FTIR Functional Group Frequency Table

One intense band in the carbonyl window. The assignment changes the whole reading of the sample, and this is the decision most often got wrong.

Published spectra place the carbonyl window by position relative to its neighbors, not by the peak on its own. Esters sit at the high end of the window, and conjugation pushes a carbonyl down by about 20 cm-1. In esterified wood the band at 1730 cm-1 was assigned to the ester C=O stretch; in plant tissue the ester carbonyl of pectin appears at 1740 cm-1 while the amide I of protein sits at 1650 cm-1; in soybean cotyledon the band at 1740 cm-1 is the ester of the lipids. The same window therefore carries at least three different structures.

• Published spectrum p. 9

  The pure SCA polymer spectra contains a large peak at 1730 cm-1, assigned to C=O stretch in ester groups [43-45].

  The two-column extraction repeated the word "contains" and moved the superscript unit onto its own line; a single copy is given here and the unit is rejoined.

  Leachability and Decay Resistance of Wood Polyesterified with Sorbitol and Citric Acid DOI: 10.3390/f11060650 · p. 9
• Published spectrum p. 5

  The (C=O) band at 1740 cm-1 is characteristic of ester bonds (stretch), arising from lipids. In the region between 1700 and 1500 cm-1 the amide I band at 1650 (protein, stretch) and the amide II band at 1545 (protein, bending) can be observed.

  The extraction encoded the bonds as "CdO" and "NsH" and split the unit "cm-1" onto its own line; the bonds and the unit are restored here and the wording is otherwise unchanged.

  Microchemical Structure of Soybean Seeds Revealed in Situ by Ultraspatially Resolved Synchrotron Fourier Transformed Infrared Microspectroscopy DOI: 10.1021/jf050608x · p. 5
• Published spectrum p. 3

  Ketones, aldehydes, acids, amides and carbonates all have absorption peaks around 1700 cm-1. Esters, chlorides and acid aldehydes tend to absorb at slightly higher wavelengths; that is 1770-1725 cm-1. Conjugation tends to lower the absorption peaks by about 20 cm-1.

  From the results and discussion of a quality-control study; the two columns were interleaved by the extraction and are given here in reading order.

  Fourier transform infrared analysis of Tamra Bhasma at different levels: A preliminary study DOI: 10.4103/0974-8520.169013 · p. 3
• Published spectrum p. 3

  Table 1. The assignment of main functional groups based on Fourier transform infrared (FTIR) spectra of susceptible (S) and resistant (R) root samples. Rows: 3410 -OH groups, NH stretching — Proteins (Amide A); 2927 C-H stretch (asym.) of CH2 — Lipid acyl chains; 1740 C=O stretching: carbonyl ester compounds — Pectin; 1650 -C=O- and -C=N- stretching — Proteins (Amide I); 1548 N-H bending and C-N stretching — Proteins (Amide II).

  Quoted from Table 1 on the same page. The extraction repeats the caption and every row in two layers and prints the three columns (wavenumber, assignment, component) side by side; the rows are serialised here as wavenumber, assignment, component, with the unit "cm-1" restored. No wording is changed.

  Evaluating Changes in Cell-Wall Components Associated with Clubroot Resistance Using Fourier Transform Infrared Spectroscopy and RT-PCR DOI: 10.3390/ijms18102058 · p. 3

What to conclude: Treat 1730 cm-1 as a starting point rather than an answer. Confirm an ester with the C-O bands at 1000-1300 cm-1, an amide with the N-H band near 3300 cm-1 together with the amide II band near 1545 cm-1, and a free carboxylic acid with the very broad O-H band running from 2500 to 3300 cm-1. If none of those partners is present, a band in this window is a ketone, an aldehyde or an amide that has shifted, and the fingerprint region has to decide.

The 3700-3000 cm-1 window carries X-H stretching from four different groups, and they overlap. A single band here is the weakest single piece of evidence on the whole spectrum.

A broad band near 3400 cm-1 was assigned to the O-H stretching vibration of hydroxyl groups in lignin, and in the same region a band at 3409 cm-1 was reported as characteristic of hydrogen-bonded N-H groups or free water, with an explicit warning that these absorptions overlap and cannot be resolved unambiguously. The same 3400-3500 cm-1 window was used for the N-H stretching of primary amines. In soybean cotyledon the broad band at 3290 cm-1 was assigned to O-H stretching from proteins and carbohydrates, while a narrow band or shoulder at 3050 cm-1 was assigned to N-H. The same paper states the general case plainly: the peaks between 3700 and 2700 cm-1 are mostly multicompound in origin and cannot be assigned to one chemical type.

• Published spectrum p. 5

  For unmodified lignin, the existence of a broad band near 3400 cm-1 corresponds to the O-H stretching vibration of hydroxyl groups [23].

  The extraction split the unit "cm-1" onto its own line; it is rejoined here and the wording is unchanged.

  Effect of Lignin Modification on Properties of Kenaf Core Fiber Reinforced Poly(Butylene Succinate) Biocomposites DOI: 10.3390/ma12244043 · p. 5
• Published spectrum p. 7

  The absorption band at 3409 cm-1 is characteristic of hydrogen-bonded -NH- groups or free H2O molecule. Unfortunately, the absorption bands at 3409 cm-1 overlap strongly and cannot be unambiguously resolved into their components.

  The two-column extraction interleaved the sentences and split the superscript unit onto its own line; one copy is given here with the unit rejoined.

