Molecular Spectroscopy. H 2 O e -

Transcription

1 Molecular Spectroscopy ν (cm -1 ) λ (cm) ν (Hz) NMR ESR microwave IR UV/Vis VUV X-Ray Gamma Ray H 2 e -

2 UV/Vis Spectroscopy absorption technique X hν * Y Y Z X Z e - e -

3 UV UV(B) UV(A) visible E = hν c = λν λ (nm) If a molecule absorbs a photon at 400 nm, how much energy is absorbed by the molecule? E = ν c hc = E = λ λ -34 J s x m s m -1 = J 5 x J/molecule x molecules/mol = 300 kj/mol

4 Single beam hν I o Instrumentation I source grating slit sample cell detector (pmt) mirror I hν I o Double beam chopper ref. cell

5 Source: visible work ( nm) uses a tungsten filament UV ( nm) uses a deuterium lamp Monochromator: prism or grating 510 ±0.5 nm intensity λ wavelength (nm)

6 absorbance absorption band of holmium oxide 'monochromatic light' wavelength (nm) Sample cells: cuvettes with path lengths of 0.1, 1.0 and 10.0 cm quartz ( nm) glass ( nm) plastic (350 nm)

7 Detectors: photomultiplier tube (pmt) ++ e e - e - hν + anode +++++

8 λ = 510 nm hν I o I %T = I/I o A = -log T T = 100 % A = 0 T= 50 % A = 0.30 T = 10 % A = 1 T = 1 % A = 2 T = 0.1 % A = 3

9 For a specific λ, Beer-Lambert Law A = εbc ε is molar absorptivity (L mol -1 cm -1 ) b is pathlength of cell (cm) c is concentration of analyte (M) A T = A 1 + A = ε 1 bc 1 + ε 2 bc Where and why UV-Vis spectroscopy is used... - easily quantifiable and good sensitivity - inexpensive and fast - reasonably selective

10 - sensitive, 10-4 to 10-7 M (depends on ε and b) - accurate (1 to 5 %) Experimental Considerations 1. Verify that A = εbc is linear by checking the calibration curve. 2. Calibration standards should match matrix of samples with respect to T, ph, and interferences I o I - reflection - scattering - absorbance due to cell and solution - absorbance due to analyte

11 3. Select λ as λ max for sensitivity and accuracy. A = ebc da is a minimum dλ wavelength (nm) A = εbc λ max 4. Calibrate A and λ 5. λ small wrt absorption band 6. Record absorbances from 0.1 to 1.5

12 A = εbc = log T c log T = 1 εb c = log e T 2 εb T 2 1 c c = log e T εbt log T εb = log e T T log T sc c = log e s T log T T If A = 1, T = 0.10 and s T = then sc log e st s = c = c T log T c log e log(0.10) = 1.3% s c A = 2, = 6.5% c

13 sc c log e st = shows a minimum when T = 1/e = 0.37 or A = 0.43 T log T Sc/c 35% 30% 25% 20% 15% 10% 5% 0% 0 1 A 2 3 Sc/c 8% 7% 6% 5% 4% 3% 2% 1% 0% A <A<1.5

14 E 1 * E E o λ λ λ

15 A B C wavelength (nm) A benzene in vapour phase, res. = 0.2 nm B benzene in vapour phase, res. = 0.5 nm C acetone in hexane A

16 A wavelength (nm) A I 2 in gas phase B I 2 in CH 2 Cl 2 B A

17 Transition metal complexes - spectra are complex d x y d 2 z 2 2 d xy d yz d xz E Cu 2+ in water A wavelength (nm)

18 A A B wavelength (nm).... A [(H 2 ) 5 Fe-S=C=N].. 2+ [(H 2 ) 5 Fe-S=C=N] B Fe(phen) 2+ 3 Fe(phen) (MLCT) (LMCT)

19 2 1.5 Ho 2 3 A wavelength (nm)

20 σ * π * n π σ E = hc λ σ σ * CH 4 λ max = 125 nm n σ *.. CH 3 H.. λ max = nm, ε = M -1 cm -1 H 2 λ max = 167 nm Me 3 N λ max = 227 nm CH 3 I λ max = 258 nm

