that most of the observed features are due to the delocalization of π electrons. Measurements of the lower limit of the effective conjugation length and of the molecular chain length are made and the structural relevance of such concepts are discussed in terms of the conformation of the oligomeric or polymeric chain. Evidence is found that the charge carrier produces ‘‘quinoid’’ type defects, hence fully coplanar.