Chem 1140; Spectroscopy UV-VIS IR NMR UV-VIS Spectroscopy

Chem 1140; Spectroscopy  UV-VIS  IR  NMR UV-VIS Spectroscopy

Chem 1140; Spectroscopy UV-VIS IR NMR UV-VIS Spectroscopy I1 TheA bso rpioLtaw ns I0 I intensity of the incident beam

Detector transmitted light Overall transmittance: T= Internal transmittance: T Usually T0 = Ti I2 i= I2 I1

I I0 (what is actually measured) (of interest to the spectroscopist) Such differences as might exist can be minimized by using match cells and setting T for the reference at 100%. The quantity I/I0 is independent of the intensity (I) of the source and proportional to the number of absorbing molecules Lambert-Beer Law: log I0

I = . . l c l = path length of the absorbing solution in [cm] c = concentration in moles/liter Log I0/I = absorbance or optical density = A = molar extinction coefficient [1000 cm2 mol-1] = A

i s a characteristic of a g iven compound, or more accurately, the light absorbing system of the compound, the so called chromophore. i s correlated to the size () of the chromophore and of course wave-length dependent. = f () f (c) (Approximation! An accurate determination of r equires determining A at various concentrations) : 10 - 105 (scales with extended -systems) (dyes) UV-Vis spectra are usually plotted as A vs. l plots:: A 2 hyps:ochromic

hyprchromic s:hifts: bathochromic hypochromic 1 pp np lmax [nm]nm] l Though there are discrete levels of electronic excited states in a

molecule, we do not observe absorption lines but broad peaks; the change in vibrational and rotational energy levels during absorption of light leads to peaks containing vibrational and rotational fine structure. Due to additional interaction with solvent molecules, this fine structure is blurred out, and a smooth curve is observed. (vapour phase: one can observe vibrational fine structure). Selection Rules The irradiation of organic compounds may or may not give rise to excitation of electrons from one orbital to another orbital. There are transitions between orbitals that are quantum mechanically forbidden. Two selection rules: -Spin-rule: The total spin S may not change during transition (S S; T ; T T) !S; T ymmtry rul: trans:itions: btwn orbitals: of idntical s:ymmtri ar not allowd.

(for ex. Even/uneven with regard to inversion).for x. Evn/uneven with regard to inversion).unvn with rgard to invrs:ion n p * vn vn In reality, these quantum mechanical rules are not rigidly observed, however, due to molecular vibrations, the intensities ( ) of "forbidden" transitions are significantly reduced (and are usually of diagnostic importance).

n * band near 300 nm of ketones; = 10-100 benzene 260 nm band with = 100-1000. Chromophores Definition: Chromophore light absorbing electron system of a compound Rules: - p or n! orbitals: that do not intract lad to a s:pctrum that is: s:um of th individual abs:orptions: of th is:olatd chromop ! th longr th conjugatd s:ys:tm, th longr th wavl th abs:orption maximum and th highr its: intns:ity. ! a bathochromic and hyprchromic ffct is: obs:rvd, wh with n!orbitals: ar dirctly attachd to a chromophor (for ex. Even/uneven with regard to inversion).!OH, -OR, NH2, SH, SR, Hal) = auxochromic groups., !OR, NH2, SH, SR, Hal) = auxochromic groups., NH, -OR, NH2, SH, SR, Hal) = auxochromic groups.2, S; T H, -OR, NH2, SH, SR, Hal) = auxochromic groups., S; T R, NH2, SH, SR, Hal) = auxochromic groups., H, -OR, NH2, SH, SR, Hal) = auxochromic groups.al) = auxochromic groups.) = auxochromic groups:.

Isolated Chromophores Of the once listed in table, only few are of practical significance (Vacuum-UV) p p h C u C E p ~ l

max = 190nmnm p Conjugated Chromophores p p h C u C E

p p ~ l max = 190nmnm 3 p A A 2 p

n 1 H H S S=Auxochrome S Benzene and aromatic compounds The UV spectra of benzenes are characterized by three major bands which have been given a variety of names. Only b is: allowd. B band is: forbiddn (for ex. Even/uneven with regard to inversion).los:s: of s:ymmtry du to molcular vibr s:hows: vibrational fin s:tructur).