21 π π * alkenes, alkynes, carbonyls λ max nm H 2 C=CH 2 λ max = 171 nm ε = M -1 cm -1 n π * λ max 280 nm ε M -1 cm -1 σ * π * n π σ A π π * λ max = 189 nm ε = 900 M -1 cm -1 H 3 C C CH wavelength (nm).... n π * λ max = 280 nm ε = 12 M -1 cm -1

22 σ * π * n π σ.. Chromophores: N, C=C, C=Ȯ... Auxochromes.. C=Ọ. X ε hyperchromic shift ε hypochromic shift λ bathochromic or red shift λ hypsochromic or blue shift λ max (π π * ) (nm) ε (M -1 cm -1 ) , , , , ,700

23 λ max (n π * ) (nm) ε (M -1 cm -1 ) R 2 C C H R - R 2 C C H + R H H C C H H H C H - H H H C C H C H+ H

24 Solvents: should be pure and transparent in region of interest Solvent Cutoff (nm) Solvent Cutoff (nm) H cyclohexane 200 CH 3 H 205 CH 3 CN 180 hexane 200 acetone acetone A wavelength (nm)

25 Polar solvents can broaden absorption bands λ max can shift H H blue shift for the n π * transition H 3 C CH 3 red shift for the π π * transition X Z Y solvent X Z Y solvent X Y Z solvent X Y Z

26 transoid cisoid λ max = 217 nm λ max = 250 nm ε = 21,000 M -1 cm -1 ε is lower Table 6.5 Empirical Rules for Dienes Parent Homoannular Heteroannular (cisoid) λ = 253 nm (transoid) λ = 214 nm Increments for: double bond extending conjugation alkyl substituent or ring residue 5 5 exocyclic double bond 5 5 polar groupings: -CCH R 6 6 -Cl, -Br 5 5 -NR

27 A B C D λ max = 214 nm (exp. = 217 nm) λ max = 229 nm (exp. = 228 nm) λ max = 234 nm (exp. = 235 nm) λ max = 353 nm (exp. = 355 nm) H 3 C Parent Homoannular Heteroannular (cisoid) λ = 253 nm (transoid) λ = 214 nm Increments for: double bond extending conjugation alkyl substituent or ring residue 5 5 exocyclic double bond 5 5 polar groupings: -CCH R 6 6 -Cl, -Br 5 5 -NR

28 .... C X π π *, λ 190 nm n π *, λ 280 nm Carbonyls As X becomes more electronegative, the n π * transition shifts to the blue. H 3 C C CH 3 λ max = 279 nm ε = 15 M -1 cm -1 H 3 C C NH 2 λ max = 214 nm ε = low H 3 C C H λ max = 204 nm ε = 41 M -1 cm C NR C + NR2 Red shift of the π π * transition

29 C R δ β C R γ α Base values six-membered ring/acyclic enone 215 nm five-membered ring 202 acylic dienone 245 Substituent Position α β γ δ double bond in conjugation 30 alkyl group/ring residue H CCH CH Cl Br NR 2 95 exocyclic double bond 5 homocyclic diene 39

30 λ max = 249 nm (exp. = 249 nm) λ max = 302 nm (exp. = 300 nm) Base values six-membered ring/acyclic enone 215 nm five-membered ring 202 acylic dienone 245 Substituent Position a b g d alkyl group/ring residue H CCH CH Cl Br NR 2 95 double bond in conjugation 30 exocyclic double bond 5 homocyclic diene 39

31 l max (nm) (e M -1 cm -1 ) primary secondary 184 (60,000) 204 (7400) 254 (204) CH (7000) 261 (300) CH (300) Cl.. CH NH C N H 210 (7600) 265 (240) 235 (9400) 287 (2600) 203 (7500) 254 (160) 230 (11,600) 273 (970) 269 (7800)

32 E 1 * E E o UV-Vis IR E o ν ν

33 %T wavenumber (cm-1) IR spectra of HBr (g) and polystyrene

34 H v o v hν 1 Br H Br r o r 1 hν H H H H θ o θ 1

35 1cm 1 = 1 wavenumbe r, ν, = 1 1cm ν = 1 λ If ν = 4000 cm cm 1 λ = λ = cm = 2500 nm E = hν c = λν ν = c λ E = hc λ E = J s x m m -34 s -1 = J 7.95 x J/molecule x molecules/mol = 45 kj/mol

