Charles L. Wilkins

Search for all self-avoiding paths for molecular graphs

Abstract An algorithm which finds all paths through a molecular skeleton (graph) is described. Programs implementing this algorithm in PL/1, BASIC and FORTRAN are described and illustrative examples are shown. The number of paths in a graph increases rapidly with the number of cycles, but for most molecular graphs counting all paths is a practical task. Input to the programs is the connectivity relationships for a molecule either explicitly or in the form of an adjacency matrix. As output, one may obtain all paths (by listing of the atoms involved) or only the total number of paths of each length. The latter is of interest for characterization of structures, and is convenient in subsequent comparisons and searches for structural similarities. It is suggested that the number of paths in a graph be considered as a useful quantitative measure of the complexity of a graph or structure.

On a graph theoretical basis for ordering of structures

Abstract Algebraic aspects of ordering of structures are examined. By using paths as the graph theoretical invariants, sequences which enumerate paths of differnet length and characterize individual molecules are derived. A rule for ordering such sequences is proposed. The partial ordering thus derived can be used for prediction of the relative magnitudes of various molecular properties of isomers.

Use of self-avoiding paths for characterization of molecular graphs with multiple bonds

Abstract Enumeration of paths in graphs with multiple connections and graphs with weighted bonds is considered. The approach involves a modification of a recently published algorithm for enumeration of paths in having only simple connections. Programs for more general enumerations have been written in PL/1, BASIC and FORTRAN. Besides connectivity, the input now requires information on the multiplicity of the connections or the weights of selected bonds. Output is a list of paths, the number of paths of different length and, in the case of weighted bonds (when no prior specifications of the weights are assumed), the list of paths involving weighted bonds as well as other bonds. Selected illustrations are given to show how path numbers may be useful for comparisons of related structures.

Coordination shifts in carbon-13 spectra of olefin and arene metal complexes

Abstract 13C coordination shifts for several Cr(0)-arene complexes have been determined. These data, as well as previous data on Ag(I), Rh(I), Pt(0), Pt(II), Pd, and Cr(0) complexes, have been reviewed and the sources of complexation effects on 13C shielding constants considered. We conclude that no single influence on these shielding constants should be expected to be universally predominant. Furthermore we show, for the Cr(0) complexes reported, that within a single compound different predominant effects are operative. A four-category qualitative classification scheme, based on the nature of the metal-ligand interaction, is proposed.

A graph-theoretic approach to quantitative structure—activity/reactivity studies

Abstract Characterization of molecular species based on the use of suitable graph invariants (graph paths, in particular) can provide a quantitative means of encoding structure; the technique is complementary to commoner approaches to studies of quantitative structure— activity relationships. Graph path encoding is here applied to quantitative studies of relationships between molecular structures and biological activity; the examples are the rates of various substrate reactions with hexoldnase, and the potential opiate-like activity of enkephalin analogs.

Evaluation of the super-modified simplex for use in chemical pattern recognition

Abstract A simplex optimization technique, the super-modified simplex (SMS), is evaluated for use in the pattern recognition analysis of low-resolution mass spectra. For the recognition of eleven functional group categories, the performances of SMS-derived weight vectors are shown to be comparable to those obtained by a previously developed modified simplex method. Data are presented which indicate that the SMS procedure requires fewer simplices and decreased computational time to converge to an optimized solution for the structural analysis problems investigated.

Experimental considerations in automated NMR relaxation time measurements by the progressive saturation method

Abstract Three important parameters in automated progressive saturation T1 measurements have been examined experimentally and compared with theory. It is found that magnetic field homogeniety must be held constant. Small deviations from the ideal 90° tipping angle were found, in agreement with theory, to be tolerable. It has been shown that, for certain progressive saturation T1 experiments ten pre-acquisition phase pulses are necessary. A homogeneity monitor gate, providing timing and logic control for field gradient pulsing and field homogeneity maintenance is described.

Fourier and Hadamard Transforms in Pattern Recognition

The advent of high-speed, stored-program, general-purpose digital computers as powerful information-handling devices has led to revolutionary developments in many fields. Among these are several fields that deal with important problems that previously appeared to have only extremely difficult, complex, or even unrealizable solutions. A large number of such problems have proven amenable to attack by a set of techniques known as pattern recognition. Pattern recognition has become a fertile area for the development of concepts and techniques now being applied routinely to problems formerly considered to be approachable only by humans. It is for this reason that pattern recognition is often considered to be a subset of the artificial or machine intelligence field.

A Computer‐Controlled Digital Frequency Synthesizer for NMR Spectroscopy

A hardware/software computer interface for control of a digital frequency synthesizer is described. This synthesizer is controlled by selection of a suitable oscillator between 2.5 and 60.2 MHz, which in turn is controlled by two 8‐bit modulo N counters. The ratio of the two integers (Q /P) selected as the moduli of the counters determines the ratio of synthesized to reference frequency. The interface described directly selects these functions and thus permits computer calculation and setting of the required ratio. Addition of a manually controlled offset frequency completes the synthesis. In this way, frequencies can be synthesized with a maximum error of approximately 10 Hz or less. The interface therefore allows rapid and accurate synthesis of any desired frequency within the 2.5–60.2 MHz range. This method is of utility in both spin‐spin decoupling experiments in NMR spectroscopy and as a central element of a variable frequency NMR spectrometer system. The algorithm for computing the necessary values ...

Computers in Microstructural Analysis

There are a number of possible approaches to a discussion of the place computers can and will play in microstructural analysis. We could, for example, devote our attention to a general discussion of “turn-key” computer/instrument systems which are supplied as part of a total analytical package by either an instrument manufacturer or an independent contractor who specializes in such developments. This sort of computer use is usually directed toward a very specific and carefully delimited problem in order to minimize, as much as possible, both the programming and hardware requirements. Alternately, we could discuss in detail a variety of very specific computer applications, developed by users to fit their particular needs. Both of these approaches are certainly useful in some contexts, but the present paper will focus, instead, on a general discussion of computer/analytical instrument interface requirements and a description of a flexible, modular approach to the whole question of using a computer in the laboratory. In particular, the advantages of using the rather inexpensive and easily programmable general purpose computer /interface systems now becoming available will be emphasized. In order to place the discussion in its appropriate context, a brief review of the backgio und of laboratory computer development and the premises upon which the present approach is based will be discussed. In this way, it will be possible to show that significant advantages can be obtained, in certain cases, by adoption of this or a similar philosophy when introducing computers into a microstructural analysis laboratory.