PRINCIPLES OF CHEMISTRY I
PRINCIPLES OF CHEMISTRY I CH 301
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Date Created: 09/07/15
Lecture 3 1HNMR Spectroscopy 361 4 3 39 39 39 39 39 39 39 39 39 39 39 39 39 H 7 Recommended Homework Read Pages 427 430 and Chapter 13 Absolutely do problems 131 132 133bampd 134 135 136 138a 1319 1327 Supplemental problems to lg 1 The resonance frequency for 1H at 47 Tesla is 200 MHZ What magnetic eld strength is required to create a 300 MHZ proton nmr spectrometer The spectrum of ethyl isopropyl ether has several resonances Please sketch a predicted spectrum The integral ratio of the high eld protons to the low eld protons is 3 1 and the quartet to the septet is 2 1 Please calculate the relative areas of the individual peaks that are marked in the in the low eld multiplets Show your work Ethyl isopropyl ether NMR Spectrosopy NMR spectrometer NMR speclrum Rndin l rrqurllc oulpu receiver oscillator pnwurul plurlrnmngnpl nr supercnnducling mngnel Nuclear Spins in Bo Nuclei spin numbers 12 and l2 Applied magnetic field B igher energy slate AEhv or v AEh Lower energy state l S in 12 aligned against the applied eld Spin 12 aligned with the applied eld Nuclear Spins in B0 0 Within a collection of 1H or 13C atoms nuclear spins are random in orientation 0 When placed in a strong external magnetic eld of strength BO interaction between nuclear spins and the applied magnetic eld is guantized with the result that only certain orientations of the nuclear magnetic moments are allowed Nuclear Spins in B0 c In an applied eld strength of 705T BIG AE between nuclear spin states for 1H is approximately 00286 calmol which corresponds to electromagnetic radiation of 300 MHz 300000000 Hz 13C is approximately 000715 calmol which corresponds to electromagnetic radiation of 7 SMHz 75000000 Hz Resonance o The transition from the lower state to the higher occurs at a unique combinations of magnetic eld and frequency of electromagnetic radiation 0 When placed in a magnetic eld of strength B a particle with a net spin can absorb a photon of frequency v The frequency v depends on the gyromagnetic ratio y of the particle v yB For hydrogen y 4258 MHZ Tesla m my c wrest v I Energy difference between allowed nuclear spin states for 1H nuclei Spin aligned against the applied field ll l callfmul Energy 000572 calr mul Spin 415 aligned with the applied eld I I I l I I is i 39 3 JAE T 139 l T Bu lZFII BSIHl H In principle we could hold eld constant and scan frequency looking for resonance However it is equally effective to scan eld strength and hold frequency constant this is how most instruments work NMR spectrometer NMR spectrum Radio frqquency Radiu frequency nnlpu receiver 4 Inplll uscillulur l nu erful Eleclr ma net 1139 supemmuumng nlnguel The 100MHz nmr Chart nu HH HH nu 50 0 300 100MHz Spectrum 0 5 0 5 2 1 1 100 150 200 250 350 400 450 T 304 Hz Hz 100MHz nmr Spectrum 2685Hz HZ Nuclear Magnetic Resonance o If we were dealing with 1H nuclei isolated from all other atoms and electrons any combination of applied eld and radiation that produces a signal for one 1H would produce a signal for all 1H The same for 13C nuclei 0 But hydrogens in organic molecules are not isolated from all other atoms they are surrounded by electrons which are caused to circulate by the presence of the applied eld o The circulation of electrons around a nucleus in an applied eld is called diamagnetic current a This current generates a eld that opposes the applied eld diarnagnetic nuclear