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What is the NIST Chemistry WebBook?
The NIST Chemistry WebBook is a free, searchable collection of critically evaluated physical and chemical data compiled by the U.S. National Institute of Standards and Technology (NIST) under its Standard Reference Data Program. It is a reference database rather than a journal, textbook, or modeling tool: you look up specific compounds, reactions, or spectra and retrieve numeric values and spectra.
What you can find
- Thermochemistry for over 7,000 organic and small inorganic compounds — enthalpy of formation, enthalpy of combustion, heat capacity, entropy, phase-transition enthalpies and temperatures, vapor pressure.
- Reaction thermochemistry for over 8,000 reactions — enthalpy and Gibbs free energy of reaction.
- Spectra: IR for over 16,000 compounds, mass spectra for over 33,000, UV/Vis for over 1,600, plus electronic and vibrational spectra for over 5,000.
- Chromatography: gas chromatography data for over 27,000 compounds.
- Ion energetics for over 16,000 compounds — ionization energy, appearance energy, electron affinity, proton affinity, gas basicity, cluster ion binding energies.
- Diatomic molecule constants for over 600 species.
- Thermophysical properties for 74 fluids — density, specific volume, Cp, Cv, enthalpy, internal energy, entropy, viscosity, thermal conductivity, Joule–Thomson coefficient, surface tension, speed of sound.
How you search
You can locate data by compound name, chemical formula, CAS registry number, molecular weight, or chemical structure, and also by selected ion energetics and spectral properties. That range of entry points is what makes it usable whether you have a bottle label, a structure drawing, or only a spectrum to match.
Who uses it, and for what
A process engineer sizing a heat exchanger needs accurate Cp and thermal conductivity for a working fluid; a graduate student assigning a mass spectrum wants reference spectra for comparison; an atmospheric chemist checking a reaction enthalpy needs a defensible number with a citation. In each case the value is the same: traceable, evaluated data instead of a figure copied from an unverified secondary source.
Trade-offs to keep in mind
- Breadth is uneven. Coverage is deep for small organic and inorganic molecules and thin or absent for large biomolecules, polymers, and mixtures.
- It is a lookup tool, not a calculator. You retrieve data; you do your own fitting, interpolation, or estimation.
- Access may change. NIST states it reserves the right to charge for access in the future, so do not build a workflow that assumes permanent free availability.
- No warranty. NIST states it makes no warranties as to quality and is not liable for damage from errors or omissions, so verify critical values against primary literature before using them in design or publication.
If you are new to the site, start with a compound you already know well, such as water or benzene, and check how the thermochemical, spectral, and fluid-property sections are organized before searching for something unfamiliar. For related reference data, the broader NIST program is at NIST.
How can I search for a compound by CAS registry number, name, or chemical formula?
Use the search form on the NIST Chemistry WebBook entry page. It accepts several identifier types, so you can look up a compound by CAS registry number, name, chemical formula, molecular weight, or chemical structure, and also by selected ion energetics and spectral properties NIST Chemistry WebBook.
The practical difference is which identifier you trust most:
| Search field | Best when | Trade-off |
|---|---|---|
| CAS registry number | You have a verified number from a label, SDS, or supplier | Most unambiguous, but a typo returns nothing |
| Name | You only have a common or IUPAC name | Names vary by synonym, so you may need to try alternatives |
| Chemical formula | You know the composition but not the identifier | Formulas match many isomers, so expect a list |
| Molecular weight | You are screening an unknown | Least specific; many compounds share a mass |
| Structure | You can draw or upload the structure | Precise, but slower than typing an identifier |
A concrete scenario: a lab notebook lists "CAS 67-64-1." Enter the number rather than the name, because names such as "acetone" and "2-propanone" both refer to the same record and you avoid guessing which synonym the database indexes. If you only have a formula like C3H6O, search that and then narrow the results using the spectra or thermochemistry data you need.
Next step: open the WebBook search page, run your most specific identifier first, and if you get multiple hits, add a second filter such as a mass spectrum or ion energetics value to isolate the right record.
Where can I find thermochemical data like enthalpy of formation or vapor pressure for a specific compound?
Use the NIST Chemistry WebBook at NIST Chemistry WebBook. It is NIST's Standard Reference Data product (SRD 69) for chemical and physical property data, and it is the fastest free starting point for compound-specific thermochemistry.
What you can look up there
For a given compound, the WebBook covers the data types you named plus related properties:
- Thermochemical data for over 7,000 organic and small inorganic compounds: enthalpy of formation, enthalpy of combustion, heat capacity, entropy, phase-transition enthalpies and temperatures, and vapor pressure.
- Reaction thermochemistry for over 8,000 reactions: enthalpy of reaction and free energy of reaction.
- Spectra and related data: IR (16,000+ compounds), mass spectra (33,000+), UV/Vis (1,600+), gas chromatography (27,000+), electronic and vibrational spectra (5,000+), and ion energetics (16,000+), including ionization energy and proton affinity.
- Thermophysical properties for 74 fluids: density, specific volume, C p, C v, enthalpy, internal energy, entropy, viscosity, thermal conductivity, Joule–Thomson coefficient, surface tension and sound speed.
How to search
You can search by name, chemical formula, CAS registry number, molecular weight, or chemical structure, and also by selected ion energetics and spectral properties. For a specific compound, the CAS number is usually the most reliable key because common names vary.
