Announcements

By supporting specimens on a thin membrane in the spectrophone wall, photoacoustic spectroscopy can be made on objects outside the acoustic chamber. Examples are presented from spectral measurements on liquid, powdered and thin anodic film samples on a 0.04 mm thick aluminium membrane. The conditions under which the technique may become practicable are briefly discussed.     

Studies on Reactive Pyridylketones Formed by Weinreb Transformations

A method has been developed to allow the preparation of reactive pure vinylpyridylketones and activated vinylketones, in general, to be used in further reactions, such as cycloadditions. The process is based on the Weinreb's amide transformation and includes a quarternary ammonium intermediate and subsequent elimination. Additionally, based on our previous results on the malonate alkylation of 3‐nitropyridines and subsequent synthetic applications, we present the studies on the transformation of pyridyl malonate derivative 3 via the Weinreb's amide 4 and reactive methylpyridyl‐ ( 17 ) and allylpyridyl‐ketone ( 6 ) into bis‐heterocyclic products 18 and 19 , and 8 , 20 , and 21 , respectively.     

The transverse energy distribution in hadron-lead collisions

The transverse energy cross-sectiondσ/dE _T has been measured in the pseudorapidity region 0.6<η<2.4 for hadron-lead collisions at 200 GeV/c incident hadron momentum. TheE _T distribution extends to 40 GeV, which is twice the kinematic limit forp-p collisions at the same incident beam momentum. The distribution ofE _T is found to shift towards low pseudorapidities with increasing total transverse energy.     

Mesostructured SBA-3 silica containing Reichardt's dye as an optical ammonia sensor

Mesostructured SBA-3 silica containing Reichardt's dye is shown to give a reversible optical response to gaseous ammonia, with a response time of a few seconds and an intensity proportional to the ammonia partial pressure, which are desirable properties for gas sensing.     

Accuracy and efficiency of atomic basis set methods versus plane wave calculations with ultrasoft pseudopotentials for DNA base molecules

Recent results from Preuss et al. (J Comput Chem 2004, 25, 112) on DNA base molecules, obtained by plane wave density functional calculations using ultrasoft pseudopotentials, are compared with calculations using Gaussian basis sets. Bond lengths and angles agree closely, but dihedral angles and vibrational frequencies show significant differences. The Gaussian basis calculations are at least an order of magnitude more efficient than the plane wave/ultrasoft pseudopotential calculations at a similar level of accuracy; the advantage is even larger if the Fourier Transform Coulomb method is used. To obtain definite benchmark values, the geometries of the four DNA bases were optimized at the MP2 level with large basis sets, up to cc-pVQZ and aug-cc-pVTZ.     

An efficient atomic orbital based second-order Møller-Plesset gradient program

Based on the orbital-invariant atomic orbital formulation of the MP2 (M?ller-Plesset second-order perturbation theory) energy and gradient [P. Pulay and S. Saeb?, Theor. Chim. Acta 69, 357 (1986)], we have derived and programmed detailed working equations for closed-shell MP2 gradients. The orbital-invariant form avoids the difficulties of other formulations with frozen orbitals, and allows the use of arbitrary occupied orbitals, an important consideration for local correlation theories, although the present program uses canonical molecular orbitals. The atomic orbital formulation offers savings both in storage and computer time. Test calculations on systems containing up to approximately 100 atoms and approximately 1000 basis functions, performed on a single personal computer, are reported. Parallelization of the code is underway.     

The Molecular Structure and the Puckering Potential Function of Octamethylcyclotetrasilane,Si4Me8, Determined by Gas Electron Diffraction and Relaxation Constraints from Ab Initio Calculations

Gas electron diffraction data are applied to determine the geometrical parameters of the octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined, taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function has been described as V() = V ₀[(/ _e )² – 1]² with V ₀ = 1.0 ± 0.5 kcal/mol and _e = 28.3 ± 1.9°, where is the puckering angle of the ring. The geometric parameters at the minimum of V() (r _a in Å, in degrees and errors given as three times the standard deviations including a scale error) are as follows: r(Si—C)_av = 1.894(3), r(Si—Si) = 2.363(3), r(C—H) = 1.104(3), CSiC = 109.5(6), SiSiSi = 88.2(2), SiCH = 111.7(6), C = 4.1, where the tilt C was estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds.     

Acetamide, a challenge to theory and experiment? On the molecular structure, conformation, potential to internal rotation of the methyl group and force fields of free acetamide as studied by quantum chemical calculations

The molecular geometry has been optimized without any constraints using different basis sets and levels of theory as: Hartree-Fock with basis sets 6–31+G**, 6–311++G**, cc-pVTZ and aug-cc-pVTZ, MP2 with basis sets 6–311++G** and cc-pVTZ, MP3 with basis set 6–311++G**, and density functional theory with basis sets 6–311++G** and cc-pVTZ. Small basis sets up to 6-31G predict the syn conformation of the methyl group to be the most stable conformation. Larger basis sets predict an unsymmetrical conformation with one of the H atoms perpendicular to the amide skeleton or an anti-like conformation. Dunnings correlation consistent polarized valence triple zeta, cc-pVTZ, basis set including MP2 predict two conformations, one perpendicular and one anti to be the most stable. The DFT calculations predict anti-like conformations. The most accurate calculations predict anti-like conformations which have not been predicted previously. The vibrational frequencies have been calculated for several basis sets and compared to the observed frequencies. The wagging frequency of the NH₂ is very dependent on the basis sets and levels of theory. Most calculations predict a planar NH₂ group in agreement with experiment. A scaled molecular force field has been determined by fitting the calculated frequencies to the observed ones for the perpendicular conformation using MP2/cc-pVTZ. The barrier heights for the methyl group have been calculated. The rotational constants, I_A + I_B − I_C values and dipole moments are compared with experimental values.     

Strategies for electron correlation calculations on large molecular systems

The two main problems in applying common methods for electron correlation to large systems are (1) the steep dependency of the computational task with the size of the system and (2) the large and often prohibitive number of two-electron integrals that must be stored and retrieved during the calculation. The direct local correlation method eliminates both these problems. This method is a combination of the local correlation method that eliminates the steep computational dependency with the size of the system and extension of the direct SCF approach to electron correlation methods. The latter method eliminates external storage of the electron repulsion integrals. In this review, the direct local correlation approach and its computer implementation will be described in detail, including computational examples and comparisons to similar methods developed in other groups. The feasibility of future applications to extended systems is discussed.