shieldin results Lenz s Law cnditi cns for Resonance o The NET field is the sum of all incident magnetic elds including those from The Giant Magnet applied eld Diamagnetic Shielding Field electrons Coupling spin elds of adjacent nuclei Other Effects credit card strips earth s eld etc Chemical Shift 0 The difference in resonance frequencies for hydrogens in CH3C1 compared to CH3Br under an applied field of 234T is only 355Hz which is 035 parts per million ppm compared with the irradiating frequency 35 Hz 035 o35 m 100x1t Hz 106 pp Here 35Hz is the resonance frequency difference in energy and 100MHz is the 1H resonance frequency for B234T and y 4258 MHz Tesla The Chemical Shift The difference betweeen TMS and CH3C1 is 304HZ for the 235T 100MHz magnet the chemical shift is 304Hz It is also 304 Hz 304 304 m 100x1t Hz 106 pp Shift in frequency from TMS Hz Frequency of spectrometer Hz 5 ppm M 133 iii 1 frijheiiiiwii iii ii Viiw ii V39ftirm ii irgh W The Chemical Shift At 705T the resonance is at 912 Hz What is the chemical shift in ppm V yB 4258 X 705 300 MHz spectrometer frequency 912 Hz 304 304 m 300 x 16quot Hz 106 pp Chemical Shift 1Hnmr Type of H 5 Type of H 6 C H3 4 Si 0 ROH 0560 RCH3 08 12 RCH2 OR 3340 RCH2 R 1214 R2 NH o55o R3 CH 1417 O RZCCRC HR2 1626 Ryzcm 2123 RC ECH 2030 O Arc H3 2245 R CH2 R 2226 ArC H2 R 2328 Chemical Shift 1HNMR Type of H 5 Type of H 8 u RCOC H3 3539 R2 CC H2 4650 quot R2 CC HR 5057 RCOC H2 R 4147 ArH 6585 RCH2 3133 RCH2 Br 3436 RltquotH 951o1 RCH2 CI 3638 II RCH2 F 4445 RCOH 1013 Chemical Shift Depends on gt electronegativity of nearby atoms gt hybridization of adjacent atoms gt magnetic induction Within an adjacent pi bond Electroneg CH3 X ativity of X 5 of H CH3 F 40 426 CH3 OH 35 347 CH3 CI 31 305 CH3 Br 28 268 CH3 25 216 CH 34 C 21 086 C H34 Si 18 000 by definition Signal Areas 0 Relative areas of signals are proportional to the number of hydrogens giving rise to each signal 0 All rnodern 1H NMR spectrometers integrate electronically and record the area of each signal Note Such integration doesn t generally work for other nuclei for a variety of reasons Integration Equivalent Hydrogens 0 Have exactly the same Chemical environment Cl Cl CH 3 A CH 3 H Cl 00 3 c H H 11Dichloro CyclopentanoneZ1Choropropene Cyclohexene ethane 2 Signals 2 signals 3 signals 3 Signals How many equivalent hydrogens 1 C1 This looks easy but it is not Chemical Shift 0 The Hybridization of C Effects 1H chemical Shift Hybrid Type of H Name 6 SP3 RCH3 alkyl 08 10 sp RCE CH acetylenic 20 30 sp2 R2 CCH 2 vinylic 46 57 I Magnetic field induced in the pi bonds if a carboncarbon triple bond Induced Inca magnetic field nf the pi electrons is against the applied field it requires a greater applied field to bring an acetylenic hydrogen into resonance Induced flow af electrons in the pi system If alkyne Induced eld opposes big magnet Applied eld Bquot Magnetic field induced in the pi bond of a carboncarbon double bond Note backwards Induced circulation of pi electrons in the alkene Induced local magnetic field of the pi electrons rein erees the applied eld and provides part of the field necessary tin bring a vinyl hydrogen into resenance Induced eld 1 helps big magnet Applied field Bu Chemical Shift 0 Magnetic induction in pi bonds of a a carboncarbon triple bond shields an acetylenic hydrogen and shifts its signal upfleld to the right to a smaller 8 value carboncarbon double bond deshields Vinylic a hydrogens and shifts their signal down eld to the left to a larger 8 value