A Guide to the NIST Chemistry WebBook

Peter J. Linstrom


  1. Introduction
  2. Finding Data
    1. Selection Pages
    2. Direct Searches
      1. Formula Search
      2. Chemical Name Search
      3. CAS Registry Number Search
      4. Ion Energetics Searches
        1. Ionization Energy Search
        2. Electron Affinity Search
        3. Proton Affinity Search
        4. Acidity Search
        5. Appearance Energy Product Search
      5. Vibrational and Electronic Spectra Searches
        1. Vibrational Energy Search
        2. Electronic Energy Level Search
      6. Structure Based Searches
        1. Applet Based Structure Search
        2. File Based Structure Search
        3. Structure Class Search
      7. Molecular Weight Search
      8. Reaction Search
      9. Author Search
    3. Indirect Searches
      1. Reaction Species Search
      2. Indirect Author Search
      3. Source Search
  3. Data Presentation
    1. Data Tables
    2. Graphical Displays
      1. Spectrum Displays
      2. Fluid Data
      3. Function Plots
  4. Data Available
    1. General Information
    2. Gas Phase Thermochemistry Data
    3. Condensed Phase Thermochemistry Data
    4. Phase Change Data
    5. Reaction Thermochemistry Data
    6. Gas Phase Ion Energetics Data
    7. Gas Phase IR Spectra
    8. Mass Spectra
    9. UV/Vis Spectra
    10. Vibrational and Electronic Spectra
    11. Constants of Diatomic Molecules
    12. Henry's Law Data
    13. Gas Chromotagraphic Retention Data
    14. Thermophysical Properties of Fluid Systems
    15. References to the Literature


The NIST Chemistry WebBook provides users with easy access to chemical and physical property data for chemical species through the internet. The data provided in the site are from collections maintained by the NIST Standard Reference Data Program and outside contributors. Data in the WebBook system are organized by chemical species. The WebBook system allows users to search for chemical species by various means. Once the desired species has been identified, the system will display data for the species.

Finding Data

Data in the NIST Chemistry WebBook can be found in two ways:

  1. Direct searches for chemical species.
  2. Indirect searches based on related data.

If only one item matching the search criteria is found, the page corresponding to the item will be displayed. If more than one item matches the search criteria, a selection page will be displayed.

Selection Pages

If any search finds more than one chemical species that match the search criteria, the matching species will be displayed on a selection page. At a minimum this page displays the name and chemical formula of each species found. If there are only a small number of species, the structure of the species will also be displayed if it is available. A user can display data on a particular species by clicking on its name. Data for other species found in the search can be viewed by returning to this page and clicking on their names.

The NIST Chemistry WebBook supports several types of direct searches for chemical species. Each search type has its own web page associated with it. Currently the following search types are available:

Most of the search types discussed above share several common features. These features include the ability to select the units to be used, the ability to select the types of data to be displayed and a common format for displaying the results of a search where more than one species match the search criterion.

Searches involving phyical quantities may be restricted based on chemical formula. This feature allows one to restrict a search to a class of compunds. For example, this feature could be used to restrict a search to hydrocarbons only. These restrictions work just like the Formula Search, except they do not allow ions to be excluded.

Each of the direct search types allows the user to select the units to be used for displaying data. The user may choose between SI units and calorie-based units. Choosing the latter option will result in energy and pressure values displayed in calories and atmospheres rather than Joules and bars.

Most of the search types allow the user to select the type of data to be displayed. The user may select any number of the following options:

If the search succeeds, data corresponding to the selected options will be displayed. If none of the data types are selected, only general information about the species will be displayed.

For all of the direct search types except the registry number and name searches, selecting one or more of the data type options will affect the search criteria. If a data type is selected, a species will be considered a match only if it contains one or more of the data types selected. For example, consider a chemical formula search with both the "Gas phase thermochemistry data" and "Mass spectrum" data types selected. This search will match all species with the given formula that have either gas phase thermochemistry data or a mass spectrum. Species that do not contain any of the selected data types will be skipped. If no data types are selected, all species matching the search criteria will be found.

The formula search routines will find all of the species in the database with the requested chemical formula. The formula may be specified in any order. Legal characters in formulas include upper and lower case letters, numbers, question marks, asterisks, and parentheses. Formulas may be specified in a semi-structural manner, but any implied structure or connectivity in the formula will be ignored. For example, (CH3)(CH2)4(CH3) will find all species in the database with the formula, C6H14, not just n-hexane.

