CHEM 1810 - Lecture 20

Lecture 20 quantized v = 0, 1, 2, 3, 4... Δv = 1, ΔE = hν the frequency of the vibration must match that of the light for it to be absorbed 40000 30000 20000 10000 0 E_v = hν(v + 1/2) allowed E in vibration a molecule must have a change in dipole moment to couple with the electric field (in sync) 1388 cm⁻¹ but peak ~ same but turned 90° peak ~700 cm⁻¹ not observed in abs. spectrum (no dipole mom.) no photon abs. energy can be transferred into w/ photon (IR) peak at 2349 cm⁻¹ hydrocarbon C-H stretches (ν ~ s-char.) alkanes: sp³, 25% s-char. C-H stretch 3000-2850 cm⁻¹ 101.1 ± 0.4 BDE alkenes: sp², 33% s-char. =C-H stretch 3100-3000 cm⁻¹ 107.1 ± 0.6 BDE alkynes: sp, 50% s-char. ≡C-H stretch 3330-3270 cm⁻¹ 133.32 ± 0.01 BDE IR silent vibrational: does not absorb IR light XPS: send in a photon, eject e-, find KE of e-; know E of photon put in, difference = work function (cost to eject e-) KE of ejected e- depends on vibrational E molecules in orbital E (ΔE = hν conversion) e- KE = E difference in vibrations Raman spectroscopy: light scatters to a higher/lower vibrational state upon interacting with a molecule. Stokes raman anti-stokes raman Rayleigh scattering scatter laser diff btw scattered and input light frequency = difference in vibration use of visible light seen even w/o dipole moment Rotational Spectroscopy quantized E that can be stored in each DOF unequal diff btw allowed E levels in rotations absorb microwaves to change E in rotations knowing ν of abs. light -> moment of inertia -> mass & bond length/angle Nuclear Magnetic Resonance Spectroscopy uses radiofrequency (lowest) photon protons have a nuclear spin (I = ±1/2); interact with mag. field E put in to move from stable, aligned state to less stable, opposed state Certainly, here's the analysis of the image you provided: Textual Content: separation btw aligned and opposed states = E of photon E cost to flip ↑ spin separation → ↑E required to flip larger mag field ↑ larger separation → ↑E required to flip 1H 400 MHz 300 MHz 14,092 7,046 applied mag. field (Bo) effective mag. field in molecule α1, α2, α3 abs. of radio frequency (RF) radiation results in flipping the nuclear spin from aligned → opposed mag. shielded mag. deshielded down field up field homotopic Hs absorb at same ν enantiotopic Hs absorb at diff. ν mirror-image environments slightly diff. environments heterotopic (no relationship) Hs abs. at diff. ν CH3-CH2Br: chemically equivalent (homo) enant: chemically equivalent closer to Br (more EN atom) homo (free to rotate on single bond): chemically equivalent diast: chemically equivalent but diff than (hetero) closer to 4 other groups diast Ha Hb Hc Hd He Ha Hb Hc Hd He 1H NMR spectrum has 3 signals Ha - Hb - Hc - Hd - He