These signals 100k weird Signal Splitting Huh Why What is going on here What a mess 0 Signal splitting splitting of an NMR signal into a set of peaks by the in uence of neighboring nonequivalent hydrogens o This splitting business is actually rich in information it is a wonderful thing Let s go back to basics What we are doing is trying to find the energy to effect a spin ip We know that this depends on the effective magnetic field which we see as difference in frequency Energy difference between allowed nuclear spin states fur 1H nuclei Spin 9392 aligned against the appHied eld 00286 eaiinml Spin 4 15 aligned with the applied eld Cnergy 0110572 calfnwl 39ll 41 T 705 TT Bu l eslui 3H iv Recall o The NET field is the sum of all incident magnetic elds including those from The Giant Magnet applied eld Diamagnetic Shielding Field electrons Coupling spin elds of adjacent nuclei As we Will see this comes about because nearby spins act as baby bar magnets and if they are close enough they modulate the effective eld of the spin you are measuring Other Effects credit card strips earth s eld etc Origins of Signal Splitting a When the chemical shift of one nucleus is in uenced by the spin of another the two are said to be coupled 0 Consider nonequivalent hydrogens Ha and Hb on adjacent carbons the chemical shift of H3 is in uenced by Whether the spin of Hb is aligned with or against the applied eld Ha lquotb Y C C X Bo Magnetic eld of H b subtracts from the applied eld Hb signal appears at a higher applied eld Magnetic eld of H b adds to the applied eld H a Signal appears at a lower applied eld 4quotquot kill 3 51L pmquot quot v quot Maw wwwwmm The signal of Ha is split into two peaks of equal area a doublet Peak when H Peak when H adds to BD subtracts from Bo I I Origins of signal splitting no neighbors 11 111 11 Relative Intensity of Peaks singlet 1 double 1 1 triplet 1 2 1 Quartet 1 3 3 l quintete 1 4 6 4 1 sextete 15101051 Pascal s triangle The binomial coef cients The N1 Rule 0 The 1HNMR signal of a hydrogen or set of equivalent hydrogens is split into N 1 peaks by a set of N equivalent neighboring hydrogens All neighboring hydrogens in the analysis must have the same chemical shift magnetically equivalent If this condition is not met a graphical tree or second order analysis must be used to predict the splitting pattern In other words things get messy u H quota Signal from the p oton of H in ence of 3 Magnetic momems of H spiil ihe signa from M 39 ks of equal In doublet U Two magne c orienla ons coth Jab m p a Inlensny a 11 Applied field no JL elm PII I signal in the absence I W at prawns 1th I llh H 1111 I TX C C C fix E xxxquot I I I I E Prawns at 11 E 5 spllit the signal possible magnetic i 5 131132 o entatinns of lac Jim J maq P prawns 01 H51 1 j 1 1 EDI Applied field HQ 39H39 Hi Ha 1 I E 1 signal In the EDSEHGE H1 ff 0f prawns Hg Hb f r I x a x u I K x z I i 1 x I f I Prawns m Hi I 39 I I split the signal L L I I inti a 13311 FE Jab Jag J b H quartet I passihla magnetic quotquot quotquot Driematinns 0f 2 prawns of Hb quot39 I q I 1 4 BD Ethyl Fragment jab Jabll Jabl Jab l Jab Signal fur Hb Signal for Ha Common Splitting Patterns CH3 X H3 H3 C E39 X CH3 CH3CH2 X CH3 H IC X CH3 3H singlet Methyl 9H singlet tbutyl 2H quartet and 3H doublet ethyl 6H doublet and 1H septet isopropyl M Splitting Patterns n1 n1 2 J H IlH H v J 2 3 333334333 3L 3 3 L H CH3 CH3 r L 3 3 LL 3334333 m3 3 3 L I3H3 CH3 r JJL 3 3 J CHs E r quLu
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