Thermochemical properties and electronic structure of boron oxides BnOm (n = 5-10, m = 1-2) and their anions

The molecular and electronic structures of a series of small boron monoxide and dioxide clusters B(n)O(m) (n = 5-10, m = 1, 2) plus their anions were predicted. The enthalpies of formation (DeltaH(f)'s), electron affinities (EAs), vertical detachment energies, and energies of different fragmentation processes are predicted using the G3B3 method. The G3B3 results were benchmarked with respect to more accurate CCSD(T)/CBS values for n = 1-4 with average deviations of +/-1.5 kcal/mol for DeltaH(f)'s and +/-0.03 eV for EAs. The results extend previous observations on the growth mechanism for boron oxide clusters: (i) The low spin electronic state is consistently favored. (ii) The most stable structure of a neutral boron monoxide B(n)O is obtained either by condensing O on a BB edge of a B(n) cycle, or by binding one BO group to a B(n-1) ring. The balance between both factors is dependent on the inherent stability of the boron cycles. (iii) A boron dioxide is formed by incorporating the second O atom into the corresponding monoxide to form BO bonds. (iv) A B(n)O(m)(-) anion is constructed with BO groups bound to the B(n-1) or B(n-2) rings (yielding the B(n-2)(BO)(2)(-) species). This becomes the preferred geometry for the larger boron dioxides, even in the neutral state. The boronyl group mainly behaves as an electron-withdrawing substituent reducing the binding energy and resonance energy of the oxides. (v) The boron oxides conserve some of the properties of the parent boron clusters such as the planarity and multiple aromaticity.

Fast reactions of hydroxycarbenes: tunneling effect versus bimolecular processes

Different uni- and bimolecular reactions of hydroxymethylene, an important intermediate in the photochemistry of formaldehyde, as well as its halogenated derivatives (XCOH, X = H, F, Cl, Br), have been considered using high-level CCSD(T)/CBS quantum chemical methods. The Wentzel-Kramers-Brillouin (WKB) and Eckart approximations were applied to estimate the tunneling rate constant of isomerization of trans-HCOH to H(2)CO, and the WKB procedure was found to perform better in this case. In agreement with recent calculations and experimental observations [Schreiner et al., Nature 2008, 453, 906], the half-life of HCOH at the low temperature limit in the absence of bimolecular processes was found to be very long (approximately 2.1 h). The corresponding half-life at room temperature was also noticeable (approximately 35 min). Bimolecular reactions of trans-hydroxymethylene with parent formaldehyde yield primarily more thermodynamically favorable glycolaldehyde via the specific mechanism involving 5-center transition state. The most preferable reaction of cis-hydroxymethylene with formaldehyde yields carbon monoxide and methanol. Due to very low activation barriers, both processes occur with nearly a collision rate. If the concentration of HCOH (and its halogenated analogues XCOH as well) is high enough, the bimolecular reactions of this species with itself become important, and H(2)CO (or X(H)CO) is then formed with a collision rate. The singlet-triplet energy separation of trans-HCOH is confirmed to be approximately -25 kcal/mol.

Production of hydrogen from reactions of methane with boranes

The reactions of methane with different hydrides have been investigated using quantum chemical calculations (MP2 and CCSD(T) methods with the aug-cc-pVnZ one-electron functions extrapolated to the basis set limits). The hydrides of the elements of the second and third row, and also GaH(3), with an electronegativity smaller than the value of hydrogen (LiH, Li(2)H(2), BeH(2), NaH, MgH(2), BH(3), AlH(3), B(2)H(6), Al(2)H(6), SiH(4), PH(4) and GaH(3)) have been considered. Reactions of CH(4) with either BH(3) or LiH are characterized by the lowest energy barriers. Reactions using the known methylated derivatives of boranes with methane follow a similar mechanism. Calculated results strongly suggest the possible use of boranes as reagents in the reactions with methane to produce molecular hydrogen.

N-terminal pro-B-type natriuretic peptide: an independent marker for coronary artery disease in asymptomatic diabetic patients

AIMS

To determine whether plasma N-terminal pro-B-type natriuretic peptide (NT-proBNP) levels, a marker for cardiac failure and potentially for the severity of coronary artery disease (CAD), predicts silent myocardial ischaemia (SMI) and silent CAD in asymptomatic high-risk diabetic patients.

METHODS

Five hundred and seventeen asymptomatic diabetic patients with > or = 1 additional cardiovascular risk factor but without heart failure were prospectively screened between 1998 and 2008 for SMI, defined as an abnormal stress myocardial scintigraphy, and subsequently for significant (> 70%) angiographic CAD. The 323 patients with interpretable echocardiography and for whom NT-proBNP was measured were included in this analysis.

