Heteronuclear correlation - HETCOR

Transcription

1 Heteronuclear correlation - HETCOR Last time we saw how the second dimension comes to be, and we analed how the COSY eperiment (homonuclear correlation) works. In a similar fashion we can perform a 2D eperiment in which we anale heteronuclear connectivit, that is, which H is connected to which C. This is called HETCOR, for HETeronuclear CORrelation spectroscop. The pulse sequence in this case involves both C and H, because we have to somehow label the intensities of the C with what we do to the populations of H. The basic sequence is as follows: C: H: t { H}

2 HETCOR (continued) We first anale what happens to the H proton (that is, we ll see how the H populations are affected), and then see how the C signal is affected. For different t values we have:, t = 0, t = J / 4, t = 3J / 4

3 HETCOR ( ) As was the case for COSY, we see that depending on the t time we use, we have a variation of the population inversion of the proton. We can clearl see that the amount of inversion depends on the J CH coupling. Although we did it on-resonance for simplicit, we can easil show that it will also depend on the H frequenc (δ). From what we know from SPI and INEPT, we can tell that the periodic variation on the H population inversion will have the same periodic effect on the polariation transfer to the C. In this case, the two-spin energ diagram is H- C: αβ 2 C 4 ββ H,2 3,4 H αα C 3 βα,3 2,4 I S Now, since the intensit of the C signal that we detect on t 2 is modulated b the frequenc of the proton coupled to it, the C FID will have information on the C and H frequencies.

4 HETCOR ( ) Again, the intensit of the C lines will depend on the H population inversion, thus on ω H. If we use a stacked plot for different t times, we get: t (ω H ) The intensit of the two C lines will var with the ω H and J CH between +5 and -3 as it did in the INEPT sequence. ω C f 2 (t 2 ) Mathematicall, the intensit of one of the C lines from the multiplet will be an equation that depends on ω C on t 2 and ω H on t, as well as J CH on both time domains: A C (t, t 2 ) trig(ω H t ) * trig(ω C t 2 ) * trig(j CH t ) * trig(j CH t 2 )

5 HETCOR ( ) Again, Fourier transformation on both time domains gives us the 2D correlation spectrum, in this case as a contour plot: ω C J CH ω H f f 2 The main difference in this case is that the 2D spectrum is not smmetrical, because one ais has C frequencies and the other H frequencies. Prett cool. Now, we still have the J CH coupling splitting all the signals of the 2D spectrum in little squares. The J CH are in the H range, so we can start having overlap of cross-peaks from different CH spin sstems. We ll see how we can get rid of them without decoupling (if we decouple we won t see H to C polariation transfer ).

6 HETCOR with no J CH coupling The idea behind it is prett much the same stuff we did with the refocused INEPT eperiment. 80 t / 2 t / 2 C: H: t 2 { H} We use a C π pulse to refocus H magnetiation, and two delas to to maimie polariation transfer from H to C and to get refocusing of C vectors before decoupling. As in INEPT, the effectiveness of the transfer will depend on the dela and the carbon tpe. We use an average value. We ll anale the case of a methine (CH) carbon...

7 HETCOR with no J CH coupling (continued) For a certain t value, the H magnetiation behavior is: α (ω H - J / 2) β (ω H + J / 2) t / 2 β α 80 C t / 2 α β β α Now, if we set to / 2J both H vectors will dephase b b eactl 80 degrees in this period. This is when we have maimum population inversion for this particular t, and no J CH effects: β β α α

8 HETCOR with no J CH coupling ( ) Now we look at the C magnetiation. After the proton π / 2 we will have the two C vectors separated in a 5/3 ratio on the <> ais. After the second dela 2 (set to / 2J) the will refocus and come together: We can now decouple H because the C magnetiation is refocused. The 2D spectrum now has no J CH couplings (but it still has the chemical shift information), and we just see a single cross-peak where formed b the two chemical shifts: ω H f f 2 ω C

9 Summar The HETCOR sequence reports on which carbon is attached to what proton and shows them both - Great for natural products stuff. The wa this is done is b inverting H population and varing the transfer of H polariation to C during the variable t. We can obtain a decoupled version b simpl lumping in an refocusing echo in the middle. Net time HOMO2DJ spectroscop. Coherence transfer and multiple quantum spectroscop. HAVE A COOL (AND SAFE) BREAK!!! (and work on the take-home )