Here we report that attempted preparation of low-valent Ca
complexes in the form of LCa-CaL (where L is a bulky β-diketiminate ligand) under dinitrogen (N
) atmosphere led to isolation of LCa(N
)CaL, ...which was characterized crystallographically. The N
- anion in this complex reacted in most cases as a very potent two-electron donor. Therefore, LCa(N
)CaL acts as a synthon for the low-valent Ca
complex LCa-CaL, which was the target of our studies. The N
- anion could also be protonated to diazene (N
H
) that disproportionated to hydrazine and N
The role of Ca d orbitals for N
activation is discussed.
Liquid crystals (LCs) are becoming increasingly important for applications in the terahertz frequency range. A detailed understanding of the spectroscopic parameters of these materials over a broad ...frequency range is crucial in order to design customized LC mixtures for improved performance. We present the frequency dependent index of refraction and the absorption coefficients of the nematic liquid crystal 5CB over a frequency range from 0.3 THz to 15 THz using a dispersion-free THz time-domain spectrometer system based on two-color plasma generation and air biased coherent detection (ABCD). We show that the spectra are dominated by multiple strong spectral features mainly at frequencies above 4 THz, originating from intramolecular vibrational modes of the weakly LC molecules.
The trans Effect in Palladium Phosphine Sulfonate Complexes Rezabal, E; Ugalde, J.M; Frenking, G
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
10/2017, Letnik:
121, Številka:
40
Journal Article
Recenzirano
Palladium phosphine sulfonate complexes constitute an efficient family of catalysts for both homopolymerization of ethylene and copolymerization of ethylene with a number of polar monomers. Their ...catalytic mechanisms have been extensively studied but not fully understood at the electronic structure level. The energy decomposition analysis, complemented with the inspection of the natural orbitals for chemical valence, reveals that their catalytic activity can be rationalized in terms of the so-called trans effect. Furthermore, our analysis shows that the competition for the σ donation of the two ligands PMe3 and L, of the palladium phosphine sulfonate complexes, to the same orbital of Pd in the trans isomer and to different orbitals in the cis isomer is the origin of the trans effect. Although the dominance of the phosphine group prevents an efficient interaction of the ligand L with the Pd atom, the large stabilization gained by the phosphine group renders a very stable trans complex.
The results of an energy decomposition analysis of various classes of donor–acceptor complexes of transition metals and main-group elements are discussed. It is shown that the nature of the chemical ...bond can be quantitatively identified in terms of Pauli repulsion, electrostatic attraction and covalent bonding. The covalent and electrostatic contributions to the interatomic attraction can be precisely given by using a well defined partitioning method in conjunction with accurate quantum chemical calculations of the geometries and bond energies. This is shown for six classes of donor–acceptor complexes: (a) transition metal carbonyl complexes; (b) transition metal complexes with Group-13 diyl ligands ER (E=BTl); (c) transition metal complexes with phosphane ligands (CO)
5TMPX
3 (TM=Cr, Mo, W; X=H, Me, F, Cl); (d) main group complexes with phosphane ligands X
3BPY
3 and X
3AlPY
3 (X=H, F, Cl; Y=F, Cl, Me, CN); (e) transition metal metallocene complexes Fe(η
5-E
5)
2 and FeCp(η
5-E
5) (E=CH, N, P, As, Sb); (f) main group metallocenes ECp
2 (E=BeBa, Zn, SiPb) and ECp (E=LiCs, BTl).
Mucopolysaccharidosis type II (MPS II; Hunter syndrome) is an X-linked, recessive, lysosomal storage disorder caused by deficiency of iduronate-2-sulfatase. It has multisystemic involvement, with ...manifestations in the brain, upper respiratory tract, heart, abdomen, joints and bones. Bone involvement leads to decreased growth velocity and short stature in nearly all patients. A therapeutic option for patients with MPS II is enzyme replacement therapy (ERT) with idursulfase (Elaprase®). We compared annual growth rates before and during ERT in 18 patients from Mainz, Germany, and Manchester, UK. Group 1 included nine patients who started ERT before 10 years of age; group 2 contained nine patients aged more than 10 years at the start of ERT. All patients had received weekly or biweekly ERT or placebo for 1 year, followed by ERT for more than 3 years. For patients in group 1, the mean (± SD) height increase was 14.6 ± 5.5 cm during 3 years of ERT. Only one patient in this group (who was below the 3rd percentile when starting ERT) deviated from the normal growth curve over this time. Patients in group 2 had a mean height increase of 8.1 ± 1.7 cm after 3 years of ERT compared with an increase of 1 cm in the year before ERT. ERT seems to have a positive influence on growth in patients with MPS II. Most benefit is seen in patients beginning ERT before the age of 10 years. This supports the recommendation that ERT should be started as early as possible in patients with MPS II.
Building a Quintuple Bond Frenking, Gernot
Science (American Association for the Advancement of Science),
11/2005, Letnik:
310, Številka:
5749
Journal Article
Recenzirano
Frenking provides his views on a study by Nguyen et al on the synthesis of a stable compound with fivefold bonding between two chromium atoms. He contends that some details of the experimental ...findings will certainly lead to controversial discussions about whether the bonding situation should be considered a true quintuple bond.