RESULTS

SMI was found in 108 (33.4%) patients, 39 of whom had CAD. NT-proBNP was higher in the patients with CAD than in the patients without CAD [45.0 (1-3199) vs. 20.0 (1-1640) pg/ml; P < 0.0001 median (range)], even after adjustment for confounding factors: age, gender, body mass index, glycated haemoglobin (HbA(1c)), retinopathy, nephropathy, hypertension, echocardiographic parameters (P < 0.05). NT-proBNP in the third tertile (> or = 38 pg/ml) predicted CAD with a sensitivity of 59% and a specificity of 67%. In a multiple logistic regression analysis including NT-proBNP > or = 38 pg/ml, age, body mass index, gender, HbA(1c), hypertension, retinopathy, nephropathy, peripheral occlusive arterial disease, left ventricular systolic dysfunction, dilatation and hypertrophy and Type 1 transmitral flow, NT-proBNP > or = 38 pg/ml was the only significant independent predictor of silent CAD [odds ratio (OR) 3.1 (95% confidence interval 1.3-7.6), P = 0.015].

CONCLUSIONS

NT-proBNP measurement helps to better define asymptomatic diabetic patients with an increased likelihood for CAD, independently of cardiac function and structure.

Hydrolysis of aspartic acid phosphoramidate nucleotides: a comparative quantum chemical study

L-Aspartic acid has recently been found to be a good leaving group during HIV reverse transcriptase catalyzed incorporation of deoxyadenosine monophosphate (dAMP) in DNA. This showed that L-Asp is a good mimic for the pyrophosphate moiety of deoxyadenosine triphosphate. The present work explores the thermochemistry and mechanism for hydrolysis of several models for L-aspartic-dAMP using B3LYP/DGDZVP, MP2/6-311++G** and G3MP2 level of theory. The effect of the new compound is gradually investigated: starting from a simple methyl amine leaving group up to the aspartic acid leaving group. The enzymatic environment was mimicked by involving two Mg(2+) ions and some important active site residues in the reaction. All reactions are compared to the corresponding O-coupled leaving group, which is methanol for methyl amine and malic acid for aspartic acid. With methyl amine as a leaving group a tautomeric associative or tautomeric dissociative mechanism is preferred and the barrier is lower than the comparable mechanism with methanol as a leaving group. The calculations on the aspartic acid in the enzymatic environment show that qualitatively the mechanism is the same as for triphosphate but the barrier for hydrolysis by the associative mechanism is higher for L-aspartic-dAMP than for L-malic-dAMP and pyrophosphate.

Remarkable blue shifts of C-H and N-H stretching frequencies in the interaction of monosubstituted formaldehyde and thioformaldehyde with nitrosyl hydride

Weak interactions of monosubstituted formaldehydes and thioformaldehydes with nitrosyl hydride were investigated by using ab initio MO calculations at the MP2/aug-cc-pVTZ level. Thirty two equilibrium structures having different complex forms were located on the corresponding potential energy surfaces (all having C(s) symmetry). Obtained binding energies, which include both ZPE and BSSE corrections, range from 7 to 14 kJ x mol(-1) and 6 to 12 kJ x mol(-1) for complexes of substituted formaldehydes and thioformaldehydes, respectively. In each geometrical structure, the (XCHO,HNO) complex is consistently more stable than the (XCHS,HNO) complex. The H-bond strength significantly increases when one H atom is replaced by a methyl group in both formaldehyde and thioformaldehyde. When replacing H by a halogen atom, the binding energy tends to decrease. It is remarkable that all the C-H and N-H bonds are shortened upon complexation, resulting in an increase of their stretching frequencies. Furthermore, the blue shifts are consistently observed for the interacting N-H bonds in N-H...X, Z, with X = F, Cl, Br, and Z = O, S; such contraction of a covalent N-H bond is extremely rare. In addition, the N-H bond length contraction and its frequency blue shift in the N-H...S complex have been revealed for the first time.

Growth mechanism and chemical bonding in scandium-doped copper clusters: experimental and theoretical study in concert

Size matters! The electronic structure and size-dependent stability of neutral and cationic scandium-doped copper clusters have been investigated by mass spectrometric studies (for the cations) and also quantum chemical computations. The proposed reaction paths ultimately lead to the most stable Frank-Kasper-shaped Cu(16)Sc(+) cluster (shown here), which could be the germ of a new crystallization process.Electronic structure and size-dependent stability of scandium-doped copper cluster cations, Cu(n)Sc(+), were investigated by using a dual-target dual-laser vaporization production scheme followed by mass spectrometric studies and also quantum chemical computations in the density functional theory framework. The neutral species also were studied by using computational methods. Enhanced abundances and dissociation energies were measured in the case of Cu(n)Sc(+) for n=4, 6, 8, 10 and 16, the last of these identified as being extraordinary stable. Neutral clusters are stable with n=5, 7, 9 and 15, which are isoelectronic with respect to the number of the valence s electrons with the stable cationic clusters; hence a simple electron count determines cluster properties to a great extent. The Cu(17)Sc cluster was found to be a superatomic molecule, containing Cu(16)Sc(+) and Cu(-) units; however, the charge separation is not as pronounced as in the case of CuLi. Cu(15)Sc was found to be a stable cluster with a large dissociation energy and a closed electronic structure; hence this can be regarded as a superatom, analogous to the noble gases. The main factors determining the growth patterns of these clusters are the central position of the scandium atom and the successive filling of the shell orbitals. For smaller clusters, the reaction paths appear to diverge yielding various products; however all paths ultimately lead to the most stable Frank-Kasper shaped Cu(16)Sc cluster, which in turn can be the germ of the crystallization process.