FTIR (Fourier-transform infrared spectroscopy) observes infrared absorption associated with molecular vibrations. Add bonds below to explore how band position and width shape a spectrum. This model does not reproduce a specific substance or identify unknown samples.
Material
Bonds
Start with PP. Switch materials or add/remove bonds to compare.
A dot marks bonds unlikely in the base structure. They remain selectable; other constituents or processing can contribute in real samples.
Use + to keep up to 5 comparison snapshots. Reset clears saved spectra too. Snapshots last until you reload.
Curves are vertically offset for comparison. Stored transmittance values are unchanged; all curves use the same vertical scale. Only Current is editable.
Hover over or tap the graph to explore possible assignments.
Dashed tags indicate possible assignments; filled tags are added bonds. Hover over a tag to preview its range, or select a circle to highlight its explanation. You can also focus the graph and use Left/Right arrows.
Polypropylene has a saturated hydrocarbon chain with methyl groups. This example includes C–H stretches and CH₂/CH₃ bending; oxidation carbonyls are not part of the base preset.
Selected bond explanations · Current
PP example: 2950 cm⁻¹ Methyl / methylene C–H · 2918 cm⁻¹ C–H stretch · 2836 cm⁻¹ C–H stretch.
PP example: 1456 cm⁻¹ CH₂ deformation.
PP example: 1376 cm⁻¹ CH₃ deformation.
In the 1500–400 cm⁻¹ fingerprint region, overlapping vibrations make single-peak assignments difficult. This is not measured data; positions, widths and strengths depend on chemical environment.
Try these comparisons
The page opens with PP and its representative C–H bands. Selecting a material rebuilds Current from its expected bonds; Custom starts with an empty baseline. Dotted bond tags indicate bonds unlikely in the base structure, but remain selectable.
- Select PET, then add O–H. Read how the ideal repeat structure differs from possible moisture, end-group and other contributions.
- Select Custom, then O–H → C–H (sp³) → C=O → C–O. Their bands combine into one curve.
- Hover over the graph, or tap it on mobile, to display the current wavenumber and possible assignments. All tags with overlapping ranges receive dashed outlines. Exploring does not add bonds.
- Hover over a tag to preview its range. Clicking adds its explanation below; clicking again removes both its band and explanation. Select a circle on the curve, or reach it with Tab and press Enter or Space, to highlight its explanation.
You can also focus the graph and explore with Left/Right arrows. Hold Shift for larger steps; Home/End move to the axis endpoints. Escape clears exploration; Reset clears saved spectra and exploration and restores the default PP example. On mobile, the tapped position remains until you tap another position or reset.
The horizontal axis decreases from 4000 on the left to 400 cm⁻¹ on the right. The vertical axis is transmittance: stronger absorption produces a downward band. In the lightly shaded 1500–400 cm⁻¹ fingerprint region, overlapping vibrations make interpretation from a single peak difficult.
Comparing with real spectra
Save and compare curves
Try PP → + → PET → + → Cellulose → + to compare material examples. Each snapshot stores its material and edited bond list; switching materials leaves saved spectra unchanged. On mobile, select an S label to inspect its material.
Select O–H + C–O and press + above the graph to save S1. Add C=O and press + again to save S2. Only Current, fixed at the top, follows the live bond tags; saved curves remain unchanged. Keep up to 5 snapshots on the current page.
Use ↑ / ↓ on the right to reorder snapshots, or × to delete one. S numbers identify creation order and never change when you reorder or delete. Select an S label to inspect its saved bonds without changing Current or its explanations.
Comparison curves share the same wavenumber axis and vertical scale, with evenly spaced offsets. Offsets affect only the display; saved transmittance values do not change. Reset clears snapshots, numbering and exploration and restores Current to PP. Reloading also discards snapshots.
Scope of the teaching model
The 20 tags distinguish bonds from vibration types: C–H stretching, CH₂/CH₃ bending and aromatic ring skeletal stretching have separate controls. Material examples use selected representative positions from the sources below. Highlight windows, widths and strengths are teaching settings, not measured uncertainty intervals or quantitative comparisons. Material explanations list the positions and assignments.
Compare broad O–H with very broad O–H (acid) and sharp ≡C–H. Ar C=C, N–O and CH₃ bend can each generate multiple bands. N–O and CH₃ bend highlight separate reference windows rather than the entire gap between them. C–O and C–O–C overlap and are not independent proof of a structure; their summed intensity does not count bonds.
Epoxy represents uncured DGEBA-type resin, not every cured epoxy. PVC plasticizer carbonyls and the cellulose absorbed-water band near 1640 cm⁻¹ are excluded from the presets. Adding/removing tags compares bands; it does not simulate a chemical reaction or structural transformation.
The displayed ranges are representative windows, not universal boundaries. O–H illustrates hydrogen-bonded alcohols and phenols; C–N spans aliphatic and aromatic amines. The 1650–1750 cm⁻¹ C=O window does not cover every carbonyl compound.
Hydrogen bonding, conjugation, crystallinity and sample state can change band positions and shapes. A single peak cannot establish a substance or functional group: interpret several bands together. Adding a bond here does not calculate the resulting changes in chemical environment.
The teaching model sums smooth Gaussian components with different widths and strengths in absorbance and converts the result to transmittance. These settings do not represent concentrations, composition or measured data. Many modes remain omitted: PE rocking, PET/PS aromatic C–H out-of-plane bending, epoxy oxirane ring deformation and cellulose C–O deformation are excluded rather than grouped under stretching tags.
References
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Photothermal Spectroscopy Corp. — Polymer IR fingerprints: PE, PS and PVC positions. This O-PTIR source differs in measurement method and intensity from our synthetic transmittance curves.
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Original PP FTIR assignments in a polymer degradation study.
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DGEBA epoxy structure and FTIR, DGEBA backbone and oxirane assignments, Comparison of DGEBA systems with different hardeners.
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UCLA WebSpectra — Table of IR Absorptions: representative bond absorption ranges.
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OpenStax — Infrared Spectra of Some Common Functional Groups: chemical environments, band shapes and companion absorptions.
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University of Colorado Boulder — IR: Amines: the C–N absorption range of aliphatic amines.
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University of Colorado Boulder — Carboxylic acids and Nitro groups: very broad acid O–H and paired nitro stretches.
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Specac — Infrared frequency lookup: terminal alkyne C–H and representative C–F, C–Cl and C–Br windows.
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University of Maryland — FTIR frequencies: C–O–C stretching.
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Barbara Stuart — Infrared Spectroscopy: Fundamentals and Applications: amine C–N and CH₂/CH₃ deformation ranges.
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Spectroscopy — Alkane chain length and overlapping bending bands.