Zum Schwingungsverhalten pentakoordinierter Zinn(IV)-organischer Verbindungen. IR- und Raman-Untersuchungen an 2,8,9-Tricarbastannatranen

Summary Results are reported of IR and Raman investigations on four 1-substituted 2,8,9-tricarbastannatranes (1,X=Cl;2,X=Br;3,X=I;4,X=Me). Group-theoretic investigations confirm the trigonal-bipyramidal configuration of the tin atoms. The vibrational frequencies of the coordination (SnN) are correlated to the distancesd (Sn-N). Besides the results on the vibrations of the coordination polyhedrons other results are reported concerning the enantiomerization of the chiral atran skeleton.

     

Collision induced dissociation of stored gold cluster ions

The stability of gold cluster ions Au _n ⁺ (2n23) has been investigated via collision induced dissociation in a Penning trap. Threshold energies and dissociation channels have been determined. The cluster stability exhibits a pronounced odd — even alternation: Clusters with an odd number of atoms,n, are more stable than the even-numbered ones. Enhanced stabilities are found for Au ₃ ⁺ , Au ₉ ⁺ , and Au ₁₉ ⁺ in accordance with the Clemenger-Nilsson and the deformed jellium model of delocalized valence electrons. Excited odd cluster ions withn15 predominantly decay by evaporation of dimers; all others decay by monomer evaporation. From the dissociation channels estimates of the binding energies are deduced.This publication comprises part of the thesis of St. Becker     

Thermodynamic transition properties of highly ordered smectic phases

A series of polyethers have been synthesized from 1-(4-hydroxy-4-biphenyl)-2-(4-hydroxyphenyl)propane and , -dibromoalkanes having different numbers of methylene units [TPPs]. Both odd- and even-numbered TPPs [TPP(n=odd)s and TPP(n=even)s) exhibit multiple transitions during cooling and heating and they show little supercooling dependence, indicating close-to-equilibrium nature of these transitions. Combining the structural characterization obtainedvia wide angle X-ray diffraction powder and fiber patterns at different temperatures and the morphological observations from microscopy techniques, not only the nematic liquid crystalline phase but also highly ordered smecticF, smectic crystalG andH phases have been identified. The phase diagrams for both TPP(n=odd)s and TPP(n=even)s have been constructed [1–3]. Thermodynamic properties (enthalpy and entropy changes) during these transitions are studied based on differential scanning calorimetry experiments. The contributions of the mesogenic groups and methylene units to each ordering process can be separated and they indicate the characteristics of these processes thereby providing estimations of the transition types.This work was supported by the SZDC's Presidential Young Investigator Award from the National Science Foundation (DMR-9175538). The generous donation of a DSC-7 from Perkin Elmer Inc. is also greatly appreciated.     

Electronic structures of organometallic complexes of f elements LXXIV: First Raman spectroscopic polarization measurements on uniformly oriented sandwich complex molecules: Bis(η-pentamethylcyclopentadienyl)ruthenium

The polarized Raman spectra of oriented single crystals as well as far and mid infrared spectra of pellets of RuCp₂* (Cp* = η⁵-C₅Me₅) (1) were recorded. Assuming local C_5v symmetry for the intra-ligand vibrations, pairs of Raman and IR bands of nearly equal energy result for the symmetric and antisymmetric modes, respectively, for the irreducible representations (irreps) a₁, e₁, and partly (the IR part is symmetry forbidden, in principle, but sometimes observed) of e₂ symmetry. By this means, intra-ligand and skeletal vibrations (where no pairs of Raman and IR bands are expected) could be separated, and the Raman active modes were assigned to irreps on the basis of the observed polarizations. The still questionable type of vibration of some intra-ligand modes could be elucidated by the comparison of the vibrational spectra of 1 with the already assigned ones of NaCp*. Transferring the results of 1 to the Raman and IR spectra of OsCp₂* (2) and FeCp₂* (3), a number of previous assignments have to be revised.     