36 ref. cell mirror I r mirror sample cell hν I s grating beam splitter Double beam, Dispersive chopper slit 100 detector 80 % T wavenumber (cm-1) 500

37 fixed mirror movable mirror Fourier Transform (FT) Spectrometer hν beam splitter sample detector P displacement

38 wavenumber (cm-1) % T wavenumber (cm-1) % T % T wavenumber (cm-1) 450 0

39 Advantages of an FT instrument 1. Speed of data collection (less than one second) 2. High sensitivity 3. High resolution (0.01 cm -1 to 0.1 cm -1 ) 4. Very accurate values for ν

40 Sources: rare-earth oxides, silicon carbide or Nichrome wire Beam splitter: KBr plate coated with Ge Monochromator: reflection grating Detectors: i) measure increase in temperature (0.005 ºC) ii) pyroelectric [tgs, triglycine sulfate, (NH 2 CH 2 CH) 3 H 2 S 4 )] iii) photoconducting, MCT, HgCdTe, 77K

41 Sampling Solids: (i) mix < 1 mg of sample with 250 mg of KBr and form a pellet (ii) as 50/50 mixture with Nujol (iii) as a solution Liquids: between two KBr plates Gases: in a tube with KBr windows

42 wavenumber (cm-1) FT-IR Spectrum of a polystyrene film % T ν I ν

43 ν = 1 2πc K µ µ = m1m2 m + m 1 2 Calculating Stretching frequencies í (cm 1 ) = 4.12 K ì K = dyne/cm single bond dyne/cm double bond dyne/cm triple bond For a C-H bond stretch: K ì í (cm ) = 4.12 = 4.12 = For a C- 2 H (C-D) bond stretch: K ì í (cm ) = 4.12 = 4.12 = 3032 cm cm -1

44 Intensity of a band For the C=X stretch of: I = 0 I = weak I = strong ν is related to hydrogen bonding

45 group ν (cm -1 ) group ν (cm -1 ) -H 3400 C C 2150 N-H 3400 C= 1715 C-H 3000 C=C 1650 C N 2250 C C-H 3000 cm -1 C cm -1 C cm -1 C= 1715 cm -1 C-Cl 750 cm -1 C 2143 cm -1

46 Alkanes CH 2 CH H 3 C CH 2 CH 3 CH 3 H C H H CH 3 C-H stretch at < 3000 cm -1 CH 2 bend at 1465 cm -1 CH 3 bend at 1375 cm -1 gem-dimethyl at 1370 and 1390 cm -1 -(CH 2 ) n>4 - bend at 720 cm -1 Nujol %T wavenumber (cm-1) 400

47 C is sp2 hybridized Alkenes C C H C-H stretch at > 3000 cm -1 (3050 to 3200 cm -1 ) C=C stretch at cm C C C X C C C X ν (C=C) 1610 cm -1 As number of alkyl substituents increase on C=C group, ν (C=C) increases. C C H H out-of-plane (P) bend

48 ut-of-plane Bending Frequencies for Alkenes R cm -1 R 2 C C H H R cm -1 H C C H R 2 R cm -1 H C C R 2 H R and 990 cm -1 H C C H H 100 ν (cm -1 ) , , % T wavenumber (cm-1) 450

49 ν (C=C) = cm -1 ν (C=C) = cm -1

50 Alkynes R-C C-H C-H stretch at 3300 cm -1 C C stretch at 2150 cm ν (cm -1 ) C C H 96 % T wavenumber (cm-1)

51 Aromatic compounds C-H stretch at 3100 cm -1 C=C stretch at 1600 and 1475 cm -1 H H H H R H P C-H bend (cm -1 ) 690 and 750 R' R' R R' R R , 780 and usually

52 vertone/combination bands at cm -1. v 3 v 2 An overtone band is approximately 2 or 3 times the frequency of a fundamental band. v 1 v CH wavenumber (cm-1) % T

53 Alcohols: R--H -H stretch, strong and broad, cm -1. If H-bonding is not present... -H stretch for primary alcohols 3640 cm -1 secondary alcohols 3630 cm -1 tertiary alcohols 3620 cm -1 phenols 3610 cm -1 C- stretch for primary alcohols 1050 cm -1 secondary alcohols 1100 cm -1 tertiary alcohols 1150 cm -1 phenols 1220 cm -1