When specifying the formula, the proper case should be used where necessary to properly identify atomic symbols. A search for a formula of no, NO, or nO will result in nitric oxide being found, while a search for No will result in nobelium being found.

A range of formulas may be searched for by using a wildcard character for an element amount. If a wildcard is used, parentheses cannot be used in the formula. There are two wildcards, the question mark and the asterisk:

A wildcard specification may be combined with a normal specification for an element. Thus C2C*H4 will search for species with two or more carbon atoms and exactly four hydrogen atoms.

The formula search may be modified by four check boxes. The effect of each check box is described below:

Exactly match the specified isotopes

When checked, this option indicates that the search will match only the isotopes specified in the search formula. When not checked, other isotopes will be matched. If one searches for H2 with this option checked, the search will find just the hydrogen molecule, H2. If this option is not checked, the search will find additional species, including D2 and T2.

Allow elements not specified in formula

When checked, this option indicates that species containing elements not specified in the formula will also be matched. For example, a search for Te2 will find all of the species which contain exactly two tellurium atoms and any number of other atoms. Checking this option alone is equivalent to the "partial formula search" previously available from this site.

This option should be used with care as it could greatly increase the number of matches found by a search. The number of matches in a search can be reduced by specifying one or more atoms that should not be present in the desired species. This is done by placing a zero after the atomic symbol for the species in the formula. For example, a search on CH2N0O0 will find species that have one carbon, two hydrogens, no oxygen, and no nitrogen, but could contain any number of other atoms.

Allow more atoms of elements in formula than specified

This option indicates that species containg more atoms than specified of a given element will be matched. For example, a search for Te with only this option checked will find all tellurium, ditellurium, tritellurium, etc.

Exclude ions from the search

When this option is checked, the search will not match ions unless a charge is explicitly indicted in the formula.

The chemical name search option looks up chemical species by their name. The search is not case sensitive, so it is not necessary to use the correct case. Spaces are also ignored by the search algorithm, so both hydrogenfluoride and Hydrogen Fluoride will find HF. The database includes some common trade names and acronyms for chemical species. Because of this, it is possible that one name may be used by more than one species in the database. In this case, a selection page containing all of the matching species will be displayed.

The name search can also match patterns. Patterns are name strings which contain the asterisk (*) character. Each asterisk in the pattern may match zero or more characters in the name. Thus a search on *heptanone will look for species with a name that ends with "heptanone". A search on *heptanoic acid will find species with names that contain the text "heptanoic acid" somewhere in the name. When using patterns it is important to remember that the set of names used by the site is incomplete, so searching on a particular pattern may not match all of the desired species (since the database may not have the name which matches the pattern for some species).

If a species with a matching name is not found, the given string is assumed to be a chemical formula and any matching species are displayed. If a matching formula is not found, the given string will be assumed to be a CAS registry number and any matching species will be displayed.

If a name search fails, it is recommended that the user attempt a formula search to find the compound, since the species may be in the database under another name.

The CAS registry number search will look for a chemical species based only on its CAS registry number. It is not necessary to enter the dashes in the registry number.

Ion Energetics Searches

Several types of searches based on ion energetics data are available. All of these searches search for species based on a given quantity, range or ion formula:

The ionization energy search searches for species with a given ionization energy. Like the molecular weight search, either a single value or a comma-separated range can be used for the search. In all cases the values must be in electron volts (eV). If a single value is entered, the system will search for species with an ionization energy within 0.05 eV of the specified value.

The search only covers those species which have an ionization energy value that has been evaluated and assigned for search purposes. Currently this amounts to only 40 percent of the species with ionization energy data. This means that this search may miss some species with experimental IE determinations within the desired value range.

The electron affinity search looks for a species with a given electron affinity. The search is carried out on either a single value or a comma separated range in electron volts (eV). If a single value is entered, the system will search for values within 0.05 eV of the given value.

Where possible, electron affinity data is listed under the neutral species. In some cases, the corresponding neutral species does not exist or is not included in our database. For these cases, the electron affinity data will be listed under the anion. The electron affinity search will find whichever species the electron affinity value is listed under.

The proton affinity search covers only those compounds in the database for which proton affinity values have been defined. Searches for a single value will look for proton affinities within 5.0 kJ/mol (or 1.2 kcal/mol) of the specified value. Unlike some of the other value-based searches, the units for the specified value or range may be selected via radio-style buttons. The units used for the search value will also be used for the display of data.

The acidity search looks for species that have a given acidity value. Acidity is the Gibbs free energy of the reaction in which an acid dissociates into an anion and a hydrogen ion. Acidity values or comma separated ranges may be specified in kJ/mol or kcal/mol. If a single value is entered, the system will search for values within 5.0 kJ/mol (1.2 kcal/mol) of the specified value.

