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  • 1
    Electronic Resource
    Electronic Resource
    Calculated Properties and Ring-Chain Rearrangements of Triphosphirane (P3H3) (2000)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 2000 (2000), S. 103-112 
    Details
    ISSN: 1434-1948
    Keywords: Ab initio calculations ; Ionization potentials ; Phosphorus ; Heterocycles ; Small ring systems ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio quantum chemical calculations have been used to explore the P3H3 potential energy surface focussing on the ring-chain rearrangements of the three-membered ring in (PH)3 (1), the parent triphosphirane. Relative energies between stationary points were estimated using the QCISD(T)/6-311G(d,p) method based on MP2/6-31G(d,p) geometries and corrected for zero-point contributions. Ring strain, proton affinities, ionization and excitation energies and heats of formation have been evaluated using larger basis sets, e.g. 6-311++G(3df,2p). The cyclic trans-triphosphirane (1a) is the most stable P3H3 isomer and lies about 40 kJ/mol below the open-chain phosphanyldi-phosphene (H2P-P=PH). The decrease of ring strain in three-membered rings when CH2 is replaced by PH is confirmed. Triphosphirane 1a is a virtually strain-free ring and even gains some stabilization relative to three separate P-P single bonds. The reduced ring strain also helps diminish the phosphorus inversion barrier to 224 kJ/mol compared to the monocyclic isomers of (CH2)(PH)2 and (CH2)2(PH). Compound 1a follows a pure ring-opening or a 1,2-hydrogen shift rather than a combined motion pathway, in fundamental contrast with corresponding processes of diphosphirane and phosphirane. This is due to the existence of an open-chain P3H3 phosphorane intermediate stabilized by allylic conjugation. The pericyclic ring-opening of 1a is the most favored process but the energy barrier in the gas phase is about 180 kJ/mol high. Electron density is largely delocalized within the three-membered P3 ring not only in the C3v-symmetric 1b (all-cis) but also in 1a (Cs). The proton affinity of 1a is similar to that of PH3. The proton affinities decrease with n in cyclo-(CH3)3 -n(PH)n and their values were obtained: PA(1a) = 777 ±10, PA(diphosphirane) = 799 ±10 and PA(phosphirane) = 802 ±10 kJ/mol. Heats of formation are evaluated as follows (ΔH°f0 at 0 K in kJ/mol): 1a, 70 ±10; cyclo-(PH)2(PH2)+ (protonated 1a), 821 ±10; diphosphirane, 85 ±10; cyclo-(CH2)(PH)(PH2)+ (protonated diphosphirane), 814 ±10; phosphirane, 86 ±10; and protonated phosphirane, 812 ±10 kJ/mol. All P rings remain cyclic following ionization to the radical cations. Adiabatic ionization energies (IEa) are estimated as: 1a and diphosphirane, 9.3 ±0.3 eV and phosphirane 9.5 ±0.3 eV. The first UV absorption band shifts toward the longer wavelength region on going from phosphirane to 1a. The GIAO/B3LYP computed magnetic shieldings for 1a and related molecules reveal a clear relationship between the narrow bond angles in the rings and their unusually strong magnetic shielding. The similarity of the predicted 31P-NMR signals in 1a and its heteroanalog diphosphirane, (CH2)(PH)2, can be rationalized in terms of a compensation of the carbon-substituent effect (downfield shift) and the bond-bending effect imposed by the ring (upfield shift).
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Transformation of 3,4-Bis(isopropylidene)-2,5-dichloro-1,2,5-thiadiborolane into nido-2,4,5-Thiadicarbahexaborane(5) Derivatives - Formation and Computational Elucidation of a Thiadicarbanonaborane(8) with a ni-9〈IV+IV〉 Configuration (1998)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 1775-1779 
    Details
    ISSN: 1434-1948
    Keywords: 3,4-Bis(isopropylidene)-1,2,5-thiadiborolane ; nido-2,4,5-Thiadicarbahexaboranes(5) ; Isomeres ; Thiadicarbanonaborane(8) ; IGLO ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 3,4-bis(isopropylidene)-2,5-dichloro-1,2,5-thiadiborolane (5b), obtained from 3,4-bis(dichloroboryl)-2,5-dimethyl-2,4-hexadiene (4) and (Me3Si)2S, reacts with Li[RBH3] (R = H, C6H5, C6Me4H) to yield the corresponding derivatives of the nido-4,5-diisopropyl-2,4,5-thiadicarbahexaboranes 2. Replacements of the chlorine atoms in 5b with two hydrido, or with one hydrido and one aryl (phenyl or duryl) group, followed by the hydroboration of the isopropylidene substituents with RBH2 (R = H, C6H5, C6Me4H) lead to four nido-2,4,5-thiadicarbahexaboranes (2a-d) in low yields. Their composition follows from MS and NMR data; not refined X-ray structural data of 2d support the proposed skeletal structure. In addition to 2a, larger thiacarboranes were detected by GC/MS and 11B NMR. The structure of the nine-vertex cluster 6a could be identified by applying the ab initio/IGLO/NMR method. Geometry optimizations at the MP2(fc)/6-31G* level rule out the Csnido-SC2B6H8 isomer 7. The C2v structure of 6 consists of a nine-vertex cluster with two elongated B-B distances. As this 22e cluster 6 represents an exception to the Wade rules, the nature of the bonding of nine-vertex clusters with different electron counts is discussed.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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