MO Diagram of Ferrocene Fe FeCp2 2 Cp- e1up a1gp 4p a2u, e1u 2gp a2up 2u 2g, 2u

4s a1g UV Spectrum of Ferrocene 300 250 4p a1g, e1g, e2g 200 150 1u a1g 2g

100 1g, 1u 50 1u 0 1g -50 200 300 400 500

600 700 800 lambda [nm] Ferrocene has a molar extinction coefficient of 96 M -1cm-1 at 442 nm a2u a1g a1g, a2u Nomenclature of Electronic Transitions; Symbols of Symmetry Classes Symbols of symmetry classes: A: sym. (according to a Cn operation)

B: antisym. (according to a Cn operation) E: 2-fold degenerate state T: 3-fold degnerate state Examples: 1A 2 1B 1u 1B 2u 1E 1u 1A 1 1A 1g 1A

1g 1A 1g Indices: g: sym. (according to an inversion operation) u: antisym. (according to an inversion operation) 1: sym. (according to a C2 axis that is orthogonal to a Cn axis) 2: antisym. (according to a C2 axis that is orthogonal to a Cn axis) : sym. (according to a plane of symmetry s:n that is orthogonal to a Cn axis) : antisym. (according to a plane of symmetry s:n that is orthogonal to a Cn axis) Calculation of spectra: Bonus Problem: Calculate UV and IR Spectra of Ferrocene and Acetylferrocene, and Compare to Experimental Data; can you design a Ferrocene derivative that is greencolored?

Infrared Spectroscopy After considering ultraviolet and visible radiation (200-800 nm) which is energetic enough to affect the electronic levels in a molecule, we shall now consider radiation which has a longer wavelength: infrared radiation which extends beyond the visible into the microwave region and is capable of affecting both the vibrational and the rotational energy levels in molecules. Range of commercial instruments: 2500 nm to 16000 nm physical chemists: 25000 160000 analytical chemists: 2.5 16 microns ( m) organic chmis:ts:: 40nm0nm0nm 625 cm!1 wavenumber n = 1/ = /c

"normalized frequency" Use: simple, rapid, reliable means for functional group identification. Vibrational modes For a molecule comprised of N atoms, there are 3N-6 normal modes of vibration (3N-5 for linear molecules). To a good approximation, however, some of these molecular vibrations are associated with the vibrations of individual bonds or functional groups (localized vibrations) while others must be considered as vibrations of the whole molecule. Localized vibrations are: Stretching modes: simple C

H coupled: H H C symmetric H H C asymmetric Bending modes:

H H H C C scissoring (sym) H H H C

wagging (sym) in plane deformations rocking (asym) H H C twisting (asym) out of plane deformations Transmission O H

C C C C N H C N C O X Y Z stretching C N C H stretching

N O stretching N H bending Absorbance 2500 4000 3000 other stretching bending and combination bands FINGERPRINT REGION 1500

2000 1000 cm -1 wave numbers Selection rules 1. In order to observe an absorption, the dipole moment of the excited vibrational state must differ from that of the ground state. Reason:oscillating dipole interacts with oscillating electric vector of h n O H C C H O

H C C H observed not observed Analysis of IR - Spectra of Unknown Compounds 1. frequency, shape, intensity of an absorption band have all to be considered in the interpretation 2. all characteristic absorption frequencies of a functional group have to be considered (band can be missing or is caused by another function) 3. first the obvious absorptions should be identified: X-H, C=O, C=C, out of plane C-H 4. subsequently the strong absorptions in the fingerprint region should be analyzed 5. possible structures can now be proposed and have to be checked with reference spectra (or data from other spectroscopic techniques)

An Introduction to NMR Spectroscopy H NMR 1 13 C NMR The types of information accessible via high resolution NMR include: 1. Functional group analysis (chemical shifts) 2. Bonding connectivity and orientation (J coupling), 3. Through space connectivity (Overhauser effect) 4. Molecular Conformations, DNA, peptide and enzyme sequence and structure. 5. Chemical dynamics (lineshapes, relaxation phenomena). http://www.chem.ucla.edu/%7Ewebspectra/