Structure and spectra of 1,3,2-dioxaphospholenes. 1. Microwave spectra of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene and its isotopomers in the ground and excited vibrational states

The microwave spectrum of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene (1) was studied in the frequency range from 7 to 53 GHz. Rotational transitions of the parent molecule in the ground and eleven excited vibrational states and those of its mono-substituted ³⁷Cl, ¹³C_Me, and ¹³C_Cycle isotopomers in the ground vibrational state were identified. Rotational constants and partial r _s-structure were obtained. The quartic centrifugal distortion constants, dipole moment components _a = 3.8D and _c = 0.24D (the total dipole moment is 3.81D), and the ³⁵Cl quadrupole coupling constants were determined for the parent molecule. The fine structure of the microwave transitions in the parent molecule was analyzed under the assumption of noninteracting methyl groups. The height of the barrier to internal rotation (V ₃₀ = V ₀₃ = 665 cm^–1) and the frequency of torsional vibrations ( = 167 cm^–1) were found. The frequencies of two lowest vibrational modes corresponding to deformation vibrations of the five-membered ring were estimated (100 cm^–1) from the relative intensities of rotational transitions for different vibrational states.     

Heavy-atom tunneling in cyclobutadiene:Ab initio calculation of the intensities ofa g Raman lines

Summary GVB/[5s3p1d/3s1p] calculations were performed on the polarizability surface (₁, ₂, ₃) of cyclobutadiene. The threea _g coordinates refer to automerization (₁), symmetric CC stretch (₂), and symmetric CCH bend (₃). This surface was used together with the previously obtained variational vibrational wave functions for the calculation of Raman intensities. The calculation predicts comparable intensities for the two split components of the automerization Raman line in an isolated molecule and disagrees with observations on matrix-isolated cyclobutadiene. The disagreement is attributed to the asymmetry of the double-well potential imposed by the effect of the Ar matrix.Dedicated to Professor Ruedenberg     

Gertrude Belle Elion: Biochemist,Nobel Prize 1988

Most chemistry courses involving group theory do not treat the infinite point groups C_v or D_h. Even the character tables themselves are not transparent looking, with the profusion of ellipses. Because of this, it is not possible to deduce the vibrational symmetry modes of CO₂, even though the symmetric, asymmetric stretch, and doubly-degenerate bending modes are discussed in almost all physical chemistry courses. In this paper we show how linear molecules such as CO₂, C₂H₂, and HCN can be treated using the point groups C_nv or D_nh for general values of n. When determining which irreducible representations comprise the normal vibrational modes of a linear molecule such as CO₂, we show that the n-dependence vanishes. The calculations presented here do not require advanced mathematical knowledge and could be incorporated into an undergraduate chemistry curriculum in which group theory is presented.     

Modellberechnugnen zu den Schwingungsspektren von Phenylsilanen und einigen verwandten Verbindungen

Normal coordinate calculations on symmetric vibrations of (C₆H₅) _n MX _4–n type molecules (M=C, Si, Ge, Sn, Pb, P andX=H, F, Cl, Br, OH) have been performed using a simplified model for M-sensitive modes of the phenyl group. A good agreement of calculated and observed frequencies is obtained with reasonable and transferable force constants. Significant effects of vibrational coupling are illustrated byPED calculations.

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Mit 3 Abbildungen     

Eigenvalues of large even annulenes in terms ofK 3,S 4 andS 5

Squares of the adjacency matrices of bipartite cycles (C_v) can be block-factored into matrices which correspond to vertex-weighted complete graphs forv = 6, vertex-weighted strongly regular graphs forv = 8 and 10, and vertex-weighted metrically regular graphs forv > 10. Using this fact and some properties of strongly and metrically regular graphs, it is shown that eigenvalues of large bipartite C _v graphs (i.e. large even annulenes) can be expressed by the general formula ± (2 ± (2 ± (... ± (2 +r _p)) ...), wherev = 2 ⁿ ×p,n is the number of surd () signs required andp = 3, 4 and 5. Here,r ₃,r ₄, andr ₅, are the eigenvalues of the complete graphK ₃ and the strongly regular graphsS ₄ andS ₅ respectively. The procedure does not require construction of characteristic polynomials for the determination of eigenvalues, and brings out a common topological origin for the two-fold degeneracies observed in the eigenvalue spectra of all even cycles and many odd cycles.     

