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π Electron Effects on Chemical Shift: Overview

π Electron Effects on Chemical Shift: Overview

An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...

Electronic Structure of Atoms

Electronic Structure of Atoms

An atom comprises protons and neutrons, which are contained inside the dense, central core called the nucleus, with electrons present around the nucleus. Taking into account the wave–particle duality of electrons and the uncertainty in position around the nucleus, quantum mechanics provides a more accurate model for the atomic structure. It describes atomic orbitals as the regions around the nucleus where electrons of discrete energy exist, characterized by four quantum...

Atomic Orbitals

Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution function indicates the total probability of finding an electron within the thin shell at a distance r from the nucleus. The atomic orbitals have distinct shapes which are determined by l, the angular momentum quantum number. The orbitals are often drawn with a boundary surface, enclosing densest regions of the cloud.

Atomic Structure

Atomic Structure

The Greek philosopher Democritus proposed that everything on Earth is made up of tiny particles called atomos, Greek for "indivisible," from which the modern term "atom" is derived. In the 19th century, John Dalton proposed the atomic theory that is still largely correct today. He put forth five postulates to explain how atoms made up the world around us. (1) All matter is composed of infinitely small particles or atoms. (2) All atoms of a given element are identical to one...

Atomic Structure

Atomic Structure

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Related Experiment Video

Updated: Apr 5, 2026

Curation of Computational Chemical Libraries Demonstrated with Alpha-Amino Acids

Curation of Computational Chemical Libraries Demonstrated with Alpha-Amino Acids

Published on: April 13, 2022

PubChem atom environments.

Volker D Hähnke¹, Evan E Bolton¹, Stephen H Bryant¹

¹Department of Health and Human Services, National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, 8600 Rockville Pike, Bethesda, MD 20894 USA.

Journal of Cheminformatics

|August 25, 2015

Summary

This summary is machine-generated.

Chemists can explore millions of novel chemical structures by combining known fragments. Analysis of PubChem reveals an eightfold increase in unique chemical fragments compared to 40 years ago, with many new combinations yet to be discovered.

Keywords:

Fragment Molecular graph PubChem SMARTS Standardization

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Area of Science:

Cheminformatics
Computational Chemistry
Chemical Information Science

Background:

Atom environments and fragments are fundamental in cheminformatics for prediction models and similarity searching.
Historical analyses of fragment distributions were limited by smaller chemical structure datasets.
Modern chemical synthesis and open databases provide access to millions of chemical structures.

Purpose of the Study:

To analyze the landscape of known chemical fragments in large, modern chemical databases.
To compare current fragment distributions with historical studies.
To identify opportunities for novel chemical compound discovery.

Main Methods:

Characterization of atoms by atomic number, charge, hydrogen count, degree, valence, and aromaticity.
Definition of bonds as single, double, triple, or aromatic.
Generation of atom environments using circular radii up to 3 (ECFP_6 equivalent).

Main Results:

Identified 28,462,319 unique fragments in 46 million PubChem structures.
Observed an eightfold increase in unique fragments compared to 40-year-old studies, with potential inflation from patent data.
Found a high proportion of singleton fragments (found in only one structure), increasing with fragment size.

Conclusions:

Significant opportunities exist for chemists to synthesize novel compounds by combining known fragments.
The growth in unique fragments reflects advances in synthetic chemistry and unexplored chemical space.
Analysis highlights the potential for fragment-based normalization across different chemical data types.