Heptet means 6 neighbor hydrogens but base on CD3OD structure, it cannot be 6
Dear Paris Do , welcome to NMR! :-)
The dominant signal that you observe in the 13C spectrum in the area of the methanol methyl is the multiplet of 13C nucleus coupled to 3 equivalent deuteria of the CD3- fully deuterated methyl group.
Now, deuterium has spin 1 and thus each deuterium splits anything-that-is-coupled-to-it into a 1:1:1 triplet. When there is another deuterium, that one will split each peak of the triplet again in a triplet, giving rise to 1:2:3:2:1 pentuplet (the thing that we observe in the 1H spectrum of deutero-methanol, where it is due to the CD2H- methyl groups). When, like in the case we are discussing, there is still another equivalent deuterium, then the latter splits into triplet each of the pentuplet peaks, giving rise to a nice 1:3:6:7:6:3:1 heptaplet, which is what you see.
The (n+1) rule that you are mentioning applies only to stuff-coupled-to-groups-of-spin-1/2-nuclei (like protons). With spin-1 nuclei (like deuterons), it becomes a (2n+1) rule - and the intensities pattern is different.
PS: When you make a question about a spectrum, please always illustrate it with a picture showing your spectrum. Even when the case is as simple as this one, you never know what other people might pick up from it. In fact, depending on your spectrum's S/N, there could be some more comments here. Nature is always a bit more tricky than what we like to think (and teach).
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