Spectroscopy MCAT

IR peak for O-H stretch

~3200–3600 cm⁻¹ (broad)

IR peak for N-H stretch

~3300 cm⁻¹ (sharp)

IR peak for sp³ C-H stretch

~2800–3000 cm⁻¹

IR peak for C=O stretch

~1700 cm⁻¹ (sharp)

IR peak for C=C stretch

~1600 cm⁻¹

More conjugation in UV-Vis spectroscopy leads to what?

Lower energy transition and longer λmax

Typical λmax range for UV-Vis spectroscopy

200–800 nm

What does the base peak in mass spectrometry represent?

The most abundant fragment ion (tallest peak)

What does the molecular ion peak (M⁺) tell you in MS?

Molecular weight of the compound

Isotopic pattern for chlorine in MS

3:1 ratio for M:M+2 peaks

Isotopic pattern for bromine in MS

1:1 ratio for M:M+2 peaks

What does chemical shift in NMR reflect?

Electronic environment of the nucleus

1H-NMR: Chemical shift of alkane protons (~CH₃)

~0.9–1.5 ppm

1H-NMR: Chemical shift of alcohol O-H proton

~1–5 ppm (broad)

1H-NMR: Chemical shift of alkene protons (=CH)

~4.5–6.5 ppm

1H-NMR: Chemical shift of aromatic protons

~6.5–8 ppm

1H-NMR: Chemical shift of aldehyde proton

~9–10 ppm

1H-NMR: Chemical shift of carboxylic acid proton

~10–12 ppm (broad)

What does integration in 1H-NMR represent?

Relative number of protons contributing to the signal

What is the (n+1) rule in NMR splitting?

n = number of adjacent hydrogens; signal splits into n+1 peaks

13C-NMR: Chemical shift range for sp³ carbon

0–50 ppm

13C-NMR: Chemical shift range for sp² carbon (alkene/aromatic)

100–160 ppm

13C-NMR: Chemical shift for carbonyl carbon

160–220 ppm

Which spectroscopy technique detects vibrational transitions?

Infrared (IR) spectroscopy

Which spectroscopy technique detects electronic transitions?

UV-Vis spectroscopy

Which spectroscopy technique detects nuclear spin states?

NMR spectroscopy

Which spectrometry technique gives molecular weight and fragmentation info?

Mass spectrometry

What causes downfield shift in NMR?

Electron-withdrawing groups (deshielding)

What causes upfield shift in NMR?

Electron-donating groups (shielding)

Effect of conjugation on UV-Vis λmax

Increases λmax (longer wavelength)

Which functional group shows a broad IR peak around 3300 cm⁻¹?

Alcohol (O-H) or Amine (N-H)