Help

This website is designed to assist the user in rapidly locating the relevant or desired content. The primary interface is a single search box located immediately underneath the depiction of a PAH molecule. When a user begins typing a PAH name or CAS number into the search box, a list of matching compounds will appear immediately below. Selecting one of these names from the list using a tap, a mouse click, or using the up/down arrow keys followed by the enter/return key will cause the information on the selected molecule to be displayed in the space immediately below the search box.

The space in which results are displayed is organized into four tabbed sections: Compound Info, 3d Model, Thermochemistry, and UV-Vis Spectra. The data that appears in each tab is described below. Navigation between the tabs is accomplished by clicking on the tabs or by highlighting the tab section by pressing the <tab> key to give focus to the tab section, using the arrow keys to select the desired tab, and then pressing enter/return to go to the selected tab.

Compound Info

A selection of identifying information including

Primary Name - A name for the molecule shown. Generally a systematic or common name, and one that may be the most commonly used to refer to the molecule.
Other Names - Additional names for the molecule. These may be systematic, common, or some other valid chemical name.
Molecular Formula - An expression of the composition of the molecule, gibing the types of atoms present in the molecule and the number of each type of atom present. The molecular formula is given in the Hill notation in which the carbon atoms are indicated first, followed by the hydrogen atoms, and all other chemical elements in alphabetical order.
Molecular Mass - The mass of 1 mol of the indicated molecule in units of g/mol.
CAS Registry Number - A unique numerical identifier assigned to the molecule by the Chemical Abstracts Service.

appears on this tab. Also, four parameters which give information on the size of the molecule

Length - Length of a minimal three-dimensional box that fully contains the molecule. Units are Angstroms (Å).
Width - Width of a minimal three-dimensional box that fully contains the molecule. Units are Angstroms (Å).
Breadth - Breadth of a minimal three-dimensional box that fully contains the molecule. Units are Angstroms (Å).
L/W Ratio - Ratio of the length to the width.

are given. This information is useful for predicting chromatographic retention times, and its use is more fully described in SP922 (pdf). The International Chemical Identifier (InChI), a unique structural identifier, is given. A link to additional compound information present in the NIST Chemistry WebBook is provided. The WebBook may contain experimental data on the selected compound or other information not provided on this web site. Links that permit download of

2d Structure (MDL MOL format) - MOL MOL representation of the 2-dimensional structure of the molecule. This format is suitable for import into many chemical structure drawing programs.
2d Structure Image (PNG format) - Graphical depiction of the 2-dimensional structure of the molecule in portable network graphics (PNG) format.
3d Structure (XYZ) - Cartesian coordinates of the molecule in Xmol format, suitable for many molecular viewers. Units of the coordinates are Angstroms (Å).

are provided. On the right hand side of the display area, a 2d graphical depiction of the PAH molecule is shown.

3d Structure

In this tab, a rotatable 3d structure of the molecule is shown. The user may rotate the molecule by clicking and dragging within the molecule display area. This functionality is powered by JSmol which provides additional functionality that is documented on their website. The structures that are displayed have been optimized at the B3LYP/cc-pVDZ level of theory.

Thermochemistry

In this tab, enthalpy of formation data in the range 0 - 6000 K is given. These data are computed from the results of an energy-extrapolated B3LYP/cc-pVDZ optimization and frequency calculation. The enthalpy of formation is computed within the ideal gas, rigid rotor, harmonic oscillator approximation, and an empirical correction based on a chemical group scheme is applied. For a complete description of how these data were derived, see "First Principles Prediction of Enthalpies of Formation for Polycyclic Aromatic Hydrocarbons and Derivatives," Thomas C. Allison and Donald R. Burgess Jr., manuscript in preparation.

UV-Vis Spectra

The data in this section is considered to be "beta" quality and is subject to change in the future. In this section, simulated UV-Vis spectra are presented. These were calculated using time-dependent density functional theory (TDDFT) based on the optimized geometries of the molecules. In the lower portion of the plot, the stick spectrum is plotted as a series of red lines, with the line height proportional to the intensity. In the main portion of the plot, the spectrum derived from broadening the individual peaks (FWHM = 75 cm^-1). At this point in time, these spectra have not been calibrated versus experimental spectra or against other theoretical calculations, so we cannot comment on their quality. Please use appropriately.

Other controls

There are a few additional control on the website which may be helpful in using the site. A menu appears in the upper left of the web site with links to information about the database (About), this help (Help), links to other web sites at NIST that may be of interest (Links), and credits for development of this web site (Credits). Also, the button immediately to the right of the search box will erase the contents of the search box when clicked.