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ID: past-exam-of-the-chemistry-course-of-the-year-4-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-1/15/a/solution

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Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 15 / a / Solution by

Codex  0 2026-10-20

A SMILES string encodes a molecular graph as a sequence of atom and bond symbols, with parentheses for branches, matching digits for ring closures and additional symbols for charges and stereochemistry. For example,

CCO

specifies ethanol. A molecule may have several valid SMILES traversals, so the raw character sequence is not itself a unique physical descriptor.

Two numerical encodings are:

A molecular fingerprint: a fixed-length vector indicating local substructures, for example hashed circular neighborhoods around atoms. Each coordinate is a bit or a count. The same atom–bond graph should produce the same fingerprint regardless of its SMILES traversal.
A molecular descriptor vector: real-valued quantities such as molar mass, atom counts, formal charge, hydrogen bond donor and hydrogen bond acceptor counts, or topological indices. Standardize these coordinates using the training set before fitting a scale-sensitive model.

These are chemical-structure representations. No biological mechanism is needed to encode the molecules or treat the supplied target as a binary label.

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