Quantum-chemical calculations at the B3LYP level have been carried out to elucidate the reaction mechanism of the epoxidation of ethylene with the molybdenum diperoxo complex MoO(O2)2OPH3. All ...relevant transition states and intermediates which belong to the reaction pathways suggested by Mimoun and by Sharpless were optimized. The calculations show that there is no reaction channel from the ethylene complex to the putative metalla-2,3-dioxolane intermediate as suggested by Mimoun. There is a transition state for the direct formation of the five-membered cyclic intermediate from ethylene and the diperoxo complex. However, the subsequent extrusion of a C2H4O species from the metalla-2,3-dioxolane does not yield the epoxide but acetaldehyde. The calculations show that the reaction of MoO(O2)2OPH3 with ethylene can directly lead to the epoxide as suggested by Sharpless. The activation energy for the latter process is 15.2 kcal/mol, which is lower than the barrier for the formation of the metalla-2,3-dioxolane (23.7 kcal/mol). Calculations with the ligand OPMe3 instead of OPH3 show an even larger preference of the pathway leading to the epoxide than the formation of the five-membered ring. The calculations strongly support the mechanism suggested by Sharpless, while the Mimoun mechanism leads to carbonyl compounds as reaction products. Examination of the electronic structure of the transition state of the epoxide formation with the Charge Decomposition Analysis shows that the reaction should be considered as nucleophilic attack of the olefin toward the σ* orbital of the peroxo bond.
The diastereo- and enantioselectivity obtained experimentally by List on the proline-catalized intramolecular aldol reaction of substituted 1,7-dicarbonylic compounds was accurately predicted using ...density functional theory methods at the B3LYP/6-31++G** level. A polarizable continuum model was used to describe solvent effects. The theoretical data agree in good extension with List’s experimental results, both in enantioselectivity and diastereoselectivity, and allow for the confirmation of our previous rationalization of the main factors contributing to the reaction selectivity. While the enantioselectivity results from an important electrostatic contact between the forming alkoxyde group and the proline moiety, the calculated diasteroselectivity results from several steric contacts that can be established between the different substituents and from their relative orientation in respect to the ring conformation. However, for dialdehydes that can originate two diastereomeric enamine intermediates, the proline attack and the immonium formation steps can also be of major importance in the rationalization of the final reaction selectivity, as is the case in two of the six studied systems. The obtained data allows for a full rationalization of the known experimental systems as well as for the extrapolation to new ones with variable substitution at the carbonylic chain.
The nature of the chemical bonds in E
2
and tetrahedral E
4
(E = N, P) has been analysed with the help of an energy decomposition method. The π bonds in P
2
are not particularly weak. On the ...contrary, the contribution of P–P π bonding to the chemical bond in P
2
is even higher than the contribution of N–N π bonding to the chemical bond in N
2
. The higher stability of P
4
(
T
d
) and the much lower stability of N
4
(
T
d
) with regard to the diatomic species come from the substantially larger weakening of the N–N σ-bonds compared with the P–P σ-bonds in the tetrahedral species. The sum of the six P–P σ-bond energies in P
4
is higher than the sum of two σ- and four π-bonds in two P
2
, but the six N–N σ-bonds in N
4
are weaker than two σ- and four π-bonds in two N
2
. The crucial factor that leads to the weak N–N bonds in N
4,
is the rather long N–N bonds which are 32.8 % longer than in N
2
. In contrast, the P–P bonds in P
4
are only 16.2 % longer than in P
2
. Since the equilibrium distances in E
2
and E
4
are determined by Pauli repulsion, it can be concluded that the origin for the different stabilities of N
4
and P
4
relative to the diatomic molecules is the exchange repulsion. The nitrogen atoms encounter stronger Pauli repulsion, because the 2
s
and 2
p
valence orbitals have a similar radius while the 3
s
and 3
p
radii are more different.
Summary
The mucopolysaccharidoses (MPS) is characterized by accumulation of glycosaminoglycans (GAGs), and mucolipidosis (ML) by accumulation of GAGs and sphingolipids. Each type of MPS accumulates ...specific GAGs. The lysosomal enzymes N‐acetylgalactosamine‐6‐sulphate sulphatase and β‐galactosidase involve the stepwise degradation of keratan sulphate (KS). Deficiency of these enzymes results in elevation of KS levels in the body fluids and in tissues, leading to MPS IV disease. In this study, we evaluated blood and urine KS levels in types of MPS and ML other than MPS IV. Eighty‐five plasma samples came from MPS I (n=18), MPS II (n=28), MPS III (n=20), MPS VI (n=3), MPS VII (n=5) and ML (n=11) patients while 127 urine samples came from MPS I (n=34), MPS II (n=34), MPS III (n=32), MPS VI (n=7), MPS VII (n=9) and ML (n=11) patients. KS levels were determined using the ELISA method. Plasma KS levels varied with age in both control and patient populations. In all age groups, the mean values of plasma KS in MPS and ML patients were significantly higher than those in the age‐matched controls. Plasma KS values in four newborn patients were above the mean + 2SD of the age‐matched controls (mean, 41 ng/ml). Overall, 85.9% of individual values in non‐type IV MPS and ML patients were above the mean + 2SD of the age‐matched controls. For urine KS levels, 24.4% of individual values in patients were above the mean + 2SD of the age‐matched controls. In conclusion, KS in blood is elevated in each type of non‐type IV MPS examined, in contrast to the conventional understanding. This finding suggests that measurement of KS level provides a new diagnostic biomarker in a wide variety of mucopolysaccharidoses and mucolipidoses in addition to MPS IV.
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