Hashing

Over and beyond the representations of chemical structures presented so far, there are others for specific applications. Some of the representations discussed in this section, e.g., fragment coding or hash coding, can also be seen as structure descriptors, but this is a more philosophical question. Structure descriptors are introduced in Chapter 8. 

Hash coding is an established method in computer science, e.g., in registration procedures [94, 95. In chemoinformatics the structure input occurs as a sequence of characters (names) or numbers (which may also be obtained, e.g., from a connection table (see Section 2.4) by conversion of a structure drawing). Both names and numbers may be quite large and may not be usable as an address... 

Figure 2-65. This fingerprint was received by hashing, whereas only one part of all the substructures is specified in the illustration. The asterisk indicates the address of a collision in the bit string, generated by the algorithm.

The ciphered code is indicated with a defined length, i.e., a fixed hit/byte length. A hash code of 32 bits could have 2 (or 4 294 976 296) possible values, whereas one of 64 bits could have 2 values, However, due to tbe fixed length, several diverse data entries could assign the same hash code ( address collision ), The probability of collision rises if the number of input data is increased in relation to the range of values (bit length). In fact, the limits of hash coding are reached with about 10 000 compounds with 32 bits and over 100 million with 64 bits, to avoid collisions in databases [97. ... 

Thus the hash code is not used as a direct way to access data rather it serves as an index or key to the filed data entry (Figure 2-66). Since hash coding receives unique codes by reducing multidimensional data to only one dimension, information gets lo.st. Thi.s los.s prevents a recon.struction of the complete data from the hash code. 

The hash code has specific characteristics and provides numerous possibilities for use ... 

Hash codes of molecules which are already pre-computed are suitable for use in fiill structure searches in database applications. The compression of the code of a chemical structure into only one number also makes it possible to compute in advance the transformation results for a whole catalog. The files can be stored and kept complete in the core memory during execution of the program, so that a search can be accomplished within seconds. 

Atomic and bond hash codes are helpful in structure manipulation programs, e.g., in reaction prediction or in synthesis design [99]. 

The stereochemistry is usually expressed in structure diagrams by wedged and hashed bonds. A wedge indicates that the substituent is in ont of a reference plane and a hashed bond indicates that the substituent is pointing away om the viewer (behind the reference plane). This projection is applied both to tetrahe-... 

A molecule editor can draw a chemical structure and save it, for example as a Molfile. Although it is possible to include stereochemical properties in the drawing as wedges and hashed bonds, or even to assign a stereocenter/stereogroup with its identifiers R/S or E/Z), the connection table of the Molfile only represents the constitution (topology) of the molecule. 

The problem of perception complete structures is related to the problem of their representation, for which the basic requirements are to represent as much as possible the functionality of the structure, to be unique, and to allow the restoration of the structure. Various approaches have been devised to this end. They comprise the use of molecular formulas, molecular weights, trade and/or trivial names, various line notations, registry numbers, constitutional diagrams 2D representations), atom coordinates (2D or 3D representations), topological indices, hash codes, and others (see Chapter 2). 

Hashed fingerprints, structural keys 2D structure See Section 12.2... 

S. D. Broadhurst and L. J. Hash, The Scrap Mica Resources of North Carolina Bulletin No. 66, North Carolina Department of Conservation and Development, 1953, p. 6. 

C. M. Butt and C. Hash, Proceedings of the 3rd International Drip Trickle Irrigation Congress, N ov. 1985, ASAE, St. Joseph, Mich., 1985. 

The Hash thermolysis of the dioxolene (124) gave diphenylketene among the products (Scheme 107) (70CC206), and it is conceivable that diphenyloxirene is the precursor of this product. 

Fire Hazards - Hash PoirU (deg. F) 60 OC 55 CC Flammable Limits in Air (%) 6.2 - 15.6 Fire Extinguishing Agents Foam, carbon dioxide, dry chemical Fire Extinguishing AgetUs Not To Be Used Water may be ineffective Special Hazards of Combustion Products Toxic and irritating gases... 

The heat transfer section of a plant was filled with oil after maintenance by opening a vent at the highest point and pumping oil into the system until it overflowed out of the vent. The overflow should have been collected in a bucket, but sometimes a bucket was not used, or the bucket was overfilled. Nobody worried about small spillages because the Hash point of the oil was above ambient temperature and its boiling point and auto-ignition temperature were both above 300°C. 

Hash-fire modeling is largely underdeveloped in the literature there are large gaps in the information base. Hardly any information is available concerning flash-fire radiation the only data available have resulted from experiments conducted to meet other objectives. Many items have not yet received sufficient attention. 

Clearly, from Table 9.12, the more dangerous case not involving hashing liquid is the 10% filled vessel. 

---