Principle of IR Spectroscopy: IR spectroscopy works by measuring the absorption of infrared radiation by a molecule. Molecules absorb IR radiation at specific frequencies that correspond to the vibrational modes of their chemical bonds. A vibration occurs when atoms are displaced from their equilibrium positions. The frequency of vibration depends on the masses of the atoms and the strength of the bond. Different functional groups exhibit characteristic vibrational frequencies, resulting in unique IR spectra. The absorption of IR radiation causes the bonds to stretch, bend, or twist. The energy absorbed corresponds to the vibrational energy levels of the molecule.

Interpretation of the 1720 cm^-1 band: A strong absorption band at 1720 cm^-1 indicates the presence of a carbonyl group (C=O) directly attached to an alkyl or aryl group (R-C=O). This is characteristic of a ketone or an aldehyde. The C=O bond stretches during vibration, and this stretching vibration occurs at a specific frequency determined by the mass of the carbon and oxygen atoms and the strength of the bond. The exact frequency is influenced by the surrounding groups; ketones typically show a stronger absorption than aldehydes due to different electronic effects.

Diagram of Vibrational Modes:

The carbonyl group exhibits several vibrational modes:

• Symmetric Stretch (ν_asym): The two C=O bonds stretch symmetrically, resulting in a vibration where the atoms move in the same plane. This typically appears as a weaker peak.
• Asymmetric Stretch (ν_sym): The two C=O bonds stretch asymmetrically, resulting in a vibration where the atoms move in opposite planes. This typically appears as a stronger peak.
• Bending (ν_bending): The oxygen atom bends out of the plane of the C=O bond. This typically appears as a weaker peak.

The position of the carbonyl stretching frequency is influenced by the substituents attached to the carbonyl group. For example, aldehydes tend to have a higher frequency (around 1700 cm^-1) than ketones due to the greater electron-donating ability of the alkyl groups in aldehydes.

Analysis of the Data:

• Molecular Ion (m/z = 200): This indicates a molecular weight of 200 amu.
• m/z = 100 (Fragment): A loss of 100 amu from the molecular ion.
• m/z = 60 (Fragment): A loss of 40 amu from the molecular ion (200 - 60 = 140 amu loss).
• m/z = 44 (Fragment): A loss of 106 amu from the molecular ion (200 - 44 = 156 amu loss).

Possible Structure and Reasoning:

The pattern of fragmentation suggests the presence of a relatively stable fragment that is 40 amu smaller than the molecular ion, and another fragment that is 100 amu smaller. A possible structure is a compound containing a benzene ring with a side chain. The 100 amu loss could represent the loss of a benzene ring, and the 40 amu loss could represent the loss of a -CH2- group. The 106 amu loss could represent the loss of a -C(CH3)2 group. Therefore, a plausible structure is 2-methyl-6-ethylbenzene (also known as *o*-xylene).

Structure Diagram:

[A diagram of 2-methyl-6-ethylbenzene would be included here. This is not possible to render in plain text. The diagram would show a benzene ring with a methyl group at position 2 and an ethyl group at position 6.]

Further Considerations: To confirm this structure, one would need to consider the possible fragmentation pathways and compare them with the experimentally observed spectrum. Other structures could also produce similar fragmentation patterns, so further analysis (e.g., NMR spectroscopy) would be required for definitive identification.

Chemical Shift and Electronic Environment: The chemical shift (δ) in ¹H NMR is a measure of the resonance frequency of a proton relative to a standard (TMS, 0 ppm). It's affected by the electron density surrounding the proton. Electronegative atoms or groups near a proton deshield the proton, causing it to resonate at a higher chemical shift (downfield). Conversely, electron-donating groups shield the proton, causing it to resonate at a lower chemical shift (upfield).

2-Methylbutane Analysis:

• Methyl Protons (0.9 ppm): The methyl protons are attached to a carbon atom that is directly bonded to a hydrogen. This carbon is only slightly affected by the adjacent methyl group. The methyl group itself has a small deshielding effect, but the overall effect is relatively small, resulting in a chemical shift of approximately 0.9 ppm. The methyl protons are in a relatively non-polar environment.
• Methine Protons (1.5 ppm): The methine protons are attached to a carbon atom that is bonded to both a hydrogen and a methyl group. The presence of the methyl group causes the methine proton to be more deshielded than a simple alkane proton. The electron density from the methyl group is transmitted through the sigma bond to the methine proton, leading to a chemical shift of around 1.5 ppm.
• Methylene Protons (2.5 ppm): The methylene protons are attached to a carbon atom that is bonded to two methyl groups and a hydrogen. The two methyl groups are electron-donating, increasing the electron density around the methylene carbon. This increased electron density deshields the methylene protons significantly, resulting in a chemical shift of approximately 2.5 ppm. The methylene protons are in a more electron-rich environment compared to the other protons.