Unlike other ion energetics related values, acidity is listed as a Gibbs free energy of reaction value in the Reaction Thermochemistry Data section. The search will provide a link to the anion; the acidity value(s) can be found by looking for Gibbs free energy values under the appropriate reaction in the reaction thermochemistry data section.

The appearance energy product search looks for appearance energy (AE) reactions that result in a given positive ion as a product. Since the actual structure of the product is often not known, this search is based only on the chemical formula for the product. This search will always look for positive ions, so it is not necessary to include a plus sign (+) in the formula.

If there are data with the given AE product, all the data will be displayed in the form of a pseudo-species that represents all of the positive ions with a given formula. This pseudo-species will only contain AE data.

There are two options available for searching the vibrational and electronic spectra available in the site. Searches may be conducted on vibrational energy values and electronic (band origin) energy levels. Please note that these searches do not search the gas phase infrared spectra available from this site.

The vibrational energy search looks for species with a specified vibrational energy. A single value or a comma separated range may be used for the search. In all cases the energy value should be in wavenumbers (cm-1). If a single value is given, the system will search for values within 5.0 cm-1 of the given value.

The electronic energy level search finds species with electronic transitions with a given band origin. A single value or a comma separated range may be used for the search. In all cases the energy value should be in cm-1. If a single value is given, the system will search for values within 5.0 cm-1 of the given value.

There are several options available for structure based searching. These searches can require substantially more computational resources than the other searches available through the site, so they will often take longer to process. All of the searches are limited to the species that have structures in the database, so it may be wise to use them in concert with other search methods. The three types of structure based searches are the applet based search, the file based search, and the structure class search.

The applet based search provides an applet that allows users to draw structures they wish to search for. The applet requires a Java capable browser to run. When the structure has been drawn the user may search for matching structures or superstructures of the structure drawn. When searching for superstructures of a given structure (substructure search) additional groups may be attached to empty valences. The number of matching substructures may be controlled by specifying the numbers of hydrogen atoms attached to an atom. For example, a substructure search on C-C will match any species with a carbon–carbon single bond while a substructure search on H3C-CH2 will match species with an ethyl group.

The file based structure search is similar to the applet based structure, except it requires a user supplied structure to be uploaded to the web server. Because of this, the search requires a browser that can upload a file to a web server. Submitted structures should be in MDL mol file format. The choice of file format does not reflect an endorsement by NIST of any company's product or file format, nor does it imply that any file format or product is more suitable for this task at hand.

The structure class search allows users to find structures based on characteristics of the structure. These characteristics are selected through a form interface. The following characteristics can be specified through this search:

The structures in this site have been pre-classified to speed up this search. It is important to note that functional groups used in this search are identified independent of each other. Because of this a carboxylic acid group is also considered to be an OH group and C=O group. For this reason, a user may wish to exclude species containing carboxylic acid groups when searching for species containing an OH group.

The molecular weight search finds species matching a specific molecular weight value or within a comma separated range. If a single value is specified, the search will find any species with an atomic weight within 0.5 units of the specified value.

The molecular weight search uses atomic weights based on the most abundant isotope for the element. This is in contrast to more commonly used atomic weight values based on an average of isotope weights. The molecular weights displayed on the species data pages are based on isotope average atomic weights. Because of this, the weights used in the search may differ from those displayed in the species data pages.

The reaction search allows one to search directly for reaction data without first searching for individual species. Reactions are selected by entering one of the following for each reactant or product of interest:

Options are supplied which allow the user to select reactions in either direction, allow reactants and products in addition to those specified, exclude reactions with ions, and select a specific reaction type. The options for matching reactions in either direction and allowing additional reactants and products are pre-selected since it is envisioned that they will be desired by a majority of users.

The author search facility allows searches of the reference database used for this site by author names. Up to three author names may be specified. The year of publication or a range of years may be specified to narrow the search. Author names should be specified as a last name followed by initials (e.g., Doe, J.A.). Since there are references in the database for which currently have no data abstracted, this search can find references for which do not otherwise show up in our database.

Indirect searches are searches that are carried out by clicking on a highlighted region of text. Unlike direct searches, these searches cannot initially be used to find data. They can only be used to find data that are related to data which have already been displayed.

A reaction species search can be carried out by clicking on the species of interest in a chemical reaction display. The search will display data for the species selected. This feature will not work for the species currently being displayed or for any species that do not have data in the database.

