摘要:

AbstractThe intramolecular cyclization reactions of alkyl fluoride and chloride anions1were investigated theoretically using the AM1 method. Formation of five‐membered ring was the most reactive and that of three‐membered ring was the least reactive process in both series. For the fluoride series the reactivity order ofn= 5>6>7>4>3 was dictated by the enthalpy (ΔH≠) effect while for the chloride series the entropy (ΔS≠) effect was found to play somewhat greater role due to the much better leaving ability of the chloride anion leading to the reactivity order ofn= 5>6>4>7>3. For both series, BEP principle held and the degree of bond formation was greater with the fluoride series owing to the poor electron acceptor ability of the clea

ISSN:0192-8651    

DOI:10.1002/jcc.540111002

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA new molecular mechanics force field for alkanes is presented. The force field aims to eliminate some identified failures of the well‐known MM2 force field. The new energy function gives an improved prediction of the rotational barriers of highly congested molecules, a better calculation of short nonbonded contacts, and the correct reproduction of bond elongation in small torsion angles. The calculation of sublimation enthalpies is also improved. The standard deviation of the formation enthalpies for a set of 54 compounds is 0.63 kcal/mol; this compares with the reported value of 0.42 calculated with MM2 and MM3 for different sets. The force field parameters were obtained using a least squares metho

ISSN:0192-8651    

DOI:10.1002/jcc.540111003

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA prescription for applying the method of molecular similarity calculations based on electrostatic potentials and fields is developed by consideration of a typical structure‐activity series. Firm conclusions are drawn about the nature of the grid of points surrounding the molecules and about the choice of geometry, but options for point charges are less clearcu

ISSN:0192-8651    

DOI:10.1002/jcc.540111004

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractTest calculations have shown that correlation energies calculated using the frozen‐core approximation in programs, such as the Gaussian series, that assume the lowest MOs to be the core orbitals may be significantly in error. Some valence orbitals in systems involving the heavier alkali metals and electronegative elements have lower energies than the highest core orbitals of the metal and are therefore erroneously omitted from the correlation energy calculation. Some examples are discusse

ISSN:0192-8651    

DOI:10.1002/jcc.540111005

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractBecause it has

ISSN:0192-8651    

DOI:10.1002/jcc.540111006

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe difference in length between two bond orders was reported by Pauling to be essentially the same, regardless of the atoms that make up the bond. To a first approximation these differences hold not only for bond orders 1, 2 and 3 but also for six membered aromatic rings containing all carbon, carbon‐nitrogen, nitrogen‐nitrogen, carbon‐phosphorous, carbon‐arsenic, and carbon‐antimony bonds. An equation was developed (based upon these differences) that relates bond order and bond length. The output of this equation was compared with those of Gordy and Pauling. Our equation as well as the Gordy equation (with revised constants) return a bond length of 1.4 Å for bond order 1.67 which is consistent with theory. (This bond order was not used in developing either the equation or the revised Gordy

ISSN:0192-8651    

DOI:10.1002/jcc.540111007

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA modification of Davidson's eigenvalue algorithm, based on the conjugate gradient method, is described. This method needs storage only for a few vectors (five to seven, depending on the implementation), making it practical for very large problems where disk storage is the limiting factor, without the necessity of restarting or discarding some expansion vectors. The convergence characteristics of the modified method are essentially identical with those of the original Davidson method if all expansion vectors are retained in the latter.

ISSN:0192-8651    

DOI:10.1002/jcc.540111008

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractMolecular dynamics simulations have been carried out for a series of systems of increasing complexity including: pure water, a model polypeptide (α‐helical decaglycine) in vacuo, a protein (Pancreatic Trypsin Inhibitor, PTI) in vacuo, and a fully solvated protein (PTI in water). The equations of motion were integrated using Andersen's velocity version of the Verlet algorithm with internal contraints (the RATTLE algorithm). The accuracy with which the equations of motion are integrated has been analyzed for several different simulation conditions. The effects of various nonbonded interaction truncation schemes on the conservation of energy have been examined, including the use of atomic cutoffs, and (neutral group) residue cutoffs. The use of a smoothing function to eliminate the discontinuities in the potential at the cutoff leads to a significant improvement in the accuracy of the integration for each of the systems studied. The accuracy with which the equations of motion are integrated using the RATTLE algorithm for pure water and for the solvated protein are found to be comparable when the nonbonded interactions are tapered with a smoothing function at the cuto

ISSN:0192-8651    

DOI:10.1002/jcc.540111009

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA protocol is given for the construction of phospholipid surfaces that possess variable head groups and thus variable net charge.Ab initioquantum mechanical calculations are performed to establish the necessary force field (AMBER) parameters. The charge distribution is defined by an electrostatic potential method consistent with theab initiowave function. As a model calculation, a monolayer surface with head groups of phosphatidylserine and phosphatidylcholine derived from the crystal structure of 1,2‐dilauroyl‐DL‐phosphatidylethanolamine (DLPE) is placed in a water bath with two Ca(II) ions present. The resultant surface is energy‐optimized followed by 64 ps of molecular dynamics integration. Evaluation of calcium ion coordination environments, characterization of the P‐N dipole inclination with respect ot the plane of the monolayer, and calculation of molecular surface area is performed and compared with experime

ISSN:0192-8651    

DOI:10.1002/jcc.540111010

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA procedure is delineated for finding the optimal superposition of a series of chemical structures. Quaternions are used to represent the absolute orientation of the structures–rotations of the structures need never be carried out explicitly. The Rational Function Optimization method is used to minimize a simultaneous superposition residual similar to the one given by Gerber and Müller. The robustness of the method is illustrated by comparing a series of conformations of a polyene caroteno

ISSN:0192-8651    

DOI:10.1002/jcc.540111011

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY