3400 cm-1 in FTIR: what a band there actually means

Direct answer

A band near 3400 cm-1 is an X-H stretching band, and in most samples it is an O-H stretch. Alcohol and phenol O-H groups that are hydrogen bonded absorb as a broad band across roughly 3200-3600 cm-1, while an O-H group that is not hydrogen bonded shows a much narrower band at the top of that window, near 3500-3650 cm-1. N-H stretches from amines and amides occupy the same region and are usually a little lower, so a 3400 cm-1 band cannot be assigned on its own. The bands that travel with it decide the case: an alcohol brings its C-H stretches at 2850-2960 cm-1, a protein brings the amide II band near 1545 cm-1, water brings its bending band at 1630-1650 cm-1, and a carboxylic acid brings a C=O band near 1730 cm-1 together with an O-H so strongly hydrogen bonded that it runs down past 3000 cm-1. That tail below 3000 cm-1 is the one clean way to separate an acid from an alcohol.

What decides the assignment

1. How broad the band is

   A hydrogen-bonded O-H is broad, and an O-H that is free of hydrogen bonds is narrow. Wood samples with many hydroxyl and phenolic groups show strong absorption between 3200 and 3500 cm-1 with a narrow peak at 3500 cm-1, while the O-H stretch of an alcohol is described as a strong and broad peak. Width therefore separates free from hydrogen-bonded, not O-H from N-H: the N-H stretch is broad as well.

2. Where it sits inside the 3200-3600 window

   The published ranges put alcohol O-H at 3650-3200 cm-1 and carboxylic acid O-H at 3300-2500 cm-1, so an acid starts lower and keeps going. In heat-treated wood the band at 3430 cm-1 broadened towards lower wavenumbers as the amount of carboxylic acid rose, which is how an acid reads in practice: the band does not stop, it runs down past 3200 and past 3000 cm-1.

3. Which bands travel with it

   Water adds a bending band at 1630-1650 cm-1: in one oxide the bands around 3400 and 1630 cm-1 accounted for absorbed molecular water, and in a biopolymer the peak centred at 1645 cm-1 was assigned to the H-O-H bending contribution. An alcohol adds C-H stretches at 2850-2960 cm-1. A protein adds the amide II band near 1545 cm-1. Look for these before naming the 3400 cm-1 band.

4. Whether the sample was dry

   Water adsorbed from the atmosphere produces a broad band centred at 3400 cm-1 on its own, and adsorbed water is described as part of the hydroxyl signal on a solid surface, so a hygroscopic sample can carry a 3400 cm-1 band that has nothing to do with the material. Drying and re-measuring settles it: a band that shrinks was moisture, a band that stays was part of the sample.

Main assignments

Check the bands around it

• 1640 cm-1

  The H-O-H bending band of water. Where it appears, the 3400 cm-1 band is at least partly moisture or water.

• 1545 cm-1

  Amide II, the band that has to accompany an amide A (N-H) stretch in a protein.

• 2925 cm-1

  The aliphatic C-H stretch of the chain that carries the O-H group.

• 1730 cm-1

  The C=O band of a carboxylic acid, the partner of the very broad acid O-H that runs down below 3000 cm-1.

• 3600 cm-1

  The O-H band of a group that is not hydrogen bonded, for example on a dry mineral surface.

Possible materials

Materials are shown only when the same literature pool supports this band and at least one additional characteristic peak.

Published spectra

• Published spectrum

  Table 1: Frequencies of IR stretching that are highly important. O-H 3650-3200, strong and broad (alcohol); O-H 3300-2500, strong and very broad (carboxylic acid); N-H 3500-3300, medium and broad; N-H 3300-2700, medium.

  Reproduced from Table 1 of the paper, which lists the IR stretching frequencies it treats as important; the reference marker behind the table title and the column headers are left out. The table cells are given here as text, in the reading order of the page.

  Infrared Spectroscopy-Based Chemometric Analysis for Lard Differentiation in Meat Samples DOI: 10.32604/cmc.2023.034164 · p. 4
• Published spectrum

  The band at 3435 cm-1 indicates OH stretching as free OH groups absorb in 3650-3400 cm-1 region. The bands between 2850 and 2959 cm-1 indicate alkyl C-H (asymmetric and symmetric) stretching

  The extraction split the unit 'cm-1' off its wavenumber; it is rejoined here and the wording is unchanged.

  Structural analysis and antioxidative properties of mutan (water-insoluble glucan) from Streptococcus mutans DOI: 10.1016/j.procbio.2020.07.006 · p. 13
• Published spectrum

  Heating seemed to reduce the absorbance of the strong and broad peak which is assigned to O-H stretching of alcohol group.

  The two columns of the page were interleaved by the extraction; the sentence is given in reading order.

  A fast and reliable 2D-IR spectroscopic technique for herbal leaves classification DOI: 10.1016/j.vibspec.2019.103014 · p. 9
• Published spectrum

  The reference spectrum of wood samples (containing multiple organic hydroxyl and phenolic groups from lignin structures) shows strong absorption between 3200 and 3500 cm-1 with a narrow peak at 3500 cm-1

  The extraction interleaved the two columns of the page; the sentence is given in reading order and truncated where it continues into the identification of the phenol group.

  Phenol Groups in Northeastern U.S. Submicrometer Aerosol Particles Produced from Gasoline and Biomass Burning DOI: 10.1021/es9032277 · p. 2
• Published spectrum

  The FTIR spectra of heat-treated pine and eucalypt wood showed a broadening to lower wavenumbers of the band at 3430 cm-1 corresponding to the O-H stretching vibration from alcohols (3600-3300 cm-1) and carboxylic acids (3300-2500 cm-1) present either in polysaccharides and lignin.

  The extraction split the unit 'cm-1' from its wavenumbers and interleaved the two columns; the units are rejoined here and the wording is unchanged.

  Chemical changes of heat treated pine and eucalyptus wood DOI: 10.4067/S0718-221X2013005000020 · p. 7
• Published spectrum

  At the 3500-3100 cm-1 peak is due to primary amine ν(NH2) and hydroxyl group ν(O-H) stretching bands.

  The same page assigns the 1564 cm-1 band to the N-H stretching band of the primary amine group in lysine. The extraction interleaved the two columns of the page and renders the Greek letter nu as a plain n; the letter is restored here and the wording is otherwise unchanged.

  Synthesis, characterization, toxicity and in vivo imaging of lysine grafted poly(HEMA) copolymers DOI: 10.1007/s10965-019-1901-7 · p. 5
• Published spectrum

  For OI3-oligomer, a broad band at 3253 cm-1 is primary stretch of amines

  Table II of the same paper lists the N-H stretch at 3295-3250 cm-1 observed, against 3500-3100 cm-1 in the literature, and the N-H bend at 1620-1590 cm-1.

  Influence of amine-terminated additives on thermal and mechanical properties of epoxy resins DOI: 10.1002/app.48404 · p. 4
• Published spectrum

  In addition, peak around 3300 cm-1 shows that vibration of O-H is overlapped with the N-H vibration.

  The two columns of the page were interleaved by the extraction; the sentence is given in reading order.

  Mechanism and kinetics of curing of diglycidyl ether of bisphenol a (DGEBA) with chitosan DOI: 10.1002/pen.24463 · p. 3
• Published spectrum

  a broad band centered at 3400 cm-1 due to the hydrogen-bonded H2O molecules

  Quoted from inside a longer sentence about water adsorbed from the atmosphere inside the pores of the framework. The extraction interleaved two copies of the column, so only the fragment that carries the assignment is quoted.

  Adsorption Properties of Ce-5(BDC)(7.5)(DMF)(4) Metal-Organic Framework: Experimental and Theoretical Studies DOI: 10.3390/inorganics8020009 · p. 5
• Published spectrum

  The bands around 3400 and 1630 cm-1 account for the presence of absorbed molecular water.

  The extraction put the unit 'cm-1' in front of the sentence and interleaved the two columns; the unit is moved back into the sentence and the wording is unchanged.

  Effect of Mn doping on the structural and optical properties of SnO2 nanoparticles DOI: 10.1016/j.jallcom.2012.01.072 · p. 3
• Published spectrum

  The broad band within the range of 3450-3290 cm-1 appeared in pristine and modified clays and can be assigned to OH and NH moieties. The OH comes from silicate sheet surfaces, trapped water molecules, or the free silanol groups of the silane coupling agents that are attached physically or chemically to the surface of nanosheets.

  The two columns of the page were interleaved by the extraction; the sentence is given in reading order. The extraction renders the dash of O-H and N-H as a letter and the unit as "cm21"; both are restored here.

  Silane functionalization of sodium montmorillonite nanoclay: The effect of dispersing media on grafting DOI: 10.1016/j.clay.2017.12.020 · p. 4
• Published spectrum

  The broad band at 3400 cm-1 pronounced in the LF, is due to OH or NH stretching typical in crop residues (Haberhauer and Gerzabek, 1999).

  The extraction interleaved the two columns of the page. The same paper, on page 6, contrasts this band with the clay OH stretching band at 3600 cm-1.

  Chemical Differences in Soil Organic Matter Fractions Determined by Diffuse-Reflectance Mid-Infrared Spectroscopy DOI: 10.2136/sssaj2009.0375 · p. 3
• Published spectrum

  The spectrum of gelatin film displayed relevant peaks ascribed to amide A stretching (N-H and O-H stretching around 3300 cm-1) coupled with C=O stretching at 1653 cm-1 (amide I), N-H bending vibration of groups and stretching vibrations of C-N groups at 1546 cm-1 (amide II)

  The two columns of the page were interleaved by the extraction; the sentence is given in reading order and truncated after the amide II band.

  Spectroscopic analyses of the influence of electron beam irradiation doses on mechanical, transport properties and microstructure of chitosan-fish gelatin blend films DOI: 10.1016/j.foodhyd.2014.09.038 · p. 10
• Published spectrum

  the peak centered at 1645 cm-1 can be assigned to the H-O-H-bending contribution

  The same page assigns the broad contribution between 3500 and 3000 cm-1 to the vibrations of carbohydrate hydroxyl groups. The extraction interleaved two copies of the column, so only the fragment that carries the assignment is quoted.

  Hydrating Capabilities of the Biopolymers Produced by the Marine Thermophilic Bacillus horneckiae SBP3 as Evaluated by ATR-FTIR Spectroscopy DOI: 10.3390/ma15175988 · p. 6
• Published spectrum

  the concomitant formation and increase of Amadori product as indicated by the appearance of carboxylic acid and aldehyde peaks at 1730 and 1720 cm-1, respectively

  The extraction renders the unit as '(cid:2)' and drops spaces inside words; both are restored here and the wording is unchanged. The paper writes the unit once, after 1720, so the quotation keeps it there.

  FTIR monitoring of oxazolidin-5-one formation and decomposition in a glycolaldehyde-phenylalanine model system by isotope labeling techniques DOI: 10.1016/j.carres.2008.10.011 · p. 7

Expert reading

The 3400 cm-1 band is where assignments are most easily over-committed, because almost every organic sample and every slightly damp powder has one. Three habits keep it honest. First, read the band as a region rather than a number: the O-H stretch of alcohols and related groups is reported at 3300, 3400, 3430, 3435 and 3500 cm-1 in the literature quoted here, so a measured 3400 cm-1 is one of several that mean the same thing. Second, always look for the partner band: a 3400 cm-1 band with no water bending at 1630-1650 cm-1, no amide II at 1545 cm-1 and no C-H at 2850-2960 cm-1 is not enough to name a group. Third, let the drying test decide between material and moisture, since adsorbed water alone produces a broad band centred at 3400 cm-1. The bands listed above link to the peak pages for the neighboring wavenumbers, where the other assignments reported at the same position are listed.