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I am currently studying the textbook Infrared and Raman Spectroscopy by Larkin. In the first chapter of the textbook, the author opens (probably prematurely) with the following statement:

Near-IR spectroscopy measures the broad overtone and combination bands of some of the fundamental vibrations (only the higher frequency modes) and is an excellent technique for rapid, accurate quantitation.

This will likely be covered in later chapters, but the statement piqued my curiosity: For someone who has not yet encountered concepts such as "broad overtone", "combination bands", and "higher frequency modes", what does this all mean? Essentially, I am seeking a brief, but clearer and more elaborate, explanation of what the author has described here.

I would greatly appreciate it if someone who has knowledge of this area would please take the time to clarify.

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Indeed that sentence should be taken as merely anticipating what the book will treat in following chapters.

Nevertheless, to satisfy your impelling curiosity the statement trace a combination of facts that leads to the actual spectrum of a given molecule.

Overtones IR absorption bands are those resulting from breaking of selection rules. Instead of having just transitions between adjacent vibrational levels, basically the 1<-- 0 one, transitions with a delta nu of 2 are observed as well. In other words the vibrations aren't those of an harmonic oscillator but those described by a Morse's potential.

Combination modes arise from the combination of normal modes, ie they happen when different vibrations are simultaneously excited. Normal modes which are unseen can contribute to these combination modes, as the coupling alter the symmetry.

High frequency: we are speaking about NIR, which is the high frequency range for molecular motions, and it is where vibrational overtones typically occur. The lower frequency part of the spectrum is dominated by the fundamental modes, and except for its fingerprints part and fine details, it has a strong bond related character rather than a molecular one.

If this does not help but instead brings out new terms or doubts, I would suggest that you treat the introductory part of the book for what it is, namely an introduction.

Alternatively you can refer to this link which provides a condensed explanation and includes schemes, further links, and definitions:

http://bit.ly/31Mc6lT

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  • $\begingroup$ Any resonant frequency above the fundamental frequency is referred to as an overtone. In the IR spectrum, overtone bands are multiples of the fundamental absorption frequency. chem.libretexts.org/Bookshelves/… $\endgroup$ – The Pointer Feb 12 '20 at 17:54
  • $\begingroup$ In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. en.wikipedia.org/wiki/Selection_rule $\endgroup$ – The Pointer Feb 12 '20 at 17:57
  • $\begingroup$ What is the "lower frequency part of the spectrum" that you are referring to? Everything at wavelengths above near-infrared? $\endgroup$ – The Pointer Feb 12 '20 at 18:02
  • $\begingroup$ Yes somehow the authors reiterates the the term NIR. $\endgroup$ – Alchimista Feb 13 '20 at 8:03

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