27 votes

Why does the weaker C–H bond have a higher wavenumber than the C=O bond?

A property of the harmonic oscillator is that the oscillation frequency, $\omega$, is dependent not only on $k$ (the spring constant) but also on the mass $m$ of the object: $$\omega = \sqrt{\frac{k}{...
orthocresol's user avatar
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18 votes
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Why does the C–C bond have extremely weak absorptions?

For vibrational spectra the primary transition under investigation is the $v = 1 \leftarrow 0$ excitation (because $\hbar\omega >> k_\mathrm{B}T$, so excited states have negligible thermal ...
orthocresol's user avatar
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17 votes

Why does ethylamine have two infra-red spectra? Are both correct?

When looking at IR spectra of hydrogen-bonding groups, always check how IR spectra was recorded. It appears that NIST spectra is recorded using gas phase, while the first one used liquid film. In ...
permeakra's user avatar
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12 votes
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Why does the carbonyl group in an acid anhydride have two stretching frequencies?

The two observed C=O frequencies are due to the symmetric and asymmetric stretching modes of the anhydride. Source: Introduction to Spectroscopy, Pavia and Lampman You can see that the lower ...
NotEvans.'s user avatar
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11 votes

Why are some molecules unable to absorb infrared radiation?

In basic terms, for a molecule to absorb radiation there has to be an oscillating dipole being produced. This can occur by nuclear motion (vibrations, rotations) or electronic motion to produce ...
porphyrin's user avatar
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Harmonic frequencies (IR) and basis set size

No, you cannot say that larger energies will necessarily give larger frequencies. Consider, for instance, a hypothetical (and purely fictional) method of determining the electronic energy of a system ...
jheindel's user avatar
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11 votes

Is it "common practice in Fourier transform spectroscopy to multiply the measured interferogram by an apodizing function"? If so, why?

You usually post interesting questions, which appear to be deceptively simple yet they are very challenging. Modern day data acquisition and signal processing is so complicated that it is almost like ...
AChem's user avatar
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Software to draw one-dimensional PES including vibrational energy levels

I have made this basic python 3 script which runs in a Jupyter notebook (or in Visual Studio Code). With different molecules you will need to adjust the plot scale. The first 10 levels are plotted ...
porphyrin's user avatar
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10 votes

Why do molecules have to have a change in dipole moment in order for them to be IR active?

There is an intuitive reason why a vibration is IR-active only if it involves a change in the dipole moment. Recall that the typical wavelength of IR radiation ($\sim 10\mu\mathrm{m}$) is much larger ...
higgsss's user avatar
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Dominant contribution of v = 0 → v = 1 transition in vibrational spectroscopy

The reference should really be to the Boltzmann distribution where the population is proportional to $\exp(-E_n/(k_\mathrm BT))$ where $E_n$ is the energy of the $n^{th}$ energy level. This shows ...
porphyrin's user avatar
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10 votes

Why does the weaker C–H bond have a higher wavenumber than the C=O bond?

According to Skoog, Analytical Chemistry: Using classical mechanics, assuming a diatomic molecule, the frequency of vibration $\nu$ may be described by $$\nu = \frac{1}{2\pi} \sqrt{\frac{k}{\mu}} $$ ...
Buttonwood's user avatar
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10 votes

How do I figure out how many hydrogens my compound actually has using a mass and NMR spectrum?

Being an NMR fan myself I would inspect that NMR spectrum: The integrals suggest you have 11 $\ce{^1H}$ or a multiple thereof (the number under each peak is the normalized integral, which is ...
Buck Thorn's user avatar
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How do I figure out how many hydrogens my compound actually has using a mass and NMR spectrum?

I would probably also use the method Buck has suggested, but let’s say the NMR broke down or somebody is measuring a $\ce{^13C}$ of $\pu{2.5mg}$ meaning it will be blocked until tomorrow; in this case,...
Jan's user avatar
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What exactly is the long chain band?

The 'long chain band' described in your textbook (Pavia) is actually a vibrational mode in which the terminal methyl groups are 'rocking'. This rocking mode is observed in open chain alkanes, -(CH2)...
NotEvans.'s user avatar
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9 votes

Is it "common practice in Fourier transform spectroscopy to multiply the measured interferogram by an apodizing function"? If so, why?

