There is an increasing awareness that the separation of over-lapping bands by the mathematical technique of curve fitting offers many pitfalls and should not be undertaken lightly. This review discusses critically the various factors involved, with particular reference to vibrational band systems; other types of overlapping band systems encountered in analytical work are considered in less detail. Five parameters are involved: the number of component bands, their positions, shapes and widths, and the form of the baseline. Curve fitting, by a least squares optimization method to a suitable goodness of fit criterion, is considerably facilitated if approximate values for some of the parameters are known at the outset. The methods available for peak finding are discussed and, although not free from problems, are reasonably effective. Similarly, band shapes can usually be defined semiquantitatively. However, it is seldom possible to obtain prior information on band widths; these should be determined during the curve fitting calculations. Although statistical goodness of fit criteria are available they should be used in conjunction with a visual plot, to locate any regions of poor fit. Furthermore, the overriding consideration must always be that the computer fit is plausible in terms of acceptable chemical species for the system being examined.

The Scope and Limitations of Curve Fitting

The use of B-splines for smoothing and differentiation of spectroscopic curves has been investigated. The theory of B-splines and their use are outlined. For single Lorentzian and Gaussian curves, optimal spline functions of degrees 3-7 inclusive, based on symmetric arrangements of 5, 6, and 7 knots, have been computed. Various schemes for the automatic selection of knots used in fitting experimental curves have been investigated and some schemes have been applied to data obtained by Raman spectroscopy.

Smoothing and Differentiation of Spectroscopic Curves Using Spline Functions

Solutions of LiNCS, NaNCS, KNCS and (Bun4N)NCS in methanol, at various concentrations and four temperatures, have been studied in the C—N stretching region by infrared spectroscopy. The spectra have been analysed to show the presence of a minimum of six species in solution, which are identified as the thiocyanate ion with 0, 1, 2 or 3 hydrogen bonds. It is proposed that the thiocyanate ion has donor sites for hydrogen bonding on both the nitrogen atom and on the π electron cloud. Some fine structure is observed in the spectra which indicates that there is also hydrogen bonding from the sulfur atom. In solutions of lithium thiocyanate an additional band was observed at ca. 2101 cm–1, which is attributed to a contact ion pair in which the nitrogen atom is attached to Li+ and the sulfur atom is hydrogen bonded, or vice versa. Equilibrium constants for this ion-pair and for hydrogen bonding have been determined from the absorption intensity at ca. 2101 cm–1. There is evidence for the existence of solvent-shared ion pairs in solutions of LiNCS, NaNCS and KNCS.

Spectrochemistry of solutions. Part 24.—Li, Na, K and Bun4N thiocyanates in methanol: infrared spectroscopic evidence for ion pairing and hydrogen bonding

Design has a long history but computer-aided design (CAD) offers a new, modern connotation. With the exception of the physical processes, such as cutting, forging, and mixing used to a manufacture items, the commonly discussed topics in computer-aided manufacturing (CAM), such as material planning, process and procedure designs, numerical control, and scheduling, can also be considered CAD because design is involved. This article reviews the history of computer-aided design beginning with ENIAC, the first electronic computer, which was developed for the army during World War II, through the recent introduction of a computer that has a speed of 100–250 million floating-point operations per second (flops). In automating chemical processes and computations, companies must decide whether to use commercial software packages or to develop in-house CAD/CAM programs. Many of the underlying principles employed in the design of the prevailing commercial software packages are discussed. These principles can also guide the development of in-house programs. The topics covered include absorption to waste management, and range from drafting isometric pipeline systems and the interactive manipulation of block diagrams involved in chemical processes to the solid modeling of molecular structures.


Keywords:

Computer application;
Computer-aided design (CAD);
Software packages;
Chemical processing;
Computer graphics;
Instruction;
Chemical structure analysis;
Pattern recognition;
Interactive graphics;
Molecular modeling;
Process flow sheet;
Process optimization;
Process control;
Modular approach

Computer‐Aided Design and Manufacturing (CAD/CAM)

Spectrochemistry of Solutions

Visually interactive parameters' estimation and refinement—A technique for data-fitting

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Visually interactive parameters' estimation and refinement—A technique for data-fitting