There is also a cross-peak at (4.20, 1.25), because that relationship goes both ways. The hydrogens at 1.25 ppm are coupled to the ones at 4.2 ppm, and that gives a "cross-peak" at (1.25, 4.20). Those peaks indicate which hydrogens are coupled to which other hydrogens. The interesting peaks are the ones that do not appear along the diagonal. There are three peaks appearing along that diagonal line these three peaks basically show us what we already saw in the regular 1H NMR spectrum. Other such peaks appear along the diagonal stretching from the upper right to the lower left through the middle of the plot. In the middle of the plot, the peaks are shown plotted against each other - that is, there is a peak at 1.25 ppm on the y axis and along the x axis, so there is a data point on the plot at (1.25, 1.25). The same peaks are also displayed along the other axis. In the following simulated COSY spectrum, the peaks are displayed along one axis. Instead of being displayed as a row of peaks, the peaks are spread out into an array. The COSY spectrum simply takes that 1H spectrum and spreads it out into two dimensions. There is a quartet and a triplet the hydrogens corresponding to those two peaks are probably beside each other in the structure. In ethyl acetate, it's pretty clear where they are. In homonuclear correlation spectroscopy (COSY), we can look for hydrogens that are coupled to each other. The 1H NMR spectrum is not very complicated.
Let's start with a simple compound, ethyl acetate.
In 2D NMR, correlation peaks are used to help establish the structure.
0 kommentar(er)