Nuclear Spin: The proton is a spinning charged particle and has also a magnetic moment. 1 H: - nuclear spin quantum number m = 1/2 B - such a nucleus is is described as having a nuclear spin I of 1/2 m = +1/2 lower energy m = -1/2 higher

energy Because nuclear charge is the opposite of electron charge, a nucleus whose magnetic moment is parallel to the magnetic field has the lower energy. The difference in energy is given by: E = gh B0nm/uneven with regard to inversion).2 p g = magntogyric ratio (for ex. Even/uneven with regard to inversion). a cons:tant, typical for a nuclus:, which s:s:ntially rfl th s:trngth of th nuclar magnt) Bo = s:trngth of th applid magntic fild h = Plancks: cons:tant (for ex. Even/uneven with regard to inversion).3.99 x 10nm!13 kJ s: mol!1) Note that as the field strength increases, the difference in energy between any two spin states increases proportionally. nucl ar s:pin quantum #

E m =!1/uneven with regard to inversion).2 Ei = !m hg Bo/uneven with regard to inversion).2 p O degenerate E = !m mNBo m = +1/uneven with regard to inversion).2 O Bo [values of = 1/ were picked for m, so that the difference in energy between two neighboring states will always be an integer multiple of Bo ( gh/uneven with regard to inversion).2 p)].

The number of nuclei in the low energy state (Na) and th numb r in th high n rgy s:tat (for ex. Even/uneven with regard to inversion).Nb) willdiff r by an am ount dt rmin d by th Boltzm ann dis:tribution: Nb/uneven with regard to inversion).Na = (for ex. Even/uneven with regard to inversion).!E/uneven with regard to inversion).kT) k = 1.3 81 x 10nm !23 JK!1 W hn a r adio frqu ncy (for ex. Even/uneven with regard to inversion).R, NH2, SH, SR, Hal) = auxochromic groups.F) s:ignalis: appli d, this: dis:tribution is: changd if th radio fr qu ncy matchs: E. E = hn = hg B 0nm /uneven with regard to inversion).2 p n = r s:onanc fr qu ncy = g B0nm /uneven with regard to inversion).2 p n is: th rfor dp ndn t upon both th appli d fi ld s:trng th and th naturof th

nucl us:. 1 H: in a 2.35 T field (earth magnetic field = 0.00006 T) = 0.999984 ~ 1 in 106 (n = 10nm0nm MH, -OR, NH2, SH, SR, Hal) = auxochromic groups.z) - The difference in population of t he two states i s exceedingly small, in the order of few parts per million. (even smaller in 13C, because is smaller). - Relatively low sensitivity of NMR compared to IR or UV - Large Bo needed to increase the population difference (usually given in MHz of 1H resonance frequency). SUMMARY - Nuclear spin is a property characteristic of each isotope and is a

function of Z and N. - Each isotope with I 0 has a characteristic magnetogyric ratio ( g) that dtrmins: th frquncy of its: prcs:s:ion in a magntic fild of s:trngth B0nm n= B0 2 It is this frequency that must be matched by the incident electromagnetic radiation for absorption to occur. - When a collection of nuclei with I 0 is immersed in a strong magnetic field, the nuclei distribute themselves among 2I + 1 spin states, the relative population of which is determined by the Boltzmann distribution, usually being near unity Nb

Na =e (-E/uneven with regard to inversion).kT) - If two (or more) spin state populations become equal, the system is said to be saturated. Obtaining an NMR Spectrum Magnet Source of RF radiation Detector + amplifier Plotter, sample The magnet: permanent cheap, stable, fixed field

1.4T electromagnet more expensive, stronger, variable field superconducting expensive, stronger, variable field 18T (24T) Strength of magnetic field shifts: lock necessary (= substance with strong, defined NMR signal) Older: reference internal, external CDCl3 TMS: 0.0 ppm singlett

Once a stable field is established, the question remains as to whether that field is completely homogeneous throughout the region between the pole faces of the magnet. N S lines of magnetic flux not uniform Sample sample has to be placed near the center of the pole gap For 2.35 T, to achieve a precision of +/- 1 Hz (10 ppb at 100 MHz) the field must be homogeneous to the extent of +/- 2.35 x 10-8 T! Such a phenomenal uniformity,

