Are IR signals affected by nearby atoms?

E.g. If there is a methyl from an aldehyde vs a methyl at the end of a hydrocarbon chain, will the difference in neighboring atoms have an effect on the location of the IR signal?




As was [already said](https://chemistry.stackexchange.com/questions/60279/are-ir-signals-affected-by-nearby-atoms#comment102088_60279),



> 
> IR works on bond lengths. So if the difference is negligible, instrument noise is going to dwarf anything useful you'll pull out.
> 
> 
> 


There *will* be a difference, and you can definitely detect it computationally, but by normal experimental means (benchtop FT-IR or ATR, not a gas-phase experiment) background noise might the same order of magnitude as any frequency shift you'd see.


For example, the carbonyl stretching frequency and carbonyl bond lengths calculated using B97-D/6-31G(d):


$$
\newcommand{\AA}{\unicode{x212B}}
\begin{array}{llll}
 & \text{bond length} \, (\AA) & \text{frequency, calc.} \, (\text{cm}^{-1}) & \text{frequency, expt.} \, (\text{cm}^{-1}) \\ \hline
\text{pentanal} & 1.218 & 1772 & 1727 \\
\text{2-hexanone} & 1.223 & 1753 & 1718
\end{array}
$$


Technically, you can distinguish between these in an experiment, but one would be more interested in the presence/absence of a large aldehyde band. This works due to the sharpness of the carbonyl stretch.


Closer to what I think you're asking about is the influence on vibrational frequencies of groups *far* from the area of substitution. These particular examples are tough because the normal modes of hydrocarbon-like species tend to couple the motion of many $\ce{C-H}$ stretches together. With that in mind, here are the highest-frequency $\ce{C-H}$ stretching modes, which happen to belong to the terminal $\ce{CH3}$ groups. The bond length is for the hydrogen eclipsed with the carbonyl.


$$
\newcommand{\AA}{\unicode{x212B}}
\begin{array}{lll}
 & \text{bond length} \, (\AA) & \text{frequency, calc.} \, (\text{cm}^{-1}) \\ \hline
\text{pentanal} & 1.102 & 3044 \\
\text{2-hexanone} & 1.102 & 3038
\end{array}
$$


"Wow, that's amazing, you *can* resolve it!" Not this time. Compare the full spectra for pentanal/valeraldehyde (top) and 2-hexanone (bottom) and try to perform the peak assignment. Is it it the 2960/2962 pair? 2935/2936?


![](https://i.stack.imgur.com/38Qun.gif)
![](https://i.stack.imgur.com/flf2u.gif)


Experimental values/images taken from the [Spectral Database for Organic Compounds](http://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng).


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