Learning Objectives Understand where certain group frequencies are Be able to read and understand the data table for IR spectroscopy As stated in the previous section, similar bonds will show up grouped near the same frequency because they tell us about the presence or absence of specific functional groups in a sample. From there, a data table of approximate frequencies for different types of bonds has been created to use to help IR spectrum analysis. Table of Common IR Absorptions. 0 1 2 3 Approximate Frequency (cm-1) Description Bond Vibration Notes 3500 - 3200 broad, round O-H much broader, lower frequency (3200-2500) if next to C=O 3400-3300 weak, triangular N-H stronger if next to C=O 3300 medium-strong =C-H (sp C-H) NaN 3100-3000 weak-medium =C-H (sp2 C-H) can get bigger if lots of bonds present 3000-2900 weak-medium -C-H (sp3 C-H) can get bigger if lots of bonds present 2800 and 2700 medium C-H in O=C-H two peaks; "alligator jaws" 2250 medium C≡N NaN 2250-2100 weak-medium C≡C stronger if near electronegative atoms 1800-1600 strong C=O lower frequency (1650-1550) if attached to O or N middle frequency if attached to C, H higher frequency (1800) if attached to Cl 1650-1450 weak-medium C=C lower frequency (1600-1450) if conjugated (i.e. C=C-C=C) often several if benzene present 1450 weak-medium H-C-H bend NaN 1300 - 1000 medium-strong C-O higher frequency (1200-1300) if conjugated (i.e. O=C-O or C=C-O) 1250-1000 medium C-N NaN 1000-650 strong C=C-H bend often several if benzene present Note: strong, medium, weak refers to the length of the peak (in the y axis direction). Note: spectra taken by ATR method (used at CSB/SJU) have weaker peaks between 4000-2500 cm -1 compared to reference spectra taken by transmittance methods (typical on SDBS and other sites). Exercise \(\PageIndex{1}\) What wavenumber range would you predict the triple bond region to be? Answer 2200 - 2500 cm -1 Exercise \(\PageIndex{2}\) How could you determine the presence of an aldehyde and rule out a ketone? Answer The aldehyde would have peaks at 2700 and 2800 cm-1, whereas the ketone would lack these. The absorption is due to the the Csp 2 -H bond. Exercise \(\PageIndex{3}\) How can you tell the difference between and alcohol and a carboxylic acid? Answer Here the main point of difference is either the presence of a carbonyl (C=O) or its absence. The carboxylic acid contains a C=O, so you would expect a peak somewhere between 1800-1600 cm -1 , whereas an alcohol would not have a peak here.