54 (v) 3331 (l) % T wavenumber (cm-1) IR spectra of 1-octanol in vapour phase and as a neat liquid

55 Ethers R--R' asymmetric C- stretch at 1120 cm -1 (R and R' are alkyl) 1250 cm -1 (R is alkyl, R' is aryl) symmetric C- stretch at 1040 cm -1 (R is alkyl, R' is aryl) C 6 H 5 --CH 2 -CH 2 -CH wavenumber (cm-1) % T

56 R C R C R C R C R C R C R C Cl H R' H R' H NR 2 Carbonyls: C= 1810 cm -1 (asymmetric), 1760 (symmetric) (free acid)

57 ν (C=) cm -1 H ν (C=) cm -1

58 Supplemental Slide: Two bands for a C= s-cis R R s-trans Effect of α halogens: H R R Cl ν (C=) 1725 cm -1 Cl R R H ν (C=) 1750 cm -1

59 Aldehydes: R C H C= stretch at 1725 cm -1 C-H stretch at 2850 and 2750 cm % T wavenumber (cm-1) IR spectrum of nonyl aldehyde

60 Ketones: R C R' ν (C=) 1715 cm wavenumber (cm-1) 50 % T IR spectrum of 3-nonanone

61 1715 cm cm -1 R 2 C C 2140 cm -1 R 1680 cm -1 Ar R Ar Ar 1685 cm cm -1 H H 3 C CH 2 CH 3 H 3 C CH CH 3 ν (C=) = 1723 and 1706 cm -1 ν (C=) = 1622 cm -1 ν (-H) = cm -1

62 Carboxylic Acids: R C H ν (C=) = cm -1 ν (-H) = cm -1 (br) ν (C-) = 1260 cm -1 -H bending at 930 cm % 80% wavenumber (cm-1) % % T 40% 20% 0% 400 IR spectrum of nonanoic acid

63 Esters: C R' C R' R' C R R ν (C=) = cm -1 ν (C-) = cm -1 (two bands) ν (C-) = cm -1, stronger and broader R ν (C-) = cm CH 3 C C 2H wavenumber (cm-1) 60 % T

64 ν (C=) = 1735 cm cm cm cm cm cm -1

65 Amides: R.. NRR' ν (C=) cm -1 Amines: R N H R N H H R N R' H R' N-H stretch N-H bend C-N stretch 3300, m, br R NH , R NHR

66 NH % T wavenumber (cm-1) NH wavenumber (cm-1) % T

67 Nitriles: R-C N ν (C N) = 2250 cm -1 (aliphatic), 2230 cm -1 (aromatic) C 3450 N wavenumber (cm-1) % T

68 Nitro: R + N ν (N-) = 1350, 1550 cm -1 (aliphatic) ν (N-) = 1300, 1500 cm -1 (aromatic) N wavenumber (cm-1) % T

69 Fluorescence -emission technique E 1 * E hν hν' E o λ

70 Intensity of Fluorescence lifetime of E 1 * is proportional to 1/ε XYZ * r f r r r others XYZ XYZ XYZ rf Φ = Quantum yield, Φ (0 to 1) r + r + r f r others

71 å >> ð-ð * å n-ð * biphenyl Φ = 0.2 fluorene Φ = 1

72 source hν grating slit sample cell attenuator grating detectors (pmt)

73 F λ excitation (nm) A / F l absorption/emission (nm)

74 Experimental Considerations 1. F = kp o εbc detection limits: 10 to 0.01 ppm self-quenching self-absorption A < 0.05 or T > 90 %

75 2. Φ decreases as i) T increases ii) viscosity decreases iii) concentration of paramagnetic molecules increases iv) concentration of halogens in solvent or on analyte increases 3. F is ph-dependent, both Φ and λ emission

76 4. For quantitative applications a calibration curve is required 5. Precision and accuracy are both approx. 3 %

77 Applications: - determination of metals 3 N H Al 3+ N N Al N - quantification of biochemical analytes such as tryptophan, NH vitamins, steroids and DNA 2 H 2 N N + ethidium bromide - analysis of PAHs benzo[a]pyrene