In many cases it may be more convenient to use the reaction search facility to search for specific reactions.

An indirect author search is carried out by clicking on an author's name in a reference citation. This search will look for all of the reference citations in the database that contain the author's name. It should be noted that this search is limited to the WebBook database and, therefore, should not be confused with a complete literature search.

A source search can be carried out by clicking on the expression, [all data], at the end of a reference citation. Such a search will display all of the species in the database that contain data from the source. If only one species contains data from the source, that species will be displayed.

Data Presentation

Most of the data available from the web site are available in tabular form. Where appropriate, however, provisions have been added for graphical viewing of some data types.

Data Tables

Many of the types of data available in the site are displayed in tables. Although these tables may vary in configuration, they share several common features.

Graphical Displays

Several types of data can be displayed with a graphical display. In some cases a Java applet is used to generate the display. In these cases, users will not be able to run the applet if they do not meet the following conditions:

The system that displays infrared mass and UV/Vis spectra has been designed to display static images of the spectra in cases where the applet cannot be loaded.

Spectrum Displays

Mass and IR spectra may be displayed as an inline image or as an applet. Inline images are only used on browsers that do not support applets or have applet support turned off.

When spectra are displayed by the applet, the user may zoom the image by selecting a region to be zoomed with the mouse. Single clicking the mouse will un-zoom the spectrum. For IR spectra, the direction of the x-axis can be reversed by toggling a button on the applet. The applet will track the coordinates of the mouse in the upper right-hand corner of the display. This feature can be toggled off and on by clicking within the applet and pressing the C (for coordinate) key on the keyboard.

Spectra displayed as images can also be zoomed, but the options for zooming are more restrictive than those of the spectra display applet. To zoom spectra images, the following steps must be carried out:

  1. The desired x-axis range must be entered. The range should be entered as two values separated by a comma. The first value will be used for the left endpoint of the x-axis and the second will be used as the right endpoint.
  2. If desired, the "automatic Y scaling" button can be checked. If the button is checked, the y-axis will be adjusted so that it is appropriate for the range of data displayed. If the button is not checked, the original y-axis range is used.
  3. The "zoom" button must be pressed. This will load a new page with the desired spectra image.

Fluid Data

Fluid property data may be optionally displayed in an applet which provides the ability to generate X-Y plots or display data in tabular form.

After the applet has loaded the data from the server, X-Y plots may be displayed by selecting the desired data types from selection boxes for each axis. The region displayed may be enlarged by selecting a region of the plot with the mouse. Data are color coded by state as noted below:

The applet has a button in the upper left hand corner that will toggle the display of a table containing the raw physical property data. The data displayed in the table can be set by selection boxes which are located above each column in the table.

Function Plots

Some data in the WebBook come in the form of function parameters that describe the behavior of a physical property over a given range. For many of these data types it is possible to select a link that will load an applet which will display the curve generated by the function. In the case where multiple determinations of the parameters are available, all of the resulting curves will be plotted together.

As with the other plot types the function plot may be zoomed by selecting a region on the plot. Depending on the type of data plotted, the data may be initially plotted using log-log, semi-log, or normal axes. Information on changing the plot format can be found by pressing the "help" button on the applet. Data from different sources will be plotted in different colors; a legend box is provided to identify the sources.

Data Available

For a chemical species there are several types of data which can be obtained from the WebBook system. However, all but a few of the chemical species in the database contain only a subset of the possible data types.

Data are broken up into types with each type separated by a horizontal line across the web page. General information about the species is always displayed at the top of the page. Reference citations, if present, and notes are displayed at the bottom of the page. If requested, groups of data are displayed between the general data section and the reference citations.

General Information

For all species, a set of general information is displayed at the top of the page. This information includes the following:

Gas Phase Thermochemistry Data

The gas phase thermochemistry data available consist of the following types of data:

The Shomate equation parameters can be used to calculate the heat capacity, molar enthalpy, and molar entropy using the equations displayed above the parameter tables.

Condensed Phase Thermochemistry Data

The following condensed (liquid, solid) phase thermochemistry data are available:

Phase Change Data

A wide variety of phase transition data are available:

For phase transitions other than vaporization, freezing, and sublimation, the phases involved in the transition are listed in the table with the data. Unless otherwise stated, all data are for species at standard pressure.

Reaction Thermochemistry Data

Reaction data consist of two types of data:

  1. Enthalpy of combustion data (ΔcH°).
  2. Properties of specific reactions.

The heat of combustion data (if available) are grouped together in a single table at the start of the section. Data for specific reactions are provided in tables following a schematic representation of the reaction. These data may include the following values:

Unless stated otherwise, all data are for reactions occurring at 298 K. The enthalpy of reaction data are grouped by reaction. Please note that for anions, some of the reactions presented correspond to heat of acidity or heat clustering values.