  Preparation of Bio-Based Polyamide Elastomer by Using Green Plasticizers DOI: 10.3390/polym8070257 · p. 7
• Published spectrum p. 5

  The broad band observed at 3290 cm-1 corresponds to O-H stretching vibrations, arising mainly from proteins and carbohydrates. In some cases (tissue dependent) a narrow band or shoulder was observed at 3050 cm-1 and is attributed to N-H proteins (amide B, bending, first overtone).

  The extraction split the unit "cm-1" and encoded the bond as "NsH"; both are restored here and the wording is otherwise unchanged.

  Microchemical Structure of Soybean Seeds Revealed in Situ by Ultraspatially Resolved Synchrotron Fourier Transformed Infrared Microspectroscopy DOI: 10.1021/jf050608x · p. 5
• Published spectrum p. 5

  The spectra obtained show differences in absorption intensity across the whole mid-IR range. However, the peaks in the region 3700-2700 cm-1 are mostly multicompound in origin and cannot be assigned to one chemical type, as can the peaks in the range 1800-1000 cm-1.

  This is the reason the table below is organised by window rather than by single band.

  Microchemical Structure of Soybean Seeds Revealed in Situ by Ultraspatially Resolved Synchrotron Fourier Transformed Infrared Microspectroscopy DOI: 10.1021/jf050608x · p. 5
• Published spectrum p. 3

  All samples gave peaks in the range 3400-3500 cm-1 that are obtained from N-H stretching vibrations and can be assigned to primary amines.

  The same window that carries the O-H band in another sample; the two columns were interleaved by the extraction and are given in reading order.

  Fourier transform infrared analysis of Tamra Bhasma at different levels: A preliminary study DOI: 10.4103/0974-8520.169013 · p. 3

What to conclude: Look at the width and the shape. A wide, rounded band is the hydrogen-bonded O-H of alcohols, phenols and water. A sharper band with a defined maximum, often a shoulder on a broader envelope, points to N-H. When both are present the window is unusable on its own, and the carbonyl next to it decides the question: 1650 cm-1 means amide, 1740 cm-1 means ester.

The fingerprint region is where the material is identified, and it is also the region where published assignments differ most between samples.

The C-O bands are the natural way to confirm a carbonyl assignment, and the fingerprint region is where material identity is settled. In soybean cotyledon the hemicellulosic polysaccharides appear at about 1370 and 1320 cm-1, and the bands at 1400 and 1246 cm-1 are attributed to cellulose. In plant roots the same window resolves further: 1246 cm-1 for hemicellulose with P-O stretching, 1151 cm-1 for C-O-C asymmetric stretching in cellulose, and 1061 cm-1 for C-O-C symmetric stretching. One paper puts the C-O-C stretch of ethers and esters at about 1200 cm-1, which is the band that has to appear alongside a 1730 cm-1 ester carbonyl.

• Published spectrum p. 5

  Although small and difficult to distinguish in Figure 2, hemicellulosic polysaccharides are characteristically seen at about 1370 and 1320 cm-1; bands at 1400 and 1246 cm-1 can be attributed to cellulose.

  The unit "cm-1" was separated by the extraction and is rejoined here; the wording is unchanged.

  Microchemical Structure of Soybean Seeds Revealed in Situ by Ultraspatially Resolved Synchrotron Fourier Transformed Infrared Microspectroscopy DOI: 10.1021/jf050608x · p. 5
• Published spectrum p. 3

  Table 1. The assignment of main functional groups based on Fourier transform infrared (FTIR) spectra of susceptible (S) and resistant (R) root samples. Rows: 1518 CH2 and CH3 methylene chain stretching — Lignin; 1246 C-O, -CH2- stretching and bending, P-O stretching — Hemicellulose; 1151 C-O-C asymmetric stretching, PO2 stretching — Cellulose; 1061 C-O-C symmetric stretching — Cellulose.

  Quoted from Table 1 on the same page. The extraction repeats the caption and every row in two layers and prints the three columns (wavenumber, assignment, component) side by side; the rows are serialised here as wavenumber, assignment, component, with the unit "cm-1" restored. No wording is changed.

  Evaluating Changes in Cell-Wall Components Associated with Clubroot Resistance Using Fourier Transform Infrared Spectroscopy and RT-PCR DOI: 10.3390/ijms18102058 · p. 3
• Published spectrum p. 3

  The C-O-C stretching vibration in ethers and esters are found at about 1200 cm-1 and the C-Cl stretching vibration at 700-800 cm-1. A number of inorganic groups such as sulfate, phosphate, nitrate and carbonate also absorb at wave numbers below 1200 cm-1.

  The two columns were interleaved by the extraction and are given here in reading order, with the unit restored.

  Fourier transform infrared analysis of Tamra Bhasma at different levels: A preliminary study DOI: 10.4103/0974-8520.169013 · p. 3

What to conclude: Read the fingerprint region as a set, not as a series of isolated absorptions. A carbonyl at 1730 cm-1 wants C-O around 1200 cm-1; a protein amide I at 1650 cm-1 wants the amide II band at about 1545 cm-1 and an N-H band above 3000 cm-1; cellulose wants 1151 and 1061 cm-1 as well as 1400 and 1246 cm-1. The bands below 1200 cm-1 are also where sulfate, phosphate, nitrate and carbonate absorb, so a filled or mineral-containing sample needs that checked before the organic assignment is trusted.