Chemistry 213 Practical Spectroscopy

Transcription

1 Chemistry 213 Practical Spectroscopy Dave Berg Elliott 314 A course in determining structure by spectroscopic methods

2 Different types of spectroscopy afford different information about molecules and their structure: INFRARED what types of functional groups? NMR what types of H s, C s, P s, F s...? - how many, what are they connected to? MASS SPEC what is molecular weight and formula? UV-VIS how many double bonds?

3 For example: 1 H NMR can tell us what kinds of environments each proton is in: H H H O O H COOH H H H

4 Same technique can also be used in medicine: MRI (NMR of water 1 H signals)

5 p. 1 Intensity Energy (E) of absorption or emission of emr emr = electromagnetic radiation Frequency ( ) or wavelength ( )

6 electromagnetic radiation emr p. 1 y wavelength ( ) x z magnetic field electric field c = λ c = speed of light = 3.00 x 10 8 ms -1 = wavelength in meters ( 1 nm = 10-9 m = 10 Å) ν = frequency in s -1 ( 1 Hz = 1 cycle / second = 1 s -1 )

7 c = = c/ = c/ p. 2 If = 1m, then = 3 x 10 8 / 1 = 3 x 10 8 Hz = 300 MHz If = 10-8 m, then = 3 x Hz UV Visible light: from about 4 to 7 x 10-7 m OR nm

8 ENERGY of Radiation: p. 3 E = h ν = h c λ where E = energy of a single photon (in Joules ) h = Planck s constant = x Js E = N h ν = N h c where E = energy of a mole of photons (in Joules per mole) N = Avogadro s number = x 10 23

9 ENERGY of Radiation: p. 3 E = h ν = h c λ E = N h ν = N h c λ single photon mole of photons h = Planck s constant = x Js N = Avogadro s number = x mol -1 For visible light, = 5 x 10-7 m E = x Js x 3 x 10 8 ms -1 5 x 10-7 m = 4 x Joules E = 4 x J x x = 2.4 x 10 5 J/mol = 240 kj/mol For uv light, = 200nm = 2 x 10-7 m E = 599 kj/mole

10 Common terminology varies with region of the spectrum: p. 4

11 p. 5 Type of Energy Region Process Instrument Nuclear Magnetism radiowave flip the nuclear magnetic spin Rotational microwave rotation of molecule m MHz 1 cm 30 GHz - NMR MRI Vibrational infrared internal vibrations bond stretch/bends cm -1 IR or Raman Electronic Vis/UV energy change of valence electrons nm UV/vis electronic nuclear X-ray -ray core electrons nuclear change 1 Angstrom 0.1 nm X-ray diff.

12 Absorbing energy causes changes dependent on the wavelength: p. 5 e.g. UV or visible light promotes an ELECTRONIC TRANSITION from the ground state E 0 to an excited state E 1 Electrons light (h ) absorption E 0 (ground state) E 1 (excited state) Molecule E 2 E 1 E 0 E = E 1 -E 0 = h only frequency can cause this excitation - Planck energy is quantized

13 INFRARED = 10 m = 10 x 10-6 m p. 6 so = c/ = (3 x 10 8 )/(1 x 10-5 ) = 3 x Hz Kind of awkward numbers, so we use a convenient stand in for the frequency instead: WAVENUMBER = 1/ where is in cm = 10,000/ where is in so for = 10 m = 1 x 10-3 cm, WN = 1000 cm -1 so the unit of cm -1 is a frequency

14 Typical IR spectrum: runs from 2.5 to 15 m p. 6 WN runs from 4000 to 600 cm -1 Note: scale is non-linear

15 SHORT WAVELENGTH LONG WAVELENGTH HIGH FREQUENCY LOW FREQUENCY HIGH ENERGY LOW ENERGY p. 6 E = Nhc/ = Nhc x WN but remember then c = 3 x cm/sec 36 kj/mol (or 9 kcal/mol) 12 kj/mol (or 3 kcal/mol)

16 HIGH ENERGY LOW ENERGY p. 6 C-H stretch C=O stretch C-H bend lighter elements Bond Stretching heavier elements Bond Bending

17 p. 6 For IR peaks to be strong (be seen) the bond dipole must change during the vibration C=O bond stretch C===O charges move apart, dipole

18 GREENHOUSE GASES p. 7 so N 2, O 2, Ar do not give IR spectra (no dipole) H O - H + +

19 p. 7 CO 2 H 2 O

20 p. 7 H 2 O CO 2 CH 4

21 Fine structure = ROTATIONS can only be seen clearly in simple molecules p. 8

22 p. 9 QUANTIZATION and SELECTION RULES Molecules are limited to specific energy levels E 3 E 2 Excited States E 1 E 0 Ground State True whether electronic, vibrational, rotational, spin

23 Molecular Energy Levels: electronic (S), vibrational (v) and rotational (J) p. 9 E N E R G Y

24 p. 9 Absorption E 1 Emission A Transition E 0 E = E 1 -E 0 = h only causes absorption Need to know: What jumps are possible? What levels are populated?

25 1] No selection rules, i.e. all jumps possible p. 10 BUT only ground state is populated E 3 E 2 E 2 -E 0 = h 2 E 1 -E 0 = h 1 E 3 -E 0 = h 3 E 1 E 0 Absorption of light frequency 2 will cause jump from E o to E 2

26 2] The IR CASE Only ground state populated p. 10 Selection rule = jump of + or 1 allowed, n = ± 1 E 3 E 2 E 1 E 1 -E 0 = h 1 E 0 Only one line, frequency 1

27 3] The Microwave (Rotational Spectra) case p. 10 All energy levels populated Selection rule is n = ± 1 E 3 E 3 -E 2 E 2 E 2 -E 1 E 1 E1 -E 0 E 0 Relative positions depend upon energy level spacings and in microwave these spacings are not all the same

28 Populations The BOLTZMANN Equation p. 11 N N upper lower e E RT E = E upper E lower J mole -1 R = JK -1 mole -1 T = temp in Kelvin NOTE: If E in J per molecule, use k = R/N = x JK -1 What happens if E is really small OR really large?

29 Rotational (microwave) spectra, E small, = 0.1 m p. 11 E = Nhc/ = (6.022 x ) x (6.626 x ) x (3 x 10 8 ) 0.1 = 1.2 J mol -1 N U N L = e -(1.2/(8.3 x 300)) = ~1 so both levels equally populated so in IR, vibrational lines not very sharp because lots of similar energy rotational levels, which give lots of similar energy lines (refer to p. 7 and slide 19)

30 Vibrational (IR) spectra, E larger, 1/ = 1700 cm -1 p. 12 E = Nhc/ = (6.022 x )x(6.626 x )x(3 x )x1700 = 20,350 J mol -1 N U N L = e -(20,350/(8.3 x 300)) = so only lower level is populated so in IR, = to so N U /N L = 0.20 to 10-7

31 p. 12 Transition Spectra E (kj/mol) N U /N L at 300K rotation microwave vibration IR 25 5 x 10-5 electronic UV-vis What happens when T increases? ASSIGNMENT 1