IR Frequency Calculator

In infrared (IR) spectroscopy, chemists use wavenumber and frequency to identify the molecular bonds in a compound — and the IR Frequency Calculator converts between the two and matches peaks to known functional groups. Enter a wavenumber (cm⁻¹) or frequency (Hz), select your calculation mode (wavenumber to frequency, frequency to wavenumber, or functional group identification), and optionally set your peak intensity and peak shape. You'll get the converted value, along with the likely functional group, wavelength (μm), bond type, and identification confidence.

cm⁻¹

Typical range: 500-4000 cm⁻¹

Hz

Alternative input to wavenumber

Results

Converted Value

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Likely Functional Group

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Wavelength

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Bond Type

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Identification Confidence

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Results Table

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Frequently Asked Questions

What is the relationship between wavenumber and frequency in IR spectroscopy?

Wavenumber (cm⁻¹) is the reciprocal of wavelength and is directly proportional to frequency. Higher wavenumbers correspond to higher frequencies and higher energy transitions.

How do I identify functional groups from IR absorption peaks?

Each functional group absorbs at characteristic frequencies. Compare your observed peak with known frequency ranges for different bonds like O-H (3200-3600 cm⁻¹), C=O (1650-1750 cm⁻¹), and C-H (2850-3000 cm⁻¹).

What does peak intensity tell us about molecular structure?

Peak intensity relates to the change in dipole moment during vibration. Polar bonds typically show stronger absorptions, while symmetric vibrations may be weak or absent in IR spectra.

Why are some peaks broad while others are sharp in IR spectra?

Peak shape depends on hydrogen bonding and molecular environment. O-H and N-H stretches are often broad due to hydrogen bonding, while C=O stretches are typically sharp.

What is the fingerprint region in IR spectroscopy?

The fingerprint region (500-1500 cm⁻¹) contains complex overlapping absorptions unique to each molecule. It's useful for compound identification but difficult to interpret for specific functional groups.

How accurate is functional group identification from a single IR peak?

A single peak provides clues but full identification requires multiple peaks, peak intensity, and shape analysis. Environmental factors and molecular context also affect absorption frequencies.

What factors can shift IR absorption frequencies?

Hydrogen bonding, conjugation, ring strain, and substituent effects can shift frequencies. For example, conjugation typically lowers C=O stretching frequencies by 20-40 cm⁻¹.

How do I convert between different IR spectral units?

Use the relationship: frequency (Hz) = wavenumber (cm⁻¹) × speed of light (3×10¹⁰ cm/s). Wavelength (μm) = 10,000 / wavenumber (cm⁻¹).