A computational mechanistic study of the deamination reaction of melamine

A detailed computational study of the deamination reaction of melamine by OH^–, n H₂O/OH^–, n H₂O (where n = 1, 2, 3), and protonated melamine with H₂O, has been carried out using density functional theory and ab initio calculations. All structures were optimized at M06/6‐31G(d) level of theory, as well as with the B3LYP functional with each of the basis sets: 6‐31G(d), 6‐31 + G(d), 6‐31G(2df,p), and 6‐311++G(3df,3pd). B3LYP, M06, and ω B97XD calculations with 6‐31 + G(d,p) have also been performed. All structures were optimized at B3LYP/6‐31 + G(d,p) level of theory for deamination simulations in an aqueous medium, using both the polarizable continuum solvation model and the solvation model based on solute electron density. Composite method calculations have been conducted at G4MP2 and CBS‐QB3. Fifteen different mechanistic pathways were explored. Most pathways consisted of two key steps: formation of a tetrahedral intermediate and in the final step, an intermediate that dissociates to products via a 1,3‐proton shift. The lowest overall activation energy, 111 kJ mol^?1 at G4MP2, was obtained for the deamination of melamine with 3H₂O/OH^?.     

First order phase transition in the frustrated triangular antiferromagnet CsNiCl3

By means of high-resolution ultrasonic velocity measurements, as a function of temperature and magnetic field, the nature of the different low temperatures magnetic phase transitions observed for the quasi-one-dimensional compound CsNiCl3 is established. Special attention has been devoted to the field-induced 120 degrees phase transition above the multicritical point in the H-T phase diagram where the elastic constant C44 reveals a steplike variation and hysteresis effects. These results represent the first experimental evidence that the 120 degrees phase transition is weakly first order and contradict the popular notion of new universality classes for chiral systems.     

Phase Space Optimization of Quantum Representations: Non-Cartesian Coordinate Spaces

In an earlier article [Found. Phys. 30, 1191 (2000)], a quasiclassical phase space approximation for quantum projection operators was presented, whose accuracy increases in the limit of large basis size (projection subspace dimensionality). In a second paper [J. Chem. Phys. 111, 4869 (1999)], this approximation was used to generate a nearly optimal direct-product basis for representing an arbitrary (Cartesian) quantum Hamiltonian, within a given energy range of interest. From a few reduced-dimensional integrals, the method determines the optimal 1D marginal Hamiltonians, whose eigenstates comprise the direct-product basis. In the present paper, this phase space optimized direct-product basis method is generalized to incorporate non-Cartesian coordinate spaces, composed of radii and angles, that arise in molecular applications. Analytical results are presented for certain standard systems, including rigid rotors, and three-body vibrators.     

The growth of and recovery from TTS in human subjects exposed to impact noise

It is now recognized that temporary threshold shift (TTS) grows to an asymptotic level (ATS) after a prolonged exposure to steady-state and intermittent noise. Few studies have been conducted to verify this process of acquisition for impact noise. However, results obtained on animals have demonstrated that an asymptote is reached after 1-2 h of exposure. These results have been confirmed in the present study on four human subjects using a 470-ms (B duration) impact noise at a rate of 1 pps. The above pattern of acquisition has been retested on two subjects exposed to the same impact for periods of 15-150 min. An ATS of 15-20 dB was measured (Lp = 102-104 dBA) after 30-60 min of exposure. Recovery curves were determined for each exposure duration. They appeared to follow a logarithmic function of post-exposure time. It took from 30-50 min for one subject and from 4-5 h for the other to completely recover. The time for total recovery did not seem to vary as a function of exposure time. Implications are drawn for damage risk criteria and for the study of the effects of impact noise on hearing.     

Synthesis of simplified hybrid inhibitors of type 1 17beta-hydroxysteroid dehydrogenase via cross-metathesis and sonogashira coupling Reactions

[structure: see text] The inhibitor of type 1 17beta-hydroxysteroid dehydrogenase EM-1745 (1) exhibits affinity for both the substrate (estrone or estradiol) and the cofactor (NAD(P)H) binding domains. However, to increase its bioavailability, this compound needs to be simplified. The efficient and convergent synthesis of simplified substrate/cofactor hybrid inhibitors (compounds 2) involving a cross-metathesis and a Sonogashira coupling reaction as key steps is described. Compounds 2a-c were also tested as enzyme inhibitors and compared to EM-1745.     

Novel 2,2-bipyridine ligand for palladium-catalyzed regioselective carbonylation

[reaction: see text] A palladium-catalyzed highly regioselective one-step carbonylation of 2,5-dibromo-3-methylpyridine is reported. A range of alkyl esters and amides can be prepared in good yield with better than 95:5 regioselectivity via this method. Key to the high regioselectivity for the formation aromatic amides is the introduction of a novel nonphosphine-based 2,2-bipyridine ligand. This novel reaction was scaled up smoothly in the plant to a 130-kg batch size and facilitated the delivery of bulk material for the clinical trials of Sch 66336, a candidate for oncologic treatments.     

Study of the quality of Gaussian basis sets for carbon and silicon: Calculations on methane and silane

A number of Gaussian basis sets for carbon and silicon have been examined in terms of the one-electron properties of methane and silane. The convergence of the properties to their limiting values is not monotonic but, in general, a representation that involves five Gaussian functions per occupied atomic orbital on the heavy atom is sufficient to closely approach the limits. A relationship between the sizes and partitioned electronic energies is shown to hold to a good approximation for the Boys spatially localized molecular orbitals employed in this study.     

Uniform quality gaussian basis sets for molecular calculations. V. Property optimization: A study on H2O

Energy optimization (Eo) and property optimization (PO) were performed on the H₂O molecule. A definition of the “optimality” κ, a dimensionless quantity of the form has been proposed where ω_i is a weighting factor, 〈ǒ〉_i is the computed observable, and O_i is the corresponding property measured experimentally. The minimization of κ leads to property optimization methods (POM) which is a useful alternative to energy optimization methods (EOM).