Calculation of the rotational constants A and C for isotopic species of slightly perturbed CH3CN symmetric top molecules

A technique which was employed earlier to calculate the rotational constants of CH₃CCH has been extended to the ground and two vibrational levels in thev ₈ vibration of CH₃CN for several isotopic species. The moments of inertia and a computer iteration technique over experimental data for each isotopic species were employed to evaluate the constantA _v in an excited vibrational state for a symmetric top molecule. This method gave good estimates forA _v for each isotope. The angle of bending and the orientation of each molecular system in reference frames, one fixed on the carbon atom at the -CN site and the other at the center of mass, were explored. These results are discussed in this paper. The method, which was applied by Tam and Roberts to the nv₁₀,n=1, 2, 3, 4, vibrations of CH₃CCH earlier and which was extended to thev ₁₀=1 vibration of CH₃CCH with¹³C isotopic species, has been applied to¹³C isotopic species of CH₃CN and seems to be a useful tool to extract the value ofA ₀. Each of these molecules shows reasonable dependency ofA _v over vibrational levels. Values ofA _v calculated from the geometrical model are in good agreement with those obtained by fitting the terms in the frequency equations, which containedA _v , to the experimental data through an iteration technique in which the value ofA was allowed to vary.     

Vibrational spectra of some o-substituted benzoic acid derivatives

The use of a semiempirical method based on the application of Group Theory to a previously established geometrical model according to the molecular structure allowed us to carry out the vibrational analysis of infrared and Raman spectra in solid phase of some o-substituted benzoic acid derivatives. Because the influence of the different substituents on the frequency of the ring vibrational normal modes is slight, the study of the ring vibrations independently of the substituent internal vibrations was possible. On this basis a tentative assignment of the ring vibrational normal modes for the o-NH₂, o-CH₃, o-Cl and o-COOH benzoic acids is proposed.     

Interpretation of vibrational and NMR spectra of allyl acrylate: An evidence for several conformers

The mid-IR, far-IR, and Raman spectra of allyl acrylate were measured and interpreted with support of the B3LYP/aug-cc-pVDZ calculated anharmonic vibrational spectra followed by the potential energy distribution analysis. The experimental ¹H and ¹³C NMR spectra of allyl acrylate dissolved in CDCl₃ or C₆D₆ were interpreted by means of the B3LYP/aug-cc-pVDZ-su2 calculated NMR chemical shifts and J(¹H,¹H) and J(¹H,¹³C) coupling constants. Exactly ten stable allyl acrylate conformers (five s-cis and five s-trans) were found after careful B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ scan of the conformational space. The experimental IR and Raman spectra are in good agreement with the theoretical spectra of the most stable conformers 1 with a presence of the second stable conformer 2, both exhibiting cis arrangement of the acrylic moiety. There are however two bands in the IR spectra, at ca. 1270 and 1260 cm⁻¹, that definitely indicate the conformers with trans arrangement of the acrylic moiety to be present in liquid allyl acrylate. The bands at ca. 2990 and 1650 cm⁻¹ are suggested to be due to Fermi resonances engaging CH and CC stretching vibrations, respectively. The careful inspection of the room temperature ¹H and ¹³C NMR spectra of allyl acrylate suggest that a dominating form of the allyl acrylate molecule in an inert solvent exhibits the cis conformation of the acrylic moiety and an extended allyl group.     

Sodium doped binary clusters I: Ionization potentials of SinNam clusters

Sodium doped silicon clusters (Si_nNa_m, 3n11, 1m4) were produced by two independent laser vaporization methods and their ionization potentials were measured by scanning the wavelength of the UV dye laser. The IPs of most Si_nNa_m clusters decrease monotonously with the number of Na atoms, but IPs of Si₇Na_m and Si₁₀Na_m clusters show an apparent even-odd alternation; odd numbers of Na atoms efficiently decrease the IP but even numbers of Na atoms never significantly decrease the IPs. In addition, the reactivity of Si_nNa_m clusters for NO molecules was investigated with a fast flow reactor, and an anti-correlation between IP and the reactivity was clearly observed; clusters having high IP show low reactivity andvice versa.     