Gas Phase Ion Energetics Data

The following types of gas phase ion energetics data are available:

Positive ion data are generally listed under the corresponding neutral species. With the exception of electron affinity data, negative ion data are provided under an entry for the anion. This means that if you want positive ion data, you must search for the neutral species, and if you want negative ion data you must search for the anion. This potentially confusing situation is a legacy of the format in which the data were collected and the fact that many of the anions in the database do not have corresponding neutral species. In those cases where the corresponding neutral is known, any electron affinity data will be listed under the neutral species, since the electron affinity may be considered a property of the neutral species.

Heat of acidity data and heat of clustering data are considered Reaction Thermochemistry Data and are shown under the anion.

In addition to the data described above, a link to search for appearance potential reactions with a product with the same formula as the current species may be displayed. Clicking on this link is equivalent to conducting an Appearance Energy Product Search using the chemical formula of the current species.

Ion Clustering Data

Data for ion clustering reactions is available in this section. This data is also available in the Reaction Thermochemistry section, but this section limits reactions to only ion clustering reactions. This makes it easier to find clustering reactions. For example, a search on a neutral species with ion clustring data specified, will find all matching species which are ligands in at least one ion clustring reaction.

Gas Phase IR Spectra

Where available, gas phase spectra from the NIST/EPA Gas-Phase Infrared Database may be displayed as a static image or through an applet. If available, supplemental data will be displayed below the spectrum.

Mass Spectra

Mass spectra are provided from the NIST Mass Spectrometry Data Center. Supplementary data, such as the source of the spectrum, instrument type, instrument parameters, and the EPA MS number are displayed below the spectrum.

UV/Vis Spectra

UV/Vis spectra are provided from the Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences. As with the other spectra, supplementary data is displayed below the spectrum.

Vibrational and Electronic Spectra

Vibrational and electronic spectra are provided in tabular format. Where appropriate, spectra are grouped by state, with the states in order of decreasing energy. Tables containing vibrational spectra contain the following data:

Some of the vibrational data tables may have more data depending on the source of the data.

Tables containing electronic energy levels contain the following data:

Both types of tables may have links to relevant notes which are displayed after the last table.

Constants of Diatomic Molecules

Constants of diatomic molecules are provided in tabular form. These data contain a large number of notes and comments. Some comments are embedded directly in the table while others are provided in a list of notes which follows the table. The following quantities are provided:

Most of the constants are derived from the Dunham series expansion:

TνJ = Ylm(ν + ½)l Jm(J + 1)m

which results in the following:

G(ν) = ωe (ν + ½) − ωexe (ν + ½)2 + ωeye (ν + ½)3 + ωeze (ν + ½)4 ...
Fν(J) = BνJ(J + 1) − DνJ2(J + 1)2 + HνJ3(J + 1)3 ...
Bν = Be − αe(ν + ½) + γe(ν + ½)2 + ...
Dν = De − βe(ν + ½) + ...

When available, references are supplied on a line directly below the values they refer to. A directional arrow (arrow facing down and to the right) is placed at the start of the reference line to indicate that they apply to the data on the line above.

Henry's Law Data

Solubility data currently available consists of Henry's law constants for the solubility of gases in water (k°H). Temperature dependence information is provided for some of the values.

Gas Chromotagraphic Retention Data

Gas chromatographic retention data consists of Kovats and Lee retention indices for a wide variery of compounds. Data are oganized based on column type (non-polar, polar, or other) and heating program (isothermal, simple ramp, or complex program). The initial set of data includes only data from non-polar (or nearly non-polar columns). It is anticpated that data from other column types will be added in the future.

Thermophysical Properties of Fluid Systems

A special facility provides fluid properties over user-specified ranges for a group of pure fluids. This facility can provide data over the following range types:

The following types of data are available:

Please note that for data along the saturation curve, all properties except temperature and pressure will be provided for both the liquid (l) and vapor (v) phases.

In order to receive data from this facility, a series of web pages must be completed that provide selections for units, range type, range parameters, and display type. There are currently two types of displays available: an applet that displays X-Y plots of selected data and a data table with user selectable axes and a HTML table that contains the raw data.

References to the Literature

Citations for the literature referenced in a page are listed in a section at the end of the page. The citations are listed in the order in which they are referenced. The reference to the citation used in the data tables is given in bold print above each citation.