even at the center of the field , can be achieved only by means of two additional techniques: Spinning of the sample ("averages" out small inhomogeneities) Variation of the contour of the field by passing extremely small currents through shim coils wound around the magnet itself: Shimming (manually, automatically) Paradox: Large sample in order to have as many nuclei as possible, small sample to increase uniformity of the field. narrow bor tubs: The Pulsed Fourier Transform Technique Further advances in S/N ratio improvement had to await the development of faster computer microprocessors: ~1970s. - RF radiation is supplied by a brief but powerful pulse of RF current through the transmitter coil. The spectral width of the pulse is chosen to cover absorption of all nuclei of interest. Th duration ofth puls: (for ex. Even/uneven with regard to inversion).tp) d t rm in s: th fr qu ncy rang cov r d (for ex. Even/uneven with regard to inversion).H, -OR, NH2, SH, SR, Hal) = auxochromic groups. is: nb rg's: unc rtainty principl : E t > h)

RF intensity S; T W ~ tp!1 ; tp (for ex. Even/uneven with regard to inversion).4 S; T W )!1 2SW Fr qu ncy no Optimum tp are obtained by trial and error and are usually in the order of 10 ms: fora = 90nm for bs:t S; T /uneven with regard to inversion).N ratio. The next step in the PFT process is to monitor the induced AC receiver signal. Digital data collection gives us the modulated free induction decay (function of Mxy). FID because the current intensity decreases with time. This decay is the result of T2 (spin-spin) relaxation.

M the microprocessor samples the voltage in the receiver coil at a regular interval, called dwell time, td. Voltage td > (2SW)-1 td t0 t0 Time In a set of nuclei with different n prcs:s:ion and T1/uneven with regard to inversion).T2, th digital FI curv bcoms: vry complx:

CH3 At this point it becomes necessary for the computer to recognize the patterns mathematically and extract the signal frequencies and relative intensities for each set of nuclei. This analysis is performed by a Fourier transformation of the FID date. f = ( x ) a

+ o { n a cos: n = 2 p

ns: x o + b s:in 2 p ns: n

x } o 1 where: a0 = constant; an = amplitude; x = period; so = fundamental frequency; xo = 1/so; and n = order of harmonic The parent function is constructed by summing together a series of sine waves.

line width: uncertainty principle: t > 1 1/2 > 1 T2* Nuclei that are slow to relax give sharp signals, nuclei that relax rapidly give broad signals (solids). 1 2 3 paramagnetic residues 0

frequency line broadening 1H NMR - Wink, D. J. J. Chem. Ed. , 1989, 66, 810. Spin-Lattice Relaxation Times in Spectroscopy. - Glasser, L. J. Chem. Ed. , 1987, 64, A228. Fourier Transforms for Chemists. - King, R. W.; Williams, K. J.R. Chem. Ed. , 1989, 66, A213, A243. The Fourier Transform in Chemistry. Summary:

A typical example of the generation of a PFT spectrum: t p: pulse time ( ms:c) t acq: th lngth of th tim a givn FI s:ignal is: actually monitord (for ex. Even/uneven with regard to inversion).rs:olution, th ability to dis:tinguis:h two narby s:ignals:, is: invr prop.acq to. R, NH2, SH, SR, Hal) = auxochromic groups.t = (for ex. Even/uneven with regard to inversion).acq t )!1 3 s:c 0nm.3 H, -OR, NH2, SH, SR, Hal) = auxochromic groups.z tw: delay time, to allow for equilibrium distribution tw =

3T1 - tacq. + dead time ( adding up, FT, spectrum (for 1H no waiting time) phasing necessary) phase correction Taking an NMR Practical Consideration - Use 5 mm tube filled with ~ 0.5 mL of solution containing 1-5 mg of sample (1H NMR). - common deuterated solvents: CCl4, CDCl3, C6D6, DMSO-d6, D2O, CD3CN, CD2Cl2, d6-acetone, CD3OD (because of HCl formation, do not leave sample in CDCl3!) - peak listings in ppm and/or Hz. - paramagnetic metal ion broad paks: Chemical shift

nH, -OR, NH2, SH, SR, Hal) = auxochromic groups. dpnds: on Bo thrfor rlativ frquncis: ar rportd: n 83 . 4 Hz act n

. 6 TMS = = = 1 . 39

x 10 6 60 x 10 o 1.39 ppm = d = downfild from TMS; T . 2

ppm (for ex. Even/uneven with regard to inversion).d) 1 0 downfi ld upfi ld d s:hi ld d s:hi ld d = 1 .