Die Schwingungsspektren einiger Pentadeuterophenylphosphorverbindungen

The vibrational spectra of C₆D₅PX ₂, (C₆D₅)₂PX (X=H, Cl), (C₆D₅)₃P and of the Cyclophosphanes (PC₆H₅) _n and (PC₆D₅) _n (n=5, 6) are reported. The spectra of the phenylphosphorouscompound D (the structure beeing unknown) are given too. The C₆H₅/C₆D₅ isotopic shift data in the lower frequency-region (600–100 cm^–1) (facilitating the recognition of vibrational coupling effects) are used for vibrational assignments.

     

IR spectroscopic study of allylperoxy radical and products of its phototransformations in the Ar matrix

Matrix IR spectra of an allylperoxy radical (1) obtained by co-condensation of allyl radicals and molecular oxygen into an argon matrix at 12 K have been studied. The bands observed in the matrix IR spectrum are assigned to normal vibrations on the basis of ab initio calculations of radical1 and its isotope-substituted analog CA 18_{O 2} (1a). The band at I128.6 cm^–1 is assigned to the stretching vibration of the 0-0 bond. The products of photodecomposition of radical1 under UV irradiation ( > 248 nm) have been studied. A vinylacyl radical (2) with a characteristic high vibrational v(C=0) frequency of 1823.I cm -1 has been observed among the products of photolysis of1. According to the data of quantum chemical calculations, some delocalization of a free electron between an oxygen atom and two C atoms is observed in radical 2.Preliminary results were presented at the XII International Conference on Physical Organic Chemistry (Padova, Italy) in 1994.¹⁵ Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. I, pp. 109–116, January, I996.     

Crystal structure and vibrational spectrum of the NaCaMg2F7 pyrochlore

The crystalline structure of NaCaMg₂F₇ was determined using single crystal X-ray diffraction. This compound crystallizes in the cubic pyrochlore structure, i.e., space group , lattice parameter: and Z=8. All atoms occupy special crystalline sites, but Na and Ca are randomly distributed in the anti-cristobalite sub-lattice of the pyrochlore structure. The vibrational spectrum was determined by polarized Raman scattering and infrared reflectance. The number of observed Raman and infrared active phonons is larger than that predicted by the factor group analysis of the pyrochlore structure. The anomalous vibrational spectrum is discussed in terms of a disorder-induced symmetry lowering mechanism.     

The lipid composition of the Western Mediterranean aerosol

Summary Marine remote aerosol samples collected in a fixed station situated at 1100 m above sea level in Mallorca Island (Western Mediterranean) have been analyzed for lipids, both neutral and acidic, by capillary gas chromatography (GC) and GC coupled to mass spectrometry. Higher plant and algal compounds are predominant. The former encompass distributions of even numbered C22–C32 n-alkan-1-ols (8–21 ng/m³) and n-aldehydes, C25–C32 odd numbered n-alkanes (3–6 ng/m³), and n-nonacosan-10-ol, -sitosterol (0–0.09 ng/m³), dehydroabietic acid and triterpenols. The latter are constituted by even numbered C14–C18 fatty acids (0.6–30 ng/m³) and cholest-5-en-3-ol (0–0.86 ng/m³). Pyrolytic polycyclic aromatic hydrocarbons are also found (86–410 pg/m³) whereas direct petrogenic inputs are not significant.     

A Raman spectroscopic study of the phase transition of BaZr(PO4)2: Evidence for a trigonal structure of the high-temperature polymorph

We have studied the structural evolution of monoclinic BaZr(PO₄)₂ during heating up to 835 K by Raman spectroscopy. In agreement with previous studies we found a first-order phase transition at about 730 K during heating while upon cooling the reverse transition occurs at 705 K. However, some disagreement about the crystal structure of the high-temperature polymorph occurs in the literature. While the space group has not yet been determined, the X-ray diffraction pattern of the high-temperature phase has been indexed on either an orthorhombic or a hexagonal unit cell. We found that the number of Raman active internal PO₄ vibrational modes decrease from nine to six during the transition. A group theoretical survey through all orthorhombic, trigonal, and hexagonal factor groups revealed that the observed number of vibrations would only be consistent with the Ba and Zr atoms located at a site, the P and two O atoms at a C_3v(3m), and six O atoms at a C_s(m) site in the D_3d factor group. Based on our Raman data, the space group of the high-temperature polymorph is thus either , , or .