Practical example
Suppose you need the enthalpy of formation and vapor pressure of a solvent at a given temperature for a process calculation. Search the compound by CAS number, open its thermochemistry page, and read the tabulated values with their temperature ranges and cited sources. If the compound also appears under fluid properties, check whether the WebBook's fluid table covers the pressure or temperature range you need — the fluid dataset is limited to a specific list of 74 fluids, so many compounds will have thermochemical data but no full fluid-property table.
What to watch for
- Units and reference states differ between tables; confirm whether a value is for the gas, liquid or solid phase before using it.
- Temperature dependence: vapor pressure, heat capacity and enthalpy are temperature-dependent. Use the tabulated range rather than extrapolating.
- Data provenance: each entry cites its original source; check the citation if the value matters for publication or design.
- Availability: NIST notes it reserves the right to charge for access in the future, so the current free access is not guaranteed indefinitely.
Next step: go to NIST Chemistry WebBook, search your compound by CAS number, and note the phase, temperature range and source for each value you record. For authoritative evaluated values beyond the WebBook, NIST's Standard Reference Data program and the primary literature cited in each entry are the appropriate follow-ups.
How do I access IR, mass, or UV/Vis spectra for a given chemical?
Enter the NIST Chemistry WebBook and search for the compound by name, chemical formula, CAS registry number, molecular weight, or chemical structure. Once you open the compound's record, look for the spectral sections; the WebBook says it holds IR spectra for over 16,000 compounds, mass spectra for over 33,000, and UV/Vis spectra for over 1,600.
NIST Chemistry WebBook
H3 Practical steps
- Go to the WebBook entry page and choose the search type that matches what you know.
- Open the compound record and scan the section headings for IR, Mass, or UV/Vis spectra.
- Note the conditions and source attached to each spectrum before comparing it with your own data.
H3 What each spectrum is useful for
- IR: identifying functional groups and checking a known sample against a reference pattern.
- Mass: confirming molecular mass and interpreting fragmentation, often alongside a chromatographic peak.
- UV/Vis: studying chromophores and electronic transitions, especially for solutions.
H3 A concrete scenario Suppose you synthesised an unknown ester and have an IR band near 1735 cm⁻¹. Search the WebBook by molecular formula or structure, open candidate records, and compare band positions and intensities. If the mass spectrum is also listed, use the molecular ion and major fragments to narrow the candidates further.
H3 Decision criteria
- If you know only the name, start with a name search, but verify the CAS number to avoid isomers.
- If you have a spectrum but no name, structure or formula searching is usually faster.
- If you need gas-phase behaviour, mass spectra and ion energetics data are more relevant; for solution-phase electronic structure, use UV/Vis.
For related reference collections, the NIST main chemistry page is at NIST Chemistry WebBook.
What thermophysical property data are available for fluids such as viscosity or thermal conductivity?
The NIST Chemistry WebBook covers thermophysical property data for 74 fluids, and viscosity and thermal conductivity are both included. For those fluids you can also find density and specific volume, heat capacity at constant pressure (Cp) and at constant volume (Cv), enthalpy, internal energy, entropy, the Joule–Thomson coefficient, surface tension (saturation curve only), and speed of sound.
How to use it
- Search by compound name, chemical formula, CAS registry number, molecular weight, or chemical structure.
- Expect fluid coverage to be narrower than the site's other collections: 74 fluids for thermophysical properties, versus thousands of compounds for spectra and thermochemistry.
- Viscosity and thermal conductivity are transport properties, so they matter most for heat transfer, pipe sizing, and process simulation; the thermodynamic entries (density, Cp, enthalpy, entropy) support energy balances and phase calculations.
- Surface tension is limited to the saturation curve, so don't treat it as a general pressure-dependent value.
A practical scenario
An engineer sizing a heat exchanger for a common refrigerant or solvent can pull viscosity and thermal conductivity for the same fluid from one source, which keeps property inputs internally consistent rather than mixing databases. The trade-off is that if a fluid isn't among the 74, you'll need another route.
If your fluid isn't listed, check the broader NIST reference-data pages, or compare against a thermophysical-properties source such as CoolProp for additional fluids and interpolated states.
Can I retrieve ion energetics data like ionization energy or proton affinity for a compound?
Yes. The NIST Chemistry WebBook covers ion energetics data for over 16,000 compounds, and the specific quantities listed include ionization energy, appearance energy, electron affinity, proton affinity, gas basicity, and cluster ion binding energies. You can search by compound name, chemical formula, CAS registry number, molecular weight, chemical structure, or by selected ion energetics and spectral properties.
What that looks like in practice
Say you are checking whether a solvent or reagent will survive in a mass-spectrometry method. You look up its ionization energy to judge how easily it gives up an electron, and its proton affinity to judge how readily it takes a proton in the gas phase. Comparing those two numbers against the analyte's values tells you whether your reagent will compete for charge or protons, which is often the difference between a clean spectrum and a confusing one.
How to get there
- Enter the compound through the site's search (name, formula, CAS number, or structure).
- Open the compound's page and look for the ion energetics section alongside thermochemistry, spectra, and other data.
- Cross-check the value against your experimental conditions. Ion energetics data are typically gas-phase quantities, so they do not automatically transfer to solution behavior.
Practical limits
The page states that NIST reserves the right to charge for access in the future, so free retrieval is not guaranteed indefinitely. NIST also states it makes no warranties as to accuracy and is not liable for errors or omissions, which is a reasonable cue to verify a critical number against a second source before publishing or betting a method on it.
If you need spectra or thermochemistry for the same compound, the same record usually carries those too, so one lookup can serve several purposes. For broader chemical and physical reference data beyond this collection, NIST maintains related programs.
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