39 ppm Integration Area under absorption peak ~ # of nuclei resonating at that n But: nucli mus:t rlax to quilibrium btwn puls:s:, not gnrally tru of 13C NMR, NH2, SH, SR, Hal) = auxochromic groups.! First-Order Spin-Spin-Coupling 1 H - 1H From previous discussions one could have gotten the impression that a typical 1H NMR spectrum exhibits just one signal for each set of equivalent 1H-nuclei and that the same thing is true for 13C spectra,

as well as for spectra of any other isotope. However, there are many more lines in a spectrum, and while these extra lines do make a spectrum more complex, they also offer valuable structural information that complements the chemical shift data. CH3CH2OH:The spin states of two hydrogens (methylene group): B0 M=-1 0 0 1 Total Magnetization Three spin states with population ratio of 1:2:1

For methyl hydrogens the net experienced field will depend on the magnetization of the neighboring methylene group! The methyl signal will be split into three lines with intensity ratio 1 : 2 : 1 (= spin-spin-coupling, homonuclear coupling because the coupling is between nuclei of the same isotope). Triplet. Accordingly, for the methylene signal, the possible spin states of the methyl group determine its multiplicity (number of lines in the signal). The spin states of three hydrogens: M=-3/2 M=3/2 4 spin states with population ratio of 1 : 3 : 3 : 1

M=1/2 M=-1/2 Quartet Accordingly, a doublet is observed for hydrogens that are coupled to a methine (CH) proton. The multiplicity of a given resonance = n+1 (n=# of neighboring equivalent nuclei). The relative intensities of the multiplet follow Pascals triangle. J J J = spacing between lines.

The slight difference in energy between the resonances is the coupling constant J [Hz]. Js are independent of instrumental parameters! Consider a 3-spin system with/without equivalent nuclei: Ha Hb Hc C C C Ha will be a doublet with 3Jab Hb will be a doublet of doublets with 3Jab and 3Jbc Hc will be a doublet with 3Jbc Jbc Jab Jab Ha

Hb Jbc Hc 13 12C (98%) has I = 0 C NMR no NMR, NH2, SH, SR, Hal) = auxochromic groups. 13C (1.1.%) has I = 1/2 NMR, NH2, SH, SR, Hal) = auxochromic groups. (E = hn = ghBo/uneven with regard to inversion).2 p gyrom agn tic ratio s:uch that nobs: 1/uneven with regard to inversion).4 that of1H, -OR, NH2, SH, SR, Hal) = auxochromic groups.

(for ex. Even/uneven with regard to inversion).3 0nm0nm MH, -OR, NH2, SH, SR, Hal) = auxochromic groups.z 75 MH, -OR, NH2, SH, SR, Hal) = auxochromic groups.z) Observe typically 0 230 ppm (rel. to TMS) Typically decouple the protons by saturating them with a second broadband RF puls (double resonance technique, second transmitter coil; white noise if h t irradiating fi ld is: s:t rong nough, notonly willth1H, -OR, NH2, SH, SR, Hal) = auxochromic groups. nucli approach s:aturation, but v irtuall y allth 1H, -OR, NH2, SH, SR, Hal) = auxochromic groups. magntization willb tipp d into h t x1y plan. S; T inc th 1H, -OR, NH2, SH, SR, Hal) = auxochromic groups. nucli ar no longr align d with or (for ex. Even/uneven with regard to inversion). a gains:t)th applid fild (for ex. Even/uneven with regard to inversion).which is: along th z axis:) thy can no long r augmn t or dimi nis:h h t

m agntic fild xp rinc d byth carbons:. As: a rs:ult, th coupling int ract ion dis:appars:, and ach 13 C multiplt coll aps:s: toa s:ingl t! (for ex. Even/uneven with regard to inversion). coupling not ffctd!) causes all 13 C resonances to be singlets affords Nuclear Overhauser Effect makes integration of 13 C spectra unreliable Aromatics: not effected by ring current 128.5 ppm substituted Cs are typically of lower intensity --

++ 176.8 176.8 209.0 102.1 85.3 Carbonyls: O O 205.1

H 199.6 O O OH 177.3 RO Cl 168.6 O O OR OEt

169.5 ~ 100 - 110 O CH3 N H 174.9 I 158.9 Heavy atom effect.

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