If there is a noisy signal that decays away, such as the FID in an NMR experiment, the signal to noise ratio is larger at shorter times than at longer ones where the noise remains but the signal is ...
porphyrin's user avatar
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Aromatic Overtones

In general there are 'fundamental' lines at frequency $v_1, v_2$ etc. Weak overtone transitions can occur for any fundamental vibration at frequency $nv_1$, $nv_2$ etc. where $n=2, 3,..$ and so on for ...
porphyrin's user avatar
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If vibrational modes are quantised, why is infrared spectroscopy smooth looking?

You should first have a look at the gas phase spectrum of water to convince yourself that vibrational levels are indeed "discrete". The textbook treatment of vibrating molecules assumes that there is ...
AChem's user avatar
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8 votes

Neither Raman nor IR Active vibrational modes

Yes it is possible. While no normal modes can be both IR and Raman active in molecules with a centre of symmetry, as mentioned in a comment molecules with silent modes exist. The easiest example is ...
Alchimista's user avatar
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8 votes

Why are there two infrared vibrational frequencies for the terminal CO ligands of Fe2(CO)9?

Despite the title of the publication, Fletcher et al. both present an IR spectrum of diiron nonacarbonyl in a matrix of ($\ce{Ar}$ + 10% $\ce{CO}$, recorded at $\pu{15 K}$) in the $\nu(\ce{C-O})$ ...
Buttonwood's user avatar
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7 votes

Why does the C–C bond have extremely weak absorptions?

If we assume the simplyfied description, i.e. atoms being tethered together by springs, the spring constant allows describe where (energy wise, along the scale of wavenumbers, the abscissa in the ...
Buttonwood's user avatar
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Raman transition and IR transition

To simplify slightly both Raman and IR spectroscopy show the vibrational modes of a molecule (though the techniques used to reveal these are very different). IR spectroscopy relies on coupling ...
matt_black's user avatar
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7 votes

How do I figure out how many hydrogens my compound actually has using a mass and NMR spectrum?

I like both answers provided before me where one has used exclusive use of internet to suggest structure by NMR spectrum, and the other has used thorough analysis of mass spectrum. Although these two ...
Mathew Mahindaratne's user avatar
7 votes

Why are there two infrared vibrational frequencies for the terminal CO ligands of Fe2(CO)9?

It basically comes down to molecular symmetry and since Buttonwood provided an answer just seconds ago, I'll leave the following as visualisations for it. An optimisation of $\ce{Fe2(CO)9}$ at the DF-...
Martin - マーチン's user avatar
7 votes

Why does the FTIR-ATR spectrum for hexamethylenediammonium adipate not have a peak for the N-H bond?

Your anticipation of absorption bands of $\ce{-NH2}$ close $3300\ldots \pu{3500 cm^-1}$ is about free and neutral amine. Your intended product is about an (organic) ammonium, $\ce{C-NH^+_3}$. Both ...
Buttonwood's user avatar
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6 votes

Why are some molecules unable to absorb infrared radiation?

Infrared radiation is absorbed by a molecule when the frequency of the radiation matches that of one of the vibrational modes of the molecule. It is also necessary that the molecule have a vibrational ...
airhuff's user avatar
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6 votes

Why does the carbonyl group in an acid anhydride have two stretching frequencies?

Just to add on to NotEvans's answer: The stretching frequencies of compounds can theoretically be predicted by group theory and symmetry considerations - some introduction can be found here. In ...
orthocresol's user avatar
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What do s, m, w, m-w, and w-s mean in the context of infrared spectroscopy?

Given the general agreement surrounding my comment on the question and the similar abbreviations and descriptions in the chart on the page linked by Tyberius in his comment, the meanings appear to be: ...
hBy2Py's user avatar
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6 votes

Is it "common practice in Fourier transform spectroscopy to multiply the measured interferogram by an apodizing function"? If so, why?

Hamming apodization function is also known as a Hamming window. If you have data that looks like this: ...
Yakk's user avatar
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6 votes

Why does ethylamine have two infra-red spectra? Are both correct?

Honestly I have doubt about your textbook version of IR spectrum. However, I can suggest by the experience I have gained that the shapes of the peaks depends on the method you have used to obtain the ...
Mathew Mahindaratne's user avatar
6 votes

Why the count of the IR band of H2O and CO2 is different?

Molecules vibrate. Some of the vibrations alter the polarizability of the electron cloud of a chemical bond, which renders them suitable for a characterization by RAMAN spectroscopy. Some of the ...
Buttonwood's user avatar
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